[Lee W. Janson] Medical Biochemistry - The Big Picture
March 17, 2017 | Author: JD Singh | Category: N/A
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Notice Medi ci ne i s a n ever-cha ngi ng s ci ence. As new res ea rch a nd cl i ni ca l experi ence broa den our knowl edge, cha nges i n trea tment a nd drug thera py a re requi red. The a uthors a nd the publ i s her of thi s work ha ve checked wi th s ources bel i eved to be rel i a bl e i n thei r efforts to provi de i nforma ti on tha t i s compl ete a nd genera l l y i n a ccord wi th the s ta nda rds a ccepted a t the ti me of publ i ca ti on. However, i n vi ew of the pos s i bi l i ty of huma n error or cha nges i n medi ca l s ci ences , nei ther the a uthors nor the publ i s her nor a ny other pa rty who ha s been i nvol ved i n the prepa ra ti on or publ i ca ti on of thi s work wa rra nts tha t the i nforma ti on conta i ned herei n i s i n every res pect a ccura te or compl ete, a nd they di s cl a i m a l l res pons i bi l i ty for a ny errors or omi s s i ons or for the res ul ts obta i ned from us e of the i nforma ti on conta i ned i n thi s work. Rea ders a re encoura ged to confi rm the i nforma ti on conta i ned herei n wi th other s ources . For exa mpl e a nd i n pa rti cul a r, rea ders a re a dvi s ed to check the product i nforma ti on s heet i ncl uded i n the pa cka ge of ea ch drug they pl a n to a dmi ni s ter to be certa i n tha t the i nforma ti on conta i ned i n thi s work i s a ccura te a nd tha t cha nges ha ve not been ma de i n the recommended dos e or i n the contra i ndi ca ti ons for a dmi ni s tra ti on. Thi s recommenda ti on i s of pa rti cul a r i mporta nce i n connecti on wi th new or i nfrequentl y us ed drugs .
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CONTENTS Dedications Acknowledgements About the Authors SECTION I: THE BASIC MOLECULES OF LIFE CHAPTER 1 Ami no Aci ds a nd Protei ns Overvi ew Ami no Aci ds —Structure a nd Functi ona l Groups Es s enti a l a nd Non-Es s enti a l Ba s i c Structure Cha ra cteri s ti cs of R-Groups Ba s i c Protei n Structure Level s of Protei n Structure Ca tegori es of Protei ns Ami no Aci d a nd Pepti de-Deri ved Hormones a nd Neurotra ns mi tters Enzymes Structura l Protei ns Motor Protei ns Tra ns port/Cha nnel Protei ns Revi ew Ques ti ons CHAPTER 2 Ca rbohydra tes Overvi ew Ba s i c Ca rbohydra te Structure a nd Functi on Monos a ccha ri des a nd Di s a ccha ri des Gl ycogen a nd Sta rches Gl ycoprotei ns Gl ycos a mi nogl yca ns Revi ew Ques ti ons CHAPTER 3 Li pi ds Overvi ew Ba s i c Li pi d Functi ons Ba s i c Membra ne Li pi d Structure Compl ex Li pi ds Gl ycol i pi ds /Sphi ngol i pi ds Ei cos a noi ds Chol es terol Li poprotei ns Bi l e Sa l ts Li pi d-Deri ved Hormones /Vi ta mi n D Corti cos teroi ds (Adrena l Gl a nd) Androgens (Tes tes ) a nd Es trogens (Ova ri es ) Vi ta mi n D Revi ew Ques ti ons CHAPTER 4 Nucl eos i des , Nucl eoti des , DNA, a nd RNA Overvi ew Nucl eos i des a nd Nucl eoti des Components of Nucl eos i des a nd Nucl eoti des Synthes i s of Puri ne Nucl eos i des a nd Nucl eoti des Synthes i s of Pyri mi di ne Nucl eos i des a nd Nucl eoti des Forma ti on of Deoxy Nucl eos i des a nd Nucl eoti des Brea kdown of Puri nes a nd Pyri mi di nes RNA a nd DNA—Ba s i c Structure a nd Functi on RNA DNA Revi ew Ques ti ons SECTION I: Integra ted USMLE-Styl e Ques ti ons a nd Ans wers Ques ti ons Ans wers
SECTION II: FUNCTIONAL BIOCHEMISTRY CHAPTER 5 Enzymes a nd Ami no Aci d/Protei n Meta bol i s m Overvi ew Enzymes Enzyme Rea cti ons Cofa ctors Regul a ti on Ami no Aci d Meta bol i s m Ami no Aci d Synthes i s Ami no Aci d Degra da ti on The Urea Cycl e Revi ew Ques ti ons CHAPTER 6 Ca rbohydra te Meta bol i s m Overvi ew Gl ycol ys i s Ci tri c Aci d Cycl e Oxi da ti ve Phos phoryl a ti on Gl uconeogenes i s The Pentos e Phos pha te Pa thwa y Gl ycogen Synthes i s Gl ycogen Brea kdown Modi fi ed Ca rbohydra tes (Gl ycoprotei ns , GAGs ) Revi ew Ques ti ons CHAPTER 7 Li pi d Meta bol i s m Overvi ew Fa tty Aci d Meta bol i s m Fa tty Aci d Synthes i s Fa tty Aci d Degra da ti on Meta bol i s m of Compl ex Li pi ds Tri a cyl gl ycerol Synthes i s Phos phogl yceri de Synthes i s Ketone Body Synthes i s Cera mi de/Sphi ngol i pi ds Synthes i s Chol es terol Synthes i s Revi ew Ques ti ons CHAPTER 8 Membra nes Overvi ew Membra ne Structure Li pi ds Protei ns Membra ne Functi ons Membra ne Cha nnel s Membra ne Si gna l i ng Revi ew Ques ti ons CHAPTER 9 DNA/RNA Functi on a nd Protei n Synthes i s Overvi ew Structure of the Nucl eus Hi s tones Nucl ea r Ma tri x/Sca ffol d Nucl eol us a nd Ri bos ome Synthes i s DNA Repl i ca ti on a nd Tra ns cri pti on DNA Repl i ca ti on Tra ns cri pti on Protei n Synthes i s Pos ttra ns l a ti ona l Tra ffi cki ng/Modi fi ca ti on Control of Gene Expres s i on Muta ti ons a nd Repa i r Mecha ni s ms Regul a ti on of Cel l Growth a nd Di fferenti a ti on Revi ew Ques ti ons SECTION II: Integra ted USMLE-Styl e Ques ti ons a nd Ans wers Ques ti ons
Ans wers SECTION III: APPLIED BIOCHEMISTRY CHAPTER 10 Meta bol i s m a nd Vi ta mi ns /Mi nera l s Co-authors/Editors: Maria L. Valencik and Cynthia C. Mastick Overvi ew Meta bol i c Rol es of Ma jor Bi ochemi ca l Mol ecul es Integra ti on a nd Regul a ti on of Meta bol i s m Gl ucos e-6-phos pha te Pyruva te Acetyl -CoA Hormona l Control of Meta bol i s m Ins ul i n Gl uca gon Ca techol a mi nes Gl ucocorti coi ds Di a betes Mel l i tus (DM) Vi ta mi ns a nd Mi nera l s Vi ta mi ns Mi nera l s Revi ew Ques ti ons CHAPTER 11 The Di ges ti ve Sys tem Editor: Kshama Jaiswal Overvi ew Summa ry of the Di ges ti ve Sys tem Mouth Stoma ch Li ver Li pi d Meta bol i s m i n the Li ver Ga l l Bl a dder Pa ncrea s Sma l l Intes ti ne (Duodenum, Jejunum, a nd Il eum) La rge Intes ti ne/Anus Revi ew Ques ti ons CHAPTER 12 Mus cl es a nd Moti l i ty Co-author/Editor: Darren Campbell Overvi ew The Ba s i c Components of Mus cl e Acti n Tropomyos i n–Troponi ns Myos i n Myos i n Li ght Cha i ns Acti n-Bi ndi ng Protei ns Exci ta ti on–Contra cti on Coupl i ng Skel eta l Mus cl e Structure a nd Genera l Overvi ew Skel eta l Mus cl e Types Ca rdi a c Mus cl e Smooth Mus cl e Energy Producti on a nd Us e i n Mus cl es Mi crotubul e-Ba s ed Moti l i ty Intermedi a te Fi l a ments Nonmus cl e Cel l s Revi ew Ques ti ons CHAPTER 13 Connecti ve Ti s s ue a nd Bone Editor: Jacques Kerr, BSc, MB, BS, FRCS, FCEM Overvi ew Connecti ve Ti s s ue Components of Bone Bone Growth a nd Remodel i ng Regul a ti on of Ca l ci um Level s
Ma rkers of Bone Forma ti on a nd Res orpti on Revi ew Ques ti ons CHAPTER 14 Bl ood Co-authors/Editors: Matthew Porteus, MD, PhD and Tina Mantanona Overvi ew Ba s i c Components of Bl ood Red Bl ood Cel l (RBC) Functi ons Di s ea s es As s oci a ted wi th Ina dequa te Synthes i s of Hemogl obi n Components Oxygen Bi ndi ng Tens e a nd Rel a xed Hgb Al l os teri c Bi ndi ng of O2 by Hgb Regul a ti on of O2 Bi ndi ng Phys i ol ogi c Res pons e to Ina dequa te O2 Del i very Si ckl e Cel l Di s ea s e (SCD) Iron Iron Meta bol i s m Tra ns ferri n Ferri ti n Regul a ti on of Iron Ava i l a bi l i ty by Hepci di n Cl otti ng Pl a tel et Pl ug Forma ti on Cons i s ts of Adhes i on, Aggrega ti on, a nd Acti va ti on of Pl a tel ets The Cl otti ng Ca s ca de The Fi bri n Mes hwork Di fference between Pl a tel et Pl ug Forma ti on a nd Cl ot Forma ti on Regul a ti on of Cl ot Forma ti on Pl a s mi n a nd Cl ot Di s s ol uti on Revi ew Ques ti ons CHAPTER 15 The Immune Sys tem Editor: Eric L. Greidinger, MD Overvi ew Overvi ew of the Immune Sys tem Anti gen Anti body Cel l s As s oci a ted wi th the Immune Sys tem T Lymphocytes B Lymphocytes /Pl a s ma Cel l s Na tura l Ki l l er (NK) Cel l s Monocytes a nd Ma cropha ges Neutrophi l s Eos i nophi l s Ba s ophi l s Dendri ti c Cel l s (DCs ) Cytoki nes Inna te Immuni ty Compl ement s ys tem Hypers ens i ti vi ty Rea cti ons Revi ew Ques ti ons CHAPTER 16 The Ca rdi ova s cul a r Sys tem Editor: Ralph V. Shohet, MD Overvi ew Ca rdi a c Mus cl e Structure a nd Functi on Si noa tri a l a nd Atri oventri cul a r Nodes The Ca rdi a c Cycl e Bl ood Ves s el s Endogenous Chol es terol /Li poprotei n Meta bol i s m a nd Tra ns port VLDL Intermedi a te-Dens i ty Li poprotei n (IDL) a nd LDL HDL Atheros cl eros i s Bi ochemi ca l Mecha ni s ms As s oci a ted wi th Hea rt Atta ck Revi ew Ques ti ons
CHAPTER 17 The Res pi ra tory Sys tem Editor: Howard J. Huang, MD Overvi ew Ba s i c Ana tomy a nd Devel opment Pul mona ry Surfa cta nt a nd the Devel opi ng Lung O2 –CO2 Excha nge i n the Lung a nd Aci d–Ba s e Ba l a nce Noni nfecti ve Di s ea s es of the Res pi ra tory Sys tem Obs tructi ve Di s ea s es —Emphys ema Obs tructi ve Di s ea s es —Bronchi ti s Obs tructi ve Di s ea s es —As thma Bi ochemi ca l Ba s i s of As thma Medi ca ti ons Res tri cti ve Di s ea s es —Acute Res pi ra tory Di s ea s e Syndrome Res tri cti ve Di s ea s es —Occupa ti ona l Expos ures Res tri cti ve Di s ea s es —Inters ti ti a l Lung Di s ea s es Infecti ve Di s ea s es of the Res pi ra tory Sys tem Revi ew Ques ti ons CHAPTER 18 The Uri na ry Sys tem Editor: Armando J. Lorenzo, MD, MSc, FRCSC, FAAP Overvi ew Ba s i c Ana tomy a nd Phys i ol ogy Rena l Fi l tra ti on The Rena l Corpus cl e Nephron Inul i n/Crea ti ni ne Cl ea ra nce Reni n–Angi otens i n–Al dos terone Sys tem (RAAS) Reni n a nd Bl ood Pres s ure Ma cul a Dens a a nd Bl ood Fl ow/Os mol a ri ty Angi otens i nogen/Angi otens i n I a nd II Al dos terone Va s opres s i n Atri a l Na tri ureti c Pepti de (ANP) Aci d–Ba s e Ba l a nce NH 3 a nd Aci d–Ba s e Ba l a nce Syntheti c Functi ons Synthes i s of Erythropoi eti n Rol e i n Vi ta mi n D Synthes i s Revi ew Ques ti ons CHAPTER 19 The Nervous Sys tem Editor: Kathryn Beck-Yoo, MD Overvi ew Components of the Nervous Sys tem Nerve Impul s e Conducti on Neuron a t Res t Nerve Impul s e Repol a ri za ti on Autonomi c Nervous Sys tem Sympa theti c Nervous Sys tem Pa ra s ympa theti c Nervous Sys tem Neurotra ns mi tters Dopa mi ne NE/Epi nephri ne Serotoni n Acetyl chol i ne (Ach) Regul a ti on of Ca techol a mi nes Gl yci ne, Gl uta ma te, a nd GABA Neuropepti des Bi ochemi s try of Vi s i on Anes thes i a Revi ew Ques ti ons CHAPTER 20 The Reproducti ve Sys tem Editor: Catrina Bubier, MD Overvi ew
Ba s i c Ana tomy a nd Devel opment Fema l e Reproducti ve Sys tem GnRH FSH LH Es trogens Proges terone hCG The Mens trua l Cycl e Mens trua ti on (Da ys 1–4) Fol l i cul a r/Prol i fera ti ve Pha s e (Da ys 5–13) The Lutea l /Secretory Pha s e (Da ys 15–28) Ferti l i za ti on Brea s t Devel opment a nd La cta ti on Oxytoci n Prol a cti n Ma l e Reproducti ve Sys tem Tes tos terone FSH a nd LH Revi ew Ques ti ons SECTION III: Integra ted USMLE-Styl e Ques ti ons a nd Ans wers Ques ti ons Ans wers SECTION IV: APPENDICES APPENDIX I: Bi ochemi ca l Ba s i s of Di s ea s es Contributing Editor: Harold Cross, MD, PhD Ami no Aci d Synthes i s /Degra da ti on Ami no Aci d Tra ns port Urea Cycl e Di s orders Structura l Protei ns Ca rbohydra tes Gl ycogen Stora ge Mi tochondri a l Enzymes (Excl udi ng Urea Cycl e a nd Fa tty Aci d Oxi da ti on) Li pi ds a nd Fa tty Aci d Oxi da ti on Errors Nucl eoti de Meta bol i s m Defecti ve DNA Bi l i rubi n Meta bol i s m Bl ood Cl otti ng Fa ctor Defects Steroi d Hormone Synthes i s Vi ta mi ns /Mi nera l s a nd El ectrol ytes APPENDIX II: Bi ochemi ca l Methods Pol ya cryl a mi de Gel El ectrophores i s (PAGE) [Sodi um Dodecyl Sul fa te (SDS)/Non-SDS] Immunoa s s a ys RIA ELISA or EIA Chroma togra phy Thi n La yer (Pa per) Chroma togra phy (TLC) Col umn Chroma togra phy Gel Fi l tra ti on Chroma togra phy Ion-Excha nge Chroma togra phy Affi ni ty Chroma togra phy Hi gh-Performa nce/Pres s ure Li qui d Chroma togra phy (HPLC) Protei n a nd Deoxyri bonucl ei c Aci d (DNA)/Ri bonucl ei c Aci d (RNA) Preci pi ta ti on DNA a nd RNA Sequenci ng Southern, Northern, a nd Wes tern Bl ots Southern Bl otti ng Northern Bl otti ng Wes tern Bl otti ng PCR Cl oni ng
Fl ow Cytometry La bora tori es APPENDIX III: Orga ni c Chemi s try Pri mer Overvi ew Introducti on The Si x Orga ni c El ements (C, H, N, O, P, a nd S) Ca rbon (C) Hydrogen (H) Ni trogen (N) Oxygen (O) Phos phorus (P) Sul fur (S) Bi ochemi ca l Functi ona l Groups (H, OH, COOH, NH 3 , PO3 , S—S, COH, a nd +
Hydrogen (Pa rti a l l y Cha rged a nd Ioni c Forms , H ) Hydroxyl Group (—OH −) Ca rboxyl Group (—COOH) Ami ne Group (—NH 2 ) Phos pha te Group (PO3 a nd PO4 ) Sul fur–Sul fur Bonds (—S—S—) Al dehyde Group (—COH) Ketone (— Summa ry Index
)
)
DEDICATIONS To my fa mi l y: my dea r wi fe, Meryl , who ha s unfa i l i ngl y s upported my profes s i ona l endea vors a nd endured my countl es s eveni ng hours a t the computer on thi s project; my da ughters , Rebecca , La ura , a nd Mi ri a m, who bri ng me i ncredi bl e “na ches ”; my mother a nd fa ther (ma y they res t i n pea ce) for thei r s upport i n my forma ti ve yea rs ; my brothers Howa rd a nd Ma tthew; my mother-i n-l a w Ma rtha for her ever-pres ent a ccol a des ; my fa ther-i n-l a w Ed (ma y he res t i n pea ce); a nd my fa ther-i n-l a w Lou for the profes s i ona l res pect a l wa ys a ccorded me. Fi na l l y to the more tha n 3000 medi ca l s tudents I ha ve ta ught who i ns pi red my s ucces s a s a tea cher a nd educa tor a nd who preceded thos e s tudents who I trus t wi l l benefi t from thi s textbook — Ma rc E. Ti s chl er To my pa rents , fa mi l y, a nd fri ends who pers evered throughout the wri ti ng, proofi ng, a nd publ i ca ti on of thi s book a s wel l a s the ma ny i ns tructors , from hi gh s chool to uni vers i ty to gra dua te s chool to medi ca l s chool a nd beyond, who i ns ti l l ed i n me not onl y a l ove for l ea rni ng but a l s o for tea chi ng. Thi s book i s pers ona l l y dedi ca ted to one s uch pers on, Ca s s i e Murphy-Cul l en, PhD (ma y s he res t i n pea ce), who s erved a s a tea cher, couns el or a nd, mos t-of-a l l , cons ta nt a nd dedi ca ted fri end to a l l of her s tudents , i ncl udi ng mys el f. — Lee W. Ja ns on
ACKNOWLEDGEMENTS Si ncere tha nks to the current a uthors of Ha rper’s Il l us tra ted Bi ochemi s try for thei r revi ews a nd comments wi th s peci a l tha nks to Dr. Robert Murra y, edi tor of Ha rper’s , for hi s pa ti ence, ki ndnes s , a nd excepti ona l efforts i n revi ewi ng thi s book. The a uthors a l s o wi s h to tha nk Andrea Agui rre, Chi neyne Ana ko, Ma rti n Benja mi n, Na ta s ha Bhuya n, Jos eph Ca rrol l , Ka tha ri ne Fl a nnery, Si l vi ja Gottes ma n, Mi cha el Ori , Cha rl es Ra ppa port, Chri s topher Ri l ey, Al a n Schuma cher, a nd Ka ren Stern, medi ca l s tudents a t the Uni vers i ty of Ari zona , who s erved on a focus group to provi de va l ua bl e i ns i ght for thi s text from a medi ca l s tudent pers pecti ve.
ABOUT THE AUTHORS Ma rc E. Ti s chl er recei ved a n undergra dua te degree i n bi ol ogy from Bos ton Uni vers i ty, a ma s ter degree i n chemi s try from the Uni vers i ty of South Ca rol i na , a nd hi s doctora te degree i n bi ochemi s try from the Uni vers i ty of Penns yl va ni a . After s ervi ng i n a pos tdoctora l pos i ti on i n phys i ol ogy a t Ha rva rd Medi ca l School , he joi ned the fa cul ty a t the Uni vers i ty of Ari zona i n 1979 where he i s currentl y a profes s or of bi ochemi s try a nd mol ecul a r bi ophys i cs hol di ng joi nt a ppoi ntments i n phys i ol ogy a nd i n i nterna l medi ci ne. Ha vi ng s erved a s coordi na tor of the medi ca l bi ochemi s try cours e for a deca de, he wa s recrui ted to pl a y a ma jor rol e i n the devel opment of the revi s ed medi ca l curri cul um a t the Uni vers i ty of Ari zona , whi ch debuted i n 2006 a nd offers a n i ntegra ted, orga n-ba s ed a pproa ch a ki n to the s econd ha l f of thi s textbook. In tha t new curri cul um, he des i gned a nd s erves a s di rector of the medi ca l bl ock enti tl ed Di ges ti on, Meta bol i s m, a nd Hormones . He ha s ta ught more tha n 3000 medi ca l s tudents duri ng hi s tenure i n Ari zona . Lee W. Ja ns on recei ved a BS i n Bi ochemi s try/mi nor i n La ti n from the Uni vers i ty of Roches ter fol l owed by a PhD i n bi ol ogi ca l s ci ences /bi ochemi s try from Ca rnegi e Mel l on Uni vers i ty. After a pos tdoctora l fel l ows hi p a t NASA-Johns on Spa ce Center doi ng res ea rch on i mmunol ogi ca l a cti va ti on i n mi crogra vi ty, he entered medi ca l s chool a t the Uni vers i ty of Texa s Southwes tern Medi ca l Center a t Da l l a s , conti nui ng wi th a fa mi l y pra cti ce res i dency i n both Da l l a s a nd Sa n Antoni o. He entered the a cti ve duty Ai r Force a nd s erved a s a fa mi l y phys i ci a n a nd a fl i ght s urgeon, i ncl udi ng tours of Korea , Ira q, a nd Afgha ni s ta n. After mi l i ta ry s ervi ce, he perma nentl y moved to Edi nburgh, Scotl a nd i n 2007, where he pra cti ces i n the Na ti ona l Hea l th Servi ce a s a n emergency room doctor a nd a genera l phys i ci a n i n the Uni ted Ki ngdom wi th occa s i ona l work i n Aus tra l i a a nd other pa rts of the worl d. In hi s free ti me, he wri tes a nd does res ea rch. Pa s t book publ i ca ti ons i ncl ude Brew Chem 101: The Basics of Home-brewing Chemistry (Storey Publ i s hi ng).
SECTION I THE BASIC MOLECULES OF LIFE
CHAPTER 1 AMINO ACIDS AND PROTEINS Ami no Aci ds —Structure a nd Functi ona l Groups Ba s i c Protei n Structure Ca tegori es of Protei ns Revi ew Ques ti ons
OVERVIEW Ami no a ci ds a re the ba s i c bui l di ng bl ocks of protei ns a nd s erve a s bi ol ogi ca l mol ecul es i n thei r own ri ght wi th a va ri ety of functi ons . Ami no a ci ds a re often ca tegori zed a s es s enti a l or non-es s enti a l , dependi ng on the a bi l i ty of the body to ma nufa cture ea ch a mi no a ci d vers us requi rement for i nges ti on i n the di et. Al though s evera l hundred a mi no a ci ds exi s t, 20 pl a y the predomi na nt rol e i n the huma n body. Ea ch a mi no a ci d ha s a cha ra cteri s ti c R-group tha t determi nes i ts chemi ca l na ture a nd, therefore, how i t wi l l i ntera ct wi th other a mi no a ci ds , other mol ecul es , a nd wi th i ts envi ronment. Ami no a ci ds l i nk together vi a pepti de bonds to form pepti des a nd protei ns . Thes e pepti des a nd protei ns fol d i nto thei r fi na l threedi mens i ona l s ha pe a s the res ul t of hydrophobi c, hydrophi l i c, hydrogen bondi ng, a nd i oni c bondi ng forces (a mong others ) tha t res ul t from the a mi no a ci ds i n the pepti de cha i n, i ncl udi ng the cha ra cteri s ti cs of thei r R-groups . Protei ns ma y be ca tegori zed a s enzymes , s tructura l protei ns , motor protei ns , a nd tra ns port/cha nnel protei ns . The s peci fi c rol es of a mi no a ci ds a nd protei ns i n the s ynthes i s of other mol ecul es a nd i n the functi ons of orga n s ys tems a nd the huma n bodi es wi l l be expl ored i n deta i l i n s ubs equent cha pters .
AMINO ACIDS—STRUCTURE AND FUNCTIONAL GROUPS ESSENTIAL AND NON-ESSENTIAL Amino acids a re the ba s i c bui l di ng bl ocks of protei ns . Twenty a mi no a ci ds ma ke up protei ns i n l i vi ng orga ni s ms ; s evera l hundred more a mi no a ci ds perform s peci a l i zed functi ons i n huma n a nd non-huma n bi ol ogy. Ami no a ci ds a re often des cri bed a s Essential (mus t be obta i ned di rectl y from food) Non-essential (the huma n body i s a bl e to produce them on i ts own). There i s s ome deba te a bout the exa ct defi ni ti ons of thes e terms , but 8–10 a mi no a ci ds a re us ua l l y deemed es s enti a l a nd 10–12 a re nones s enti a l (s ee Ta bl e 1-1).
TABLE 1-1. Ami no Aci ds —R-Group Cl a s s i fi ca ti ons Succota s h, a combi na ti on of corn a nd l i ma bea ns , wa s us ed by Na ti ve Ameri ca n hunters a nd wa rri ors . Li ght i n wei ght, ea s y to ca rry, a nd s i mpl e to prepa re, s uccota s h conta i ns a l l the es s enti a l requi red a mi no a ci ds needed by huma ns a nd kept thes e tra vel ers wel l nouri s hed a nd hea l thy duri ng l ong tri ps a wa y from thei r s ettl ements .
BASIC STRUCTURE Every a mi no a ci d ha s four components l i nked together wi th a centra l ca rbon a tom (Fi gure 1-1): 1. Ami no group 2. Ca rboxyl i c a ci d group 3. Hydrogen a tom 4. R-group, whi ch va ri es wi th ea ch a mi no a ci d
Figure 1-1. Basic Structure of an Amino Acid. A central alpha (α) carbon is bonded to an amino group (NH 2 ), a carboxyl group (COOH), and an R group. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] CHARACTERISTICS OF R-GROUPS As a mi no a ci ds a re i denti ca l except for thei r R-group, four R-group cha ra cteri s ti cs cl a s s i fy the a mi no a ci ds (s ee a l s o Ta bl e 1-1). 1. Hydrophobic (Water Hating) R-groups: Thes e a mi no a ci ds prefer to be i ns i de a fol ded protei n or covered by a nother pa rt of a protei n or a l i pi d membra ne (Cha pter 8) where they a re hi dden from the externa l wa ter envi ronment. 2. Hydrophilic (Water Loving) R-groups: Wi th pa rti a l cha rges from the hydroxyl (OH −) or a mi no (NH 3 +) pa rts of thes e R-groups , thes e a mi no a ci ds prefer to be a t or nea r the s urfa ce of a protei n where they i ntera ct wi th s urroundi ng wa ter mol ecul es . An excepti on woul d be the s urfa ce porti on of a membra ne protei n tha t i ntera cts wi th the hydrophobi c regi on of the phos phol i pi d bi l a yer (Cha pter 8). Hydrophi l i c R-groups a re often i mporta nt a t the a cti ve s i te of a n enzyme (Cha pter 5). 3. Charged R-groups: Ei ther pos i ti vel y or nega ti vel y cha rged, thes e a mi no a ci ds prefer to be a t the s urfa ce of a fol ded protei n or i n conta ct wi th other cha rged a toms /mol ecul es . 4. Special R-groups: There a re four a mi no a ci ds wi th s peci a l qua l i ty R-groups . • Prol i ne ha s a gl uta ma te R-group tha t ha s bonded onto i ts el f (s ee Fi gure 1-2A) formi ng a n “i mi no” a ci d. Prol i ne i s often found a t s ha rp turns of fol ded protei ns (Fi gure 1-2B).
Figure 1-2. A–B. Proline. A. Prol i ne forms from the a mi no a ci d gl uta ma te when the a mi no group ni trogen (bl ue) crea tes a bond wi th ca rbon 2 on the R-group (red), ma ki ng a n a mi no a ci d tha t i s “fol ded” onto i ts el f. The ca rbon a toms a re i ndi vi dua l l y numbered to a l l ow the rea der to fol l ow the proces s tha t ta kes three s teps (i ndi ca ted by a rrows ). B. Prol i ne’s s peci a l s tructure a l l ows the forma ti on of a “ha i rpi n” β-turn vi a forma ti on of a n i mi no bond. The a mi no group from prol i ne i s s ha ded i n bl ue. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] • Cys tei ne ha s a s ul fhydryl group tha t ca n bond wi th a nother cys tei ne s ul fhydryl group to form a cys ti ne “di s ul fi de” bond (Fi gure 1-3). Thi s bond ca n be ei ther wi thi n one protei n or between two di fferent protei ns a nd i s i mporta nt i n ma ki ng s trong s tructures s uch a s the protei n kera ti n found i n fi ngerna i l s .
Figure 1-3. Formation of Cystine Disulfide Bond. The s ul fhydryl groups (SH) from two cys tei ne a mi no a ci d res i dues from di fferent pa rts of a s i ngl e or two s epa ra te a mi no a ci d s equences ma y l os e one hydrogen a tom ea ch to become a cys ti ne res i due by the forma ti on of a di s ul fi de bond (S —S). [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] • Methi oni ne ha s a s ul fur a tom conta i ned wi thi n i ts R-group. Al though i t does not ma ke di s ul fi de bonds , thi s s ul fur i s s een a t the s i te of s ome enzyme rea cti ons or a t s peci a l a rea s of protei n s tructure. • Hi s ti di ne ha s a uni que i mi da zol e ri ng conta i ni ng two ni trogen a toms , whi ch ca n be uncha rged or pos i ti vel y cha rged. Thi s uni que cha ra cteri s ti c ma kes the hi s ti di ne R-group i mporta nt i n enzyme rea cti ons tha t ma ke or brea k bonds . Hair and Cystine Double Bonding: Ha i r, whi ch i s compos ed of l i nea r protei n s equences , i s curl y or s tra i ght dependi ng on the number of cys ti ne di s ul fi de bonds . The number a nd l oca ti on of cys tei ne res i dues a nd a cces s ory protei ns s peci fi c for ea ch pers on a ffect the number of di s ul fi de bonds res ul ti ng i n thi s very i ndi vi dua l i zed cha ra cteri s ti c. Chemi ca l s tha t promote thes e di s ul fi de bonds a re us ed for “perms ” a nd, a l terna ti vel y, chemi ca l s tha t brea k thes e bonds a re us ed i n ha i r s tra i ghteni ng trea tments .
BASIC PROTEIN STRUCTURE Severa l fa ctors a ffect ba s i c protei n s tructure, i ncl udi ng the fol l owi ng: Amino Acid Composition: Whether the R-group of ea ch a mi no a ci d wa nts to be a wa y from, nea r, or i n conta ct wi th wa ter a t the s urfa ce of the fol ded protei n. Special Amino Acids: Prol i ne, cys tei ne, a nd/or methi oni ne exert effects on protei n fol di ng due to res ul ti ng bends a nd di s ul fi de bonds . Functional Sites: Structura l protei ns or protei ns tha t ca ta l yze a rea cti on (i .e., enzymes ; Cha pter 5) us ua l l y conta i n s peci fi c a mi no a ci ds tha t a re i mporta nt for tha t protei n’s pa rti cul a r functi on. The R-groups a t thes e s i tes wi l l a ffect the protei n’s fi na l fol ded s ha pe. Final Modifications: Mos t protei ns a re i ni ti a l l y ma de wi th extra a mi no a ci ds a t thei r begi nni ng a nd/or end. Thes e extra a mi no a ci ds ma y be modi fi ed or removed duri ng the ma tura ti on of the protei n, res ul ti ng i n cha nges to the fi na l s tructure. Final Destination: Whether the protei n wi l l end up i n a n a queous s ol uti on or i n a membra ne wi l l a l s o cha nge the fol di ng, a s protei ns tha t go to membra nes wi l l ul ti ma tel y wa nt thei r hydro-phobi c, “wa ter ha ti ng” R-groups on the outs i de i n conta ct wi th the hydrophobi c membra ne (s ee bel ow, Cha pter 8). LEVELS OF PROTEIN STRUCTURE When des cri bi ng protei n s tructure, we us e the fol l owi ng terms : pri ma ry (1°), s econda ry (2°), terti a ry (3°), a nd/or qua terna ry (4°) s tructure. Primary (1°) Structure: The pa rti cul a r s equence of a mi no a ci ds i n a protei n (a l s o ca l l ed a polypeptide) i s termed a s primary structure. The a mi no a ci ds wi thi n a pol ypepti de a re termed residues a nd a re l i nked vi a peptide bonds (Fi gure 1-4A), formed when the ca rboxyl i c a ci d group of one a mi no a ci d i ntera cts wi th the a mi no group of a s econd a mi no a ci d. The combi na ti on produces the pepti de bond a nd one mol ecul e of wa ter. Repea ted pepti de bonds form a cha i n of pepti de bond l i nka ges wi th the ni trogen from the a mi no group, the centra l ca rbon from the a mi no a ci d, a nd the ca rbon from the ca rboxyl i c a ci d formi ng the protei n “ba ckbone.” The pri ma ry s tructure i s often s hown a s “bea ds on a s tri ng” wi th ea ch bea d repres enti ng a n a mi no a ci d res i due (Fi gure 1-4B).
Figure 1-4. A. Formation of a Peptide Bond. A pepti de bond i s formed between the ca rboxyl i c a ci d (COOH) group a nd the a mi no ni trogen (HHN) group to form a new C—N bond. The proces s rel ea s es one mol ecul e of wa ter (OH a nd H a s s hown i n pi nk ci rcl e). By conventi on a protei n i s a l wa ys dra wn s ta rti ng wi th the N-termi nus on the l eft. [Reproduced wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] B. Primary (1°) Structure. The pri ma ry s tructure of a protei n i s the cha i n of a mi no a ci ds from the Ntermi na l (NH 2 group of a mi no a ci d 1) to i ts C-termi na l end (COOH group of the fi na l a mi no a ci d). Indi vi dua l a mi no a ci ds a re denoted a s green “bea ds ” a nd bonds a re red “s tri ngs ” i n thi s s tyl i zed repres enta ti on. Protei ns va ry grea tl y i n l ength from a few a mi no a ci ds to s evera l hundred. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Secondary (2°) Structure: From the l i nea r cha i n of a mi no a ci ds , the C—N a nd C—C bonds rota te a round the centra l ca rbon a tom (s ee Fi gure 1-1). Thi s rota ti on forms secondary structures (Fi gure 1-5A–C) ca l l ed a n α-helix, β-strand, or β-turn (s ee a l s o Fi gure 1-2B a bove) dependi ng on the
hydrophi l i c a nd hydrophobi c cha rge a nd s i ze i nfl uences of the R-groups a s wel l a s hydrogen a nd i oni c bondi ng. Pa rts of the pepti de tha t do not form conventi ona l s econda ry s tructures a re referred to a s “ra ndom coi l .”
Figure 1-5. A–C. Secondary (2°) Structure. A. The α-hel i x s tructure i s s ta bi l i zed by hydrogen bonds (bl ue dotted l i ne) between the N—H of one a mi no a ci d a nd the of a nother a mi no a ci d. The hydrophobi c a nd hydrophi l i c na ture of the R-groups from ea ch a mi no a ci d i n the hel i x a l s o pl a ys a pa rt i n forma ti on of the α-hel i x. B. The β-s tra nds form when the one s equence of s ucces s i ve a mi no a ci ds form hydrogen bonds between the N—H a nd the C═O of a nother group of s ucces s i ve a mi no a ci ds . R-groups of thes e a mi no a ci ds a l s o i nfl uence the forma ti on of βs tra nds by both cha rge a nd s teri c forces . β-Stra nds ca n be ei ther pa ra l l el (l eft) or a nti pa ra l l el (ri ght) a s noted by the a rrows . C. A β-turn forms a t the juncture between two a nti pa ra l l el β-s tra nds a nd us ua l l y i nvol ves a mi no a ci ds wi th s ma l l R-groups , i ncl udi ng gl yci ne, a l a ni ne, a nd va l i ne. β-turns ca n a l s o form vi a prol i ne’s uni que s tructure (Fi gure 1-2B). [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Tertiary (3°) Structure: Seconda ry s tructure a nd the rema i ni ng a mi no a ci d s equences conti nue to fol d a nd i ntera ct wi th other pa rts of the a mi no a ci d s equence to form terti a ry (3°) s tructure. Thes e proces s es a ga i n a re dri ven by hydrophobi c a nd hydrophi l i c forces of the i ndi vi dua l pa rts of the s equence, a s wel l a s hydrogen bondi ng a nd i oni c bondi ng between cha rged a mi no a ci d R-groups . In pa rti cul a r, R-groups a nd the pa rti a l pos i ti ve cha rge of the ni trogen a nd nega ti ve cha rge of oxygen (OH − a nd C═O−) mol ecul es often form hydrophi l i c a nd hydrophobi c “s i des ” of the α-hel i x or β-s tra nd. As a res ul t, thes e s econda ry s tructures wi l l pos i ti on thems el ves wi th ea ch other a nd wi th other s i mi l a r a rea s of the fol ded protei n to keep thei r hydrophobi c a rea s a wa y from a nd thei r hydrophi l i c a rea s expos ed to the externa l wa ter envi ronment. Exa mpl es a re s hown i n Fi gure 1-6. Addi ti ona l l y, terti a ry s tructure s ta rts to devel op a t a cti ve s i tes of protei ns where cri ti ca l a cti ons a nd i ntera cti ons wi l l ta ke pl a ce. Thes e a cti ve s i tes wi l l be di s cus s ed i n s ubs equent cha pters .
Figure 1-6. Examples of Tertiary (3°) Structure. Seconda ry s tructures , i ncl udi ng α-hel i ces , β-s heets , β-turns /bends , a nd l oop regi ons combi ne to form terti a ry s tructure doma i ns . Severa l common forms of terti a ry s tructure ha ve been cha ra cteri zed a nd a re i l l us tra ted i n the fi gure. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Globular vs. Fibrous Proteins: Protei ns a re often grouped i nto the broa d ca tegori es of “gl obul a r”—ha vi ng a n a pproxi ma tel y s pheri ca l threedi mens i ona l s ha pe—or “fi brous ”—bei ng l ong a nd fa i rl y s tra i ght overa l l . A va s t ma jori ty of protei ns a re gl obul a r, wi th fi brous protei ns often pl a yi ng s tructura l or very s peci a l i zed functi ona l rol es . Exa mpl es of fi brous protei ns i ncl ude a cti n, col l a gen, a nd kera ti n, whi ch pl a y
s tructura l rol es i n mus cl e, connecti ve ti s s ue, a nd s ki n/na i l s . Quaternary (4°) Structure: Al though ma ny protei ns a re ma de of onl y one pepti de cha i n (ca l l ed “monomers ” or “s ubuni ts ”), two or more fol ded cha i ns ma y combi ne together to ma ke a fi na l a cti ve “ol i gomer” protei n. The s ubuni ts i n a mul ti meri c protei n ma y ha ve i denti ca l (homooligomer) or di fferent (hetero-oligomer) a mi no a ci d s equences a nd i ntera ct to opti mi ze hydrophobi c a nd hydrophi l i c a rea s a nd hydrogen/ i oni c bondi ng. The combi na ti on of the mul ti pl e protei n s ubuni ts ma kes qua terna ry (4°) s tructure (Fi gure 1-7).
Figure 1-7. Quaternary (4°) Structure. Protei n monomers ma y joi n, hel d together by hydrogen bondi ng (s hown) a nd/or hydrophobi c–hydrophi l i c i ntera cti ons , to ma ke ol i gomers . Identi ca l protei n s ubuni t monomers ma y joi n together to ma ke a homo-ol i gomer. Noni denti ca l protei n s ubuni t monomers ma y joi n together to ma ke a hetero-ol i gomer. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] The proces s of protei n fol di ng i s not a trul y l i nea r one; a protei n never exi s ts a s a l ong a mi no a ci d s tri ng. Ins tea d, protei n fol di ng i s a compl ex i ntera cti on of thes e proces s es a nd s econda ry a nd terti a ry s tructures a ctua l l y form s omewha t i n pa ra l l el . Ma ny more compl ex fa ctors not di s cus s ed here a l s o occur to hel p i n determi ni ng the fi na l protei n conforma ti on. The proces s of ma ki ng a nd tra ffi cki ng a protei n wi l l be expl ored more ful l y i n Cha pters 5 a nd 9. Dystrophin and Muscular Dystrophy: Mus cul a r dys trophi es (i ncl udi ng Duchenne a nd Becker mus cul a r dys trophi es ) a re di s ea s es i n whi ch s kel eta l mus cl e ra pi dl y brea ks down, res ul ti ng i n mus cl e wea knes s a nd wa s ti ng, decrea s ed motor s ki l l s , a nd, eventua l l y, the i na bi l i ty to wa l k (us ua l l y by the a ge of 12 yea rs ). Of a pproxi ma tel y, 30 di fferent types of mus cul a r dys trophi es , the Duchenne a nd Becker forms s how X chromos ome-l i nked i nheri ta nce a nd, therefore, a l mos t a l wa ys a ffect ma l es a t a ra te of a pproxi ma tel y one i n 3500 boys worl dwi de. Both a re ca us ed by the a bs ence or muta ti on of the protei n dystrophin, whi ch provi des s trength for s kel eta l mus cl e cel l s by a nchori ng the i nterna l cel l components to the extra cel l ul a r ma tri x.
Reproduced wi th permi s s i on from Ka ndel l E, et a l .: Pri nci pl es of Neuros ci ence, 4th edi ti on, McGra w-Hi l l , 2000. The i ntera cti on of dys trophi n wi th s evera l other protei ns i nvol ved i n thi s s tructura l s upport network i s a pri me exa mpl e of protei n– protei n i ntera cti ons i nvol vi ng the forces of thei r hydrophobi c a nd hydrophi l i c regi ons . More i mporta ntl y, the muta ti ons i n the dys trophi n protei n a re bel i eved to s i gni fi ca ntl y a l ter one or more of thes e regi ons , res ul ti ng i n brea kdown of the a nchori ng compl ex a nd thus di s ea s e.
CATEGORIES OF PROTEINS AMINO ACID AND PEPTIDE-DERIVED HORMONES AND NEUROTRANSMITTERS Ami no a ci ds a nd the pepti des /protei ns tha t they form s erve s evera l cri ti ca l rol es i n huma n bi ochemi s try a nd l i fe. The ma jor rol e of a mi no a ci ds i s to provi de the bui l di ng bl ocks for protei ns a nd a va s t ma jori ty of the body’s a mi no a ci ds a re i nvol ved i n thi s functi on. However, a mi no a ci ds a re a l s o the ma jor precurs ors of s evera l bi ol ogi ca l l y i mporta nt mol ecul es , a s noted i n Ta bl e 1-2.
TABLE 1-2. Ami no Aci ds —Components of Va ri ous Bi ol ogi ca l Mol ecul es Quaternary (4°) Structure: Qua terna ry (4°) s tructure i s exempl i fi ed i n ma ny other wa ys tha n thos e gi ven a bove, where two s econda ry s tructura l el ements a re s i mpl y i n cl os e proxi mi ty to ea ch other. Ma ny fi brous protei ns expa nd on thi s concept, wi th two or more α-hel i ces wound a round ea ch other for mos t of thei r a mi no a ci d s equence. Exa mpl es i ncl ude F-a cti n a nd the ta i l s ecti on of myos i n, kera ti n i n ha i r, a nd i ntermedi a te fi l a ments , whi ch pl a y a n i mporta nt s tructura l rol e i ns i de a l l cel l s .
Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010 Collagen, a nother fi brous s tructura l protei n of s ki n a nd connecti ve ti s s ue, i s compos ed of three i ntertwi ned hel i ca l protei n s equences (s ee the fi gure on ri ght), whi ch di ffer from the α-hel i ca l s tructure. Unl i ke a n α-hel i x where hydrogen bondi ng wi thi n the s a me protei n s equence predomi na tes , col l a gen uti l i zes hydrogen bondi ng between the three hel i ca l protei n cha i ns . Col l a gen opti mi zes thi s very uni que s tructure by ha vi ng the s ma l l a mi no a ci d gl yci ne a s every thi rd a mi no a ci d to a l l ow the three hel i ces to fi t very cl os e together (s ee the fi gure bel ow). In a ddi ti on, col l a gen ha s a n a bunda nce of prol i ne res i dues to promote the hel i ca l protei n s tructure a nd a s peci a l form of prol i ne (hydroxyprol i ne) wi th a n extra hydroxyl (OH –) to form the i nterprotei n hydrogen bonds wi th the gl yci ne NH groups . Hydroxyl ys i ne res i dues hel p to s ta bi l i ze the s tructure a s wel l .
Ada pted (l eft) a nd reproduced (ri ght) wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009. Osteogenesis imperfecta (OI) i s a geneti c di s ea s e tha t a ffects col l a gen-conta i ni ng ti s s ues s uch a s bone, s ki n, joi nts , eyes , ea rs , a nd teeth beca us e of poi nt muta ti ons tha t des ta bi l i ze or a l ter col l a gen’s i mporta nt tri pl e hel i x s tructure. Pa ti ents wi th OI often di s pl a y frequent fra ctures a nd ea s y brui s i ng (s ometi mes mi s ta ken for chi l d a bus e); wea k joi nts ; a bl ui s h col or to the norma l l y whi te pa rt of thei r eyes ; hea ri ng l os s due pa rtl y to a bnorma l i ti es of the i nner ea r bones ; a nd poorl y s ha ped, s ma l l , bl ue-yel l ow teeth. As a res ul t, ei ther the exces s or defi ci ency of a mi no a ci ds ca n l ea d di rectl y to di s ea s e tha t ma y res ul t i n centra l nervous s ys tem defects , di eta ry a nd meta bol i s m probl ems , l i ver a nd ki dney fa i l ure, s ki n a nd eye l es i ons , a nd even dea th. ENZYMES Enzymes a re s peci a l i zed protei ns tha t a ccel era te a chemi ca l rea cti on by s ervi ng a s a bi ol ogi ca l ca ta l ys t. By ca ta l yzi ng thes e rea cti ons , enzymes ca us e them to ta ke pl a ce one mi l l i on or more ti mes fa s ter tha n i n thei r a bs ence. Enzymes a re us ua l l y i denti fi ed by the endi ng of “a s e” to thei r na me (e.g., hexoki na s e, the fi rs t enzyme i n the brea kdown of gl ucos e). There a re excepti ons for enzymes tha t were di s covered before thi s na mi ng s cheme wa s a dopted (e.g., tryps i n, peps i n, a nd thrombi n). Enzyme rea cti ons wi l l be di s cus s ed i n grea ter deta i l i n Cha pter 5. STRUCTURAL PROTEINS Protei ns a l s o s erve a n i mporta nt rol e a s s tructura l el ements of cel l s a nd ti s s ues . The bes t exa mpl es of thes e protei ns a re a cti n a nd tubul i n, whi ch form a cti n fi l a ments a nd mi crotubul es , res pecti vel y (Fi gure 1-8A-B). In s kel eta l mus cl e, a cti n fi l a ments provi de the “s ca ffol di ng” a ga i ns t whi ch the motor protei n myos i n ca n genera te force to produce mus cl e contra cti on. In s mooth mus cl e a nd non-mus cl e (e.g., s ki n a nd i mmune s ys tem), a cti n fi l a ments crea te the mecha ni ca l s tructure of the cel l a nd a re di rectl y a s s oci a ted wi th l i nka ges to s urroundi ng cel l s a l l owi ng i ntercel l ul a r s i gna l i ng. The a cti n fi l a ments a l s o provi de tra cks on whi ch s peci a l i zed myos i n mol ecul es move ves i cl es a nd orga nel l es (s ee Cha pter 12). Fi na l l y, a cti n fi l a ments a re i nti ma tel y i nvol ved i n cel l moti l i ty, a wi de a rra y of cel l ul a r movements s uch a s wound hea l i ng (the movement of s ki n cel l s i nto cuts ), the i mmune res pons e (the proces s of whi te bl ood cel l s conta cti ng a nd recogni zi ng ea ch other i n the hi ghl y s el ecti ve proces s of i mmune rea cti ons ), a nd cytoki nes i s (the di vi s i on of one cel l i nto two duri ng mi tos i s ).
Figure 1-8. A–B. A. Actin and B. Tubulin in Monomer and Filamentous Forms. [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] The s tructura l protei n tubul i n crea tes mi crotubul e tra cks for the movement by two mol ecul a r motor protei ns , dynei n a nd ki nes i n, of ves i cl es , gra nul es , orga nel l es , a nd chromos omes . Mi crotubul es a re a l s o the s tructura l component i n fl a gel l a a nd ci l i a i nvol ved i n functi ons s uch a s s perm moti l i ty, the movement of the egg down the Fa l l opi a n tubes , a nd the expul s i on of di rt a nd mucus out of the l ungs a nd tra chea . Nonmoti l e ci l i a a re a l s o i mporta nt i n rod cel l s i n the eye a nd neurons i nvol ved i n ol fa cti on (s mel l ). Mi crotubul es a l s o s erve a mecha ni ca l – s tructura l rol e i n the cel l s i mi l a r to a cti n mi crofi l a ments a nd a re res pons i bl e for the movement a nd s epa ra ti on of chromos omes duri ng mi tos i s (s ee Cha pter 12). MOTOR PROTEINS
“Motor” protei ns tra ns port mol ecul es i ns i de a cel l , provi de movement of certa i n pa rts of i ndi vi dua l cel l s i nvol ved i n s peci a l i zed functi ons (e.g., i mmune res pons es a nd wound hea l i ng), genera te l a rger s ca l e movements of fl ui ds a nd s emi s ol i ds s uch a s the ci rcul a ti on of bl ood a nd movement of food through the di ges ti ve tra ct, a nd fi na l l y provi de movement of the huma n body through thei r rol es i n s kel eta l mus cl es . Myos i n i s a protei n wi th a hydrophobi c ta i l ; a hea d group, whi ch ca n a tta ch a nd deta ch from a cti n fi l a ments ; a nd a “hi nge” s ecti on, whi ch moves the hea d group ba ck a nd forth res ul ti ng i n movement. Two ma jor types of myos i n exi s t. Myos i n I i s compos ed of one mol ecul e wi th a n a ddi ti ona l a rea on i ts s hort ta i l tha t ca n bi nd to other protei ns a nd membra nes . Myos i n II (s evera l s ubtypes exi s t) i s compos ed of a l ong ta i l tha t bi nds vi a hydrophobi c i ntera cti ons to other myos i n II mol ecul es , res ul ti ng i n a compos i te mol ecul e tha t ca n s horten s kel eta l mus cl es by i ts i ntera cti on wi th a cti n fi l a ments i n thos e mus cl es . Ki nes i n a nd dynei n a re very much l i ke myos i n I i n form a nd functi on (s ee Cha pter 12). Microtubules, Cilia, and Flagella—Roles in Disease Processes: Al though us ua l l y ra re, defects i n ci l i a /fl a gel l a , known a s ci l i opa thi es , l ea d to s evera l di s ea s es /s yndromes i ncl udi ng the fol l owi ng: Kartagener Syndrome/Primary Ciliary Dyskinesia—defecti ve ci l i a i n the res pi ra tory tra ct, Eus ta chi a n tube, a nd Fa l l opi a n tubes l ea di ng to chroni c l ung i nfecti ons , ea r i nfecti ons a nd hea ri ng l os s , a nd i nferti l i ty. Pos s i bl e a s s oci a ti on wi th “s i tus i nvers us ,” a condi ti on i n whi ch ma jor i nterna l orga ns a re “fl i pped” l eft to ri ght. Senior–Loken Syndrome/Nephronophthisis—eye di s ea s e a nd forma ti on of cys ts i n the ki dneys l ea di ng to rena l fa i l ure. Bardet–Biedl Syndrome—dys functi on of ci l i a throughout the body l ea di ng to obes i ty due to i na bi l i ty to s ens e s a ti a ti on, l os s of eye pi gment/vi s ua l l os s a nd/or bl i ndnes s , extra di gi ts a nd/or webbi ng of fi ngers a nd toes , menta l a nd growth reta rda ti on a nd beha vi ora l /s oci a l probl ems , s ma l l a nd/or mi s s ha ped geni ta l i a (ma l e a nd fema l e), enl a rged a nd da ma ged hea rt mus cl e, a nd ki dney fa i l ure. Alstrom Syndrome—chi l dhood obes i ty, brea kdown of the reti na l ea di ng to bl i ndnes s , hea ri ng l os s , a nd type 2 di a betes . Meckel–Gruber Syndrome—forma ti on of cys ts i n ki dneys a nd bra i n l ea di ng to rena l fa i l ure a nd neurol ogi ca l defi ci ts , extra di gi ts a nd bowi ng/s horteni ng of the l i mbs . Increased Ectopic (Tubal) Pregnancies/Male Infertility—defi ci ent ci l i a i n Fa l l opi a n tubes or defi ci ent fl a gel l a / s perm ta i l moti l i ty. Autosomal Recessive Polycystic Kidney Disease—much ra rer tha n the a utos oma l domi na nt form, dys functi on of ba s a l bodi es a nd ci l i a i n rena l cel l s l ea ds to a l tera ti ons of the l ungs a nd ki dneys l ea di ng to a va ri ety of s econda ry medi ca l condi ti ons a nd often dea th. Parkinson’s and Alzheimer’s diseases—Al though work i s s ti l l ongoi ng, res ea rchers now feel tha t s ome forms of Pa rki ns on’s a nd Al zhei mer’s di s ea s es ma y res ul t, i n pa rt, from da ma ge to mi crotubul es a nd a s s oci a ted protei ns . Trea tments a i med a t s ta bi l i zi ng mi crotubul es ma y hel p ma ny s ufferers of thes e ma l a di es . TRANSPORT/CHANNEL PROTEINS Another group of protei ns fol d i nto a terti a ry or qua terna ry s tructure tha t crea tes cha nnel s for the movement of mol ecul es i nto a nd out of the nucl eus , va ri ous cel l orga nel l es , a nd from cel l s to thei r outs i de envi ronment s uch a s the bl ood s trea m. The hydrophobi c a nd hydrophi l i c na ture of the a mi no a ci ds tha t ma ke up thes e cha nnel protei ns a l l ows a n exteri or of the protei n tha t ca n exi s t i ns i de the extremel y hydrophobi c envi ronment of a membra ne bi l a yer a nd a hydrophi l i c i nteri or tha t ca n a l l ow cha rged mol ecul es to move through the membra ne (Cha pter 8). Cha nnel s a re es s enti a l for the tra ns porta ti on of nutri ents i nto a nd out of cel l s a s wel l a s for nerve s i gna l s a nd the s el ecti ve fi l tra ti on of mol ecul es i n the ki dneys . Thes e s peci a l i zed functi ons of cha nnel s wi l l be di s cus s ed i n deta i l i n Cha pter 8 a nd Secti on III.
REVIEW QUESTIONS 1. Wha t i s the mea ni ng a nd s i gni fi ca nce of es s enti a l a nd non-es s enti a l a mi no a ci ds ? 2. Wha t i s the s i gni fi ca nce of ea ch a mi no a ci d R-group (hydrophobi c, hydrophi l i c, a nd cha rged)? 3. Wha t a re the four ma jor types of s tructura l el ements of protei ns a nd how a re they defi ned? 4. Wha t i s a n enzyme a nd how do the terms ca ta l ys t a nd a cti ve s i te rel a te to enzymes ? 5. Wha t i s the ba s i c s tructure of a mi no a ci ds ? 6. How do the el ements pepti de bond, pepti des , α-hel i x, β-s tra nd, β-turn, ha i rpi n turn, a nd di s ul fi de bond rel a te to the s tructure of protei ns ? 7. How does a n a mi no a ci d s equence fol d? 8. Wha t a re the rol es of R-groups a nd pri ma ry to qua terna ry s tructure i n the fi na l conforma ti on of protei ns ? 9. Wha t a re the di fferent ca tegori es of protei ns a nd how a re they defi ned?
CHAPTER 2 CARBOHYDRATES Ba s i c Ca rbohydra te Structure a nd Functi on Monos a ccha ri des a nd Di s a ccha ri des Gl ycogen a nd Sta rches Gl ycoprotei ns Gl ycos a mi nogl yca ns Revi ew Ques ti ons
OVERVIEW Ca rbohydra tes a re va s tl y i mporta nt i n huma n bi ol ogy, i ncl udi ng rol es a s a ma jor energy s ource, s tructura l mol ecul es when combi ned wi th other ca rbohydra tes , protei ns , a nd other mol ecul es , a nd bi ndi ng a nd s i gna l i ng between mol ecul es a nd cel l s . As a res ul t of a l l thes e i mporta nt functi ons , ca rbohydra te bi ochemi s try i s i nvol ved i n a l a rge number of di s ea s e s ta tes . Al though mul ti pl e ca rbohydra tes exi s t, onl y a few s uga r mol ecul es a nd pol ys a ccha ri des a re i mporta nt to huma n phys i ol ogy (e.g. onl y ei ght di fferent ca rbohydra tes a re found a s cons ti tuents of gl ycoprotei ns a nd gl ycol i pi ds ). However, a number of a ddi ti ona l mol ecul es crea ted by l i nka ges of ca rbohydra tes to protei ns pl a y va ri ous rol es i n cel l –cel l i ntera cti ons a nd bi ol ogi ca l s tructures .
BASIC CARBOHYDRATE STRUCTURE AND FUNCTION Carbohydrates, whos e na mes end i n “-ose,” ha ve a formul a of (CH 2 O) x where x i s a number from three to s even (gi vi ng the na mes of triose, tetros e, pentose, hexose, a nd heptose). Al l ca rbohydra tes conta i n a ketone or a n a l dehyde group, a s wel l a s one or more hydroxyl groups (Fi gure 2-1A–B; Appendi x III). The oxygen a toms of the ketone a nd a l dehyde groups ha ve s i mi l a r rea cti ve qua l i ti es to tha t of the ca rboxyl i c a ci d group s een i n a mi no a ci ds a nd a re the s i tes of chemi ca l rea cti ons wi thi n the ca rbohydra te mol ecul e, a s wel l a s wi th other ca rbohydra te, protei n, or l i pi d mol ecul es . Often, the ketone or a l dehyde rea cts wi th a hydroxyl group from the s a me s uga r mol ecul e to form a ca rbohydra te ri ng s tructure a s s hown.
Figure 2-1. A–B. Basic Carbohydrate Structures. A. The rea cti ve ketone group of ca rbon 2 (green ca rbon group) from the hexos e fructos e rea cts wi th the hydroxyl group of ca rbon 5 to form a new bond a nd a fi ve-s i ded (pentos e) ri ng s tructure. Al l ca rbon a toms a re numbered for cl a ri ty. Thi s rea cti on i s ful l y revers i bl e a s i ndi ca ted by the bi di recti ona l a rrows . As a res ul t, the l i nea r a nd ri ng s tructures a re cons ta ntl y cha ngi ng i n s ol uti on. B. The rea cti ve a l dehyde group of ca rbon 1 (green ca rbon group) from the hexos e gl ucos e rea cts wi th the hydroxyl group of ca rbon 5 to form a new bond a nd a s i x-s i ded (hexos e) ri ng s tructure. Al l ca rbon a toms a re numbered for cl a ri ty. Thi s rea cti on i s ful l y revers i bl e a s i ndi ca ted by the bi di recti ona l a rrows . As a res ul t, the l i nea r a nd ri ng s tructures a re cons ta ntl y cha ngi ng i n s ol uti on. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Ca rbohydra tes pl a y a ma jor rol e i n huma ns a s energy s ources a nd s tora ge, a nd thei r rol e i n di et a nd nutri ti on, a l though s ometi mes controvers i a l , i s a l wa ys one of s upreme i mporta nce. However, ca rbohydra tes pl a y other rol es a s noted i n Ta bl e 2-1.
TABLE 2-1. Bi ochemi ca l Rol es of Ca rbohydra tes
MONOSACCHARIDES AND DISACCHARIDES Al though there a re mul ti pl e tri os es , pentos es , hexos es a nd heptos es dependi ng on the va ri ous a rra ngements of hydroxyl groups a nd hydrogens a round the centra l ca rbon ba ckbone, onl y a few of thes e s i ngl e res i due s uga rs a re commonl y s een i n huma n bi ol ogy. Common s i ngl e s uga r groups , ca l l ed monosaccharides, i ncl ude the tri os e glyceraldehyde; the pentos e ribose; a nd the hexos es fructose, glucose, a nd galactose (s hown i n Fi gure 2-2).
Figure 2-2. Common Monosaccharides in Human Biology. The common monos a ccha ri de ca rbohydra tes found i n huma ns a re s hown a bove, i ncl udi ng the three-ca rbon tri os e gl ycera l dehyde; the fi ve-ca rbon pentos e ri bos e; a nd the s i x-ca rbon hexos es fructos e, gl ucos e, a nd ga l a ctos e. Note the onl y s tructura l di fference between gl ucos e a nd ga l a ctos e i s the pl a cement of the hydrogen a tom a nd hydroxyl group a t ca rbon 4. Ca rbon a toms a re numbered for cl a ri ty. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Hydroxyl groups (OH) a re often l oca ti ons of enzyme rea cti ons , es peci a l l y the forma ti on of a new bond between two ca rbohydra te mol ecul es wi th the res ul ti ng rel ea s e of a wa ter mol ecul e. When monos a ccha ri des form s uch bonds , the res ul ti ng mol ecul es a re a disaccharide, trisaccharide, a nd s o on. The va ri ous combi na ti ons of a l l the di fferent monos a ccha -ri des woul d produce a va s t mi xture of thes e new mol ecul es but, i n fa ct, onl y a few a re common i n huma ns (Fi gure 2-3), na mel y lactose (the pri ma ry s uga r found i n mi l k), trehalose [found i n pl a nts (e.g., s unfl ower s eeds ), a ni ma l s (e.g., s hri mp), Ba ker’s yea s t, a nd s evera l types of mus hrooms )], maltose [(found i n ma ny foods ma de from gra i ns (e.g., ba rl ey)], a nd sucrose (found na tura l l y i n pl a nts ; us ua l l y a rti fi ci a l l y ma de for huma n cons umpti on a s common ta bl e s uga r). The ri ng s tructure of monos a ccha ri des a l s o ha s rea cti ve hydroxyl groups a t ea ch of thei r ca rbon a toms , es peci a l l y a t the fi rs t a nd s i xth ca rbons , whi ch ca n bond to other s uga r mol ecul es a nd a mi no groups a nd protei ns (di s cus s ed bel ow a nd i n the fol l owi ng cha pters ).
Figure 2-3. Common Disaccharides in Human Biology. The common di s a ccha ri de ca rbohydra tes found i n huma ns a re s hown a bove, i ncl udi ng l a ctos e, treha l os e, ma l tos e, a nd s ucros e. Component monos a ccha ri des a nd s peci fi c α- a nd β-bond confi gura ti ons a re i ndi ca ted i mmedi a tel y under ea ch di s a ccha ri de. Note tha t the s econd gl ucos e mol ecul e i n treha l os e a nd the fructos e mol ecul e i n s ucros e a re fl i pped hori zonta l l y i n the fi na l di s a ccha ri de mol ecul e (s ee ca rbon numbers ). [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Carbohydrate Intolerance: The di ges ti on of ca rbohydra tes from food i nvol ves s evera l proces s es rel yi ng on protei ns , both enzymes a nd tra ns port/cha nnel types . Defects i n a ny of thes e protei ns l ea d to di s ea s e s ta tes i n whi ch a pa rti cul a r ca rbohydra te ca nnot be tol era ted i n the pa ti ent’s di et. One of the bes t known exa mpl es i s lactose intolerance, whi ch devel ops i n a dol es cence or a dul thood a nd i s ca us ed by the i na bi l i ty to di ges t the s uga r l a ctos e, the pri ma ry ca rbohydra te i n cow’s mi l k. Decrea s ed di ges ti on i ncrea s es ba cteri a l fermenta ti on of the exces s s uga r mol ecul es produci ng i ntes ti na l ga s , bl oa ti ng, na us ea , a nd pa i nful cra mpi ng. The os moti c effect of the exces s monos a ccha ri de a nd di s a ccha ri de mol ecul es l ea ds to i ncrea s ed wa ter retenti on a nd a bs orpti on i n the l a rge i ntes ti ne, ca us i ng wa tery di a rrhea . In fa ct, hi s tori ca l evi dence i ndi ca tes tha t huma ns onl y recentl y evol ved the a bi l i ty to di ges t l a ctos e when cow’s mi l k beca me a s ta pl e of thei r di et. In a ddi ti on, s ome ra ces of huma ns a re l es s a bl e to di ges t l a ctos e, l ea di ng to i ncrea s ed i nci dence of l a ctos e i ntol era nce i n tha t ra ci a l group. Other types of ca rbohydra te i ntol era nce i ncl ude enzyme defi ci enci es i n the di ges ti on of s ucros e, ma l tos e, a nd treha l os e—the l a tter s een predomi na tel y i n Inui t a nd Greenl a nd popul a ti ons —a nd the a bs ence or decrea s ed a cti on of tra ns porter/cha nnel protei ns tha t ca n ca us e profound effects on di ges ti on of gl ucos e, fructos e, a nd ga l a ctos e. Some ca rbohydra te i ntol era nce ca n l ea d to s eri ous probl ems of fa i l ure to thri ve a nd ki dney a nd/or l i ver di s ea s e. Trea tment of ca rbohydra te i ntol era nces i s norma l by a voi da nce of the offendi ng s uga r or by s uppl ementa ti on of the a ffected enzyme.
GLYCOGEN AND STARCHES Li nka ges of monos a ccha ri des a nd di s a ccha ri des form l ong ca rbohydra te cha i ns ca l l ed pol ys a ccha ri des . The common pol ys a ccha ri des found i n na ture a re glycogen a nd starch. In fa ct, over ha l f of a l l the ca rbohydra tes i n the huma n di et a re s ta rch mol ecul es . Al though gl ycogen i s a l wa ys a bra nched pol ys a ccha ri de mol ecul e, s ta rch ca n be ei ther bra nched (ca l l ed amylopectin) or unbra nched (ca l l ed amylose). The gl ycogen mol ecul e a nd s ta rch mol ecul es a re s hown i n Fi gure 2-4.
Figure 2-4. A–B. Glycogen and the Plant Starch Forms Amylopectin and Amylose. A. Gl ucos e mol ecul es , bondi ng both l i nea rl y between the ca rbons 1 a nd 4 (green bonds ) a nd a s bra nches between ca rbons 1 a nd 6 (red bond) of s ucces s i ve gl ucos e mol ecul es , crea te gl ycogen (bra nches a pproxi ma tel y every 10 gl ucos e res i dues ) or the form of pl a nt s ta rch ca l l ed a myl opecti n (bra nches a pproxi ma tel y every 30 gl ucos e mol ecul es ). Gl ycogen a nd s ta rch a re us ua l l y thous a nds of gl ucos e mol ecul es l ong a nd a re extremel y i mporta nt forms of ca rbohydra te s tora ge i n the huma n body a nd pl a nts , res pecti vel y. B. Gl ucos e mol ecul es , bondi ng onl y l i nea rl y between the ca rbons 1 a nd 4 (green bonds ) of s ucces s i ve gl ucos e mol ecul es , crea te the form of pl a nt s ta rch ca l l ed a myl os e, whi ch ha s no bra nchi ng. Amyl os e i s a n i mporta nt form of ca rbohydra te s tora ge i n pl a nts . [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Glycogen Storage Diseases: Gl ycogen s tora ge di s ea s es i ncl ude 11 cl a s s es of i nborn errors i n the producti on a nd brea kdown of thes e l ong ca rbohydra te cha i ns . The geneti c errors , a l though ra re, res ul t i n a n i na bi l i ty of the body to res pond to the i ncrea s ed need for gl ucos e mol ecul es . As a res ul t, pa ti ents s ufferi ng from gl ycogen s tora ge di s ea s es a re l i mi ted i n thei r a bi l i ty to exerci s e or devel op l ow bl ood s uga r a fter a rel a ti vel y s hort peri od of food depri va ti on. Thes e gl ycogen s tora ge di s ea s es wi l l be expl ored i n ful l er deta i l i n l a ter cha pters . Cellulose, the a nother ma jor pl a nt pol ys a ccha ri de bes i des s ta rch, di ffers from amylose onl y i n the bonds between ca rbons 1 a nd 4. Cha nges i n the wa y the a l dehyde a nd hydroxyl groups of the two gl ucos e mol ecul es form the bond res ul ts i n ei ther a n a myl os e α-bond [i ndi ca ted by a downwa rd bond (Fi gure 2-4A)] or a cel l ul os e β-bond [i ndi ca ted by a n upwa rd bond (Fi gure 2-5)]. Al though the di fference ma y s eem i ns i gni fi ca nt, thi s β-bond res ul ts i n a ma rkedl y di fferent overa l l s tructure of the pol ys a ccha ri de. Al though α-l i nka ges crea te a n overa l l hel i ca l s tructure of the cha i n, β-l i nka ges crea te a s tra i ght cha i n. The α-l i nked hel i ca l s tructure i s i mporta nt for a cces s i ng the ca rbohydra te mol ecul es for meta bol i s m, wherea s the β-l i nked l i nea r cha i n i s s tronger a nd, therefore, wel l s ui ted for cel l ul os e-conta i ni ng s tructures s uch a s pl a nt wa l l s (e.g., wood). In a ddi ti on, the β-l i nka ges ca nnot be di ges ted by huma ns but ca n be di ges ted by a ni ma l s s uch a s cows a nd termi tes , a nd i t i s the rea s on huma ns ca nnot l i ve on gra s s or wood, both of whi ch a re compos ed of β-bonded gl ucos e mol ecul es .
Figure 2-5. Cellulose. Note the β-l i nka ges (upwa rd-goi ng green bond) between ca rbons 1 a nd 4 of the two gl ucos e mol ecul es tha t di ffer from the α-l i nka ges s een i n gl ycogen a nd s ta rches . Cel l ul os e i s compos ed of mul ti pl e repea ts of thi s ba s i c uni t a s i ndi ca ted by the s ubs cri pt “n.” [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
GLYCOPROTEINS Ca rbohydra tes ma y a l s o form bonds wi th protei ns vi a a ny of the ca rbohydra te hydroxyl groups combi ni ng wi th the a mi no a ci d hydroxyl groups of s eri ne or threoni ne or the a mi ne ni trogen of a s pa ra gi ne. The res ul ti ng ca rbohydra te–protei n mol ecul es a re referred to a s glycoproteins, a nd a pproxi ma tel y ha l f of the protei ns i n the huma n body a re es ti ma ted to be gl ycoprotei ns . The di vers i ty of gl ycoprotei ns s een i n na ture a nd huma ns i s i mmens e wi th compl ex mi xtures of protei ns , a mi no a ci ds , a nd s uga rs (monos a ccha ri des , di s a ccha ri des , a nd tri s a ccha -ri des ) l i nked
together i n a va s t number of l i nea r a nd bra nched forma ti ons generi ca l l y ca l l ed oligosaccharides. Thes e ol i gos a ccha ri des pl a y a di vers e number of functi ona l rol es i n bi ndi ng, s i gna l i ng, a nd regul a ti on tha t wi l l be more ful l y expl ored i n Secti on III.
GLYCOSAMINOGLYCANS Even more compl ex tha n s i mpl e ol i gos a ccha ri des a re the glycosaminoglycans (often referred to a s “GAGs”), whi ch conta i n repea ti ng di s a ccha ri de cha i ns of a modi fi ed gl ucos e a nd/or ga l a ctos e. Pri or to thei r l i nka ge to the GAG mol ecul e, thes e ca rbohydra te mol ecul es ha ve a n a mi ne group a dded wi th a n a ddi ti ona l a cetyl (e.g., NHCOCH 3 ) or nega ti vel y cha rged s ul fa te (e.g., NHSO3 −) group, produci ng glucosamine a nd galactosamine mol ecul es . In a ddi ti on, nega ti vel y cha rged s ul fa te (SO3 −), vi a the rea cti ve hydroxyl groups , a nd/or ca rboxyl a te (COO−) groups a re l i nked to a t l ea s t one of the s uga rs of the repea ti ng cha i n (Fi gure 2-6). A very wel l known GAG i s heparin, a potent i nhi bi tor of bl ood cl ot forma ti on, us ed i n pa ti ents wi th hea rt a tta cks , s trokes , a nd cl otti ng di s ea s es . Other GAGs i ncl ude chondroitin a nd hyaluronic acid (the l onges t of the GAGs ), whi ch bond wi th a centra l l i nea r core protei n to crea te proteoglycans. Thes e mol ecul es a re i mporta nt a s s trong s tructura l el ements i n connecti ve ti s s ue a nd ca rti l a ge (Fi gure 2-6C a nd Ta bl e 2-2).
Figure 2-6. A–C. Examples of Glycosaminoglycans (GAGs) and of Proteoglycan Associated with Collagen. A. Hepa ri n, a GAG a nd a n i mporta nt mol ecul e i n bl ood cl ot regul a ti on, i s compos ed of two ca rbohydra te mol ecul es l i nked vi a a β-l i nka ge between ca rbons 1 a nd 4 a nd the a ddi ti on of a COO− group, a s wel l a s s ul fa te a nd ni trogen a nd s ul fa te groups a t va ri ous other ca rbon a toms . Al though mul ti pl e forms of hepa ri n exi s t, dependi ng on the pa rti cul a r ca rbon tha t i s modi fi ed, one of the mos t common i s s hown a bove. B. Chondroi ti n, a GAG i mporta nt i n ca rti l a ge, tendon, a nd bone s tructure, i s compos ed of a l terna ti ng ca rbohydra te mol ecul es (gl ucuroni c a ci d N-a cetyl -ga l a ctos a mi ne; note COO− a nd ni trogen groups ) l i nked vi a a n α-l i nka ge between ca rbons 1 a nd 3. Ea ch ca rbohydra te mol ecul e ma y be s ul fa ted (once or twi ce) or l eft uns ul fa ted. The more common s ul fa ted forms a re referred to a s chondroi ti n s ul fa te a nd a re fel t to be ma i nl y res pons i bl e for the mol ecul e’s bi ol ogi ca l a cti vi ty. Chondroi ti n ha s recentl y become popul a r to i nges t i n pi l l form by s ome pa ti ents wi th knee a nd other joi nt pa i n i n hopes of “repl a ci ng” ca rti l a ge tha t ha s been depl eted by ti me a nd wea r a nd tea r. C. Col l a gen, the ma i n protei n i n connecti ve ti s s ue, i s s trongl y a s s oci a ted wi th proteogl yca ns compos ed of s evera l GAGs . Common components i ncl ude hya l uroni c a ci d, a nd l ong, l i nea r cha i ns of repeti ti ve uni ts of uroni c a ci ds (l eft) a nd chondroi ti n s ul fa te or kera ta n s ul fa te bound together by l i nk a nd core protei ns . The res ul ti ng protei n a nd ca rbohydra te mol ecul es a s wel l a s i ntera cti ons between the cha rged GAG groups a nd s urroundi ng wa ter mol ecul es crea te a n overa l l s tructure (ri ght) tha t i s both s trong a nd ri gi d but a l s o fl exi bl e a nd conforma bl e. Thes e qua l i ti es hel p to ma ke col l a gen the perfect s ubs ta nce to provi de s tructure but a l s o a l l ow movement i n joi nts . [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
TABLE 2-2. Bi ochemi ca l Rol es of Gl ycos a mi nogl yca ns Gl ycos a mi nogl yca ns a re res pons i bl e for a va s t number of functi ons i n huma n bi ol ogy, mos tl y i nvol vi ng s tructure outs i de the cel l a nd/or a tta chment of cel l s to externa l s tructures . An exa mpl e of thi s s tructura l moti f i s s hown i n Fi gure 2-7.
Figure 2-7. Example of Extracellular Matrix Structure. The rol e of col l a gen, chondroi ti n, proteogl yca ns , a nd l i nk protei ns i n the forma ti on of the extra cel l ul a r ma tri x i s i l l us tra ted. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] Carbohydrates and Fertilization: The ferti l i za ti on of a huma n egg rel i es on ca rbohydra te bi ndi ng a nd s i gna l i ng. Bi ndi ng of a receptor on the s perm’s s urfa ce to a galactose mol ecul e wi thi n a n ol i gos a ccha ri de on the egg’s s urfa ce s i gna l s the s perm to rel ea s e mol ecul es tha t a l l ow s perm entry i nto the egg, res ul ti ng i n ferti l i za ti on.
Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.
The l i s t bel ow i l l us tra tes jus t a few of thes e mol ecul es a nd thei r functi ons , whi ch wi l l be expl ored further i n l a ter cha pters .
REVIEW QUESTIONS 1. Wha t a re tri os e, pentos e, a nd hexos e ca rbohydra tes a nd thei r key fea tures ? 2. Wha t a re ketone a nd a l dehyde groups a nd thei r key fea tures ? 3. Wha t a re monos a ccha ri des a nd di s a ccha ri des a nd thei r key fea tures ? 4. Wha t a re the ba s i c s tructures of gl ycera l dehyde, ri bos e, gl ucos e, ga l a ctos e, fructos e, ma l tos e, l a ctos e, s ucros e, gl ycogen, s ta rch, a myl opecti n, a myl a s e, gl ycoprotei ns , a nd gl ycos a mi nogl yca ns ? 5. Wha t a re the ba s i c rol es a nd functi ons of ca rbohydra te mol ecul es , i ncl udi ng the va ri ous other types of mol ecul es to whi ch they ma y bond a nd the res ul ti ng s tructura l cha ra cteri s ti cs ?
CHAPTER 3 LIPIDS Ba s i c Li pi d Functi ons Ba s i c Membra ne Li pi d Structure Compl ex Li pi ds Li pi d-deri ved Hormones /Vi ta mi n D Revi ew Ques ti ons
OVERVIEW Li pi ds a re the thi rd ma jor type of bi ochemi ca l mol ecul e found i n huma ns . Al though one of thei r ma jor functi ons rel a tes to the forma ti on of bi ol ogi ca l membra nes (phos phol i pi ds a nd chol es terol ), l i pi d mol ecul es a re a l s o es s enti a l for energy s tora ge a nd tra ns port (tri a cyl gl ycerol s ), cel l ul a r bi ndi ng a nd recogni ti on a nd other bi ol ogi ca l proces s es (gl ycol i pi ds ), s i gna l i ng (s teroi d hormones ), di ges ti on (bi l e s a l ts ), a nd meta bol i s m (fa tty a ci ds , ketone bodi es , a nd vi ta mi n D). Li pi d mol ecul es a re ma i nl y hydrophobi c a nd a re, therefore, found i n a rea s a wa y from wa ter mol ecul es or a re i nvol ved i n mecha ni s ms s uch a s l i poprotei n compl exes tha t a l l ow thei r movement i n a nd through wa ter envi ronments . The s ma l l er hydrophi l i c pa rts of l i pi ds a re, thems el ves , i mporta nt i n forma ti on of bi ol ogi ca l membra nes a nd i n the s evera l s peci fi c functi ons of l i pi ds a nd l i pi d-deri ved mol ecul es .
BASIC LIPID FUNCTIONS A ma jor rol e of l i pi d mol ecul es i s to provi de the bui l di ng bl ocks for bi ol ogi ca l membra nes , i ncl udi ng phospholipids, glycolipids, a nd cholesterol. However, other l i pi ds known a s triacylglycerols (a l s o referred to a s tri gl yceri des or fa ts ) functi on i n the s tora ge of bi ol ogi ca l energy a nd bile salts, deri ved i n the l i ver from chol es terol a nd s erve i n the di ges ti on of di eta ry fa t. Fi na l l y, s evera l l i pi d-deri ved mol ecul es s erve a s i mporta nt hormones a nd i ntra cel l ul a r mes s engers . It i s i mporta nt to note tha t the ma jor pa rt of every l i pi d mol ecul e i s hydrophobi c i n na ture a nd, l i ke the hydrophobi c pa rts of protei ns di s cus s ed ea rl i er (Cha pter 1), prefers to be a wa y from a nd protected a ga i ns t i ntera cti on wi th wa ter mol ecul es . Thi s hydrophobi c cha ra cter i s funda menta l i n membra ne forma ti on, l i pi d tra ns port, a nd i n ma ny of the functi ons tha t the va ri ous types of l i pi d mol ecul es perform.
BASIC MEMBRANE LIPID STRUCTURE A membra ne l i pi d i s compos ed of three ba s i c components tha t a re a s fol l ows : 1. Fatty acids a re compos ed of l ong cha i ns of ca rbon mol ecul es wi th a ca rboxyl i c a ci d (COOH) a t ca rbon 1 a nd a CH 3 (methyl ) group a t the end of the cha i n (Fi gure 3-1A). The ca rboxyl i c a ci d group i s i nvol ved i n bondi ng of the fa tty a ci d to the other components of a l i pi d mol ecul e. In huma ns , fa tty a ci ds a re us ua l l y 12–24 ca rbons l ong a nd mos t often the number of ca rbons i n the fa tty a ci d ba ckbone i s even. Fa tty a ci ds ca n conta i n s i ngl e (C—C), doubl e (C═C), or tri pl e
ca rbon–ca rbon bonds .
Figure 3-1. A. Common Saturated and Unsaturated Fatty Acids. Pa l mi ta te (16-ca rbon), s tea ra te (18-ca rbon), a nd a ra chi da te (20-ca rbon), fa tty a ci ds , s hown by the ca rbon ba ckbone a nd i n “s ti ck di a gra m” form for a ra chi da te often us ed for s i mpl i ci ty. The ca rbon a toms of fa tty a ci ds a re numbered from the ca rboxyl i c a ci d (COOH) to the termi na l methyl (CH 3 ) group. Hydrogen a toms a re not s hown for cl a ri ty. B. Fatty Acid Chain Double Bonding. Deta i l of uns a tura ted fa tty a ci d ca rbon cha i n, i l l us tra ti ng trans doubl e bond (l eft: hydrogen a toms on oppos i te s i des of the bond a nd res ul ti ng l i nea r ca rbon cha i n) a nd uns a tura ted cis doubl e bond (ri ght: hydrogen a toms on the s a me s i de of the bond a nd res ul ti ng “ki nked” ca rbon cha i n). C. Common Unsaturated Fatty Acids. (Top a nd mi ddl e) Two 18-ca rbon uns a tura ted a ci ds s howi ng a cis doubl e bond (ol ei c a ci d) a nd a trans doubl e bond (el a i di c a ci d) both a t ca rbon 9. Arrows i l l us tra te di fferent conforma ti ons of fa tty a ci d cha i n tha t res ul t from the two types of s a tura ted bonds . (Bottom) An 18-ca rbon uns a tura ted fa tty a ci ds wi th two cis doubl e bonds (l i nol ei c a ci d) a t ca rbons 9 a nd 12 (s ee a rrows ). Note how the a ddi ti on of a s econd cis doubl e bond crea tes a n even more nonl i nea r fa tty a ci d cha i n, whi ch ca us es i ncrea s ed di s order of pa cki ng a nd, therefore, i ncrea s ed fl ui di ty of bi ol ogi ca l membra nes . [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] • Saturated fa tty a ci ds conta i n onl y s i ngl e ca rbon–ca rbon bonds , a nd a l l of the ca rbon mol ecul es a re bonded to the ma xi mum number of hydrogen mol ecul es . • Unsaturated fa tty a ci ds ha ve a t l ea s t one doubl e ca rbon–ca rbon bond wi th the potenti a l for a ddi ti ona l hydrogen a tom bondi ng s ti l l exi s ti ng for s ome of the ca rbon a toms i n the ba ckbone cha i n. If more tha n one doubl e bond i s pres ent, the term pol yuns a tura ted i s us ed. Thes e doubl e bonds ca n exi s t i n ei ther a “ki nked” cis doubl e bond or a more l i nea r trans doubl e bond (Fi gure 3-1B-C). 2. Glycerol i s a s i mpl e three-ca rbon mol ecul e wi th hydroxyl groups a t ea ch ca rbon (Fi gure 3-2A). Thes e hydroxyl groups a re the rea cti ve l oca ti on where fa tty a ci ds a nd other components of a l i pi d mol ecul e bond to form di a cyl gl ycerol (Fi gure 3-2A) a nd tri a cyl gl ycerol (Fi gure 3-2B) mol ecul es .
Figure 3-2. A–B. A. Glycerol, Diacylglycerol, and Triacylglycerol. Gl ycerol i s a s i mpl e, three-ca rbon cha i n mol ecul e (green) wi th a hydroxyl group (OH) bonded to ea ch of the ca rbon a toms . The hydroxyl groups a t ca rbons 1 a nd 2 of gl ycerol bond rea ct wi th the ca rboxyl i c a ci d groups (COO–) of the fa tty a ci d cha i ns , res ul ti ng i n two new bonds a nd two wa ter (H 2 O) mol ecul es . In genera l , uns a tura ted fa tty a ci ds bond to ca rbon 1, wherea s s a tura ted fa tty a ci ds bond to ca rbon 2. The res ul ti ng mol ecul e i s ca l l ed “di a cyl gl ycerol ” a nd i s i nvol ved i n i mporta nt s i gna l i ng pa thwa ys (Cha pter 8). B. Tri a cyl gl ycerol i s formed when a thi rd fa tty a ci d bonds to the thi rd gl ycerol hydroxyl group. Thi s res ul ta nt mol ecul e i s a n i mporta nt s tora ge form of energy (Cha pter 8). [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Double Bonds and Melting Temperatures: The number a nd type of doubl e bonds i n a pa rti cul a r l i pi d mol ecul e’s fa tty a ci d a ffects how tha t fa tty a ci d “pa cks ” wi th other fa tty a ci ds a nd, therefore, the tempera ture a t whi ch the pa rti cul a r l i pi d mol ecul e mel ts . For exa mpl e, the s a tura ted fa tty a ci ds l i s ted i n Ta bl e 3-1 ha ve mel ti ng poi nts between 44°C a nd 77°C. The uns a tura ted fa tty a ci ds ha ve fa r l ower mel ti ng poi nts , ra ngi ng from 13°C to –50°C, decrea s i ng a s the number of doubl e bonds (pol yuns a tura ti on) i ncrea s es . “Ki nked” cis doubl e bonds ma ke the pa cki ng of fa tty a ci ds even more di s orga ni zed a nd l ower the mel ti ng tempera ture even further. The effects of s a tura ted/uns a tura ted/pol yun-s a tura ted a nd cis/trans fa tty a ci ds a re rea di l y s een i n the di fferent mel ti ng tempera tures of butter, compos ed of a hi ghl y s a tura ted l i pi ds , a nd ma rga ri ne, compos ed of uns a tura ted l i pi ds . The l i nea r na ture of trans fa tty a ci ds ma kes them s i mi l a r to the s tructure of s a tura ted fa tty a ci ds . Thi s s tructura l fea ture, rel a ti ve to the cis confi gura ti on, s eems to ma ke meta bol i s m of trans fa ts di ffi cul t. Cons equentl y, trans fa ts rema i n l onger i n the ci rcul a ti on, thereby contri buti ng to a rteri a l depos i ti on a nd s ubs equent devel opment of corona ry hea rt di s ea s e. In genera l , uns a tura ted fa ts a re hea l thi er for the huma n body a nd, therefore, di eta ry fa ts compos ed mos tl y of cis doubl e bonded, pol yuns a tura ted fa ts a re recommended by di eti ci a ns a nd cl i ni ci a ns to hel p a voi d hea rt di s ea s e a nd other medi ca l probl ems .
TABLE 3-1. Common Fa tty Aci ds Found i n Huma ns
Essential Fatty Acids: Much l i ke there a re essential a mi no a ci ds tha t the body ca n onl y get from di eta ry s ources , certa i n fa tty a ci ds a re a l s o deemed es s enti a l . Two pa rti cul a r es s enti a l fa tty a ci ds , l i nol ea te a nd l i nol ena te, ha ve doubl e bonds a t the s i xth a nd thi rd ca rbon a toms counti ng from the methyl end of thei r cha i ns , res pecti vel y, a nd a re needed to produce certa i n 20-ca rbon l ong fa tty a ci ds conta i ni ng doubl e bonds . Thes e two fa tty a ci ds a re known a s omega-6 (ω-6) a nd omega-3 (ω-3) fa tty a ci ds . Huma ns do not ha ve the a bi l i ty to produce doubl e bonds a t thes e l oca ti ons a nd, therefore, mus t obta i n thes e two requi red fa tty a ci d bui l di ng bl ocks from vegeta bl e oi l s . Ara chi dona te wi th a 20-ca rbon cha i n a nd four cis doubl e bonds i s a l s o a n es s enti a l fa tty a ci d i nvol ved i n s evera l i mporta nt bi ol ogi ca l functi ons . Recentl y, l onger ca rbon cha i n ω-3 fa tty a ci ds ha ve been propos ed to decrea s e hea rt a tta cks a nd s trokes ; s uppl ements a nd s ome food products a re now a va i l a bl e, whi ch conta i n thes e fa tty a ci ds . Interes ti ngl y, exces s i ve di eta ry i nta ke of ω-6 fa tty a ci ds ha s been i mpl i ca ted i n a n i ncrea s ed ri s k of hea rt a tta cks , s trokes , s ome ca ncers , a nd even depres s i on. 3. Head Group The fi na l component of a l i pi d mol ecul e va ri es wi th ea ch type of l i pi d a nd, a l ong wi th the two s peci fi c fa tty a ci ds , defi nes ea ch pa rti cul a r l i pi d. Thi s thi rd pa rt of the l i pi d mol ecul e i s often ca l l ed the “hea d group,” a ptl y na med i f one envi s i ons the end methyl group of the fa tty a ci d cha i ns to be the ta i l of the l i pi d mol ecul e (Fi gure 3-3A). Mos t l i pi ds i n a bi ol ogi ca l membra ne ha ve a phos pha te group (PO4 –3 ) a tta ched to the thi rd gl ycerol ca rbon a nd a re, therefore, ca l l ed phospholipids. Us ua l l y, a n a ddi ti ona l mol ecul e (s evera l common exa mpl es found i n huma ns a nd the res ul ti ng phos phol i pi d mol ecul es a re s hown i n Fi gure 3-3B) i s a tta ched to the phos pha te mol ecul e, res ul ti ng i n the fi na l hea d group of the l i pi d mol ecul e. Thi s hea d group i s us ua l l y cha rged, crea ti ng a pa rt of the l i pi d tha t i s hydrophi l i c, a nd wa nts to be nea r wa ter, a qua l i ty tha t i s es s enti a l for the forma ti on of bi ol ogi ca l membra nes (Cha pter 8) a nd ma ny l i pi d functi ons .
Figure 3-3. A. Phospholipid Components and Formation. Bondi ng between hydroxyl (OH) of a phos pha te group wi th the hydroxyl (OH) of gl ycerol ca rbon 3 (green) res ul ts i n a phos phol i pi d mol ecul e a nd one wa ter mol ecul e. The ba s i c phos phol i pi d i l l us tra ted a bove i s termed phos pha ti di c a ci d a nd i s the bui l di ng bl ock of common phos phol i pi ds found i n the cel l membra ne (s ee bel ow). Fa tty a ci d cha i ns a re depi cted i n bl a ck. B. Common Phospholipids Found in Humans. Common hea d groups , whi ch form phos phol i pi ds (top a nd mi ddl e) a nd a re s hown i n bl ue, bonded to the hydroxyl (OH) of a phos pha te group (purpl e), whi ch i ts el f i s bonded to the hydroxyl (OH) of gl ycerol ca rbon 3 (green). Thi s s tructure res ul ts i n a phos phol i pi d mol ecul e tha t i s genera l l y found i n membra nes . Li pi d mol ecul es a re often repres ented i n “s ti ck di a gra m” form (bottom) wi th the cha rged phos pha te group a nd hydrophi l i c hea d group s hown a s a ci rcl e a nd ova l a nd wi th the fa tty a ci d “ta i l s ,” depi cted a s ei ther s tra i ght or ja gged l i nes , formi ng the hydrophobi c regi on. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] The common fa tty a ci ds found i n huma ns a re l i s ted i n Ta bl e 3-1.
COMPLEX LIPIDS GLYCOLIPIDS/SPHINGOLIPIDS Jus t a s ca rbohydra te a nd protei n mol ecul es ca n bi nd together (di s cus s ed i n Cha pter 2), ca rbohydra tes ca n a l s o bi nd to l i pi ds to form a glycolipid. However, i n a huma n gl ycol i pi d, the gl ycerol ba ckbone i s genera l l y repl a ced by a ba ckbone of sphingosine [ma de from the a mi no a ci d s eri ne a nd the 16-ca rbon fa tty a ci d pa l mi ta te (Fi gure 3-4A)] a nd i s , therefore, referred to a s a sphingolipid. Sphi ngos i ne ca n bi nd two other mol ecul es wi th the rema i ni ng hydroxyl (OH) a nd a mi no (NH 3 ) groups from the s eri ne a mi no a ci d. In huma n s phi ngol i pi ds , the a mi no group i s a l wa ys bound to a nother fa tty a ci d to ma ke the mol ecul e ceramide. From cera mi de, the pa rti cul a r mol ecul e(s ) a tta ched to the rema i ni ng hydroxyl group defi nes both the na me a nd the cha ra cteri s ti cs of the res ul ti ng s phi ngol i pi d. For exa mpl e, the mol ecul e sphingomyelin, whi ch ca n ma ke up to 20% of the tota l phos phol i pi d i n ma ny bi ol ogi ca l membra nes , i s ma de of cera mi de a nd a phos phoryl chol i ne hea d group (Fi gure 3-4B).
Figure 3-4. A-B. Ceramide and Sphingolipids. A. Cera mi de i s produced from the combi na ti on of the a mi no a ci d s eri ne a nd the 16-ca rbon, fa tty a ci d
pa l mi ta te (green) wi th the s ubs equent a ddi ti on of a s econd fa tty a ci d (bl a ck) to the a mi no (NH 3 ) group (referred to a s N-a cetyl a ti on). Other mol ecul es ca n then bi nd to the cera mi de hydroxyl (OH) to produce a wi de ra nge of s phi ngol i pi ds i mporta nt to huma ns , a n exa mpl e of whi ch i s s phi ngomyel i n (B) s hown i n the ri ght pa nel a nd di s cus s ed i n the text. C. Common Cerebrosides. Cerebros i des a re compos ed of s phi ngos i ne a nd a s i ngl e gl ucos e or ga l a ctos e mol ecul e a tta ched vi a the hydroxyl group a t ca rbon 4. D. A Sulfatide. Sul fa ti des a re s i mpl y cerebros i des wi th the a ddi ti on of a ca rbohydra te-l i nked s ul fa te mol ecul e. Compa re wi th ga l a ctocerebros i de i n Fi gure 3-4B. E. A Globoside and GalNAc Carbohydrate. Gl obos i des a re compos ed of cera mi de bonded to a fa tty a ci d vi a the NH group (green) a nd s evera l ca rbohydra te mol ecul es , i ncl udi ng Ga l NAc (l i ght bl ue s qua re), bonded vi a the s eri ne OH (upper pa nel ). The s tructure of N-a cetyl -ga l a ctos a mi ne or Ga l NAc i s s hown i n the l ower pa nel wi th the NH a tta chment hi ghl i ghted i n green. F. A Ganglioside and NANA. The s tructure of N-a cetyl neura mi ni c (NANA), a type of s i a l i c a ci d, i s s hown i n the l ower pa nel (N-a cetyl group i ndi ca ted i n bl ue). [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] From the ba s e mol ecul e of a s phi ngol i pi d (e.g., cera mi de), ca rbohydra te mol ecul es ma y a l s o be a tta ched to form a glycosphingolipid. In genera l , huma n gl ycos phi ngol i pi ds a re grouped i nto four ca tegori es . 1. Cerebrosides—Cera mi de (s ee a bove) a tta ched to onl y a single gl ucos e or ga l a ctos e res i due, produci ng gl ucos yl cera mi de or ga l a ctos yl cera mi de, res pecti vel y (Fi gure 3-4C). Cerebros i des a re i mporta nt i n the membra nes of mus cl e a nd nerve cel l s a nd a re l oca ted i n myel i n, whi ch covers nerve a xons a nd ena bl es fa s t a nd effi ci ent conducti on of nerve i mpul s es . Cerebros i des ma y a l s o be i nvol ved i n the bi ndi ng of morphi ne a nd other opi a tes . 2. Sulfatides—Ga l a ctos e-ba s ed gl ycos phi ngol i pi d mol ecul es , whi ch conta i n a s ul fur a tom-conta i ni ng s ul fa te group i n pl a ce of the phos phorous a tom (Fi gure 3-4D). Sul fa ti des a re found ma i nl y i n the bra i n a nd centra l a nd peri phera l nervous s ys tems but a re s een i n tra ce a mounts i n other ti s s ues . Sul fa ti des a re bel i eved to be i nvol ved i n the regul a ti on of cel l growth a nd s i gna l i ng a nd ma y s erve to both hel p form a nd, a l terna ti vel y, brea k down bl ood cl ots pos s i bl y by a ffecti ng the tra ns porta ti on of s odi um a nd pota s s i um i n a nd out of cel l s s uch a s pl a tel ets . Sul fa ti des a l s o ma y pl a y rol es a s a n a dhes i on mol ecul e, i ncl udi ng the recrui tment of i mmune cel l s to i nfl a med ti s s ue a nd the bi ndi ng a nd repl i ca ti on of i nfl uenza vi rus es . Cha nges i n the producti on of s ul fa ti des ha ve been noted a s one of the ea rl i es t i ndi ca tors of Al zhei mer’s di s ea s e. 3. Globosides—Gl ycos phi ngol i pi d mol ecul es wi th the ca rbohydra te mol ecul e N-acetyl-galactosamine (a .k.a . Ga l NAc) a l ong wi th two or more other ca rbohydra te mol ecul es (Fi gure 3-4E). Gl obos i des a re found i n s evera l orga ns i ncl udi ng red bl ood cel l s , s erum, l i ver, a nd s pl een. Al though the functi ons of gl obos i des a re not wel l unders tood, they a re bel i eved to pl a y a n i mporta nt rol e i n cel l receptors . Interes ti ngl y, the bi ndi ng of Escherichia coli, a ba cteri um common i n uri na ry tra ct i nfecti ons , to cel l s i n the uri na ry tra ct i s bel i eved to occur through gl obos i des . 4. Gangliosides—Gl ycos phi ngol i pi d mol ecul es wi th one or more a tta ched sialic acid mol ecul es , mos t often N-acetylneuraminic acid (a.k.a. “NANA”), a compl ex, ni ne-ca rbon ca rbohydra te mol ecul e (Fi gure 3-4F). There a re a l a rge number of di fferent ga ngl i os i des , whi ch di ffer i n thei r s tructure dependi ng on the number a nd l oca ti on of the ca rbohydra te a nd NANA mol ecul es . Ga ngl i os i des a re i nvol ved i n bi ndi ng, recogni ti on, a nd s i gna l i ng between cel l s . Al though ga ngl i os i des a re a bunda nt i n the nervous s ys tem, they a re a l s o bel i eved to pl a y a n i mporta nt rol e i n bi ndi ng i mmune cel l s . Ga ngl i os i des a re a l s o found i n l es s a bunda nce i n other cel l types . Ga ngl i os i des a re a l s o bel i eved to pl a y a rol e i n the bi ndi ng a nd entra nce i nto cel l s of the i nfl uenza vi rus a nd the toxi n tha t ca us es chol era . EICOSANOIDS Eicosanoid i s the genera l term for mol ecul es tha t a re a l l compos ed of 20-ca rbon fa tty a ci ds a nd i nvol ved i n s i gna l i ng. The four ma jor groups of ei cos a noi ds i ncl ude the prostaglandins (PGs), the prostacyclins (PGIs), the thromboxanes (TXs), a nd the leukotrienes (LTs). Functi ons of ei cos a noi ds i ncl ude promoti on of i nfl a mma ti on (us ua l l y ω-6 deri ved), i mmune res pons e, neurol ogi ca l s i gna l i ng, regul a ti on of bl ood pres s ure, control of pl a tel et a ggrega ti on/ di s a ggrega ti on, a nd modul a ti on of l evel s of tri a cyl gl ycerol s . They a l s o ha ve di rect/i ndi rect effects on a va ri ety of di s ea s es , i ncl udi ng ca rdi ova s cul a r a nd rheuma toi d pa thol ogi es , a mong other rol es . The s i gna l i ng a cti ons of ei cos a noi ds a re ma i nl y tra ns mi tted vi a G-protei n receptors (Cha pter 8). Al l ei cos a noi ds a re produced from ei ther ω-6 acids (dihomo-gamma-linolenic acid (DGLA), arachidonic acid (AA), or a n ω-3 acid [eicosapentaenoic acid (EPA)]. The predomi na nt s yntheti c pa thwa y for ei cos a noi ds i s vi a the ω-6, AA pa thwa y whos e products a re denoted by the s ubs cri pt “2” (Fi gure 3-5). LTs a re produced from AA vi a a di fferent pa thwa y i l l us tra ted i n the fi gure. Importa ntl y, ei cos a noi ds deri ved from ω-6/DGLA a nd the ω-3/EPA precurs ors a re much l es s i nfl a mma tory/a nti -i nfl a mma tory i n na ture. Increa s ed i nta ke of DGLA (di eta ry s uppl ements ), es peci a l l y EPA (“ω-3” fi s h oi l s ), res ul ts i n l owered a s s oci a ted di s ea s es vi a di rect competi ti on wi th the AA pa thwa y. Medi ca ti ons s uch a s a s pi ri n, nons teroi da l a nti -i nfl a mma tory drugs , a nd cycl ooxygena s e (COX)-2 i nhi bi tors a l s o decrea s e the i nfl a mma tory effects of PG, PGI, a nd TX (col l ecti vel y known a s prostanoids) by di rect i nhi bi ti on of COX-1 or -2, whi ch a re key enzymes i n pros ta noi d s ynthes i s (Fi gure 3-5). Corti cos teroi ds (e.g., predni s ol one) i nhi bi t phos phol i pa s e A2 (Fi gure 3-5).
Figure 3-5. Overview of Eicosanoid Synthetic Pathway. Phos phol i pi ds genera ted a s noted a bove (s ee a l s o Fi gure 3-3A) provi de the ba s i c bui l di ng bl ocks for the forma ti on of a ra chi doni c a ci d (AA), the s ource of a l l ei cos a noi ds . The va ryi ng s yntheti c pa thwa ys comi ng from AA a re s hown. See a l s o Ta bl e 3-2 for a revi ew of the ma jor known ei cos a noi ds a nd thei r functi ons . [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
TABLE 3-2. Overvi ew of Ei cos a noi ds More s peci fi ca l l y, PGs a re ei cos a noi ds whos e ma jor functi ons i ncl ude regul a ti on of i nfl a mma ti on, s mooth mus cl e contra cti on (bl ood ves s el s , ga s troi ntes ti na l , bronchi a l , a nd uterus ), neurol ogi ca l pa i n, pl a tel et functi on, hormone a cti vi ty, a nd cel l growth. PGIs a re deri ved di rectl y from a pros ta gl a ndi n precurs or (PGH 2 ) a nd a re i mporta nt i n decrea s i ng pl a tel et functi on a nd, therefore, bl ood cl ot forma ti on a s wel l a s di l a ti on of bl ood ves s el s . TXs a re a l s o deri ved from PGH 2 a nd, oppos i te of PGIs , a ct a s va s ocons tri ctors to promote pl a tel et a ggrega ti on a nd bl ood cl ot forma ti on. LTs , the fourth cl a s s of ei cos a noi ds , a re produced vi a a pa thwa y tha t devi a tes from the other ei cos a noi ds ’ producti on a t AA (Fi gure 3-5). LTs a re ma i nl y s een i n i nfl a mma tory proces s es of the l ung, i ncl udi ng a s thma ti c rea cti ons a nd the i nfl a mma ti on a s s oci a ted wi th bronchi ti s . Speci fi c functi ons of the ma jor ei cos a noi ds a nd thei r rol e i n di s ea s e a nd trea tments a re revi ewed i n Ta bl e 3-2 a nd wi l l be covered further i n Secti on III. ABO Blood Groups: The huma n bl ood groups O, A, B, a nd AB a re determi ned by s peci fi c gl ycos phi ngol i pi ds found i n the red cel l membra ne. The ba s i c gl ycos phi ngol i pi d, ca l l ed “H-substance,” i s compos ed of a s phi ngol i pi d connected to three ca rbohydra te mol ecul es a nd a membra ne-bound protei n. The a ddi ti on of s peci fi c, extra ca rbohydra te mol ecul e to a ga l a ctos e res i due on H-s ubs ta nce produces the four common bl ood types a s fol l ows :
Eicosanoids and Inflammation: Infl a mma ti on i s often cha ra cteri zed by the four s i gns of ca l or, dol or, tumor, a nd rubor. Ea ch of thes e fea tures
res ul ts , a t l ea s t i n pa rt, from the a cti on of one or more ei cos a noi ds . Calor (wa rmth) i s ca us ed by the pros ta gl a ndi n PGE2 . Dolor (pa i n) i s hei ghtened vi a the a cti on of PGE2 , whi ch a l s o i ncrea s es the s ens i ti vi ty of neurons . Tumor (s wel l i ng) res ul ts from the l ea ka ge of pl a s ma from bl ood ves s el s whos e permea bi l i ty i s i ncrea s ed by the l eukotri ene LTB 4 . Infl a mma tory rubor (rednes s ) res ul ts from the a cti on of thromboxa ne TXA2 , i ni ti a l l y rel ea s ed a fter i njury, whi ch s ubs equentl y i ncrea s es the concentra ti on a nd, therefore, the bl ood ves s el di l a ti on a cti vi ty of PGE2 a nd LTB 4 , res ul ti ng i n engorged ves s el s a nd reddeni ng. CHOLESTEROL Cholesterol i s a n extremel y i mporta nt mol ecul e found onl y i n euka ryoti c orga ni s ms wi th a va ri ety of functi ons i n the huma n body. Al though huma ns ca n ma ke chol es terol , they ha ve to rel y on a compl ex mecha ni s m of bi ndi ng a nd modi fi ca ti ons by other mol ecul es s o tha t chol es terol ma y be el i mi na ted from the body (s ee bel ow). As a res ul t, properl y control l i ng the da y-toda y a mount of chol es terol i n the body—a ba l a nce between producti on, el i mi na ti on, a nd the externa l i nfl uence of di eta ry i nta ke—ca n pl a y a key rol e i n hea l th a nd i l l nes s . The functi ons of chol es terol a re l i s ted bel ow. 1. Chol es terol i s one of the es s enti a l l i pi d components of bi ol ogi ca l membra nes where i t i s a modul a tor of the fl ui di ty of membra nes . The a bi l i ty of membra nes to modi fy thei r s tructure a nd/or the a bi l i ty for other mol ecul es to be a bl e to move wi thi n the membra ne i s cri ti ca l for cel l s i gna l i ng, bi ndi ng, wound hea l i ng, i mmune res pons e, a nd s o on. 2. Chol es terol s erves a s the pri ma ry s ource for the producti on of s teroi d hormones (s ee next s ecti on bel ow), bi l e s a l ts , a nd even vi ta mi n D. 3. Chol es terol meta bol i s m i s a l s o i mporta nt i n the regul a ti on of the tra ns porta ti on of l i pi ds throughout the body, a nd di s turba nce of thi s s ys tem l ea ds to the depos i ti on of chol es terol i n the a rtery wa l l ca us i ng a theros cl eros i s , whi ch ul ti ma tel y l ea ds to hea rt a tta cks a nd s trokes . An unders ta ndi ng of thi s regul a ti on ha s l ed to i mporta nt trea tments for decrea s i ng the ri s k of thes e di s ea s es . The chol es terol mol ecul e ha s four ri ngs ma de of ca rbon a toms —three ri ngs ha ve s i x s i des a nd one ha s fi ve s i des —wi th a s i x-ca rbon ri ng ta i l (Fi gure 3-6A). Mos t of the ca rbons a re s i ngl e bonded a nd, therefore, ha ve thei r ful l compl ement of hydrogen a toms . The three-di mens i ona l s tructure of chol es terol i s a pproxi ma tel y a fl a t pl a ne, expos i ng a l l of the hydrophobi c pa rts of the chol es terol mol ecul e to the envi ronment. Onl y one hydroxyl group wi th i ts hydrophi l i c na ture crea tes a ny cha rged qua l i ty to the chol es terol mol ecul e. As a res ul t, chol es terol does not l i ke to be expos ed to wa ter envi ronments , preferri ng to be s hi el ded by other hydrophobi c mol ecul es s uch a s l i pi ds or hydrophobi c pa rts of protei ns . The i mpa ct of chol es terol ’s uni que s tructure a nd i ts rol e i n pl a s ma membra ne s tructure a nd fl ui di ty wi l l be exa mi ned further i n Cha pter 8.
Figure 3-6. Cholesterol Molecule (A) Unesterified and (B) Esterified. A. Unes teri fi ed chol es terol mol ecul e wi th free end hydroxyl group crea ti ng a hydrophi l i c cha rge i mporta nt i n bi ol ogi ca l membra ne pa cki ng. B. Es teri fi ed chol es terol mol ecul e wi th R group bonded by es ter bond to oxygen from end hydroxyl group. The R group i s us ua l l y a fa tty a ci d bonded to chol es terol by the enzyme l eci thi n chol es terol a cyl tra ns fera s e (LCAT). [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] However, s ome chol es terol i s a l s o found outs i de of bi ol ogi ca l membra nes i n a drena l gl a nds , bl ood, a nd other ti s s ues where i t i s often bonded vi a the hydroxyl group to a l ong fa tty a ci d a s a chol es terol es ter (Fi gure 3-6B). Thes e chol es terol es ters a re hi ghl y hydrophobi c a nd i ns ol ubl e a nd ca n form fa tty l es i ons or “pl a ques ” i n the a rtery wa l l tha t ca n l ea d to hea rt a tta cks or s trokes . Fortuna tel y, the huma n body ha s devel oped a uni que wa y of a ddres s i ng thi s probl em, na mel y by formi ng l i poprotei ns (s ee bel ow a nd Fi gure 3-7).
Figure 3-7. Basic Lipoprotein Structure. Li poprotei n compl exes a re compos ed of a centra l l i pi d core (chol es terol es ter a nd tri a cyl gl ycerol mol ecul es ), a cha rged l i pi d outer s hel l (phos phol i pi d a nd chol es terol mol ecul es ), a nd s urroundi ng a poprotei ns tha t hel p to tra ns port hydrophobi c l i pi d mol ecul es throughout the body. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.]
Cholesterol, Phospholipids, Gangliosides, and Cancer: Emergi ng res ea rch ha s s hown tha t s ome ca ncer cel l s (e.g., meni ngos a rcoma a nd certa i n l eukemi a s ) ha ve s i gni fi ca nt cha nges i n thei r cel l membra ne chol es terol , phos phol i pi d, a nd ga ngl i os i de content tha t a l ters the membra ne fl ui di ty a s wel l a s l i pi d s i gna l i ng tha t produces unregul a ted growth or ca rci nogenes i s . Thes e cha nges ca n a l s o ca us e res i s ta nce to a gents us ed to trea t ca ncer. Unders ta ndi ng how cha ngi ng membra ne compos i ti on l ea ds to thes e fi ndi ngs ma y hel p to devel op new a nd di fferent ca ncer-fi ghti ng a gents wi th a uni que mecha ni s m of l i pi d a tta ck. LIPOPROTEINS Lipoproteins, a s the na me i mpl i es , a re formed from the compl exes of l i pi d a nd protei n mol ecul es . Unl i ke gl ycol i pi ds , i n whi ch s i mpl e bonds connect the pri nci pa l component mol ecul es , l i poprotei ns a re fa r more compl ex a nd form l a rge pa rti cl es conta i ni ng s evera l l i pi d cl a s s es a nd protei n. Thei r compl exi ty emerges from the pri ma ry functi on of l i poprotei ns , na mel y the tra ns porta ti on a nd del i very of fa tty a ci ds , tri a cyl gl ycerol , a nd chol es terol to a nd from ta rget cel l s i n a va ri ety of orga ns (s ee a l s o Cha pter 7 for further di s cus s i on). Thus , a l though gl ycol i pi ds once produced a nd i n thei r fi na l l oca ti on rema i n there for rel a ti vel y l ong peri ods of ti me, l i po-protei ns a re more tra ns i ent. The bondi ng a nd s tructure of l i poprotei ns refl ect thi s cha ra cteri s ti c. A s i mpl i fi ed model of a l i poprotei n i ncl udes a center core compos ed of chol es terol es ter a nd tri a cyl gl ycerol mol ecul es s urrounded by a n outer s hel l of phos phol i pi ds a nd chol es terol mol ecul es wi th thei r hydrophobi c a rea s i nwa rd towa rd the l i pi d core a nd thei r cha rged, hydrophi l i c a rea s fa ci ng outwa rd towa rd the a queous envi ronment (Fi gure 3-7). Speci a l i zed protei ns , known a s apoproteins, wra p a round the outer s hel l of the l i po-protei n pa rti cl e a l s o i nvol ved i n i ntera cti ons wi th externa l wa ter. The compos i ti on of l i poprotei n pa rti cl es va ri es but they ca n be broa dl y ca tegori zed dependi ng on thei r dens i ty a s s hown i n Ta bl e 3-3. Li poprotei ns , thei r components , functi on, meta bol i s m, a nd medi ca l i mpl i ca ti ons wi l l be di s cus s ed i n much more deta i l i n Cha pters 11 a nd 16.
TABLE 3-3. Ba s i c Li poprotei n Cha ra cteri s ti cs BILE SALTS Bile salts, compos ed of bi l e a ci ds conjuga ted wi th gl yci ne or ta uri ne (Fi gure 3-8A), a re produced i n the l i ver di rectl y from chol es terol a nd a re i mporta nt i n s ol ubi l i zi ng di eta ry fa ts i n the ma i nl y wa tery envi ronment of the s ma l l i ntes ti ne. After producti on i n the l i ver a nd pri or to s ecreti on i nto the ga l l bl a dder a nd/or di ges ti ve s ys tem, they a re often bonded to the a mi no a ci d gl yci ne (Cha pter 1) or ta uri ne, a deri va ti ve of the common a mi no a ci d cys ti ne (Cha pter 1), to i ncrea s e thei r wa ter s ol ubi l i ty (Fi gure 3-8B). Gl yco- a nd ta uro-bi l e a ci ds a re a l s o ca l l ed a s conjuga ted bi l e a ci ds .
Figure 3-8. A-B. Major Bile Acids and Bile Salts Found in Humans. A. Bi l e s a l ts , i ncl udi ng the pri ma ry forms chol i c a ci d a nd chenodeoxychol i c a ci d, a re produced by the l i ver a nd commonl y found i n huma ns . Remova l of hydroxyl groups (OH) by i ntes ti na l ba cteri a produces the s econda ry bi l e a ci ds deoxychol i c a ci d a nd l i thochol i c a ci d. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] B. The a ddi ti on of the a mi no a ci d gl yci ne (l eft) or the cys ti ne-rel a ted ta uri ne (bottom ri ght) i ncrea s es the s ol ubi l i ty of the bi l e s a l ts . [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Bile Salt Sequestrants: Medi ci nes tha t bi nd wi th or “s eques ter” a nd hel p excrete bi l e s a l ts a re s ometi mes us ed i n pa ti ents wi th hi gh chol es terol l evel s i n thei r bl ood. The remova l of thes e bi l e s a l ts ca us es further producti on of repl a cement bi l e a ci ds from the body’s chol es terol s tore to a l l ow fa t di ges ti on, thereby l oweri ng the tota l chol es terol i n thes e pa ti ents . However, s uch trea tments ca n i nterfere wi th norma l l i pi d a bs orpti on a ffecti ng di eta ry requi rements a s wel l a s res ul ti ng i n the unpl ea s a nt s i de effect of fa tty, foul -s mel l i ng bowel movements .
LIPID-DERIVED HORMONES/VITAMIN D Steroid hormones , whi ch a re a l l produced from chol es terol (Fi gure 3-9), perform a va ri ety of di fferent functi ons i n the huma n body a s l i s ted bel ow.
Figure 3-9. Major Steroid Hormones Found in Humans. Steroi d hormones , deri ved from chol es terol (upper l eft corner), control s evera l i mporta nt bodi l y functi ons es s enti a l to l i fe. Thei r s ynthes i s i nvol ves mul ti pl e, i nterrel a ted s teps , whi ch occur i n the a drena l cortex (mi nera l ocorti coi ds a nd gl ucocorti coi ds ), tes tes (a ndrogens ), a nd ova ri es (es trogens ). The s ynthes i s of the va ri ous s teroi d-deri ved hormones ca n a l s o be ca tegori zed by the s peci fi c enzyme (yel l ow boxes ) i nvol ved a nd/or the number of ca rbon mol ecul es i n the s ubs tra te or product. Bl ue hi ghl i ght i ndi ca tes pos i ti on of chemi ca l cha nge. Severa l di s ea s es rel a ted to s teroi d producti on i nvol ve thes e enzymes . [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] CORTICOSTEROIDS (ADRENAL GLAND) Mineralocorticoids. The pri ma ry mi nera l ocorti coi d i n huma ns i s aldosterone, whi ch i s produced i n the outer l a yer of cel l s of the a drena l cortex. Synthes i s a nd s ecreti on of a l dos terone i s control l ed by the reni n–a ngi otens i n s ys tem (s ee Cha pter 18). In certa i n di s ea s e s ta tes , a mi nor mi nera l ocorti coi d, 11-deoxycorticosterone, ma y i ns tea d be produced i n exces s i n the a drena l cortex a nd s ubs ti tute for a l dos terone. The rol e of the mi nera l ocorti coi ds i s to ma i nta i n bl ood vol ume a nd bl ood pres s ure by i ncrea s i ng the rea bs orpti on of Na + from the uri ne to the bl ood. The i ncrea s e of Na + rea bs orpti on i s a ccompa ni ed by the excreti on of K+ a nd protons (H +) a nd l ea ds to rea bs orpti on of wa ter by the a cti on of antidiuretic hormone (s ee Cha pter 18). Glucocorticoids. The pri ma ry gl ucocorti coi d i n huma ns i s cortisol, whi ch i s produced i n the mi ddl e a nd i nnermos t l a yer of cel l s of the a drena l cortex. Gl ucocorti coi ds ha ve a va ri ety of uni que functi ons . They pl a y a n i mporta nt rol e i n fuel homeos ta s i s duri ng s ta rva ti on a nd other s tres s rel a ted condi ti ons by promoti ng the mobi l i za ti on of fa tty a ci ds from tri a cyl gl ycerol s (Fi gure 3-2B) s tored i n fa t cel l s , promote the brea kdown of mus cl e protei n, a nd i ncrea s e l i ver gl ucos e s ynthes i s (gl uconeogenes i s ) us i ng the a mi no a ci ds deri ved from mus cl e a nd l a cti c a ci d (s ee Cha pter 10). Addi ti ona l l y, they di mi ni s h i nfl a mma tory res pons es by decrea s i ng producti on of PGs (s ee Fi gure 3-5). Androgens. The cel l s i n the i nnermos t l a yer of the a drena l cortex produce a ndrogens , pri ma ri l y androstenedione, a s a ma jor product. The a drena l cortex i s the s ol e s ource of a ndrogens i n fema l es but a mi nor s ource i n ma l es who produce l a rge a mounts of a ndrogens i n the tes tes (s ee Cha pter 20). Adrena l a ndrogens , pa rti cul a rl y i n fema l es , contri bute to certa i n s econda ry s exua l cha ra cteri s ti cs s uch a s the growth of a xi l l a ry a nd pubi c ha i r, a s wel l a s contri bute to l i bi do. Progestogens. The pri ma ry proges togen i n huma ns i s progesterone. Proges terone produced i n the a drena l cortex i s pri ma ri l y a n i ntermedi a te i n the producti on of the other a drena l s teroi d hormones . Its phys i ol ogi ca l rol e i s i mpl a nta ti on of a ferti l i zed egg i n the uteri ne l i ni ng a nd ma i ntena nce of pregna ncy. Its pri ma ry producti on occurs i n the corpus l uteum of the ova ri es a nd the pl a centa fol l owi ng ferti l i za ti on. ANDROGENS (TESTES) AND ESTROGENS (OVARIES) Res pons i bl e for s exua l functi ons s uch a s puberty (ons et, devel opment, a nd cha nges i n s econda ry s exua l cha ra cteri s ti cs ), menopa us e, ovul a ti on, a nd s perm forma ti on. VITAMIN D Vitamin D, one of the fa t-s ol ubl e vi ta mi ns , i s requi red for ca l ci um meta bol i s m. A s ma l l a mount of vi ta mi n D i s obta i ned from di eta ry s ources but a ma jori ty i s produced by the convers i on of chol es terol through a uni que proces s i nvol vi ng s unl i ght a nd three s epa ra te orga ns (Fi gure 310). Chol es terol , now dehydrochol es terol , ha vi ng ha d a s econd ca rbon–ca rbon doubl e bond formed i n the l i ver, i s tra ns ported to the s ki n where ul tra vi ol et ra ys brea k open the s econd a nd thi rd ri ngs . Tra vel i ng ba ck to the l i ver, a s econd hydroxyl group i s a dded to the formerl y hydrophobi c “ta i l .” Fi na l l y, the mol ecul e tra vel s to the ki dney where a nother hydroxyl group i s a dded nea r the s i te of the l one chol es terol hydroxyl group. The res ul ti ng mol ecul e, “ca l ci trol ,” i s the a cti ve form of vi ta mi n D whos e functi ons wi l l be di s cus s ed more ful l y i n Cha pters 10 a nd 13.
Figure 3-10. Production of Vitamin D from Cholesterol. Acti ve vi ta mi n D (ca l ci trol ) i s produced from chol es terol by mea ns of four s teps ta ki ng pl a ce i n the l i ver, s ki n, l i ver, a nd ki dney, res pecti vel y. Di eta ry s ources ca n a l s o provi de chol eca l ci ferol di rectl y. Sma l l , da rk a rrows i ndi ca te the l oca ti on where mol ecul a r cha nge ta kes pl a ce i n ea ch s tep. Added hydroxyl groups (OH) a re i ndi ca ted i n green. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.]
REVIEW QUESTIONS 1. Wha t a re fa tty a ci ds , tri a cyl gl ycerol s , a nd phos phol i pi ds ? 2. Wha t a re l i pi d hea d groups a nd the key cha ra cteri s ti cs of common ones found i n ma n? 3. Wha t a re cerebros i des , s ul fa ti des , gl obos i de, a nd ga ngl i o-s i des a nd thei r key fea tures ? 4. Wha t a re l i poprotei ns , chol es terol , bi l e s a l t (a ci d), s teroi d hormones , a nd ca l ci trol a nd thei r key fea tures ? 5. Wha t a re the ba s i c s tructures a nd cha ra cteri s ti cs of s a tura ted, uns a tura ted, ci s , tra ns fa tty a ci ds , gl ycerol , a nd phos phol i pi d? 6. Wha t a re the ba s i c s tructures a nd cha ra cteri s ti cs of the va ri ous gl ycol i pi ds (i .e., cerebros i des , s ul fa ti des , gl obo-s i des , a nd ga ngl i os i des )? 7. Wha t a re the ba s i c s tructures a nd cha ra cteri s ti cs of the l i poprotei ns (hi gh-dens i ty l i poprotei ns , l ow-dens i ty l i poprotei n, i ntermedi a tedens i ty l i poprotei ns , a nd very-l ow-dens i ty l i poprotei n)? 8. Wha t a re the ba s i c s tructures a nd cha ra cteri s ti cs of bi l e s a l ts , s teroi d hormones , a nd vi ta mi n D? 9. Wha t a re the ba s i c functi ons of the four types of gl ycol i pi ds ? 10. How do l i poprotei ns tra ns port chol es terol a nd l i pi d mol ecul es throughout the body? 11. Wha t a re the ba s i c functi ons of bi l e s a l ts , s teroi d hormones , a nd vi ta mi n D tha t a re produced from chol es terol ?
CHAPTER 4 NUCLEOSIDES, NUCLEOTIDES, DNA, AND RNA Nucl eos i des a nd Nucl eoti des RNA a nd DNA—Ba s i c Structure a nd Functi on Revi ew Ques ti ons
OVERVIEW Nucl eos i des a nd nucl eoti des a re the fourth a nd fi na l ma jor group of bi ochemi ca l mol ecul es a nd a re es s enti a l for numerous bi ol ogi ca l functi ons i n huma ns , i ncl udi ng ma i nta i ni ng a nd tra ns ferri ng geneti c i nforma ti on, pl a yi ng a ma jor rol e i n energy s tora ge, a nd a cti ng a s s i gna l i ng mol ecul es . Thes e mol ecul es ca n be di vi ded i nto two ma jor fa mi l i es —puri nes , whi ch i ncl ude a denos i ne a nd gua ni ne, a nd pyri mi di nes , whi ch i ncl ude cyto-s i ne, thymi di ne, a nd ura ci l . The uni que s tructures a nd i ntera cti ons of thes e mol ecul es s erve a s the funda menta l bui l di ng bl ock of RNA a nd DNA mol ecul es a nd a l l ow funda menta l proces s es of gene repl i ca ti on a nd protei n s ynthes i s to occur. Ma ny other functi ons of the va ri ous nucl eos i des a nd nucl eoti des wi l l be expl ored i n l a ter cha pters .
NUCLEOSIDES AND NUCLEOTIDES Nucleosides a nd nucleotides a re cl os el y i nvol ved i n the pres erva ti on a nd tra ns mi s s i on of the geneti c i nforma ti on of a l l l i vi ng crea tures . In a ddi ti on, they pl a y rol es i n bi ol ogi ca l energy s tora ge a nd tra ns mi s s i on, s i gna l i ng, regul a ti on of va ri ous a s pects of meta bol i s m, a nd even a n i mporta nt rol e a s a n a nti oxi da nt. Mi s ta kes or defi ci enci es i n thei r s ynthes i s us ua l l y l ea d to dea th. Overproducti on or decrea s ed el i mi na ti on of nucl ei c a ci d deri va tes a l s o l ea d di rectl y to medi ca l condi ti ons . Nucl eos i des ha ve a ni trogenous ba s e a nd a fi ve-ca rbon ca rbohydra te group, us ua l l y a ri bos e mol ecul e (s ee Cha pter 2). Nucl eoti des a re s i mpl y a nucl eos i de wi th one or more phos pha te groups a tta ched (Fi gure 4-1). The res ul ti ng mol ecul e i s found i n ribonucleic acid or RNA. If one hydroxyl (OH) group ha s been removed from the ri bos e, the deoxy vers i ons of the nucl eos i de a nd nucl eoti de form the bui l di ng bl ocks of deoxyribonucleic acid or DNA (Fi gure 4-1). Ea ch component of nucl eos i des a nd nucl eoti des i s di s cus s ed bel ow.
Figure 4-1. Basic Structure of Nucleosides and Nucleotides. Fi ve ma jor nucl eos i de ba s es a re common i n huma n bi ol ogy, i ncl udi ng the puri nes (twori ng s tructure) a deni ne a nd gua ni ne (top) a nd the pyri mi di nes (one-ri ng s tructure) cytos i ne, ura ci l , a nd thymi ne (mi ddl e). Nucl eos i des (bottom) a re ma de of a ni trogenous ba s e, us ua l l y ei ther a puri ne or pyri mi di ne, a nd a fi ve-ca rbon ca rbohydra te, us ua l l y ri bos e. A nucl eoti de i s s i mpl y a nucl eos i de wi th a n a ddi ti ona l phos pha te group or groups (bl ue); pol ynucl eoti des conta i ni ng the ca rbohydra te ri bos e a re known a s ri bonucl eoti de or RNA. If 2′ hydroxyl group (OH) i s removed, the pol ynucl eoti de deoxy ri bonucl ei c a ci d (DNA) res ul ts . [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
COMPONENTS OF NUCLEOSIDES AND NUCLEOTIDES 1. Ni trogenous ba s e—The ni trogenous ba s e of a nucl eos i de or nucl eoti de (na med beca us e of the ni trogen a toms found i n i ts s tructure) ma y be ei ther a puri ne or a pyri mi di ne. Purines, i ncl udi ng inosine (I), adenine (A), a nd guanine (G), a re two-ri ng s tructures a nd pyrimidines, i ncl udi ng uracil (U), cytosine (C), a nd thymine (T), ha ve onl y one ri ng (Fi gure 4-1). Both puri ne a nd pyri mi di ne ni trogenous ba s es a re ma de, i n pa rt, from a mi no a ci ds a s s hown i n Fi gures 4-2 a nd 4-3, res pecti vel y. 2. Ca rbohydra te—The ca rbohydra te component of nucl eos i des a nd nucl eoti des i s us ua l l y the s uga r ribose. When the hydroxyl group i s removed from ca rbon 2 (norma l l y removed after the a ddi ti on of the ni trogenous ba s e to the ca rbohydra te), deoxyribose, the s uga r mol ecul e found i n DNA, res ul ts . 3. Phos pha te Group—One or more phos pha te groups (PO4 –3 ) ma y be a tta ched to the ca rbon 5 of the ca rbohydra te mol ecul e. The phos pha te mol ecul es a re i mporta nt i n energy s tora ge a nd s i gna l i ng functi ons of nucl eos i des a nd nucl eoti des .
Figure 4-2. A. Constituents of the Purine Ring. The s ources of ca rbon, ni trogen, a nd oxygen mol ecul es tha t form the ni trogenous ba s e of i nos i ne a re i ndi ca ted. Atoms from gl yci ne a re i ndi ca ted i n bl ue s ha de. B. Synthesis of Adenosine and Guanosine. The nucl eos i des a nd nucl eoti des a denos i ne a nd gua nos i ne a re produced from the mol ecul e IMP. Synthes i s of a denos i ne requi res one a ddi ti ona l ni trogen mol ecul e from the a mi no a ci d a s pa rta te (bl ue s ha de). Gua nos i ne s ynthes i s uti l i zes one a ddi ti ona l ni trogen mol ecul e from the a mi no a ci d gl uta mi ne (bl ue s ha de). [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] SYNTHESIS OF PURINE NUCLEOSIDES AND NUCLEOTIDES The a denos i ne- a nd gua ni ne-ba s ed nucl eos i des a nd nucl eoti des a re formed by fi rs t produci ng a nother nucl eoti de ca l l ed inosine monophosphate (IMP). IMP i s produced from a ri bos e, pa rts of one gl yci ne, one a s pa rta te, two gl uta mi ne a mi no a ci ds , two tetra hydrofol a te mol ecul es (a modi fi ed form of the vi ta mi n fol a te), a nd one ca rbon di oxi de (CO2 ) mol ecul e (Fi gure 4-2A). After IMP i s formed, adenosine monophosphate or guanosine monophosphate ma y be produced dependi ng on the body’s needs (Fi gure 4-2B). The s ynthes i s of puri nes s ta rts by tra ns ference of a phos pha te group from a n exi s ti ng a denos i ne tri phos pha te (ATP) mol ecul e to a new ri bos e mol ecul e. As a res ul t of thi s requi rement, the concentra ti ons of ATP mus t be hi gh for puri ne s ynthes i s to proceed—a method of regul a ti ng when to produce or not produce more of thes e mol ecul es . Thus , the body onl y produces more nucl eos i des a nd nucl eoti des when energy l evel s (from food) a re hi gh. Puri nes ca n be produced a s a bove, or ca n be obta i ned from the di et or by brea ki ng down a nd recycl i ng exi s ti ng nucl eos i des a nd nucl eoti des . Ma mma l s , i ncl udi ng huma ns , preferenti a l l y us e the recycl i ng pa thwa y when a ppropri a te s ta rti ng ma teri a l s a re a va i l a bl e. SYNTHESIS OF PYRIMIDINE NUCLEOSIDES AND NUCLEOTIDES Unl i ke puri nes , the pyri mi di ne ni trogenous ba s es , ura ci l a nd cytos i ne, a re formed before bondi ng to the ca rbohydra te porti on of the nucl eoti de. The proces s s ta rts by produci ng the ura ci l ri ng (Fi gure 4-3A) i n a three-s tep proces s uti l i zi ng one ea ch of the a mi no a ci ds gl uta mi ne a nd a s pa rta te a nd one bi ca rbona te mol ecul e (HCO3 –). Next, cytidine triphosphate (CTP) i s deri ved from the nucl eoti de uri di ne monophos pha te (UMP) a fter the convers i on of UMP to the tri phos pha te form (uri di ne tri phos pha te); a n extra ni trogen i s ga i ned from one a ddi ti ona l gl uta mi ne a mi no a ci d (Fi gure 4-3B, fa r ri ght). Cyti di ne monophos pha te (CMP) a nd di phos pha te (CDP) ca n be produced from (CTP) by the s ubs equent l os s of two or one phos pha te groups , res pecti vel y. The fi na l pyri mi di ne-deri ved nucl eoti de, thymi di ne, found onl y i n DNA, i s produced by a s epa ra te proces s i nvol vi ng the deoxy form of UMP (dUMP). Thi s proces s wi l l be di s cus s ed bel ow.
Figure 4-3. Synthesis of Pyrimidine-Derived Nucleosides Uridine and Cytidine. A. Constituents of the Pyrimidine Uracil Ring. The nucl eos i de uri di ne i s produced from one a s pa rta te a mi no a ci d, one gl uta mi ne a mi no a ci d, a nd a bi ca rbona te mol ecul e from CO2 res pi ra ti on. Atoms from a s pa rta te a re i ndi ca ted i n bl ue s ha de. B. Uridine and Cytidine Nucleoside Synthesis. Cyti di ne s ynthes i s uti l i zes one a ddi ti ona l ni trogen mol ecul e from the a mi no a ci d gl uta mi ne (i ndi ca ted by a rrow) a fter the a ddi ti on of two a ddi ti ona l phos pha te groups to the UMP mol ecul e. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.]
Figure 4-4. Synthesis of Thymidine Deoxyribonucleotide. UMP i s deoxygena ted a t the 2′ hydroxyl of the ri bos e ri ng (s ee Fi gure 4-1 a bove) a nd s ubs equentl y converted to dTMP (TMP) by the a ddi ti on of a methyl group (i ndi ca ted by a rrow) from methyl ene tetra hydrofol a te. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] FORMATION OF DEOXY NUCLEOSIDES AND NUCLEOTIDES Adenos i ne, gua nos i ne, a nd cyti di ne deoxyri bonucl eoti des found i n DNA a re formed di rectl y from thei r corres pondi ng, doubl e-phos phoryl a ted (nucl eos i de di phos pha tes ) ri bonucl eoti des by remova l of the hydroxyl (OH –) group from the ca rbon 2 (e.g., ADP → dADP, GDP → dGDP, CDP → dCDP). Thymidine deoxyriboncleotide (dTMP), found onl y i n DNA, i s formed from dUMP (UMP → dUMP → dTMP) by the a ddi ti on of a methyl (CH 3 ) group (Fi gure 4-4). Beca us e there i s no oxyri bo-nucl eoti de form of dTMP, the “d” des i gna ti on i s often not us ed (TMP). See s umma ry i n Ta bl e 4-1.
TABLE 4-1. Summa ry of Ni trogenous Ba s es , Nucl eos i des , a nd Nucl eoti des Nucleoside and Nucleotide Analogues as Chemotherapy Agents and Antibiotics: Nucl eos i des a nd/or nucl eoti des a re es s enti a l components of geneti c ma teri a l a nd a re, therefore, i nti ma tel y i nvol ved i n the prol i fera ti on of cel l s . One method of ca ncer trea tment i s to s top the reproducti on of ca ncerous cel l s by i nhi bi ti ng the producti on of nucl eos i des a nd/or nucl eoti des . Exa mpl es i ncl ude the pyri mi di ne a na l ogue of dUMP, fluorodeoxyuridine (FdUMP), whi ch bl ocks the producti on of dTDP. Beca us e dTDP ha s i ts own s peci a l s yntheti c proces s , no other nucl eoti des a nd nucl eos i des a re a ffected by FdUMP. Another a na l ogue, fluorocytosine, ca n be us ed a s a n a nti bi oti c beca us e i t i s converted i nto a n a cti ve “nucl eos i de or nucl eoti de” onl y i n ba cteri a —huma n cel l s a re not a ffected by i ts pres ence. Aminopterin a nd methotrexate a re a l s o chemothera py a gents , whi ch a ct by bl ocki ng fol a te a ddi ti on to puri ne ri ngs . Unl i ke fl uorocytos i ne, though, thes e a gents a l s o a ffect ra pi dl y di vi di ng huma n cel l s s uch a s ha i r a nd i ntes ti nes , res ul ti ng i n the common chemothera py s i de effects of ha i r l os s a nd na us ea / vomi ti ng. BREAKDOWN OF PURINES AND PYRIMIDINES Nucl eoti des ca n be broken down i nto thei r puri ne or pyri mi -di ne ni trogenous ba s es by remova l of the phos pha te groups to form the res pecti ve nucl eos i des a nd remova l a nd tra ns fer of the ri bos e ca rbohydra te ba ck i nto ca rbohydra te meta bol i s m. Once the free puri ne ba s es rema i n, they a re cha nged i nto a s l i ghtl y di fferent puri ne ca l l ed xanthine, whi ch i s s ubs equentl y converted i nto uri c a ci d a nd excreted ma i nl y i n uri ne by the ki dneys . Brea kdown of pyri mi di ne ni trogenous ba s es i s s i mpl y a revers e of the s teps of s ynthes i s wi th the res ul ti ng mol ecul es bei ng di rected i nto regul a r meta bol i s m. Gout and Lesch–Nyhan Syndrome: Gout i s a medi ca l condi ti on typi fi ed by exces s i ve a mounts of uri c a ci d i n the body. Thes e hi gh l evel s res ul t
i n the forma ti on of needl e-s ha ped uri c a ci d crys ta l s , whi ch then become l odged i n s oft ti s s ues , es peci a l l y joi nts s uch a s thos e i n the fi rs t toe, res ul ti ng i n s evere “gouty a rthri ti s ” pa i n. Gout i s trea ted by s evera l di fferent types of medi ca ti ons , i ncl udi ng a l l opuri nol , whi ch di rectl y decrea s es the convers i on of puri ne ni trogenous ba s es to xa nthi ne a nd uri c a ci d. Uloric (febuxostat) ha s been a pproved for us e i n the trea tment of chroni c hyperuri cemi a . Thi s drug a ppea rs to be more effecti ve tha n a l l opuri nol i n preventi ng a cute a tta cks a nd reduci ng the s i ze of the crys ta l depos i ts . Lesch–Nyhan syndrome i s a geneti c di s ea s e, a ffecti ng a l mos t s ol el y ma l es , of exces s i ve s ynthes i s of puri nes beca us e of defecti ve recycl i ng a nd, therefore, uri c a ci d producti on from thei r brea kdown. Les ch–Nyha n s yndrome i s cha ra cteri zed by gouty a rthri ti s but, i n a ddi ti on, a ffects the bra i n, res ul ti ng i n menta l reta rda ti on, l os s of control of a rm/l eg/fa ce movements , a ggres s i ve beha vi or, a nd s el f-muti l a ti on by bi ti ng a nd s cra tchi ng. Succes s ful trea tment of thi s di s order i s s ti l l bei ng s ought. Nucl eoti de mol ecul es a re a bl e to form RNA a nd DNA s tra nds by the bondi ng of the phos pha te group of one nucl eoti de a nd the ri bos e s uga r mol ecul e of the next nucl eoti de (Fi gure 4-5). Thi s l i nka ge i s ca l l ed a “phosphodiester bond” a nd hel ps to form the funda menta l s tructure es s enti a l for the s tora ge, ma i ntena nce, a nd tra ns mi s s i on of the geneti c code of l i vi ng crea tures . For purpos es of nomencl a ture, RNA a nd DNA mol ecul es ha ve a 5′ a nd 3′ end, dependi ng on the number of the ca rbon bonded to the rema i ni ng phos pha te group (Fi gure 4-5).
Figure 4-5. Structure of Phosphodiester Bond Found in RNA and DNA. Phos phodi es ter bond (bol d a rrows ) formed i n RNA a nd DNA between the hydroxyl on the thi rd ca rbon of ri bos e a nd the hydroxyl from the fi fth ca rbon. Succes s i ve bonds form the ba ckbone of thes e mol ecul es a nd l i nk s ucces s i ve nucl eoti des to form the orga ni s m’s geneti c code. Forma ti on of the bond res ul ts i n the producti on of one mol ecul e of wa ter (not s hown). Both RNA a nd DNA a re wri tten by conventi on 5′ a nd 3′ ends (i ndi ca ted). [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
RNA AND DNA—BASIC STRUCTURE AND FUNCTION RNA RNA mol ecul es a re s i ngl e s tra nds containing the nucl eoti des : a deni ne (A), gua ni ne (G), cytos i ne (C), a nd ura ci l (U). RNA mol ecul es often form s econda ry (2°) s tructures much l i ke protei ns a nd ma y i ntera ct wi th DNA, other RNA mol ecul es , a nd protei ns . Thes e i ntera cti ons hel p to defi ne the pa rti cul a r functi on of ea ch type of RNA. In genera l , there a re four di s ti nct types of RNA mol ecul es , ea ch wi th a pa rti cul a r functi on (Ta bl e 4-2). Messenger RNA (mRNA) mol ecul es ,
whi ch ca n be 100s –1000s of nucl eoti des l ong, functi on a s the tra ns mi tter of geneti c i nforma ti on from the DNA geneti c code to the res ul ti ng protei n. In thi s ta s k, mRNA i s often bound by protei ns tha t hel p to protect a nd regul a te thes e i mporta nt geneti c mes s a ges . Transfer RNA (tRNA) mol ecul es , norma l l y 65–110 nucl eoti des l ong, ca rry i ndi vi dua l a mi no a ci ds a nd ma tch them wi th a s peci fi c mRNA s equence duri ng protei n s ynthes i s . tRNA mol ecul es ha ve a very cha ra cteri s ti c “T” s ha pe tha t i s opti mi zed for thi s functi on. Ribosomal RNA (rRNA) i s a s s oci a ted wi th protei ns a nd ma kes up the a ctua l worki ng “ma chi nery” res pons i bl e for the s ynthes i s of protei n mol ecul es . The exa ct mecha ni s m of protei n s ynthes i s a nd the rol es of ea ch i ndi vi dua l type of RNA wi l l be di s cus s ed i n more deta i l i n Secti on II. A fourth type of RNA, often referred to a s regulatory RNA, i s i nvol ved i n regul a ti on of DNA expres s i on, pos ttra ns cri pti ona l mRNA proces s i ng, a nd the a cti vi ty of the tra ns cri bed mRNA mes s a ge (s ee Cha pter 9).
TABLE 4-2. Cha ra cteri s ti cs of mRNA, tRNA, a nd rRNA DNA DNA mol ecul es a re compos ed of two s i ngl e s tra nds of deoxynucl eoti des , adenine (A), guanine (G), thymidine (T), a nd cytosine (C), i n whi ch the two s tra nds a re pa i red to form a l a dder-type mol ecul e referred to a s a double helix (Fi gure 4-6A). Thi s doubl e hel i x forms when a toms i n the ni trogenous ba s es of the nucl eoti des form hydrogen bonds (G bondi ng wi th C a nd A bondi ng wi th T) (Fi gure 4-6B) whi l e the hydrophi l i c phos pha te a nd hydroxyl groups of the s uga r “ba ckbone” a re expos ed to the wa ter envi ronment.
Figure 4-6. Basic Structure of DNA. A. Doubl e hel i x model (l eft fi gure), i l l us tra ti ng the wi ndi ng “l a dder” s tructure wi th the i ns i de “rungs ” crea ted by nucl eoti de pa i ri ng a nd the outs i de “runners ” crea ted by cha rged phos pha te groups . B. Deta i l (top) of how the doubl e hel i x s tructure protects the i nner hydrophobi c a rea a nd a l l ows hydrogen bondi ng between pa i red G–C a nd A–T nucl eoti des whi l e expos i ng the outer hydrophi l i c deoxyri bos e a nd phos pha te groups to the wa ter envi ronment. Bondi ng (bottom) i n DNA onl y between T a nd A a nd G a nd C, whi ch hel ps to di cta te the geneti c code a nd fi del i ty i n i ts repl i ca ti on a nd expres s i on. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Deta i l (bottom) of hydrogen bondi ng (a rrows ) between puri ne a nd pyri mdi ne pa i rs . C. Mecha ni s m of DNA Repl i ca ti on. Unwi ndi ng of DNA to a l l ow a cces s to new nucl eoti des duri ng DNA repl i ca ti on. [Reproduced wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] The double-helix s tructure of DNA i s es s enti a l for i ts functi on beca us e thes e two bonded s tra nds ca n tempora ri l y s epa ra te a t s peci fi c pa rts of the DNA mol ecul e to a l l ow for DNA repl i ca ti on (s hown i n Fi gure 4-6C). mRNAs a nd a s s oci a ted protei ns ca n a l s o a cces s the DNA s tra nds to copy the conta i ned geneti c s equence a s the fi rs t s tep, l ea di ng to protei n s ynthes i s (s ee Cha pter 9). Thi s proces s i s di s cus s ed i n more deta i l i n Secti on II. The uni que doubl e hel i x ca n then rebond to a ga i n protect the vi ta l DNA mes s a ge. A s epa ra te type of DNA i s found i n mi tochondri a . Thi s mitochondrial DNA (mtDNA) codes for onl y a few protei ns (13 tota l protei ns i n huma n mi tochondri a ) i nvol ved i n the producti on of energy wi thi n the mi tochondri a . Unl i ke DNA found i n the nucl eus , mtDNA forms a s ma l l ci rcul a r s tructure wi th the s a me bondi ng a nd s tra nd s epa ra ti on s een i n l i nea r doubl e-hel i x DNA mol ecul es . The s peci fi c s equence of A, G, C, a nd T nucl eoti des or “genome” defi nes a l l the functi ona l mol ecul es of a l i vi ng crea ture a nd, a s s uch, DNA i s the bl uepri nt of l i fe. The genome of huma ns i s es ti ma ted to conta i n a pproxi ma tel y 20,000–25,000 di fferent genes . Conta i ned wi thi n the chromos omes (Fi gure 4-7) a re the nucl eoti de s tra nds , whi ch conta i n the mes s a ge or “code” for every s i ngl e protei n. The s equences of A’s , G’s , C’s , a nd T’s or “genes” tha t code for mRNA mol ecul es a nd, s ubs equentl y, thes e protei ns a re referred to a s “expres s ed s equences ” or “exons.” Sequences tha t do not code for a protei n a re ca l l ed “interveni ng s equences ” or “introns.” In fa ct, i ntrons compri s e over 90% of the tota l DNA s equence found i n huma ns a nd a re bel i eved to be l eftover, nonfuncti ona l remna nts of evol uti ona ry cha nges or, perha ps , i mporta nt regul a tory s equences whos e functi ons a re yet to be determi ned. Introns a re removed or “s pl i ced” from the s equence duri ng the ea rl y s ta ges of protei n s ynthes i s (s ee Secti on II).
Figure 4-7. Relationship between Chromosomes (right) and a Gene. Ea ch huma n chromos ome (fa r l eft, whos e DNA content i s gra phi ca l l y i l l us tra ted a s fa r l eft ba r) conta i ns a pproxi ma tel y 1000–2000 genes a rra nged i n cl us ters of a pproxi ma tel y 20 genes . Wi thi n the gene a re “expres s ed s equences ” (exons , da rk ora nge) a nd “i nterveni ng s equences ” (i ntrons , l i ght ora nge). The i nterveni ng s equences a re removed duri ng expres s i on of the pri ma ry mRNA tra ns cri pt a nd proces s i ng to the fi na l mRNA product. Introns do not a ppea r to code for a ny protei n a nd thei r rol e, i f a ny, i s s ti l l unknown. Length of mol ecul es i s i ndi ca ted a s bp (ba s e pa i rs ) or nt (nucl eoti des ). [(Left pa rt) Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010. (Ri ght pa rt) Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Ribozymes: Ribozymes (ribonucleic acid enzymes) repres ent a uni que depa rture from the ori gi na l thought tha t enzymes ca n onl y be protei ns . Li ke thei r protei n counterpa rts , pa rti cul a r RNA s equences pos s es s s econda ry or terti a ry s tructure tha t ena bl es them to ca ta l yze a rea cti on. Mos t ri bozymes a ct on ei ther thems el ves or a nother RNA mol ecul e. However, s ome ri bozymes , i ncl udi ng thos e i n ri bos omes (s ee Cha pter 9), ca ta l yze the tra ns fer of a mi no groups to a growi ng protei n s equence a nd a s s i s t new protei ns to fol d i nto thei r a ppropri a te conforma ti on. The devel opment of potenti a l s ci enti fi c a nd medi ca l a ppl i ca ti ons of ri bozymes i s ongoi ng, i ncl udi ng pos s i bl e trea tments a ga i ns t i ni ti a l HIV i nfecti on. Theori es s ugges ti ng tha t RNA, not DNA, mol ecul es were the ori gi na l geneti c code mol ecul es a l s o i nfer tha t ri bozymes ma y ha ve been s ome of the i ni ti a l enzyma ti c mol ecul es tha t a l l owed propa ga ti on of ea rl y l i fe. Knowi ng the exa ct s equence of a n orga ni s m’s DNA genome woul d a l l ow one to dupl i ca te or “cl one” tha t orga ni s m exa ctl y. “Cloning” i s s i mpl y the a bi l i ty to copy the DNA s equence a nd repl i ca te i t to form the pa rti cul a r orga ni s m. Is ol a ti ng s peci fi c s equences tha t a re the code for a pa rti cul a r protei n a l l ows res ea rchers to s tudy a nd ma ni pul a te thes e protei ns (s ee Appendi x II), a nd i s vi ta l for s ci enti fi c a nd medi ca l purpos es . Adenosine Deaminase Deficiency and Gene Therapy: Defi ci enci es i n the brea kdown of nucl eoti des a nd nucl eos i des l ea d to often fa ta l di s ea s es ea rl y i n l i fe i n whi ch the i mmune res pons e i s ma rkedl y decrea s ed. Severe combined immunodeficiency syndromes encompa s s s evera l exa mpl es of thes e di s ea s es but a l mos t ha l f of pa ti ents ha ve a defi ci ency i n the brea kdown of a denos i ne by adenosine deaminase. Pa ti ents ha ve been trea ted by bone ma rrow tra ns pl a nt but a l s o by the emergi ng technol ogy of gene thera py wi th s ome s ucces s . Gene thera py offers the potenti a l for produci ng a “good” copy of a gene a nd us es i t to repl a ce the defecti ve copy. The i mpl i ca ti ons of thi s trea tment a re va s t.
REVIEW QUESTIONS 1. Wha t a re nucl eos i des , nucl eoti des , deoxynucl eoti des , puri nes , a nd pyri mi di nes , thei r key fea tures a nd ba s i c s tructure? 2. How do you di s ti ngui s h between a deni ne, gua ni ne, cytos i ne, thymi di ne, a nd ura ci l s tructure? 3. Wha t a re ri bonucl ei c a ci d (RNA) a nd deoxyri bonucl ei c a ci d (DNA), thei r ba s i c s tructure, a nd thei r rol es i n huma n bi ol ogy? 4. Wha t a re genes , exons , a nd i ntrons a nd thei r key fea tures ? 5. Wha t i s the rel a ti ons hi p between a mi no a ci ds a nd the ni trogenous ba s es of puri nes a nd pyri mi di nes ? 6. Wha t i s the rol e of fol a te i n the s ynthes i s of ni trogenous ba s es of puri nes ? 7. Wha t i s the overa l l pa thwa y of s ynthes i s of the fi ve ma jor nucl eos i des a nd nucl eoti des i ncl udi ng the deoxy forms ?
SECTION I INTEGRATED USMLE-STYLE QUESTIONS AND ANSWERS QUESTIONS I-1. Severa l fa mi l i es were s tudi ed whos e a ffected i ndi vi dua l s ha ve nephrogeni c di a betes i ns i pi dus . Thi s di s ea s e ca us es chi l dhood s ymptoms of pol yuri a (frequent uri na ti on), pol ydi ps i a (cons ta nt thi rs t a nd frequent dri nki ng), poor growth, a nd hyperna tremi a (i ncrea s ed s erum s odi um concentra ti on). Admi ni s tra ti on of a nti di ureti c hormone wa s not cura ti ve, focus i ng a ttenti on on a rena l wa ter l os s due to a tra ns port defect. A gene na med a qua pori n-2 wa s cl oned from rena l tubul a r epi thel i um, i ts a mi no a ci d s equence deri ved, a nd s tructura l doma i ns hypothes i zed to fa ci l i ta te s epa ra ti on of muta ti ons from beni gn va ri a nts . The hypothes i zed s tructure conta i ned s evera l tra ns membra ne doma i ns dema rca ted by β-turns , a nd thes e potenti a l wa ter cha nnel s were found to be muta ted i n a ffected i ndi vi dua l s . Whi ch of the fol l owi ng a mi no a ci ds i s mos t s ugges ti ve of β-turns ? A. Argi ni ne a nd l ys i ne B. As pa rti c a ci d a nd gl uta mi c a ci d C. Gl yci ne a nd prol i ne D. Leuci ne a nd va l i ne E. Tryptopha n a nd tyros i ne I-2. A 7-yea r-ol d fema l e pres ents wi th dehydra ti on a fter 3 da ys of expl os i ve di a rrhea a fter cons umi ng a l a rge a mount of i ce crea m. Ana l ys i s of her feca l ma tter revea l s a l a rge a mount of l a ctos e. Whi ch of the fol l owi ng s uga r pa i rs woul d be found i n thi s di s a ccha ri de? A. Fructos e a nd gl ucos e joi ned by a α-1, 2 bond B. Ga l a ctos e a nd fructos e joi ned by a β-1, 2 bond C. Ga l a ctos e a nd gl ucos e joi ned by a β-1, 4 bond D. Two mol ecul es of gl ucos e joi ned by a α-1, 1 bond E. Two mol ecul es of gl ucos e joi ned by a α-1, 4 bond I-3. A defi ci ency of whi ch of the fol l owi ng vi ta mi ns woul d a l ter meta bol i s m of ca l ci um? A. A B. C C. D D. E E. K I-4. Whi ch of the fol l owi ng cha ra cteri s ti cs di s ti ngui s h mos t RNA mol ecul es from DNA? A. 3′-phos pha te group l i nked to a pentos e s uga r B. 5′-phos pha te group l i nked to a pentos e s uga r C. Adeni ne ba s e on RNA, wherea s a ura ci l ba s e on DNA D. Puri ne or pyri mi di ne ba s e l i nked to a pentos e s uga r E. Uri di ne ba s e on RNA, wherea s a thymi di ne ba s e on DNA I-5. A s evere form of os teogenes i s i mperfecta (OI) wa s noted, i n whi ch newborn i nfa nts ha ve extremel y deformed l i mbs a nd ches t, ca us i ng them to di e s hortl y a fter bi rth beca us e thei r ches t wa l l wa s not a dequa te for res pi ra ti on. Thi s “type II” s evere form wa s i ni ti a l l y thought to be a utos oma l reces s i ve, but wa s l a ter s hown to i nvol ve muta ti ons i n type I col l a gen l i ke other forms of OI. Col l a gen i s a fi brous protei n cons i s ti ng of three pepti de cha i ns entwi ned i n a tri pl e hel i x, formed by a repea ti ng a mi no a ci d moti f (where X or Y ca n be a ny a mi no a ci d). Whi ch of the fol l owi ng s hows tha t the repea ti ng 3-a mi no a ci d moti f i s mos t compa ti bl e wi th col l a gen tri pl e hel i x forma ti on a nd the muta ti on mos t l i kel y to ca us e s evere OI? A. Al a –X–Y muta ted to Gl y–X–Y i n one repea t B. Al a –X–Y muta ted to Leu–X–Y i n one repea t C. Gl y–X–Y muta ted to Al a –X–Y i n one repea t D. Gl y–X–Y muta ted to Pro–X–Y i n one repea t E. Pro-X-Y muta ted to Gl y-X-Y i n one repea t I-6. A 60-yea r-ol d ma n i s brought to hi s phys i ci a n from a n i ns ti tuti on for s evere menta l defi ci ency. The phys i ci a n revi ews hi s fa mi l y hi s tory a nd fi nds he ha s a n ol der s i s ter i n the s a me i ns ti tuti on. Thei r pa rents a re decea s ed but reportedl y ha d norma l i ntel l i gence a nd no chroni c di s ea s es . The ma n s i ts i n a n odd pos i ti on a s though he wa s s ewi ng, prompti ng the phys i ci a n to obta i n a ferri c chl ori de tes t on the ma n’s uri ne. Thi s tes t turns col or wi th a roma ti c (ri ng) compounds , i ncl udi ng certa i n a mi no a ci ds , a nd a green col or confi rms the phys i ci a n’s di a gnos i s . Whi ch of the fol l owi ng a mi no a ci ds wa s mos t l i kel y detected i n the ma n’s uri ne? A. Gl uta mi ne B. Gl yci ne C. Methi oni ne D. Phenyl a l a ni ne E. Seri ne I-7. Chi l dren wi th s evere vi ta mi n A defi ci ency devel op a ccumul a ti on of a gl ycos a mi nogl yca n on the cornea l epi thel i um a nd i s a ccompa ni ed by “dry eye.” Whi ch of the fol l owi ng types of gl ycos a mi nogl yca n woul d you expect to fi nd a ccumul a ted on the s urfa ce of the cornea ? A. Chondroi ti n B. Hepa ra n C. Hepa ri n D. Kera ti n E. Perl eca n I-8. Whi ch of the fol l owi ng i s a n es s enti a l fa tty a ci d? A. Ara chi doni c a ci d (C-20:4-Δ5,8,11,14 ) B. Ei cos a tetra enoi c a ci d (C-20:3-Δ8,11,14 ) C. Li nol ei c a ci d (C-18:2-Δ9,12 ) D. Ol ei c a ci d (C-18:1-Δ9 ) E. Pa l mi ti c a ci d (C-16:0)
I-9. A 47-yea r-ol d ma n compl a i ns of pa i n i n the joi nts of hi s l eft bi g toe, whi ch a re obvi ous l y s wol l en a nd tender. The pa i n ha s been chroni c but beca me i ntol era bl e the da y a fter Tha nks gi vi ng when he ha d a l a rge mea l a nd s evera l gl a s s es of red wi ne. He i s obes e, a nd hi s pa s t medi ca l hi s tory i s s i gni fi ca nt for remova l of ki dney s tones . Whi ch of the fol l owi ng i s i nvol ved i n the pa thophys i ol ogy of thi s pa ti ent’s condi ti on? A. Defi ci ency of fol i c a ci d B. El eva ted oroti c a ci d C. El eva ted uri c a ci d D. Los s of red bl ood cel l s E. Low bl ood gl ucos e I-10. A chi l d pres ents wi th s evere vomi ti ng, dehydra ti on, a nd fever. Ini ti a l bl ood s tudi es s how a ci dos i s wi th l ow bi ca rbona te. Prel i mi na ry res ul ts from the bl ood a mi no a ci d s creen s how two el eva ted a mi no a ci ds , both wi th nonpol a r s i de cha i ns . A ti tra ti on curve performed on one of the el eva ted s peci es s hows onl y two i oni za bl e groups : one tha t i s a ci di c a nd the other tha t i s ba s i c (i .e., no cha rged s i de cha i n). Whi ch of the fol l owi ng pa i rs of el eva ted a mi no a ci ds i s mos t l i kel y el eva ted? A. Argi ni ne a nd i s ol euci ne B. As pa rti c a ci d a nd gl uta mi ne C. Gl uta mi c a ci d a nd threoni ne D. Hi s ti di ne a nd va l i ne E. Leuci ne a nd i s ol euci ne I-11. Certa i n a mi no a ci ds a re not pa rt of the pri ma ry s tructure of protei ns but a re modi fi ed a fter tra ns l a ti on. In s curvy, whi ch of the fol l owi ng a mi no a ci ds tha t i s norma l l y pa rt of col l a gen ca nnot be hydroxyl a ted a fter tra ns l a ti on? A. Al a ni ne B. Hi s ti di ne C. Prol i ne D. Tryptopha n E. Tyros i ne I-12. A woma n returns from a yea r l ong tri p a broa d wi th her 2-week-ol d i nfa nt, whom s he i s brea s tfeedi ng. The chi l d s oon s ta rts to exhi bi t l etha rgy, di a rrhea , vomi ti ng, ja undi ce, a nd a n enl a rged l i ver. The pedi a tri ci a n pres cri bed a s wi tch from brea s t mi l k to i nfa nt formul a conta i ni ng s ucros e a s the s ol e ca rbohydra te. The ba by’s s ymptoms res ol ve wi thi n a few da ys . Whi ch of the fol l owi ng wa s the mos t l i kel y di a gnos i s ? A. Defi ci ency of a n enzyme i n the meta bol i s m of pentos e s uga rs B. Defi ci ency of a n enzyme i n the pa thwa y tha t meta bol i zes gl ucos e C. Ga l a ctos emi a D. Intol era nce to di eta ry fructos e E. Intol era nce to l a cta s e I-13. Des pi te the fa ct tha t trans fa tty a ci ds a re uns a tura ted, thei r contri buti ons to a theros cl eros i s a re s i mi l a r to thos e of s a tura ted fa ts . Thi s s i mi l a ri ty i n phys i ol ogi ca l a cti on ca n be a ttri buted to whi ch of the fol l owi ng? A. Rel a ti vel y l i nea r s tructures B. Si mi l a r ra tes of meta bol i s m C. Si mi l a r ti s s ue di s tri buti ons D. Sol ubi l i ti es i n wa ter E. Tendency to form tri gl yceri des I-14. Methotrexa te i s a potent a nti ca ncer a gent tha t s ta rves di vi di ng cel l s of deoxyri bonucl eoti des through di rect i nhi bi ti on of whi ch of the fol l owi ng proces s es ? A. Degra da ti on of puri ne ba s es to uri c a ci d B. Deoxygena ti on of the ri bos e ri ng to produce deoxyri -bos e for DNA C. Meta bol i s m of fol i c a ci d D. Synthes i s of puri ne ba s es E. Synthes i s of thymi di ne I-15. An i nfa nt i s norma l a t bi rth but becomes l etha rgi c a fter s evera l feedi ngs ; the medi ca l s tudent des cri bes a n unus ua l s mel l to the uri ne but i s i gnored. Infecti on (s eps i s ) i s s us pected, a nd bl ood tes ts s how norma l whi te bl ood cel l counts wi th a s erum pH of 7.0. Eva l ua ti on for a n i nborn error of meta bol i s m s hows a n a bnorma l a mi no a ci d s creen. The report s ta tes tha t bra nch-cha i n a mi no a ci ds a re s tri ki ngl y el eva ted. Whi ch of the fol l owi ng a mi no a ci ds does the report refer to? A. Argi ni ne B. As pa rti c a ci d C. Is ol euci ne D. Lys i ne E. Threoni ne I-16. A 10-month-ol d whi te boy i s bei ng eva l ua ted for wea knes s , pa l l or, hemorrha ges under the fi ngerna i l s , a nd bl eedi ng gums . Ra di ogra phs i ndi ca te tha t bone nea r the growth pl a tes s hows reduced os teoi d forma ti on a nd gros s l y defecti ve col l a gen s tructure. Wha t woul d be the mos t effecti ve trea tment for thi s pa ti ent’s condi ti on? A. Excl us i on of da i ry products from the di et B. Growth hormone trea tment C. Ora l i ron s uppl ementa ti on D. Ora l vi ta mi n A E. Ora l vi ta mi n C I-17. A 9-month-ol d gi rl i s s ufferi ng from vomi ti ng, l etha rgy, a nd poor feedi ng beha vi or. Her mother reports tha t the s ymptoms bega n s hortl y a fter the ba by wa s gi ven a porti on of a pops i cl e a nd ma s hed ba na na s by her gra ndpa rents . The ba by’s di s comfort s eemed to res ol ve a fter brea s tfeedi ng wa s res umed. Whi ch of the fol l owi ng i s the mos t l i kel y di a gnos i s ? A. Defi ci ency of a n enzyme i n the meta bol i s m of pentos e s uga rs B. Defi ci ency of a n enzyme i n the pa thwa y tha t meta bol i zes gl ucos e C. Ga l a ctos emi a
D. Intol era nce to di eta ry fructos e I-18. Duri ng s ta rva ti on gl uca gon, a pepti de hormone, i s i mporta nt for a cti va ti ng gl ucos e s ynthes i s a nd mobi l i zi ng fa ts . Bes i des gl uca gon, whi ch of the fol l owi ng i s a l i pi d-deri ved hormone tha t regul a tes s uga r a nd fa t meta bol i s m i n s ta rva ti on a nd other s tres s s ta tes ? A. Gl ucocorti coi ds B. Mi nera l ocorti coi ds C. Pros ta gens D. Vi ta mi n A E. Vi ta mi n D I-19. A 2-yea r-ol d boy’s mother i s concerned a bout hi s tendency to bi te hi ms el f to the poi nt of bl eedi ng. The boy’s fi ngers s how s ca rri ng a nd s evera l s ca bs , a nd hi s l i ps a re s wol l en a nd brui s ed. He exhi bi ts poor coordi na ti on, poor mus cl e tone, a nd frequent jerki ng movements of hi s a rms a nd l egs . He i s s i gni fi ca ntl y del a yed i n s peech. Hi s uri ne i s ora nge i n col or a nd “gri tty.” Whi ch of the fol l owi ng i s the mos t l i kel y di a gnos i s ? A. Adenos i ne dea mi na s e defi ci ency B. Cerebra l pa l s y C. Gout D. Les ch–Nyha n s yndrome E. Ta y–Sa chs di s ea s e
ANSWERS I-1. The answer is C. A β-turn s tructure cons i s ts of four a mi no a ci ds i n whi ch the fi rs t res i due i s hydrogen bonded to the fourth res i due of the turn (s ee Fi gure 1-5C). Gl yci ne res i dues a re s ma l l a nd fl exi bl e, wherea s prol i ne res i dues a s s ume a cis or fl a ttened conforma ti on, ma ki ng thes e res i dues a mena bl e to ti ght turns . Tra ns port protei ns often ha ve s evera l membra ne-s pa nni ng doma i ns dema rca ted by β-turns tha t a l l ow them to exi t a nd return ba ck i nto the membra ne. Thes e tra ns membra ne doma i ns form cha nnel s tha t regul a te tra ns port of i ons a nd wa ter i n orga ns s uch a s l ung, gut, a nd ki dney. Nephrogeni c di a betes i ns i pi dus res ul ts when the ki dney i s l es s res pons i ve to a nti di ureti c hormone excreted by the pos teri or pi tui ta ry, ca us i ng a bnorma l wa ter excreti on, dehydra ti on, a nd el ectrol yte di s turba nces . I-2. The answer is C. Di s a ccha ri des a re ca rbohydra tes compos ed of two s uga r mol ecul es . La ctos e i n thi s ca s e i s compos ed of ga l a ctos e a nd gl ucos e joi ned by a β-1,4 gl ycos i di c bond. Sucros e i s a di s a ccha ri de of a fructos e a nd a gl ucos e mol ecul e joi ned by a α-1,2 gl ycos i di c bond. Ma l tos e i s a di s a ccha ri de of two gl ucos e mol ecul es joi ned by a α-1,4 gl ycos i di c bond. There i s no di s a ccha ri de compos ed of fructos e a nd ga l a ctos e (s ee Fi gure 2-3). I-3. The answer is C. Vi ta mi n D i s a fa t-s ol ubl e vi ta mi n es s enti a l for the a bs orpti on of di eta ry ca l ci um a nd the rea bs orpti on of ca l ci um by the ki dney. Vi ta mi n A i s a l s o a fa t-s ol ubl e vi ta mi n es s enti a l for vi s i on, reproducti on, a nd devel opment but does not s peci fi ca l l y a ffect a ny mi nera l s . Vi ta mi n C i s a wa ter-s ol ubl e vi ta mi n i mporta nt i n two enzymes i n col l a gen forma ti on a nd a s a n a nti oxi da nt. Vi ta mi n E i s a fa ts ol ubl e vi ta mi n i mporta nt a s a n a nti -oxi da nt. Vi ta mi n K i s fa t-s ol ubl e a nd i mporta nt i n bl ood cl otti ng. I-4. The answer is E. RNA a nd DNA a re compos ed of nucl eos i de uni ts where a puri ne or pyri mi di ne ba s e i s l i nked to a pentos e s uga r. The 1′ ca rbon of the pentos e i s l i nked to the ni trogen of the ba s e. In DNA, 2′-deoxyri bos e s uga rs a re us ed; i n RNA, ri bos e s uga rs a re us ed tha t conta i n 2′-a nd 3′-hydroxyl s . The ni trogenous ba s es a re a deni ne, thymi ne, gua ni ne, a nd cytos i ne i n DNA, wi th thymi ne repl a ced by uri di ne i n RNA. Nucl eoti de pol ymers a re cha i ns of nucl eoti des wi th s i ngl e phos pha te groups , joi ned by bonds between the 3′-hydroxyl of the precedi ng pentos e a nd the 5′-phos pha te of the next pentos e. Pol ymeri za ti on requi res hi gh-energy nucl eoti de tri phos pha te precurs ors tha t l i bera te pyrophos pha te (broken down to phos pha te) duri ng joi ni ng. The pol ymeri za ti on rea cti on i s gi ven s peci fi ci ty by compl ementa ry RNA or DNA templ a tes a nd ra pi di ty by enzyme ca ta l ys ts ca l l ed pol ymera s es . I-5. The answer is D. Col l a gen i s the mos t a bunda nt fi brous protei n found i n connecti ve ti s s ue, bone, ca rti l a ge, s ki n, l i ga ments , a nd tendons a s wel l a s other ti s s ues . Col l a gen cons i s ts of two s tra nds of α1 a nd one s tra nd of α2 col l a gen i ntertwi ned i n a tri pl e hel i x. The tri pl e hel i x ha s 3.3 res i dues per turn. The tri pl e hel i x i s s ta bi l i zed by hydrogen bonds between res i dues i n di fferent s tra nds . The pres ence of gl yci ne i n every thi rd res i due a l l ows for very ti ght pa cki ng of ea ch s tra nd i n the tri pl e hel i x beca us e gl yci ne res i dues a re s ma l l a nd confer fl exi bi l i ty. Muta ti ons s ubs ti tuti ng a l a rger or di fferentl y confi gured a mi no a ci d for the gl yci ne of one repea t woul d di s rupt the ti ght pa cki ng a t tha t pos i ti on of the col l a gen s tra nd a nd a l ter tri pl e hel i x conforma ti on; prol i ne s ubs ti tuti ons for gl yci ne (a ns wer D) woul d a l ter cha i n di recti on a nd be more di s rupti ve tha n thos e wi th a l a ni ne (a ns wer C). Subs ti tuti ons of a mi no a ci ds a t the X–Y pos i ti ons of a repea t woul d ha ve l es s i nfl uence on pa cki ng, better pres ervi ng the tri pl e hel i x s tructure. Muta ti ons a t pos i ti ons i n the α1 cha i n woul d a l s o be more s evere beca us e two of thes e s tra nds a re i ncorpora ted wi th ea ch tri pl e hel i x. Mi l der muta ti ons a l ter bone col l a gen to decrea s e bone thi cknes s a nd ca us e s us cepti bi l i ty to fra ctures . More s evere muta ti ons i nterfere wi th bone forma ti on duri ng devel opment, ca us i ng s evere deformi ti es of l i mbs a nd ches t wa l l tha t a re i ncompa ti bl e wi th l i fe. Thes e muta ti ons a l s o i nhi bi t col l a gen s ynthes i s i n the s cl era e (whi tes of the eyes ), ca us i ng them to be thi nner s o the underl yi ng bl ue choroi d s hows through. I-6. The answer is D. Before phenyl ketonuri a wa s recogni zed a nd before the a dvent of routi ne newborn meta bol i c s creeni ng, chi l dren wi th thi s a utos oma l reces s i ve di s order devel oped s evere menta l reta rda ti on wi thout other i denti fyi ng s ymptoms . Al though few chi l dren wi th devel opmenta l di s a bi l i ty a re bei ng pl a ced i n i ns ti tuti ons toda y, the movement for rel ea s e i nto home or ha l fwa y hous e ca re i n the 1960s a nd 1970s wa s compl i ca ted by l ong-term res i dents who ha d not devel oped l i fe s ki l l s . Thus , s ome ol der i ndi vi dua l s i n i ns ti tuti ons ha ve di s orders tha t coul d ha ve been detected by modern s creeni ng a nd woul d never ha ve prompted i ns ti tuti ona l i za ti on wi th modern ca re. Phenyl a l a ni ne, wi th i ts benzene ri ng, i s a n es s enti a l a mi no a ci d tha t i s converted to tyros i ne by phenyl a l a ni ne hydroxyl a s e, the enzyme tha t i s defi ci ent i n phenyl ketonuri a . Thi s di s ea s e ca n a l s o be ca us ed by a defi ci ency of the enzyme’s cofa ctor, bi opteri n. I-7. The answer is D. Kera ti n i s a component of ca rti l a ge a nd cornea of the eye. In vi ta mi n A defi ci ency, kera ti ni za ti on of the s ki n a nd of the mucous membra nes i n the res pi ra tory, GI, a nd uri na ry tra cts ca n a l s o occur. Dryi ng, s ca l i ng, a nd fol l i cul a r thi ckeni ng of the s ki n a nd res pi ra tory i nfecti ons ca n res ul t from thi s defi ci ency. Chondroi ti n i s a s tructura l component a nd a tta chment poi nt to col l a gen. Hepa ra n i s found a tta ched to col l a gen for va ri ous cel l s , i ncl udi ng l i ver cel l s . Hepa ri n regul a tes the i mmune res pons e a nd bl ood cl ot forma ti on. Perl eca n i s a s tructura l component of the ki dney membra ne. I-8. The answer is C. The es s enti a l fa tty a ci d l i nol ei c a ci d (C-18:2-Δ9,12 ), wi th 18 ca rbons a nd two doubl e bonds a t ca rbons 9 a nd 18, i s des a tura ted to form β-l i nol eni c a ci d (C-18:3-Δ6,9,12 ), whi ch i s s equenti a l l y el onga ted a nd des a tura ted to form ei cos a tri enoi c a ci d (C-20:3Δ8,11,14 ) a nd a ra chi doni c a ci d (C-20:4-Δ5,8,11,14 ). Ma ny of the ei cos a noi ds (20-ca rbon compounds )—pros ta gl a ndi ns , thromboxa nes , a nd l eukotri enes —a re deri ved from a ra chi doni c a ci d. Ara chi doni c a ci d ca n onl y be s ynthes i zed from es s enti a l fa tty a ci ds obta i ned from the di et. Pa l mi ti c a ci d (C-16:0) a nd ol ei c a ci d (C-18:1-Δ9 ) ca n be s ynthes i zed by the ti s s ues (Ta bl e 3-1). I-9. The answer is C. Thi s pa ti ent s hows ma ny s ymptoms cons i s tent wi th a n epi s ode of gout. Hi s joi nt pa i n i s due to gouty a rthri ti s , a n i nfl a mma tory condi ti on a ri s i ng from depos i ti on of s odi um ura te crys ta l s . The s wel l i ng i n the joi nts of hi s bi g toe (topha ceous gout) i s a l s o a ma ni fes ta ti on of thi s phenomenon. In hi s ca s e, the epi s ode s eems to ha ve been tri ggered by exces s i ve ea ti ng a t Tha nks gi vi ng di nner a l ong wi th a l cohol cons umpti on, l ea di ng to degra da ti on of l a rge qua nti ti es of puri ne nucl eoti des a nd cons equent i ncrea s ed fl ux
through the pa thwa y tha t produces uri c a ci d. Whether hi s gout a ri s es from i mpa i red excreti on of uri c a ci d or i s due to a muta ti on of phos phori bos yl pyrophos pha te s yntheta s e ca nnot be determi ned from the da ta . Ana l ys i s of hi s bl ood ma y confi rm the gout i f hi gh concentra ti ons of uri c a ci d (hyperuri cemi a ) a re pres ent. I-10. The answer is E. Leuci ne a nd i s ol euci ne ha ve nonpol a r methyl groups a s s i de cha i ns . As for a ny a mi no a ci d, ti tra ti on curves obta i ned by noti ng the cha nge i n pH woul d s how a n a ci di c i oni za bl e group for the pri ma ry ca rboxyl group a nd a ba s i c i oni za bl e group 5 for the pri ma ry a mi no group; there woul d be no a ddi ti ona l i oni za bl e s i de cha i n. As pa rti c a nd gl uta mi c a ci ds (s econd ca rboxyl group), hi s ti di ne (ba s i c i mi no group), gl uta mi ne (s econd a mi no group), a nd threoni ne (hydroxyl group) a l l ha ve i oni za bl e s i de cha i ns tha t woul d s how a n a ddi ti ona l group on the ti tra ti on curve. The l i kel y di a gnos i s here i s ma pl e s yrup uri ne di s ea s e, whi ch i nvol ves el eva ted i s ol euci ne, l euci ne, a nd va l i ne together wi th thei r ketoa ci d deri va ti ves . The ketoa ci d deri va ti ves ca us e the a ci dos i s , a nd the fever s ugges ts tha t the meta bol i c i mba l a nce wa s wors ened by a n i nfecti on. I-11. The answer is C. Prol i ne a nd l ys i ne a re hydroxyl a ted a fter the s ynthes i s of new col l a gen mol ecul es . The hydroxyl a ti on of prol i ne a nd l ys i ne res i dues occurs i n rea cti ons ca ta l yzed by prol yl a nd l ys yl hydroxyl a s e enzymes tha t requi re the reduci ng a gent a s corbi c a ci d (vi ta mi n C). In s curvy, whi ch res ul ts from a defi ci ency of vi ta mi n C, i ns uffi ci ent hydroxyl a ti on of col l a gen ca us es a bnorma l col l a gen fi bri l s . The wea kened col l a gen i n teeth, bone, a nd bl ood ves s el s ca us es tooth l os s , bri ttl e bones wi th fra ctures , a nd bl eedi ng tendenci es wi th brui s i ng a nd bl eedi ng gums . I-12. The answer is C. The pa ti ent’s s ymptoms a nd cours e i n res pons e to a l a ctos e-conta i ni ng formul a a re cons i s tent wi th a di a gnos i s of ga l a ctos emi a . Beca us e the chi l d ca n cons ume s ucros e, tha t conta i ns gl ucos e a nd fructos e, the probl em ca nnot be i n pa thwa ys tha t meta bol i ze ei ther gl ucos e or pentos e s uga rs . Addi ti ona l l y, the chi l d ca n tol era te di eta ry fructos e a s evi denced by the a bi l i ty to cons ume s ucros e. Fi na l l y, whi l e one ca n be i ntol era nt to l a ctos e, the s uga r, the s a me i s not true rega rdi ng l a cta s e, the enzyme. Al though geneti c s creeni ng tes ts requi red i n mos t s ta tes i denti fy newborns wi th ga l a ctos emi a , thes e tes ts ma y not ha ve been performed on a chi l d born outs i de the Uni ted Sta tes . I-13. The answer is A. Sa tura ted fa tty a ci ds a nd trans fa tty a ci ds a re s tructura l l y s i mi l a r; thei r hydroca rbon ta i l s a re rel a ti vel y l i nea r. Thi s a l l ows them to pa ck ti ghtl y together i n s emi crys ta l l i ne a rra ys s uch a s the membra ne bi l a yer. Such a rra ys ha ve s i mi l a r bi ochemi ca l properti es i n terms of mel ti ng tempera ture (fl ui di ty). Al though s ome of the other properti es l i s ted a re a l s o s ha red by s a tura ted a nd trans fa ts , they a re not thought to a ccount for the tendency of thes e fa ts to contri bute to a theros cl eros i s (Fi gure 3-1B). I-14. The answer is C. Methotrexa te i s a n a na l og of fol i c a ci d tha t bi nds wi th very hi gh a ffi ni ty to the s ubs tra te-bi ndi ng s i te of di hydrofol a te reducta s e, the enzyme tha t ca ta l yzes convers i on of di hydrofol a te to tetra hydrofol a te, whi ch i s us ed i n va ri ous forms by enzymes of both the puri ne a nd pyri mi di ne de novo s yntheti c pa thwa ys . Thus , s ynthes i s of deoxythymi di ne monophos pha te from deoxyuri di ne monophos pha te ca ta l yzed by thymi dyl a te s yntheta s e a nd s evera l s teps i n puri ne s ynthes i s ca ta l yzed by formyl tra ns fera s e a re i ndi rectl y bl ocked by the a cti on of methotrexa te beca us e both thos e enzymes requi re tetra hydrofura n coenzymes . Xa nthi ne oxi da s e tha t ca ta l yzes degra da ti on of puri ne ba s es to uri c a ci d i s una ffected. I-15. The answer is C. Ami no a ci ds a re cl a s s i fi ed a s a ci di c, neutra l hydrophobi c, neutra l hydrophi l i c, or ba s i c, dependi ng on the cha rge or pa rti a l cha rge on the R-group a t pH 7. Hydrophobi c (wa ter-ha ti ng) groups a re ca rbon–hydrogen cha i ns l i ke thos e of l euci ne, i s ol euci ne, gl yci ne, or va l i ne. Ba s i c R-groups , s uch a s thos e of l ys i ne a nd a rgi ni ne, ca rry a pos i ti ve cha rge a t phys i ol ogi c pH owi ng to protona ted a mi de groups , wherea s a ci di c R-groups , s uch a s gl uta mi c a ci d, ca rry a nega ti ve cha rge owi ng to i oni zed ca rboxyl groups . Threoni ne wi th i ts hydroxyl s i de cha i n i s neutra l a t phys i ol ogi c pH. Leuci ne, i s ol euci ne, a nd va l i ne a re a mi no a ci ds wi th bra nched s i de groups , a nd they s ha re a pa thwa y for degra da ti on tha t i s defi ci ent i n chi l dren wi th ma pl e s yrup uri ne di s ea s e. Thei r a mi no groups ca n be removed, but the res ul ti ng ca rboxyl i c a ci ds a ccumul a te wi th res ul ti ng a ci dos i s , coma , a nd dea th unl es s a di et free of bra nch-cha i ned a mi no a ci ds i s i ns ti tuted. I-16. The answer is E. The pa ti ent s hows ma ny s i gns of vi ta mi n C defi ci ency or s curvy, whi ch i s s een mos t frequentl y i n i nfa nts , the el derl y, a nd i n a l cohol i c pa ti ents . Pa rti cul a rl y i ndi ca ti ve of vi ta mi n C defi ci ency a re the mul ti pl e s ma l l hemorrha ges tha t occur under the s ki n (petechi a e) a nd na i l s a nd s urroundi ng ha i r fol l i cl es . Bl eedi ng gums a re a cl a s s i c i ndi ca tor of s curvy. I-17. The answer is D. The ma i n s uga r i n mother’s mi l k i s l a ctos e. When the ba by wa s gi ven the frui t a nd the a rti fi ci a l l y s weetened pops i cl e, s he wa s expos ed to fructos e for the fi rs t ti me a nd a ppa rentl y i s i ntol era nt to di eta ry fructos e. Beca us e the chi l d ca n cons ume l a ctos e, whi ch conta i ns gl ucos e a nd ga l a ctos e, the probl em ca nnot be i n pa thwa ys tha t meta bol i ze ei ther gl ucos e or pentos e s uga rs . The s ymptoms a re a l s o cons i s tent wi th ga l a ctos emi a , but woul d be expected a s a rea cti on to l a ctos e i nta ke. Thi s di a gnos i s of fructos e i ntol era nce s houl d be confi rmed by geneti c tes ti ng. Es s enti a l fructos uri a i s a beni gn condi ti on tha t woul d not ha ve produced s uch s evere s ymptoms . I-18. The answer is A. Gl ucocorti coi ds a re ma de i n the a drena l cortex, the pri nci pa l one bei ng corti s ol i n huma ns . Corti s ol promotes brea kdown of protei ns i n s ta rva ti on to provi de precurs ors for the s ynthes i s of gl ucos e (bl ood s uga r) a nd a l s o promotes the brea kdown of s tored l i pi ds (tri gl yceri des ) to provi de the energy for thi s proces s . Corti s ol performs a s i mi l a r functi on i n chroni c s tres s . Mi nera l ocorti coi ds , the pri nci pa l one bei ng a l dos terone, control s s odi um reupta ke by the ki dney i n excha nge for pota s s i um a nd thereby regul a tes a nd cha nges bl ood vol ume a nd pres s ure. Pros ta gens a re requi red for i mpl a nta ti on of a ferti l i zed egg i n the uteri ne l i ni ng a nd ma i ntena nce of a pregna ncy. Vi ta mi n A i s not a l i pi d-deri ved hormone. It i s i nvol ved i n vi s i on, reproducti on, a nd ti s s ue devel opment. Vi ta mi n D control s the bl ood ca l ci um–phos pha te ma tri x a nd thereby promotes bone forma ti on. I-19. The answer is D. Thi s pa ti ent’s s el f-muti l a ti on beha vi or, neurol ogi c s ymptoms , a nd devel opmenta l del a y a re cons i s tent wi th di a gnos i s of Les ch–Nyha n s yndrome. Thi s di s order i s due to the defi ci ency of hypoxa nthi ne–gua ni ne phos phori bos yl tra ns fera s e, whi ch prevents s a l va ge of hypoxa nthi ne a nd gua ni ne to thei r res pecti ve nucl eoti des , i nos i ne monophos pha te a nd gua nos i ne monophos pha te. Thi s l ea ds i n turn to hypera cti vi ty of the puri ne s ynthes i s pa thwa y, exces s i ve puri ne degra da ti on, a nd overproducti on of uri c a ci d. The gri tty s ubs ta nce a nd ora nge col or of the pa ti ent’s uri ne a re beca us e of excreti on of both di s s ol ved uri c a ci d a nd preci pi ta ted s odi um ura te. Gout mi ght a ccount for the exces s i ve uri c a ci d producti on but not the neurol ogi c s ymptoms . Sel f-muti l a ti on i s not cha ra cteri s ti c of Ta y– Sa chs di s ea s e, cerebra l pa l s y, or a denos i ne dea mi na s e defi ci ency.
SECTION II FUNCTIONAL BIOCHEMISTRY
CHAPTER 5 ENZYMES AND AMINO ACID/PROTEIN METABOLISM Enzymes Ami no Aci d Meta bol i s m The Urea Cycl e Revi ew Ques ti ons
OVERVIEW Enzymes a re s peci a l i zed protei ns , whi ch a ccel era te or catalyze a bi ochemi ca l rea cti on. Ea ch enzyme ca ta l yzes a s peci fi c rea cti on a nd i s regul a ted by competi ti ve a nd noncompeti ti ve i nhi bi tors a nd/or by a l l os teri c mol ecul es . Mul ti pl e enzymes ca n ca ta l yze a s eri es of cons ecuti ve rea cti ons , known a s pathways, to produce a nd/or brea k down compl ex bi ol ogi ca l mol ecul es . Exa mpl es i ncl ude a mi no a ci d s ynthes i s a nd degra da ti on, the coordi na ted rea cti ons i nvol ved i n protei n s ynthes i s , a nd the urea cycl e. Probl ems wi th enzyme pa thwa ys ca n not onl y l ea d to di s ea s e but a l s o offer the opportuni ty for di s ea s e trea tment vi a medi ca ti ons , whi ch ta rget s peci fi c poi nts i n thes e pa thwa ys .
ENZYMES ENZYME REACTIONS Enzymes bri ng together one or more mol ecul es , ca l l ed substrates, to form a res ul ti ng mol ecul e ca l l ed a product (Fi gure 5-1). Mos t enzymes ca ta l yze one s peci fi c rea cti on. However, s ome mul ti pa rt enzyme compl exes ca ta l yze a s eri es of s tep-by-s tep rea cti ons —the fi rs t enzyme pa s s es i ts product, now a new s ubs tra te, to a s econd enzyme tha t i s pa rt of the compl ex, the s econd pa s s es i ts product to a thi rd a nd s o on. Enzymes a re res pons i bl e for ma ny es s enti a l rea cti ons i n the huma n body; i n fa ct, there a re from 20,000 to 25,000 tota l huma n genes , wi th a bout 25% of them produci ng enzymes . It i s not s urpri s i ng tha t probl ems wi th enzymes a re ca us ed by or res ul t i n di s ea s es .
Figure 5-1. Illustration of Simple One and Two Substrate Enzyme Reaction. Components i ncl ude enzyme (E), s ubs ta nces (S 1 , S 2 , S 3 ), a nd product (P). If the s ubs ta nce ca n bi nd to the enzyme’s s ubs tra te-bi ndi ng s i te (e.g., S 1 , top fi gure, a nd S 1 a nd S 2 , bottom fi gure), then i t a cts a s a s ubs tra te. Mol ecul a r s ha pe a s determi ned by s econda ry, terti a ry, a nd qua terna ry s tructure a s wel l a s the hydrophobi c/hydrophi l i c a nd neutra l or cha rged na ture i nfl uences whi ch s ubs tra te mol ecul es ca n bi nd a s s ubs tra tes (repres ented gra phi ca l l y by di fferi ng s ha pes of S 1 , S 2 , a nd S 3 a nd the tri a ngul a r a nd ci rcul a r bi ndi ng pockets ). The enzyme bi nds the s ubs tra te mol ecul e(s ), ca ta l yzes the enzyma ti c rea cti on, a nd rel ea s es the product a fter whi ch the enzyme i s rea dy to ca ta l yze the s a me rea cti on a ga i n. The ra te of the overa l l rea cti on i s i nfl uenced by ea ch s tep i n the proces s , i ncl udi ng s ubs tra te bi ndi ng, ra te of the rea cti on, a nd product rel ea s e a s di s cus s ed i n the text bel ow. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] The concept of enzyme kinetics a l l ows a n exa ct des cri pti on of the enzyma ti c rea cti on, i ncl udi ng the i nfl uence of s ubs tra te a nd product mol ecul es a nd how fa s t the enzyme ca ta l yzes the rea cti on a nd the i mpa ct. More a dva nced enzyme ki neti cs a l l ows the ma thema ti ca l expres s i on of how other mol ecul es s uch a s cofactors, inhibitors, a nd activators (s ee bel ow) a ffect the enzyme rea cti on. For exa mpl e, a s di s cus s ed i n Cha pter 1, s peci fi c a mi no a ci ds form a n enzyme’s pri ma ry s tructure a nd, therefore, s econda ry to qua terna ry s tructure. Thes e s tructures , i n turn, form substrate-binding sites (a l s o known a s the “active site”) to a ccommoda te the s ubs tra tes , thei r chemi ca l rea cti on, a nd the depa rture of the product. Subs tra tes a nd products ca n both be a hydrophobic, hydrophilic, charged, uncharged, or neutral mol ecul e, or a combi na ti on of the a bove. Muta ti ons tha t res ul t i n the cha nge of a n a mi no a ci d or a mi no a ci ds tha t ma ke the s ubs tra te pocket ca n dra s ti ca l l y cha nge the enzyme’s a cti vi ty. For exa mpl e, a hydrophobi c s ubs tra te wi l l ea s i l y enter a n enzyme pocket tha t i s l i ned wi th hydrophobi c a mi no a ci ds . However, i f one of thes e hydrophobi c a mi no a ci ds i s cha nged to a hi ghl y cha rged a mi no a ci d, the s ubs tra te ma y not be a bl e to enter the pocket a nd the enzyme ma y no l onger functi on. How wel l the s ubs tra te i ntera cts wi th the s ubs tra te-bi ndi ng s i te repres ents the “affinity” of the enzyme for the s ubs tra te. The s tronger the a ffi ni ty, the l es s s ubs tra te i s needed to a chi eve a certa i n ra te of rea cti on. Thi s concept i s i mporta nt i f a muta ti on cha nges the s ubs tra te-bi ndi ng s i te a nd l owers the a ffi ni ty. Thi s concept i s i l l us tra ted i n Fi gure 5-1. Substrate Concentration and Enzyme-based Diseases: Phenylketonuria (PKU) i s a n i mporta nt a utos oma l reces s i ve di s ea s e occurri ng i n a n a vera ge of 1 i n 15,000 bi rths . Thi s geneti c di s ea s e i s us ua l l y ca us ed by the defi ci ency of the enzyme phenylalanine hydroxylase, requi red to convert phenyl a l a ni ne to tyros i ne (s ee rea cti on bel ow; hydroxyl group i ndi ca ted by s ma l l a rrowhea d), i mporta nt i n the producti on of neurotra ns mi tters a nd s ki n pi gment (mel a ni n). However, the di s ea s e does not res ul t onl y from the decrea s e of tyros i ne l evel s ; ra ther, the res ul ti ng hi gh concentration of phenyl a l a ni ne l ea ds to the a cti vi ty of a n otherwi s e mi nor enzyma ti c pa thwa y, whi ch produces phenyl ketones , i ncl udi ng phenyl a ceta te, phenyl pyruva te, a nd phenyl ethyl a mi ne. The pres ence of thes e phenyl ketones i s norma l l y s creened for i n the bl ood a s pa rt of s ta nda rd neona ta l tes ti ng a nd, a ga i n, a t 2 weeks of a ge.
[Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Pa ti ents wi th PKU us ua l l y expres s tra i ts of a l bi ni s m, i .e., very fa i r s ki n; whi te-bl ond ha i r, a nd nota bl e, pa l e bl ue eyes ; a “mus ty” odor from phenyl a ceta te i n thei r s wea t, uri ne, s ki n, a nd ha i r; a nd s ometi mes devel op eczema . Untrea ted PKU res ul ts i n decrea s ed bra i n devel opment (mi crocepha l y), hypera cti vi ty, bra i n da ma ge, s ei zures , a nd s evere l ea rni ng di s a bi l i ti es /menta l reta rda ti on. PKU i s us ua l l y trea ted wi th a di et l ow i n phenyl a l a ni ne content, a l though s ome feel tha t res i dua l phenyl a l a ni ne l evel s ma y s ti l l ca us e neurol ogi ca l da ma ge. As a res ul t, a ddi ti ona l trea tments a re bei ng devel oped, whi ch further decrea s e phenyl a l a ni ne l evel s . The number of mol ecul es of s ubs tra te i ncrea s es or decrea s es the l i kel i hood of the s ubs tra te i ntera cti ng wi th the enzyme a nd, therefore, the ra te of the rea cti on. When a l l of the enzyme mol ecul es ha ve s ubs tra te occupyi ng the bi ndi ng s i tes , the enzyme i s s a i d to be saturated by s ubs tra te a nd the ra te of the rea cti on i s ma xi mi zed. If l ower a mounts of s ubs tra te a re pres ent, the rea cti on woul d a l s o be s l ower tha n norma l , potenti a l l y l ea di ng to cl i ni ca l ma ni fes ta ti ons . If a di s ea s e s ta te ca us es a mol ecul e to a bnorma l l y i ncrea s e i n concentra ti on, i t ma y become a s ubs tra te for a n enzyme wi th whi ch i t woul d norma l l y not rea ct. The fi na l fa ctor tha t deci des ma xi mum ra te of the rea cti on i s the effi ci ency a t whi ch a n enzyme ca n ca ta l yze a s i ngl e rea cti on. Enzyme muta ti ons ca n di mi ni s h thi s effi ci ency, a l though, i n ra re i ns ta nces , a muta ti on ca n i ncrea s e a cti vi ty. Li kewi s e, the cel l ha s regul a tory mol ecul es tha t ca n cha nge the ra te of the rea cti on by decrea s i ng or i ncrea s i ng the effi ci ency of the enzyme. COFACTORS Enzyme rea cti ons often requi re a ddi ti ona l mol ecul es ca l l ed cofactors (Fi gure 5-2). Exa mpl es of thes e cofa ctors i ncl ude vi ta mi ns , ni coti na mi de a deni ne di nucl eoti de (NADH), ni coti n-a mi de a deni ne di nucl eoti de phos pha te (NADPH), fl a vi n a deni ne di nucl eoti de (FADH 2 ), a nd coenzyme A (CoA). For exa mpl e, duri ng a n enzyme rea cti on, NADH or NADPH dona tes a hydrogen a tom a s wel l a s el ectrons to the product a nd becomes NAD + or NADP+, res pecti vel y, a nd a re then rel ea s ed. FADH 2 a l s o dona tes el ectrons a nd two hydrogen a toms to produce FAD. Convers el y, NAD +, NADP+, a nd FAD a ccept el ectrons to become the res pecti ve reduced forms . Some i norga ni c meta l i ons s uch a s i ron (Fe), copper (Cu), ca l ci um (Ca ), ma nga nes e (Mn), ma gnes i um (Mg), or zi nc (Zn) a l s o s erve a s cofa ctors for rea cti ons . Meta l i ons ma y contri bute or a ccept a n el ectron (e.g., Fe 2+ → Fe 3+) or thei r a s s oci a ted cha rge ma y s i mpl y provi de es s enti a l s ta bi l i za ti on for the chemi ca l rea cti on wi thout cha ngi ng the meta l .
Figure 5-2. Enzymatic Reaction Involving a Cofactor. Components i ncl ude enzyme (E), s ubs tra tes (S 1 , S 2 ), cofa ctor (Co), a nd product (P). The cofa ctor i s neces s a ry for the crea ti on of the fi na l product but i s uncha nged a t the end of the rea cti on. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Diseases of Copper Deficiency: The i mporta nce of proper l evel s of copper (Cu 2+) to the huma n body i s often not a ppreci a ted. For exa mpl e, the i ncorpora ti on of copper i nto s evera l enzymes i n mi tochondri a i s es s enti a l for oxi da ti ve phos phoryl a ti on (Cha pter 6), a nd l ow l evel s ca n a dvers el y a ffect the producti on of ATP. The oppos i te, a n exces s of copper, l ea ds to the i mporta nt cl i ni ca l condi ti ons of Wilson’s disease a nd Menke’s “kinky hair” disease. Wi l s on’s di s ea s e res ul ts from l ow a cti vi ty of ATPase, Cu2+ transporting, beta polypeptide (ATP7B), whi ch i s res pons i bl e for copper tra ns port out of cel l s . The tra ns ported copper i s ca rri ed by cerul opl a s mi n i n the ci rcul a ti on. In Wi l s on’s di s ea s e, copper i on concentra ti ons a re i ncrea s ed i n l i ver, bra i n, eyes , a nd other ti s s ues . Li ver depos i ts res ul t i n fa ti gue, confus i on from hi gh a mmoni a l evel s (hepa ti c encepha l opa thy), hei ghtened bl ood pres s ure (porta l hypertens i on), a n i ncrea s ed ri s k of bl eedi ng from bl ood ves s el s i n the es opha gus (es opha gea l va ri ces ), a nd, eventua l l y, l i ver fa i l ure a nd/or ca ncer. Accumul a ti on i n the bra i n ca n l ea d to deteri ora ti on of memory a nd thought proces s es , l os s of mus cl e control a nd tone, s ei zures , mi gra i nes , depres s i on, a nxi ety, ps ychos i s , a nd, often, the tremors a nd s l ow movement of Pa rki ns on’s di s ea s e. Depos i ti on i n eyes res ul ts i n the di a gnos ti c “Ka ys er–Fl ei s cher ri ng,” a browni s h-green ri ng evi dent i n the cornea . Ki dneys , hea rt, a nd pa ra thyroi d gl a nds ca n a l s o be a dvers el y a ffected by copper depos i ts . Menke’s “ki nky ha i r” di s ea s e i s a very ra re, X-l i nked reces s i ve di s order tha t muta tes ATPase, Cu2+ transporting, alpha polypeptide (ATP7A) a nd, therefore, a ffects ma l es more often tha n fema l es . The di s ea s e us ua l l y s tri kes i nfa nts (a l though a chi l dhood vers i on i s a l s o known), s l owl y ca us i ng growth fa i l ure, devel opmenta l del a y, a nd menta l reta rda ti on. The s ubtl e ons et often l ea ds to no trea tment a nd dea th i n the fi rs t deca de. Exces s copper a ccumul a ti on ma i nl y i n the ki dneys a nd the di ges ti ve tra ct l ea ds to s ymptoms s i mi l a r to Wi l s on’s di s ea s e. However, l ow l evel s of copper i n bone, bra i n, s ki n, bl ood ves s el s , a nd ha i r a ffect copper-conta i ni ng enzymes . The ATP7A enzyme i s a bs ent i n the l i ver. Other s ymptoms pa rti cul a r to Menke’s di s ea s e i ncl ude a bnorma l body tempera ture; rupture or bl ocka ge of a rteri es i n the bra i n; wea kened bones a nd i ncrea s ed fra ctures ; a nd the s pa rs e, coa rs e, fra gi l e, ki nky, col orl es s , or s teel -col ored ha i r tha t gi ves the di s ea s e i ts a l terna ti ve na me. The onl y trea tment i s da i l y i njecti on of copper a s wel l a s s ymptoma ti c a nd s upporti ve ca re. REGULATION The s peed or “rate” a t whi ch a n enzyme ca ta l yzes a rea cti on depends on numerous fa ctors , whi ch ca n i ncrea s e (activate) or decrea s e (inhibit) thi s ra te. Thi s effect i s ca l l ed regulation a nd i s es s enti a l not onl y for s i ngl e enzyme rea cti ons but a l s o for the overa l l coordi na ti on of mul ti pl e, a nd often competi ng, rea cti ons i n the huma n body. Li ke a ny compl i ca ted proces s , res ources need to be cons erved a nd us ed i n a n effi ci ent ma nner. It i s thi s regul a ti on tha t a l l ows the body to us e energy or s tore energy a nd/or produce certa i n protei ns or ca rbohydra tes or l i pi d mol ecul es when pa rti cul a r needs a ri s e. Enzyme regul a ti on ta kes ma ny forms . The s i mpl es t i s by va ryi ng the concentra ti on of the s ubs tra te. As di s cus s ed a bove, the i ncrea s e or decrea s e of the a mount of a va i l a bl e s ta rti ng ma teri a l i ncrea s es or s l ows the rea cti on, res pecti vel y, a nd forms the ba s i s of not onl y the s i ngl e enzyme regul a ti on but a l s o gl oba l regul a ti on of meta bol i s m a s wel l . Exa mpl es wi l l be s een i n other cha pters i n Secti ons II a nd III.
Enzyme regul a ti on ca n a l s o occur a s the res ul t of a mol ecul e other tha n s ubs tra te or product i ntera cti ng s epa ra tel y wi th the enzyme to modi fy i ts rea cti on ra te. Thi s mol ecul e, whether a n a cti va tor or i nhi bi tor, works i n one of two wa ys : (1) competitive—bi ndi ng to the s a me s i te a s the s ubs tra te mol ecul e(s ) or (2) noncompetitive—bi ndi ng to a compl etel y s epa ra te pa rt of the enzyme, whi ch cha nges the s ha pe of the enzyme a nd, a s a res ul t, the effi ci ency of the rea cti on (Fi gure 5-3). A thi rd type of enzyme competi ti on, uncompetitive, i s ra re a nd wi l l not be covered here. The bi ndi ng of a regul a tory mol ecul e ma y ei ther i ncrea s e or s l ow the rea cti on by i ncrea s i ng/decrea s i ng s ubs tra te bi ndi ng a nd/or the a ctua l chemi ca l rea cti on a nd/or product rel ea s e. (Note: The terms “competi ti ve” a nd “noncompeti ti ve” a re norma l l y not us ed to des cri be a cti va ti on; however, for s i mpl i ci ty, thes e terms a re us ed here.) Both a cti va tors a nd i nhi bi tors of enzyma ti c rea cti ons a re i mporta nt i n di s ea s e proces s es . However, they a re even more i mporta nt i n di s ea s e trea tment beca us e mos t medi ci nes a re des i gned to i ncrea s e the producti on of a benefi ci a l product or decrea s e/s top the forma ti on of a ha rmful product.
Figure 5-3. Competitive and Noncompetitive Inhibition. Competi ti ve i nhi bi tor (upper pa nel ) bi nds to a nd bl ocks the enzyme’s s ubs tra te-bi ndi ng s i te a nd hence the bi ndi ng of the s ubs tra te (S 1 ), thereby preventi ng producti on of the product. Noncompeti ti ve i nhi bi tor (bottom pa nel ) bi nds to a s i te other tha n the s ubs tra te-bi ndi ng s i te, l ea di ng to a conforma ti ona l cha nge of the enzyme, whi ch a l ters the norma l s ubs tra te-bi ndi ng s i te. Thi s cha nge bl ocks s ubs tra te (S 1 ) bi ndi ng a nd producti on of the product. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] β-Lactam Antibiotics and Competitive Inhibition: Gra m-pos i ti ve ba cteri a ha ve cel l wa l l s compos ed of up to 40 l a yers of pepti dogl yca ns , s peci a l i zed ca rbohydra te, a nd a mi no a ci d s tructures , whi ch offer s upport a nd protecti on. Synthes i s of the pepti dogl yca n cel l wa l l i nvol ves a fi na l s tep, whi ch l i nks the i ndi vi dua l pepti dogl yca ns together a nd i nvol ves the cofa ctor peni ci l l i n-bi ndi ng protei n (PBP). β-Lactam antibiotics (s ee the fi gure bel ow), of whi ch peni ci l l i n i s the a rchetype, mi mi c the end a mi no a ci d s equence of the unl i nked pepti dogl yca ns a nd i rrevers i bl y bi nd to the s ubs tra te s i te to competi ti vel y bl ock the fi na l cros s -l i nki ng rea cti on of the PBPs . The di s rupti on of thi s fi na l s tep of cel l wa l l s ynthes i s l ea ds to dea th of the ba cteri a . Ma ny types of a nti bi oti cs a re β-l a cta ms , i ncl udi ng cepha l os pori ns , monoba cta ms , ca rba penems , a nd β-l a cta ma s e i nhi bi tors (e.g., cl a vul a ni c a ci d, s ul ba cta m, a nd ta zoba cta m).
The structure of penicillins (left) and cephalosporins (right) includes the β-lactam component shown in red. These antibiotics mimic the terminal end of proteoglycans and block Gram-positive cell wall synthesis. [Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.] Feedback regulation i s a pa rti cul a r type of s i mpl e regul a ti on i n whi ch a product of a s i ngl e or a s eri es of enzyme rea cti ons ca n bi nd to a nd a cti va te or i nhi bi t i ts el f or a n ea rl i er enzyme i n the s eri es (Fi gure 5-4). Hi gher or l ower concentra ti ons of thi s product offer “feed ba ck” rega rdi ng l evel s of the product a nd the need to i ncrea s e or decrea s e the ra te of the rea cti on(s ) a ccordi ng to the needs of the body. Severa l exa mpl es of feedba ck regul a ti on a re s een i n metabolism—the brea kdown of bi ochemi ca l mol ecul es di s cus s ed i n l a ter cha pters i n thi s s ecti on a nd Cha pter 10.
Figure 5-4. Feedback Inhibition and Activation. The product ca n bi nd to i ts own enzyme or a nother enzyme a nd ei ther i nhi bi t or a cti va te the enzyma ti c rea cti on a ccordi ng to the body’s needs . Components i ncl ude enzyme, s ubs tra te (S 1 ), a nd products (P). [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] The a cti vi ty of a n enzyme ca n a l s o be control l ed by the concept of precurs or or proenzymes. Proenzymes a re often, but not a l wa ys , denoted by the s uffi x “-ogen.” Proenzymes conta i n the a cti ve enzyme a nd a n a ddi ti ona l a mi no a ci d s equence tha t keeps the a cti vi ty “turned off.” A s epa ra te enzyme, s peci fi c for the proenzyme to enzyme tra ns forma ti on, removes thi s a ddi ti ona l s equence to a l l ow the enzyme functi on to begi n. Ma ny enzymes i nvol ved i n the di ges ti on of food a re control l ed thi s wa y a nd a re onl y turned on when s peci fi c s i gna l s a re pres ent (e.g., the pres ence of a recentl y i nges ted food s ource or a cl ot forma ti on). Exa mpl es of proenzymes i ncl ude peps i nogen → peps i n, fi bri nogen → fi bri n, a nd proca rboxypepti da s e → ca rboxypepti da s e. Isozymes (a l s o known a s i s oenzymes ) a re two or more enzymes i n the s a me i ndi vi dua l tha t di ffer i n a mi no a ci d s equence but whi ch ca ta l yze the exa ct s a me chemi ca l rea cti on. Is oenzymes a re us ua l l y found i n di fferent ti s s ues a s wel l a s i n di fferent cel l orga nel l es , where they ca n be di fferentl y regul a ted. Exa mpl es of proenzymes a nd i s ozymes wi l l be s een i n future cha pters . Isozymes: Isozymes a l l ow the body to di fferenti a l l y regul a te the s a me enzyme rea cti on i n di fferent ti s s ues of a n i ndi vi dua l . One of the bes t exa mpl es i s hexoki na s e a nd gl ucoki na s e (s ee Cha pter 6). Hexokinase, found i n a l l huma n cel l s , i s a bl e to bi nd gl ucos e even a t very l ow concentra ti ons . Glucokinase, found onl y i n pa ncrea s , l i ver, s ma l l i ntes ti ne, a nd bra i n, requi res gl ucos e concentra ti ons 100 ti mes hi gher for bi ndi ng a nd i s not i nhi bi ted by gl ucos e-6-phos pha te. Even though hexoki na s e a nd gl ucoki na s e ca ta l yze the s a me rea cti on, the di fferent bi ndi ng of gl ucos e a nd regul a ti on a l l ows the body to properl y regul a te gl ucos e us e (e.g., rel ea s e of i ns ul i n by beta cel l s of the pa ncrea s ) a nd s tora ge (e.g., l i ver cel l s ). Fi na l l y, allosteric regulation i nvol ves the bi ndi ng of a n effector mol ecul e to a pa rt of a s i ngl e s ubuni t (“monomeri c”) enzyme, or to one of s evera l s ubuni ts , i n the ca s e of enzyme compl exes wi th more tha n s ubuni t (“mul ti meri c”). For the more common a l l os teri c regul a ti on of mul ti meri c enzymes , the effector i s often the s ubs tra te for more tha n one a cti ve s i te conta i ned wi thi n the compl ex. Bi ndi ng of the effector ca us es a conforma ti ona l cha nge, whi ch a ffects the a cti ve s i te of a n enzyme (Fi gure 5-5). The bi ndi ng ei ther a cti va tes (pos i ti ve coopera ti vi ty) or i nhi bi ts (nega ti ve coopera ti vi ty) a ddi ti ona l s ubs tra te bi ndi ng or enzyme a cti vi ty—a n effect known a s a l l os teri c regul a ti on. Al l os teri c regul a ti on ha s been cl a s s i ca l l y i l l us tra ted i n the s equenti a l l y i ncrea s i ng bi ndi ng of oxygen by the four s ubuni ts of hemogl obi n a s wel l a s the decrea s ed bi ndi ng of oxygen by the s ubuni ts by the mol ecul e 2,3-bi s phos phogl ycera te (both a l l ow i ncrea s ed but regul a ted rel ea s e of oxygen nea r ti s s ues l ow i n oxygen). An exa mpl e of a l l os teri c regul a ti on of monomeri c enzymes i ncl udes the effect of l a cta te produced duri ng peri ods of prol onged phys i ca l effort to decrea s e oxygen bi ndi ng by myogl obi n to hel p ma i nta i n ATP concentra ti on. Severa l drugs , i ncl udi ng i buprofen a nd a ceta mi nophen, a l s o a l l os teri ca l l y a ffect the bi ndi ng of the a nti -a nxi ety cl a s s of drugs ca l l ed benzodi a zepi nes to a l bumi n, thereby i nfl uenci ng the a ctua l concentra ti on of thi s medi ca ti on i n the bl ood.
Figure 5-5. Allosteric Regulation of Enzyme Activity. Bi ndi ng of a pos i ti ve a l l os teri c modul a tor (upper pa nel ) l ea ds to a conforma ti ona l cha nge of the enzyme (E), whi ch enha nces enzyme a cti vi ty (bol d a rrows ) by i ncrea s ed s ubs tra te bi ndi ng (s ma l l curved a rrow) a nd/or i ncrea s ed ra te of product (P) forma ti on. Bi ndi ng of a nega ti ve a l l os teri c modul a tor (l ower pa nel ) l ea ds to a conforma ti ona l cha nge, whi ch decrea s es enzyme a cti vi ty (thi n a rrows ) by decrea s i ng s ubs tra te bi ndi ng (s ma l l curved a rrow) a nd/or l ea di ng to reduced or no product forma ti on. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
AMINO ACID METABOLISM
AMINO ACID SYNTHESIS The s ynthes i s of a mi no a ci ds i s of pa ra mount i mporta nce to the huma n body. Once s ynthes i zed or i nges ted, a mi no a ci ds a re us ed a s bui l di ng bl ocks not onl y for protei ns but a l s o for s evera l other cri ti ca l bi ol ogi ca l mol ecul es s uch a s nucl ei c a ci ds (both puri nes a nd pyri mi di nes ), hormones , neurotra ns mi tters , a nti -oxi da nts , a nd va ri ous s i gna l i ng mol ecul es (Fi gure 5-6; s ee a l s o Cha pter 1 a nd l a ter cha pters ).
Figure 5-6. Sources and Fates of Amino Acids. Ami no a ci ds , from endogenous or di eta ry protei ns a s wel l a s de novo s ynthes i s , a re us ed for s ynthes i s of a va ri ety of ni trogenous compounds a s wel l a s new protei ns . Thei r ca rbon s kel eton ca n ma ke a va ri ety of fuel s , dependi ng on energy needs , or be burned a s a fuel , wherea s the toxi c ni trogen wa s te i s l a rgel y excreted a s urea . [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Al though i ndi vi dua l a mi no a ci ds a re us ua l l y obta i ned from the di et, es peci a l l y the es s enti a l a mi no a ci ds , nones s enti a l a mi no a ci ds ca n a l s o be produced from other s ources . The non-es s enti a l a mi no a ci ds a re a l l ma de from precurs ors obta i ned from the meta bol i s m of ca rbohydra tes to be s tudi ed i n the next cha pter. Thes e a mi no a ci ds a nd thei r precurs or mol ecul e(s ) a re s hown i n Fi gure 5-7.
Figure 5-7. Synthesis of Amino Acids. Precurs ors of the 20 a mi no a ci ds found i n huma ns a re predomi na tel y from ca rbohydra te meta bol i s m (gl ycol ys i s a nd ci tri c a ci d cycl e, s ee Cha pter 6). Es s enti a l a mi no a ci ds requi red from the di et a re i ndi ca ted by a s teri s ks (*). Hi s ti di ne i s deri ved
pa rtl y from the nucl ei c a ci d precurs or phos phori bos yl pyrophos pha te a nd the a mi no a ci d gl uta ma te. Speci fi c medi ca l condi ti ons a s s oci a ted wi th the s ynthes i s of a mi no a ci ds a re s hown i n Ta bl e 5-1.
TABLE 5-1. Di s ea s es of Ami no Aci d Meta bol i s m AMINO ACID DEGRADATION Protei n a nd a mi no a ci d brea kdown, known a s “degradation,” i s es s enti a l a s a n energy s ource when ca rbohydra te a nd/or l i pi d s ources a re l ow, a s wel l a s to produce a toms a nd mol ecul es requi red for the s ynthes i s of other mol ecul es . The brea kdown of pa rti cul a r a mi no a ci ds a l s o hel ps the body to regul a te how much ni trogen i t conta i ns a nd to excrete exces s ni trogen i n the form of urea i n uri ne. Ea ch a mi no a ci d undergoes a s peci fi c s eri es of enzyma ti c rea cti ons , l ea di ng to mol ecul es i nvol ved wi th a nd, therefore, l ea di ng i nto ca rbohydra te or l i pi d meta bol i s m a nd energy producti on (Fi gure 5-8). As wi th a mi no a ci d s ynthes i s , defi ci enci es a nd decrea s ed a cti vi ty of the enzymes i nvol ved i n thes e brea kdown pa thwa ys l ea d to di s ea s e s ta tes . Thes e di s ea s es a re us ua l l y pres ent a t bi rth a nd requi re a dherence to s peci fi c di eta ry regi mens to control the i l l nes s .
Figure 5-8. Overview of Amino Acid Degradation Pathways. Ami no a ci ds enter i nto a nd a re meta bol i zed vi a the ca rbohydra te meta bol i c pa thwa ys (ci tri c a ci d cycl e a nd gl ycol ys i s , s ee Cha pter 6). [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Severa l di s ea s es i nvol vi ng the a bs ence or i na cti vi ty of a n enzyme i nvol ved i n a mi no a ci d meta bol i s m a re l i s ted i n Ta bl e 5-1. Thes e defi ci enci es i n enzymes res pons i bl e for the s ynthes i s , degra da ti on, a bs orpti on, a nd tra ns port of the a mi no a ci ds genera l l y ca us e fa i l ure or a bnorma l devel opment, neurol ogi ca l /ps ychi a tri c probl ems , s ome qui te uni que (repeti ti ve s el f-huggi ng a nd “l i ck a nd fl i p” a cti vi ty—l i cki ng of fi ngers a nd fl i ppi ng of book a nd ma ga zi ne pa ges ), mus cul os kel eta l di s orders , a bnorma l functi on or fa i l ure of orga n a nd orga n s ys tems , s evera l uni que odors [“ma pl e s yrup,” “mous y,” “ca bba ge-l i ke,” “oa s thous e” (bui l di ng for dryi ng hops )], a nd col ors of uri ne (bl a ck a nd bl ue). Thes e condi ti ons ma y a l s o l ea d to dea th of the fetus or i nfa nt. A more compl ete des cri pti on of ea ch of thes e condi ti ons i s found i n Appendi x I.
THE UREA CYCLE Urea provi des a cri ti ca l mea ns of ri ddi ng the body of ni trogen wa s te. The urea cycle i s the s eri es of enzyme rea cti ons i n l i ver cel l mi tochondri a a nd cytopl a s m res pons i bl e for the remova l of exces s ni trogen a nd the potenti a l l y toxi c by-product a mmoni a vi a the producti on of urea . More i mporta ntl y, the urea cycl e hel ps keep i n ba l a nce ni trogen a toms i n the huma n body. The urea cycl e a ccepts ni trogen a toms i n the form of a mi no groups excl us i vel y from the a mi no a ci ds gl uta ma te a nd a s pa rta te (Fi gure 5-9A). Other s ources of exces s ni trogen, i ncl udi ng from other a mi no a ci ds , feed through the gl uta ma te a nd a s pa rta te pa thwa ys .
Figure 5-9A. Urea Cycle. A. The urea cycl e converts two a mi no groups (NH 3 ) from gl uta ma te a nd a s pa rta te i nto urea a s pa rt of the body’s ni trogen ba l a nce. Gl uta ma te dona tes the fi rs t ni trogen a tom (1) vi a the mol ecul e ca rba moyl phos pha te tha t i s s ynthes i zed vi a the regul a ted enzyme CPS. Thi s enzyme i s a cti va ted by i ncrea s ed gl uta ma te, N-a cetyl gl uta mi c a ci d, a nd/or a rgi ni ne concentra ti ons [i ndi ca ted by (+)]. Ca rba moyl phos pha te then enters the urea cycl e by combi na ti on wi th orni thi ne vi a orni thi ne tra ns ca rba moyl a s e. As pa rta te dona tes the s econd ni trogen (3). Other a mi no a ci ds contri bute a mi no groups a fter convers i on i nto gl uta ma te or a s pa rta te. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] B. α-ketogl uta ra te/Gl uta ma te/Gl uta mi ne Ammoni a Bufferi ng Sys tem. The i nterconvers i on of α-ketogl uta ra te to/from gl uta ma te to/from gl uta mi ne offers the body a n i mporta nt s ys tem to uti l i ze, genera te, a nd/or s tore a mmoni a . Fi na l l y, the body a l s o rel i es on a n a mmoni a bufferi ng s ys tem of α-ketogl uta ra te, gl uta ma te, a nd gl uta mi ne for remova l , producti on, a nd/or s tora ge of a mmoni a groups (Fi gure 5-9B). Regul a ti on of the urea cycl e i s vi a the enzyme ca rba moyl phos pha te s yntheta s e I (CPS-I), l oca ted wi thi n mi tochondri a . Thi s enzyme ca ta l yzes the rea cti on tha t converts a mmoni a from gl uta ma te i nto ca rba moyl phos pha te, whi ch then combi nes wi th orni thi ne to form ci trul l i ne. CPS i s regul a ted by i ncrea s ed concentra ti ons of N-a cetyl gl uta mi c a ci d, produced from the combi na ti on of a cetyl -CoA a nd gl uta ma te, a s wel l a s i ncrea s ed concentra ti on of a rgi ni ne. Both of thes e a mi no a ci ds a re di rect or i ndi rect pa rti ci pa nts i n the urea cycl e, a nd i ncrea s ed concentra ti ons of ei ther refl ect hi gher a mi no a ci d concentra ti ons .
REVIEW QUESTIONS 1. Wha t i s a n enzyme a nd how do the terms ca ta l yze, s ubs tra te, a nd product rel a te to enzymes ? 2. Wha t i s s i mpl e feedba ck a nd a l l os teri c regul a ti on of enzymes ? 3. Wha t i s mea nt by a mi no a ci d s ynthes i s a nd a mi no a ci d degra da ti on? 4. Wha t a re the rel a ti ons hi ps between a mi no a ci d s ynthes i s a nd degra da ti on a nd other meta bol i c pa thwa ys ? 5. For ea ch ba s i c enzyme rea cti on wha t i s the rol e of cofa ctors a nd regul a ti on? 6. Wha t i s the functi on of the urea cycl e a nd how woul d you di a gra m thi s pa thwa y?
CHAPTER 6 CARBOHYDRATE METABOLISM Gl ycol ys i s Ci tri c Aci d Cycl e Oxi da ti ve Phos phoryl a ti on Gl uconeogenes i s The Pentos e Phos pha te Pa thwa y Gl ycogen Synthes i s Gl ycogen Brea kdown Modi fi ed Ca rbohydra tes (Gl ycoprotei ns , Ga gs ) Revi ew Ques ti ons
OVERVIEW The brea kdown (catabolism) a nd s ynthes i s (anabolism) of ca rbohydra te mol ecul es repres ent the pri ma ry mea ns for the huma n body to s tore a nd uti l i ze energy a nd to provi de bui l di ng bl ocks for mol ecul es s uch a s nucl eoti des (Fi gure 6-1). The enzyme rea cti ons tha t form the meta bol i c pa thwa ys for monos a ccha ri de ca rbohydra tes (Cha pter 2) i ncl ude glycolysis, the citric acid cycle, a nd oxidative phosphorylation a s the ma i n mea ns to produce the energy mol ecul e a denos i ne tri phos pha te (ATP). Gluconeogenesis a nd the pentose phosphate pathway repres ent the two ma i n a na bol i c pa thwa ys to produce new ca rbohydra te mol ecul es . Gl ycogen ha s i ts own meta bol i c pa thwa y for l engtheni ng, s horteni ng, a nd/or a ddi ng bra nch poi nts i n the ca rbohydra te cha i n(s ). Not s urpri s i ngl y, a l l of thes e proces s es a re hi ghl y regul a ted a t mul ti pl e poi nts to a l l ow the huma n body to effi ci entl y uti l i ze thes e i mporta nt bi omol ecul es . Fi na l l y, ma ny modi fi ed ca rbohydra tes a re pa rt of a va ri ety of s urfa ce a nd cytos ol i c s i gna l i ng mol ecul es , i ncl udi ng gl ycoprotei ns a nd gl ycos a mi nogl yca ns (GAGs ) (Cha pter 2). Thes e i mporta nt ca rbohydra te mol ecul es a nd the control poi nts i n ca rbohydra te a nd gl ycoprotei n meta bol i s m, therefore, pres ent cl i ni ci a ns wi th opportuni ti es to modi fy thes e ma ny rea cti ons to i mprove hea l th or to fi ght di s ea s e.
Figure 6-1. Overview of Carbohydrate Metabolism. Gl ucos e from the di et ca n be meta bol i zed vi a gl ycol ys i s or gl ycogenes i s . Res ul ti ng meta bol i c
products ca n return to gl ucos e vi a gl uconeogenes i s or gl ycogenol ys i s , res pecti vel y, or proceed further a l ong ca rbohydra te meta bol i s m to the ci tri c a ci d cycl e. Al terna ti vel y, gl ucos e products ca n be s hunted off to fa t or a mi no a ci d meta bol i s m a s i ndi ca ted. Deta i l s a re di s cus s ed i n the text a nd other cha pters . [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.]
GLYCOLYSIS Gl ycol ys i s i s the meta bol i c pa thwa y tha t brea ks down (ca ta bol i s m) hexos e (s i x-ca rbon) monos a ccha ri des s uch a s gl ucos e, fructos e, a nd ga l a ctos e i nto two mol ecul es of pyruva te, two mol ecul es of ATP, two mol ecul es of NADH, two wa ter (H 2 O) mol ecul es , a nd two hydrogen i ons (H +) (Fi gure 6-2). Gl ycol ys i s i nvol ves 10 enzyme-medi a ted s teps a nd i s bes t envi s i oned i n two pha s es —phosphorylation a nd energy production— a l l of whi ch occur i n the cytopl a s m. The phos phoryl a ti on pha s e (s ometi mes referred to a s the prepa ra tory pha s e) s ta rts wi th the s i x-ca rbon ca rbohydra te gl ucos e a nd i nvol ves two phos phoryl a ti ons from ATP a nd the cl ea va ge i nto two mol ecul es of the tri s a ccha ri de (three-ca rbon s uga r) gl ycera l dehyde-3-phos pha te. The energy producti on pha s e i nvol ves the next fi ve s teps duri ng whi ch the two mol ecul es of gl ycera l dehyde-3-phos pha te a re converted to two pyruva te mol ecul es wi th the producti on of two NADH mol ecul es a nd four ATP mol ecul es . Gl ucos e-6-phos pha te, the fi rs t i ntermedi a te of gl ycol ys i s , ca nnot exi t the cel l -l i ke gl ucos e, s o i t a l s o tra ps the gl ucos e mol ecul e i n the cel l for energy producti on vi a gl ycol ys i s or gl ycogen s ynthes i s (s ee bel ow). NADH repres ents a n a l terna ti ve energy s tora ge form tha n ATP, whi ch ma y be uti l i zed by the oxidative phosphorylation pa thwa y.
Figure 6-2. Glycolysis. The pa thwa y of gl ycol ys i s i ncl udes 10 enzyme s teps , whi ch brea k down one mol ecul e of gl ucos e to two mol ecul es of pyruva te, yi el di ng two ATP a nd two NADH mol ecul es . Pyruva te s ubs equentl y enters the mi tochondri a a nd the ci tri c a ci d cycl e (s ee bel ow). Gl ycol ys i s ca n be di vi ded i nto phos phoryl a ti on (Steps 1–5) a nd energy producti on (Steps 6–10) pha s es for better comprehens i on. Enzymes a re
i ndi ca ted by yel l ow-s ha ded boxes . [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Not s urpri s i ngl y, a l l three regul a tory s teps i nvol ve ei ther a n i nves tment or producti on of ATP mol ecul es , a commi tti ng proces s i n the ba l a nce of energy producti on a nd s tora ge. The fi rs t rea cti on of gl ycol ys i s (ca ta l yzed by hexokinase) res ul ts i n the a ddi ti on of a phos pha te group to form the mol ecul e gl ucos e-6-phos pha te. In the l i ver a nd pa ncrea s , hexoki na s e i s repl a ced by the enzyme glucokinase. Phosphofructokinase-1 a dds a s econd phos pha te group to produce fructos e-1,6-bi s phos pha te. Pyruvate kinase, the fi na l rea cti on i n gl ycol ys i s , produces a n ATP mol ecul e. Al though not regul a ted a t the enzyme l evel s uch a s the a bove enzymes , phosphoglycerate kinase i s i nvol ved i n the i ni ti a l producti on of ATP from gl ycol ys i s a nd repres ents the “brea k even” poi nt when ATP i nves ted i n the prepa ra ti on pha s e i s ga i ned ba ck. Al though regul a ted i n ma ny di fferent wa ys a nd by ma ny di fferent mol ecul es , a theme emerges tha t, when energy s tores a re l ow, gl ycol ys i s i s a cti va ted to produce more ATP. When energy l evel s a re hi gh, ei ther the gl ycol yti c pa thwa y i s i nhi bi ted or a l terna ti ve pa thwa ys a re promoted for s tora ge of the gl ucos e mol ecul e for l a ter us e. Regul a ti on of ca rbohydra te meta bol i s m a nd thes e i mporta nt regul a tory enzymes wi l l be di s cus s ed i n further deta i l i n l a ter cha pters (e.g., Cha pter 10). Phosphofructokinase Deficiency: Beca us e of i ts i mporta nce i n energy producti on, di s ea s es i nvol vi ng enzymes from gl ycol ys i s a re ra re. One exa mpl e, though, i s the defi ci ency of phos phofructoki na s e, a l s o known a s gl ycogen s tora ge di s ea s e type VII or Tarui’s disease. It i s i nheri ted i n a n a utos oma l reces s i ve ma nner. Thi s di s ea s e l ea ds to a bui l dup of gl ycogen i n cel l s a nd i mpa i rs va ri ous red bl ood a nd mus cl e cel l s from us i ng gl ucos e a nd ma ny other monos a ccha ri des to produce ATP. There a re three di fferent va ri a ti ons of the di s ea s e, dependi ng on the s everi ty of i mpa i rment. The fi rs t, termed “i nfa nti l e,” pres ents i n the fi rs t yea r of l i fe us ua l l y s ta rti ng wi th bl i ndnes s /ca ta ra cts a nd reta rda ti on wi th dea th by the a ge of 4 yea rs . A s econd “cl a s s i c” va ri a nt pres ents i n chi l dhood wi th mus cl e cra mpi ng a nd wea knes s a fter exerci s e a nd dea th/brea kdown of mus cl e cel l s a s wel l a s l ow red bl ood cel l count (a nemi a ). The thi rd “l a te ons et” va ri a nt pres ents i n ea rl y a dul thood wi th progres s i ve wea knes s of a rms a nd l egs but wi thout mus cl e cra mpi ng/brea kdown a s wel l a s a nemi a . Di a gnos i s i s by noti ng hi gh l evel s of gl ycogen i n mus cl e s a mpl es a nd mea s uri ng the a cti vi ty of phos phofructoki na s e. The onl y trea tment i s a voi da nce of hi gh ca rbohydra te mea l s a nd a ny vi gorous exerci s e. ATP i s the pri ma ry energy mol ecul e us ed by the huma n body (Fi gure 6-3). Three enzymes i nvol ved i n gl ycol ys i s , hexoki na s e, phos phofructoki na s e-1, a nd pyruva te ki na s e, a re regul a ted to a l l ow the body to deci de both whether to us e i ts ca rbohydra te res ources for ATP producti on a nd how much to produce. At ea ch s tep, the cel l deci des to conti nue energy producti on or to cha nnel the res ul ti ng i ntermedi a tes i nto other mol ecul es s uch a s gl ycogen, l i pi ds , a nd/or protei ns . The a bi l i ty of the huma n body to s ens e i ts bi ochemi ca l s urroundi ngs a nd then res pond to tha t envi ronment ena bl es i ntel l i gent a nd effi ci ent us e of i ts l i mi ted res ources .
Figure 6-3. ATP Structure with Its Magnesium Cofactor. ATP s erves a s the pri ma ry energy mol ecul e of the huma n body uti l i zi ng the energy conta i ned i n the phos pha te bonds . Ma gnes i um (Mg2+) s ta bi l i zes thes e bonds a nd hence i s neces s a ry for ATP to be bi ol ogi ca l l y functi ona l . [Reproduced wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.]
CITRIC ACID CYCLE The citric acid cycle i s the s econd meta bol i c pa thwa y i nvol ved i n the ca ta bol i s m of ca rbohydra tes i nto energy a nd i nvol ves ei ght enzyme s teps , s ta rti ng a nd endi ng wi th the mol ecul e oxaloace-tate (Fi gure 6-4). In huma ns , a l l of the s teps of the ci tri c a ci d cycl e, i ncl udi ng the l i nki ng rea cti on of pyruva te convers i on to a cetyl -CoA, ta ke pl a ce wi thi n the mi tochondri a l ma tri x, the a rea wi thi n the i nner mi tochondri a l membra ne. Pyruvate enters the ci tri c a ci d cycl e fol l owi ng i ts convers i on to a cetyl -CoA; the rea cti on i s ca ta l yzed by pyruva te dehydrogena s e tha t l i nks gl ycol ys i s to the ci tri c a ci d cycl e a nd produces one mol ecul e of NADH. By mea ns of a cetyl -CoA a nd i ntermedi a tes i n thi s cycl e, the body ca n a l s o di vert fa ts a nd a mi no a ci ds i nto energy producti on (di s cus s ed l a ter i n thi s cha pter). In a ddi ti on, the ci tri c a ci d cycl e provi des precurs or mol ecul es for the s ynthes i s of a mi no a ci ds a nd/or recei ves thes e a mi no a ci ds when protei ns a re us ed for energy producti on (s ee Cha pter 5, Fi gure 5-7).
Figure 6-4. Citric Acid Cycle. The ci tri c a ci d cycl e i ncl udes ei ght enzyma ti c s teps (ta n-s ha ded boxes ), whi ch recei ve pyruva te from gl ycol ys i s vi a the a cetyl -CoA mol ecul e. The cycl e produces one GTP, three NADH, a nd one FADH 2 a s i t progres s es to oxa l oa ceta te a t whi ch poi nt the cycl e repea ts . [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Acetyl-CoA, a two-ca rbon mol ecul e bonds to the four-ca rbon mol ecul e oxa l oa ceta te to form ci tra te, the na mes a ke of thi s pa thwa y. Thi s pa thwa y i s a l s o known a s the tri ca rboxyl i c a ci d cycl e, beca us e ci tra te conta i ns three ca rboxyl i c a ci d groups , or the Krebs cycl e a fter the phys i ci a n/bi ochemi s t Si r Ha ns Krebs , who ea rned a Nobel Pri ze for hi s work on el uci da ti ng thi s s equence of rea cti ons . Further enzyme rea cti ons remove ca rbon mol ecul es i n the form of ca rbon di oxi de (CO2 ) a nd H 2 O. By doi ng s o, the cycl e produces three mol ecul es of NADH, one mol ecul e of FADH 2 , a nd one mol ecul e of gua nos i ne tri phos pha te (GTP), a l l of whi ch ma y be us ed i n energy producti on a nd s tora ge by oxi da ti ve phos phoryl a ti on (s ee bel ow) a nd convers i on to ATP. The regul a ti on of the ci tri c a ci d cycl e i s ma i nl y vi a the a va i l a bi l i ty of ea ch enzyme’s s ubs tra te a s wel l a s i nhi bi ti on when the concentra ti on of a ny enzyme’s product gets too hi gh. One pri me exa mpl e i s ci tra te, the product of the fi rs t rea cti on, whi ch i nhi bi ts not onl y the cycl e but a l s o phos phofructoki na s e a cti vi ty i n gl ycol ys i s . Tha t ci tra te i s the fi rs t i ntermedi a te of the cycl e a nd i s i mporta nt i n the control of the body’s ca rbohydra te res ources . As wi th gl ycol ys i s , the s a me genera l rul es a l s o a ppl y: i f energy l evel s (i n thi s ca s e NADH or FADH 2 , whi ch a re converted to ATP vi a the l a ter oxi da ti ve phos phoryl a ti on pa thwa y) a re hi gh, the ci tri c a ci d cycl e i s s l owed or i ntermedi a tes a re di verted to other purpos es . Red Blood Cells (Erythrocytes) and Glycolysis: Red bl ood cel l s a re uni que i n the huma n body i n tha t they l os e thei r nucl ei a nd orga nel l es , i ncl udi ng mi tochondri a , ea rl y i n thei r devel opment (Cha pter 14). As a res ul t, red bl ood cel l s a re una bl e to uti l i ze the ci tri c a ci d cycl e or oxi da ti ve phos phoryl a ti on a nd a re s ol el y rel i a nt on ATP producti on from gl ycol ys i s . NADH produced from gl ycol ys i s i s us ed to produce l a cta te from pyruva te; the res ul ti ng l a cta te di ffus es from the red bl ood cel l s a nd i s tra ns ported i nto a nd us ed by ti s s ues s uch a s hea rt mus cl e a nd l i ver. In a ddi ti on, red bl ood cel l s do not res pond to i ns ul i n l i ke other ti s s ues i n the huma n body a nd a re, therefore, not regul a ted by thi s i mporta nt hormona l s i gna l tha t control s ca rbohydra te meta bol i s m.
OXIDATIVE PHOSPHORYLATION The thi rd a nd fi na l pa thwa y of convers i on of ca rbohydra tes to energy i s oxi da ti ve phos phoryl a ti on (Fi gure 6-5). Thi s meta bol i c pa thwa y ta kes pl a ce excl us i vel y i ns i de the i nner mi tochondri a l membra ne, ta ki ng a dva nta ge of di fferences i n concentra ti on of the products of the ci tri c a ci d cycl e to dri ve the forma ti on of ATP.
Figure 6-5. Overview of Oxidative Phosphorylation. El ectrons a re a ccepted from NADH (Compl ex I), s ucci na te (Compl ex II), reduced Q protei n vi a Compl ex II-genera ted FADH2 (Compl ex III), a nd from cytochrome c (Compl ex IV). The res ul ti ng exces s of hydrogen dri ves ATPa s e s yntha s e (Compl ex V) to produce ATP. The proces s us es mol ecul a r oxygen (O2 ) a nd the fi na l product of thi s s eri es of rea cti ons i s wa ter (H 2 O). [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Speci fi ca l l y, oxi da ti ve phos phoryl a ti on i ncl udes both the res pi ra tory cha i n (i .e., oxi da ti ve) a nd ATP s ynthes i s vi a ATP s yntha s e [i .e., phos phoryl a ti on of a denos i ne di phos pha te (ADP)]. The res pi ra tory cha i n porti on i nvol ves four compl exes (compl exes I–IV) a nd a Q protei n tha t hel p to tra ns port H + from the i ns i de of the mi tochondri a to the i ntermembra ne s pa ce between the i nner a nd outer membra nes . Thes e H + i ons ca n then rea di l y di ffus e to the cytopl a s m s o tha t the pH i n the i ntermembra ne s pa ce a nd the cytopl a s m become equi va l ent. Compl ex II i s a ctua l l y the s a me enzyme s een i n the ci tri c a ci d cycl e (s ucci na te dehydrogena s e; Fi gure 6-4) a nd i s res pons i bl e for the genera ti on of one FADH 2 a nd a mol ecul e of fuma ra te for ea ch mol ecul e of s ucci na te. Complex I a ccepts el ectrons from NADH, complex II a ccepts el ectrons from s ucci na te, a nd complex III a ccepts el ectrons from reduced Q protein tha t i ts el f recei ves el ectrons from FADH 2 genera ted from compl ex II. Complex IV a ccepts el ectrons from cytochrome c mol ecul es (s ee Fi gure 6-5). The proces s us es mol ecul a r oxygen (O2 ) a nd the fi na l product of thi s s eri es of rea cti ons i s H 2 O. The four compl exes a re thought to be a s s oci a ted i nto a “s uper” compl ex, a l l owi ng the H + to be cha nnel ed from one compl ex to a nother; i f thi s concept i s correct, oxi da ti ve phos phoryl a ti on i l l us tra tes how a s s oci a ti on of s evera l enzymes together ca n i ncrea s e thei r effi ci ency, a theme tha t wi l l be repea ted s evera l ti mes i n huma n bi ochemi s try. ATP s yntha s e (a l s o ca l l ed compl ex V) then tra ns ports the H + ba ck through the i nner membra ne i nto the ma tri x, uti l i zi ng the energy from the H + concentra ti on gra di ent to form ATP (Fi gure 6-5). Toxins and Poisons—Blocking Oxidative Phosphorylation: Beca us e oxi da ti ve phos phoryl a ti on repres ents a key s tep i n the producti on of bi ol ogi ca l energy, i t i s a l s o the ta rget of va ri ous chemi ca l a gents . Cyanide a nd carbon monoxide, both toxi c s ubs ta nces , i nterfere wi th hemogl obi n mol ecul es but a l s o s peci fi ca l l y i nhi bi t compl ex IV, bl ocki ng the res pi ra tory cha i n. Ti s s ues hi ghl y dependent on energy producti on (e.g., nervous a nd hea rt mus cl es ) a re pa rti cul a rl y a ffected, res ul ti ng i n hea da che, di zzi nes s , s ei zures , coma , a nd ca rdi a c a rres t l ea di ng to dea th. Anti dotes (e.g., ni tri tes a nd s odi um thi os ul fa te) work by ca us i ng the rel ea s e of compl ex IV from the cya ni de mol ecul e fol l owed by meta bol i s m of the nontoxi c product. Some i ns ecti ci des (e.g., rotenone) a l s o work vi a bl ocki ng s peci fi c compl exes of oxi da ti ve phos phoryl a ti on.
ATP Synthase: The enzyme res pons i bl e for producti on of ATP offers uni que i ns i ghts i nto protei n conforma ti ona l cha nges i n a membra nebound protei n. ATP synthase i s compos ed of two s ubuni ts , a cyl i ndri ca l -s ha ped F 0 protei n wi th a hol l ow centra l cha nnel a nd a s ta l k a nd ba l l -s ha ped F 1 headpiece s ubuni t (s ee the fi gure bel ow). The enti re compl ex a ppea rs s i mi l a r to a nd, i n fa ct, works much l i ke a n el ectri ca l genera tor wi th H + repl a ci ng the fl ow of H 2 O. The bi ochemi ca l genera tor i s compl ete wi th a thi rd rod-s ha ped “s ta tor” s ubuni t, whi ch hol ds the F 0 a nd F 1 s ubuni ts together (s ee the fi gure). The F 0 s ubuni t i s compos ed of s evera l s ma l l er protei ns , wi th thei r hydrophi l i c a mi no a ci ds on the i nteri or a nd hydrophobi c a mi no a ci ds a dja cent to the membra ne. When H + fl ow ba ck i nto the i nteri or mi tochondri a l membra ne s pa ce, they ca us e rota ti on of the F 0 s ubuni t wi thi n the the the the
s urroundi ng membra ne. Studi es i ndi ca te tha t the H + ma y i nfl uence pa rti cul a r a mi no a ci d R-groups of the i nterna l F 0 s tructure to crea te rota ti on. The F 1 hea dpi ece s ta l k, connected di rectl y to the F 0 s ubuni t, s pi ns i ns i de the s ta ti ona ry externa l ba l l porti on (hel d i n pl a ce by s ta tor protei n s ubuni t), produci ng a conforma ti ona l cha nge tha t bi nds ADP, ca ta l yzes the rea cti on to form ATP, a nd then rel ea s es ATP. At end of the rota ti ng cycl e, the ba l l -s ha ped s ubuni t i s rea dy for a repea t round of ATP s ynthes i s .*
[Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] *Controversy surrounds the actual number of ATP molecules generated by oxidative phosphorylation with suggestions ranging from two to three ATP per NADH (and numbers in between). FADH 2 produces only about two ATP molecules because of its later entry at complex III into oxidative phosphorylation. Some scientists feel that the number may actually change depending on cellular conditions. This book has adopted the convention of exactly three ATP per NADH and exactly two ATP per FADH 2 .
NADH i s often produced outs i de the mitochondrial matrix (e.g., gl ycol ys i s , convers i on of l a cta te to pyruva te) but ca nnot cros s the i nner mi tochondri a l membra ne for convers i on to ATP. To a ddres s thi s probl em, the body uti l i zes two mol ecul a r “s huttl e” mecha ni s ms . The fi rs t rel i es on the enzyme glycerol-3-phosphate dehydrogenase, whi ch ha s both cytopl a s mi c a nd i nner mi tochondri a l membra ne i s oforms . The cytos ol i c enzyme converts gl ycerol -3-phos pha te i nto di hydroxya cetone phos pha te by oxi di zi ng NADH to NAD +. The mi tochondri a l enzyme ca ptures the energy by ca ta l yzi ng the oxi da ti on of di hydroxya cetone phos pha te vi a the convers i on of fl a vi n a deni ne di nucl eoti de to FADH 2 . FADH 2 then conti nues through the oxi da ti ve phos phoryl a ti on pa thwa y a s a bove to yi el d two ATP mol ecul es . A more effi ci ent s huttl e i nvol ves convers i on of oxa l oa ceta te to ma l a te by reducti on wi th NADH i n the cytos ol ; thi s rea cti on i s ca ta l yzed by a n i s ozyme of ma l a te dehydrogena s e from the ci tri c a ci d cycl e except i n revers e (Fi gure 6-4). The ma l a te i s tra ns ported i nto the mi tochondri a l ma tri x where the revers e rea cti on ta kes pl a ce, res ul ti ng i n NADH tha t yi el ds three ATP mol ecul es . Ma l a te i s ul ti ma tel y converted to a s pa rta te, whi ch returns the ca rbons to the cytos ol .
GLUCONEOGENESIS Gluconeogenesis, the crea ti on of new gl ucos e, cons i s ts of 11 s teps (s ome s i mpl y the revers e of rea cti ons found i n gl ycol ys i s ) a nd i s the fi rs t a na bol i c or “s yntheti c” ca rbohydra te pa thwa y to be di s cus s ed. The proces s ta kes pl a ce a l mos t excl us i vel y i n the l i ver, a l though s ome producti on of new gl ucos e occurs i n the ki dney. Other ti s s ues (e.g., mus cl es ) conta i n ma ny of the gl uconeogeni c enzymes wi th the excepti on of the fi na l s tep tha t a ctua l l y produces gl ucos e. In mus cl es , thes e rea cti ons a l l ow for the convers i on of pyruva te to gl ycogen. Gl uconeogenes i s occurs when the body ha s l ow energy s tores (e.g., s ta rva ti on, l engthy peri ods of exerci s e) a nd el ects to di vert nonca rbohydra te mol ecul es [e.g., l a cti c a ci d, gl ycerol , a nd a mi no a ci ds (except l ys i ne a nd l euci ne)] i nto ATP mol ecul es (s ee Cha pter 10). Poi nts of entra nce by thes e mol ecul es a re s hown i n Fi gure 6-6. Any of the i ntermedi a tes of the ci tri c a ci d cycl e a nd mol ecul es tha t feed i nto the cycl e ca n a l s o be di verted to gl uconeogenes i s vi a oxa l oa ceta te. The ba s i c gl uconeogenes i s pa thwa y (Fi gure 6-6) s hows the convers i on of pyruva te to gl ucos e. Referri ng ba ck to the gl ycol ys i s pa thwa y a bove (Fi gure 6-2), two pyruva te mol ecul es a re formed by the enzyma ti c brea kdown of ea ch gl ucos e mol ecul e. Therefore, two pyruva te mol ecul es a re requi red by gl uconeogenes i s to form one gl ucos e mol ecul e.
Figure 6-6. Gluconeogenesis. The pa thwa y of gl uconeogenes i s i ncl udes 11 enzyma ti c s teps , whi ch form one mol ecul e of gl ucos e from two mol ecul es of pyruva te. Rea cti ons a nd enzymes s peci fi c onl y to gl uconeogenes i s a re s hown i n red. Irrevers i bl e rea cti ons s peci fi c for gl ycol ys i s a re s hown i n green. Addi ti ona l s ubs tra tes for gl uconeogenes i s a re s hown i n bl ue a nd ma y a l s o enter gl uconeogenes i s a s denoted by the a s s oci a ted a rrows . [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] However, gl uconeogenes i s i s not s i mpl y a revers a l of gl ycol ys i s . Fi rs t, gl uconeogenes i s i s a fa r more energy-expens i ve pa thwa y, uti l i zi ng four ATP (two a t ea ch of two s teps ), two GTP, a nd two NADH (the equi va l ent of three a ddi ti ona l ATP mol ecul es per NADH) per gl ucos e produced; a s i mpl e revers a l of gl ycol ys i s woul d us e two fewer ATP a nd GTP mol ecul es ea ch. Thi s energy i nves tment does not i ncl ude tha t us ed by the s ubs tra te mol ecul e to enter gl uconeogenes i s . Second, four i ns tea d of three di fferent enzymes a re s een i n gl uconeogenes i s a t rea cti ons i mporta nt to dri ve forma ti on a nd regul a ti on of gl ucos e producti on. In order to dri ve the energeti ca l l y fa vora bl e gl ycol ys i s pa thwa y i n revers e, the convers i on of ea ch pyruva te to phos phoenol pyruva te requi res one ATP a nd one GTP mol ecul e per pyruva te (i .e., four tota l hi gh-energy phos pha te mol ecul es per gl ucos e mol ecul e produced) a s wel l a s two enzymes s peci fi c to the gl uconeogeni c pa thwa y, the enzymes pyruva te ca rboxyl a s e a nd phos phoenol pyruva te ca rboxyki na s e. Thi rd, the next s teps to fructos e-1,6-bi s phos pha te a re gl ycol ys i s i n revers e a nd, hence,
occur i n the cytopl a s m. Al though gl ycol ys i s genera tes NADH a nd ATP wi thi n thi s porti on of the pa thwa y, gl uconeogenes i s requi res the net cons umpti on of two ATP a nd two NADH mol ecul es per gl ucos e mol ecul e produced. Fourth, beca us e gl uconeogenes i s ca nnot regenera te the ATP us ed i n the revers e di recti on (phos phofructoki na s e-1) duri ng gl ycol ys i s , the convers i on of fructos e-1,6-bi s phos pha te ba ck to fructos e-6phos pha te requi res a thi rd new gl uconeogeni c enzyme, fructos e-1,6-bi s phos pha ta s e. Thi s rea cti on i s dri ven by the energy from cl ea va ge a nd rel ea s e of a n i norga ni c phos pha te (Pi ). The next to l a s t s tep i s a s i mpl e revers a l of the gl ycol ys i s rea cti on. La s tl y, the fi na l convers i on of gl ucos e-6-phos pha te to gl ucos e occurs i n the l umen of the endopl a s mi c reti cul um, wi th gl ucos e bei ng tra ns ported i nto the cytopl a s m a nd, s ubs equentl y, i nto the bl ood to ma i nta i n i ts concentra ti on duri ng s ta rva ti on or s tres s . Aga i n, energy rel ea s e of Pi dri ves the rea cti on forwa rd. As the body does not wa nt to “wa s te” energy-converti ng mol ecul es i nto gl ucos e; gl uconeogenes i s i s hi ghl y regul a ted, i n pa rti cul a r wi th res pect to gl ycol ys i s (s ee Cha pter 10). Not s urpri s i ngl y, gl uconeogenes i s i s regul a ted a t the exa ct poi nts i mporta nt for gl ycol ys i s but by di fferent enzymes . Pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, a nd glucose-6-phosphatase repl a ce the gl ycol ys i s enzymes pyruva te ki na s e, phos phofructoki na s e-1, a nd hexoki na s e or gl ucoki na s e. Hi gh a cetyl -CoA a nd ATP, repres enta ti ve of hi ghenergy s tores i n the body, a cti va te pyruva te ca rboxyl a s e, the fi rs t s tep i n gl uconeogenes i s , wherea s the a na l ogous gl ycol yti c s tep, pyruva te ki na s e, the fi na l s tep i n gl ycol ys i s i s i nhi bi ted, i ns uri ng tha t onl y one pa thwa y—gl ycol ys i s or gl uconeogenes i s —i s functi ona l a t a ny one ti me. Pri ma ry regul a ti on of gl uconeogenes i s i n huma ns occurs a t phos phoenol pyruva te ca rboxyki na s e vi a corti s ol a nd gl uca gon effects on DNA tra ns cri pti on l evel s of the enzyme. Hi gh concentra ti ons of ci tra te, evi dence of pl enti ful ATP producti on a nd ca rbon a va i l a bi l i ty, a nd l ow a mounts of fructos e-2,6 bi s phos pha te ca us e i ncrea s ed a cti vi ty of the enzyme, fructos e-1,6-bi s phos pha ta s e, hel pi ng to regul a te gl uconeogenes i s i n ma mma l s ; i n non-ma mma l s peci es , thi s enzyme i s , i n fa ct, the pri ma ry regul a tor of gl uconeogenes i s . Further deta i l s of regul a ti on a re di s cus s ed i n Cha pter 10. Gluconeogenesis and Metformin: Mos t pa ti ents wi th type 2 diabetes mellitus ha ve gl uconeogenes i s ra tes i n the l i ver three ti mes tha t of pers ons wi thout thi s di s ea s e. Metformin, one of the fa mi l y of biguanide medi ca ti ons , works i n the l i ver by s peci fi ca l l y i nhi bi ti ng gl uconeogenes i s . Al though s evera l mecha ni s ms of bi gua ni de a cti on ma y be a t work, evi dence poi nts to a n i ncrea s e i n cytos ol i c a denos i ne monophos pha te, i ndi ca ti ng to the body a l ow ATP concentra ti on a nd prompti ng us e of gl ucos e vi a gl ycol ys i s . Metformi n a l s o i ncrea s es the body’s s ens i ti vi ty to i ns ul i n, i ncrea s es cel l ul a r upta ke of ci rcul a ti ng gl ucos e, the brea kdown of fa tty a ci ds (a l l whi ch promote gl ucos e us e i ns tea d of new producti on), a nd decrea s es gl ucos e a bs orpti on by the i ntes ti nes .
THE PENTOSE PHOSPHATE PATHWAY The s econd “s yntheti c” ca rbohydra te pa thwa y to be di s cus s ed i s the pentose phosphate pathway, s ometi mes referred to a s the pentos e phos pha te s hunt. Thi s pa thwa y ta kes pl a ce i n the cytopl a s m a nd produces fi ve-ca rbon, pentos e ca rbohydra te mol ecul es for nucl eoti de a nd nucl ei c a ci d s ynthes i s . It i s a l s o the ma jor pa thwa y for the producti on of the i mporta nt reduci ng mol ecul e nicotinamide adenine dinucleotide phosphate (NADPH), a s i mi l a r mol ecul e to NADH but wi th a very di fferent functi on. NADPH ha s a n a ddi ti ona l phos pha te group on the ca rbon 2 pos i ti on of the a deni ne ri bos e mol ecul e. It i s the pri ma ry mol ecul e i ndi rectl y res pons i bl e for el i mi na ti ng toxi c oxygen “ra di ca l s ” (e.g., hydrogen peroxi de)—hi ghl y rea cti ve oxygen mol ecul es produced by s ome rea cti ons i n the body, whi ch ca n ca us e s i gni fi ca nt da ma ge to ti s s ues . NADPH i s a l s o requi red for certa i n rea cti ons i n the s ynthes i s of nucl ei c a ci ds (Cha pter 3) a s wel l a s fa tty a ci ds , chol es terol a nd s teroi ds , a nd nucl ei c a ci ds (Cha pter 7), whi ch requi re a reduci ng mol ecul e. The pentos e phos pha te pa thwa y occurs i n two pha s es . The fi rs t, ca l l ed the oxidative phase, produces NADPH a nd ri bu-l os e-5-phos pha te, a pentos e s uga r mol ecul e (Fi gure 6-7A). The s econd, ca l l ed the nonoxidative phase, produces products for gl ycol ys i s (Fi gure 6-7B) or for nucl eoti de s ynthes i s (Fi gure 6-7C). Regul a ti on of thi s pa thwa y i s vi a NADPH concentra ti on—when NADP+ concentra ti on i s hi gh, the oxi da ti ve pa thwa y i s a ccel era ted; when NADPH concentra ti on i s hi gh, the oxi da ti ve pa thwa y i s i nhi bi ted. Norma l concentra ti ons of NADPH i n the cytopl a s m a re 100 ti mes tha t of NADP, s o the pa thwa y wi l l onl y be turned on when NADPH s tores a re us ed.
Figure 6-7. A. Pentose Phosphate Pathway, Oxidative Phase. Gl ucos e-6-phos pha te i s converted i nto ri bul os e-5-phos pha te wi th genera ti on of two mol ecul es of NADPH a nd one mol ecul e of CO2 (bl ue s ha di ng). Va ri ous ca ti ons (Mg2+, Mn 2+, a nd Ca 2+) pl a y rol es a s cofa ctors i n thes e rea cti ons . The overa l l rea cti on for thi s proces s i s : gl ucos e-6-phos pha te + 2 NADP+ + H 2 O → ri bul os e-5-phos pha te + 2 NADPH + 2 H + + CO2 . B–C. Pentose Phosphate Pathway, Nonoxidative Phase. Ri bul os e-5-phos pha te i s further converted to 3-, 4-, 5-, 6-, a nd 7-ca rbon ca rbohydra tes , whi ch ca n be us ed for gl ycol ys i s (B), or i n a revers a l of rea cti ons , gl ycol yti c i ntermedi a tes ca n be us ed for nucl eoti de s ynthes i s (C). Enzymes i nvol ved i n the rea cti ons a re i ndi ca ted. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Glucose-6-phosphate Dehydrogenase (G6PDH) Deficiency: Abnorma l l y l ow l evel s of G6PDH i s the mos t common i nheri ted di s ea s e of enzyme defi ci ency, a ffecti ng a n es ti ma ted 400 mi l l i on pers ons worl dwi de, ma i nl y thos e of Afri ca n, Mi ddl e Ea s tern, or South As i a n des cent. The di s ea s e i s i nheri ted vi a the X chromos ome i n a reces s i ve ma nner a nd, therefore, a ffects ma l es much more often tha n fema l es . However, fema l es wi th two a ffected X chromos omes s uffer from a n i mmune di s order ca l l ed chronic granulomatous disease, whi ch bl ocks the a cti on of i mmune cel l s tha t uti l i ze the oxygen ra di ca l s to ki l l ba cteri a , often l ea di ng to dea th due to i nfecti on. Mul ti pl e muta ti ons ca n a ffect G6PDH a cti vi ty wi th fi ve di fferent cl a s s es of the di s ea s e cl i ni ca l l y defi ned. Thi s pa thwa y i s the onl y wa y tha t red bl ood cel l s ma y el i mi na te rea cti ve oxygen mol ecul es , whi ch ca n s eri ous l y da ma ge the red bl ood cel l membra ne a nd cel l wa l l a nd l ea d to red bl ood cel l dea th. As a res ul t, a ny probl ems wi th the pentos e phos pha te pa thwa y, i ncl udi ng G6PDH defi ci ency, ca n be detri menta l to red bl ood cel l s a nd the body. Al though mos t i ndi vi dua l s wi th G6PDH defi ci ency a re a s ymptoma ti c, s everel y a ffected pa ti ents di s pl a y s ymptoms of red bl ood cel l brea kdown res ul ti ng i n l i ver a nd ki dney probl ems . In newborn ba bi es , thi s ma y res ul t i n da ma ge to the bra i n, termed kernicterus, a nd dea th. Di a gnos i s i s by s i gns of red bl ood cel l brea kdown a nd l i ver probl ems a nd i s confi rmed by a di rect a s s a y of G6PDH a cti vi ty. Indi vi dua l s di a gnos ed wi th the defi ci ency mus t a voi d us e of a l a rge number of sulfa drugs, i ncl udi ng va ri ous a nti bi oti cs ; a burn medi ca ti on; s evera l a nti convul s a nts ; thi a zi de a nd l oop di ureti cs (us ed to trea t hi gh bl ood pres s ure), a cl a s s of medi ca ti ons us ed to trea t di a betes ; s ome gl a ucoma medi ca ti ons ; certa i n pa i n medi ca ti ons ; a nd, fi na l l y, s evera l a nti ma l a ri a l medi ca ti ons . Trea tment of G6PDH defi ci ency i s a voi da nce of a gents , whi ch ca us e red bl ood cel l des tructi on a nd, i n s ome ca s es , bl ood tra ns fus i ons a nd/or remova l of the s pl een, the orga n res pons i bl e for remova l of da ma ged a nd des troyed red bl ood cel l s . However, one a dva nta ge of modera te G6PDH defi ci ency ha s been noted i n res i s ta nce to ma l a ri a l i nfecti on by Plasmodium falciparum. In certa i n i ndi vi dua l s , us ua l l y of Afri ca n or Medi terra nea n des cent, a decrea s ed l evel of G6PDH a cti vi ty l ea ds to wea keni ng of the red bl ood cel l membra ne, whi ch does not a ffect the pers on but does s top reproducti on a nd further i nfecti on by the ma l a ri a pa ra s i te. Pers ons wi th s i ckl e cel l di s ea s e a l s o enjoy a hei ghtened res i s ta nce to ma l a ri a i nfecti ons . A s ugges ted mecha ni s m i s the s el ected cl ea ra nce by the s pl een of the red bl ood cel l s pa rti a l l y da ma ged by the ma l a ri a l pa ra s i te, wherea s uni nfected red bl ood cel l s a re pres erved.
Arrows i ndi ca te ri ng pha s e of Plasmodium falciparum i n i nfected cel l s . [Reproduced wi th permi s s i on from Cha mberl a i n NR: The Bi g Pi cture: Medi ca l Mi crobi ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.]
GLYCOGEN SYNTHESIS Step 1 of gl ycol ys i s , producti on of glucose-6-phosphate ca ta l yzed by hexokinase, i nvol ves a n i nves tment of one ATP mol ecul e a nd i s , therefore, a commi tti ng s tep towa rd uti l i za ti on of the gl ucos e mol ecul e. If energy (ATP) s tores a re hi gh, the body ma y deci de to s tore the gl ucos e mol ecul e i n the form of glycogen (Cha pters 2 a nd 10; Fi gure 6-8). The fi rs t s tep of thi s proces s i s the convers i on of gl ucos e-6-phos pha te to gl ucos e-1phos pha te. Gl ucos e-1-phos pha te rea cts wi th the nucl eoti de uri di ne tri phos pha te (one of the nucl eoti des i n RNA) to form uri di ne di phos pha te
gl ucos e (UDP gl ucos e), the rea cti on depends on the energy rel ea s ed from the two phos pha te bonds hydrol yzed duri ng thi s rea cti on (Fi gure 68A). Gl ucos e from UDP gl ucos e i s then a tta ched to a preexi s ti ng gl ycogen mol ecul e (Fi gure 6-8B), whi ch i ncl udes a protei n ca l l ed glycogenin a s the core of the growi ng gl ycogen cha i n. The new bond i s formed between ca rbon 1 of the gl ycogen mol ecul e a nd ca rbon 4 of the new gl ucos e. A s epa ra te “bra nchi ng” enzyme forms bonds between ca rbons 1 a nd 6, a pproxi ma tel y every 10 gl ucos e mol ecul es .
Figure 6-8. A–B. Glycogen Synthesis. A. The s ynthes i s of gl ycogen begi ns wi th the convers i on of gl ucos e to gl ucos e-1-phos pha te a nd s ubs equent bondi ng to uri di ne tri phos pha te (UTP) to form uri di ne di phos pha te–gl ucos e (UDP gl ucos e) a nd two phos pha te groups . B. UDP gl ucos e s erves a s the s ource for the a ddi ti on of new gl ucos e mol ecul es to a n exi s ti ng gl ycogen mol ecul e ei ther vi a a 1,4-bond or a 1,6-bond wi th a res ul ti ng uri di ne monophos pha te (UMP) mol ecul e. Enzymes a re s hown i n bl ue-s ha ded boxes . [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Glycosyltransferase Inhibitors: The enzymes tha t l i nk ca rbohydra te mol ecul es together to form gl ycogen a nd s i mi l a r compl ex s uga r mol ecul es a re col l ecti vel y known a s glycosyltransferases. Thi s fa mi l y of enzymes i s es s enti a l to the effi ci ent s tora ge of ca rbohydra te energy, a s wel l a s the s ynthes i s of i mporta nt s tructura l protei ns . As a res ul t, i nhi bi tors of gl ycos yl tra ns fera s es ha ve been devel oped for ma ny medi ca l a nd other us es . The drug caspofungin ki l l s fungi by i nhi bi ti ng bonds between ca rbons 1 a nd 3 of gl ucos e mol ecul es i n funga l cel l wa l l s , l ea di ng to thei r dea th. Ethambutol bl ocks growth of the ba cteri a tha t ca us e tubercul os i s by bl ocki ng ca rbon 5 on the ca rbohydra tes of the ba cteri a ’s cel l wa l l . Other gl ycos yl tra ns fera s e i nhi bi tors a re a l s o bei ng devel oped a s a gents a ga i ns t ca ncer a nd vi ra l i nfecti ons , i ncl udi ng a ga i ns t the huma n i mmunodefi ci ency vi rus (HIV).
GLYCOGEN BREAKDOWN Glycogenolysis, the brea kdown of gl ycogen to rel ea s e gl ucos e, i s a n i mporta nt pa thwa y i n energy us e a nd rea cti ons tha t uti l i ze ca rbohydra tes . Gl ucos e-6-phos pha te i s the entry poi nt of s uga rs l i bera ted from gl ycogen ca ta bol i s m i nto gl ycol ys i s , thus bypa s s i ng the fi rs t regul a tory s tep of hexoki na s e. Gl ucos e from the di ges ti on of s ta rch mol ecul es , a ma jor pa rt of the huma n di et, a l s o enters gl ycol ys i s a t thi s poi nt. Gl ycogenol ys i s i nvol ves three s teps (Fi gure 6-9). Fi rs t, the repeti ti ve remova l of gl ucos e res i dues by phosphorylase brea ki ng bonds between ca rbons 1 a nd 4, produci ng a gl ucos e-1-phos pha te mol ecul e. However, thi s proces s s tops when four gl ucos e res i dues a re l eft before the ca rbons 1 a nd 6 bra nch poi nts . Second, a glucan transferase enzyme moves three res i dues to the l i nea r pa rt of the gl ycogen cha i n, l ea vi ng onl y the fi na l bra nched gl ucos e res i due. Thi rd, a debranching enzyme cl ea ves the ca rbons 1 a nd 6 bond. A fi na l enzyme converts the res ul ti ng gl ucos e-1-phos pha te mol ecul es i nto gl ucos e-6-phos pha te (not s hown) for entry i nto gl ycol ys i s .
Figure 6-9. Glycogen Breakdown. The brea kdown of gl ycogen proceeds by gl ycogen phos phoryl a s e cl ea va ge of the α 1,4-bonds unti l four res i dues rema i n before the α 1,6-bond. Gl uca n tra ns fera s e then moves three res i dues to the other cha i n. Fi na l l y, the rema i ni ng 1,6-bound gl ucos e i s rel ea s ed a s free gl ucos e by a debra nchi ng enzyme. Res ul ti ng gl ucos e-1-phos pha te mol ecul es a re s ubs equentl y converted to gl ucos e-6phos pha te for reentry i nto gl ycol ys i s (not s hown). [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Removi ng gl ucos e res i dues from the l i nea r cha i n a nd bra nch poi nt res i dues conti nues a s l ong a s the body requi res them. The proces s i s control l ed by the a ddi ti on of a phos pha te to (l ea di ng to a cti va ti on) or remova l of a phos pha te from (l ea di ng to i nhi bi ti on) the fi rs t enzyme i n the proces s . Phos phoryl a ti on of thi s enzyme i s , i n turn, under the control of the hormones epinephrine a nd glucagon, both of whi ch promote a cti va ti on of gl ycogenol ys i s to produce new gl ucos e i n l i ver. The l i ver i s the pri ma ry orga n i nvol ved i n gl ucos e uti l i za ti on or s tora ge for the body (s ee Cha pters 10 a nd 11 for further di s cus s i on). As a res ul t, the l i ver conta i ns a n enzyme, gl ucos e-6-phos pha ta s e, whi ch removes the phos pha te from gl ucos e, a l l owi ng i t to l ea ve the l i ver cel l a nd be tra ns ported to other ti s s ues . Mus cl es , too, ca n s tore gl ucos e i n the form of gl ycogen but do not conta i n gl ucos e-6-phos pha ta s e. Gl ucos e rel ea s ed duri ng gl ycogenol ys i s i s , therefore, us ed by the mus cl e cel l s under
control by epi nephri ne but not by gl uca gon. Glycogenolysis Inhibitors: As wi th gl ycos yl tra ns fera s es , the enzymes res pons i bl e for brea kdown of gl ycogen a nd other compl ex ca rbohydra te mol ecul es a re a l s o the ta rgets of s evera l medi ca ti ons . Acarbose a nd miglitol, two medi ca ti ons us ed i n the trea tment of di a betes , i nhi bi t the rel ea s e of new gl ucos e res i dues i n the s ma l l i ntes ti ne a nd pa ncrea s , thereby reduci ng the a va i l a bi l i ty of thes e gl ucos e mol ecul es to the body. Zanamivir (tra de na me Relenza) a nd oseltamivir (tra de na me Tamiflu) a re both i nhi bi tors of a rea cti on i nvol vi ng cl ea va ge of ca rbohydra te bonds tha t a l l ows new i nfl uenza A a nd B vi rus pa rti cl es to be rel ea s ed from a n i nfected cel l . As a res ul t of thes e medi ca ti ons , the new vi ra l pa rti cl es a re tra pped a nd s oon di e. Beca us e of the mecha ni s m i nvol ved, thes e medi ca ti ons mus t be ta ken a t the very begi nni ng of a s us pected i nfl uenza i nfecti on before repl i ca ti on a nd rel ea s e of new vi rus es ta kes pl a ce. Beca us e of the i mporta nce to the proper us e of ca rbohydra te a nd other energy s ources , defects i n the pa thwa ys of gl ycogen s ynthes i s a nd brea kdown wi thi n the l i ver a nd mus cl es , col l ecti vel y known a s glycogen storage diseases (GSDs ), ca n ca us e ma jor medi ca l probl ems . As further res ea rch ha s i denti fi ed s peci fi c enzyme defi ci ts , s evera l GSDs ha ve been combi ned. Ten GSDs ha ve been i denti fi ed a nd a re l i s ted i n Ta bl e 61.
TABLE 6-1. Gl ycogen Stora ge Di s ea s es
MODIFIED CARBOHYDRATES (GLYCOPROTEINS, GAGS) Glycoproteins a nd GAGs, di s cus s ed i n Cha pter 2, a re protei ns wi th a cha i n of ca rbohydra tes , known a s a n oligosaccharide, a tta ched to a core protei n a fter protei n s ynthes i s . Thes e ca rbohydra te mol ecul es ca n be a tta ched to the a mi no (NH 3 ) group of a n a s pa ra gi ne a mi no a ci d (known a s “N-glycosylation”) or to the hydroxyl (OH) group of a mi no a ci ds s eri ne, threoni ne, or s peci a l l y modi fi ed hydroxyl ys i ne or hydroxyprol i ne res i dues (known a s “O-glycosylation”). Enzymes from the fa mi l y of gl ycos yl tra ns fera s es , a l s o s een i n gl ycogen s ynthes i s (s ee a bove), ca ta l yze the a ddi ti ons of ca rbohydra tes to the protei n mol ecul es . Onl y ei ght ca rbohydra te mol ecul es a re us ua l l y s een i n gl ycoprotei ns —gl ucos e,
ga l a ctos e, ma nnos e, fructos e, xyl os e, N-a cetyl ga l a ctos a mi ne, N-a cetyl gl ucos a mi ne, a nd N-a cetyl neura mi ni c a ci d. GAG ca rbohydra tes ca n va ry wi del y but us ua l l y i ncl ude ga l a ctos e, gl ucos a mi ne, gl ucuroni c a ci d, a nd/or i duroni c a ci d. Speci fi c gl ycos yl tra ns fera s es a re res pons i bl e for a ddi ti on of a pa rti cul a r ca rbohydra te mol ecul e to a s peci fi c a mi no a ci d res i due or to the growi ng ol i gos a ccha ri de cha i n. The res ul ti ng gl ycoprotei ns /GAGs perform i mporta nt functi ons both i n the cytopl a s m a nd i n the membra ne (gl ycos yl a ted porti ons of the protei n us ua l l y poi nt out i nto the extra cel l ul a r envi ronment), the l a tter i s i mporta nt a s cel l –cel l s i gna l i ng mol ecul es . Functi ons of GAGs a re l i s ted i n Ta bl e 2-2; gl ycoprotei n functi ons a re revi ewed i n Ta bl e 6-2.
TABLE 6-2. Bi ochemi ca l Rol es of Gl ycoprotei ns Glucosamine/Chondroitin and Osteoarthritis: Certa i n GAGs s erve s tructura l rol es i n joi nt ca rti l a ge, pos s i bl y a cti ng a s mol ecul a r “s hock a bs orbers .” Osteoarthritis i s a di s ea s e cha ra cteri zed by decrea s ed ca rti l a ge i n wei ght-bea ri ng joi nts s uch a s knees , hi ps , a nd ba ck due to “wea r-a nd-tea r” on thes e joi nts . One thera py for os teoa rthri ti s i nvol ves the ora l repl a cement of glucosamine a nd chondroitin by ta bl ets . Al though evi dence i s l a cki ng for true effecti venes s , s tudi es ha ve s hown tha t thes e ta bl ets pos e no hea l th threa t to pa ti ents a nd ma ny note ma rked i mprovement i n os teoa rthri ti s pa i n.
REVIEW QUESTIONS 1. Wha t a re the key fea tures of gl ycol ys i s a nd gl uconeogenes i s a nd how a re they s i mi l a r a nd di fferent? 2. Wha t a re the key fea tures of the ci tri c a ci d cycl e? 3. Wha t a re the key fea tures of oxi da ti ve phos phoryl a ti on a nd i ts rel a ti ons hi p to the ci tri c a ci d cycl e a nd gl ycol ys i s ? 4. Wha t a re the key fea tures of the pentos e phos pha te pa thwa y? 5. Wha t a re the key fea tures of gl ycogen meta bol i s m a nd how does i t rel a te to gl ycol ys i s a nd gl uconeogenes i s ? 6. How i s ca rbohydra te meta bol i s m regul a ted a nd how does thi s regul a ti on rel a te to s ynthes i s a nd degra da ti on of the va ri ous ca rbohydra tes i mporta nt to huma ns a nd choi ces between other meta bol i c pa thwa ys ?
CHAPTER 7 LIPID METABOLISM Fa tty Aci d Meta bol i s m Meta bol i s m of Compl ex Li pi ds Revi ew Ques ti ons
OVERVIEW Li pi ds perform s evera l es s enti a l functi ons , i ncl udi ng formi ng bi ol ogi ca l membra nes , effi ci ent s tora ge of energy, a nd a s components of s evera l i mporta nt s tructura l a nd functi ona l mol ecul es . Li pi d meta bol i s m i ncl udes both the s ynthes i s a nd degra da ti on of fa tty a ci ds a nd/or more compl ex l i pi d mol ecul es . The choi ce between s ynthes i s a nd degra da ti on repres ents a n i mporta nt regul a tory s tep i n huma n bi ol ogy a nd refl ects the l evel of food a nd, therefore, energy s tores a va i l a bl e to the body. Severa l proces s es a re i nvol ved i n thi s deci s i on of produci ng fa tty a ci ds / l i pi ds or, i ns tea d, di recti ng thei r precurs ors to energy producti on vi a ca rbohydra te meta bol i c pa thwa ys . Sepa ra te but s ti l l dependent on thi s proces s , the producti on of chol es terol a nd s evera l l i pi d-deri ved hormones a nd s i gna l i ng mol ecul es i s es s enti a l for mul ti pl e functi ons i n the huma n body. As i n mos t meta bol i c pa thwa ys , defi ci enci es or probl ems ca n l ea d to s eri ous di s ea s e s ta tes . As s een i n other cha pters , unders ta ndi ng of the exa ct mecha ni s m of thes e probl ema ti c meta bol i c s teps a l s o a l l ows trea tment of thes e di s ea s es .
FATTY ACID METABOLISM FATTY ACID SYNTHESIS Al though mos t fa tty a ci ds needed by huma ns a re s uppl i ed i n the di et, fa tty a ci d s ynthes i s pl a ys a rol e i n certa i n ti s s ues to convert a ny exces s of s uga r mol ecul es i nto the more effi ci ent s tora ge form of fa tty a ci d/l i pi d mol ecul es a nd/or to produce s peci a l i zed l i pi d mol ecul es . The l i nk between s uga r a nd fa tty a ci d/l i pi d meta bol i s m i s s een a t a cetyl coenzyme A (CoA), the i ntermedi a te between gl ycol ys i s a nd the ci tri c a ci d cycl e. As wi l l be s een bel ow, a cetyl -CoA i s a l s o the i ni ti a l mol ecul e of fa tty a ci d meta bol i s m. Therefore, a cetyl -CoA i s a n i mporta nt bra nch poi nt of huma n meta bol i s m a nd i ts us e i s hi ghl y control l ed to refl ect the body’s nutri ti ona l s ta te a nd needs . As a n i l l us tra ti on of thi s fa ct, the fi rs t a nd commi tti ng s tep of fa tty a ci d s ynthes i s i s the a ddi ti on of a ca rboxyl (CO2 −) group, dona ted by HCO3 −, to a cetyl -CoA to produce malonylCoA.
Al though the rea cti on i s s i mpl e, the choi ce to commi t the potenti a l energy producti on from ca rbohydra tes to produce l i pi d mol ecul es refl ects s evera l i mporta nt bi ol ogi ca l pri nci pl es i n huma n meta bol i s m. Fi rs t, the enzyme tha t ca ta l yzes thi s rea cti on, acetyl-CoA carboxylase, i s produced a s s ma l l er, i na cti ve protei n compl exes . When the concentra ti on of citrate i s hi gh (the fi rs t i ntermedi a te of the ci tri c a ci d cycl e a nd a s i gn of a bunda nt s uga r res ources ), thes e s ma l l er compl exes joi n to form a cti ve enzyma ti c pol ymers (Fi gure 7-1). Therefore, ci tra te i ncrea s es fa tty a ci d s ynthes i s when the body ha s pl enti ful energy a nd needs to s tore thi s energy i n a n effi ci ent fa s hi on. If l i pi d s tores a re hi gh, though, i ncrea s ed concentra ti on of palmitoyl-CoA (the fi na l product of fa tty a ci d s ynthes i s ) decrea s es fa tty a ci d s ynthes i s by depol ymeri zi ng the a cti ve enzyme compl ex i nto the ori gi na l s ma l l er, i na cti ve compl exes . The ba l a nce between ci tra te a nd pa l mi toyl -CoA or, more s i mpl y, a mea s ure of s uga r vers us fa t l evel s (e.g., hi gh s uga r/fa t i ndi ca tes pl enti ful food s uppl i es ; l ow s uga r/fa t i ndi ca tes l ow food a nd/or s ta rva ti on condi ti ons ) control s the a cti vi ty of thi s commi tti ng s tep a nd, therefore, uti l i zes a va i l a bl e energy s uppl i es mos t effi ci entl y.
Figure 7-1. Regulation of the Acetyl-CoA Carboxylase and Production of Malonyl-CoA by Hormones. Pa l mi ta te a nd ci tra te regul a te ma l onyl -CoA producti on a l l os teri ca l l y by enha nci ng the forma ti on of the l ow-a cti vi ty monomer or the hi ghl y a cti ve pol ymer form of a cetyl -CoA ca rboxyl a s e, res pecti vel y. Both forms a re unphos phoryl a ted the phos phoryl a ti on (i na cti va ti on,
. Ins ul i n, epi nephri ne, a nd gl uca gon regul a te a cetyl -coA ca rboxyl a s e a cti vi ty by i nfl uenci ng
) or dephos phoryl a ti on (a cti va ti on,
) of the monomer. [Ada pted wi th permi s s i on from Na i k P:
Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] There i s a s econd, l onger term, hormona l regul a ti on tha t a l l ows the huma n body even further control of energy s ources . Glucagon, the hormone tha t promotes the brea kdown of gl ycogen to gl ucos e, a nd/or epinephrine, rel ea s ed i n ti mes of s tres s when energy requi rements a re hi gher, a cti va te phos phor-yl a ti on of a cetyl -CoA ca rboxyl a s e, ca us i ng i nhi bi ti on of fa tty a ci d s ynthes i s (Fi gure 7-1). Al terna ti vel y, insulin, the hormone tha t promotes upta ke a nd s tora ge of s uga rs (Cha pter 10), ca us es dephos phoryl a ti on, whi ch i ncrea s es producti on of ma l onyl -CoA a nd, therefore, fa tty a ci d s ynthes i s . Therefore, gl uca gon a nd epi nephri ne dri ve a cetyl -CoA mol ecul es a wa y from l i pi d s tora ge a nd towa rd the producti on of energy vi a the ci tri c a ci d cycl e. Addi ti ona l l y, food depri va ti on or chroni c hi gh-fa t di et decrea s es the a mount of the enzyme tha t produces ma l onyl -CoA. Fol l owi ng the commi tti ng s tep of ma l onyl -CoA producti on, fa tty a ci d s ynthes i s i s a rel a ti vel y s i mpl e pa thwa y tha t ta kes pl a ce on a mul ti enzyme–protei n compl ex i n the cytopl a s m. Thi s enzyme compl ex, ca l l ed fatty acid synthase, conta i ns s even s epa ra te s ubuni ts , ca ta l yzi ng l i nked enzyma ti c rea cti ons , a nd a n a cyl ca rri er protei n “a rm,” whi ch ca rri es the growi ng fa tty a ci d cha i n through to pa l mi ta te, the 16-ca rbonl ong fa tty a ci d wi th s i ngl e ca rbon–ca rbon bonds . The overa l l rea cti on for producti on of one pa l mi ta te mol ecul e, i ncl udi ng the s even a denos i ne tri phos pha te (ATP) mol ecul es to genera te ma l onyl -CoA vi a a cetyl -CoA ca rboxyl a s e, i s a s fol l ows :
Other fa tty a ci ds a re deri ved from pa l mi ta te by s peci fi c enzyma ti c rea cti ons . Producti on of fa tty a ci ds wi th a n odd number of ca rbon a toms rel i es on the s ubs ti tuti on of the three-ca rbon, propionyl-CoA, mol ecul e for a cetyl -CoA to i ni ti a te the fa tty a ci d s yntha s e rea cti on. The ba s i c proces s of fa tty a ci d producti on, whi ch rel i es on the cofa ctor ni coti na mi de a deni ne di nucl eoti de phos pha te (NADPH) to dri ve the rea cti on, i s s hown i n Fi gure 7-2. Si mi l a r to the ma l onyl -CoA rea cti on, the qua nti ty of the enzyme compl ex i s pos i ti vel y regul a ted by hi gher s uga r l evel s a nd nega ti vel y regul a ted by hi gh fa tty a ci d/ fa t l evel s i n the di et.
Figure 7-2. Fatty Acid Synthase. The huma n fa tty a ci d s yntha s e enzyme compl ex i s compos ed of two i denti ca l monomers (I a nd II, “s ubuni t di vi s i on”), whi ch a re, thems el ves , s pl i t i nto condens i ng a nd reduci ng enzyme ha l ves (“functi ona l di vi s i on”). The enzymes res pons i bl e for the growi ng fa tty a ci d cha i n a re l i ned up (a l though not i n the l i nea r order of rea cti ons ), s ta rti ng a t ketoa cyl s yntha s e. As one rea cti on concl udes , the product i s ca rri ed to the next enzyme by a n “a cyl ca rri er protei n” (ACP), a s ecti on of the enzyme compl ex, whi ch ca n fl i p ba ck a nd forth between the s ucces s i ve enzyme rea cti ons . Bui l di ng bl ocks of a cyl or ma l onyl s ubuni ts a re a tta ched to the ACP by a cetyl tra ns a cyl a s e or ma l onyl tra ns a cyl a s e, res pecti vel y. Reduci ng enzymes crea tes the fi na l ca rbon–ca rbon bond of the growi ng fa tty a ci d cha i n. The new fa tty a ci d product ei ther returns to the s ta rt of the proces s or, once a 16-ca rbon, pa l mi ta te mol ecul e i s a chi eved, i s rel ea s ed by thi oes tera s e. The exa ct terti a ry a nd qua terna ry s tructure of fa tty a ci d s yntha s e i s s ti l l i n ques ti on but a l l model s a gree wi th the s ucces s i ve enzyma ti c proces s es a nd the rol e of the ACP. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Pa l mi ta te i s us ed to produce l onger fa tty a ci d cha i ns by the a ddi ti on of more two-ca rbon uni ts from ma l onyl -CoA a s i ndi ca ted by the equa ti on bel ow. Thi s rea cti on ma i nl y occurs i n the endopl a s mi c reti cul um by a four-s tep proces s i ncl udi ng the enzyme fatty acid elongase wi th conti nued us e of NADPH a s a s ource of both energy a nd reduci ng power. Fa tty a ci d el onga ti on ca n a l s o occur i n the mi tochondri a (NADH dependent) a nd i n s el ected ti s s ues (s ee Si deba r) where pa rti cul a r fa tty a ci d types a re requi red. Fi na l l y, the producti on of ca rbon–ca rbon doubl e bonds i n fa tty a ci ds ta kes pl a ce vi a a desaturase enzyme, uti l i zi ng NADH, i n the endopl a s mi c reti cul um.
[Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.
[Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Lipid Synthesis in Selected Tissues: New s ynthes i s of fa tty a ci ds i n the huma n body ta kes pl a ce i n l i ver, a di pos e (fa t) ti s s ue, ki dney, bra i n, a nd the ma mma ry gl a nds . The l i ver i s i nti ma tel y i nvol ved i n ma i nta i ni ng the ba l a nce of l i pi ds , i ncl udi ng chol es terol (Cha pter 11). Adi pos e ti s s ue, commonl y referred to a s fa t cel l s , pl a ys a key rol e i n the s tora ge of l i pi ds i n the form of tri a cyl gl ycerol mol ecul es . Triacylglycerol ca n s tore 9 ki l oca l ori es of energy per gra m (kca l /g) vers us onl y 4 kca l /g for carbohydrates, ma ki ng tri a cyl gl ycerol the mos t effi ci ent form of energy s tora ge i n the body. Fa tty a ci d, tri a cyl gl ycerol , a nd chol es terol s ynthes es a re a l s o found i n ki dneys , a nd i ncrea s ed l evel s of l i pi ds a re s een i n va ri ous ki dney di s ea s es (Cha pter 18). The bra i n i s one of onl y a few ti s s ues tha t us e ketone bodi es (revi ewed bel ow) a s a n energy s ource. Beca us e the bra i n us es a pproxi ma tel y 25% of the body’s tota l energy producti on, thi s a l terna ti ve energy s ource to ca rbohydra tes mos t certa i nl y i l l us tra tes a n es s enti a l a da pta ti on for s urvi va l duri ng peri ods of reduced food i nta ke. The bra i n a l s o s ynthes i zes l i pi d mol ecul es es s enti a l i n the growth a nd ma tura ti on of the nervous s ys tem from i nfa ncy to a dul thood. La ck of thes e s yntheti c functi ons l ea ds to a number of neurol ogi ca l di s ea s es . Ma mma ry gl a nds a l s o res pond duri ng pregna ncy by produci ng tri a cyl gl ycerol conta i ni ng medi umcha i n fa tty a ci ds found i n huma n mi l k. Thes e l i pi ds , whi ch ma ke up 3%–5% of huma n brea s t mi l k, a re es s enti a l for the devel opment of the newborn.
Essential Unsaturated Fatty Acids: Huma ns ca nnot produce des a tura ted bonds beyond the ni nth a nd tenth ca rbons a nd, therefore, need to a cqui re fa tty a ci ds wi th more di s ta l doubl e bonds from di eta ry s ources (Cha pter 3). Thes e es s enti a l , uns a tura ted fa tty a ci ds a re requi red for producti on of mol ecul es s uch a s pros ta gl a ndi ns (i mporta nt i n i nfl a mma tory rea cti ons , va ri ous a s pects of pregna ncy, the s prea d of s ome ca ncers , control of bl ood fl ow to the ki dney, regul a ti on of i on fl ow a cros s membra nes , the conducti on of nerve i mpul s es , a nd modul a ti on of s l eep), thromboxa nes (requi red for bl ood cl otti ng a nd pos s i bl y i nvol ved i n cons tri cti on of bl ood ves s el s , e.g., Pri nzmeta l ’s a ngi na ), a nd l eukotri enes (i mporta nt mol ecul es i n i mmune rea cti ons i ncl udi ng a s thma ti c a nd a l l ergi c rea cti ons a s wel l a s certa i n ca rdi ova s cul a r a nd neurops ychi a tri c di s ea s es ). FATTY ACID DEGRADATION The proces s of the brea kdown or degra da ti on of fa tty a ci ds s ta rts i n the cytopl a s m wi th the l i nka ge of a fa tty a ci d to CoA formi ng a fatty acylCoA mol ecul e. Fol l owi ng thi s l i nka ge, fa tty a ci ds less tha n 12 ca rbons i n l ength cros s the mi tochondri a l membra nes wi thout a s s i s ta nce. However, fa tty a ci ds l onger tha n 12 ca rbons a re tra ns ported i nto the mi tochondri a vi a a uni que proces s i nvol vi ng the mol ecul e ca rni ti ne. In the fi rs t of three s teps , the fa tty a cyl -CoA l i nks to ca rni ti ne by the enzyme carnitine palmitoyltransferase I (CPT I), l oca ted on the outer mi tochondri a l membra ne (Fi gure 7-3). The new mol ecul e moves a cros s the mi tochondri a l i nner membra ne vi a a s peci fi c membra ne protei n ca l l ed tra ns l oca s e. Ins i de the mi tochondri a , CPT II, l oca ted on the i ns i de of the i nner mi tochondri a l membra ne (Fi gure 7-3), removes the ca rni ti ne mol ecul e. Thi s ca rni ti ne mol ecul e movi ng out of the mi tochondri a excha nges wi th a new mol ecul e of pa l mi toyl ca rni ti ne movi ng i nto the mi tochondri a . Hi gh concentra ti on of ma l onyl -CoA, the mol ecul e repres enti ng commi tment to the s ynthes i s of fa tty a ci ds , prevents the tra ns port of fa tty a ci ds i nto mi tochondri a by i nhi bi ti ng CPT I. Increa s ed a mounts of fa tty a cyl -CoA revers e thi s i nhi bi ti on a nd s ti mul a tes fa tty a ci d degra da ti on.
Figure 7-3. Transport of Fatty Acids Longer than 12 Carbons Cross the Mitochondrial Membrane Using Carnitine Palmitoyltransferase I and II. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] CPT Diseases: Not s urpri s i ngl y, defi ci enci es i n CPT I or II l ea d to s evere di s ea s e s ta tes . CPT I defi ci ency i s a n a utos oma l reces s i ve di s order tha t us ua l l y s tri kes i n i nfa ncy or ea rl y chi l dhood a nd ca us es l ow s uga r, l ow ketone l evel s , a nd i ncrea s ed a mmoni a l evel s i n a s s oci a ti on wi th fa s ti ng or i l l nes s (i .e., when the body a ttempts to us e fa tty a ci ds a s a n energy s ource). Pa ti ents wi th CPT I defi ci ency ca n devel op da ma ged a nd enl a rged l i vers (pa rtl y due to the bui l dup of fa tty a ci ds ), ki dney probl ems , mus cl e brea kdown, s ei zures , coma , a nd i mpa i red growth. Wi thout a di et of onl y medi um-cha i n tri gl yceri des (whos e fa tty a ci ds ca n cros s the mi tochondri a l membra nes i ndependent of ca rni ti ne) a nd the a voi da nce of fa s ti ng, dea th i s common. CPT II defi ci ency occurs i n three di fferent forms a nd i s the mos t common i nheri ted di s order of mi tochondri a l l ong-cha i n fa tty a ci d oxi da ti on. Inheri ta nce ca n be a utos oma l reces s i ve or due to mul ti pl e forms of heterozygous muta ti ons tha t a ffect CPT II a cti vi ty. Overa l l , CPT II defi ci ency occurs more often i n ma l es tha n i n fema l es . The three forms i ncl ude a l etha l neona ta l form, a s evere i nfa nti l e hepa toca rdi omus cul a r form, a nd a mi l der form, whi ch ma y occur from i nfa ncy to a dul thood. The fi rs t two forms ca us e l ow s uga r, l ow ketone l evel s , l i ver fa i l ure, hea rt da ma ge, fa ti gue, s ei zures , a nd, often, ea rl y dea th. The s econd a nd thi rd forms i nvol ve brea kdown of mus cl e duri ng peri ods of exerci s e, fa s ti ng, or i nfecti on; reddi s h-brown uri ne us ua l l y res ul ts from the res ul ti ng mus cl e brea kdown products . Trea tment of CPT II defi ci ency i ncl udes a hi gh-ca rbohydra te/l ow-fa t di et to encoura ge onl y ca rbohydra te meta bol i s m (i ncl udi ng i nfus i on of gl ucos e duri ng i nfecti ons , us e of medi um-cha i n fa tty a ci ds i n the di et, ma i nta i ni ng cons ta nt feedi ng s o a s to a voi d fa s ti ng s ta tes , a nd res tri cti ng l engthy peri ods of exerci s e), a dequa te hydra ti on to hel p fl us h brea kdown products through the ki dneys wi thout da ma ge, a nd the a ddi ti on of ca rni ti ne to the di et to convert potenti a l l y toxi c, l ong-cha i n fa tty a ci ds to mol ecul es , whi ch ca n be el i mi na ted vi a other meta bol i c pa thwa ys . Once i ns i de the mi tochondri a , fa tty a ci d brea kdown conti nues i n a four-s tep proces s ca l l ed the β-oxidation cycle, whi ch removes two ca rbons from the fa tty a ci d wi th ea ch cycl e (Fi gure 7-4). The fi rs t s tep i nvol ves the acyl-CoA dehydrogenase enzyme, whi ch removes el ectrons from the two end ca rbons to form a ca rbon–ca rbon doubl e bond. The el ectrons a re tra ns ferred to a fl a vi n a deni ne di nucl eoti de (FAD) cofa ctor mol ecul e a s s oci a ted wi th the enzyme tha t l a ter produces ATP vi a the el ectron tra ns port cha i n. In the s econd s tep, enoyl-CoA hydratase res a tura tes the doubl e bond wi th hydrogen (H +) a nd a hydroxyl (OH −) group from a wa ter mol ecul e. Next, the bond i s oxi di zed to a ketone by L-3-hydroxyacyl-CoA dehydrogenase, wi th the tra ns fer of a nother el ectron to the cofa ctor NAD +, whi ch l a ter produces ATP vi a the el ectron tra ns port cha i n. In the fourth a nd fi na l s tep, the potenti a l l y rea cti ve ketone a nd a nother CoA mol ecul e rea ct vi a the enzyme β-ketothiolase (s ometi mes ca l l ed a cetyl CoA a cyl tra ns fera s e) to form a two-ca rbon a cetyl -CoA fra gment a nd a new fa tty a cyl -CoA tha t i s now two ca rbons s horter. The a cetyl -CoA fra gment i s then us ed for other meta bol i c pa thwa ys for energy producti on or s ynthes i s of new mol ecul es . The four-s tep cycl e repea ts unti l the enti re fa tty a ci d cha i n i s degra ded i nto a cetyl -CoA uni ts . In the fi na l s tep for a n even-cha i n fa tty a ci d, two a cetyl -CoA mol ecul es a re formed from the rema i ni ng four-ca rbon fra gment.
Figure 7-4. Fatty Acid Degradation (β-Oxidation, see the text for details). [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Fa tty a ci d cha i ns wi th a n odd number of ca rbon a toms a re broken down s i mi l a rl y (Fi gure 7-5), a l though the fi na l cycl e produces a threeca rbon propi onyl -CoA mol ecul e a nd a n a cetyl -CoA mol ecul e. A ca rboxyl (CO2 −) group i s then a dded to the propi onyl -CoA mol ecul e i n a two-s tep proces s to produce s ucci nyl -CoA, one of the ma jor mol ecul es of the ci tri c a ci d cycl e (Cha pter 6).
Figure 7-5. Odd Number Fatty Acid Degradation (β-Oxidation, see the text for details). [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Mitochondrial Trifunctional Protein (MTP): Three of the enzymes i nvol ved i n β-oxi da ti on, na mel y 2,3-enoyl-CoA hydrase, 3-hydroxyacyl-CoA dehydrogenase, a nd 3-ketoacyl-CoA thiolase (s ee i ns i de box i n fi gure), a re pa rt of the MTP. Thi s enzyme compl ex ca ta l yzes the fi na l three s teps of fa tty a ci d degra da ti on (s ee the fi gure bel ow).
[Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] MTP defi ci ency res ul ts when a l l three enzyme a cti vi ti es a re defi ci ent. Severa l muta ti ons ha ve been found to ca us e thi s di s ea s e, whi ch s tops the convers i on of l i pi ds /fa ts to energy. Symptoms i ncl ude l ow s uga r, progres s i ve des tructi on of nerves i n the l i mbs a nd di gi ts , brea kdown of mus cl e ti s s ue, a nd l i ver or hea rt da ma ge, whi ch ca n res ul t i n ea rl y dea th. Muta ti ons a l s o a ffect i s ol a ted enzyme a cti vi ti es of the MTP compl ex. Low l evel s of the fi rs t enzyme enoyl -CoA hydra s e res ul t i n a bnorma l l y s ma l l body, a rms , fi ngers , a nd hea d. Defi ci enci es of the s econd enzyme 3-hydroxya cyl -CoA dehydrogena s e a ctua l l y i nvol ve two s epa ra te enzymes , whi ch degra de fa tty a ci ds of a pa rti cul a r l ength. Medi um- a nd s hort-cha i n 3-hydroxya cyl -CoA dehydrogena s e defi ci ency or l ong-cha i n 3-hydroxya cyl -CoA dehydrogena s e (LCHAD) defi ci ency i nvol ves l os s of β-oxi da ti on of medi um- a nd s hort-cha i n fa tty a ci ds or l ongcha i n fa tty a ci ds , res pecti vel y. As a res ul t, the body ca nnot convert thes e pa rti cul a r fa tty a ci d cha i ns to energy a nd i ncrea s ed l evel s bui l d up i n pa rti cul a r ti s s ues a nd orga ns ca us i ng da ma ge. Thes e di s ea s es a re pres ent i n i nfa ncy or l a ter i n l i fe. Symptoms of the i nfa nt di s ea s e i ncl ude poor feedi ng, vomi ti ng/di a rrhea , fa ti gue, l ow bl ood s uga r, mus cl e wea knes s , hea rt a nd l ung probl ems , cha nges i n the reti na , coma , a nd dea th. The di s ea s e tha t pres ents i n l a ter l i fe i s us ua l l y l es s s evere wi th s ymptoms i ncl udi ng poor mus cl e tone a nd wea knes s , i ncrea s ed brea kdown of mus cl es , a nd probl ems wi th nerves i n the a rms a nd l egs . Mothers ca rryi ng a fetus wi th LCHAD defi ci ency ma y a l s o ha ve l i ver di s ea s e a s wel l a s HELLP syndrome, whi ch i ncl udes red bl ood cel l des tructi on (Hemol ys i s ), El eva ted L i ver enzymes (i ndi ca ti ve of l i ver i njury), a nd Low Pl a tel ets . Di s ea s es i nvol vi ng i s ol a ted defi ci enci es of 3-ketoa cyl -CoA thi ol a s e ha ve not been des cri bed. Speci fi c a tta cks of a l l thes e di s ea s e s ta tes ca n be brought on by decrea s ed food i nta ke a nd s tres s es s uch a s vi ra l i nfecti ons . Therefore, trea tment i nvol ves di et res tri cti ons to a voi d the a ffected fa tty a ci d cha i n(s ), a ddi ti on of ca rni ti ne to the di et to promote a l terna ti ve meta bol i c pa thwa ys , a nd the a voi da nce of fa s ti ng a nd i nfecti ve s tres s es . Degra da ti on of uns a tura ted fa tty a ci ds wi th ca rbon–ca rbon doubl e bonds depends on the exa ct conforma ti on of the doubl e bond. Beca us e
pa rt of the proces s of produci ng the a cetyl -CoA fra gment i nvol ves the crea ti on of a doubl e bond (Step 1), s ome uns a tura ted fa tty a ci ds ca n s i mpl y be meta bol i zed by the s a me enzyma ti c proces s . However, i f the doubl e bond i s not i n the correct conforma ti on or a t the wrong ca rbon for the a cyl -CoA dehydrogena s e or the enoyl -CoA hydra ta s e, two other enzymes provi de a s s i s ta nce (Fi gure 7-6). If the doubl e bond i s not i n the proper conforma ti on, enoyl-CoA isomerase converts i t to the correct s tructure, fol l owed by the norma l rea cti on ca ta l yzed by enoyl-CoA hydratase. If the doubl e bond i s i n the wrong l oca ti on, 2,4-dienoyl-CoA reductase cha nges the pos i ti on of the doubl e bond uti l i zi ng NADPH a nd, i f needed, the enoyl -CoA i s omera s e i ns ures i t i s the correct conforma ti on.
Figure 7-6. Alteration of Carbon–Carbon Double Bonds. Enoyl -CoA i s omera s e cha nges a trans-doubl e bond a t ca rbon 3 to a cis-doubl e bond a t ca rbon 2, a nd 2,4-di enoyl reducta s e cha nges the doubl e bond a t ca rbon 4 to a s i ngl e bond. Thes e a l tera ti ons a l l ow norma l fa tty a ci d degra da ti on by enoyl -CoA hydra ta s e to conti nue. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Two poi nts a re worth empha s i zi ng a bout the brea kdown of fa tty a ci ds . Fi rs t, for every two-ca rbon a cetyl -CoA fra gment produced, one FADH 2 a nd one NADH a re produced, res ul ti ng i n the l a ter producti on of two a nd three ATP mol ecul es , res pecti vel y. Thus , one mol ecul e of pa l mi ta te converted to ei ght a cetyl -CoA mol ecul es ca n be us ed to produce 35 ATP mol ecul es . Ea ch a cetyl -CoA mol ecul e ca n then potenti a l l y genera te a n a ddi ti ona l 12 ATP mol ecul es vi a oxi da ti on by the ci tri c a ci d cycl e. Second, thi s proces s i l l us tra tes the es s enti a l rol es of FAD + a nd NAD + a nd thei r a s s oci a ted vi ta mi ns ri bofl a vi n (vi ta mi n B 2 ) a nd ni coti ni c a ci d (ni a ci n) i n the tempora ry s tora ge a nd s ubs equent tra ns fer of bi ol ogi ca l energy. Multiple Acyl-CoA Dehydrogenase Deficiency (MADD): The fi rs t enzyma ti c s tep i n the brea kdown of fa tty a ci ds i s a ctua l l y vi a a cti vi ty of one of three s epa ra te enzymes , dependi ng on the i ni ti a l l ength of the mol ecul e. Long-chain acyl-CoA dehydrogenase (LCAD) hel ps to degra de fa tty a ci ds l onger tha n 12 ca rbons . Medium-chain acyl-CoA dehydrogenase (MCAD) degra des fa tty a ci ds between 6 a nd 12 ca rbons i n l ength. Fi na l l y, short-chain acyl-CoA dehydrogenase (SCAD) hel ps to brea k down fa tty a ci ds s horter tha n s i x ca rbons . MADD i s the generi c term referri ng to the defi ci ency of a ny of the three enzymes (LCAD, MCAD, or SCAD) ca ta l yzi ng the fi rs t s tep i n the degra da ti on of fa tty a ci ds . Thi s i s a n a utos oma l reces s i ve di s ea s e wi th a va ryi ng occurrence ra te dependi ng on the s peci fi c enzyme i nvol ved (MCAD defi ci ency i s the mos t common di s order of the three enzymes , occurri ng i n a pproxi ma tel y 1 i n 17,000 peopl e). An enzyme defi ci ency l ea ds to a ha l t i n the brea kdown of fa tty a ci ds a nd a requi rement for energy producti on from ca rbohydra te s tores . In a ddi ti on, ti s s ues tha t uti l i ze ketone bodi es a re una bl e to produce thi s a l terna ti ve energy s ource. As a res ul t, l evel s of s uga r (hypogl ycemi a ) a nd ketone (hypoketonuri a ) decrea s e a nd a bnorma l by-products of the “s tuck” fa tty a ci d degra da ti on i ncrea s e. Some of thes e a bnorma l fa tty a ci ds i mpede norma l a mmoni a meta bol i s m, l ea di ng to toxi c a mmoni a l evel s , a nd dra ma ti ca l l y decrea s e the body’s a bi l i ty for gl uconeogenes i s — the producti on of new ca rbohydra te mol ecul es from a l terna ti ve s ources s uch a s a mi no a ci ds . Pa ti ents wi th MADD ha ve a number of cl i ni ca l s ymptoms i ncl udi ng s wea ty feet a nd s ta l e brea th odor (from the a bnorma l fa tty a ci ds ), na us ea a nd vomi ti ng, i ncrea s ed l i ver s i ze (hepa tomega l y) a nd l i ver da ma ge (s ometi mes res ul ti ng i n yel l owi ng of the s ki n a l s o known a s ja undi ce), di s torti ons of the s kul l a nd fa ce (e.g., forehea d, nos e, l i ps , a nd ea rs ), mus cl e wea knes s , brea thi ng probl ems a nd l ung da ma ge, a nd defecti ve ki dneys . Dea th i n i nfa ncy i s common.
Diseases of the Peroxisome: Peroxisomes a re often-forgotten, i ntra cel l ul a r orga nel l es tha t el i mi na te toxi c s ubs ta nces , brea k down s peci a l types of fa tty a ci ds , s ynthes i ze bi l e a ci ds a nd pl a s ma l ogen (a n i mporta nt type of phos phol i pi d i n the myel i n membra ne s urroundi ng nerve cel l s ), a nd perform pos ttra ns l a ti ona l proces s i ng of protei ns . Defi ci enci es i n peroxi s omes or thei r va ri ous functi ons l ea d to s eri ous di s ea s es . One s uch di s order, Zellweger syndrome, l ea ds to decrea s ed numbers of peroxi s omes a nd/or defi ci enci es i n a ny of a va ri ety of peroxi s oma l enzymes . The s yndrome us ua l l y res ul ts from muta ti ons i n genes codi ng for a ny of s evera l peroxin protei ns , whi ch recogni ze s peci fi c a mi no a ci d s equences on newl y tra ns l a ted protei ns a nd l a bel them for tra ns port to peroxi s omes . Two cl os el y rel a ted di s ea s es of peroxi s ome functi on, adrenoleukodystrophy (ALD) (a l s o ca l l ed Addi s on–Schi l der Di s ea s e or Si emerl i ng– Creutzfel dt Di s ea s e) a nd Refsum disease, combi ne to form a tri o ca l l ed the “Zel l weger s pectrum.” ALD i s ca us ed by the defi ci ency of a membra ne protei n tha t tra ns ports VLCFA i nto the peroxi s ome. ALD i s us ua l l y reces s i vel y i nheri ted vi a the X chromos ome a nd, therefore, s tri kes onl y young ma l es (s ymptoms us ua l l y become evi dent between the a ge of 4 yea rs a nd 10 yea rs ) a t a ra te of 1 i n 20,000. However, women ca rryi ng a s i ngl e muta ti on ma y devel op l es s s eri ous s ymptoms i n a dul thood (ca l l ed a drenomyel oneuropa thy). Refs um di s ea s e i s ca us ed by a s peci fi c muta ti on on ei ther chromos ome 6 or 10, whi ch decrea s es the brea kdown of phyta ni c a ci d, a 16-ca rbon fa tty a ci d found norma l l y i n the huma n di et. Pa ti ents wi th Refs um di s ea s e us ua l l y pres ent wi th s ymptoms i n chi l dhood or ea rl y a dol es cence. The three di s ea s es of the Zel l weger s pectrum s ha re ma ny s i mi l a r s ymptoms . Abs ent β-oxi da ti on of VLCFA l ea ds to da ma gi ng i ncrea s es i n l evel s of 24- to 30-ca rbon fa tty a ci ds , whi ch res ul t i n a n enl a rged l i ver, ja undi ce, a nd i ntes ti na l bl eedi ng. Wi thout producti on of pl a s ma l ogen, the i ns ul a ti ng myel i n membra ne i s compromi s ed, l ea di ng to progres s i ve bra i n a nd nerve da ma ge wi th s ei zures , l os s of vi s i on a nd hea ri ng, decrea s i ng mus cl e tone a nd s trength cul mi na ti ng i n the i na bi l i ty to move (a ga i n due to decrea s ed producti on of the myel i n s hea th), a nd, i n i nfa nts , poor or a bs ent s uckl i ng or s wa l l owi ng a bi l i ty. ALD pa ti ents often devel op a drena l gl a nd fa i l ure s econda ry to bui l dup of the VLCFAs i n thes e orga ns . A va ri a nt of ALD predomi na tel y s tri kes the s pi na l cord wi th s ymptoms i ncl udi ng wea knes s a nd numbnes s of the l i mbs a nd probl ems wi th uri na ti on a nd defeca ti on. Dea th us ua l l y occurs i n chi l dhood or ea rl y a dol es cence due to probl ems occurri ng i n the a ffected orga ns . Trea tment for Zel l weger s yndrome i s ma i nl y s upporti ve, i ncl udi ng preventi on of i nfecti ons ; however, dea th us ua l l y occurs before the fi rs t bi rthda y. Some s ucces s i n the trea tment of ALD ha s been reported wi th bone ma rrow tra ns pl a nta ti on a nd a di et wi th l ow i nta ke of VLCFA a nd i ncl us i on of Lorenzo’s oil, a mi xture of 18- a nd 22-ca rbon tri gl yceri des . Further res ea rch i s a ttempti ng to provi de further s upport for thi s trea tment a nd to el uci da te the mecha ni s m of Lorenzo’s oi l . Fi na l l y, pa ti ents wi th Refs um di s ea s e a re ma i nta i ned on a di et wi th no phyta ni c a ci d (found i n beef, l a mb, tuna , cod, a nd ha ddock); a ttempts to fi nd a l terna ti ve/na tura l thera pi es a re a l s o ongoi ng.
Fi na l l y, mi tochondri a a re una bl e to degra de fa tty a ci ds grea ter tha n 22 ca rbons . In the ca s e of very-long-chain fatty acids (VLCFA), brea kdown occurs i n peroxi s omes , orga nel l es found i n a l l euka ryotes tha t provi de s peci a l i zed l i pi d meta bol i s m a s wel l a s proces s i ng of toxi c s ubs ta nces . Peroxi s omes meta bol i ze VLCFA down to a n ei ght-ca rbon octa nyl -CoA, whi ch i s then further proces s ed by mi tochondri a a s des cri bed a bove. Peroxi s oma l oxi da ti on of fa tty a ci ds i s dri ven not by ATP but ra ther by the producti on of hydrogen peroxi de (H 2 O2 ), a hi ghl y energi zed mol ecul e, whi ch i s converted to wa ter a nd oxygen by the enzyme ca ta l a s e found onl y i n peroxi s omes . Si mi l a rl y, though, the fa tty a ci d s ubs tra te i s tra ns ported i nto the peroxi s ome by a ca rni ti ne a cyl tra ns fera s e a nd the fi na l s tep i n the proces s i s vi a a s i mi l a r peroxi s oma l β-ketothi ol a s e.
METABOLISM OF COMPLEX LIPIDS TRIACYLGLYCEROL SYNTHESIS Triacylglycerols, commonl y referred to a s tri gl yceri des , a re the predomi na nt s tora ge form of l i pi ds (Cha pter 3, Fi gure 3-2B. In fa ct, the term “fa t” i s a ctua l l y a na me for tri a cyl gl ycerol s tores . As noted previ ous l y, tri a cyl gl ycerol s tores 9 kca l /g vers us onl y 4 kca l /g for ca rbohydra tes . Synthes i s of tri a cyl gl ycerol s ma i nl y ta kes pl a ce on the s mooth endopl a s mi c reti cul um of the l i ver but ca n a l s o be genera ted i n a di pos e (fa t) cel l s . Rega rdl es s of the l oca ti on of s ynthes i s , the s ta rti ng mol ecul e i s gl ycerol -3-phos pha te produced i n l i ver from gl ycerol s tores or i n a di pos e cel l s from di hydroxya cetone phos pha te, the product of the fourth s tep of gl ycol ys i s (Cha pter 6). Tri a cyl gl ycerol mol ecul es form vi a a s i mpl e s eri es of enzyma ti c rea cti ons ca ta l yzed by a triacylglycerol synthase enzyme complex. Thi s col l ecti on of enzymes s equenti a l l y a tta ches fa tty a ci d cha i ns to gl ycerol -3-phos pha te to produce a tri a cyl gl ycerol mol ecul e. As a l wa ys , the choi ce to di vert s tored l i ver gl ycerol or the potenti a l energy from ca rbohydra te mol ecul es depends on whether or not ca rbohydra te l evel s a re hi gh (fed) or l ow (fa s ti ng). Once s ynthes i zed, tri a cyl gl ycerol s produced i n the l i ver combi ne wi th chol es terol a nd phos phol i pi ds together wi th a l i poprotei n ca l l ed a pol i poprotei n B to form a compl ex referred to a s very-low-density lipoprotein (VLDL), whi ch i s rel ea s ed i nto the bl ood for tra ns port to other ti s s ues . VLDL pl a ys a n i mporta nt rol e i n tri a cyl gl ycerol tra ns port a nd l ow-dens i ty l i poprotei n (LDL) l evel s a nd i s a l s o one of the l i pi ds mea s ured when a s s es s i ng a pa ti ent’s chol es terol a nd l i pi d s ta tus . VLDL wi l l be exa mi ned i n much more deta i l i n Cha pters 11 a nd 16. PHOSPHOGLYCERIDE SYNTHESIS Phosphoglycerides a re l i pi d mol ecul es wi th two fa tty a ci d cha i ns a nd a s peci fi c “hea d group,” whi ch defi nes tha t pa rti cul a r phos phogl yceri de mol ecul e. The va ri ous phos phogl yceri des a nd thei r functi ons a re di s cus s ed i n Cha pters 3 a nd 8. The s ynthes i s of s eri ne-, i nos i tol -, a nd etha nol a mi ne-phos phogl yceri des fol l ows the s a me enzyma ti c pa thwa y a s tri a cyl gl ycerol s up to a nd i ncl udi ng the a ddi ti on of the s econd fa tty a ci d mol ecul e to form phosphatidic acid (Fi gure 7-7). Fol l owi ng thi s rea cti on, a n a cti va ted cyti di ne di phos phogl ycerol i ntermedi a te formed from the nucl eoti de cyti di ne tri phos pha te (CTP) bi nds a t the phos pha te group a nd then i s repl a ced by the s eri ne or i nos i tol hea d group (Fi gure 77A). Phos pha ti dyl etha nol a mi ne i s formed by the l os s of the COO− group from the s eri ne (Fi gure 7-7B). Phos pha ti dyl chol i ne i s from di eta ry s ources by ATP-dri ven phos phoryl a ti on of the hea d group, a tta chment to CTP, a nd fi na l a tta chment to the di a cyl gl ycerol mol ecul e. Thi s i s a l s o a s econd a nd a l terna ti ve s yntheti c pa thwa y for phos pha ti dyl etha nol a mi ne (Fi gure 7-7C).
Figure 7-7. A–C. Synthetic Pathways of Phosphatidyl Serine, Inositol, Ethanolamine, and Choline. Phos pha ti dyl s eri ne a nd i nos i tol a re produced from phos pha ti di c a ci d, uti l i zi ng cyti di ne tri phos pha te (CTP). Los s of a ca rbon di oxi de mol ecul e from phos pha ti dyl s eri ne produces phos pha ti dyl etha nol a mi ne. Phos pha ti dyl chol i ne a nd etha nol a mi ne s ynthes i s a l s o uti l i ze CTP fol l owi ng ATP phos phoryl a ti on of the chol i ne or
etha nol a mi ne hea d group. CDP, cyti di ne 5′-di phos pha te. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] KETONE BODY SYNTHESIS If ca rbohydra te l evel s a re l ow, the huma n body wi l l often revert to a n a l terna ti ve s ource of energy i n the form of ketone bodi es . Ketone bodies, produced i n the l i ver from a cetyl -CoA, a re a col l ecti ve term for the mol ecul es acetoacetate a nd D-3-hydroxy-butyrate, a s wel l a s acetone tha t i s produced by the s ponta neous degra da ti on of a cetoa ceta te (Fi gure 7-8A).
Figure 7-8. A. Ketone Body Synthesis. B. Conversion of Ketone Bodies to Acetyl-CoA for Subsequent Entrance into the Citric Acid Cycle. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] The body ca n convert s tored ketone bodi es ba ck i nto oxi da ti ve meta bol i s m by a s eri es of enzyma ti c rea cti ons tha t mi mi c the revers e of ketone body s ynthes i s . Acetoa ceta te a nd D-3-hydroxybutyra te a re produced by the l i ver from a cetyl uni ts . Hi gh concentra ti ons es peci a l l y of D-3hydroxybutyra te a nd, therefore, a cetyl uni ts a va i l a bl e for the ci tri c a ci d cycl e decrea s e thi s rel ea s e of a di pos e-deri ved fa tty a ci ds . In the uti l i za ti on proces s , whi ch occurs i n ti s s ues other tha n the l i ver a nd red cel l s , ketone bodi es a re converted ba ck to two a cetyl -CoA mol ecul es by the a ddi ti on of s ucci nyl -CoA a nd then di rected i nto the ci tri c a ci d cycl e (Fi gure 7-8B). In ti mes of l ow ca rbohydra te l evel s a nd s ta rva ti on, thi s s eri es of rea cti ons i s us ed a s the pri ma ry s ource of energy by the hea rt, ki dneys , mus cl es , a nd bra i n. Ketone body s ynthes i s /degra da ti on i s regul a ted by the concentra ti on of oxa l oa ceta te, the l a s t mol ecul e of the ci tri c a ci d cycl e (Cha pter 6), whi ch combi nes wi th a cetyl -CoA to conti nue the proces s of producti on of energy from ca rbohydra tes . Therefore, i f oxa l oa ceta te concentra ti on i s l ow, i ndi ca ti ve of l ow ca rbohydra te l evel s , the body ca n di rect i ts res ources to the producti on of new gl ucos e mol ecul es (gl uconeogenes i s , Cha pter 6) a nd ketone bodi es to produce energy. At the hormona l l evel , gl uca gon a nd epi nephri ne a l s o i ncrea s e brea kdown of tri a cyl gl ycerol s a nd the producti on of fa tty a ci ds , a cetyl -CoA, a nd, a s a res ul t, ketone bodi es . The hormone corti s ol ha s the s a me but l onger l a s ti ng effects . If oxa l oa ceta te concentra ti on i s hi gh, entry i nto the ci tri c a ci d cycl e wi l l i ncrea s e beca us e the a cetyl uni ts wi l l condens e wi th the oxa l oa ceta te. Ins ul i n ha s the oppos i te effect, promoti ng s tora ge of fa tty a ci ds i n the form of tri a cyl gl ycerol s a nd i nhi bi ti ng ketone body s ynthes i s ; the effect of i ns ul i n i s decrea s ed or nega ted by hi gh corti s ol l evel s s een i n Cus hi ng’s s yndrome. Not s urpri s i ngl y, hi gh l evel s of ketone bodi es a re s een i n forms of poorl y trea ted di a betes (Cha pter 10), l ow-chol es terol /hi gh-fa t di ets , a nd fa s ti ng/s ta rva ti on. Cushing’s syndrome: Uncontrol l ed a nd exces s i ve producti on of the hormone corti s ol by the a drena l gl a nds , a condi ti on known a s Cushing’s syndrome, i s ca us ed by a tumor i n the cortex porti on of the a drena l gl a nds . Exces s i ve us e of s ome medi ca ti ons ca n ca us e a s i mi l a r condi ti on known a s hypera drenocorti ci s m or hypercorti ci s m. Pa ti ents wi th thes e di s ea s es exhi bi t a “Cus hi ngoi d” a ppea ra nce from i ncrea s ed a nd uncontrol l ed depos i ti on of fa t, res ul ti ng i n ra pi d wei ght ga i n ma i nl y i n the trunk of the body, roundi ng of the fa ce known a s “moon fa ce,” a l a rge fa t pa d i n the ba ck of the neck known a s a “buffa l o hump,” a nd s tretch ma rks i n the s ki n from the ra pi d wei ght i ncrea s e. Pa ti ents ma y a l s o experi ence other hormona l s ymptoms i ncl udi ng exces s s wea ti ng, ma l e-pa ttern ha i r growth i n fema l es (hi rs uti s m), i mpotence, i nferti l i ty, a nd ces s a ti on of mens es a nd va ri ous ps ychi a tri c probl ems s uch a s depres s i on, a nxi ety, a nd ps ychos i s . Trea tment i s us ua l l y by s urgi ca l remova l of the tumor. Interes ti ngl y, other ma mma l s a l s o often s uffer from Cus hi ng’s s yndrome, i ncl udi ng dogs , hors es , a nd ferrets . CERAMIDE/SPHINGOLIPIDS SYNTHESIS Sphingosine i s the precurs or mol ecul e for cera mi de, whi ch repl a ces gl ycerol a s the “ba ckbone” of s phi ngol i pi d mol ecul es , i ncl udi ng cerebros i des , s ul fa ti des , gl obos i des , a nd ga ngl i os i des (Cha pter 3), es s enti a l i n ma ny neurol ogi ca l a nd mus cl e functi ons a nd i n cel l –cel l recogni ti on a nd i ntera cti ons . Sphi ngos i ne i s produced from a pa l mi toyl -CoA a nd a s eri ne mol ecul e i n a rea cti on tha t i nvol ves NADPH a nd FAD (Fi gure 3-4A), a ga i n s howi ng the i mporta nce of thes e mol ecul es a nd thei r res pecti ve vi ta mi ns i n l i pi d producti on. Sphi ngos i ne, i n turn, l i nks to a l ong-cha i n fa tty a ci d a cti va ted by CoA to form the mol ecul e ceramide. Cera mi de i s then uti l i zed by s evera l i ndependent enzyma ti c rea cti ons
to produce a va ri ety of s phi ngol i pi ds mol ecul es of va ryi ng functi on. Ga ngl i os i des a re the mos t compl ex s phi ngol i pi ds , bei ng bui l t by the s ucces s i ve a ddi ti on of ca rbohydra te mol ecul es vi a s peci fi c gl ycos yl tra ns fera s e enzymes . Thes e enzymes a dd gl ucos e, ga l a ctos e, a nd other modi fi ed s uga rs i n a defi ned order a nd preci s e pos i ti ons (Fi gure 3-4F) to crea te one of more tha n 15 ga ngl i os i de mol ecul es . The functi on of thes e mol ecul es i s revi ewed i n Cha pter 3 a nd wi l l be di s cus s ed i n more deta i l i n l a ter cha pters . Tay–Sachs disease: As di s cus s ed i n Cha pter 3, ga ngl i o-s i des a re found i n hi ghes t concentra ti on i n cel l s of the nervous s ys tem a nd a re norma l l y broken down i ns i de l ys os omes by orderl y remova l of the s uga r res i dues , pa rtl y by the a cti on of the enzyme β-Nacetylhexosaminidase. When thi s enzyme i s mi s s i ng or defi ci ent, ga ngl i os i de GM2 ca nnot be broken down a nd Tay–Sachs disease res ul ts . Symptoms of wea knes s a nd del a yed devel opment of mus cl e s ki l l s a re us ua l l y evi dent before the a ge of 1 yea r wi th s ubs equent bl i ndnes s a nd dea th, us ua l l y by the a ge of 3 yea rs . Neurons of a ffected pa ti ents a re l i tera l l y s wol l en wi th l ys os omes fi l l ed wi th GM2. Ta y–Sa chs di s ea s e occurs 100 ti mes more often i n As hkena zi (Ea s tern Europea n) Jews . CHOLESTEROL SYNTHESIS Al though a bout ha l f of the body’s requi rement for chol es terol i s us ua l l y obta i ned from the di et, the rema i nder mus t be s ynthes i zed. Any defi ci enci es i n thi s s ynthes i s a re, therefore, i mporta nt i n norma l huma n phys i ol ogy a nd di s ea s e. The ma i n l oca ti on of chol es terol s ynthes i s i n huma ns i s the l i ver/ i ntes ti na l tra ct. Chol es terol recei ves a l l of i ts ca rbon mol ecul es from the two-ca rbon mol ecul e a cetyl -CoA, once a ga i n s howi ng the i mporta nce of thi s i ntermedi a te i n ca rbohydra te a nd l i pi d meta bol i s m (Fi gure 7-9). The fi rs t s tep i nvol ves the forma ti on of 3hydroxy-3-methyl-glutaryl-CoA (3-HMG-CoA) from one mol ecul e of a cetyl -CoA, one mol ecul e of a cetoa cetyl -CoA, a nd a wa ter mol ecul e. Interes ti ngl y, a ny 3-HMG-CoA ma de i n the mi tochondri a i s us ed to produce ketone bodi es (s ee a bove), wherea s 3-HMG-CoA ma de i n the cytopl a s m forms chol es terol . 3-HMG-CoA i s converted to the s i x-ca rbon meva l ona te i n a rea cti on ca ta l yzed by the HMG-CoA reducta s e enzyme. Thi s s tep i n chol es terol s ynthes i s i s i rrevers i bl e a nd i s the commi tti ng s tep i n the pa thwa y. Thi s s tep i s a l s o the pri ma ry ta rget of medi ca l thera py for hi gh chol es terol di s ea s es (s ee Si deba r). The s i x-ca rbon mevalonate i s converted to the fi ve-ca rbon isopentenyl pyrophosphate through a s eri es of three rea cti ons tha t i nvol ve two mol ecul es of NADPH a nd one of ATP. Is opentenyl pyrophos pha te mol ecul es then joi n s ucces s i vel y together to form a 10-ca rbon, 15-ca rbon (farnesyl pyrophosphate), a nd fi na l l y a 30-ca rbon squalene mol ecul e. Thi s s tep a ga i n i nvol ves NADPH. At thi s poi nt, the growi ng mol ecul e i s s ti l l a l i nea r cha i n of ca rbon mol ecul es . For the fi na l s ta ge i n chol es terol s ynthes i s , termed “cycl i za ti on,” three s epa ra te s teps uti l i ze NADPH a nd one a tom of mol ecul a r oxygen (O2 ) to form new s i ngl e- a nd doubl e-ca rbon bonds , remove three methyl (CH 3 ) groups , a nd crea te a hydroxyl (OH −) group (Fi gure 7-9). The fi rs t cycl i c i ntermedi a te i s l a nos terol , a nd thi s i s converted to chol es terol vi a a compl ex s eri es of rea cti ons .
Figure 7-9. Cholesterol Synthesis. Summa ry of s teps i n the s ynthes i s of chol es terol (top). The mol ecul a r s tructures of meva l oni c a ci d a nd the fi na l chol es terol mol ecul e a re s hown i n the bottom pa nel . [Reproduced wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] Regul a ti on of new chol es terol s ynthes i s a nd, therefore, l evel s i n the huma n body rel y on HMG-CoA reductase, the enzyme ca ta l yzi ng the fi rs t a nd commi tti ng s tep of produci ng meva l ona te (Fi gure 7-9). Speci fi ca l l y, the a cti vi ty of thi s enzyme i s decrea s ed by hi gh concentra ti ons of meva l ona te, chol es terol , a nd i ts products . Decrea s ed HMG-CoA reducta s e a cti vi ty res ul ts from (a ) decrea s ed DNA to mes s enger RNA (mRNA) tra ns cri pti on, (b) l owered mRNA for protei n tra ns l a ti on, a nd (c) i ncrea s ed degra da ti on of exi s ti ng HMG-CoA reducta s e enzyme compl exes . The enzyme i s a l s o under di rect hormona l control wi th glucagon a nd epinephrine decrea s i ng a cti vi ty vi a phos phoryl a ti on, wherea s i ns ul i n i ncrea s es a cti vi ty by dephos phoryl a ti on. In rel a ti on to the hormona l control , l ow concentra ti ons of ATP decrea s e chol es terol s ynthes i s a s the body di rects i ts res ources towa rd energy producti on ra ther tha n l i pi d s ynthes i s . Statins: Statins a re a n i mporta nt cl a s s of medi ca ti ons , whi ch reduce chol es terol i n pa ti ents wi th hi gh chol es terol a nd hea rt di s ea s e. Sta ti ns work by i nhi bi ti ng the cri ti ca l a nd commi tti ng s tep i n chol es terol s ynthes i s , the enzyme HMG-CoA reductase, res pons i bl e for the convers i on of HMG-CoA i nto meva l ona te. Sta ti ns , whos e s tructure i s s i mi l a r to HMG-CoA, mi mi c the s ubs tra te a nd competi ti vel y i nhi bi t the enzyme. Decrea s ed producti on of meva l ona te a nd, therefore, chol es terol a l s o ca us es i ncrea s ed producti on of receptors for LDL chol es terol , the mos t ha rmful chol es terol -tra ns porti ng l i po-protei n, i ncrea s i ng cl ea ra nce of LDL from the bl oods trea m.
REVIEW QUESTIONS 1. Wha t a re the key fea tures of fa tty a ci d s ynthes i s a nd how i s i t regul a ted? 2. Wha t i s the rol e of fa tty a ci d s yntha s e i n fa tty a ci d s ynthes i s ?
3. How a re fa tty a ci ds wi th a n odd number of ca rbons s ynthes i zed? 4. How a re uns a tura ted fa tty a ci ds produced? 5. Wha t a re the functi ons of ca rni ti ne pa l mi toyl tra ns fera s e I a nd II (CPT I a nd II) a nd mi tochondri a l tri functi ona l protei n (MTP) i n the degra da ti on of fa tty a ci ds ? 6. Wha t a re the key fea tures of ketone body s ynthes i s a nd how do ketone bodi es contri bute to meeti ng energy dema nds i n the body? 7. Wha t a re the key fea tures a nd regul a ti on of tri a cyl gl ycerol , phos phogl yceri des , a nd cera mi de s ynthes i s ? 8. Wha t a re the key fea tures a nd regul a ti on of chol es terol s ynthes i s ? 9. Wha t rol e do HMG-CoA a nd HMG-CoA reducta s e pl a y i n chol es terol s ynthes i s ?
CHAPTER 8 MEMBRANES Membra ne Structure Membra ne Functi ons Membra ne Cha nnel s Membra ne Si gna l i ng Revi ew Ques ti ons
OVERVIEW Li pi ds , dri ven by thei r hydrophobi c a nd hydrophi l i c porti ons , form the bi ol ogi ca l membra nes found i n a l l l i vi ng crea tures . Thes e i ncl ude the cel l a nd nucl ea r membra nes a s wel l a s membra nes tha t a re pa rt of orga nel l es s uch a s mi tochondri a a nd the endopl a s mi c reti cul um (ER). Membra nes provi de s epa ra ti on of di fferent envi ronments to permi t a va ri ety of bi ol ogi ca l functi ons . Membra nes a re not s ta ti c s tructures , though, ra ther they a re dyna mi c a nd fl ui d a nd a l l ow s el ecti ve movement of i ons , energy s ources , vi ta mi ns a nd cofa ctors , a nd wa s te. Compl ex l i pi ds s uch a s chol es terol a nd s phi ngol i pi ds both a ffect the s tructure of membra nes where they a re found a nd a re a l s o i nvol ved i n s peci fi c functi ons . The va ri ety of l i pi d a nd protei n mol ecul es , whi ch ma ke up membra nes , a re res pons i bl e for es s enti a l functi ons s uch a s cha nnel s a nd tra ns port a cros s the membra ne a s wel l a s s i gna l i ng. Membra ne receptors , a l ong wi th thei r cytopl a s mi c pa rtners , tra ns mi t s i gna l s vi a s teroi d hormones . An i mporta nt type of membra ne receptors a re G-protei ns wi th i ntri ns i c enzyme a cti vi ty. The res ul ti ng s econda ry mes s enger mol ecul es a mpl i fy a nd tra ns mi t thi s s i gna l to va ri ous pa rts of the cytopl a s m a nd nucl eus . Thi s a bi l i ty to tra ns mi t a mes s a ge a cros s a membra ne i s pa ra mount to not onl y the norma l functi ons of cel l s but a l s o thei r a bi l i ty to perform s peci a l i zed functi ons , whi ch defi ne the huma n body. Not s urpri s i ngl y, l i pi ds a nd membra ne s tructure a nd functi on a re i mporta nt i n di s ea s e proces s es a s wel l a s trea tments . Any defi ci enci es or probl ems wi th l i pi d s ynthes i s or brea kdown l ea d to a s eri ous di s ea s e s ta te a nd/or dea th. Modul a ti on of membra ne fl ui di ty i s , i ts el f, i mporta nt i n membra ne functi ons a nd, a s a res ul t, a va ri ety of di s ea s es . For exa mpl e, mul ti pl e ba cteri a a nd vi rus es a re i nfecti ve a nd s evera l medi ca ti ons work s i mpl y beca us e thei r hydrophobi c na ture a ffects a nd di rectl y ta rgets l i pi ds a nd membra nes .
MEMBRANE STRUCTURE LIPIDS Membra nes a re compos ed of l i pi ds a rra nged i n a lipid bilayer, wi th the hydrophi l i c gl ycerol a nd phos pha te “hea d” groups of the l i pi d mol ecul es formi ng the two outs i de l a yers a nd the hydrophobi c “ta i l ” groups a rra nged i ns i de (Fi gure 8-1).
Figure 8-1. Simplified Representation of Lipid Bilayer/Membrane. The l i pi d bi l a yer i s compos ed of l i pi d mol ecul es , wi th hydrophi l i c hea d groups (e.g., phos pha te) formi ng the outer s urfa ces a nd hydrophobi c ta i l s grouped together i n the hydrophobi c center. See Cha pter 3 for more deta i l ed di s cus s i on of l i pi d mol ecul e s tructures . [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] A ma jori ty of membra ne l i pi ds a re phospholipids, us ua l l y wi th 16- or 18-ca rbon ta i l s , s ome s a tura ted a nd s ome uns a tura ted. Other types of l i pi ds , i ncl udi ng chol es terol , modul a te the s tructure a nd, therefore, the fl ui di ty of the membra ne a nd, i n turn, a ffect s ome membra ne functi ons . Pa rti cul a r l i pi ds a re found i n s peci fi c membra nes a nd a re res pons i bl e for s peci fi c functi on a s s umma ri zed bel ow: Cardiolipin (Diphosphatidylglycerol)—Severa l l oca ti ons i ncl udi ng the i nner mi tochondri a l membra ne. It ha s a nega ti ve cha rge a nd ma y be i nvol ved i n (a ) decrea s i ng bl ood cl ots , (b) s ta bi l i zi ng i mporta nt res pi ra tory cha i n enzymes i n the mi tochondri a , (c) movi ng protei ns a nd chol es terol from the outer to the i nner mi tochondri a l membra ne, (d) a s s i s ti ng i n the proper fol di ng of mi tochondri a l protei ns (Cha pter 9) a s a cha perone, a nd (e) pos s i bl y regul a ti ng deoxyri bonucl ei c a ci d (DNA) s ynthes i s . Phosphatidylserine (PS)—Loca ted i n the i nner pl a tel et membra ne a nd, duri ng a cti va ti on, the exteri or pl a tel et membra ne. It ca rri es a nega ti ve cha rge a nd i s the pri ma ry phos phol i pi d tha t promotes the a nti coa gul a nt protei n C pa thwa y, provi di ng feedba ck i nhi bi ti on of thrombi n forma ti on. Phosphatidylethanolamine (PE)—Loca ted i n both the i nteri or a nd exteri or of cel l membra nes . It ca rri es a neutra l cha rge a nd promotes the a nti coa gul a nt protei n C pa thwa y, but to a l es s er degree tha n PS. Phosphatidylcholine (PC)—Loca ted i n the i nteri or a nd exteri or of cel l membra nes . It ca rri es a neutra l cha rge a nd promotes the a nti coa gul a nt protei n C pa thwa y, but to a l es s er degree tha n PS. Phosphatidylinositol —Loca ted i n the i nteri or a nd exteri or of cel l membra nes a s wel l a s the nucl ea r membra ne. It ca rri es a pos i ti ve cha rge a nd promotes the a nti coa gul a nt protei n C pa thwa y, but to a l es s er degree tha n PS, a nd i s i mporta nt i n s i gna l i ng proces s es for i ni ti a ti on of DNA repl i ca ti on. Cardiolipin and Disease: Cardiolipin ha s been s ugges ted to pl a y s evera l i mporta nt rol es i n a va ri ety of ti s s ues , a nd defi ci enci es i n the producti on a nd modi fi ca ti on of thi s l i pi d, not s urpri s i ngl y, l ea d to s evera l huma n di s orders . Barth syndrome i s a n X-l i nked di s ea s e res ul ti ng from defects i n a n enzyme tha t remodel s ca rdi ol i pi n. The di s ea s e res ul ts i n decrea s ed phos phoryl a ti on of a denos i ne di phos pha te mol ecul es , pres uma bl y rel a ted to ca rdi ol i pi n’s propos ed rol e i n s ta bi l i zi ng res pi ra tory cha i n enzymes . Cha nges i n the a mount or compos i ti on of ca rdi ol i pi n i n hea rt a nd bra i n mi tochondri a ma y a l s o pl a y a rol e i n di s ea s es s uch a s heart failure, diabetes, Alzheimer’s disease, a nd Parkinson’s disease. Fi na l l y, Trepenoma pallidum, the ba cteri um res pons i bl e for the di s ea s e syphilis, produces a nti bodi es a ga i ns t ca rdi ol i pi n; the bi ol ogi ca l a nd di s ea s e i mpl i ca ti ons of thi s a re s ti l l not wel l unders tood.
The s ma l l er hea d groups of phos phol i pi ds s uch a s PS a nd PE a re preferred on the i nner s i de of the membra ne bi l a yer. On the other ha nd, PS i s often l oca ted on the outs i de of the bi l a yer where i t ca n i ncrea s e a dherence to other cel l s a nd ti s s ues . A membra ne-bound enzyme ca l l ed “flippase” ca ta l yzes the proces s of movi ng pa rti cul a r phos phol i pi d mol ecul es from one s i de of the bi l a yer to the other when requi red. Other phos phol i pi ds , beca us e of thei r hea d group s i ze a nd cha rges a s wel l a s the number a nd l oca ti on of thei r ta i l doubl e bonds , crea te a more fl ui d membra ne more a mena bl e to a rea s of cel l curva ture or cel l –cel l connecti ons (Fi gure 8-2).
Figure 8-2. Membrane Packing of Saturated and Unsaturated Lipid Tails. The l i pi d bi l a yer i s compos ed of l i pi d mol ecul es whos e ta i l groups ca n be s a tura ted (“S”) or uns a tura ted (“U”). Sa tura ted ta i l s pa ck cl os e together, wherea s uns a tura ted ta i l s ha ve a l oos er pa cki ng s tructure, l ea di ng to a more fl ui d l i pi d membra ne. Increa s ed a mounts of s a tura ted or uns a tura ted l i pi ds ca n ma rkedl y a ffect membra ne fl ui di ty a nd, therefore, the functi on of the membra ne a t tha t l oca ti on. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Compos i ti on of the pl a s ma membra ne a nd, therefore, fl ui di ty ha s been i mpl i ca ted i n s evera l di s ea s e proces s es , i ncl udi ng the fol l owi ng: 1. Duri ng s ta ges of i nfecti on wi th ma l a ri a , the compos i ti on of i nfected red bl ood cel l s i s ma rkedl y cha nged to mi mi c tha t of the i nfecti ng pa ra s i te. Thi s ma y a s s i s t i n the rupture of red bl ood cel l s a nd the s ubs equent s prea d of the pa ra s i tes tha t i s a ha l l ma rk of thi s di s ea s e. 2. Va ri ous ba cteri a a nd vi rus es a re bel i eved to cha nge the pl a s ma membra ne of ta rget cel l s to a l l ow the ba cteri a /vi rus to fus e a nd, therefore, i nfect the modi fi ed cel l s . 3. Huma n i mmunodefi ci ency vi rus (HIV) fus es wi th s peci fi c pa rts of a n i nfected cel l ’s membra ne s i mi l a r to i ts own vi ra l membra ne i n compos i ti on a nd fl ui di ty, a l l owi ng new vi ra l pa rti cl es to emerge a nd s prea d to other cel l s . Res ea rch ha s s hown tha t cha ngi ng membra ne compos i ti on of ta rget cel l s ma y a l l ow protecti on from HIV i nfecti on. 4. The pa ra s i te Entamoeba histolytica produces protei ns tha t bi nd to a nd form hol es i n huma n cel l membra nes , ki l l i ng thes e ta rget cel l s . However, the pa ra s i te membra ne ha s a ma rkedl y di fferent membra ne compos i ti on, i ncl udi ng di fferent phos phol i pi ds a nd a hi gh l evel of chol es terol , whi ch protects i ts el f from thes e dea dl y protei ns . 5. Membra ne compos i ti on i s ma rkedl y cha nged i n the pl a tel ets of pa ti ents wi th frequent mi gra i nes . Al though the mecha ni s m i s not known, thes e cha nges ma y be pa rt of the ca us e of mi gra i ne hea da ches a nd a better unders ta ndi ng coul d, therefore, l ea d to a novel trea tment. 6. Vi rgi n ol i ve oi l a l ters the compos i ti on of pl a s ma membra nes a nd a ffects regul a tory protei ns i n the l i pi d bi l a yer, whi ch res ul ts i n benefi ci a l cha nges i n ca rbohydra te a nd l i pi d meta bol i s m. The pres ence a nd a mount of chol es terol a l s o i nfl uence the fl ui di ty of membra nes . Chol es terol ’s hydroxyl group rea di l y i ntera cts wi th phos phol i pi d hea d groups or exteri or wa ter mol ecul es a nd the rema i ni ng hydrophobi c tetra cycl i c s tructure a nd “ta i l ” a l s o na tura l l y i ns ert i ns i de the l i pi d bi l a yer (Fi gure 8-3). Hydrogen bondi ng s ta bi l i zes thes e i ntera cti ons . Dependi ng on the a dja cent l i pi ds , chol es terol ei ther i ncrea s es or decrea s es the l i pi d ta i l pa cki ng. Loca l i zed membra ne “ra fts ” conta i ni ng hi gh l evel s of chol es terol ha ve been noted by res ea rchers a nd a re fel t to di rectl y i nfl uence not onl y membra ne functi ons but a l s o the functi ons of the membra ne protei ns wi thi n them. Polyene Antifungals: Anti funga l medi ca ti ons i ncl udi ng Nystatin a nd Amphotericin B* (s ee the fi gures bel ow) s ha re the s tructura l ba s i s of the pol yene mol ecul e (i .e., mul ti pl e ca rbon=ca rbon doubl e bonds ) oppos i te a n a ddi ti ona l ca rbon cha i n conta i ni ng mul ti pl e hydroxyl groups (OH). Thi s s tructure enha nces bi ndi ng to ergosterol, a chol es terol -l i ke, compl ex l i pi d found i n funga l cel l membra nes . Bi ndi ng of thes e a nti funga l s to ergos terol decrea s es membra ne fl ui di ty a nd l ea ds to l ea ka ge of the i nner contents a nd funga l cel l dea th. Huma n cel l pl a s ma membra nes a re not a ffected bea cus e they do not conta i n ergos terol , a nd the a nti funga l s tructure does not bi nd to chol es terol .
[Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.]
[Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] *Amphotericin B is well known for a variety of serious and even fatal side effects, which are believed to be due to mechanisms other than its effects on fungal membrane fluidity.
Figure 8-3. Cholesterol Interaction with Membrane Lipids. Il l us tra ti on of the pos i ti oni ng of chol es terol wi thi n the l i pi d membra ne. The hydroxyl group on one end of the mol ecul e prefers to be nea r the pol a r, l i pi d hea d groups of the externa l a queous envi ronment, wherea s the rema i nder of the chol es terol mol ecul e prefers to be wi thi n the hydrophobi c l i pi d ta i l s . [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
PROTEINS Protei ns a re the s econd ma jor pa rt of bi ol ogi ca l membra nes a nd ma ke up a pproxi ma tel y 20%–80% of both the s tructura l a nd functi ona l components of thes e membra nes . Membra ne protei ns a re cl a s s i fi ed a s peripheral—predomi na tel y on the i nner or the outer s urfa ce of the membra ne bi l a yer—a nd integral—predomi na tel y wi thi n the membra ne (Fi gure 8-4). Peri phera l membra ne protei ns a re bel i eved to ma i nl y a ct a s a nchor poi nts for a tta chment to externa l s tructures (e.g., the extra cel l ul a r ma tri x) a nd i nterna l poi nts (e.g., the a cti n, mi crotubul e, a nd i ntermedi a te fi l a ment cytoma tri x; Cha pter 12). Integra l protei ns a re us ua l l y compos ed of uncha rged, hydrophobi c a mi no a ci ds s o they ca n enter a nd s ta y i n the hydrophobi c envi ronment of the l i pi d bi l a yer. Integra l protei ns s erve s evera l functi ons i ncl udi ng cha nnel s , “ca rri er protei ns ” (tra ns porti ng mol ecul es through the membra ne), or a s a s i gna l i ng protei n (cha ngi ng a porti on of thei r i nterna l cytopl a s mi c s tructure i n res pons e to a s i gna l a t a n expos ed externa l pa rt of thei r s tructure to a cti va te a ddi ti ona l , cl os el y a s s oci a ted mol ecul es , l ea di ng to a cha nge of functi on, e.g., turni ng on a gene).
Figure 8-4. Peripheral and Integral Membrane Proteins. The rel a ti ve pos i ti ons of peri phera l a nd i ntegra l membra ne protei ns i n the l i pi d bi l a yer a re i l l us tra ted. Membra ne protei ns ca n a l s o conta i n ca rbohydra tes res ul ti ng i n membra ne gl ycoprotei ns i mporta nt i n membra ne s i gna l i ng a nd functi ons . The pres ence a nd pos i ti oni ng of chol es terol a re a l s o i l l us tra ted. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] As a l rea dy i l l us tra ted, the membra ne content of chol es terol a nd s a tura ted/uns a tura ted l i pi ds i mpa cts membra ne fl ui di ty. Membra ne protei ns form connecti ons to the extra cel l ul a r ma tri x, the i nterna l cytoma tri x, a nd even other cel l s tha t ca n a l l ow pa rti cul a r protei ns a nd l i pi ds to col l ect a t s peci a l a rea s of a bi ol ogi ca l membra ne. Thus , a cel l ca n orches tra te a pa rti cul a r “a rea ” or “s i de” where i t ca n recei ve s i gna l s a nd/or tra ns port pa rti cul a r mol ecul es both i n a nd out a nd/ or other es s enti a l functi ons . Thes e s peci a l i zed “doma i ns ” wi thi n membra nes a re ca l l ed “l i pi d ra fts ” (Fi gure 8-5). Va ri ous exa mpl es of thes e membra ne protei n functi ons a re di s cus s ed bel ow.
Figure 8-5. Diagram of Lipid Raft. Li pi d ra fts a re s omewha t thi cker a nd conta i n hi gher a mounts of s peci a l ty l i pi ds [e.g., s phi ngomyel i n, ga ngl i os i des (not s hown), s a tura ted phos phol i pi ds (non zi g-za g ta i l s ), a nd chol es terol ]. Membra ne protei ns ca n a l s o conta i n ca rbohydra tes res ul ti ng i n membra ne gl ycoprotei ns i mporta nt i n membra ne s i gna l i ng a nd functi ons . [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Hemoglobinopathies: Di s ea s es of red bl ood cel l s , known col l ecti vel y a s “hemoglobinopathies,” often res ul t from cha nges i n l i pi d compos i ti on a nd/or defects i n protei ns a s s oci a ted wi th the red cel l membra ne. Elliptocytosis a nd spherocytosis (s ee the Fi gure) a re two s uch di s ea s es tha t res ul t from defects i n s pectri n a nd a nkyri n a nd other red cel l membra ne protei ns i mporta nt i n the s ta bi l i za ti on of the norma l bi conca ve s ha pe of red bl ood cel l s .
Normal erythrocytes surrounding elliptocyte/spherocyte shown in the center [Courtes y of Dr. Wa l ter Kemp, Monta na Sta te Depa rtment of Jus ti ce] Hemogl obi nopa thi es i nvol vi ng cha nges i n l i pi d compos i ti on of the red cel l membra ne a re a l s o common. Red bl ood cel l s ca reful l y ma i nta i n a s peci fi c, a s ymmetri c di s tri buti on of hundreds of phos phol i pi ds a nd protei ns i n the l i pi d bi l a yer. Speci fi c defects i n ei ther number or bi l a yer compos i ti on of PS, PC, s phi ngomyel i n, a nd chol es terol a re s een i n di s ea s es s uch a s s i ckl e cel l , tha l a s s emi a s , a nemi a s , a nd l i ver di s ea s e. Thes e cha nges a l s o l ea d to des tructi on or remova l of thes e bl ood cel l s a s pa rt of the di s ea s e proces s es .
MEMBRANE FUNCTIONS A pri ma ry functi on of bi ol ogi ca l membra nes i s to ma i nta i n the s tructura l i ntegri ty a nd the i ndi vi dua l functi ons of cel l s , the nucl eus , a nd orga nel l es . Sepa ra ti on of envi ronments a l l ows di fferences i n concentra ti on of i ons a nd mol ecul es tha t l i tera l l y a l l ow l i fe to exi s t. As exa mpl es , i f bi ol ogi ca l mol ecul es s uch a s ca rbohydra tes pa s s ed ea s i l y through membra nes , cel l s coul d not perform a ny s i gni fi ca nt bi ol ogi ca l functi ons . If cel l s were una bl e to s el ecti vel y i ncrea s e a nd ma i nta i n the concentra ti on of certa i n i ons on the i ns i de a nd outs i de of the membra ne, they coul d not ma i nta i n thei r requi red i nterna l envi ronment or perform functi ons s uch a s conducti on of a nerve i mpul s e. However, the fa ct tha t mol ecul es s uch a s oxygen, ca rbon di oxi de, ni trogen, a nd urea a re a bl e to move through the l i pi d bi l a yer wi thout a ny s peci a l i zed tra ns port protei ns i s a ctua l l y benefi ci a l to the huma n body (Fi gure 8-6A). Speci a l ci rcums ta nces of nons el ecti ve tra ns port of mol ecul es ha ve a l s o been devel oped. For exa mpl e, beca us e of i ts rel i a nce on ketones a s a n energy s ource i n ti mes of l ow ca rbohydra te l evel s , bra i n membra nes a l l ow free fl ow of ketone bodi es wi thout the need of a tra ns port protei n.
Figure 8-6. A-D. Illustration of Membrane Transport Proteins. Si mpl e di ffus i on through (A) l i pi d bi l a yer or (B) s i mpl e cha nnel . Fa ci l i ta ted di ffus i on vi a a (C) cha nnel a nd (D) a cti ve tra ns port ei ther i nto or out of the cel l . See the text for further des cri pti on. E. Na+–K + ATPase Membrane Channel. Three s odi um i ons (Na +) i ns i de the cel l bi nd to the Na +–K+ ATPa s e pump, whi ch promotes phos phoryl a ti on of the protei n by a bound a denos i ne tri phos pha te (ATP) mol ecul e. Thi s phos phoryl a ti on ca us es a conforma ti ona l cha nge of the pump, whi ch ca rri es a nd rel ea s es the Na + to the outs i de of the cel l . Adenos i ne di phos pha te (ADP) i s rel ea s ed from the protei n a nd two pota s s i um i ons (K+) bi nd to the new conforma ti on of the pump. Bi ndi ng of K+ promote dephos phoryl a ti on of the protei n, ca us i ng a revers e conforma ti ona l cha nge, whi ch ca rri es a nd rel ea s es the K+ to the i ns i de of the cel l . ATP rebi nds to the pump, whi ch a l s o hel ps a ccel era te rel ea s e of the K+, returni ng the pump to i ts ori gi na l s ta te. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
MEMBRANE CHANNELS Protei ns tha t crea te membrane channels often ha ve mul ti pl e α-hel i ca l a nd/or β-s tra nd s econda ry s tructures tha t form a tube-l i ke cha nnel through the membra ne. Hydrophobi c a mi no a ci ds on the outs i de of the protei n’s terti a ry a nd qua terna ry s tructure (Cha pter 1) form a n exteri or tha t rea di l y i ns erts i ts el f i nto the membra ne a nd s ta ys wi thi n the hydrophobi c l i pi d ta i l s . Hydrophi l i c or cha rged a mi no a ci ds on the i ns i de of the cha nnel form a s ui ta bl e pa s s a gewa y tha t a l l ows cha rged a nd/or uncha rged mol ecul es to move through the previ ous l y i mpedi ng l i pi d membra ne. Simple channels (Fi gure 8-6B) ha ve no a cti ve control over the mol ecul es tha t ca n enter through them except by the di a meter of the tube a nd, s ometi mes , the exa ct hydrophobi c/ hydrophi l i c envi ronment i n the i nteri or of the tube. Si mpl e cha nnel s , therefore, onl y work vi a a concentra ti on gra di ent tha t provi des the dri ve for mol ecul es to cros s the membra ne. An exa mpl e of a s i mpl e cha nnel i s the gap junction tha t crea tes a pa s s a gewa y i n the ga p between two cel l s a l l owi ng movement of i ons , s uga rs , a mi no a ci ds , a nd nucl eoti des between the cel l s . Ga p juncti ons a re i mporta nt i n orga ns s uch a s the hea rt, bone a nd eye l ens , a nd i n devel opi ng ti s s ues . Facilitated protein channels (Fi gure 8-6C) a re s i mi l a r to s i mpl e cha nnel s (no energy i s us ed, dri ven by a concentra ti on gra di ent) but the tra ns port i s s ped up or fa ci l i ta ted from pa rti cul a r a mi no a ci ds tha t hel p to form the cha nnel . Thes e cha nnel s ca n a l s o ha ve wel l -pos i ti oned a mi no a ci ds (often cha rged) i n the tube tha t s erve a s a “ga te” to regul a te the movement of s peci fi c mol ecul es through the cha nnel . Thes e gated channels (not s hown) ha ve a mi no a ci ds tha t bl ock tra ns port but a s i gna l (e.g., bi ndi ng of a s peci fi c mol ecul e or a cha nge i n vol ta ge)
ca us es the protei n to cha nge conforma ti on, movi ng the ga tekeeper a mi no a ci d(s ) out of the wa y. Thes e a mi no a ci ds ca n be s peci fi ca l l y hydrophobi c or hydrophi l i c or even cha rged (Cha pter 1), dependi ng on the mol ecul e to be tra ns ported. Ionophore Antibiotics/Antifungals: The i mporta nt functi on of membra ne cha nnel s i n ma i nta i ni ng the correct ba l a nce of va ri ous i ons i ns i de a nd outs i de of cel l s ha s been uti l i zed i n the devel opment of a n i mporta nt cl a s s of a nti ba cteri a l a nd a nti funga l medi ca ti ons —the “ionophores.” Thi s cl a s s of drugs i ncl udes pepti de- or l i pi d-ba s ed mol ecul es tha t i ns ert i nto bi ol ogi ca l membra nes . Al though mecha ni s ms di ffer s l i ghtl y, a l l di s rupt the i oni c ba l a nce of the ta rget cel l by a l l owi ng unregul a ted or a bnorma l movement or permea bi l i ty of i ons , l ea di ng to cel l dea th. The s tructure of certa i n i onophores l ends i ts el f to s el ecti ve i ns erti on i nto onl y ba cteri a l or yea s t cel l s wi th l i mi ted or no effect on huma n cel l s , thereby a l l owi ng trea tment of the i nfecti on wi thout a dvers e effects on the huma n pa ti ent. Active transport (Fi gure 8-6D) i nvol ves a ca rri er protei n tha t undergoes a conforma ti ona l cha nge vi a the rel ea s e of energy or phos phoryl a ti on from nucl eoti de mol ecul es . Acti ve tra ns port ca n move mol ecul es i nto or out of a cel l . A pri me exa mpl e of a cti ve tra ns port i s the sodium– potassium ATPase (Na+–K +-ATPase) pump (Fi gure 8-6E). The Na +–K+ ATPa s e pump hel ps to es ta bl i s h a hi gh concentra ti on of Na + outs i de a nd a hi gh concentra ti on of K+ i ns i de a cel l . Thi s Na +–K+ gra di ent i s requi red for nerve a nd mus cl e functi on a s wel l a s to dri ve other cha nnel s to tra ns port ca rbohydra tes a nd a mi no a ci ds a nd nutri ents i nto cel l s . Proper Na + a nd K+ cel l ul a r concentra ti ons a l s o hel p to ma i nta i n the correct cel l vol ume beca us e thes e i ons a ffect the movement of wa ter mol ecul es i nto a nd out of cel l s . Other mol ecul a r pumps wi l l be di s cus s ed i n l a ter cha pters . Membrane Channels and Channel Blockers: Membra ne cha nnel s of a l l types a re found throughout the huma n body performi ng a wi de va ri ety of functi ons . Thes e cha nnel s a re pri me ta rgets for the devel opment of va ri ous medi ca ti ons to trea t di s ea s es a s s oci a ted wi th membra ne cha nnel functi on a nd/or dys functi on. One exa mpl e i s ca l ci um (Ca 2+) cha nnel s , whi ch a re i mporta nt i n the ra te a nd force of hea rt contra cti on a s wel l a s contra cti on/rel a xa ti on of s mooth mus cl e i n bl ood ves s el s . Pa ti ents wi th hi gh bl ood pres s ure a re often trea ted by a cl a s s of medi ca ti ons known a s calcium channel blockers (CCBs), ea s i l y i denti fi ed by the s uffi x “-di pi ne.” CCBs decrea s e the tota l tra ns port of Ca 2+ i nto mus cl e cel l s , thereby decrea s i ng hea rt contra cti on (ra te a nd force) a nd i ncrea s i ng the di a meter of bl ood ves s el s l ea di ng to l ower bl ood pres s ure. Histamine (H 2 ) receptors a nd the H +–K + ATPase proton pump a re both res pons i bl e for the tra ns port of a ci di c hydrogen i ons (H +) i nto the s toma ch to a i d di ges ti on. In di s ea s es s uch a s hea rtburn, ga s tri ti s , pepti c ul cer di s ea s e, refl ux di s ea s e, Ba rrett’s es opha gi ti s , ga s tri noma s , a nd Zol l i nger–El l i s on s yndrome, i t i s hel pful to l ower the a mount of a ci d content i n the s toma ch. In ea ch of thes e ca s es , s peci fi c H 2 blockers wi th the s uffi x “ti di ne” a nd proton pump inhibitors, endi ng wi th the s uffi x “pra zol e,” bl ock H + tra ns port. A “ca rri er protei n” ca n a l s o fa ci l i ta te the movement of l a rge or cha rged mol ecul es through l i pi d membra nes . In thi s i ns ta nce, the cha rged mol ecul e to be tra ns ported bi nds to the membra ne protei n a nd, vi a a conforma ti ona l cha nge, i s s urrounded by the ca rri er protei n’s hydrophobi c exteri or. Thi s ca rri er protei n ca n then cros s from one s i de of the l i pi d bi l a yer to the other wi th i ts mol ecul a r pa s s enger s a fel y s hi el ded from the l i pi d ta i l groups onl y to emerge on the other s i de where i t ca n rel ea s e the mol ecul e i nto a fri endl i er hydrophi l i c envi ronment. Aquaporins: Aquaporins a re a fa mi l y of i ntegra l membra ne protei ns found i n the ki dney a nd other orga ns , whi ch hel p to regul a te wa ter fl ow i nto a nd out of cel l s . Defects of one of thes e cha nnel protei ns aquaporin-2 (AQP2) ca us e exces s i ve l os s of wa ter i n s ome pa ti ents wi th nephrogenic diabetes insipidus or i ncrea s ed wa ter retenti on i n pregnancy a nd congestive heart failure. Us e of the ps ychi a tri c drug l i thi um ca n i nduce probl ems by decrea s i ng the a mount of AQP2, wherea s the pepti de hormone va s opres s i n works vi a i ncrea s i ng AQP2 concentra ti ons i n membra nes .
MEMBRANE SIGNALING Ma ny i ntegra l membra ne protei ns do not form a cha nnel or phys i ca l l y move through the membra ne but, i ns tea d, tra ns mi t a mes s a ge (s i gna l ) from one s i de of the l i pi d bi l a yer to the other. The signaling functi on of membra ne protei ns i s s een frequentl y a nd i s va s tl y i mporta nt i n huma n bi ol ogy i n mul ti pl e cel l types . Thes e membra ne protei ns functi on by a cha nge i n thei r own conforma ti on from externa l bi ndi ng of a mol ecul e, phos phor-yl a ti on of one of i ts a mi no a ci ds , a nd/or then i ntera cti on wi th other protei ns , a nd s o on. Thi s conforma ti ona l cha nge a ffects the i nterna l porti on of the s i gna l i ng protei n a nd ca n then a cti va te i nterna l protei ns , l ea di ng to a s el ected functi on(s ). There a re s evera l di fferent types of membra ne protei n s i gna l i ng, s umma ri zed i n Ta bl e 8-1.
TABLE 8-1. Summa ry of Membra ne Si gna l i ng Receptors The fi rs t exa mpl e of s i gna l i ng mol ecul es i n Ta bl e 8-1 a re the steroid hormones (s ee Cha pter 3) a nd ca n be further di vi ded i nto fi ve ca tegori es , dependi ng on thei r pa rti cul a r receptor—androgens, estrogens, glucocorticoids, mineralocorticoids, a nd progestagens (precurs ors of a va ri ety of proges terones ). Vi ta mi n D, a l though not techni ca l l y a s teroi d, tra ns duces s i gna l s s uch a s s teroi d hormones a nd i s , therefore, us ua l l y i ncl uded wi th thi s group. The s teroi d hormones work i n two di fferent wa ys (Fi gure 8-7). Fi rs t, the hormone ma y bi nd to a membra ne s urfa ce receptor, whi ch then ei ther a cti va tes other s i gna l i ng pa thwa ys i ns i de the cel l or moves from the membra ne i nto the nucl eus to a cti va te DNA tra ns cri pti on fa ctors . Second, a nd more commonl y, s teroi ds a re l i pi d s ol ubl e a nd ca n, therefore, pa s s ea s i l y through the cel l membra ne wi thout the a i d of a membra ne receptor. Once i ns i de the cel l , the hormone ca n bi nd wi th cytopl a s mi c receptors to a cti va te s i gna l i ng or ma y conti nue to receptors i n the nucl eus to a cti va te tra ns cri pti on fa ctors a nd DNA s ynthes i s . Steroi d hormones tha t a ffect DNA s ynthes i s a re ca l l ed genomic (i .e., a ffect genes), wherea s thos e tha t do not a ffect tra ns cri pti on a re referred to a s nongenomic.
Figure 8-7. Activation by Steroid Hormone Receptor. A l i pi d-deri ved s teroi d hormone tra vers es the membra ne to bi nd to a cytopl a s mi c receptor (bottom l eft), a nd the hormone–receptor compl ex enters the nucl eus a s a homodi mer (upper). Al terna ti vel y, the unbound hormone ma y tra vers e through the nucl ea r membra ne wi th s ubs equent bi ndi ng to a hormone receptor to form a homodi mer i n the nucl eus . Ei ther
mecha ni s m l ea ds to bi ndi ng to s peci fi c pa rts of deoxyri bonucl ei c a ci d (DNA) known a s s teroi d res pons e el ements a nd a cti va ti on of tra ns cri pti on of s el ected DNA. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] One of the mos t predomi na nt a nd bes t unders tood membra ne s i gna l i ng mecha ni s ms i s vi a the G-protein fa mi l y (s ee Ta bl e 8-1). Al l of thes e receptors rel y on the conforma ti ona l cha nge res ul ti ng from the excha nge/convers i on of the nucl eoti des gua nos i ne di phos pha te (GDP) a nd gua nos i ne tri phos pha te (GTP) to convey a n externa l s i gna l to the i ns i de of the cel l . G-protei ns a re a l l compos ed of three i nterna l peri phera l protei n s ubuni ts α-, β-, a nd γ-s ubuni ts , whi ch a s s oci a te wi th a n i ntegra l membra ne protei n receptor. The α- a nd β-s ubuni ts a re cl os el y bound a nd a re repres ented a s the di mer α/γ. G-protei ns ca n be di vi ded i nto fi ve cl a s s es , Gs , Gi, Gq, G12/13 , a nd Gt, dependi ng on di fferences i n the αs ubuni t, whi ch a ffects i ntera cti on wi th the externa l s i gna l i ng mol ecul e (Ta bl e 8-2). Des pi te the di fferences i n the α-s ubuni t a nd effects a mong the fi ve cl a s s es , the genera l s i gna l i ng mecha ni s m i s a l mos t i denti ca l a nd i s s umma ri zed i n Fi gure 8-8 a nd a s fol l ows :
Figure 8-8. Function of G-proteins. The l i ga nd ( ) bi nds to i ts receptor, whi ch i ntera cts wi th the i na cti ve α/β/γ compl ex wi th bound GDP (l eft) a nd ca us es a n excha nge of GTP for the GDP on the α-s ubuni t wi th di s s oci a ti on of the β- a nd γ-s ubuni ts (ri ght). Thes e s ubuni ts ca n then move wi thi n the membra ne a nd s erve a s “effectors ” to a cti va te other protei n s i gna l i ng pa thwa ys . GTP ca n be hydrol yzed ba ck to GDP vi a a GTPa s e ca us i ng re-a s s oci a ti on of the α–β–γ compl ex a nd termi na ti on of the s i gna l . [Reproduced wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.]
TABLE 8-2. G-Protei n Receptor Cl a s s es 1. The α-s ubuni t wi th a bound GDP i ntera cts wi th the i nterna l porti on of the receptor a s wel l a s the β/γ-s ubuni ts . 2. Upon bi ndi ng of the s i gna l i ng protei n to the exteri or of the receptor, a GTP mol ecul e di s pl a ces the GDP on the α-s ubuni t, ca us i ng the di s s oci a ti on of the β-s ubuni t from the β/γ-s ubuni ts . 3. The freed α-s ubuni t ca n then i ntera ct wi th other membra ne-bound protei ns (effectors), l ea di ng to thei r a cti va ti on (G s , G q, G 12/13 , a nd G t) or i nhi bi ti on (G i). Increa s i ng evi dence a l s o s ugges ts tha t the β/γ-s ubuni ts ca n a cti va te di fferent effector protei n s i gna l i ng pa thwa ys (e.g., L-type Ca 2+ cha nnel s i nvol ved i n va ri ous CCB medi ca ti ons ). 4. The α-s ubuni t’s i nherent GTPa s e a cti vi ty, often enha nced by a n a cces s ory GTPa s e a cti va ti ng protei n or s ometi mes by the effector protei ns , eventua l l y converts GTP ba ck to GDP, a l l owi ng thi s s ubuni t to rea s s oci a te wi th the β- a nd γ-s ubuni ts , thereby turni ng the a cti va ti on off. The va ri ous G-protei ns a cti va te s evera l i mporta nt membra ne protei ns , whi ch l ea d to the conveya nce of the s i gna l vi a second messenger molecules. Exa mpl es i ncl ude cyclic adenosine monophosphate (cAMP) (Fi gure 8-9A), genera ted by adenyl cyclase (a l s o known a s a denyl yl or a denyl a te cycl a s e), whi ch l ea ds to phos phoryl a ti on of a ny of s evera l mol ecul es by protein kinase A a nd (Fi gure 8-9B) cl ea va ge of the membra ne l i pi d phosphatidylinositol 4,5-bisphosphate by phospholipase C to form diacylglycerol a nd inositol triphosphate (IP3 ). IP3 s ubs equentl y l ea ds to the
rel ea s e of Ca 2+ genera ted from i nterna l cel l ul a r s tores (us ua l l y i ns i de the ER) by s ubs equentl y a cti va ted i on cha nnel s a nd/or enzymes . Thes e s econd mes s engers l ea d to mul ti pl e membra ne, cytopl a s mi c, a nd nucl ea r effects . For exa mpl e, the ca l modul i n ki na s es a cti va te myos i n mol ecul es ca us i ng mus cl e contra cti on, hel p to regul a te s ecreti on of neurotra ns mi tter mol ecul es , regul a te a va ri ety of tra ns cri pti on fa ctors , modul a te ca rbohydra te s tora ge a nd us e, a nd ma y be es s enti a l to bra i n functi on. Protein kinase C i s known to cha nge membra ne s tructure, regul a te tra ns cri pti on a nd cel l growth, a s s i s t i n i mmune res pons es , a nd provi de key a cti va ti on of protei ns i nvol ved i n l ea rni ng a nd memory. Importa ntl y, the mul ti s tep, s i gna l i ng proces s a l l ows s el ected a mpl i fi ca ti on of a s ma l l s i gna l a t the exteri or of the cel l membra ne i nto a potenti a l l y l a rge res pons e wi thi n the cel l .
Figure 8-9. A. Cyclic adenosine monophosphate (cAMP) Signaling. Acti va ti on of a denyl yl cycl a s e by the α-s ubuni t res ul ts i n the convers i on of a denos i ne tri phos pha te (ATP) to cAMP whi ch then a cti va tes protei n ki na s e A a nd phos phoryl a ti on of va ri ous s i gna l i ng protei ns tha t el i ci t phys i ol ogi ca l effects s uch a s the expres s i on of s peci fi c genes . Sti mul a tory l i ga nds a cti va te vi a G s protei ns , wherea s i nhi bi tory l i ga nds a ct vi a G i protei ns . B. Phospholipase C–Protein Kinase C (PKC) Signaling. Acti va ti on of phos phol i pa s e C, us ua l l y vi a the α-s ubuni t of the G-protei n G q, res ul ts i n the cl ea va ge of the membra ne l i pi d phos pha ti dyl i nos i tol 4,5-bi s phos pha te (PIP2 ) i nto di a cyl gl ycerol (DAG) a nd i nos i tol tri phos pha te (IP3 ). Hydrophi l i c IP3 l ea ves the membra ne a nd enters the cytopl a s m to rel ea s e ca l ci um (Ca 2+) from the endopl a s mi c reti cul um (ER). Ca 2+ s ubs equentl y a cti va tes Ca 2+ bi ndi ng protei ns (Ca BP) l ea di ng to phys i ol ogi ca l effects s uch a s a cti va ti on of enzymes a nd/or expres s i on of s peci fi c gene products . Hydrophobi c DAG rema i ns i n the membra ne a nd ca n a cti va te PKC l ea di ng to s epa ra te phos phoryl a ti on of protei ns a nd res ul ti ng phys i ol ogi ca l effects . Ca 2+ i s a l s o requi red to s ynergi s ti ca l l y ma xi mi ze thi s effect of DAG on PKC. C. Janus Kinase and Signal Transducer and Activator of Transcription (JAK–STAT) Signaling. The l i ga nd bi nds to the monomeri c form of the receptor (1), l ea di ng to di meri za ti on a nd cros s -
l i nki ng of the two receptors . Ea ch JAK cros s -phos phoryl a tes (s ee a rrows ) the tyros i ne res i due of other JAK, l ea di ng to thei r a cti va ti on (2). The a cti va ted, di meri c JAKs cros s -phos phoryl a te (s ee a rrows ) tyro-s i ne res i dues on the other receptor (3). The phos phoryl a ted receptors now conta i n a n a ppropri a te s i te for bi ndi ng wi th a number of other s i gna l i ng mol ecul es s uch a s STAT (note s emi ci rcul a r bi ndi ng pockets on STAT mol ecul es ), l ea di ng to STAT phos phoryl a ti on (4). The phos phoryl a ted, di meri c STAT mol ecul es s ubs equentl y ma y tra vel to the nucl eus a nd bi nd to res pons e el ements i n deoxyri bonucl ei c a ci d (DNA) wi th propa ga ti on of the i ni ti a l s i gna l by the expres s i on of certa i n genes (5). [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] The fi na l group of membra ne receptors (s ee Ta bl e 8-1) i s the “s ol ubl e receptor-a s s oci a ted tyros i ne ki na s es ,” i ncl udi ng the receptor tyrosine kinases, Janus kinases, a nd integral guanyl cyclase (a l s o known a s gua nyl yl or gua nyl a te cycl a s e) receptors (Fi gure 8-9C). Al l three receptors ha ve the a bi l i ty, a fter a cti va ti on by bi ndi ng of the s i gna l i ng mol ecul e, to ca rry out a s peci fi c enzyma ti c functi on (i .e., phos phoryl a ti on of i ts own s el ected tyros i ne res i dues , producti on of cycl i c gua nos i ne monophos pha te). Thes e enzyma ti c cha nges then l ea d to a cti va ti on of a s i gna l i ng pa thwa y s peci fi c for the s i gna l i ng mol ecul e.
REVIEW QUESTIONS 1. Wha t a re the ba s i c s tructure, key fea tures , a nd i mporta nt components of the l i pi d bi l a yer a nd bi ol ogi ca l membra nes ? 2. How do s peci fi c phos phol i pi ds a nd chol es terol mol ecul es a ffect the functi oni ng of bi ol ogi ca l membra nes ? 3. Wha t i s mea nt by peri phera l a nd i ntegra l membra ne protei ns a nd wha t functi ons ca n they pl a y? 4. How woul d you des cri be s i mpl e, ga ted, a nd ATPa s e-dependent membra ne cha nnel s a nd wha t i s thei r s i gni fi ca nce? 5. Wha t i s the s i gni fi ca nce of s i gna l i ng protei ns , ca rri er protei ns , a nd s econd mes s enger mol ecul es ? 6. Wha t a re the ma jor cl a s s es of receptors a nd s econd mes s enger mol ecul es ?
CHAPTER 9 DNA/RNA FUNCTION AND PROTEIN SYNTHESIS Structure of the Nucl eus DNA Repl i ca ti on a nd Tra ns cri pti on Protei n Synthes i s Pos ttra ns l a ti ona l Tra ffi cki ng/Modi fi ca ti on Control of Gene Expres s i on Muta ti ons a nd Repa i r Mecha ni s ms Regul a ti on of Cel l Growth a nd Di fferenti a ti on Revi ew Ques ti ons
OVERVIEW The nucl eus i s often repres ented a s a rel a ti vel y empty s tructure, conta i ni ng onl y deoxyri bonucl ei c a ci d (DNA) bei ng repl i ca ted a nd tra ns cri bed a l ong wi th a few a cces s ory mol ecul es to hel p i n the proces s . To the contra ry, the nucl eus i s a ctua l l y a hi ghl y orga ni zed, membra ne-bound s tructure tha t i s l i tera l l y fi l l ed wi th protei ns , nucl eoti des , ca rbohydra tes , a nd l i pi ds wi th mul ti pl e functi ons . Va ri ous protei ns a re i nvol ved, a l ong wi th the nucl ea r membra ne, i n the orga ni za ti on of chromos omes , whi ch a l s o hel ps to regul a te the proces s es of DNA repl i ca ti on a nd tra ns cri pti on, a nd, s ubs equentl y, protei n s ynthes i s . Other protei ns di rectl y i nfl uence the expres s i on of genes vi a di rect i ntera cti ons wi th s peci fi c nucl eoti de s equences . Pos ttra ns l a ti ona l modi fi ca ti ons a ffect both protei n functi on a nd di rect pa rti cul a r protei ns to i ntra cel l ul a r a nd/or extra cel l ul a r des ti na ti ons .
STRUCTURE OF THE NUCLEUS The nucl eus i s s urrounded by a doubl e membra ne ca l l ed the nuclear membrane (a l s o known a s the nucl ea r envel ope), wi th the outer l a yer bei ng conti nuous wi th the endopl a s mi c reti cul um (ER) i n the cytopl a s m a nd, l i ke the ER, conta i ni ng ri bos omes a nd newl y s ynthes i zed protei ns . The i nner a nd outer nucl ea r membra nes a re a l s o conti nuous a t the s i tes of nuclear pores. Approxi ma tel y 2000 nuclear pores a re conta i ned wi thi n the nucl ea r membra ne, a nd ea ch pore ca n a l l ow movement of a bout 1000 mol ecul es i n a nd out of the nucl eus per s econd. Thes e nucl ea r pores , compos ed of protei ns ca l l ed nucleoporins, a re di rectl y a na l ogous to the membra ne cha nnel s di s cus s ed i n Cha pter 8 a nd tra ns port s evera l types of mol ecul es , i ncl udi ng ri bonucl ei c a ci d (RNA) a nd ri bos omes , protei ns , ca rbohydra tes , a nd l i pi ds . Sma l l er mol ecul es a nd i ons pa s s through the nucl ea r pore by s i mpl e di ffus i on, but l a rger protei ns a nd RNA mol ecul es a re bl ocked by a s poke-l i ke ga te i ns i de the cha nnel a nd mus t be a cti vel y a s s i s ted by ca rri er protei ns ca l l ed “importins” or “exportins” by a proces s tha t requi res two gua nos i ne tri phos pha te (GTP) mol ecul es . Ea ch type of RNA mol ecul e [mes s enger RNA (mRNA) a nd tra ns fer RNA (tRNA)] tha t mus t be tra ns ported i nto the cytopl a s m ha s a n exporti n a nd a s peci fi c a mi no a ci d (AA), nuclear export sequence, whi ch di rects them to a s peci a l i zed nucl ea r pore for thei r s el ected tra ns port out of the nucl eus . Leptomycins: Leptomycins A and B, ori gi na l l y devel oped a s a nti funga l drugs , s peci fi ca l l y a l kyl a te a n i mporti n, whi ch res ul ts i n i nhi bi ti on of the nucl ea r export of s evera l RNAs a nd tra ns cri pti on regul a tors i n cel l cycl e control . An a ddi ti ona l l eptomyci n i s HIV-1 “regulator, which allows HIV to take over host protein synthesis.” The l eptomyci ns a l s o s ta bi l i ze p53, known to s uppres s tumor devel opment/growth. Beca us e of thei r cel l cycl e effects (s ee bel ow), l eptomyci ns a re now bei ng cons i dered for ca ncer thera py. HISTONES The mos t i mporta nt s tructure i ns i de the nucl eus i s chromatin, cons i s ti ng, i n huma ns , of the 46 chromos omes a nd thei r a s s oci a ted protei ns . Thes e protei ns ena bl e not onl y effi ci ent pa cki ng of over 12 bi l l i on nucl eoti des i n huma n DNA, but a l s o s el ecti ve unwi ndi ng of thes e chromos omes to expos e genes for DNA repl i ca ti on, DNA to RNA tra ns cri pti on a nd mRNA proces s i ng. Histones a re one ma jor exa mpl e of thes e a s s oci a ted protei ns , a nd a re s epa ra ted i nto s i x cl a s s es : H1, H2A, H2B, H3, H4, a nd H5. Two ea ch of hi s tones H2A, H2B, H3, a nd H4 a s s embl e i nto a n ei ght-s ubuni t nucleosome a nd wra p 146 or 147 DNA ba s e pa i rs of the s i mpl e, doubl e hel i x s tructure (Fi gure 9-1) a round the compl ex.
Figure 9-1. Histone Structure. The hi s tone conta i ns a core of hi s tone mol ecul es , i ncl udi ng pa i rs of H2A, H2B, H3, a nd H4, wra pped by doubl e-hel i x DNA a nd hel d together by hi s tone H1. See text for further deta i l s . DNA, deoxyri bonucl ei c a ci d. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.]
Severa l of thes e nucl eos omes , l i nked together by a bout 50 DNA ba s e pa i r s equences (Fi gure 9-2A), crea te a n a pproxi ma tel y 11-nm “bea ds on a s tri ng” chroma ti n s tructure (Fi gure 9-2B). Addi ti on of the H1 hi s tone (Fi gure 9-1) covers the entry a nd exi t poi nts of the DNA mol ecul e a nd a l l ows DNA to form coi l ed-coi l s tructures , i ncl udi ng the 30-nm chroma ti n fi ber (Fi gure 9-2C). The 30-nm fi ber ca n rea di l y unwi nd i nto i ts 11-nm component to a l l ow DNA repl i ca ti on a nd tra ns cri pti on. Ana l ogous protei ns ca l l ed protamines a re found i n s perm a nd a l l ow even dens er pa cki ng of DNA i n the s perm hea d tha n hi s tones .
Figure 9-2. A-F. DNA Structure from Chromosome to Double-Helix. Ini ti a l DNA s tructure beyond the s i mpl e, doubl e hel i x (A) res ul ts from hi s tones H2A, H2B, H3, a nd H4 i ntera cti ons wi th DNA to crea te (B) 11-nm “bea ds on a s tri ng” s tructure. (C) The s ubs equent a ddi ti on of hi s tone H1 crea tes the 30-nm fi ber. Further terti a ry a nd qua terna ry s tructures of DNA a re crea ted by the a ddi ti on of va ri ous s ca ffol di ng protei ns , whi ch condens e DNA i nto va ryi ng compa ct chromos ome s tructures . The l evel of s tructure va ri es dependi ng on the cel l cycl e s ta ge a nd, a s a res ul t, the requi rement for DNA tra ns cri pti on or repl i ca ti on [e.g, (D) noncondens ed, i nterpha s e s tructures (300 nm) a nd (E) condens ed, meta pha s e s tructure (700 nm), endi ng wi th (F) the hi ghl y compa cted s tructure of the meta pha s e chromos ome]. DNA, deoxyri bonucl ei c a ci d. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Importa nt protei n a ttri butes (Cha pter 1) i ncl ude pos i ti ve/ nega ti ve/uncha rged AAs (pri ma ry s tructure), a l pha hel i ces (s econda ry s tructure), a nd grooves formed between s epa ra te hi s tone protei ns tha t provi de a nd opti mi ze es s enti a l i ntera cti on poi nts between the hi s tones a nd the DNA mol ecul es . The properti es of nucl eoti des a l s o contri bute to thi s bi ndi ng beca us e i ncrea s ed numbers of a denos i ne (A) a nd thymi ne (T) res i dues a t the mi nor groove of DNA a l l ow i mproved nucl eos ome bi ndi ng. Not s urpri s i ngl y, hi s tone AA s equences a nd, therefore, hi gher order s tructures a re hi ghl y cons erved i n bi ol ogy. However, hi s tones do much more tha n jus t wi nd DNA to a ma na gea bl e s i ze. Severa l hi s tones a re modi fi ed on thei r N-termi na l ta i l s a nd gl obul a r a rea s (s ee pos ttra ns l a ti ona l modi fi ca ti ons bel ow). Thes e modi fi ca ti ons i ncl ude the a ddi ti on of methyl , a cetyl , phos pha te, ubi qui ti n, a nd a denos i ne di phos pha te groups a mong others . The modi fi ca ti ons cha nge how the protei ns i ntera ct wi th the DNA mol ecul e, to expos e (e.g., a cetyl a ti on) a nd concea l (e.g., methyl a ti on) pa rti cul a r pa rts of the DNA s tra nd a s a pa rt of a cti va ti on a nd i nhi bi ti on of gene tra ns cri pti on (gene regul a ti on), a s wel l a s the repa i r of DNA errors from both DNA repl i ca ti on a nd ongoi ng proces s es tha t l ea d to nucl eoti de muta ti ons . NUCLEAR MATRIX/SCAFFOLD Further compa cti on of the s tructure i s provi ded by the nuclear matrix or “scaffold” protei ns , whi ch hel p i n ma i nta i ni ng the s tructure a nd i ntegri ty of the DNA mol ecul es . The exa ct s tructure a nd functi on of thi s s tructura l el ement of the nucl eus a re s ti l l hi ghl y di s puted. However, the exi s tence i s genera l l y a ccepted of a s upporti ve, hi ghl y dyna mi c network tha t produces both chroma ti n s tructure a nd, thereby, i ts functi ons by i nfl uenci ng the mol ecul es wi th whi ch i t i ntera cts . In corrobora ti on of the concept of a nucl ea r ma tri x, “scaffold/matrix attachment elements” (S/MARs) ha ve been propos ed/ i denti fi ed i n whi ch s peci fi c DNA s equences woul d be a bl e to connect to the s ca ffol di ng protei ns . The exi s tence a nd functi on of S/MARs i s a l s o now cons i dered i n bi otechnol ogy i nvol ved i n gene thera py a nd modul a ti on of DNA expres s i on to modi fy di s ea s e s ta tes .
The nucl ea r ma tri x or lamina i s a s tructure thought to be a three-di mens i ona l network of intermediate filaments, a thi rd type of s tructura l protei n (Cha pter 12), a s wel l a s peri phera l a nd i ntegra l membra ne protei ns a tta ched to the i nner l i pi d bi l a yer. Wi thi n the nucl eus , a ddi ti ona l chroma ti n s tructure i s crea ted from the s ca ffol di ng protei ns by the forma ti on of further s uper-coi l ed s tructures . Thes e hi gher orders of s tructure i ncl ude a 30-nm zi gza g forma ti on a nd a 100-nm fi ber (not s hown), a s wel l a s a 300-nm fi ber (Fi gure 9-2D) wi th noncondens ed a nd, therefore, a cces s i bl e l oops a nd a hi ghl y condens ed 700-nm fi ber (Fi gure 9-2E) tha t i s pa rt of the fi na l meta pha s e chroma ti n s tructure (Fi gure 92F) found i n cel l s . Other “s ca ffol di ng” protei ns l i nk the chroma ti n to the nucl ea r ma tri x (es s enti a l for chromos ome movement duri ng cel l repl i ca ti on) a nd the cytopl a s mi c ma tri x to the nucl ea r membra ne a nd i nterna l nucl ea r s tructures . Of cours e, chroma ti n s tructure cha nges dra ma ti ca l l y from i nterpha s e throughout DNA repl i ca ti on. Laminopathies: More tha n 2000 va ri a nts of medi ca l di s orders of the nucl ea r membra ne a re known, ma ny of whi ch a re ca us ed by muta ti ons tha t a ffect the protei ns of the nucl ea r membra ne or l a mi na . Ea ch a ffects a n otherwi s e i mporta nt rol e of protei ns i nvol ved i n tra ns port or s tructure of the membra ne bi l a yer or i ts membra ne protei ns , whi ch di rectl y a ffect DNA repl i ca ti on a nd tra ns l a ti on a nd the proces s es of protei n tra ns l a ti on a nd tra ns port. Thes e “laminopathies” ha ve fa r-rea chi ng effects , often res ul ti ng i n chi l dhood or a dol es cent di s orders of s kel eta l a nd ca rdi a c mus cl es , l i pi d, s ki n, nerves , a nd whi te bl ood cel l s , a s wel l a s ca ncers , di a betes , prema ture a gi ng, a nd even ea rl y dea th. The nucl ea r ma tri x i s not onl y a cti vel y i nvol ved i n chromos ome s epa ra ti on a nd cel l di vi s i on but a l s o hel ps to ma i nta i n the requi red s tructure for ea ch of thes e functi ons . The nucl ea r ma tri x crea tes a chroma ti n s tructure i n the nucl eus tha t i s ordered a nd cons tra i ned i n di s crete terri tori es tha t refl ect s peci fi c functi ons , but whi ch, a t ti mes of need s uch a s mi tos i s , ca n a l ter dra ma ti ca l l y whi l e s ti l l ma i nta i ni ng control a nd orga ni za ti on. The res ul t of the pri ma ry, s econda ry, terti a ry, a nd qua terna ry fol di ng of the DNA genome i s the compa cti on of a bout 1.8 m of huma n DNA a bout 40,000-fol d s o i t ca n fi t i ns i de a mi cros copi c nucl eus . Of cours e, thi s l evel of compa cti on cha nges throughout the cel l cycl e, bei ng ma xi ma l l y compa cted duri ng mi tos i s a nd mi ni ma l l y compa cted duri ng i nterpha s e. Fi na l l y, cha nges i n the DNA s tructure, i ncl udi ng the ma rked condens a ti on a nd expa ns i on of chroma ti n, occur duri ng the norma l cel l cycl e. Changing Nuclear Matrix in Cancer: Recent res ea rch ha s s hown tha t ea rl y cha nges i n ma ny huma n ca ncers i ncl ude a nota bl e a nd mea s ura bl e cha nge i n the protei ns tha t ma ke up the nucl ea r ma tri x. Al though eva l ua ti on of thes e cha nges i s s ti l l ongoi ng, the a bi l i ty to detect thes e cha nges i n the nucl ea r ma tri x ma y offer a rel i a bl e wa y to detect ca ncers a t very ea rl y s ta ges of the di s ea s e. Revers a l of thes e cha nges ma y a l s o offer cl i ni ci a ns , i ns i ghts i nto prognos i s a nd cures . NUCLEOLUS AND RIBOSOME SYNTHESIS Al s o found wi thi n the nucl eus i s the nucleolus, ma de of protei ns a nd nucl ei c a ci ds , where ribosomal RNA (rRNA) i s produced a nd ribosomes a re a s s embl ed for export i nto the cytopl a s m. The protei n components of ri bos omes , often ca l l ed “r-proteins,” a re ma de i n the cytopl a s m a nd tra ns ported to the nucl eol us vi a a connected network of nucl ea r membra ne pores a nd nucl eol a r cha nnel s . rRNA i s tra ns cri bed i n the nucl eus by RNA polymer-ases pol I, II, and III, a nd often methyl a ted or s hortened. The res ul ti ng rRNA s equences a re ta rgeted to the nucl eol us where they joi n wi th the r-protei ns to ma ke the 40S a nd 60S s ubuni ts of ma mma l i a n ri bos ome, whi ch i s s ubs equentl y exported through a nucl ea r pore to the cytopl a s m.
DNA REPLICATION AND TRANSCRIPTION The convers i on of i nforma ti on i n the geneti c code to functi oni ng protei ns a nd nucl ei c a ci ds rel i es on the a bi l i ty of the DNA to repl i ca te i ts el f a nd to tra ns cri be i ts code to mRNA for us e i n protei n s ynthes i s . DNA repl i ca ti on res ul ts i n very few mi s ma tched nucl ei c a ci d pa i rs , wi th a n a pproxi ma te error ra te of onl y one mi s ma tched nucl ei c a ci d per 10 mi l l i on nucl eoti des . The proces s of tra ns cri pti on a l s o ha s mecha ni s ms tha t reduce errors , but thes e a re fa r l es s effi ci ent tha n tha t s een i n DNA repl i ca ti on. Both proces s es i nvol ve s evera l protei ns es s enti a l to the proces s a s functi ona l or regul a tory el ements . DNA REPLICATION The proces s of uncoi l i ng doubl e-s tra nded DNA, fa i thful l y copyi ng ea ch DNA s tra nd a nd then s epa ra ti ng the two, new, doubl e-s tra nded copi es , i s ca l l ed replication. The proces s s ta rts a t a n origin of replication (ori), a pa rti cul a r s equence of nucl ei c a ci ds a t whi ch a pre-replication complex (pre-RC) ca n bi nd a nd the repl i ca ti on ca n s ta rt. Approxi ma tel y 100,000 ori gi ns of repl i ca ti on ca n be found i n ea ch huma n cel l , a l l owi ng the copyi ng of DNA to proceed i n a pa ra l l el fa s hi on from a l l of thes e poi nts , thereby s peedi ng the proces s of DNA repl i ca ti on. The pre-RC i s compos ed of the fol l owi ng four protei ns : (a ) a s i x-s ubuni t origin recognition complex bi nds fi rs t to the ori gi n of repl i ca ti on; (b) two cel l cycl e regul a tory protei ns , Cdc6 a nd Cdt1, ens ure tha t the cel l i s prepa red for DNA repl i ca ti on; a nd (c) the mini-chromosome maintenance complex, whi ch i s bel i eved to conta i n protei ns es s enti a l for the es ta bl i s hment of a replication fork. The repl i ca ti on fork i s the poi nt where two DNA s tra nds , one termed the leading strand a nd the other the lagging strand, a re s epa ra ted a nd DNA copyi ng occurs . The coi l ed-coi l , doubl e-hel i ca l DNA s tructure exa mi ned i n Cha pter 4 i s i ni ti a l l y unwound by the enzyme DNA helicase (pos s i bl y pa rt of the mi ni chromos ome ma i ntena nce compl ex) by brea ki ng the hydrogen bonds between compl ementa ry nucl ei c a ci ds . Single-stranded binding proteins a tta ch to the new DNA s tra nds to keep them s epa ra ted. An enzyme termed primase then produces a s hort s tra nd of RNA (s ometi mes wi th DNA) to s erve a s a pri mer for the rema i nder of the proces s . The enzyme DNA polymerase repl i ca tes ea ch DNA s tra nd i n the 5′ to 3′ di recti on by a ddi ng the correct, ma tchi ng nucl eoti de tri phos pha te to the 3′-hydroxyl end of the pri mer s tra nd. As ea ch new nucl ei c a ci d i s a dded, a new phos phodi es ter bond i s formed, uti l i zi ng the energy conta i ned i n the rema i ni ng di phos pha te group. Thi s proces s i s conti nuous on the l ea di ng s tra nd but, a s DNA pol ymera s e ca n onl y a dd i n the 5′ to 3′ di recti on, s hort cha i ns of newl y a dded nucl ei c a ci ds , ca l l ed Okazaki fragments, a re genera ted on the l a ggi ng s tra nd. The enzyme DNA ligase joi ns the Oka za ki fra gments together a s l a ggi ng s tra nd repl i ca ti on proceeds . The proces s of repl i ca ti on a l ong the coi l edcoi l s tructure of DNA s oon l ea ds to a n unfa vora bl e DNA conforma ti on tha t i s wound a bout i ts el f. To rel i eve thi s probl em, a DNA topoisomerase effi ci entl y cuts the phos pha te ba ckbone, “unta ngl es ” the DNA s tra nds , a nd then repa i rs the cut, l ea vi ng the DNA otherwi s e una l tered. Thi s enti re repl i ca ti on proces s i s depi cted i n Fi gure 9-3.
Figure 9-3. Overview of DNA Replication. Summa ry of DNA repl i ca ti on, i l l us tra ti ng the repl i ca ti on fork, l ea di ng a nd l a ggi ng s tra nd s ynthes i s , a nd the va ri ous protei ns i nvol ved i n repl i ca ti on a nd unwi ndi ng. A deta i l ed des cri pti on i s offered i n the text. DNA, deoxyri bonucl ei c a ci d; RNA, ri bonucl ei c a ci d. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] TRANSCRIPTION Transcription i s the proces s whereby geneti c i nforma ti on from a DNA s equence i s uti l i zed to crea te a n equi va l ent RNA—mRNA i f the gene codes for a protei n, tRNA for a tRNA, rRNA for a s s embl y of a ri bos ome, or even ca ta l yti c RNA, termed ribozyme. Thi s DNA s equence conta i ns not onl y the gene for the RNA (coding sequence) but a l s o regul a tory s equences tha t di cta te when a nd how the pa rti cul a r RNA wi l l be produced. Aga i n, the DNA i s tra ns cri bed or “rea d” from 3′ to 5′ a nd occurs onl y on one of the DNA s tra nds , known a s the template strand. Tra ns cri pti on s ta rts wi th bi ndi ng of the enzyme RNA polymerase to a promoter s equence on the DNA, us ua l l y l oca ted from 10 to 35 ba s es before the s ta rt of the a ctua l gene (Fi gure 9-4). RNA pol ymera s e bi ndi ng depends on a va ri ety of protei n transcription factors. Tra ns cri pti on fa ctors va ry i n type a nd a cti vi ty, but a l l i ntera ct i n s ome wa y wi th the DNA (ei ther di rectl y or i ndi rectl y) a nd the RNA pol ymera s e to ei ther enha nce or bl ock bi ndi ng to the DNA mol ecul e. The va ri a ti on of tra ns cri pti on fa ctors i s pa rt of the regul a tory mecha ni s m tha t a l l ows di fferenti a l expres s i on of genes . Once bound, the RNA pol ymera s e tra vel s down the DNA from 3′ to 5′ whi l e ma tchi ng the a ppropri a te RNA to i ts DNA counterpa rt, uti l i zi ng ura ci l ma tched wi th a deni ne i ns tea d of thymi ne. As i n DNA repl i ca ti on, energy for the forma ti on of the phos phodi es ter bond i s deri ved from hydrol ys i s of the two termi na l phos pha te bonds of the nucl eos i de tri phos pha te. Unl i ke repl i ca ti on, mul ti pl e RNA pol ymera s es ca n tra ns cri be on a s i ngl e DNA gene s equence, a l l owi ng ra pi d producti on of the RNA product. The termi na ti on of tra ns cri pti on i s s ti l l not wel l unders tood but i s fol l owed by the a ddi ti on of s evera l a deni ne uni ts to the 3′-end of the new RNA (polyadenylation, a l s o known a s a pol y-A ta i l ). Next, the new RNA i s us ua l l y s pl i ced a nd rea nnea l ed to remove noncodi ng regi ons , a nd a 7methyl gua nos i ne nucl ei c a ci d i s a dded to the 5′-end (known a s a 5′ cap) to protect the RNA from va ri ous enzymes (exonucl ea s es ) duri ng i ts s ubs equent a cti vi ti es . If the RNA i s des ti ned for protei n s ynthes i s , the mol ecul e i s exported from the nucl eus to the cytopl a s m or ER, dependi ng on the fi na l des ti na ti on of the protei n (s ee Protei n Tra ffi cki ng bel ow). The 5′ ca p a l s o a i ds i n the recogni ti on of mRNA by the ri bos omes .
Figure 9-4. mRNA Processing and Nuclear Export. Fol l owi ng tra ns cri pti on of DNA to produce the i ni ti a l compl ementa ry mRNA mol ecul e, s ubs equent proces s i ng occurs , i ncl udi ng s pl i ci ng, pol ya denyl a ti on, a nd a ddi ti on of the 5 ca p. The mRNA mol ecul e bound for protei n s ynthes i s i s then exported through nucl ea r pores i nto the cytopl a s m. See the text for further deta i l s . CTD, ca rboxy termi na l doma i n; DNA,
deoxyri bonucl ei c a ci d; mRNA, mes s enger ri bonucl ei c a ci d. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Reverse Transcriptase and Retroviral Therapy: Severa l types of vi rus es , i ncl udi ng thos e tha t ca us e huma n i mmunodefi ci ency vi rus (HIV)/a cqui red i mmune defi ci ency s yndrome, ha ve the a bi l i ty to tra ns cri be thei r RNA i nto DNA a s pa rt of the proces s of i nfecti ng a hos t cel l a nd uti l i zi ng tha t cel l ’s repl i ca ti on ma chi nery. The vi ra l enzyme tha t performs thi s RNA to DNA repl i ca ti on i s known a s reverse transcriptase. Cl i ni ci a ns ca n uti l i ze thi s uni que a bi l i ty of vi rus es a ga i ns t them by us i ng a cl a s s of medi ca ti ons known a s reverse transcriptase inhibitors, commonl y ca l l ed retrovirals. One s uch retrovi ra l i s Zidovudine or Azidothymidine (AZT), a n a na l og of thymi di ne (s ee fi gure bel ow). Beca us e revers e tra ns cri pta s e enzymes a re onl y found i n thes e vi rus es a nd the thymi -di ne a na l og i s 100 ti mes l es s s peci fi c for the pa ti ent’s huma n DNA pol ymera s es , the medi ca ti on preferenti a l l y bl ocks vi rus repl i ca ti on. The a zi do group a l s o ha s a hydrophobi c qua l i ty tha t a l l ows AZT to cros s through the pl a s ma membra ne for ea s y del i very to i nfected cel l s , a fter whi ch i nterna l cel l ul a r enzymes convert the AZT mol ecul e i nto a n a cti ve 5′-tri phos pha te form tha t ca n i ncorpora te i nto a nd termi na te revers e tra ns cri pti on.
PROTEIN SYNTHESIS Protei ns a re cha i ns of AAs connected by pepti de bonds formed between the ca rboxyl i c a ci d (COOH) group of one AA a nd the a mi no group (NH 3 ) of the s econd AA (Cha pter 1, Fi gure 1-4A). Beca us e ea ch protei n ha s a s peci fi c AA s equence, mRNA copi ed from the DNA gene templ a te for tha t s peci fi c protei n provi des the nucl eoti de s equence/codon (Cha pter 4) i ns tructi ons to l i nk the proper AAs together. Enzymes bri ng together the RNA a nd the neces s a ry s ubs tra tes a l ong wi th a n energy s ource, ATP, for effi ci ent producti on of protei ns . Thi s proces s , ca l l ed “translation,” i s bri efl y des cri bed bel ow a nd s umma ri zed i n Fi gure 9-5.
Figure 9-5. A. Formation of the Ribosome Complex for “Translation.” The components of protei n s ynthes i s i ncl ude the 60S a nd 40S ri bos oma l s ubuni ts , mRNA, a nd tRNA–AA mol ecul es , the fi rs t of whi ch codes for methi oni ne (AUG mRNA s equence wi th a na l ogous UAC tRNA s equence). Severa l i ni ti a ti on fa ctors (IFs ) a nd el onga ti on fa ctors (EFs ) a re i nvol ved i n the forma ti on of the a cti ve 80S ri bos oma l compl ex. B. Protein Synthesis (“Translation”). Si mpl i fi ed model of protei n s ynthes i s . The fi rs t tRNA–AA i s bound to the P-s i te fol l owed by bi ndi ng of the s econd tRNA–AA to the A-s i te [a i ded by el onga ti on fa ctor 1α(eEF 1α)]. A pepti de bond i s formed between the two AAs , a nd the mRNA moves down the ri bos ome [a i ded by el onga ti on fa ctor 2 (eEF 2 )], rel ea s i ng the fi rs t tRNA mol ecul e a nd freei ng the A-s i te, s o a nother tRNA–AA ma y bi nd. Thi s proces s i s repea ted unti l a nons ens e codon i s rea ched i n the mRNA s equence, s i gna l i ng the end of the protei n. Termi na ti on i s fol l owed by di s s oci a ti on of the ri bos oma l compl ex (a i ded by eRF) a nd rel ea s e of the new pepti de. AA, a mi no a ci d; GTP, gua nos i ne tri phos pha te; mRNA, mes s enger ri bonucl ei c a ci d; tRNA, tra ns fer ri bonucl ei c a ci d. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Protei n s ynthes i s requi res the concerted i ntera cti on of mRNA, tRNA, s evera l a cces s ory protei ns , ca l l ed initiation factor (IF) a nd elongation (EF) factor, a nd ribosomes. The mRNA mol ecul e i s copi ed from the DNA gene a nd conta i ns the copy of the nucl eoti de s equence for the chos en protei n. Indi vi dua l tRNA mol ecul es ca n bi nd to onl y one s peci fi c type of AA. Thi s tRNA–AA bi nds to the mRNA mol ecul e vi a anticodons us i ng the s a me bi ndi ng rul es a s DNA doubl e s tra nds (e.g., a tRNA tha t bi nds the s ta rti ng mRNA codon AUG ha s a n a nti codon s equence of UAC), i ns uri ng the s peci fi c order of AAs requi red for proper producti on of the protei n. The IF a nd EF a cces s ory protei ns s erve a number of rol es , i ncl udi ng ena bl i ng bi ndi ng of the mRNA mol ecul e to the ri bos ome, movement of the mRNA a l ong the ri bos ome to the s ta rt poi nt of the s ynthes i s , docki ng of the tRNA–AA, joi ni ng of the 60S s ubuni t to the 40S–mRNA compl ex (Fi gure 9-5A), a nd movement of the mRNA a nd growi ng pepti de cha i n (Fi gure 9-5B), a s wel l a s a ccura cy a s s ura nce. IF a nd EF rol es wi l l not be di s cus s ed further. Elongation Factors, Bacteria, and Disease: Corynebacteria diptheriae ca us es the di s ea s e diphtheria, whi ch, unti l effecti ve i mmuni za ti on wa s a va i l a bl e, ca us ed up to 15,000 dea ths a yea r, ma i nl y i n chi l dren a nd ol der a dul ts . The ba cteri a Pseudomonas aeruginosa ca us es frequent i nfecti ons of burns , wounds , a nd medi ca l devi ces (ca theters , venti l a tors , etc.), a s wel l a s s eri ous probl ems i n i mmunocompromi s ed pers ons . Rotaviruses ca us e mi l l i ons of chi l dhood dea ths ea ch yea r from s eri ous di a rrhea . Al l three of thes e i nfecti ve a gents work by mi mi cki ng huma n a cces s ory protei ns a nd bi ndi ng to el onga ti on fa ctors . Al though the two ba cteri a i nhi bi t a n el onga ti on fa ctor, rota vi rus es a ctua l l y “hi ja ck” the fa ctor a nd us e the hos t’s protei n s ynthes i s ma chi nery for reproducti on a nd further i nfecti on. The ri bos ome i s a two-pa rt s tructure compos ed of protei ns a nd four s tra nds of rRNA. The fi rs t pa rt of the ri bos ome i s a 40S s ubuni t (“S” s ta nds for “Svedberg,” a uni t of mol ecul a r s i ze) a l ong wi th one rRNA, whi ch hel ps i n es ta bl i s hi ng the proper AUG codon s ta rt s i te on the mRNA. The 40S s ubuni t a l s o conta i ns the bi ndi ng s i tes for the mRNA s tra nd a nd two bi ndi ng s i tes for i ncomi ng AAs —the “P-site” (na med for the pepti de cha i n tha t grows a t thi s s i te) a nd the “A-site” (na med beca us e new AAs bi nd here). The s econd pa rt of the ri bos ome i s a 60S s ubuni t wi th three rRNA mol ecul es , whi ch hel p i n s ta bi l i za ti on of the enti re ri bos ome, pepti de bond forma ti on, a nd movement of the ri bo-s ome, a l ong the mRNA s tra nd. The 60S s ubuni t bi nds to the 40S s ubuni t onl y a fter the mRNA i s i n pl a ce, the fi rs t tRNA–AA i s bound a t the P-s i te, a nd the AUG s ta rt codon i s found a nd correctl y a l i gned. As s embl y of thi s compl ex rel i es on the energy obta i ned from GTP. Fol l owi ng the forma ti on of the mRNA–fi rs t tRNA–AA–ri bos ome compl ex, a s econd tRNA wi th i ts s peci fi c, bound AA bi nds to the A-s i te. A pepti de bond i s formed between the two AAs a nd the ri bos ome moves down the mRNA (a proces s tha t uti l i zes ATP energy), the P-s i te tRNA i s rel ea s ed, a nd the proces s repea ts i ts el f unti l the end of the protei n i s rea ched. Macrolides and the Machinery of Protein Synthesis: The cl a s s of drugs ca l l ed ma crol i des i ncl udes the a nti bi oti cs azithromycin, clarithromycin, erythromycin, a nd roxithromycin (us ed for ma ny res pi ra tory, uri na ry, a nd s oft-ti s s ue i nfecti ons ), a nd the i mmunos uppres s a nt drugs tacrolimus a nd sirolimus (us ed a fter orga n tra ns pl a nts to reduce orga n rejecti on). Thes e medi ca ti ons bl ock the movement of the ri bos ome a l ong the mRNA mol ecul e. Ma crol i des ta ke a dva nta ge of the di fferent 50S ri bos oma l s ubuni t found i n ba cteri a a nd do not a ffect the huma n 60S s ubuni t. Ma crol i des a l s o tend to concentra te i n whi te bl ood cel l s a nd a re, therefore, conveni entl y tra ns ported to the s peci fi c s i tes of i nfecti on. As a res ul t, ma crol i des s el ecti vel y a nd effi ci entl y trea t ba cteri a l i nfecti ons wi thout ha rm to the pa ti ent. Ma ny other cl a s s es of a nti bi oti cs ta ke a dva nta ge of the di fferences i n huma n a nd ba cteri a l protei n tra ns l a ti on for thei r s el ecti ve a cti ons .
POSTTRANSLATIONAL TRAFFICKING/ MODIFICATION Inherent i n the proces s of protei n s ynthes i s /tra ns l a ti on i s the producti on of a s tri ng of AAs , the pri ma ry protei n s tructure, whi ch mus t fol d i nto the correct s econda ry, terti a ry, a nd qua terna ry s tructures to produce a functi oni ng fi na l protei n a nd/ or protei n compl ex (Cha pter 1). Al though s ome s ma l l er protei ns a re a bl e to s i mpl y fol d a s tra ns l a ti on occurs a nd the AA s equence i s genera ted, mos t of the l a rger a nd/or more compl ex protei ns requi re s peci a l i zed protei ns , known a s chaperones. Thes e cha perones a s s i s t i n the res ul ti ng fol di ng a nd unfol di ng of s i ngl e, s equence protei ns (i .e., s econda ry a nd terti a ry s tructures ) a nd the a s s embl y of mul ti pl e protei ns (qua terna ry s tructure). Some cha perone protei ns a re known a s “heat shock” proteins beca us e they a re s ometi mes preferenti a l l y uti l i zed to s ta bi l i ze protei n fol di ng a t ti mes of cel l ul a r s tres s es s uch a s a n i ncrea s e i n tempera ture. In fa ct, a ma jor rol e of cha perones i s to tempora ri l y s ta bi l i ze the newl y s ynthes i zed AA cha i n, es peci a l l y i n a cytopl a s m a l rea dy fi l l ed wi th other protei ns , s o i t does not ra ndoml y a ggrega te i nto a nonfuncti ona l s tructure. Other cha perones a re di rectl y i nvol ved i n the a ctua l fol di ng, di s s oci a ti ng from thei r a s s oci a ted protei n a fter the fi na l s tructure i s a chi eved. Sti l l other cha perone protei ns a re i nvol ved i n s ome membra ne tra ns port a nd even the brea kdown of certa i n protei ns . Beca us e of thei r cl os e a s s oci a ti on wi th protei n tra ns l a ti on, cha perones a re often found i n or nea r the ER. Protein trafficking or targeting di rects a protei n ma de for a pa rti cul a r l oca ti on (e.g., nucl eus , cytopl a s m, membra nes , orga nel l es , or extra cel l ul a r) to i ts correct des ti na ti on. Thi s proces s rel i es on signal sequences norma l l y conta i ned i n the fi rs t 20–30 AAs of the protei n. Excepti ons i ncl ude protei ns bound for the nucl eus , whi ch ha ve s i gna l s throughout thei r enti re s equence, s ome peroxi s ome-bound protei ns wi th a s i gna l s equence i n the l a s t three AAs , a nd mi tochondri a l protei ns , whi ch ha ve two s i gna l s equences to di rect them to ei ther the mi tochondri a l ma tri x or the i ntermembra ne s pa ce. In a ddi ti on, protei n s ynthes i s for cytopl a s mi c-, nucl eus -, a nd mi tochondri a -bound protei ns us ua l l y ta kes pl a ce on free ribosomes, wherea s extra cel l ul a r-, peroxi s ome-, a nd membra ne-bound protei ns a re us ua l l y s ynthes i zed on ERbound ribosomes. Acces s ory protei ns ca l l ed sequence recognition particles (SRPs) a nd SRP docking protein a l s o hel p to di rect protei ns ta rgeted for s ecreti on/ peroxi s ome/membra nes to the ER a nd then the Gol gi a ppa ra tus for further s orti ng a nd routi ng. The fi na l s tep i n the s ynthes i s of ma ny protei ns i s post-translational modification, the a ddi ti on or remova l of AAs (e.g., s i gna l s equence a nd i ni ti a ti on methi oni ne), ca rbohydra tes (e.g., s peci fi c ca rbohydra te s equences for gl ycoprotei ns /gl ycos a mi nogl yca ns ), or chemi ca l modi fi ca ti ons (e.g., a ddi ti on of l i pi ds , phos pha te, hydroxyl or methyl groups , a nd/ or cys tei ne–cys tei ne di s ul fi de bondi ng). Thes e cha nges us ua l l y i nvol ve extra cel l ul a r or membra ne-bound protei ns a nd, thus , genera l l y occur i n the ER a nd/or Gol gi a ppa ra tus . Proenzymes (Cha pter 5) ca n a l s o be converted to thei r a cti ve form a t thi s ti me. Protein Folding, Chaperones, and Mad Cow Disease: Mad cow disease/Bovine spongioform encephalopathy a nd i ts huma n equi va l ent Creutzfeldt–Jakob disease res ul t from i nfecti ous , mi s fol ded protei ns known a s prions (protei na ceous a nd infecti ous vi ri ons). Al though current res ea rch i s s ti l l
ongoi ng a nd i s often very controvers i a l , pri ons a re fel t to i nfect thei r hos t a s a norma l l y fol ded protei n, whi ch then repl i ca tes a nd mi s fol ds i nto a ti ghtl y pa cked s tructure of β-s heets , referred to a s a n “a myl oi d fol d.” Thes e protei ns a ggrega te i nto s tructures known a s a myl oi d pl a ques , whi ch crea te “hol es ” i n the norma l bra i n ti s s ue a nd a res ul ti ng “s pongi oform” a ppea ra nce tha t i s typi ca l of the di s ea s e (s ee pi cture bel ow). Pri ons a ffect the bra i n/nervous ti s s ue l ea di ng to detri menta l a nd fa ta l neurol ogi ca l s ymptoms , i ncl udi ng dementi a , memory/s peech/movement/ba l a nce probl ems , ma rked pers ona l i ty cha nges , ha l l uci na ti ons , a nd s ei zures . Some res ea rchers a re l ooki ng i nto the pos s i bl e rol e of cha perone protei ns i n the preventi on a nd/or trea tment of thi s di s ea s e.
Courtes y of Dr. Denni s K. Burns , Depa rtment of Pa thol ogy, Uni vers i ty of Texa s Southwes tern Medi ca l Center
CONTROL OF GENE EXPRESSION The a bi l i ty of the body to control the expres s i on of genes a nd thei r res ul ti ng mRNAs a nd protei ns a l l ows the s el ecti ve i ni ti a ti on a nd ces s a ti on of the va ri ous bi ol ogi ca l rea cti ons tha t a l l ow l i fe. Beca us e cons erva ti on of res ources i s i mporta nt, mos t regul a ti on occurs a t the l evel of gene tra ns cri pti on, a l though tra ns l a ti ona l control a s wel l a s pos ttra ns l a ti ona l a cti va ti on vi a protei n modi fi ca ti ons a l s o pl a ys a n i mporta nt pa rt i n the overa l l regul a ti on. Tra ns cri pti ona l control (DNA → RNA) rel i es on promoter regi ons of DNA, us ua l l y, but not a l wa ys , l oca ted cl os e to the s ta rti ng poi nt of the tra ns cri pti on. Thes e promoter regi ons a l l ow bi ndi ng of the RNA pol ymera s e a nd regul a tory transcription factor a nd enhancer protei ns . DNA s equences tha t a ppea r i n s evera l di fferent promoter regi ons ha ve been i denti fi ed, s uch a s the TATA box, a n ei ght ba s e-pa i r s equence often excl us i vel y cons i s ti ng of a deni ne a nd thymi ne nucl eoti des . Ma ny tra ns cri pti on fa ctors a l s o s ha re cons erved s econda ry a nd terti a ry protei n s tructures tha t a l l ow them to bi nd to pa rti cul a r pa rts of the DNA s equence; exa mpl es a re s hown i n Ta bl e 9-1). The bi ndi ng of di fferent tra ns cri pti on fa ctors a nd/ or enha ncers to thes e s equences a re es s enti a l for tra ns cri pti on a nd ca n i ncrea s e or decrea s e the tra ns cri pti on of a s peci fi c gene. Steroid hormones (Cha pter 3) a re s peci fi c type of tra ns cri pti on fa ctors tha t bi nd often to uni que receptors (cytopl a s mi c or nucl ea r). Thes e s teroi d receptors s ubs equentl y bi nd to s peci fi c DNA s equences ca l l ed steroid response elements for s teroi d a cti va ti on of pa rti cul a r genes . Steroi d receptors a l wa ys conta i n a n a rea for s teroi d bi ndi ng, a modi fi ed zi nc fi nger s tructure for DNA bi ndi ng, a s equence tha t a cti va tes tra ns cri pti on, a nd a n a rea tha t a l l ows receptor di meri za ti on. Exa mpl es of s teroi ds a nd thei r a cti on wi l l be covered i n Secti on III.
TABLE 9-1. Exa mpl es of Some Common DNA Bi ndi ng Moti fs Iron Response Element (IRE): IRE i s a s peci fi c s pa n of a pproxi ma tel y 25–30 nucl eoti des found i n the mRNA for protei ns s uch a s ferritin (res pons i bl e for control l ed i ron s tora ge) a nd transferrin (res pons i bl e for i ron tra ns port). Thes e nucl eoti des bi nd to form a s econda ry ha i rpi n s tructure, whi ch ca n then bi nd protei ns i nvol ved i n regul a ti on of i ron concentra ti on i n the huma n body. Duri ng ti mes of l ow i ron concentra ti on, thes e protei ns bi nd to the IRE of ferri ti n a nd decrea s e i ts tra ns l a ti on to i ncrea s e the a va i l a bl e i ron by decrea s i ng i ts s tora ge. Thes e s a me protei ns bi nd to res pons e el ements found i n tra ns ferri n, whi ch s ta bi l i zes the mRNA l ea di ng to i ncrea s ed tra ns l a ti on a nd, therefore, i ncrea s ed i ron a cqui s i ti on. IREs , therefore, a l l ow regul a ti on a t the RNA l evel of concentra ti ons of free a nd s tored i ron. Gene expres s i on i s a l s o control l ed by the revers i bl e a cetyl a ti on of hi s tone, l ys i ne AA res i dues i n nucl eos omes , ca rri ed out by s peci fi c enzymes known a s histone acetyl transferases (HATs) a nd histone deacetylases. The a ddi ti on of a cetyl groups a l l ows a cces s of RNA pol ymera s es to the DNA, wherea s the remova l of the a cetyl groups bl ocks thi s a cces s . Some tra ns cri pti on fa ctors or thei r a s s oci a ted protei ns a ctua l l y conta i n HAT a cti vi ty to a l l ow them a cces s to thei r ta rget genes . In a s i mi l a r wa y, the a ddi ti on of methyl groups to cytos i ne nucl eoti de res i dues us ua l l y res ul ts i n decrea s ed tra ns cri pti ona l a cti vi ty, a nd the remova l of the methyl group i s us ua l l y requi red for expres s i on of tha t gene. Fi na l l y, s evera l a l terna ti ve forms of gene expres s i on a re a l s o s een. In s ome i ns ta nces , promoters onl y occur i n pa rti cul a r ti s s ue or cel l types . Al terna ti ve s pl i ci ng/ remova l /rea nnea l i ng ca n a l s o occur, a ffecti ng mRNA mol ecul es tha t conta i n i ntrons tha t crea te s evera l di fferent rel a ted mRNAs / protei ns wi th di fferi ng regul a ti on (up to 30% of huma n genes ma y be a ffected by thi s proces s ). Dea cti va ti on of one a l l el e of a gene or even a n enti re chromos ome ca n a l s o occur a s i s s een by the i na cti va ti on of one of the X chromos omes i n fema l es duri ng i ni ti a l embryo devel opment.
MUTATIONS AND REPAIR MECHANISMS Acci denta l cha nges or “mutations” i n DNA or RNA s equence (Fi gure 9-6) a re es ti ma ted to occur a t a n a pproxi ma te ra te of 1000–1,000,000 per cel l per da y i n the huma n genome, a nd every new cel l i s bel i eved to conta i n a pproxi ma tel y 120 new muta ti ons . One ki nd of muta ti on i s point mutation, whi ch i ncl udes two di fferent types (Fi gure 9-6B): transition occurs when a s i ngl e puri ne nucl eoti de i s cha nged to a di fferent puri ne (A ↔ G) or a pyri mi di ne nucl eoti de i s cha nged to a di fferent pyri mi di ne nucl eoti de [C ↔ T(U)] a nd transversion occurs when the ori enta ti on of a s i ngl e puri ne a nd pyri mi di ne nucl eoti de i s revers ed [A/G ↔ C/T(U)]. Poi nt muta ti ons ca n be further cl a s s i fi ed a s silent (when the s a me AA i s coded), mis-sense (when a di fferent AA i s coded), neutral (when a n AA cha nge occurs but does not a ffect the protei n’s s tructure or functi on), or nonsense (when a s top codon res ul ts , termi na ti ng tra ns l a ti on a nd s horteni ng the res ul ti ng protei n). Insertion a nd deletion muta ti ons i nvol ve more tha n one nucl eoti de a nd ca n not onl y cha nge the tra ns l a ted protei n, but, i f not occurri ng i n mul ti pl es of three nucl eoti des , ca n a l s o cha nge the enti re rea di ng of the mRNA s equence (frameshift muta ti on). Chemi ca l -, ra di a ti on-, a nd even vi ra l -ca us ed muta ti ons ca n a l s o ca us e s i ngl e- or doubl e-strand breaks a nd/or nucl eoti de–nucl eoti de or nucl eoti de–protei n cros s -l i nki ng. A common exa mpl e of cros s -l i nki ng occurs between a dja cent thymi di ne di mers whos e puri ne ba s es ca n form a cycl obuta ne ri ng when expos ed to ul tra vi ol et l i ght (Fi gure 9-6A–C).
Figure 9-6. A-C. DNA Mutations. Overvi ew of muta ti ons a nd s ome of thei r cl i ni ca l i mpl i ca ti ons (A a nd B) a nd exa mpl e thymi di ne di mer (C). See text for further di s cus s i on. DNA, deoxyri bonucl ei c a ci d. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Al though s ome of thes e muta ti ons a re benefi ci a l , offeri ng res i s ta nce to di s ea s e or i mproved s tructure a nd/or functi on, they ca n often l ea d to di s ea s e a nd/or dea th of the cel l or orga ni s m. The huma n body ha s , therefore, devel oped a va ri ety of repa i r mecha ni s ms to a ttempt to res tore the a l tered nucl eoti de s equence. In the ca s e of s i ngl e-nucl eoti de muta ti ons , pa rti cul a r endonuclease enzymes s ens e a nd remove (excision) the i ncorrect ba s e a nd the correct nucl eoti de i s repl a ced by a pol ymera s e a nd l i ga s e. A pa rti cul a r exa mpl e i s the endonucl ea s e s peci fi c for the repa i r of thymi ne di mers . Stra nd brea ks a re repa i red by l i ga s es or, when compl ex, by exci s i on a nd s ubs equent retra ns l a ti on/l i ga ti on. Cel l cycl e check poi nts (s ee bel ow) a l s o offer the body the cha nce to veri fy the a ccura cy of the DNA s equence pri or to commi tment to cel l repl i ca ti on a nd di vi s i on. Thymidine Dimer Repair and Xeroderma Pigmentosum: Al though a rel a ti vel y uncommon di s order Xeroderma pigmentosa i s a di rect exa mpl e of how a n a utos oma l reces s i ve muta ti on i n the enzyme(s ) requi red for the repa i r of thymi di ne di mers ca n l ea d to a di s ea s e. The l os s of thi s i mporta nt repa i r mecha ni s m res ul ts i n di s ea s es s uch a s basal cell carcinoma, squamous cell carcinoma, a nd malignant melanoma—a l l a s a res ul t of unrepa i red, ul tra vi ol et l i ght da ma ge. The onl y trea tment for a ffected i ndi vi dua l s i s reduci ng the expos ure to s unl i ght a nd s upporti ve ca re, a l though mos t pa ti ents di e by the a ge of 20 yea rs .
REGULATION OF CELL GROWTH AND DIFFERENTIATION Cell cycle i s the ordered pa thwa y underta ken by a l l ma mma l i a n cel l s , whi ch defi nes not onl y ti mes of both cel l di vi s i on a nd repl i ca ti on, but a l s o thos e ti mes of cel l growth a nd functi ona l a cti vi ti es . The cel l cycl e i s di vi ded i nto G1 (gap 1), S (synthesis), G2 (gap 2/interphase), a nd M (mitosis) pha s es . Cel l s ca n a l s o enter a G0 (“resting”) pha s e. The progres s i on through the cel l cycl e depends on a va ri ety of protei ns , whi ch i ns ure tha t a cel l i s rea dy for the next cel l cycl e pha s e (“checkpoi nts ”). Thes e protei ns i ncl ude two types , the cyclins a nd the cyclin-dependent kinase (CDKs), whi ch joi n i n di fferent heterodi mer combi na ti ons tha t regul a te va ri ous pa rts of the cel l cycl e. The cycl i n component of thi s protei n compl ex regul a tes the functi on of the phos phoryl a ti on a cti vi ty of the bound CDK. Di fferent concentra ti ons of cycl i ns duri ng the cel l cycl e, a s determi ned by regul a ti on of thei r protei n s ynthes i s i n res pons e to va ri ous mol ecul a r s i gna l s , l ea d to a cti va ti on or i nhi bi ti on of a CDK. An overvi ew of the cel l cycl e a nd the rol e of cycl i ns a nd CDKs a re s hown i n Ta bl e 9-2 a nd Fi gure 9-7.
TABLE 9-2. Overvi ew of the Cel l Cycl e
Figure 9-7. Overview of Cell Cycle Control. Revi ew of va ri ous s i gna l s tha t regul a te progres s i on through the cel l cycl e. Checkpoi nts i ncl ude G 1 –S a nd G 2 –M tra ns i ti on poi nts where DNA da ma ge i s detected a nd repa i red, S-pha s e where compl eti on of repl i ca ti on i s a s s es s ed, a nd M-pha s e where the i ntegri ty of the s pi ndl e a ppa ra tus i s veri fi ed. Cycl i ns A, D, E, CDK2, a nd CDK4 a nd the protei n regul a tors p16, p 27, a nd p53, a s wel l a s DNA i rra di a ti on da ma ge a nd extra cel l ul a r s i gna l s pl a y rol es a s i l l us tra ted. Tra ns formi ng growth fa ctor-β a l s o i ncrea s es p27 i nhi bi tory a cti vi ty (not s hown). CDK, cycl i n-dependent ki na s e; DNA, deoxyri bonucl ei c a ci d. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Microtubules in the Cell Cycle and Cancer Treatment Strategies: The growth of mi crotubul es duri ng G 2 a nd thei r s ubs equent brea kdown duri ng M pha s es of the cel l cycl e ha ve l ed to va ryi ng s tra tegi es i n the trea tment of ca ncers . One s uch ca ncer a gent i s vincristine, whi ch bi nds to tubul i n protei n a nd bl ocks mi crotubul e pol ymeri za ti on, s toppi ng mi tos i s a t meta pha s e. Vi ncri s ti ne i s us ed i n non-Hodgkin’s lymphoma, Wilm’s tumor, a nd acute lymphoblastic leukemia trea tments a mong others . Taxol i s a nother i mporta nt ca ncer trea tment medi ca ti on obta i ned from the Pa ci fi c yew tree a nd i s us ed for pa ti ents s ufferi ng from ca ncers of the brea s t, l ung, ova ri es , hea d a nd neck, a nd for Ka pos i ’s s a rcoma . Ta xol works by s ta bi l i zi ng mi crotubul es , thereby i nhi bi ti ng the progres s i on of ca ncer cel l s undergoi ng s i s ter chroma ti d s epa ra ti on. Vi ncri s ti ne a nd ta xol a l s o a ffect a pa ti ent’s hea l thy cel l s , but to a l es s er extent tha n the much fa s ter repl i ca ti ng a nd di vi di ng ca ncer cel l s . Va ri ous i nhi bi tors ca n s top the cel l cycl e by i na cti va ti ng the cycl i n–CDK heterodi mer compl ex. When DNA da ma ge i s pres ent, the tumor s uppres s or protei n p53 s ens es the probl em a nd a cti va tes the i nhi bi tor p21, whi ch bl ocks the cycl i n D–CDK4 compl ex a nd, s ubs equentl y, the G 1 to S tra ns i ti on. The i nhi bi tors p16 a nd p27, the l a tter a cti va ted by the growth i nhi bi tor tra ns formi ng growth fa ctor-β (not s hown i n fi gure), ca n a l s o i nhi bi t the cel l cycl e by i nhi bi ti ng the cycl i n D–CDK4 or cycl i n E–CDK2 compl ex, res pecti vel y. The overa l l control of the cel l cycl e by cycl i ns a nd CDKs i s s hown i n Fi gure 9-7.
REVIEW QUESTIONS 1. Wha t a re 2. Wha t a re 3. Wha t a re 4. Wha t a re
the the the the
ba s i c s tructura l components of the nucl eus a nd thei r rol es ? key events i nvol ved i n DNA repl i ca ti on a nd the rol e of rel eva nt protei ns a nd enzymes ? key events i nvol ved i n tra ns cri pti on a nd the rol e of rel eva nt protei ns a nd enzymes ? key events i nvol ved i n protei n s ynthes i s i ncl udi ng the rol es of mRNA, tRNA, a nd a cces s ory protei ns ?
5. Wha t i s the rol e of cha perones ? 6. Wha t i s the s i gni fi ca nce of protei n tra ffi cki ng? 7. Wha t a re the key a s pects i n the control of gene expres s i on? 8. Wha t a re the va ri ous types of muta ti ons a nd how do repa i r mecha ni s ms rel a te to thes e muta ti ons ? 9. Wha t a re the va ri ous pha s es of the cel l cycl e a nd thei r functi ons ?
SECTION II INTEGRATED USMLE-STYLE QUESTIONS AND ANSWERS QUESTIONS II-1. A previ ous l y norma l 2-month-ol d fema l e pres ents wi th ji ttery s pel l s s evera l hours a fter mea l s . She ha s l ow bl ood gl ucos e. Phys i ca l exa m revea l s a l i ver edge 4 cm bel ow the ri ght cos ta l ma rgi n. Percus s i on of the ri ght ches t a nd a bdomen confi rms hepa tomega l y. The i nfa nt i ncrea s es her bl ood gl ucos e a fter brea s tfeedi ng but i t i s not ma i nta i ned a t norma l l evel s upon fa s ti ng. Whi ch of the fol l owi ng i s the mos t l i kel y di a gnos i s of thi s pa ti ent? A. Fructos emi a wi th i na bi l i ty to l i bera te s ucros e from gl ucos e B. Ga l a ctos emi a wi th i na bi l i ty to convert l a ctos e to gl ucos e C. Gl ycogen s tora ge di s ea s e D. Growth hormone defi ci ency wi th i na bi l i ty to ma i nta i n gl ucos e E. Intes ti na l ma l a bs orpti on of l a ctos e II-2. A wel l , 2-yea r-ol d gi rl contra cted a vi ra l i l l nes s a t da y ca re wi th vomi ti ng, di a rrhea , a nd progres s i ve l etha rgy. She pres ents to the cl i ni c wi th di s ori enta ti on, a ba rel y rous a bl e s ens ori um, cra cked l i ps , s unken eyes , l a ck of tea rs , fl a cci d s ki n wi th “tenti ng” wea k pul s e wi th l ow bl ood pres s ure, a nd i ncrea s ed deep tendon refl exes . La bora tory tes ts s how l ow bl ood gl ucos e, norma l el ectrol ytes , el eva ted l i ver enzymes , a nd (on ches t X-ra y) a di l a ted hea rt. Uri na l ys i s revea l s no i nfecti on a nd no ketones . The chi l d i s hos pi ta l i zed a nd s ta bi l i zed wi th 10% gl ucos e i nfus i on. Admi s s i on l a bora tori es s how el eva ted medi um-cha i n fa tty a cyl ca rni ti nes i n bl ood a nd 6–8 ca rbon di ca rboxyl i c a ci ds i n the uri ne. Whi ch of the fol l owi ng a bnorma l i ti es i s the mos t l i kel y di a gnos i s i n thi s chi l d? A. Ca rni ti ne defi ci ency B. Defect of medi um-cha i n coenzyme A dehydrogena s e C. Defect of medi um-cha i n fa tty a cyl s yntheta s e D. Mi tochondri a l defect i n fa tty a ci d tra ns port E. Mi tochondri a l defect i n the el ectron tra ns port cha i n II-3. Whi ch of the fol l owi ng i s the correct order of the fol l owi ng s teps i n protei n s ynthes i s ? 1. A pepti de bond i s formed. 2. The s ma l l ri bos oma l s ubuni t i s l oa ded wi th i ni ti a ti on fa ctors , mes s enger ri bonucl ei c a ci d (mRNA), a nd i ni ti a ti on a mi noa cyl –tra ns fer RNA (tRNA). 3. The i nta ct ri bos ome s l i des forwa rd three ba s es to rea d a new codon. 4. The pri med s ma l l ri bos oma l s ubuni t bi nds wi th the l a rge ri bos oma l s ubuni t. 5. El onga ti on fa ctors del i ver a mi noa cyl –tRNA to bi nd to the A s i te. A. 1, 2, 5, 4, 3 B. 2, 3, 4, 5, 1 C. 2, 4, 5, 1, 3 D. 3, 2, 4, 5, 1 E. 4, 5, 1, 2, 3 II-4. A mi ddl e-Ea s tern fa mi l y pres ents for eva l ua ti on beca us e thei r i nfa nt s on di ed i n the nurs ery wi th s evere hemol ys i s a nd ja undi ce. The coupl e ha d two pri or fema l e i nfa nts who a re a l i ve a nd wel l , a nd the wi fe rel a tes tha t s he l os t a brother i n i nfa ncy wi th s evere hemol ys i s i nduced a fter a vi ra l i nfecti on. The phys i ci a n s us pects gl ucos e-6-phos pha te dehydrogena s e defi ci ency, i mpl yi ng defecti ve s ynthes i s of whi ch of the fol l owi ng compounds ? A. Deoxyri bos e a nd ni coti na mi de a deni ne di nucl eoti de phos pha te (NADP) B. Gl ucos e a nd l a cta te C. La ctos e a nd NADPH D. Ri bos e a nd NADPH E. Sucros e a nd ni coti na mi de a deni ne di nucl eoti de II-5. Whi ch of the fol l owi ng i s a n i mporta nt i ntermedi a te i n the bi os ynthes i s of unes teri fi ed fa tty a ci ds ? A. Ca rni ti ne B. Chol es terol C. Fa tty a cyl -coenzyme A (CoA) D. Gl ucos e E. Ma l onyl -CoA II-6. Severa l di s orders , s uch a s one form of α1 -a nti tryps i n defi ci ency, ca n res ul t from mi s ta rgeti ng of protei ns i nto the wrong cel l ul a r compa rtments . New protei ns des ti ned for s ecreti on a re s ynthes i zed i n whi ch of the fol l owi ng? A. Free pol ys omes B. Gol gi a ppa ra tus C. Nucl eus D. Rough endopl a s mi c reti cul um E. Smooth endopl a s mi c reti cul um II-7. Chi l dren wi th urea cycl e di s orders pres ent wi th el eva ted s erum a mmoni a a nd cons equent neurol ogi c s ymptoms i ncl udi ng a l tered res pi ra ti on, l etha rgy, a nd coma . Severa l a mi no a ci ds a re i ntermedi a tes of the urea cycl e, ha vi ng s i de a mmoni a groups tha t joi n wi th free ca rbon di oxi de (CO2 ) a nd a mmoni a to produce net excreti on of a mmoni a a s urea (NH 2 CONH 2 ). Whi ch of the fol l owi ng a mi no a ci ds ha s a n a mmoni a group i n i ts s i de cha i n a nd i s thus l i kel y to be a n i ntermedi a te of the urea cycl e? A. Argi ni ne B. As pa rta te C. Gl uta ma te D. Methi oni ne E. Phenyl a l a ni ne II-8. Chol era toxi n ca us es ma s s i ve a nd often fa ta l di a rrhea by ca us i ng the conti nua l s ynthes i s of cycl i c a denos i ne monophos pha te (cAMP). Whi ch of the fol l owi ng mecha ni s ms woul d a ccount for thi s effect? A. Acti va ti on of G i protei n
B. Irrevers i bl y a cti va ti on of a denyl yl cycl a s e C. Locki ng of G s protei n i nto a n i na cti ve form D. Preventi on of gua nos i ne tri phos pha te (GTP) from i ntera cti ng wi th G q protei n E. Ra pi d hydrol ys i s of G protei n GTP to gua nos i ne di phos pha te (GDP) II-9. Whi ch of the fol l owi ng events occur duri ng the forma ti on of phos phoenol pyruva te from pyruva te duri ng gl uconeogenes i s ? A. Acetyl -CoA i s uti l i zed B. Adenos i ne tri phos pha te (ATP) i s genera ted C. CO2 i s requi red D. GTP i s genera ted E. Inorga ni c phos pha te i s cons umed II-10. Regul a ti on of whi ch of the fol l owi ng enzymes i s mos t i mporta nt i n control l i ng l i pogenes i s ? A. Acetyl -CoA ca rboxyl a s e B. Acyl -CoA s yntheta s e C. Ca rni ti ne–a cyl ca rni ti ne tra ns l oca s e D. Ca rni ti ne–pa l mi toyl tra ns fera s e E. Fa tty a ci d s yntha s e II-11. The s equence of the templ a te deoxyri bonucl ei c a ci d (DNA) s tra nd i s 5′-GATATCCATTAGTGAC-3′. Wha t i s the s equence of the RNA produced? A. 5′-CAGUGAUUACCUAUAG-3′ B. 5′-CTATAGGTAATCACTG-3′ C. 5′-CUAUAGGUAAUCACUG-3′ D. 5′-GTCACTAATGGATATC-3′ E. 5′-GUCACUAAUGGAUAUC-3′ II-12. Whi ch of the fol l owi ng i s a n enzyme tha t i s a cti va ted by hydrol ys i s of a proenzyme form? A. Hepa ri n B. Kera ti n C. La cta s e D. Peps i n E. Phenyl a l a ni ne hydroxyl a s e II-13. Whi ch of the fol l owi ng i s a n a ccura te des cri pti on of s i gna l tra ns ducti on i n res pons e to a pepti de hormone? A. Ca l ci um a cti va ti on of ca l modul i n-dependent ki na s e. B. Di a cyl gl ycerol (DAG) ca us i ng a n i ncrea s e of i ntra cel l ul a r Ca 2+. C. Inos i tol tri s phos pha te (IP3 ) bi ndi ng to a nd a cti va ti ng protei n ki na s e B. D. Phos phol i pa s e A2 ca ta l yzi ng the cl ea va ge of membra ne phos phol i pi ds to rel ea s e DAG. E. Rel ea s e of pota s s i um i ons from the endopl a s mi c reti cul um. II-14. Whi ch of the fol l owi ng i s a n energy-requi ri ng s tep of gl ycol ys i s ? A. Hexoki na s e B. Phos phoenol pyruva te ca rboxyki na s e C. Phos phogl ycera te ki na s e D. Pyruva te ca rboxyl a s e E. Pyruva te ki na s e (PK) II-15. Whi ch of the fol l owi ng proces s es genera tes the mos t ATP? A. Ci tri c a ci d cycl e B. Fa tty a ci d oxi da ti on C. Gl ycogenol ys i s D. Gl ycol ys i s E. Pentos e phos pha te pa thwa y II-16. Whi ch of the fol l owi ng s ta tements a ccura tel y des cri bes fea tures of ri bos omes ? A. An i ntegra l pa rt of tra ns cri pti on B. Bound together s o ti ghtl y they ca nnot di s s oci a te under phys i ol ogi c condi ti ons C. Compos ed of RNA, DNA, a nd protei n D. Compos ed of three s ubuni ts of unequa l s i ze E. Found both free i n the cytopl a s m a nd bound to membra nes II-17. Chi l dren wi th cys ti nos i s ha ve growth del a y, photos ens i ti vi ty wi th crys ta l s i n the l ens of thei r eyes , a nd progres s i ve rena l fa i l ure beca us e of a ccumul a ti on of cys ti ne i n cel l ul a r l ys os omes . The defect i nvol ves a s peci fi c l ys os oma l membra ne receptor tha t fa ci l i ta tes cys ti ne egres s , a nd a n effecti ve thera py ha s been found us i ng ora l cys tea mi ne, a compound s i mi l a r i n s tructure to cys ti ne. Thi s thera py refl ects the genera l pri nci pl e tha t competi ti ve i nhi bi tors typi ca l l y res embl e the s tructure of whi ch of the fol l owi ng? A. An a l l os teri c regul a tor of enzyme/receptor a cti vi ty B. Enzyme or receptor protei n C. Enzyme rea cti on products D. Subs tra tes or l i ga nds tha t bi nd the enzyme/receptor E. The cofa ctor II-18. Whi ch of the fol l owi ng ki na s es pl a ys a rol e i n s i gna l tra ns ducti on for growth hormone, l epti n, a nd prol a cti n? A. Adenyl a te ki na s e B. Ja nus ki na s e C. Mi togen-a cti va ted protei n (MAP) ki na s e D. Protei n ki na s e A E. Pyruva te ki na s e (PK) II-19. A 2-month-ol d boy i s brought to the emergency depa rtment i n a coma a fter s l eepi ng through the ni ght a nd fa i l i ng to a wa ken i n the
morni ng. He i s gi ven i ntra venous gl ucos e a nd a wa kens . Serum l evel s of pyruva te, l a cta te, a nd a l a ni ne a re el eva ted, wherea s a s pa rti c a ci d l evel s a re reduced. A mus cl e bi ops y s hows no a bnorma l i ti es , a nd vi ta mi n s uppl ementa ti on i s i neffecti ve. Whi ch of the fol l owi ng i s the mos t l i kel y di a gnos i s i n thi s pa ti ent? A. Is oci tra te dehydrogena s e defi ci ency B. Phos phofructoki na s e (PFK) defi ci ency C. Pyruva te ca rboxyl a s e defi ci ency D. Pyruva te dehydrogena s e (PDH) compl ex defi ci ency E. Pyruva te ki na s e defi ci ency II-20. Whi ch of the fol l owi ng bes t expl a i ns why s ta ti n thera py i s effecti ve for i ndi vi dua l s wi th hyperchol es terol emi a ? A. Bi nd to l ow-dens i ty l i poprotei n (LDL) receptor, di s pl a ci ng chol es terol a nd i nhi bi ti ng chol es terol s ynthes i s . B. Inhi bi t 3-hydroxy-3-methyl gl uta ryl (HMG)-CoA reducta s e, a key regul a tor of chol es terol s ynthes i s . C. Inhi bi t HMG-CoA s yntha s e, key s tep for s ynthes i s of meva l ona te tha t i nhi bi ts fa tty a ci d s ynthes i s . D. Sti mul a te s ynthes i s of trans-uns a tura ted fa tty a ci ds . E. Sti mul a te thi ol a s e, thus ma ki ng more ma l onyl -CoA for i nhi bi ti on of the tri ca rboxyl i c a ci d cycl e. II-21. GTP i s requi red by whi ch of the fol l owi ng s teps i n protei n s ynthes i s ? A. Ami noa cyl –tRNA s yntheta s e a cti va ti on of a mi no a ci ds B. Atta chment of mRNA to ri bos omes C. Atta chment of ri bos omes to endopl a s mi c reti cul um D. Atta chment of s i gna l recogni ti on protei n to ri bos omes E. Tra ns l oca ti on of tRNA–na s cent protei n compl ex from A to P s i tes II-22. Inheri ted defi ci ency of the enzyme methyl ma l onyl -CoA (MMA-CoA) muta s e ca us es s erum a nd uri ne a ccumul a ti on of methyl ma l oni c a ci d. Recogni ti on tha t perni ci ous a nemi a (due to defi ci ency of vi ta mi n B 12 ) ca n i nvol ve a ccumul a ti on of methyl ma l oni c a ci d l ed to s ucces s ful trea tment of s ome pa ti ents wi th MMA-CoA muta s e defi ci ency us i ng exces s B 12 . Studi es of puri fi ed MMA-CoA muta s e enzyme from norma l i ndi vi dua l s then s howed enha nced muta s e a cti vi ty when B 12 wa s a dded to the rea cti on mi xture. Thes e fa cts a re bes t reconci l ed by whi ch of the fol l owi ng rol es for Vi ta mi n B 12 ? A. Cofa ctor for the MMA- CoA muta s e enzyme B. Competi ti ve i nhi bi tor of MMA-CoA muta s e enzyme C. Cova l entl y a tta ched group for the enzyme methyl ma l oni c a ci d-CoA muta s e D. Feedba ck i nhi bi tor of MMA-CoA muta s e enzyme E. Precurs or for methyl ma l oni c a ci d s ynthes i s II-23. Whi ch of the fol l owi ng s ta tements des cri bes where i ntegra l protei ns a re l oca ted? A. As s oci a ted wi th DNA i n the nucl eus B. In the l umen of the endopl a s mi c reti cul um C. In the mi tochondri a l ma tri x D. Mos tl y i n the bl ood E. Predomi na ntl y wi thi n cel l membra nes II-24. A 7-yea r-ol d boy a rri ves a t the emergency depa rtment a s l eep i n hi s fa ther’s a rms . The boy’s mother expl a i ns tha t the boy s pent the ni ght throwi ng up a nd experi enci ng s evere di a rrhea . She i s concerned a bout the vomi ti ng a nd hi s i na bi l i ty to s ta y a wa ke. Hi s tory i ndi ca tes the boy wa s hea l thy yes terda y, but beca me i l l a t di nnerti me a fter s pendi ng ti me pl a yi ng i n the ba s ement of thei r a pa rtment compl ex tha t a fternoon. Further i nqui ry revea l s tha t a n extermi na tor ha d been hi red to ta ke ca re of a ra t probl em i n the a pa rtment. He ha d us ed a poi s on (Rotenone) tha t bl ocks compl ex I i n the res pi ra tory cha i n of oxi da ti ve phos phoryl a ti on. The boy i s pa l e a nd not cya noti c. An a na l ys i s of thi s pa ti ent’s meta bol i s m woul d l i kel y i ndi ca te i mpa i red functi on of whi ch of the fol l owi ng enzymes ? A. Gl ucos e-6-phos pha te dehydrogena s e B. PFK C. Pyruva te ca rboxyl a s e D. PDH compl ex E. Succi na te dehydrogena s e II-25. A pri me di a gnos ti c i ndi ca tor for the fi rs t pres enta ti on of di a betes mel l i tus type 1 i s the pres ence of ketonuri a a nd s evere a ci dos i s ; the a ci dos i s produces exa ggera ted a ttempts a t res pi ra tory compens a ti on known a s Kus s ma ul brea thi ng. Whi ch of the fol l owi ng a re “ketone bodi es ” tha t woul d be el eva ted i n s erum a nd uri ne from a chi l d wi th di a beti c ketoa ci dos i s ? A. Acetone a nd etha nol B. Fuma ra te a nd s ucci na te C. Oxa l oa ceta te a nd pyruva te D. Pyruva te a nd l a cta te E. β-Hydroxybutyra te a nd a cetoa ceta te II-26. Ami noa cyl –tRNA s yntheta s es mus t be ca pa bl e of recogni zi ng whi ch of the fol l owi ng? A. A s peci fi c a mi no a ci d a nd the 40S ri bos oma l s ubuni t B. A s peci fi c a mi no a ci d a nd the 60S ri bos oma l s ubuni t C. A s peci fi c ri bos oma l RNA (rRNA) a nd a s peci fi c a mi no a ci d D. A s peci fi c tRNA a nd a s peci fi c a mi no a ci d E. A s peci fi c tRNA a nd the 40S ri bos oma l s ubuni t II-27. Your pa ti ent pres ents wi th a defi ci ency of the enzyme tha t ca ta l yzes s ynthes i s of N-a cetyl gl uta mi c a ci d. Whi ch of the fol l owi ng woul d be a cons equence of thi s defi ci ency i n the pa ti ent? A. Decrea s ed di ges ti on of di eta ry protei n B. Decrea s ed excreti on of uri c a ci d C. Increa s ed a mi no a ci ds i n the bl ood D. Increa s ed fa tty a ci ds i n the bl ood E. Increa s ed gl ucos e i n the bl ood II-28. A pa ti ent pres ents wi th a nemi a a nd hyperbi l i rubi nemi a , refl ecti ng exces s i ve red bl ood cel l hemol ys i s . Her s ymptoms i ncl ude fa ti gue, pa l l or, a nd ja undi ce. Further eva l ua ti on s hows a defect i n s pectri n of her red bl ood cel l membra nes . Whi ch of the fol l owi ng i s the l i kel y di a gnos i s ?
A. Es s enti a l fa tty a ci d (EFA) defi ci ency B. Pyruva te ki na s e defi ci ency C. Spherocytos i s D. Ta rui ’s di s ea s e E. Zel l weger s yndrome II-29. Whi ch of the fol l owi ng des cri bes the ma l a te s huttl e s ys tem? A. Ca rri es NADH from the cytopl a s m di rectl y i nto the mi tochondri a l ma tri x. B. Genera tes three mol ecul es of ATP i n mi tochondri a per ea ch NADH from gl ycol ys i s . C. Moves NADH from the mi tochondri a to the cytopl a s m. D. Rel i es on ma l a te dehydrogena s e i n the i nner mi tochondri a l membra ne. E. Requi res reducti on of pyruva te to l a cta te for oxi di zi ng NADH. II-30. Whi ch of the fol l owi ng i s the fi na l pri ma ry product of the fa tty a ci d s yntha s e rea cti on i n a di pos e ti s s ue? A. Acetyl -CoA B. Ma l onyl -CoA C. Pa l mi ti c a ci d D. Pa l mi toyl -CoA E. Propi oni c a ci d II-31. A muta ti on tha t res ul ts i n a va l i ne repl a cement for gl uta mi c a ci d a t pos i ti on 6 of the β-cha i n of hemogl obi n S hi nders norma l hemogl obi n functi on a nd res ul ts i n s i ckl e cel l a nemi a when the pa ti ent i s homozygous for thi s muta ti on. Thi s i s a n exa mpl e of whi ch of the fol l owi ng types of muta ti on? A. Del eti on B. Fra mes hi ft C. Ins erti on D. Mi s s ens e E. Nons ens e II-32. In l i ver, whi ch of the fol l owi ng i nhi bi tory effects i s the key regul a tory event tha t ens ures newl y s ynthes i zed pa l mi toyl -CoA i s not i mmedi a tel y oxi di zed? A. Acyl -CoA s yntheta s e by ma l onyl -CoA. B. Ca rni ti ne–a cyl ca rni ti ne tra ns l oca s e (CAT) by pa l mi toyl -CoA. C. Ca rni ti ne–pa l mi toyl tra ns fera s e-I (CPT-I) by ma l onyl -CoA. D. CPT-I by pa l mi toyl ca rni ti ne. E. Ca rni ti ne–pa l mi toyl tra ns fera s e-II (CPT-II) by a cetyl -CoA. II-33. Some pa ti ents wi th fa mi l i a l hyperchol es terol emi a produce a trunca ted form of the LDL receptor, termed the “Leba nes e” a l l el e, whi ch l a cks three of the fi ve doma i ns of the protei n a nd ca us es i t to be reta i ned i n the endopl a s mi c reti cul um. Ana l ys i s of the muta nt gene i ndi ca ted tha t the s equence of the protei n wa s norma l up to the poi nt where i t termi na ted. The geneti c cha nge tha t produced the muta nt LDL receptor i n thes e ca s es ca n be cl a s s i fi ed a s whi ch type of muta ti on? A. Del eti on B. Ins erti on C. Mi s s ens e D. Nons ens e E. Si l ent
ANSWERS II-1. The answer is C. Importa nt ca rbohydra tes i ncl ude the di s a ccha ri des ma l tos e (gl ucos e–gl ucos e), s ucros e (gl ucos e–fructos e), a nd l a ctos e (ga l a ctos e–gl ucos e), a nd the gl ucos e pol ymers s ta rch (cerea l s , pota toes , a nd vegeta bl es ) a nd gl ycogen (a ni ma l ti s s ues ). Huma ns mus t convert di eta ry ca rbohydra tes to s i mpl e s uga rs (ma i nl y gl ucos e) for fuel , empl oyi ng i ntes ti na l enzymes a nd tra ns port s ys tems for enzyma ti c di ges ti on a nd a bs orpti on. Si mpl e s uga rs (ga l a ctos e a nd fructos e) a re converted to gl ucos e by l i ver enzymes , a nd the gl ucos e i s revers i bl y s tored a s gl ycogen. Enzyma ti c defi ci enci es i n i ntes ti na l di ges ti on (e.g., l a cta s e defi ci ency i n thos e wi th l a ctos e i ntol era nce), i n s uga r to gl ucos e convers i on (e.g., ga l a ctos e to gl ucos e convers i on i n ga l a ctos emi a ), or i n gl ycogenes i s /gl ycogenol ys i s (e.g., i n thos e gl ycogen s tora ge di s ea s es ) res ul t i n gl ucos e defi ci enci es (l ow bl ood gl ucos e or hypogl ycemi a ) wi th potenti a l a ccumul a ti on a nd toxi ci ty to hepa ti c ti s s ues . The i nfa nt ha d been norma l duri ng brea s tfeedi ng, excl udi ng l ow gl ucos e due to growth hormone defi ci ency, a nd coul d rea di l y di ges t brea s t mi l k l a ctos e wi th a bs orpti on a nd convers i on to gl ucos e. Low gl ucos e duri ng fa s ti ng a nd l i ver enl a rgement i mpl i es a l tered regul a ti on of gl ycogen s ynthes i s / rel ea s e due to one of the enzyme defi ci enci es wi thi n the ca tegory of gl ycogen s tora ge di s ea s e. The hepa tomega l y res ul ts from the a ccumul a ti on of exces s i ve a mounts of gl ycogen (Ta bl e 6-1; Fi gure 6-9). II-2. The answer is B. Fa tty a ci d oxi da ti on i s a ma jor s ource of energy a fter gl ycogen i s depl eted duri ng fa s ti ng. Fa tty a ci ds a re fi rs t coupl ed wi th CoA, tra ns ferred for mi tochondri a l i mport a s a cyl ca rni ti nes , a nd degra ded i n s teps tha t remove two ca rbons . The fa tty a cyl -CoA dehydrogena s es , enoyl hydra ta s es , hydroxya cyl -CoA dehydrogena s es , a nd thi ol a s es tha t ca rry out ea ch oxi da ti on s tep a re pres ent i n three groups wi th s peci fi ci ti es for very l ong-/l ong-, medi um-, a nd s hort-cha i n fa tty a cyl es ters . As woul d be expected, defi ci enci es of l ong-cha i n oxi di zi ng enzymes ha ve more s evere cons equences tha n thos e for s hort cha i ns beca us e they i mpa i r ma ny more cycl es of two-ca rbon remova l . Long-cha i n defi ci enci es ma y be l etha l i n the newborn peri od, wherea s medi um- or s hort-cha i n defi ci enci es ma y be undetected unti l a chi l d goes wi thout food for a prol onged ti me a nd mus t res ort to extens i ve fa tty a ci d oxi da ti on for energy. Medi um-cha i n CoA dehydrogena s e defi ci ency ca n be fa ta l i f not recogni zed, a nd s ometi mes pres ents a s s udden unexpl a i ned dea th s yndrome (us ua l l y a t ol der a ges tha n s udden i nfa nt dea th s yndrome, tha t i s , mos tl y from res pi ra tory probl ems ). The defi ci t of a cetyl CoA from fa tty a ci d oxi da ti on i mpa cts gl uconeogenes i s wi th hypogl ycemi a , a nd the energy defi ci t l ea ds to hea rt, l i ver, a nd mus cl e di s ea s e tha t ma y be l etha l . Unl i ke mos t ca us es of hypogl ycemi a , the i mpa i red fa tty a ci d oxi da ti on does not produce ketones (nonketoti c hypogl ycemi a ). Ca rni ti ne i s ti ed up a s medi um-cha i n a cyl ca rni ti nes a nd i s s econda ri l y defi ci ent i n fa tty a ci d oxi da ti on di s orders . Ra re pri ma ry ca rni ti ne defi ci enci es [a s i n a ns wer opti on (e)] i mpa i r oxi da ti on of a l l fa tty a ci ds beca us e they ca nnot be i mported i nto mi tochondri a . II-3. The answer is C. Des pi te s ome di fferences , protei n s ynthes i s i n proka ryotes a nd euka ryotes i s qui te s i mi l a r. The s ma l l ri bos oma l s ubuni t i s 30S i n proka ryotes a nd 40S i n euka ryotes . The l a rge ri bos oma l s ubuni t i s 50S i n proka ryotes a nd 60S i n euka ryotes . The i nta ct ri bos ome i s cons equentl y l a rger i n euka ryotes (80S) a nd s ma l l er i n proka ryotes (70S). At the s ta rt of tra ns l a ti on, i ni ti a ti on fa ctors ,
mRNA, a nd i ni ti a ti on a mi noa cyl –tRNA bi nd to the di s s oci a ted s ma l l ri bos oma l s ubuni t. The i ni ti a ti on tRNA i n proka ryotes i s N-formyl methi oni ne i n proka ryotes a nd s i mpl y methi oni ne i n euka ryotes . Onl y a fter the s ma l l ri bos oma l s ubuni t i s pri med wi th mRNA a nd i ni ti a ti on a mi noa cyl –tRNA does the l a rge ri bos oma l s ubuni t bi nd to i t. Once thi s ha ppens , el onga ti on fa ctors bri ng the fi rs t a mi noa cyl – tRNA of the na s cent protei n to the A s i te. Then pepti dyl tra ns fera s e forges a pepti de bond between the i ni ti a ti on a mi no a ci d a nd the fi rs t a mi no a ci d of the formi ng pepti de. Now uncha rged i ni ti a ti on tRNA l ea ves the P s i te a nd the pepti dyl –tRNA from the A s i te moves to the now va ca nt P s i te wi th the two a mi no a ci ds a tta ched. The ri bos ome a dva nces three ba s es to rea d the next codon a nd the proces s repea ts . When the s top s i gna l i s rea ched a fter the compl ete pol ypepti de ha s been s ynthes i zed, rel ea s i ng fa ctors bi nd to the s top s i gna l , ca us i ng pepti dyl tra ns fera s e to hydro-l yze the bond tha t joi ns the pol ypepti de a t the A s i te to the tRNA. Fa ctors prevent the rea s s oci a ti on of ri bos oma l s ubuni ts i n the a bs ence of new i ni ti a ti on compl ex (Fi gure 9-5). II-4. The answer is D. Gl ucos e-6-phos pha te dehydrogena s e (G6PD) i s the fi rs t enzyme of the pentos e phos pha te pa thwa y, a s i de pa thwa y for gl ucos e meta bol i s m whos e pri ma ry purpos e i s to produce ri bos e a nd NADPH. Its defi ci ency i s the mos t common enzymopa thy, a ffecti ng 400 mi l l i on peopl e worl dwi de. It contra s ts wi th gl ycol ys i s i n i ts us e of NADP ra ther tha n NAD for oxi da ti on, i ts producti on of CO2 , i ts producti on of pentos es (ri bos e, ri bul os e, a nd xyl ul os e), a nd i ts producti on of the hi gh-energy compound (5-phos phori bos yl -1pyrophos pha te) ra ther tha n ATP. Producti on of NADPH by the pentos e phos pha te pa thwa y i s cruci a l for reducti on of gl uta thi one, whi ch i n turn removes hydrogen peroxi de vi a gl uta thi one peroxi da s e. Erythrocytes a re pa rti cul a rl y s us cepti bl e to hydrogen peroxi de a ccumul a ti on, whi ch oxi di zes red bl ood cel l membra nes a nd produces hemol ys i s . Stres s es s uch a s newborn a djus tment, i nfecti on, or certa i n drugs ca n i ncrea s e red bl ood cel l hemol ys i s i n G6PD-defi ci ent i ndi vi dua l s , l ea di ng to s evere a nemi a , ja undi ce, pl uggi ng of rena l tubul es wi th rel ea s ed hemogl obi n, rena l fa i l ure, hea rt fa i l ure, a nd dea th. Beca us e the l ocus encodi ng G6PD i s on the X chromos ome, the defi ci ency exhi bi ts X-l i nked reces s i ve i nheri ta nce wi th s evere a ffl i cti on i n ma l es a nd tra ns mi s s i on through a s ymptoma ti c fema l e ca rri ers . Ri bos e-5-phos pha te produced by the pentos e phos pha te pa thwa y i s a n i mporta nt precurs or for ri bonucl eoti de s ynthes i s , but a l terna ti ve routes from fructos e-6-phos pha te a l l ow ri bos e s ynthes i s i n ti s s ues wi thout the compl ete cohort of pentos e phos pha te enzymes or wi th G6PD defi ci ency. The compl ete pentos e phos pha te pa thwa y i s a cti ve i n l i ver, a di pos e ti s s ue, a drena l cortex, thyroi d, erythrocytes , tes ti s , a nd l a cta ti ng ma mma ry gl a nd. Skel eta l mus cl e ha s onl y l ow l evel s of s ome of the enzymes of the pa thwa y but i s s ti l l a bl e to s ynthes i ze ri bos e through fructos e-6-phos pha te (Fi gure 6-7). II-5. The answer is E. Acetyl -CoA i s ca rboxyl a ted to form ma l onyl -CoA through the a ddi ti on of CO2 by a cetyl -CoA ca rboxyl a s e. The a cetyl - a nd ma l onyl -CoA groups a re a dded to s ul fydryl groups of fa tty a ci d s yntha s e mul ti enzyme compl ex (one on ea ch s ubuni t) through tra ns a cyl a ti on rea cti ons . Condens a ti on forms a cetoa cetyl -S-enzyme on one s ubuni t a nd a free s ul fhydryl group of the other s ubuni t—a s equence of enzyme rea cti ons then converts the a cetoa cetyl -S-enzyme to a cyl (a cetyl ) enzyme. A s econd round of two-ca rbon a ddi ti on begi ns , a s a nother ma l onyl -CoA res i due di s pl a ces the a cyl -S-enzyme to the other s ul fhydryl group, a nd then condens es to extend the a cyl group by two ca rbons . Fa tty a ci d s ynthes i s then proceeds by s ucces s i ve a ddi ti on of ma l onyl -CoA res i dues wi th condens a ti on, ca us i ng the a cyl cha i n to grow by two ca rbons wi th ea ch cycl e (Fi gure 7-1). II-6. The answer is D. Protei n s ynthes i s occurs i n the cytopl a s m, on groups of free ri bos omes ca l l ed pol ys omes , a nd on ri bos omes a s s oci a ted wi th membra nes , termed the rough endopl a s mi c reti cul um. However, protei ns des ti ned for s ecreti on a re onl y s ynthes i zed on ri bos omes of the endopl a s mi c reti cul um a nd a re s ynthes i zed i n s uch a ma nner tha t they end up i ns i de the l umen of the endopl a s mi c reti cul um. From there, the s ecretory protei ns a re pa cka ged i n ves i cl es . The Gol gi a ppa ra tus i s i nvol ved i n the O-gl ycos yl a ti on a nd pa cka gi ng of ma cromol ecul es i nto membra nes for s ecreti on. II-7. The answer is A. Argi ni ne i s a n a mi no a ci d us ed i n protei ns tha t i s a l s o pa rt of the urea cycl e. Ci trul l i ne a nd orni thi ne a re a mi no a ci ds not us ed i n protei ns but i mporta nt a s urea cycl e i ntermedi a tes . As pa rta te i s condens ed wi th ci trul l i ne to form a rgi ni nos ucci na te i n the urea cycl e, a nd a cetyl gl uta ma te i s a cofa ctor i n the joi ni ng of CO2 wi th a mmoni a to form ca rba moyl phos pha te a t the begi nni ng of the urea cycl e (Fi gure 5-9A; a l s o s ee Ta bl e 1-1). II-8. The answer is B. Chol era toxi n i s a n 87-kDa protei n produced by Vibrio cholerae, a Gra m nega ti ve ba cteri um. The toxi n enters i ntes ti na l mucos a l cel l s by bi ndi ng to G M1 ga ngl i os i de. It i ntera cts wi th G s protei n, whi ch s ti mul a tes a denyl cycl a s e. By ADP-ri bos yl a ti on of G s , the toxi n bl ocks i ts ca pa ci ty to hydrol yze bound GTP to GDP. Thus , the G protei n i s l ocked i n a n a cti ve form, a nd a denyl cycl a s e s ta ys i rrevers i bl y a cti va ted. Under norma l condi ti ons , i na cti va ted G protei n conta i ns GDP, whi ch i s produced by a phos pha ta s e ca ta l yzi ng the hydrol ys i s of GTP to GDP. When GDP i s s o bound to the G protei n, the a denyl cycl a s e i s i na cti ve. Upon hormone bi ndi ng to the receptor, GTP i s excha nged for GDP a nd the G protei n i s i n a n a cti ve s ta te, a l l owi ng a denyl cycl a s e to produce cAMP. Beca us e chol era toxi n prevents the hydrol ys i s of GTP to GDP, the a denyl cycl a s e rema i ns i n a n i rrevers i bl y a cti ve s ta te, conti nuous l y produci ng cAMP i n the i ntes ti na l mucos a l cel l s . Thi s l ea ds to a ma s s i ve l os s of body fl ui d i nto the i ntes ti ne wi thi n a few hours (Fi gures 8-8, 8-9A; Ta bl e 8-2). II-9. The answer is C. In the forma ti on of phos phoenol pyruva te duri ng gl uconeogenes i s , oxa l oa ceta te i s a n i ntermedi a te. In the fi rs t s tep, ca ta l yzed by pyruva te ca rboxyl a s e, pyruva te i s ca rboxyl a ted wi th the uti l i za ti on of one hi gh-energy ATP phos pha te bond: Pyruva te + ATP + CO2 → Oxa l oa ceta te + ADP + Pi The pyruva te i n gl uconeogenes i s i s deri ved mos tl y from l a cta te a nd to a l a rge extent from a l a ni ne, a s wel l a s to a l es s er extent from s ome other a mi no a ci ds . In the s econd s tep, ca ta l yzed by phos phoenol pyruva te ca rboxyki na s e, a hi gh-energy phos pha te bond of GTP dri ves the deca rboxyl a ti on of oxa l oa ceta te: Oxa l oa ceta te + GTP → Phos phoenol pyruva te + GDP + CO2 In contra s t to gl uconeogenes i s , the forma ti on of pyruva te from phos phoenol pyruva te duri ng gl ycol ys i s requi res onl y PK, a nd ATP i s produced (Fi gure 6-6). II-10. The answer is A. Acetyl -CoA ca rboxyl a s e ca ta l yzes the fi rs t s tep of l i pogenes i s i n whi ch a cetyl -CoA i s l i nked to ma l onyl -CoA. Thi s enzyme i s a cti va ted by ci tra te vi a pol ymeri za ti on a nd i na cti va ted by pa l mi toyl -CoA tha t ca us es depol ymeri za ti on to the monomeri c form. The monomeri c form ca n undergo phos phoryl a ti on by epi nephri ne or gl uca gon to put i t i nto a n i na cti ve conforma ti on tha t ca nnot rea di l y pol ymeri ze. Ins ul i n a cti va ti on of a phos pha ta s e revers es thi s cova l ent modi fi ca ti on. Acetyl -CoA does not rea di l y cros s the mi tochondri a l membra ne. Ins tea d, ci tra te tra ns l oca tes to the cytos ol where i t i s cl ea ved to a cetyl -CoA a nd oxa l oa ce-ta te by ATP-ci tra te l ya s e. Ci tra te i ncrea s es i n the fed s ta te a nd i ndi ca tes a n a bunda nt s uppl y of a cetyl -CoA for l i pogenes i s . Al though fa tty a ci d s yntha s e i s i n the pa thwa y, i ts regul a ti on i s fa r l es s i mporta nt. Acyl -CoA s yntheta s e i s not i n the l i pogenes i s pa thwa y per s e beca us e i t a cti va tes fa tty a ci ds to the fa tty a cyl -CoA form rega rdl es s of thei r s ource. Ca rni ti ne–a cyl ca rni ti ne tra ns l oca s e a nd ca rni ti ne–pa l mi toyl tra ns fera s e a re both i nvol ved i n the proces s of fa tty a ci d degra da ti on (Fi gure 7-1). II-11. The answer is E. The templ a te s tra nd refers to the DNA s tra nd tha t i s tra ns cri bed i nto mRNA. As for DNA, mRNA i s s ynthes i zed i n the 5′ to 3′ di recti on. The templ a te s tra nd i s a l wa ys rea d i n the 3′ to 5′ di recti on. The oppos i te DNA s tra nd i s known a s the codi ng s tra nd a nd ha s the s a me s equence a s the mRNA tra ns cri pt, except tha t U repl a ces T i n mRNA. Choi ces b a nd d a re DNAs , a s they conta i n T i ns tea d of U. II-12. The answer is D. Peps i n i s s ecreted i n a proenzyme form (peps i nogen) i n the s toma ch. Unl i ke the ma jori ty of proenzymes , i t i s not a cti va ted by protea s e hydrol ys i s but i ns tea d by s ponta neous a ci d hydrol ys i s . Hydrochl ori c a ci d s ecreted by the s toma ch l i ni ng crea tes the a ci d envi ronment. Al l the enzymes s ecreted by the pa ncrea s exi s t a l s o i n a proenzyme form i ncl udi ng tryps i nogen, chymotryps i nogen, proca rboxypepti da s e, a nd proel a s ta s e. La cta s e a nd phenyl a l a ni ne hydroxyl a s e a re a cti ve i n thei r na ti ve forms . Hepa ri n a nd kera ti n a re not enzymes .
II-13. The answer is A. Hormone s i gna l tra ns ducti on ul ti ma tel y l ea ds to the a cti va ti on of a va ri ety of ki na s es . Some of thes e depend on ca l ci um for thei r a cti vi ty. One of thes e i nvol ves ca l ci um fi rs t a tta chi ng to a ca l ci um-bi ndi ng protei n, ca l modul i n. Hence, thi s ki na s e i s referred to a s ca l modul i n-dependent ki na s e. Protei n ki na s e C depends on i ntera cti on wi th both ca l ci um a nd DAG for i ts ful l a cti vi ty. The DAG i s genera ted by the cl ea va ge of membra ne phos phol i pi ds ca ta l yzed by the enzyme phos phol i pa s e C. The other product of thi s cl ea va ge i s IP3 tha t ca us es the rel ea s e of ca l ci um from the endopl a s mi c reti cul um where i t i s s tored (Fi gure 8-9B). II-14. The answer is A. Hexoki na s e ca ta l yzes the convers i on of gl ucos e to gl ucos e-6-phos pha te i n the energy-requi ri ng fi rs t s tep of gl ycol ys i s . ATP i s a l s o requi red i n the convers i on of fructos e-6-phos pha te to fructos e 1,6-bi s phos pha te by PFK. ATP i s genera ted i n the convers i on of 1,3-bi s phos phogl ycera te to 3-phos phogl ycera te by phos phogl ycera te ki na s e a nd i n the convers i on of phos phoenol pyruva te to pyruva te by PK. Both phos phoenol pyruva te ca rboxyki na s e a nd pyruva te ca rboxyl a s e a re energy-requi ri ng rea cti ons except tha t thes e occur i n the gl uconeogenes i s pa thwa y (Fi gure 6-2). II-15. The answer is B. The pentos e phos pha te pa thwa y does not genera te a ny ATP but i ns tea d forms NADPH a nd ri bos e phos pha te. Gl ycol ys i s produces a net two ATP mol ecul es per gl ucos e. The ci tri c a ci d cycl e produces a net 12 ATP per turn of the cycl e. Fa tty a ci d oxi da ti on of pa l mi ta te res ul ts i n a tota l of 129 ATP. El ectron tra ns port i n the res pi ra tory cha i n res ul ts i n fi ve ATP for ea ch of the fi rs t s even a cetyl -CoA produced by the oxi da ti on of pa l mi ta te for a tota l of 35 ATP. Ea ch of the ei ght a cetyl -CoA mol ecul es produced from pa l mi ta te res ul ts i n 12 ATP from the ci tri c a ci d cycl e for 96 tota l ATP. Thi s gi ves a tota l of 131 ATP per pa l mi ta te oxi di zed, mi nus two ATP for the i ni ti a l a cti va ti on of pa l mi ta te for a gra nd tota l of 129 ATP per pa l mi ta te. II-16. The answer is E. The two s ubuni ts of ri bos omes a re compos ed of protei ns a nd rRNA. Ri bos omes a re found i n the cytopl a s m, mi tochondri a , a nd bound to the endopl a s mi c reti cul um. Tra ns cri pti on refers to the s ynthes i s of RNA compl ementa ry to a DNA templ a te a nd ha s nothi ng i mmedi a tel y to do wi th ri bos omes . II-17. The answer is D. Li ga nd–receptor a nd s ubs tra te–enzyme rea cti ons a re both s a tura bl e proces s es wi th s i mi l a r dependence of rea cti on ra te on l i ga nd/s ubs tra te a nd receptor/s ubs tra te concentra ti ons . Competi ti ve i nhi bi tors functi on by bi ndi ng to the s ubs tra te or l i ga ndbi ndi ng porti on of the a cti ve s i te a nd thereby bl ock a cces s to the s ubs tra te (Fi gure 5-3). Thus , the s tructures of competi ti ve i nhi bi tors tend to res embl e the s tructures of the s ubs tra te a nd a re often ca l l ed s ubs tra te or l i ga nd a na l ogs . The effects of competi ti ve i nhi bi tors ca n be overcome by ra i s i ng the concentra ti on of the s ubs tra te. The a mount the s ubs tra te mus t be i ncrea s ed i s dependent on the concentra ti on of the i nhi bi tor, the a ffi ni ty of the i nhi bi tor for the enzyme, a nd the a ffi ni ty of the s ubs tra te for the enzyme. For membra ne receptors s uch a s tha t i n the l ys os ome tha t i s defecti ve i n cys ti nos i s , the rea cti on ma y be one of membra ne tra ns port s uch tha t i nterna l s ubs tra te/l i ga nd i s i n equi l i bri um wi th externa l s ubs tra te/l i ga nd. Thus , l ys os ome–i nterna l cys ti ne i s s ubs tra te a nd l ys os oma l –externa l cys ti ne a product, i n a s ens e, s uch tha t l ys os oma l –externa l cys tea mi ne wi l l effecti vel y decrea s e externa l cys ti ne concentra ti on a nd l ea d to egres s of l ys os oma l cys ti ne through i ts defecti ve tra ns porter. II-18. The answer is B. Ja nus ki na s e i s a s ol ubl e receptor-a s s oci a ted tyros i ne ki na s e tha t hel ps ful l y a cti va te the receptor a fter bi ndi ng of the a ppropri a te hormone (e.g., growth hormone, prol a cti n, a nd l epti n). Adenyl a te ki na s e ca ta l yzes the revers i bl e i nterconvers i on of 2 ADP ↔ ATP + AMP. MAP ki na s e i s a s s oci a ted wi th tra ns ducti on of s i gna l s from growth fa ctors (e.g., epi derma l growth fa ctor, fi brobl a s t growth fa ctor, i ns ul i n-l i ke growth fa ctor, a nd other cytoki nes a nd growth fa ctors ) but not from growth hormone, whi ch i s more s i mi l a r to the meta bol i c hormones tha n the growth fa ctors . Protei n ki na s e A i s i nvol ved i n tra ns duci ng s i gna l s from hormones tha t a ct vi a G s protei n (e.g., ca techol a mi nes , gl uca gon, fol l i cl e-s ti mul a ti ng hormone, pa ra thyroi d hormone, a mong others ). PK i s the l a s t enzyme of the gl ycol yti c pa thwa y (Fi gure 8-9C; Ta bl e 8-1). II-19. The answer is C. Defi ci enci es of PK a nd PFK a re rul ed out by the el eva ted s erum l a cta te l evel s , a s thes e a re gl ycol yti c enzymes . The coma i s a s s oci a ted wi th a fa s ti ng hypogl ycemi a , whi ch i s i ndi ca ti ve of pyruva te ca rboxyl a s e defi ci ency. The el eva ted l a cta te a nd a l a ni ne occurs beca us e the pyruva te requi red for pyruva te ca rboxyl a s e i n gl uconeogenes i s i s deri ved mos tl y from l a cta te a nd to a l a rge extent a l s o from a l a ni ne, a s wel l a s to a l es s er extent s ome other a mi no a ci ds . The reducti on of a s pa rti c a ci d l evel s occurs beca us e there i s reduced forma ti on of oxa l oa ceta te, the product of the pyruva te ca rboxyl a s e rea cti on a nd oxa l oa ceta te provi des the ca rbon ba ckbone for s ynthes i s of a s pa rta te (Fi gure 6-6). II-20. The answer is B. Chol es terol i s formed i n fi ve s teps . The fi rs t s tep, bi os ynthes i s of meva l ona te, i s ca ta l yzed by three enzymes —a cetyl -CoA thi ol a s e, HMG-CoA s yntha s e, a nd HMG-CoA reducta s e. Thi ol a s e ca ta l yzes the condens a ti on of two mol ecul es of a cetyl -CoA to form a cetoa cetyl -CoA. HMG-CoA s yntha s e ca ta l yzes the a ddi ti on of a thi rd mol ecul e of a cetyl -CoA to form HMG-CoA. Thi s compound i s reduced to meva l ona te by HMG-CoA reducta s e. Thi s enzyme i s the pri nci pa l regul a tory s tep i n the pa thwa y. In the s econd s tep of chol es terol s ynthes i s , meva l ona te i s phos phoryl a ted a nd deca rboxyl a ted to produce i s opentyl di phos pha te. Si x of thes e i s oprenoi d uni ts a re condens ed to form s qua l ene i n the thi rd s tep. La nos terol i s formed i n the fourth s tep a nd i s s ubs equentl y converted to chol es terol (Fi gure 7-9). II-21. The answer is E. Two mol ecul es of GTP a re us ed i n the forma ti on of ea ch pepti de bond on the ri bos ome. In the el onga ti on cycl e, bi ndi ng of a mi noa cyl –tRNA del i vered by EF-2 to the A s i te requi res hydrol ys i s of one GTP. Pepti de bond forma ti on then occurs . Tra ns l oca ti on of the na s cent pepti de cha i n on tRNA to the P s i te requi res hydrol ys i s of a s econd GTP. The a cti va ti on of a mi no a ci ds wi th a mi noa cyl –tRNA s yntheta s e requi res hydrol ys i s of ATP to AMP pl us PPi. II-22. The answer is A. Sma l l mol ecul es ma y be i ntegra l (cova l entl y a tta ched) pa rts of enzymes (pros theti c groups ) or cofa ctors tha t pa rti ci pa te i n enzyme–s ubs tra te i ntera cti on or convers i on. Pros theti c groups ca nnot be di s s oci a ted from the enzyme by di l uti on a nd thus wi l l not be obvi ous components of the enzyme rea cti on when recons ti tuted i n the tes t tube. Cofa ctors , s uch a s vi ta mi n B 12 for MMA-CoA muta s e, a s s oci a te revers i bl y wi th enzymes or s ubs tra tes a nd ca n be a dded i n vi tro to obta i n enha ncement of the ca ta l yzed rea cti on(s ). Competi ti ve or feedba ck i nhi bi tors i ntera ct a t s ubs tra te or a l l os teri c bi ndi ng s i tes of the enzyme, reduci ng effecti ve s ubs tra te concentra ti on a nd rea cti on ra te or converti ng the enzyme to a l es s a cti ve conforma ti on. Vi ta mi n B 12 (cya nocoba l a mi n) i s a cofa ctor for MMA-CoA muta s e, a ccel era ti ng the convers i on of methyl ma l oni c a ci d to s ucci nyl -CoA through a cti vi ty of i ts coba l t group. Certa i n defects i n MMA-CoA muta s e ca n be a mel i ora ted by i ntra mus cul a r B 12 i njecti ons s o tha t effecti ve B 12 concentra ti on a nd muta s e a cti vi ty a re i ncrea s ed. II-23. The answer is E. Membra ne protei ns a re cl a s s i fi ed a s extrinsic—predomi na ntl y on the i nner or the outer s urfa ce of the membra ne bi l a yer —a nd intrinsic—predomi na ntl y wi thi n the membra ne. Intri ns i c protei ns a re us ua l l y compos ed of uncha rged, hydrophobi c a mi no a ci ds s o they ca n enter a nd s ta y i n the hydrophobi c envi ronment of the l i pi d bi l a yer. Intri ns i c protei ns s erve s evera l functi ons i ncl udi ng cha nnel s , “ca rri er protei ns ”—tra ns porti ng mol ecul es through the membra ne a s a “ca rri er protei n” or a s a s i gna l i ng protei n—cha ngi ng a porti on of thei r i nterna l cytopl a s mi c s tructure i n res pons e to a s i gna l a t a n expos ed externa l pa rt of thei r s tructure to a cti va te a ddi ti ona l , cl os el y a s s oci a ted mol ecul es l ea di ng to a res ul ti ng functi on (e.g., turni ng on a gene) (Fi gure 8-4). II-24. The answer is D. Thi s pa ti ent exhi bi ts s evera l s i gns of a cute compl ex I poi s oni ng. Beca us e compl ex I of oxi da ti ve phos phoryl a ti on i s a t l ea s t pa rti a l l y i nhi bi ted by the chi l d cons umi ng thi s poi s on, the a bi l i ty of mi tochondri a l NADH to be oxi di zed i s i mpa i red. Cons equentl y, NAD wi l l become l i mi ti ng for the PDH compl ex (Cha pter 6). Al though gl ycol ys i s a l s o requi res regenera ti ng NAD, enzymes i n thi s pa thwa y ca n opera te norma l l y beca us e NADH ca n be oxi di zed by the convers i on of pyruva te to l a cta te. Gl ucos e-6-phos pha te dehydrogena s e i n the pentos e phos pha te pa thwa y wi l l be una ffected beca us e i t us es NADP not NAD (Fi gure 6-7A). Li kel y pyruva te ca rboxyl a s e wi l l ha ve i ncrea s ed a cti vi ty beca us e pyruva te not us ed by the PDH compl ex coul d be proces s ed to oxa l oa ceta te. Fi na l l y, s ucci na te dehydrogena s e wi l l be una ffected beca us e i t produces FADH 2 ra ther tha n NADH. Hence, i t feeds el ectrons i nto compl ex II i ns tea d of compl ex I of the res pi ra tory cha i n.
II-25. The answer is E. The ketone bodi es , β-hydroxybutyra te a nd a cetoa ceta te, a re s ynthes i zed i n l i ver mi tochondri a from a cetyl -CoA. The l i ver produces ketone bodi es under condi ti ons of fa s ti ng a s s oci a ted wi th hi gh ra tes of fa tty a ci d oxi da ti on. The i na bi l i ty to get gl ucos e i nto extra hepa ti c cel l s beca us e of i ns ul i n defi ci ency i n di a betes a l s o i ncrea s es fa tty a ci d oxi da ti on a nd ketogenes i s . The a ci d groups of βhydroxybutyra te a nd a cetoa ceta te ca us e a ci dos i s a nd a n a ni on ga p (s um of s erum s odi um a nd pota s s i um mi nus the s um of chl ori de a nd bi ca rbona te) tha t i s grea ter tha n norma l (over 8–15). In the ca s e of di a betes , the “hi dden a ni ons ” tha t a dd to bi ca rbona te i n ba l a nci ng the ca ti ons ca n be recogni zed a s ketones through uri ne ketos ti x tes ti ng. In meta bol i c di s orders s uch a s methyl ma l oni c a ci duri a or fa tty a ci d oxi da ti on defects , there a re s ca nty a bnorma l or no ketones (i f fa t ca nnot be oxi di zed) s o the hi dden a ni ons mus t be i denti fi ed by pl a s ma a cyl ca rni ti ne or uri ne orga ni c a ci d profi l es . Acetone i s a ketone body produced i n di a betes tha t produces a n a ci d brea th duri ng ketoa ci dos i s (Fi gure 7-8A). II-26. The answer is D. Ami noa cyl –tRNA s yntheta s es a re res pons i bl e for cha rgi ng a tRNA wi th the a ppropri a te a mi no a ci d for tra ns l a ti on. Cha rgi ng a tRNA i s a two-s tep rea cti on. In the fi rs t s tep, the enzyme forms a n a mi noa cyl –AMP enzyme compl ex i n a rea cti on tha t requi res one ATP. In the s econd s tep, the a cti va ted a mi no a ci d i s a tta ched to the a ppropri a te tRNA a nd the enzyme a nd AMP a re rel ea s ed. II-27. The answer is C. N-a cetyl gl uta mi c a ci d i s a s ti mul a nt of the urea cycl e. A pa ti ent wi th a defi ci ency of the enzyme tha t ca ta l yzes i ts forma ti on woul d ha ve a decrea s ed a cti vi ty of the urea cycl e. Beca us e the ni trogen for the cycl e l a rgel y comes from a mi no a ci ds , the bl ood concentra ti on of a mi no a ci ds woul d i ncrea s e. A decrea s ed a bi l i ty to proces s thes e a mi no a ci ds i n fa s ti ng/s ta rva ti on for gl ucos e producti on coul d potenti a l l y l ea d to decrea s ed bl ood gl ucos e. Fa tty a ci d us a ge woul d be una ffected. Al though the urea cycl e proces s es the ni trogen from a mi no a ci ds deri ved from di eta ry protei ns , thei r di ges ti on woul d be una ffected. Ins tea d, bl ood a mi no a ci ds woul d a l s o ri s e a fter a mea l . Al though s ecreti on of urea woul d be di mi ni s hed, uri c a ci d i s deri ved from the degra da ti on of puri ne nucl eoti des a nd tha t woul d be una ffected. II-28. The answer is C. Di s ea s es of red bl ood cel l s often res ul t from cha nges i n l i pi d compos i ti on a nd/or defects i n protei ns a s s oci a ted wi th the red bl ood cel l membra ne. Spherocytos i s i s one s uch di s ea s e tha t res ul ts from defects i n s pectri n, a nkyri n, or other red bl ood cel l membra ne protei ns i mporta nt i n the s ta bi l i za ti on of the norma l bi conca ve s ha pe of red bl ood cel l s . Thi s uni que s ha pe i s requi red for red bl ood cel l s to tra vel ea s i l y a nd unda ma ged through bl ood ves s el s . As the na me i mpl i es , red bl ood cel l s a ppea r a s s pheres a nd pa ti ents often s uffer from l ow red bl ood cel l numbers (a nemi a ) due to the res ul ti ng des tructi on of the a ffected cel l s . EFA defi ci ency, PK defi ci ency, a nd Ta rui ’s di s ea s e (PFK defi ci ency) ca n a l l l ea d to a nemi a for va ri ous rea s ons . EFA defi ci ency ca us es i ns ta bi l i ty of the red bl ood cel l membra ne a nd hence hemol ys i s . Both PK a nd PFK defi ci enci es res ul t i n i ns uffi ci ent energy producti on to ma i nta i n red bl ood cel l membra ne i ntegri ty. Al though Zel l weger s yndrome ca n l ea d to ja undi ce, thi s i s rel a ted to di mi ni s hed l i ver functi on a nd not to red bl ood cel l i s s ues . II-29. The answer is B. The rol e of the ma l a te s huttl e i s , under a erobi c condi ti ons , to oxi di ze cytos ol i c NADH produced duri ng gl ycol ys i s to regenera te NAD to keep the gl ycol yti c pa thwa y opera ti ng. Beca us e there i s no tra ns porter to ca rry NADH i nto the mi tochondri a , the el ectrons retri eved duri ng the cytos ol i c oxi da ti on of NADH mus t be ca rri ed a cros s the mi tochondri a l i nner membra ne to the ma tri x a s ma l a te. In the ma tri x, ma l a te dehydrogena s e i n the ci tri c a ci d cycl e (Fi gure 6-4) us es the ma l a te to produce NADH tha t i s s ubs equentl y proces s ed vi a oxi da ti ve phos phoryl a ti on (Fi gure 6-5) to produce three mol ecul es of ATP. Under a na erobi c condi ti ons , NAD i s regenera ted for gl ycol ys i s by the reducti on of pyruva te to l a cta te. II-30. The answer is C. The pri ma ry fa tty a ci d tha t i s s tored for energy purpos es i s the 16-ca rbon pa l mi ti c a ci d. The product of fa tty a ci d s yntha s e i s a l wa ys the free (unes teri fi ed) fa tty a ci d. In a s ubs equent s tep ca ta l yzed by a cyl -CoA s yntheta s e, the fa tty a ci d i s a cti va ted to i ts fa tty a cyl -CoA form. Acetyl -CoA i s a precurs or for the s ynthes i s of fa tty a ci ds , a nd ma l onyl -CoA i s a n i ntermedi a te i n the proces s . Propi oni c a ci d i s a product of odd-cha i n fa tty a ci d degra da ti on (Fi gure 7-2). II-31. The answer is D. Mi s s ens e muta ti ons a re thos e i n whi ch a s i ngl e ba s e cha nge (poi nt muta ti on) res ul ts i n a codon tha t encodes for a di fferent a mi no a ci d res i due. The effects of thes e types of muta ti ons ca n ra nge from very mi nor or even undetecta bl e to ma jor, dependi ng on the i mporta nce of the a l tered res i due to protei n fol di ng a nd functi on. Nons ens e muta ti ons a re a l s o poi nt muta ti ons i n whi ch the a ffected codon i s a l tered to a s top (nons ens e) codon, res ul ti ng i n a trunca ted protei n. Fra mes hi ft muta ti ons a re due to one or two ba s e pa i r i ns erti ons or del eti ons s uch tha t the rea di ng fra me i s a l tered. Thes e muta ti ons genera l l y l ea d to trunca ted protei ns a s wel l beca us e, i n mos t protei n codi ng regi ons , the unus ed rea di ng fra mes conta i n numerous s top codons . II-32. The answer is C. Ma l onyl -CoA i s a uni que i ntermedi a te i n the pa thwa y for fa tty a ci d s ynthes i s a nd wi l l onl y be hi gh i n concentra ti on i n the cel l when fa tty a ci d s ynthes i s i s a cti ve. When the cel l i s s ynthes i zi ng fa tty a ci ds , i t woul d be energeti ca l l y i l l ogi ca l to i mmedi a tel y oxi di ze the newl y formed fa tty a ci d. The s i mpl es t mea ns of s l owi ng fa tty a ci d oxi da ti on i s to prevent the forma ti on of the fa tty a cyl ca rni ti ne deri va ti ve tha t mus t be formed for tra ns port of the fa tty a ci d i nto the mi tochondri a l ma tri x for oxi da ti on. Inhi bi ti on of a cyl CoA s yntheta s e woul d not work beca us e the fa tty a cyl -CoA form mus t be produced for es teri fi ca ti on of gl ycerol to form tri a cyl gl ycerol for s tora ge. Any control by pa l mi toyl -CoA woul d be i neffecti ve beca us e thi s i s the s ubs tra te for fa tty a ci d oxi da ti on. Inhi bi ti on of CAT woul d be i l l ogi ca l beca us e the fa tty a cyl ca rni ti ne deri va ti ves woul d a ccumul a te a nd be us el es s for a ny other functi on. Inhi bi ti on of ca rni ti ne– pa l mi toyl tra ns fera s e-I by pa l mi toyl ca rni ti ne woul d be feedba ck i nhi bi ti on by i ts product a nd woul d not be effecti ve. Inhi bi ti on of ca rni ti ne–pa l mi toyl tra ns fera s e-II by a cetyl -CoA woul d ca us e a ccumul a ti on of pa l mi toyl ca rni ti ne i n the mi tochondri a l ma tri x, l ea di ng to a depl eti on of ca rni ti ne (Fi gure 7-3). II-33. The answer is D. Producti on of a trunca ted protei n i ndi ca tes tha t a muta ti on ha s occurred, but thi s phenomenon ma y ha ve a ri s en from a fra mes hi ft muta ti on (i ns erti on or del eti on) or by a nons ens e muta ti on. The mos t l i kel y pos s i bi l i ty i s a nons ens e muta ti on beca us e s equence a na l ys i s of the trunca ted protei n s howed tha t i t ha d norma l (wi l d-type) s equence. Ins erti on a nd del eti on events often produce a s tretch of ga rbl ed or a bnorma l protei n s equence a t the C-termi na l end of the trunca ted protei n a ri s i ng from out-of-fra me tra ns l a ti on of the mRNA downs trea m of the muta ti on unti l a s top codon i s encountered.
SECTION III APPLIED BIOCHEMISTRY
CHAPTER 10 METABOLISM AND VITAMINS/MINERALS Co-authors/Editors: Maria L. Valencik and Cynthia C. Mastick Uni vers i ty of Neva da School of Medi ci ne, Depa rtment of Bi ochemi s try, Reno, NV
Meta bol i c Rol es of Ma jor Bi ochemi ca l Mol ecul es Integra ti on a nd Regul a ti on of Meta bol i s m Hormona l Control of Meta bol i s m Vi ta mi ns a nd Mi nera l s Revi ew Ques ti ons
OVERVIEW The i ntegra ti on of meta bol i s m i s a s tory of s uppl y a nd dema nd. Food i s i nges ted to s uppl y energy but mus t be converted to the ca rbohydra te, l i pi d, a nd a mi no a ci d forms the body ca n us e, pri ma ri l y gl ucos e a nd fa tty a ci ds . Indi vi dua l cel l s then convert the fuel s to us a bl e energy, a denos i ne tri phos pha te (ATP) a nd ni coti na mi de a deni ne di nucl eoti de (NADH). The body dema nds energy to functi on but i ndi vi dua l orga ns a nd ti s s ues requi re pa rti cul a r s ources of energy under va ryi ng condi ti ons . To convert cons umed food i nto the needed energy, the body us es a va ri ety of orga ns , ea ch wi th uni que meta bol i c profi l es , to i ntegra te a nd regul a te the us e a nd s tora ge of energy. Speci fi c regul a tory poi nts of bi ochemi ca l pa thwa ys provi de i mmedi a te control of the us a ge, convers i on, or s tora ge of food energy. Va ri ous hormones ca n a l s o regul a te thes e bi ochemi ca l pa thwa ys to provi de l onger term control of food convers i on a nd energy us a ge. Es s enti a l to both of thes e proces s es i s the ma i ntena nce of gl ucos e homeos ta s i s . Fi na l l y, vi ta mi ns a nd mi nera l s s erve i mporta nt functi ons a s cofa ctors i n ma ny of thes e meta bol i c rea cti ons . Thei r defi ci ency or exces s ca n l ea d to numerous di s ea s e s ta tes .
METABOLIC ROLES OF MAJOR BIOCHEMICAL MOLECULES The fi rs t cons i dera ti on i s the ma jor s ources of energy tha t ca n be us ed by the body, the nutri ents requi red for thei r meta bol i s m, a nd the bi ochemi ca l pa thwa ys tha t i ntegra te them. Amino acids (Cha pter 1, Fi gure 10-1) provi de s evera l ma jor bi ochemi ca l functi ons , i ncl udi ng s ervi ng a s (1) the bui l di ng bl ocks of protei ns ; (2) the precurs ors of hormones , neurotra ns mi tters , a nd other i mporta nt s i gna l i ng mol ecul es (s uch a s ni trous oxi de); a nd (3) contri butors to the puri ne a nd pyri mi -di ne components of nucl ei c a ci ds , co-enzymes [NADH a nd fl a vi n a deni ne di nucl eoti de (FADH 2 )], a nd other funda menta l bi ol ogi ca l mol ecul es . Addi ti ona l l y, exces s a mi no a ci ds ca n enter the ci tri c a ci d cycl e a nd ca n be us ed to genera te or s tore bi ol ogi ca l energy (Cha pter 5). Furthermore, the meta bol i s m of s ome a mi no a ci ds ca n be funnel ed i nto gl ucos e s ynthes i s (gl uconeogenes i s ) duri ng food depri va ti on.
Figure 10-1. Summary of Amino Acid Metabolism. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Carbohydrates (Cha pter 2, Fi gure 10-2) perform a funda menta l rol e a s the pri ma ry energy-producti on s ource for the huma n body. Gl ycol ys i s a nd the s ubs equent meta bol i c pa thwa ys form the pri ma ry energy mol ecul es ATP, NADH, a nd FADH 2 vi a the oxi da ti on of gl ucos e a nd other ca rbohydra tes (Cha pter 6). Stora ge of ca rbohydra tes a s gl ycogen offers a rea di l y a va i l a bl e s ource of energy when di eta ry ca rbohydra te i nta ke i s l ow (Cha pter 2). Ca rbohydra tes a re a l s o i mporta nt i n the s ynthes i s of ni coti na mi de a deni ne di nucl eoti de phos pha te (NADPH) (Cha pter 6) a nd nucl ei c a ci ds (Cha pter 4).
Figure 10-2. Transport and Fate of Major Carbohydrates and Amino Acids. [Reproduced wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Lipids (Cha pter 3, Fi gure 10-3) a re nonpol a r bi omol ecul es . In mos t ti s s ues , they s erve a pri ma ry s tructura l rol e a s the components of bi ol ogi ca l membra nes , crea ti ng a l i pi d bi l a yer vi a thei r hydrophobi c a nd hydrophi l i c enti ti es (Cha pters 7 a nd 8). Thei r rol es i n membra nes a s wel l a s i n pa thol ogi ca l proces s es s uch a s a theros cl eros i s (Cha pter 16) ha ve ra i s ed the a wa renes s of s a tura ted, mono-uns a tura ted, a nd pol yuns a tura ted forms wi th rega rd to thei r rol e i n di et. However, i n a di pos e ti s s ue, tri gl yceri des a re the ma jor s tora ge form of bi ol ogi ca l energy a nd thei r oxi da ti on yi el ds more energy per ca rbon tha n ca rbohydra tes (Cha pter 7). Li pol ys i s of tri gl yceri des mobi l i zes fa tty a ci ds tha t genera te energy through β-oxi da ti on a nd produces the s ubs tra tes neces s a ry for ketone body (a cetoa ceta te a nd β-hydroxybutyra te) s ynthes i s , a n es s enti a l fuel s ource duri ng prol onged s ta rva ti on. Oxi da ti on of both fa tty a ci ds a nd ketone bodi es s pa res gl ucos e by preventi ng i ts oxi da ti on. The cons umpti on of di eta ry chol es terol a nd fa ts ha s a l a rge i mpa ct on l i pi d meta bol i s m through the genera ti on of pl a s ma l i poprotei ns [chyl omi crons a nd l ow-dens i ty l i poprotei n (LDL) vi a very-l ow-dens i ty l i poprotei n (VLDL)]. The res ul ta nt el eva ti on of ha rmful l i pi ds /l i poprotei ns (dys l i pi demi a ) ha s nega ti ve meta bol i c cons equences tha t di rectl y i mpa ct hea l th a nd di s ea s e throughout a l l s oci oeconomi c cl a s s es of modern s oci ety.
Figure 10-3. Transport and Fate of Major Lipid Substrates and Metabolites. FFA, free fa tty a ci ds ; LPL, l i poprotei n l i pa s e; MG, monoa cyl gl ycerol ; TG, tri a cyl gl ycerol ; a nd VLDL, very-l ow-dens i ty l i poprotei n. [Reproduced wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.]
Vitamins, both l i pi d a nd nonl i pi d deri ved, s erve i mporta nt rol es a s cofa ctors i n meta bol i c pa thwa ys a nd rea cti ons (s ee end of thi s cha pter). Severa l di s ea s es , i ncl udi ng s curvy, ri ckets , a nd Werni cke–Kors a koff s yndrome, res ul t di rectl y from defi ci enci es of vi ta mi ns or, a s i n perni ci ous a nemi a , from the body’s i na bi l i ty to properl y a bs orb them. Minerals, i ncl udi ng s odi um, pota s s i um, chl ori de, ca l ci um, phos pha te, i ron, a nd others , pl a y ma jor rol es i n the regul a ti on of meta bol i c enzymes i nvol ved i n di ges ti on, i n the us e a nd/or s tora ge of food meta bol i tes , a nd i n the el i mi na ti on of wa s te products . Even more i mporta nt i s the i ntegra ti on of meta bol i s m of thes e mol ecul es i n the huma n body a nd how regul a ti on ca n be ma i nta i ned by i nterrel a ti ons hi ps between thei r a na bol i c a nd ca ta bol i c meta bol i s m. In thi s rega rd, the body’s a bi l i ty to s ens e energy l evel s , res pond to hormone s i gna l i ng, a nd upregul a te a nd downregul a te pa rti cul a r meta bol i c pa thwa ys i s pa ra mount for the body to ma i nta i n the proper a nd control l ed l evel of meta bol i c functi on a nd for the myri a d of s tructura l a nd functi ona l proces s es to occur, whi ch a l l ow l i fe.
INTEGRATION AND REGULATION OF METABOLISM 2+
ATP, a s s oci a ted wi th ma gnes i um (Mg ) for s ta bi l i ty, i s the pri ma ry form of bi ol ogi ca l energy uti l i zed by the huma n body (Fi gure 10-4).
Figure 10-4. Adenosine Triphosphate Structure with Its Magnesium Cofactor. [Reproduced wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] As s uch, the ca ta bol i c oxi da ti on of ca rbohydra tes (gl ycol ys i s , ci tri c a ci d cycl e, a nd oxi da ti ve phos phoryl a ti on), fa tty a ci ds / l i pi ds /ketone bodi es (fa tty a ci d degra da ti on), a nd a mi no a ci ds a l l l ea d eventua l l y to the producti on of ATP. In contra s t, a na bol i c meta bol i c proces s es (gl uconeogenes i s , gl ycogen s ynthes i s , l i pi d s ynthes i s , tri gl yceri de s ynthes i s , a nd a mi no a ci d s ynthes i s ) cons ume ATP, NADH, a nd/or NADPH to s tore energy (gl ucos e), to s tore energy, or to bui l d es s enti a l bi omol ecul es . Coupl ed to a l l of thes e proces s es i s the need to el i mi na te wa s te products , i ncl udi ng CO2 (exha l a ti on, a ci d–ba s e ba l a nce), rea cti ve a nd/or free-ra di ca l s peci es (a nti oxi da nts ), a nd urea (urea cycl e). Thes e concepts a re s umma ri zed i n Fi gure 10-5.
Figure 10-5. Interrelationship Between Proteins, Carbohydrates, and Fats. ATP, a denos i ne tri phos pha te; CoA, coenzyme A. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Thes e meta bol i c pa thwa ys a re i nti ma tel y l i nked a t s evera l poi nts i n bi ochemi ca l pa thwa ys , but a re a l s o s epa ra ted i nto di s ti nct compa rtments a nd/or orga nel l es (e.g., cytopl a s m vers us mi tochondri a vers us nucl eus , etc.) to a l l ow the neces s a ry regul a ti on a nd control . Addi ti ona l l y, ea ch orga n ha s uni que meta bol i c needs a nd functi ons a s s umma ri zed i n Fi gure 10-6. Thes e functi ons a nd needs mus t be coordi na ted i n a va ri ety of orga ns to ma i nta i n a cons ta nt s uppl y of energy whi l e pres ervi ng s ome energy for the future. The body a ccompl i s hes thi s goa l by us i ng the nervous s ys tem a nd hormona l s i gna l s to di fferenti a l l y s ti mul a te a nd i nhi bi t bi ochemi ca l pa thwa ys wi thi n va ri ous orga ns i n res pons e to s uppl y a nd dema nd. The ma i n s i gna l s us ed to regul a te meta bol i s m a re i ns ul i n, gl uca gon, ca techol a mi nes , gl ucocorti coi ds , a nd growth hormone (i n chi l dren).
Figure 10-6. Integration of Metabolism Among Major Organs. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] The rema i nder of thi s cha pter wi l l focus on the meta bol i s m i n three ma jor ti s s ues , the l i ver, a di pos e ti s s ue, a nd s kel eta l mus cl e (Fi gure 106). The l i ver a cti vel y provi des the qui ck fuel (gl ucos e) your body needs , wherea s a di pos e ti s s ue provi des l ong-term energy s tora ge. Fi na l l y, s kel eta l mus cl e a nd the res t of your body cons ta ntl y dema nd thi s energy. For exa mpl e, the bra i n cons umes a pproxi ma tel y 90 g of gl ucos e i n a da y, 20% of the a vera ge di et. The s uppl y a nd dema nd of energy mus t be conti nuous l y provi ded vi a di eta ry i nta ke or brea kdown of s tores to ba l a nce wi th the energy requi rements of res pi ra ti on, tra ns port, moti l i ty, a nd s ynthes i s of cel l s a nd ti s s ues (Fi gure 10-7). Overa l l , the a vera ge a dul t us es a pproxi ma tel y 24 kca l of energy per ki l ogra m of body ma s s to i ns ure proper hea l th a nd to ma i nta i n proper wei ght.
Figure 10-7. Factors Affecting Blood Glucose. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
Severa l key bi omol ecul es (gl ucos e-6-phos pha te or G6-P, pyruva te, a nd a cetyl coenzyme A or a cetyl -CoA) l i nk the bi ochemi ca l pa thwa ys for ca rbohydra tes , l i pi ds , a nd a mi no a ci ds / protei ns a nd the pa thwa ys they funnel i nto a re ti ghtl y regul a ted a nd ti s s ue s peci fi c (Fi gure 10-8). G6P, pyruva te, a nd a cetyl -CoA l i nk the a na bol i c a nd ca ta bol i c pa thwa ys of ca rbohydra te meta bol i s m to ma i nta i n a cons ta nt s uppl y of energy to ma i nta i n homeos ta s i s under cons ta ntl y cha ngi ng condi ti ons . The pa rti cul a r pa thwa ys a nd regul a ti on a l s o depend on the s peci fi c functi ons a nd needs of ea ch ti s s ue type.
Figure 10-8. Summary of Important Control Points of Metabolism. The three i mporta nt i ntermedi a ri es , gl ucos e-6-phos pha te, pyruva te, a nd a cetyl CoA a re i ndi ca ted. Meta bol i c pa thwa ys of i mporta nce a re i ndi ca ted i n red. See text for ful l di s cus s i on. ATP, a denos i ne tri phos pha te; CoA, coenzyme A; NADPH, ni coti na mi de a deni ne di nucl eoti de phos pha te. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] GLUCOSE-6-PHOSPHATE Meta bol i c regul a ti on a t thi s fi rs t bra nch poi nt, G6-P, i s cl ea rl y i l l us tra ted i n the l i ver (Fi gure 10-8). After the i nges ti on of ca rbohydra tes , gl ucos e ta ken up by the l i ver i s converted to G6-P by gl ucoki na s e. Thi s phos phoryl a ti on us es one ATP mol ecul e a nd tra ps the gl ucos e wi thi n l i ver cel l s (hepa tocytes ). Subs equentl y, G6-P i s meta bol i zed vi a one of the fol l owi ng three pa thwa ys : (a ) glycogenesis—the s tora ge of ca rbohydra tes a s gl ycogen, (b) glycolysis—the producti on of ATP, or (c) the pentose phosphate pathway—the producti on of NADPH a nd/ or fi ve-ca rbon (pentos e) s uga rs (Cha pter 6, Fi gure 10-8). The pa thwa y chos en depends upon the a cti va ti on s ta te of key enzymes (glycogen synthase a nd phosphofructokinase-1), s ubs tra te a va i l a bi l i ty (G6-P, ATP, a nd NADP+), a nd a l l os teri c effectors [ATP, a denos i ne monophos pha te (AMP), fructos e 2,6-bi s phos pha te (F2,6BP), hydrogen i ons (H +), a nd ci tra te]. The key enzymes i n gl ycogenes i s a nd gl ycol ys i s a re predomi na ntl y regul a ted by hormone-s ti mul a ted, cova l ent modi fi ca ti on (phosphorylation), wherea s the a l l os teri c effectors fi ne-tune the fl ow of ca rbons through thes e pa thwa ys . In contra s t, the pentos e phos pha te pa thwa y i s pri ma ri l y regul a ted by the a va i l a bi l i ty of G6-P a nd NADP+ (Cha pter 6). In the wel l -fed s ta te, when ATP a nd ci tra te concentra ti ons a re hi gh, phosphofructokinase-1 i s a l l os teri ca l l y i nhi bi ted, s l owi ng down the commi tted s tep of gl ycol ys i s (the producti on of fructose 1,6-bisphosphate) l ea di ng to i ncrea s ed concentra ti ons of G6-P. The i ncrea s ed concentra ti on of G6-P ca n s ti mul a te ca rbohydra te s tora ge i n two wa ys . Fi rs t, G6-P i s a pos i ti ve a l l os teri c effector of glycogen synthase, l ea di ng to the forma ti on of gl ycogen. Second, G6-P i ndi rectl y i nhi bi ts glycogen phosphorylase thereby i nhi bi ti ng glycogenolysis (gl ycogen degra da ti on). Al terna ti vel y, when the ra ti o of NADP+ to NADPH i s hi gh, G6-P ca n be s huttl ed i nto the pentos e phos pha te pa thwa y to genera te NADPH (reducti ve energy). Thi s reduci ng power i s us ed to s ynthes i ze a va ri ety of bi omol ecul es s uch a s , fa tty a ci ds , chol es terol , nucl eoti des a nd other cofa ctors a s needed. Under condi ti ons where the ra ti o of NADP+ to NADPH i s l ow, the pentos e pa thwa y wi l l not opera te rega rdl es s of the concentra ti on of G6-P. The producti on of gl ycogen i n the l i ver i s further control l ed by the hormones , i ns ul i n a nd gl uca gon (s ee bel ow), a nd the res ul ti ng phos phoryl a ti on or dephos phoryl a ti on of gl ycogen s yntha s e. Degra da ti on of gl ycogen i s decrea s ed concomi ta ntl y by counterregul a tory dephos phoryl a ti on or phos phoryl a ti on of gl ycogen phos phoryl a s e. Ea ti ng brea kfa s t, a fter a n overni ght fa s t, s ti mul a tes gl ycogen s ynthes i s (a nd i nhi bi ts gl ycogen brea kdown) i n prepa ra ti on for the next peri od of fa s ti ng. Thi s repl eni s hment i s control l ed by the fa vora bl e hi gh ra ti o of i ns ul i n to gl uca gon, l ea di ng to a cti va ti on of gl ycogen s yntha s e a cti vi ty a nd decrea s ed gl ycogen phos phoryl a s e a cti vi ty. Under thes e condi ti ons , the dema nd for de novo s ynthes i s of l i pi d wi l l ri s e a fter the gl ycogen i s repl a ced, us i ng ca rbons from exces s di eta ry ca rbohydra te (to s ynthes i ze fa tty a ci ds ). Addi ti ona l l y, i f chol es terol bi os ynthes i s i s a cti ve, exces s a cetyl Co A from fa tty a ci d ca ta bol i s m ca n be s ynthes i zed i nto chol es terol . Once l i pi d bi os ynthes i s commences , the uti l i za ti on of NADPH i ncrea s es the NADP+/NADPH ra ti o fa vori ng fl ux through the pentos e phos pha te pa thwa y. After a mea l , the key regul a tor tha t res ta rts gl ycol ys i s i n l i ver i s F2,6BP. F2,6BP concentra ti on i s control l ed by a bifunctional enzyme tha t i ncl udes both ki na s e a nd phos pha ta s e a cti ve s i tes . Under condi ti ons of a hi gh ra ti o of i ns ul i n to gl uca gon, the bi functi ona l enzyme (phosphofructokinase-2/fructose 2,6-bisphosphatase) i s dephos phoryl a ted, l ea di ng to s ti mul a ti on of phos phofructoki na s e-2. The res ul ti ng F2,6BP formed a l l os teri ca l l y a cti va tes phos phofructoki na s e-1 a nd hence i ncrea s es gl ycol ys i s whi l e s i mul ta neous l y i nhi bi ti ng fructos e 1,6 bi s phos pha ta s e, therefore s hutti ng down gl uconeogenes i s . Fol l owi ng food depri va ti on, thes e events a re revers ed wi th a hi gh ra ti o of gl uca gon to i ns ul i n, fa vori ng phos phoryl a ti on of the bi functi ona l enzyme s ti mul a ti ng the fructos e 2,6-bi s phos pha ta s e a cti vi ty a nd l ea di ng to decrea s ed phos phofructoki na s e-1 a cti vi ty. PYRUVATE The s econd ma jor bra nch poi nt i n the i ntegra ti on of meta bol i s m i s a t pyruvate (Cha pter 6, Fi gure 10-8). Pyruva te ca n be converted i nto four di fferent s ubs tra tes : l a cta te, a l a ni ne, oxa l o-a ceta te, a nd a cetyl -CoA, dependi ng upon the energy needs of a cel l . Therefore, i t i s a n i mporta nt i ntegra ti on poi nt where ca rbons a re s huttl ed between energy s tora ge, energy genera ti on, a nd/or bi os yntheti c rea cti ons . In the l i ver, pyruva te ca n undergo oxi da ti ve deca rboxyl a ti on to enter the ci tri c a ci d cycl e a nd ul ti ma tel y genera te ATP when energy l evel s a re l ow. Speci fi ca l l y, l ow energy l evel s i nhi bi t the a cti vi ty of a n i mporta nt regul a tory enzyme, pyruvate dehydrogenase kinase. Thi s i nhi bi ti on prevents phos phoryl a ti on of
the pyruva te dehydrogena s e compl ex to a n i na cti ve s ta te. Furthermore, thi s ki na s e i s i nhi bi ted by NAD +, pyruva te, a nd s ul fhydryl form of CoA (non-a cetyl a ted), s ubs tra tes of pyruva te dehydrogena s e. Therefore, when s ubs tra tes a re pl enti ful , pyruva te i s oxi da ti vel y deca rboxyl a ted to a cetyl -CoA. In the l i ver, pyruva te i s a l s o the poi nt where l a cta te a nd a l a ni ne (s ee bel ow) ca n be a cti vel y funnel ed i nto ei ther the ci tri c a ci d cycl e or gl uconeogenes i s vi a pyruva te ca rboxyl a ti on to oxa l oa ceti c a ci d (Cha pter 6) when l i ver gl ycogen or bl ood gl ucos e l evel s a re l ow. Duri ng s ta rva ti on, gl uconeogenes i s ca n produce up to 160 g of gl ucos e i n a da y, ha l f of thi s from a mi no a ci ds . Ha l f of thi s gl ucos e wi l l be us ed by the bra i n. As bl ood gl ucos e l evel s s ta bi l i ze a nd gl uconeogenes i s i s no l onger requi red, oxa l oa ceta te ca n re-enter the gl ycol yti c pa thwa y a t phos phoenol pyruva te or s huttl e ba ck i nto the mi tochondri a , a s ma l a te, to be us ed i n the ci tri c a ci d cycl e. If energy i s a bunda nt, hi gh NADH a nd a cetyl -CoA concentra ti ons a cti va te pyruva te dehydrogena s e ki na s e a nd a l s o s erve a s a l l os teri c i nhi bi tors of enzyma ti c a cti vi ti es wi thi n the PDH compl ex. Thi s effecti vel y turns off the pyruva te dehydrogena s e compl ex by phos phoryl a ti on a nd a l l os teri c control a nd s huts down the ci tri c a ci d cycl e. Hi gh ATP a nd a cetyl -CoA concentra ti ons a l s o s ti mul a te pyruvate carboxylase, the fi rs t s tep of gl uconeogenes i s (hormone regul a ti on of gl uconeogenes i s i s even more i mporta nt; s ee bel ow). Skel eta l mus cl e i l l us tra tes a nother i mporta nt wa y tha t pyruva te ca n be meta bol i zed (Fi gure 10-9). If oxygen l evel s a re l ow a nd anaerobic respiration becomes i mporta nt (s uch a s duri ng a qui ck s pri nt), pyruva te ca n be converted to lactate by lactate dehydrogenase wi th a n oxi da ti on of one NADH to NAD +, the l a tter bei ng es s enti a l for s us ta i ni ng gl ycol ys i s . In thi s s cena ri o, ATP i s s ol el y deri ved from a na erobi c gl ycol ys i s . La cta te ca n s ubs equentl y be converted ba ck to gl ucos e for energy producti on vi a the Cori cycle (i n the l i ver); when l a cta te concentra ti ons get too hi gh, feedba ck i nhi bi ti on bl ocks further convers i on of pyruva te to l a cta te. Hi gh l a cta te concentra ti ons a l s o crea te the s ens a ti on of “burni ng” i n mus cl es , whi ch s erves a s a s i gna l to the body to l i mi t further us e of thes e mus cl es . Furthermore, pyruva te ca n a l s o be converted i n mus cl e ti s s ue to the a mi no a ci d alanine vi a the alanine transaminase rea cti on. In a ma nner a na l ogous to the Cori cycl e, the alanine cycle then converts thi s a l a ni ne ba ck to pyruva te i n the l i ver where i t i s us ed to produce new gl ucos e vi a gl uconeogenes i s a s a s ource of energy for a na erobi c gl ycol ys i s i n mus cl e. Once oxygen l evel s a re res tored i n s kel eta l mus cl e, producti on of ATP vi a ci tri c a ci d cycl e/oxi da ti ve phos phoryl a ti on res umes .
Figure 10-9. The Lactic Acid (Cori) and Glucose–Alanine Cycles. Ca rbons from gl ucos e meta bol i s m i n mus cl e a re recycl ed to the l i ver ei ther a s l a cta te or a l a ni ne for reconvers i on to gl ucos e. Hence, when thes e cycl es opera te gl ucos e ca rbons a re s pa red. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] ACETYL-COA Acetyl-CoA i s the thi rd bra nch poi nt of pri ma ry meta bol i c control , a nd coordi na tes ca rbohydra te, ketone, a nd fa t/l i pi d pa thwa ys (Cha pter 6, Fi gures 10-8 a nd 10-10). Acetyl -CoA i s a s ubs tra te for the ci tri c a ci d cycl e a nd ca n be oxi di zed to genera te energy. However, when energy l evel s a re hi gh (hi gh NADH/ NAD + ra ti o), NADH i nhi bi ts the ci tri c a ci d cycl e a t the i s oci tra te dehydrogena s e a nd α-ketogl uta ra te dehydrogena s e s teps . Accumul a ti on of FADH 2 a l s o occurs , l ea di ng to a n i ncrea s e i n s ucci nyl -CoA tha t i nhi bi ts the cycl e a s wel l . Hormones a l s o pl a y a key, l onger term rol e i n the regul a ti on of fa tty a ci d s ynthes i s a nd degra da ti on (s ee bel ow). Acetyl -CoA i s a l s o requi red for producti on of the neurotra ns mi tter a cetyl chol i ne (s ee bel ow a nd Cha pter 19).
Figure 10-10. Overview of Acetyl-CoA Metabolism. ATP, a denos i ne tri phos pha te; CoA, coenzyme A; TCA, tri ca rboxyl i c a ci d. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] In the fed s ta te, exces s a cetyl -CoA ca n be di rected towa rd s ynthes i s of chol es terol a nd/or fa ts /tri a cyl gl ycerol s i n the l i ver. Duri ng s ta rva ti on, fa tty a ci d oxi da ti on s uppl i es energy for hepa tocytes to dri ve gl uconeogenes i s . Furthermore, a ny exces s a cetyl -CoA genera ted wi l l be us ed for the s ynthes i s of ketone bodi es . The ketones ca nnot be oxi di zed by the l i ver a nd a re exported a nd us ed a s a n a l terna te fuel for the bra i n, hea rt, a nd mus cl es . In fa ct, duri ng fa s ti ng/s ta rva ti on, the bra i n wi l l be hea vi l y rel i a nt on ketone bodi es , us i ng them for up to 70% of i ts energy requi rements , es peci a l l y i n prol onged s ta rva ti on. Hormones a l s o regul a te ketone body s ynthes i s (s ee bel ow). In both nutri ti ona l ci rcums ta nces , a cetyl -CoA ma y a cti va te pyruvate carboxylase, a l though for di fferent purpos es . In the fed s ta te, pyruva te ca rboxyl a s e converts pyruva te to oxa l oa ce-ta te, whi ch condens es wi th a cetyl -CoA produci ng ci tra te, the fi rs t product of the ci tri c a ci d cycl e. The ci tra te i s tra ns ported to the cytopl a s m for fa tty a ci d s ynthes i s . Hi gh ci tra te a cti va tes acetyl-CoA carboxylase to promote the forma ti on of fa tty a ci ds (Cha pter 7). Ci tra te, when too hi gh, i nhi bi ts phos phofructoki na s e-1, thus bl ocki ng gl ycol ys i s to prevent unneces s a ry meta bol i s m of more gl ucos e to pyruva te. The G6-P tha t ba cks up ca n be cycl ed through the pentos e pa thwa y to provi de NADPH for fa tty a ci d s ynthes i s , a s des cri bed a bove, or ma y be di rected towa rd gl ycogen s ynthes i s . In the s ta rva ti on s ta te, hi gh a cetyl -CoA from oxi da ti on of fa tty a ci ds s ti mul a tes pyruva te ca rboxyl a s e to promote gl uconeogenes i s . Low concentra ti ons of citrate a nd the other i ntermedi a tes of the ci tri c a ci d cycl e a s wel l a s l ow ATP/NADH/FADH 2 promote conti nua ti on of the ci tri c a ci d cycl e a nd oxi da ti ve phos phoryl a ti on. The ci tri c a ci d cycl e i ntermedi a tes ca n a l s o be us ed for the producti on of a mi no a ci ds or a s a n energy s ource (Cha pter 5). Low ci tra te/hi gh pa l mi toyl -CoA (from l i pol ys i s ) concentra ti ons prevent fa tty a ci d s ynthes i s . The res ul ta nt decrea s e of malonyl-CoA, a n a l l os teri c i nhi bi tor of ca rni ti ne pa l mi toyl tra ns fera s e 1 (CPT1), fa vors forma ti on of pa l mi toyl ca rni ti ne by CPT1, wi th s ubs equent tra ns port a cros s the mi tochondri a l membra ne a nd oxi da ti on i n the mi tochondri a .
HORMONAL CONTROL OF METABOLISM The coordi na ti on of meta bol i c pa thwa ys to a chi eve thi s es s enti a l ba l a nce pri ma ri l y depends on hormone; nerve a nd s i gna l i ng pa thwa ys , i ncl udi ng i ns ul i n, gl uca gon, ca techol a mi nes (Cha pter 19), gl ucocorti coi ds (s l ower, s tres s -rel a ted cha nges ); a nd cytoki nes . Erra nt control l ea ds to di s ea s e s ta tes i f gl ucos e l evel s a re hi gh (di a betes mel l i tus or DM) or l ow (hypogl ycemi a ) a nd, i f too l ow, even dea th due to coma . INSULIN Ins ul i n (Fi gure 10-11) i s the a na bol i c hormone of the wel l -fed s ta te a nd a n i mporta nt s i gna l to s ti mul a te s tora ge of exces s nutri ents a s gl ycogen a nd tri gl yceri des (fa t i n a di pos e ti s s ue).
Figure 10-11. Preproinsulin Processing. Preproi ns ul i n (top) i s compos ed of a l ea der s equence (bl ue), A (green) a nd B (yel l ow) i ns ul i n cha i ns , a nd C (red) pepti de. Remova l of the l ea der s equence produces proi ns ul i n (bottom l eft). Cl ea va ge of C-pepti de from proi ns ul i n l ea ds to the producti on of a cti ve i ns ul i n. Beca us e C-pepti de i s produced i n equa l a mounts to i ns ul i n, i t ha s become a n i mporta nt mea s ure of i ns ul i n producti on. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] The a cti on of i ns ul i n (Fi gure 10-12) i s experi enced by three ma i n ta rgets , the l i ver, a di pos e ti s s ue, a nd s tri a ted mus cl e. The s ynthes i s a nd rel ea s e of i ns ul i n i s s ti mul a ted by gl ucos e a nd potenti a ted by a mi no a ci ds . In the l i ver, i ns ul i n s ti mul a tes gl ycogenes i s (gl ycogen s ynthes i s ), fa tty a ci d s ynthes i s , gl ycol ys i s , a nd the pentos e phos pha te pa thwa y. In the a di pos e ti s s ue, i t s ti mul a tes gl ucos e a nd fa tty a ci d upta ke a nd tri gl yceri de s ynthes i s (energy s tora ge). Si mi l a rl y, i n s kel eta l mus cl e, i t s ti mul a tes gl ucos e upta ke, gl ycogenes i s , a nd protei n s ynthes i s . It i s noteworthy tha t i ns ul i n does not i nfl uence gl ucos e meta bol i s m i n ei ther the bra i n or red bl ood cel l s .
Figure 10-12. Metabolic Systems Affected by Insulin. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] The rel ea s e of i ns ul i n from the pa ncrea ti c β-cel l s (Fi gure 10-13) i s the res ul t of i ncrea s ed bl ood gl ucos e concentra ti ons . Gl ucos e enters the β-cel l s vi a the gl ucos e tra ns porter 2 (GLUT2) (pa s s i ve tra ns port). The GLUT2 ha s a wea k a ffi ni ty for gl ucos e s o tha t i t fa vors gl ucos e upta ke onl y a fter a mea l , when bl ood gl ucos e l evel s a re hi gh, ra ther tha n i n the fa s ted s ta te. Fol l owi ng gl ucos e oxi da ti on, the i ncrea s ed ATP concentra ti on s ti mul a tes K+ cha nnel s a nd depol a ri zes the cel l membra ne. Thi s depol a ri za ti on opens vol ta ge-ga ted Ca 2+ cha nnel s . Other s i gna l s rel a ted to producti on of i nos i tol tri s phos pha te, a s econd mes s enger, s ti mul a te Ca 2+ rel ea s e from the endopl a s mi c reti cul um, res ul ti ng i n hi gh i ntercel l ul a r Ca 2+ concentra ti on a nd tri ggeri ng the rel ea s e of i ns ul i n.
Figure 10-13. Regulation of Secretion of Insulin via Glucose. Pa ncrea ti c β-cel l s a re i nduced to s ecrete i ns ul i n by (1) the upta ke of gl ucos e a nd i ts oxi da ti ve meta bol i s m i n mi tochondri a , whi ch produces i ncrea s ed concentra ti on of ATP. (2) Increa s ed ATP ca us es cl os ure of the ATP-s ens i ti ve K+ cha nnel s , l ea di ng to depol a ri za ti on. (3) The depol a ri za ti on of the cel l ca us es i n i nfl ux of Ca 2+ from vol ta ge-ga ted Ca 2+ cha nnel s . (4) The i ncrea s ed Ca 2+ a l ong wi th IP3 a nd other s i gna l i ng i nduces further rel ea s e of Ca 2+ from the endopl a s mi c reti cul um (ER), whi ch prompts exocytos i s of i ns ul i n, produced i n the ER, a nd s ubs equentl y proces s ed a nd rel ea s ed from s ecretory gra nul es of the Gol gi a ppa ra tus . ATP, a denos i ne tri phos pha te; GLUT2, gl ucos e tra ns porter 2; IP3 , i nos i tol tri s phos pha te. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Ins ul i n a ffects the meta bol i s m of cel l s tha t ha ve i ns ul i n receptors : l i ver cel l s (hepa tocytes ), fa t cel l s (a di pocytes ), a nd mus cl e cel l s (Ta bl e 10-1). The bra i n a nd red bl ood cel l s a re not a ffected by i ns ul i n. Ins ul i n works vi a a tyros i ne ki na s e receptor, whi ch phos phoryl a tes ta rget protei ns tha t l ea d to a number of meta bol i c effects . One effect i s the ra pi d tra ns l oca ti on of a gl ucos e tra ns porter 4, GLUT4, from ves i cl es to the cel l s urfa ce of s kel eta l a nd ca rdi a c mus cl e a nd fa t cel l s , i ncrea s i ng gl ucos e tra ns port i nto thes e cel l s . Ins ul i n a l s o regul a tes meta bol i c
enzymes s uch a s gl ycogen s yntha s e a nd phos phoryl a s e through a cti va ti on of type I phos pha ta s e a nd dephos phoryl a ti on.
TABLE 10-1. Ins ul i n Effects on Meta bol i s m GLUCAGON Gl uca gon (Fi gure 10-14) i s the hormone of fa s ti ng produced by pa ncrea ti c α-cel l s , a dja cent to the i ns ul i n-produci ng α-cel l s . Gl uca gon s i gna l s vi a G-protei n coupl ed receptors a nd the s econda ry mes s enger mol ecul e cycl i c AMP. In contra s t to ma ny ma mma l s , gl uca gon a cts a l mos t excl us i vel y on the l i ver i n huma ns (Ta bl e 10-2). Pri ma ri l y, i t s ti mul a tes gl ycogenol ys i s , gl uconeogenes i s , a nd fa tty a ci d oxi da ti on. Gl uca gon l evel s i ncrea s e two-to threefol d i n res pons e to hypogl ycemi a , a nd the l i ver begi ns producti on of gl ucos e from gl ycogen. Duri ng ti mes of hi gh bl ood gl ucos e, gl uca gon i s reduced to ha l f of i ts norma l l evel . Gl uca gon a l s o s ti mul a tes the rel ea s e of i ns ul i n, thereby a l l owi ng i ns ul i ns ens i ti ve cel l s to ta ke up the rel ea s ed gl ucos e. The del i ca te ba l a nce of gl uca gon a nd i ns ul i n l evel s i s how the body ma i nta i ns gl ucos e homeos ta s i s under va ryi ng condi ti ons .
Figure 10-14. Preproglucagon. Preprogl uca gon, produced by pa ncrea ti c α-cel l s , i s proces s ed to a cti ve gl uca gon (ora nge). [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.]
TABLE 10-2. Gl uca gon Effects on Meta bol i s m CATECHOLAMINES Ca techol a mi nes , i ncl udi ng norepi nephri ne a nd epi nephri ne, the l a tter bei ng pri ma ri l y the hormone res pons i bl e for the “fi ght or fl i ght” res pons e to externa l s tres s es , ca n provi de a l mos t i mmedi a te (wi thi n s econds ) regul a ti on of meta bol i s m (Fi gure 10-15). Speci fi ca l l y, they s ti mul a te gl ycogenol ys i s a nd gl ycol ys i s for the producti on of ATP i n the mus cl e. At the s a me ti me, they i nhi bi t gl ycol ys i s i n the l i ver a nd s ti mul a te gl ycogenol ys i s to provi de gl ucos e for the bl ood. More recentl y, s yna pti ca l l y rel ea s ed ca techol a mi nes ha ve emerged a s the ma i n phys i ol ogi ca l pa thwa y for the a cti va ti on of l i pol ys i s under condi ti ons of fa s ti ng (a condi ti on of chroni c s tres s ). The effects of epi nephri ne on meta bol i s m a re s umma ri zed i n Ta bl e 10-3.
Figure 10-15. Integrated Control of Blood Glucose Concentration. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.]
TABLE 10-3. Epi nephri ne Effects on Meta bol i s m GLUCOCORTICOIDS Corti s ol , a glucocorticoid, i s a chroni c s tres s hormone tha t a l s o regul a tes meta bol i s m but i n the ti me fra me of hours to da ys . Wi th prol onged s tres s , the hypotha l a mus i ncrea s es s ecreti on of corti cotrophi n-rel ea s i ng fa ctor, whi ch s ubs equentl y l ea ds to producti on a nd s ecreti on of a drenocorti cotropi c hormone from the a nteri or pi tui ta ry gl a nd a nd then corti s ol from the a drena l gl a nds . Corti s ol ha s much of the s a me i nfl uence on meta bol i s m a s epi nephri ne but functi ons vi a a cti va ti on of tra ns cri pti on a nd tra ns l a ti on of genes ra ther tha n modul a ti on of enzyme a cti vi ty. Under condi ti ons where i ns ul i n decl i nes a nd/ or corti s ol l evel s ri s e, corti s ol s ti mul a tes tra ns cri pti on of l i pa s es i nvol ved i n l i pogenes i s (gl ucos e s pa ri ng), enzymes i nvol ved i n gl uconeogenes i s a nd gl ycogenes i s i n the l i ver, a nd i n the brea kdown of mus cl e protei n. The net effect i s res tored bl ood gl ucos e a nd l a rger gl ycogen s tores i n the l i ver. However, thi s i ncrea s e i s a t the expens e of mus cl e a nd bone a nd ul ti ma tel y i mpa i rs i mmunol ogi ca l functi on. DIABETES MELLITUS (DM) DM i s a condi ti on cha ra cteri zed by ei ther the tota l l a ck of i ns ul i n (Type 1) or res i s ta nce of peri phera l ti s s ues to the effects of i ns ul i n (Type 2). Both di s ea s es l a ck the s i gna l i ng effect of i ns ul i n i n the pres ence of norma l or hi gh gl uca gon a nd other meta bol i c s i gna l s (Fi gure 10-16). The di s ea s e of DM i s due to the i mba l a nce i n ca rbohydra te meta bol i s m a nd i ts effects on other meta bol i c pa thwa ys .
Figure 10-16. Metabolic Events Occurring in Diabetes Mellitus. Overvi ew of effects i ncurred by the defi ci ency of i ns ul i n a nd exces s gl uca gon, i ncl udi ng thos e on gl ucos e, protei ns /a mi no a ci ds , a nd l i pi ds . Al l effects l ea d to dehydra ti on a nd the condi ti on of di a beti c ketoa ci dos i s , whi ch ca n be fa ta l . [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] In type 1 DM, a utoi mmune des tructi on of the pa ncrea ti c β-cel l s l ea ds to a compl ete l os s of i ns ul i n producti on. Al though the l i ver ca n ma ke gl ucos e, gl ycogen s ynthes i s i s i mpeded. In the a bs ence of i ns ul i n, gl uconeogenes i s i s unres tra i ned, el eva ti ng bl ood gl ucos e (Fi gure 10-17). However, mus cl e a nd fa t cel l s ca nnot ta ke up a va i l a bl e bl ood gl ucos e vi a the GLUT4. Thus , the body i s una bl e to cl ea r the el eva ted bl ood gl ucos e, a nd the peri phera l ti s s ues (mus cl e a nd fa t) a re s ta rved for gl ucos e even when pres ent a t very hi gh l evel s i n the bl ood. Furthermore, i n the a bs ence of i ns ul i n, gl uca gon s ecreti on i s uncoupl ed from the bl ood gl ucos e l evel s (i ns ul i n i s a n i mporta nt phys i ol ogi ca l regul a tor of gl uca gon s ecreti on). Unoppos ed gl uca gon, together wi th the other counter regul a tory hormones (ca techol a mi nes , corti s ol , a nd growth hormone), i nhi bi ts gl ycogen s ynthes i s a nd s ti mul a tes gl uconeogenes i s , gl ycogenol ys i s , a nd l i pol ys i s . Increa s ed l i pol ys i s l ea ds to el eva ti on of free fatty acids i n the bl ood s trea m. Thes e fa tty a ci d mol ecul es a re pa rtl y ta ken up by l i ver a nd i ncorpora ted i nto lipoproteins to i ncrea s e VLDL a nd LDL l evel s , a ri s k fa ctor for hea rt di s ea s e. Ketone bodies a re a l s o produced beca us e of the exces s of l i pol ys i s , whi ch ca nnot be i nhi bi ted i n the a bs ence of i ns ul i n. Thi s ca n res ul t i n the da ngerous condi ti on ketoa ci dos i s , i f the ketone body l evel becomes too el eva ted. The onl y a va i l a bl e trea tment i s the i njecti on of exogenous i ns ul i n i nto the body. However, even wi th opti ma l control , the da ma gi ng effects of el eva ted gl ucos e a nd l i pi ds eventua l l y l ea d to medi ca l compl i ca ti ons .
Figure 10-17. Type 1 Diabetes Mellitus. The effect of thi s di s ea s e on orga ns a nd ma jor meta bol i c pa thwa ys i s i l l us tra ted. The a bs ence of i ns ul i n i n type 1 di a betes i nhi bi ts (red ba rs ) the upta ke/convers i on of gl ucos e by mus cl e a nd a di pos e ti s s ue, l ea di ng to a n i ncrea s e i n gl ucos e (red a rrow) s ynonymous wi th the di s ea s e. Upta ke of fa tty a ci ds from tri a cyl gl ycerol s by a di pos e ti s s ue i s a l s o i nhi bi ted (red ba rs ), l ea di ng to a n i ncrea s e i n i ts l evel s (red a rrow). Res ul ti ng cha nges i n meta bol i s m l ea d to i ncrea s ed fa tty a ci ds a nd ketone bodi es (red a rrows ), the l a tter of whi ch contri butes to di a beti c ketoa ci dos i s s een i n type 1 di a beti c pa ti ents . See the text for further di s cus s i on. VLDL, very-l ow-dens i ty l i poprotei n. [Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.] Diabetes and the Polyol Pathway. DM i s a di s ea s e wi th the ha l l ma rk of el eva ted bl ood gl ucos e. Al though the mecha ni s m i s not compl etel y a greed, the hi gh gl ucos e l evel s a re s peci fi ca l l y detri menta l to the kidneys, retina, a nd nerves beca us e of thei r a bi l i ty to tra ns port gl ucos e wi thout the a i d of i ns ul i n. In thes e ti s s ues , exces s gl ucos e enters the sorbitol–aldolase reductase pathway (a l s o known a s the polyol pathway) where i t i s reduced to s orbi tol a nd then fructos e, oxi di zi ng NADPH to NADP+ a nd reduci ng NAD + to NADH, duri ng the enzyma ti c rea cti ons .
Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009. When gl ucos e concentra ti ons a re norma l , thi s pa thwa y i s mi ni ma l l y a cti ve beca us e of l ow a ffi ni ty of gl ucos e for the enzyme aldolase reductase. However, when gl ucos e l evel s a re hi gh, thi s rea cti on i s more promi nent. The res ul ti ng decrea s e i n NADPH a nd i ncrea s e i n NADH a ffects other enzyma ti c rea cti ons tha t us e thes e mol ecul es a s cofa ctors . NADPH i s requi red for producti on of reduced glutathione a nd ni tri c oxi de requi red for detoxi fi ca ti on of reactive oxygen species. Lowered NAD + a l s o l ea ds to a ddi ti ona l rea cti ve oxygen s peci es , a nd producti on of inositol (s i gna l i ng, i ncl udi ng i ns ul i n receptor) i s a l s o decrea s ed. The effects of l owered NADPH a nd NAD + res ul t i n conti nua l da ma ge to thos e ti s s ues where the pol yol pa thwa y i s mos t promi nent, ca us i ng ki dney, eye, a nd nerve probl ems s een i n ma ny di a beti c pa ti ents . Type 2 DM i s cha ra cteri zed by the producti on of i ns ul i n but res i s ta nce of i ts effects on ta rget ti s s ues . As a res ul t of thi s res i s ta nce, the huma n body a cts a s i f there i s a rel a ti ve defi ci ency of i ns ul i n, even when pres ent a t hi gh l evel s (Fi gure 10-18). The di s ea s e s ha res ma ny tra i ts wi th type 1 DM. As i n type 1, gl uconeogenes i s i s unres tra i ned, a nd mus cl e a nd fa t cel l s do not ta ke up gl ucos e vi a the GLUT4. As a res ul t, hi gh l evel s of bl ood gl ucos e a re pres ent. However, the l i ver s ti l l ca n ma ke gl ycogen, a nd l i pol ys i s i s kept i n check beca us e of decrea s ed but pres ent i ns ul i n. However, pl a s ma l i poprotei ns a re typi ca l l y el eva ted, often a s a cons equence of obes i ty a nd poor nutri ti on. Ketoa ci dos i s i s not a common s equel a to type 2 DM. However, i t ca n occur i n type 2 DM pa ti ents under condi ti ons of a ddi ti ona l meta bol i c s tres s a nd a fter pa ncrea ti c fa i l ure l ea ds to decrea s ed producti on a nd s ecreti on of i ns ul i n. Some ol der peopl e wi th type 2 DM ma y experi ence a di fferent s eri ous condi ti on ca l l ed Hyperosmolar hyperglycemic nonketotic syndrome (HHNS), a condi ti on i n whi ch the body tri es to get ri d of exces s s uga r by pa s s i ng i t i nto the uri ne. HHNS i s us ua l l y brought on by a n i l l nes s , i nfecti on, or other fa ctors .
Figure 10-18. Type 2 Diabetes Mellitus. The effect of thi s di s ea s e on orga ns a nd ma jor meta bol i c pa thwa ys i s i l l us tra ted. Ins ul i n res i s ta nce i n type 2 di a betes i nhi bi ts (red ba rs ) the upta ke/convers i on of gl ucos e by mus cl e a nd a di pos e ti s s ue, l ea di ng to a n i ncrea s e i n gl ucos e (red a rrow) s ynonymous wi th the di s ea s e. Upta ke of fa tty a ci ds from tri a cyl gl ycerol s by a di pos e ti s s ue i s a l s o i nhi bi ted (red ba rs ), l ea di ng to i ncrea s e i n i ts l evel s (red a rrow). Unl i ke type 1 di a betes , fa tty a ci ds a nd ketone bodi es a re not i ncrea s ed (not s hown) a nd di a beti c ketoa ci dos i s i s , therefore, ra re. See the text for further di s cus s i on. VLDL, very-l ow-dens i ty l i poprotei n. [Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.] DM and Advanced Glycation End Products: Increa s ed l evel s of s orbi tol a nd fructos e i n di a beti c pa ti ents l ea d to nons peci fi c a tta chment onto protei ns of ca rbohydra te mol ecul es vi a ca rbohydra te–ni trogen l i nks (s ee the fi gure bel ow), for exa mpl e, l ys i ne a nd a rgi ni ne a mi no a ci ds , proporti ona l to the l evel of gl ucos e i n the body. Thes e erroneous l y modi fi ed protei ns a re referred to a s advanced glycated (also known as glycosylation) endproducts (AGEs). The bes t known AGE i s hemogl obi n A1C i n ci rcul a ti ng red bl ood cel l s , whi ch forms the ba s i s for tes ti ng of di a beti c control . AGE protei ns a nd thei r brea kdown products ca us e oxi da nt da ma ge to the ki dney a nd a l s o i ncrea s e the producti on of cytoki nes (e.g., tumor necros i s fa ctor-β), whi ch da ma ges the gl omerul us . In a ddi ti on, they i ncrea s e the permeability of blood vessels, i ncrea s e oxidized LDL l evel s a nd i ncrea s e cytoki ne-rel a ted oxidative stress.
Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009. AGE protei ns a l s o bi nd to a receptor for advanced glycation endproducts (RAGE). Acti va ti on of RAGE l ea ds to nuclear factor kappa B i nducti on of s evera l i nfl a mma tory gene products res ul ti ng i n a chroni c i nfl a mma tory envi ronment. Thi s l ong-term i nfl a mma ti on promotes s uch va ri ed di s ea s e s ta tes a s a theros cl eros i s wi th a ccompa nyi ng hea rt a tta cks a nd s trokes . Da ma ge to the nerves a nd reti na i s a l s o promi nent,
l ea di ng to diabetic neuropathy a nd retinopathy, the l a tter of whi ch often l ea ds to a voi da bl e bl i ndnes s .
VITAMINS AND MINERALS VITAMINS Vi ta mi ns a nd mi nera l s s erve a s i mporta nt cofa ctors i n s evera l , es s enti a l enzyma ti c rea cti ons , i ncl udi ng protei n, ca rbohydra te, a nd fa t meta bol i s m a s wel l a s the forma ti on of ti s s ues . Vi ta mi ns a re us ua l l y di vi ded i nto fa t s ol ubl e (vi ta mi ns A, D, E, a nd K) a nd wa ter s ol ubl e (vi ta mi ns B a nd C). Al though the huma n body or res i dent, benevol ent ba cteri a ca n produce s ome vi ta mi ns , mos t, a l ong wi th the requi red mi nera l s , a re obta i ned from the di et. Some vi ta mi ns ca n a l s o be s tored i n the body for di fferent ti mes . Defi ci enci es of pa rti cul a r vi ta mi ns i n the di et, therefore, often do not ma ni fes t thems el ves for a l engthy peri od of ti me. Vi ta mi ns A, D, a nd B 12 , for exa mpl e, a re s tored i n l a rge enough qua nti ti es s o tha t a bs ence i n the di et wi l l not be noti ced for months or yea rs . Defi ci enci es i n vi ta mi ns a nd mi nera l s ma y l ea d to ti s s ue da ma ge a nd/or a number of di s ea s es (e.g., beri beri , s curvy, ni ght bl i ndnes s , pel l a gra , a nd ri ckets ), ma ny of them bei ng fa ta l . At the other end of the s pectrum, exces s a mounts of l i pi d-s ol ubl e vi ta mi ns ca n be toxi c. The contri buti on of vi ta mi ns duri ng feta l devel opment i s es peci a l l y i mporta nt; exces s ma terna l vi ta mi n A duri ng pregna ncy ca n l ea d to s i gni fi ca nt bi rth defects . Vi ta mi ns requi red by huma ns a re s umma ri zed i n Ta bl e 10-4.
TABLE 10-4. Summa ry of Vi ta mi ns MINERALS Mi nera l s , s peci fi ca l l y thos e es s enti a l a toms i n the huma n di et, a re s i mpl e chemi ca l el ements requi red for the exi s tence a nd s ubs i s tence of l i fe. The ba s i c chemi ca l el ements i ncl ude ca rbon, hydrogen, ni trogen, oxygen, phos pha te, a nd s ul fur. Addi ti ona l mi nera l s tha t pl a y a s ma l l er but s ti l l i mporta nt rol e a re s odi um, pota s s i um, ca l ci um, ma gnes i um, i odi ne, a nd zi nc. Other mi nera l s a re requi red by the huma n body, a l though the exa ct number of es s enti a l mi nera l s i s s ti l l controvers i a l . A s umma ry of mi nera l s i mporta nt to bi ochemi ca l functi ons of the huma n body i s i ncl uded i n Ta bl e 10-5.
TABLE 10-5. Summa ry of Importa nt Mi nera l s
REVIEW QUESTIONS 1. Wha t a re the genera l meta bol i c rol es of the ma jor bi ochemi ca l mol ecul es —ca rbohydra tes , l i pi ds , a nd a mi no a ci ds ? 2. Wha t a re the pri ma ry meta bol i c contri buti ons of s ma l l i ntes ti ne, l i ver, a di pos e ti s s ue, pa ncrea s , a nd mus cl e? 3. Why a re gl ucos e-6-phos pha te, pyruva te, a nd a cetyl -CoA cons i dered pri ma ry i ntermedi a tes i n meta bol i s m of the ma jor bi ochemi ca l mol ecul es ? 4. How does the concentra ti on of gl ucos e-6-phos pha te determi ne the fa te of gl ucos e a fter entry i nto the l i ver cel l or mus cl e cel l ? 5. Wha t i s the rol e of fructos e 2,6-bi s phos pha te i n the control of l i ver ca rbohydra te meta bol i s m? 6. Wha t fa ctors i n the fed a nd s ta rved s ta tes determi ne the fa te of pyruva te i n terms of i ts oxi da ti on, us e for gl ucos e s ynthes i s , or rol e i n fa tty a ci d forma ti on? How do thes e fa ctors ba l a nce ea ch other? 7. Wha t fa ctors i n the fed a nd s ta rved s ta tes determi ne the fa te of a cetyl CoA i n l i ver i n terms of i ts oxi da ti on, us e for fa tty a ci d s ynthes i s , or convers i on to ketone bodi es ? How do thes e fa ctors ba l a nce ea ch other? 8. How does the ra ti o of i ns ul i n to gl uca gon determi ne the ba l a nce of ca rbohydra te meta bol i s m i n the body i n terms of effects on meta bol i c pa thwa ys i n s peci fi c ti s s ues ? 9. How does the ra ti o of i ns ul i n to gl uca gon determi ne the ba l a nce of fa t meta bol i s m i n the body i n terms of effects on meta bol i c pa thwa ys i n s peci fi c ti s s ues ? 10. Wha t a re the key effects of epi nephri ne on meta bol i s m a nd how do thes e rel a te to the body’s needs i n the fi ght or fl i ght res pons e? 11. Wha t a re the rol es of gl ucocorti coi ds i n s tres s or s ta rva ti on? 12. Wha t a re the key meta bol i c di s turba nces a s s oci a ted wi th type 1 a nd type 2 di a betes ? 13. Wha t a re the fa t-s ol ubl e vi ta mi ns , thei r functi ons , a nd di s ea s es of defi ci ency? 14. Wha t a re the B vi ta mi ns , thei r functi ons , a nd di s ea s es of defi ci ency? 15. Wha t a re the functi ons of vi ta mi n C a nd di s ea s es of defi ci ency? 16. Wha t a re the functi ons of the fol l owi ng mi nera l s a nd the cons equences of thei r defi ci ency a nd/or exces s —s odi um, pota s s i um, ca l ci um, ma gnes i um, phos phorus , i ron, a nd i odi ne?
CHAPTER 11 THE DIGESTIVE SYSTEM Editor: Kshama Jaiswal Depa rtment of Surgery, Denver Hea l th Medi ca l Center, Denver, Col ora do
Summa ry of the Di ges ti ve Sys tem Mouth Stoma ch Li ver Ga l l Bl a dder Pa ncrea s Sma l l Intes ti ne (Duodenum, Jejunum, a nd Il eum) La rge Intes ti ne/Anus Revi ew Ques ti ons
OVERVIEW The di ges ti ve or ga s troi ntes ti na l s ys tem i s roughl y defi ned a s the a na tomi ca l component from the mouth to the a nus , i ncl udi ng orga ns res pons i bl e for tra ns i t, mecha ni ca l brea kdown, di ges ti on a nd a bs orpti on of foods tuffs , a s wel l a s the effi ci ent el i mi na ti on of s ol i d wa s te. Incl uded a re the mouth a nd denti ti a , pha rynx a nd es opha gus , s toma ch, s ma l l i ntes ti ne, l i ver, ga l l bl a dder, pa ncrea s , l a rge i ntes ti ne, rectum, a nd a nus . As wi th the compl ex i ntegra ti on a nd control of meta bol i s m, the di ges ti ve s ys tem i s , i ts el f, under the i nfl uence of neurol ogi ca l a nd hormona l regul a ti on tha t both a cti va tes a nd i nhi bi ts ma ny of i ts compl ex a cti ons . Mos t of thes e compl ex a cti ons a re, thems el ves , s i mpl e bi ochemi ca l proces s es of protei ns , ca rbohydra tes , l i pi ds , a nd nucl eos i des / nucl eoti des to i ncl ude enzyme rea cti ons wi th a s s oci a ted a cti va ti ng a nd i nhi bi tory mol ecul es , membra ne-s pa nni ng protei n cha nnel s , a nd pumps a l l l ea di ng to the producti on a nd s tora ge of energy a nd es s enti a l bui l di ng bl ocks for current or future us e.
SUMMARY OF THE DIGESTIVE SYSTEM The di ges ti ve s ys tem i s the col l ecti on of orga ns res pons i bl e for the di ges ti on of i nges ted foods a nd l i qui ds (Fi gure 11-1). Thi s s ys tem i s cl a s s i ca l l y cons i dered to s ta rt a t the mouth a nd conti nue vi a the es opha gus , s toma ch, s ma l l a nd l a rge i ntes ti nes , a nd end a t the rectum/a nus . Bes i des thei r mecha ni ca l a nd enzyma ti c brea kdown of foods tuffs , cons i dera bl e contri buti ons towa rd di ges ti ons a re s uppl i ed from the l i ver a nd pa ncrea s .
Figure 11-1. Overview of the Digestive System. Components of di ges ti on a nd tra ns port of food from the mouth to the rectum/a nus a re s hown, i ncl udi ng a s umma ry of thei r contri buti ons a nd the a vera ge a mount of ti me for food to rea ch thei r l oca ti on a fter i nges ti on. [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.]
MOUTH Teeth provi de the i ni ti a l mecha ni ca l brea kdown of food vi a chewi ng or ma s ti ca ti on a nd, therefore, the forma ti on a nd hea l th of teeth i s i mporta nt. Protei ns s erve a s a n i mporta nt s tructura l component of teeth ena mel . Li ke bone, teeth requi re the col l a gen-l i ke protei ns amelogenin (over 90% compos ed of prol i ne, gl uta mi ne, a nd hi s ti di ne a mi no a ci ds ), ameloblastin, enamelin, a nd tuftelin (a phos phoryl a ted
gl ycoprotei n), whi ch orga ni ze, i ni ti a te, a nd di rect ca l ci um phos pha te crys ta l forma ti on a nd hel p a nchor teeth to the gums . The pos s i bl e brea kdown of ena mel i s equa l l y i mporta nt a nd i s a l s o dependent on bi ochemi ca l proces s es . Denta l pl a que forma ti on a nd tooth deca y both rel y on enzyma ti c proces s es for forma ti on a s wel l a s preventi on. Pl a que i s ca us ed ma i nl y by the norma l ora l ba cteri a Streptococcus mutans, Lactobacillus acidophilus, Fusobacterium nucleatum, Actinomyces viscosus, a nd Nocardia spp. When thes e orga ni s ms form a l a yer on teeth, thos e cl os es t to the teeth exi s t i n a n oxygen-defi ci ent envi ronment a nd convert to a na erobi c res pi ra ti on for energy producti on. Thi s enzyma ti c proces s turns ca rbohydra tes i nto l a cti c a ci d from pyruva te (Cha pter 6) a nd res ul ts i n a pH of bel ow 5.5, l ea di ng to tooth deca y, the demi nera l i za ti on proces s tha t ca us es ca vi ti es . Gl ucos e, fructos e, a nd es peci a l l y s ucros e (common ta bl e s uga r) a re the ma i n ca rbohydra te s ources . The mouth i s a l s o the l oca ti on for the s ta rt of the proces s es of ca rbohydra te, l i pi d, a nd protei n di ges ti on vi a i mporta nt enzymes conta i ned i n s a l i va . Sa l i va producti on, a l ong wi th feel i ngs of hunger a nd s a ti ety, i s control l ed by a va ri ety of neuro-bi ochemi ca l proces s es , whi ch s ta rt wi th the i ni ti a l thoughts or cephalic phase of ea ti ng. The phys i ca l pres ence a nd a ct of chewi ng a nd ta s ti ng food, known a s the oros ens ory or gustatory phase, el i ci ts further s i gna l s tha t enha nce s a l i va forma ti on a nd expres s i on. The va ri ous fa ctors a ffecti ng hunger a nd s a ti ety a re di s cus s ed i n Cha pter 19. Sa l i va a l s o tra ps mol ecul es produced by norma l ora l ba cteri a tha t a dds to the ta s te s ens a ti on of otherwi s e odorl es s a nd ta s tel es s food compounds . The hormone gustin, produced i n s a l i va a nd a n a cti va tor of a ca l modul i n-dependent cycl i c a denos i ne monophos pha te (cAMP) phos phodi es tera s e (Cha pter 8), i s a l s o thought to pl a y a n i mporta nt pa rt i n ta s te bud forma ti on. Di ges ti ve enzymes conta i ned i n s a l i va a nd thei r rol es i n di ges ti on a re l i s ted i n Ta bl e 11-1.
TABLE 11-1. Compos i ti on of Sa l i va “Meth Mouth”: Pa ti ents who a bus e methamphetamines a re prone to ma rked denta l deca y, known col l oqui a l l y a s “Meth Mouth.” Metha mpheta mi ne a cts on the αadrenergic receptors of the va s cul a ture of the s a l i va ry gl a nds , ca us i ng va s ocons tri cti on a nd reduci ng s a l i va ry fl ow, depri vi ng the ora l envi ronment of s a l i va ’s bufferi ng a cti vi ty to countera ct a ci di ty a nd prevent demi nera l i za ti on of ena mel . Metha mpheta mi ne-i nduced vomi ti ng a l s o expos es teeth to a ci ds . In a ddi ti on, metha mpheta mi ne overs ti mul a tes the s ympa theti c nervous s ys tem, eventua l l y depl eti ng norepi nephri ne a nd dopa mi ne a nd a l teri ng concentra ti ons of other centra l nervous s ys tem (CNS) neurotra ns mi tters s uch a s s erotoni n, a cetyl chol i ne, a nd gl uta ma te. Thi s reducti on i ncrea s es the dema nd for a denos i ne tri phos pha te (ATP); metha mpheta mi ne us ers ma y compens a te by cons umi ng more ca rbohydra tes i n the form of s uga rs a nd s ta rches . Obs ervers s peci fi ca l l y report a hi gh i nta ke of ca rbona ted s oft dri nks a mong meth us ers . At the s a me ti me, us ers typi ca l l y a ba ndon ora l hygi ene. In s hort, metha mpheta mi ne us e encoura ges a n envi ronment tha t ma xi mi zes ca ri es ri s k—decrea s ed s a l i va , frequent expos ure to s uga r, a nd l a ck of pl a que control .
STOMACH Stoma ch i s the l oca ti on of conti nued mecha ni ca l brea kdown of food vi a the a cti ons of i ts s mooth mus cl e l a yers but, perha ps more i mporta ntl y, the s i te of the a cti va ti on a nd a cti vi ty of a number of regul a ted enzymes a nd expos ure to a ci d (pH 1–2) tha t i ni ti a tes meta bol i s m. The other ma i n functi on of the s toma ch i s the regul a ted a nd coordi na ted s ecreti on of hydrogen a toms a nd va ri ous di ges ti ve enzymes under the control of the a utonomi c nervous s ys tem a nd s evera l hormones . Thes e mol ecul es a re produced from a va ri ety of cel l types found i n va ri ous pa rts of the s toma ch a s s hown i n Fi gure 11-2 a nd s umma ri zed bel ow. Hormones tha t a ffect the s toma ch a re des cri bed i n Ta bl e 11-2.
Figure 11-2. Hormone Production from the Stomach. Speci fi c porti ons of the s toma ch a re res pons i bl e for producti on of hormones a s i l l us tra ted a bove (s ee text for further des cri pti on). [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.]
TABLE 11-2. Hormones Affecti ng the Stoma ch 1. Mucus (Neck) Cells. Found i n a l l pa rts of the s toma ch, thes e cel l s produce a vi s cous mi xture of protecti ve enzymes a nd mucins, l a rge gl ycoprotei ns (Cha pter 1), vi a s ti mul a ti on of a myristylated alanine-rich C kinase. Muci ns ha ve l ow gl ycos yl a ti on on the a mi no a nd ca rboxy termi na l ends but very hi gh l evel s of gl ycos yl a ti on (vi a s eri ne, threoni ne, a nd a s pa ra gi ne a mi no a ci ds ) i n the centra l pa rt of i ts a mi no a ci d s equence. Thes e muci n gl ycoprotei ns a re a l s o i nterl i nked by cys tei ne–cys tei ne di s ul fi de bonds (Cha pter 1) tha t form l a rge a ggrega te gel s fi l l ed wi th wa ter a nd protected from enzymes by thei r dens e ca rbohydra te coa ti ng. The s ecreted mucus provi des a protecti ve ba rri er a ga i ns t the di ges ti ve enzymes a nd hi ghl y a ci di c condi ti ons found i n the s toma ch. 2. Parietal (Oxyntic) Cells. Found i n a l l pa rts of the s toma ch, pa ri eta l cel l s a re s ti mul a ted by the hormones histamine vi a the H 2 receptor (G s — a denyl cycl a s e/cAMP) a nd gastrin vi a a CCK2 receptor [G q—phos phol i pa s e C/i nos i tol tri phos pha te (IP3 )/Ca 2+] a s wel l a s by the va gus (pa ra s ympa theti c) nerve vi a a cetyl chol i ne a nd the M3 receptor (G q—phos phol i pa s e C/IP3 /Ca 2+). Sti mul a ti on of a denyl cycl a s e i s known to s peci fi ca l l y a cti va te the H +/K+ ATPa s e a cti ve tra ns port cha nnel a nd ga s tri c a ci d producti on. Thes e cel l s produce three ma i n components tha t a re a s fol l ows : a. Gastric acid [ma i nl y hydrogen (H +) a nd chl ori de (Cl −) i ons ] vi a a uni que H +/K + ATPase active transport channel tha t pumps hydrogen i ons i nto the s toma ch a ga i ns t a very hi gh concentra ti on gra di ent (a pprox. 3 mi l l i on to 1). Ga s tri c a ci d functi ons i n protei n dena turi za ti on, peps i nogen a cti va ti on (s ee bel ow), a nd i nhi bi ti on of ba cteri a l growth. b. Bicarbonate ion (HCO3 −). Excreted i nto the bl ood a s pa rt of the overa l l H + pumpi ng mecha ni s m. c. Intrinsic factor. Requi red for i ntes ti na l a bs orpti on of vi ta mi n B 12 (s ee Cha pter 10). Achlorydia: The des tructi on or da ma ge of parietal cells l ea ds to a ma rked reducti on i n the producti on of ga s tri c a ci d, es s enti a l for the i ni ti a l brea kdown of food a nd a cti va ti on of s toma ch enzymes (e.g., peps i n). Al though s evera l di s ea s e s ta tes a nd/or s urgi ca l i nterventi ons ca n a ffect pa ri eta l cel l s , i mmune des tructi on of the cel l s l ea ds to the condi ti on of achlorydia/hypochloridia i n whi ch decrea s ed a mounts of hydrochl ori c a ci d a re produced. The res ul ti ng hi gher s toma ch pH l ea ds to s ymptoms of ga s troes opha gea l refl ux, pa i n a nd ful l nes s from i na dequa te di ges ti on, a nd i ncrea s ed growth of ba cteri a , norma l l y l i mi ted by l ow pH, whi ch ca n l ea d to di a rrhea a nd decrea s ed a bs orpti on of es s enti a l i ons (e.g., ma gnes i um a nd zi nc) a nd vi ta mi ns (e.g., C, K, B-compl ex), whi ch thems el ves l ea d to other di s ea s e s ta tes . Trea tment i s vi a s uppl ementa ti on wi th Betaine HCl, a form of hydrochl ori c a ci d, whi ch s urvi ves i nto the s toma ch, a nd a ny requi red vi ta mi ns a nd mi nera l s . 3. Chief (Zymogenic) Cells. Produce pepsinogen, the proenzyme form of the i mporta nt enzyme pepsin, whi ch cl ea ves pepti de bonds , prefera bl y a t hydrophobi c a nd a roma ti c [phenyl a l a ni ne (Phe), tryptopha n, a nd tyros i ne] a mi no a ci ds . Sti mul a ti on of chi ef cel l s ecreti ons i s by the va gus nerve, a ci di c condi ti ons per ga s tri c a ci d, or by the hormones ga s tri n or secretin (produced i n the duodenum). In i nfa ncy, Chi ef cel l s a l s o
produce the enzyme rennin, whi ch a i ds mi l k a bs orpti on by brea ki ng the Phe–methi oni ne pepti de bond i n mi l k protei n ka ppa -ca s ei n. Secreti on of renni n i s s ti mul a ted by i nges ti on of mi l k by the huma n i nfa nt but the gene product i s turned off pa s t thi s s ta ge. 4. Enterochromaffin-like Cells (ELCs). Found i n the ga s tri c gl a nds . Thes e cel l s s ecreted hi s ta mi ne, whi ch a cti va tes pa ri eta l cel l s a nd ga s tri c a ci d producti on. ELCs a re, thems el ves , a cti va ted by the hormone ga s tri n, pi tui ta ry a denyl cycl a s e-a cti va ti ng pepti de, a nd va gus nerve. ELCs a re i nhi bi ted by somatostatin. 5. G Cells. Found i n the a ntrum. Secrete the hormone gastrin (s ee Ta bl e 11-2), whi ch both i ncrea s es the s ecreti on of a nd works a l ong wi th hi s ta mi ne to s ti mul a te pa ri eta l cel l s to produce hydrochl ori c a ci d a nd Chi ef cel l s to produce peps i n. Secreti on of ga s tri n i s i ncrea s ed by pa ra s ympa theti c va gus nerve a cti vi ty vi a rel ea s e of gastrin-releasing peptide or by the pres ence of a mi no a ci ds i n the s toma ch. 6. Prostaglandin E2 . Bi ndi ng to i ts receptor s ti mul a tes s mooth mus cl e contra cti on of the ga s troi ntes ti na l tra ct a nd decrea s es pa ri eta l cel l s ecreti on of ga s tri c a ci d whi l e i ncrea s i ng mucus producti on. The a cti on i s vi a the G i protei n receptor, whi ch i nhi bi ts the producti on of cAMP by a denyl cycl a s e a nd, therefore, pa ri eta l cel l H +/K+ ATPa s e pump a cti vi ty. Misoprostol and Gastric Ulcers. Misoprostol (s ee the fi gure), a s yntheti ca l l y produced prostaglandin E1 , i s s ometi mes us ed to prevent ga s tri c ul cers beca us e of i ts a bi l i ty to i nhi bi t pa ri eta l cel l producti on of ga s tri c a ci d. Mi s opros tol i s norma l l y us ed onl y for trea tment of or prophyl a xi s a ga i ns t nons teroi da l a nti -i nfl a mma tory drug-i nduced pepti c ul cers beca us e other medi ca ti on cl a s s es (H 2 -receptor bl ocker a nd protei n pump i nhi bi tors , PPIs ) a re more effecti ve for l ong-term ca re of a ci d refl ux a nd s i mi l a r di s orders .
Reproduced wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.
The a cti ons of ea ch of the a bove cel l types a nd, therefore, the envi ronment of the s toma ch a re control l ed by va gus nerve s i gna l s a s wel l a s the hormones l i s ted bel ow. Of note i s the fa ct tha t s evera l hormones tha t a ffect the s toma ch a re produced a nd a l s o a ct on orga ns outs i de the s toma ch (e.g., s ma l l i ntes ti ne) to decrea s e s toma ch moti l i ty a nd/or di ges ti on (Fi gure 11-3). In thi s ma nner, the body not onl y “turns on” the s toma ch when food i s pres ent but a l s o turns i t “off” when food ha s tra vel ed further a l ong the di ges ti ve tra ct.
Figure 11-3. Hormone Production by the Digestive System. Si tes of producti on of the fi ve ma jor ga s troi ntes ti na l hormones a l ong the l ength of the ga s troi ntes ti na l tra ct. The wi dth of the ba rs i ndi ca tes the rel a ti ve a bunda nce a t ea ch l oca ti on. CCK, chol ecys toki ni n; GIP, ga s tri c i nhi bi tory pepti de. [Reproduced wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.]
Acid Reflux (Gastroesophageal Reflux Disease or GERD), Barrett’s esophagus, and the Vagus Nerve. GERD a ffects a l a rge percenta ge of the popul a ti on ei ther a cutel y or chroni ca l l y. Untrea ted GERD ca n s ometi mes l ea d to preca ncerous cha nges i n the l ower pa rt of the es opha gus ca l l ed Barrett’s esophagus, whi ch ca n progres s to potenti a l l y fa ta l adenocarcinoma of the es opha gus . The s urgi ca l s everi ng of the va gus nerve (vagal nervectomy or vagotomy) wa s once us ed for trea tment due to thi s nerve’s promi nent rol e i n pa ri eta l a nd G cel l (producers of ga s tri n), s ecretory a cti vi ty. The procedure, a l though us ua l l y effecti ve for GERD a nd pepti c ul cer di s ea s e, a l s o ca rri ed a number of unwa nted s i de effects beca us e of the i nnerva ti on of other orga ns by the va gus nerve unl es s ca reful a nd s el ecti ve s urgery wa s performed. Ma ny trea tment methods ha ve s i nce been devel oped, i ncl udi ng the H 2 blocker cl a s s of medi ca ti ons , whi ch i nhi bi t the promoti on of a ci d s ecreti on by hi s ta mi ne a nd PPIs, whi ch bl ock the uni que H +/K+ ATPa s e pump found i n the s toma ch. As a res ul t, a va gotomy i s now ra rel y performed for GERD trea tment.
LIVER The l i ver pl a ys mul ti pl e, cri ti ca l rol es (s ee Ta bl e 11-3) i n s uch di vers e functi ons a s a ma jor pa rt i n ca rbohydra te, protei n, a nd l i pi d meta bol i s m/regul a ti on; s ynthes i s a nd s ecreti on of s evera l bl ood cl otti ng (coa gul a ti on) fa ctors ; s ynthes i s of bi l e; the degra da ti on of hemogl obi n/bi l i rubi n from decompos i ng red bl ood cel l s (Fi gure 11-4); s ynthes i s a nd cl ea ra nce of chol es terol ; s tora ge of gl ycogen, vi ta mi ns , A, D, B 12 , i ron, a nd copper; brea kdown a nd el i mi na ti on of a va ri ety of toxi c s ubs ta nces (i ncl udi ng a l cohol ); a nd the s ynthes i s of a va ri ety of mol ecul es from a l bumi n to i ns ul i n-l i ke growth fa ctor 1 to a ngi otens i n (s ee Cha pters 12 a nd 16). Thes e functi ons a re ca rri ed out by hepa tocytes , the functi ona l cel l s of the l i ver.
TABLE 11-3. Summa ry of Li ver Functi ons
Figure 11-4. Degradation of Hemoglobin to Bilirubin. Overvi ew of brea kdown i n the reti cul oendothel i a l s ys tem of heme mol ecul es tha t a re rel ea s ed from decompos i ng red bl ood cel l s . Steps produci ng bi l i verdi n a nd bi l i rubi n a re s hown. The gra dua l brea kdown of heme res ul ts i n s evera l i ntermedi a te products tha t crea te the col ors s een i n a n evol vi ng a nd hea l i ng brui s e. NADPH, ni coti na mi de a deni ne di nucl eoti de phos pha te. [Reproduced wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] Bilirubin Measurement and Disease: La bora tory mea s urement of bilirubin i s a l wa ys reported a s the conjugated (direct), unconjugated (indirect), a nd total bilirubin. A dye us ed to detect bi l i rubi n qui ckl y produces a red-vi ol et a zobi l i rubi n compound wi th conjuga ted bi l i rubi ns ; thes e a re,
therefore, referred to a s “di rect” bi l i rubi n. The a ddi ti on of etha nol to the s a mpl e ma kes a l l bi l i rubi ns (conjuga ted a nd unconjuga ted) rea ct qui ckl y wi th the dye a nd yi el ds “tota l bi l i rubi ns .” Unconjuga ted bi l i rubi n l evel s a re ca l cul a ted by s ubtra cti ng di rect bi l i rubi n from tota l bi l i rubi n a nd a re, therefore, ca l l ed “i ndi rect” bi l i rubi n. Urobilinogen i s mea s ured by the a ddi ti on of a rea gent conta i ni ng pdi methyl a mi nobenza l dehyde, whi ch turns a da rk pi nk/red proporti ona l to the urobi l i nogen i n the tes t s a mpl e. Urobi l i n does not rea ct wi th thi s compound a nd, therefore, us ua l l y onl y urobi l i nogen i s mea s ured. Stercobl i n, found excl us i vel y i n feces , i s us ua l l y not mea s ured. Di s ea s es of the l i ver, ga l l bl a dder, a nd red bl ood cel l s a l l a ffect the a mount of unconjuga ted/conjuga ted bi l i rubi n a s wel l a s the a mounts of res ul ta nt urobi l i nogen a nd s tercobi l i n. Si mpl e obs erva ti ons a nd l a bora tory mea s urement of thes e mol ecul es ca n grea tl y a s s i s t i n determi ni ng thes e di s ea s e s ta tes . For exa mpl e, ga l l s tones , chroni c l i ver di s ea s e wi th ci rrhos i s , or other di s ea s es tha t bl ock or decrea s e the s ecreti on of conjuga ted bi l i rubi n i nto the bi l e wi l l l ea d to a n i ncrea s ed l ea ka ge of bi l i rubi n from l i ver cel l s a nd a res ul ti ng ri s e i n uri ne bi l i rubi n. Thi s ri s e ca n both be mea s ured a nd s een i n uri ne by the res ul ti ng da rk a mber col or. The l a ck of bi l i rubi n s ecreti on i nto the i ntes ti ne wi l l a l s o decrea s e the a mount of s tercobi l i n crea ti ng whi te or pa l e feces . Di s ea s es tha t ca us e i ncrea s ed brea kdown of red bl ood cel l s (e.g., hemol yti c a nemi a s ) wi l l i ncrea s e the a mount of unconju-ga ted bi l i rubi n i n bl ood tes ts ; however, a s the unconjuga ted form i s not s ol ubl e i n wa ter, uri ne bi l i rubi n wi l l not i ncrea s e a nd uri ne col or wi l l rema i n the s a me. However, the exces s heme from the des troyed red bl ood cel l s wi l l i ncrea s e the a mount of urobi l i nogen/urobi l i n tha t ca n be mea s ured. In newborns , the i ni ti a l l a ck of l i ver enzymes res pons i bl e for conjuga ti on a nd i ntes ti na l ba cteri a , whi ch convert bi l i rubi n to urobi l i nogen, ma y l ea d to the di s ea s es of hyperbilirubinemia a nd potenti a l l y l etha l kernicterus. Thes e s a me condi ti ons a l s o l ea d to pa l er feces i n newborns a nd el eva ted l evel s of bi l i rubi n crea te a yel l ow s ki n col or (ja undi ce). LIPID METABOLISM IN THE LIVER The l i ver pl a ys a centra l rol e i n l i pi d meta bol i s m. It i s the onl y orga n ca pa bl e of the di s pos a l of s i gni fi ca nt qua nti ti es of cholesterol, ei ther vi a excreti on i nto the bi l e or by meta bol i s m to bi l e a ci ds , both of whi ch a re l os t to s ome degree i n the feces . Di eta ry chol es terol , whi ch i s pa cka ged i nto chyl omi crons i n the i ntes ti ne for tra ns port i n the bl ood, ul ti ma tel y ends up i n the l i ver where i t combi nes wi th the pool of chol es terol s ynthes i zed from a cetyl coenzyme A deri ved from β-oxi da ti on of s a tura ted fa tty a ci ds a nd i s ei ther excreted vi a the bi l e a s bi l e s a l ts or di s tri buted to other ti s s ues vi a low-density lipoprotein (LDL). In the fed s ta te, the l i ver converts exces s di eta ry gl ucos e to fa tty a ci ds tha t a re es teri fi ed to gl ycerol (Cha pter 7). The res ul ti ng triglycerides a re pa cka ged, wi th chol es terol , i nto very-low-density lipoprotein (VLDL), whi ch i s s ecreted i nto the bl ood to del i ver fa tty a ci ds pri ma ri l y to a di pos e cel l s a nd mus cl e. Fi na l l y, bi l e s a l ts ma de i n the l i ver a re needed for the a bs orpti on of di eta ry l i pi ds (e.g., tri gl yceri des a nd chol es terol ) a nd fa t-s ol ubl e vi ta mi ns . The l i ver i s the s ource of the l i poprotei n VLDL a nd mos t a pol i poprotei ns (a po) a nd i s a cti vel y i nvol ved i n the endogenous tra ns port/meta bol i c pa thwa y of chol es terol a nd a s s oci a ted l i poprotei ns (hi gh-dens i ty l i poprotei n, LDL, a nd VLDL; s ee Cha pter 3). Its ma jor functi on i s to tra ns port the endogenous l y s ynthes i zed tri a cyl gl ycerol from the l i ver to extra hepa ti c ti s s ues . VLDL s ecreted i nto the ci rcul a ti on a l s o conta i ns a po B100 (a l s o s ynthes i zed i n l i ver) tha t i s requi red for the proper a s s embl y a nd export of VLDL pa rti cl es . The s econd ma jor functi on of VLDL i s to ca rry l i ver-genera ted chol es terol es ters to peri phera l cel l s a fter i ts convers i on to LDL (s ee Cha pter 16). The further s teps i n chol es terol tra ns port a nd del i very to peri phera l ti s s ues a re des cri bed i n Cha pter 16.
GALL BLADDER The ga l l bl a dder s erves to s tore a nd concentra te bile produced i n the l i ver. Bi l e i s rel ea s ed i nto the duodenum a t the ampulla of Vater by s mooth mus cl e contra cti on of the muscularis externa l a yer a nd rel a xa ti on of the Sphincter of Oddi. Rel ea s e i s i n res pons e to s ecreti on of cholecystokinin (CCK), the na me of a group of rel a ted pepti de hormones s ecreted from I-cel l s i n the duodenum wi th s i mi l a r s tructure to ga s tri n (s ee a bove). CCK s ecreti on i s i ncrea s ed by entra nce of fa t- or protei n-conta i ni ng food i nto the duodenum but decrea s ed by the a cti ons of s oma tos ta ti n (s ee a bove). Bi l e i s compos ed of wa ter, a va ri ety of i ons [Na +, K+, Ca 2+, Cl −, a nd HCO3 − (bi ca rbona te)], l i pi ds (fa tty a ci ds , phos phol i pi ds , a nd chol es terol ), protei ns , a nd, mos t i mporta ntl y for di ges ti on, more tha n 30 g/l of a ni on forms of bi l e a ci ds (s ee Cha pter 3). As noted i n Cha pter 3, gl yci ne- a nd ta uri ne-conjuga ted bi l e a ci ds a re the ma jor forms . Thes e bi l e a ci ds ha ve detergent properti es ena bl i ng them to s urround a nd brea k up tri gl yceri des a nd phos phol i pi ds fa t pa rti cl es i n food (Fi gure 11-5), a l l owi ng l i pi d-degra di ng enzymes (e.g., pa ncrea ti c l i pa s e) to a ct. Bi l e a ci ds a l s o promote i mproved a bs orpti on of fa ts , i ncl udi ng the fa t-s ol ubl e vi ta mi ns A, D, E, a nd K (Cha pters 3 a nd 10).
Figure 11-5. A–B. Role of Bile Salts in Triglycerides Metabolism. A. Bi l e s a l ts form mi cel l es wi th fa tty a ci ds rel ea s ed from di eta ry tri gl yceri des by pa ncrea ti c l i pa s e. The hydrophobi c, cha rged s i de cha i n a nd hydroxyl (OH) groups of bi l e s a l t monomers (s ee Cha pter 3) a re wel l s ui ted for mi cel l e forma ti on. [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] B. Thes e mi cel l es promote i ncrea s ed degra da ti on of the tri a cyl gl ycerol , by the l i pa s e/col i pa s e compl ex, to two fa tty a ci ds a nd monoa cyl gl ycerol fol l owed by tra ns port of thes e products to the mucos a , a s pa rt of new mi cel l es , where they a re a bs orbed for rea s s embl y of tri a cyl gl ycerol s . Bes i des expres s i on of bi l e from the ga l l bl a dder, CCK s erves a n i mporta nt rol e i n s ti mul a ti ng the pa ncrea s to rel ea s e di ges ti ve enzymes , i ncl udi ng trypsin, chymotrypsin, pancreatic lipase, a nd pancreatic amylase, to a i d i n fa t a nd protei n di ges ti on. CCK a l s o s l ows down s toma ch emptyi ng to a l l ow proper di ges ti on of thes e food pa rti cl es a nd decrea s es the producti on of ga s tri c a ci d (s ee a bove). Fi na l l y, CCK s erves a s a neuropep-ti de regul a ti ng hunger/s a ti ety worki ng vi a CCK receptors found i n the CNS. As di s cus s ed a bove, the CCK receptor a l s o bi nds ga s tri n a nd functi ons vi a a G q protei n-coupl ed s i gna l i ng proces s , whi ch a cti va tes phos phol i pa s e C wi th s ubs equent producti on of IP3 a nd Ca 2+. In the
CNS, CCK works vi a regul a ti on of the a cti vi ty of dopa mi ne a nd pos s i bl y GABA (Cha pter 19). CCK a cti vi ty i s a l s o i nvol ved i n the a cti vi ty of opi oi ds i n the CNS. Cholesterol Gallstones: The forma ti on of chol es terol ga l l s tones , one type of chol el i thi a s i s , occurs mos t often i n women ol der tha n 40 yea rs who ha ve ha d s evera l pregna nci es . Obes i ty i ncrea s es the ri s k. Ga l l s tones a re a l s o i ncrea s ed i n women of Ca uca s i a n a nd Hi s pa ni c ori gi n. Thes e ri s k fa ctors a re s ometi mes des cri bed by the fi ve “Fs ”—fema l e, forty, ferti l e, fa t, a nd fa i r—but the mecha ni s m a ppea rs to be fa r more compl i ca ted tha n thes e s i mpl e fa ctors . Hi gh chol es terol a nd rel a ti vel y l ow bi l e s a l t l evel s a re known ri s k fa ctors a s wel l a s bi l e s ta s i s i n the ga l l bl a dder due to i nfrequent, wea k, a nd i ncompl ete contra cti ons a nd emptyi ng of i ts contents . Pa rti cul a r protei ns found i n bi l e ha ve a l s o been noted to enha nce or i nhi bi t the preci pi ta ti on of chol es terol i nto s tones . Interes ti ngl y, l evel s of the di eta ry s uppl ement mel a toni n ma y a l s o pl a y a rol e i n ga l l s tone forma ti on a s , a mong ma ny s ugges ted functi ons , i t a l s o promotes l ower l evel s of chol es terol a nd reduces oxi da ti on l evel s i n the ga l l bl a dder.
PANCREAS The pa ncrea s pl a ys a predomi na nt rol e i n the di ges ti on of fa ts a nd protei ns beca us e of the pa ncrea ti c jui ce a nd va ri ous di ges ti ve enzymes tha t i t produces a nd s ecretes i nto the duodenum (exocrine functions), s ha ri ng the a mpul l a of Va ter wi th the common bi l e duct. In a ddi ti on, the pa ncrea s produces s evera l hormones (endocrine functions). The exocri ne functi ons occur i n the pancreatic acini, s ma l l cl us ters of cel l s a nd ducts . One functi on of thes e a ci ni i s the producti on of bi ca rbona te i ons (HCO3 −), under control of the hormone secretin, for a l ka l i ni za ti on of the a ci d contents l ea vi ng the s toma ch (Fi gure 11-6).
Figure 11-6. Mechanism for Production of Bicarbonate Ions (HCO3 −) by the Pancreas. Ach, a cetyl chol i ne. [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] A s econd functi on, ma i nl y under the control of CCK, i s the s ynthes i s a nd s ecreti on of enzyme protea s es , i ncl udi ng the proenzymes tryps i nogen a nd chymotryps i nogen a s wel l a s pa ncrea ti c l i pa s e, pa ncrea ti c a myl a s e, phos phol i pa s e A2, l ys ophos phol i pa s e, a nd chol es terol es tera s e (Ta bl e 11-4).
TABLE 11-4. Exocri ne Pa ncrea ti c Enzymes a nd Thei r Functi ons
As a n endocri ne orga n, the pa ncrea s produces a nd s ecretes s evera l hormones from fi ve di fferent cel l types formi ng the Is l ets of La ngerha ns , s umma ri zed i n Ta bl e 11-5.
TABLE 11-5. Endocri ne Pa ncrea ti c Hormones , Cel l s ource, a nd Functi ons The endocri ne pa ncrea s i s regul a ted not onl y by other hormones but a l s o by the a utonomi c nervous s ys tem (Cha pter 19). Sympa theti c (a drenergi c) i nnerva ti on s peci fi ca l l y i ncrea s es α-cel l s ecreti ons whi l e decrea s i ng tha t from β-cel l s . Pa ra s ympa theti c (mus ca ri ni c) i nnerva ti on i ncrea s es s ecreti ons from both α- a nd β-cel l s .
SMALL INTESTINE (DUODENUM, JEJUNUM, AND ILEUM) The small intestine i s the a pproxi ma tel y 16–20 ft. l ong s ecti on of the di ges ti ve tra ct i n whi ch a ma jori ty of the di ges ti on of food a nd a bs orpti on of nutri ents occurs . The effecti ve functi ona l l ength of the s ma l l i ntes ti ne i s i ncrea s ed by a fa ctor of a bout 500 by fol ds / i nva gi na ti ons (plicae circulares a nd ruga e) of the i ntes ti na l wa l l a nd a l s o the projecti ons (vi l l i ) of the enterocyte cel l borders whi ch l i ne i t. The three di fferent pa rts of the s ma l l i ntes ti ne—the duodenum, the jejunum, a nd the ileum—perform di fferent functi ons , dependi ng on the pres ence of di ges ti ve enzymes a nd a bs orpti ve ca pa bi l i ti es of thei r pa rti cul a r cel l types (Fi gure 11-7).
Figure 11-7. Summary of Absorption by Small Intestine. Abs orpti on of va ri ous nutri ents i s noted for the duodenum, jejunum, a nd i l eum. [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Short Gut Syndrome: Short gut (bowel) syndrome res ul ts from ei ther congeni ta l or s urgi ca l l y ca us ed s horteni ng of the s ma l l i ntes ti ne wi th res ul ti ng l os s of the es s enti a l di ges ti ve a nd a bs orpti ve functi ons . Surgery for Crohn’s di s ea s e, ca ncers , tra uma , a nd ba ri a tri c (obes i ty) trea tments a re the pri ma ry ca us es . Symptoms whi ch i ncl ude di a rrhea , pa i n, a nd s econda ry di s orders from l i mi ted vi ta mi n a nd nutri ent a bs orpti on us ua l l y do not pres ent unl es s more tha n two-thi rds of the s ma l l i ntes ti ne (i .e., l es s tha n 7 ft. rema i ni ng) i s a ffected. Some pa ti ents ca n i mprove vi a enl a rgement a nd i ncrea s ed functi oni ng of the rema i ni ng i ntes ti na l l ength a nd/or s l owi ng of movement of food through the i ntes ti ne to opti mi ze di ges ti on a nd a bs orpti on. Trea tment i s s ymptoma ti c a nd by s uppl ementa ti on of requi red nutri ents , a l though s urgi ca l a ttempts a t i ntes ti na l l engtheni ng or tra ns pl a nt a re bei ng a ttempted.
Infectious/Inflammatory Diseases of the Small Intestine: The cons ta nt expos ure of the di ges ti ve tra ct a nd, i n pa rti cul a r, the s ma l l i ntes ti ne to i nges ted mi croorga ni s ms l ea ds to mul ti pl e occurrences of gastroenteritis or i nfl a mma ti on of the s toma ch a nd/or s ma l l i ntes ti ne. Al though a ma jori ty of ca us es a re vi ra l (e.g., rota vi rus ), va ri ous ba cteri a (e.g., Escherichia coli, Shigella, Salmonella, Campylobacter, Vibrio cholerae, a nd Clostridium), protozoa n (e.g., Gi a rdi a ), a nd pa ra s i tes (e.g., Ascaris lumbricoides, fl a t-worms , a nd ta peworms ) ca n a l s o i nfect the s ma l l i ntes ti ne. Al l i nfecti ons l ea d to a cute cha nges i n the a bi l i ty of the i ntes ti na l l i ni ng to di ges t a nd a bs orb nutri ents a nd wa ter, l ea di ng to a cute di a rrhea a nd nutri ent defi ci enci es . Di ges ti on a nd a bs orpti on of ea ch of the three pa rts of the s ma l l i ntes ti ne a re s umma ri zed i n Ta bl e 11-6.
TABLE 11-6. Overvi ew of Functi ons of the Sma l l Intes ti ne Pernicious Anemia: Pernicious anemia i s a decrea s e i n red bl ood cel l s count ca us ed by a decrea s e of a bs orbed vitamin B12 due to the l a ck of intrinsic factor. Intri ns i c fa ctor i s norma l l y produced by the pa ri eta l cel l s of the s toma ch a nd i s es s enti a l i n a l l owi ng the a bs orpti on of thi s es s enti a l vi ta mi n i n the i l eum. The ca us e of perni ci ous a nemi a i s norma l l y by atrophic gastritis a nd res ul ti ng i mmune a tta ck a ga i ns t i ntri ns i c fa ctor a nd thes e cel l s . Ga s tri c bypa s s s urgery ca n a l s o ca us e a n a rti fi ci a l l y produced form of nonfuncti ona l pa ri eta l cel l s . Symptoms i ncl ude l os s or a l tera ti on of nerve s ens a ti on (paresthesia) us ua l l y i n the fi ngers a nd toes , i nfl a mma ti on of the tongue (glossitis), a nd wea knes s (s econda ry to the a nemi a ) a mong others . Perni ci ous a nemi a i s one of the megaloblastic anemias, one ca us e of whi ch i s fol a te defi ci ency, l ea di ng to defects i n red bl ood cel l s DNA s ynthes i s a nd a bnorma l l y enl a rged red bl ood cel l s . Trea tment i s by ora l a nd s ubl i ngua l s uppl ementa ti ons , both of whi ch a l l ow a bs orpti on of vi ta mi n B 12 vi a l oca ti ons other tha n the i l eum, or by i njecti ons or other a bs orpti ve methods .
LARGE INTESTINE/ANUS The fi na l pa rt of the ga s troi ntes ti na l tra ct i s the a pproxi ma tel y 5 ft. l ong, l a rge i ntes ti ne a nd a nus /rectum, whi ch ma i nl y s erves a s a l oca ti on for conti nued, pa s s i ve wa ter rea bs orpti on, vi ta mi n a bs orpti on, a nd tra ns port of i ndi ges ti bl e food for el i mi na ti on a s feces (Fi gure 11-8). Ma ny of thes e functi ons a re dependent on “gut fl ora ,” a va ri ety of from 300 to 1000 di fferent, s ymbi oti c ba cteri a l s peci es a nd four or more funga l s peci es tha t l i ve i n the i ntes ti ne a nd a l l ow a l a rge number of bi ochemi ca l rea cti ons tha t a re es s enti a l to l i fe. The Bacteroides genus of ba cteri a a ppea rs to be mos t a bunda nt a nd pl a y a n es peci a l l y i mporta nt rol e.
Figure 11-8. Summary of Functions of the Large Intestine. Abs orpti on of wa ter a nd i ons by the a s cendi ng a nd proxi ma l tra ns vers e col on a nd s tora ge a nd tra ns port of wa s te ma teri a l s by the di s ta l tra ns vers e, des cendi ng, a nd s i gmoi d col on a nd the rectum i s i l l us tra ted. [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] One rol e of gut fl ora i s the fi na l di ges ti on of dietary fiber, s ta rches , a nd ca rbohydra tes tha t the huma n body ca nnot meta bol i ze, i ncl udi ng l a ctos e i n l a ctos e-i ntol era nt i ndi vi dua l s . Thes e pol ys a ccha ri de cha i ns a re converted to short-chain fatty acids (e.g., a ceti c a ci d, propi oni c a ci d, butyri c a ci d, l a cti c a ci d, a nd i s ova l eri c a ci d) by ba cteri a l fermentation a nd pa s s i vel y a bs orbed i nto the bl ood s trea m. Indi ges ti bl e protei ns (e.g., col l a gen a nd el a s ti n) a re a l s o broken down by ba cteri a l fermenta ti on pa thwa ys . Bi ca rbona te i ons s ecreted by the l a rge i ntes ti ne hel p to reduce a ci di ty produced by the fermenta ti on rea cti ons . In a ddi ti on, s ymbi oti c, i ntes ti na l ba cteri a a l s o i ncrea s e the a bs orpti on of rema i ni ng l i pi ds a nd mi nera l s s uch a s ca l ci um, ma gnes i um, a nd i ron. Thes e ba cteri a l proces s es not onl y a l l ow uti l i za ti on of thes e energy s ources but a l s o i ncrea s e wa ter a bs orpti on a nd reduce l evel s of da ngerous i ntes ti na l ba cteri a whi l e i ncrea s i ng growth of benefi ci a l ba cteri a . Gut fl ora a l s o a ugment l evel s of vitamin K a nd biotin (vitamin B7 ), whi ch a re a bs orbed by the l a rge i ntes ti ne, a s a norma l by-product of thei r meta bol i s m. Gut fl ora a re a l s o res pons i bl e for producti on of s ome modified (secondary) bile salts. In a ddi ti on, thes e ba cteri a a re i mporta nt i n the devel opment a nd growth of es s enti a l , i ntes ti na l lymph tissue; the devel opment of antibodies to ha rmful pa thogens ; meta bol i s m a nd el i mi na ti on of i nges ted carcinogens; the conti nued repl i ca ti on a nd growth of the cel l s l i ni ng the i ntes ti ne; a nd protecti ve cha nges i n the expres s i on of cel l -s urfa ce mol ecul es on thes e cel l s . Antibiotic Use and Clostridium difficile: Gut fl ora provi des a number of es s enti a l functi ons , whi ch i ncl udes keepi ng the growth of unwa nted ba cteri a a nd yea s t reduced. Thi s a bi l i ty to prevent ha rmful s peci es from overproduci ng i n the huma n i ntes ti ne i s s ometi mes ca l l ed the “ba rri er effect.” Anti bi oti cs us ed i n the trea tment of di s ea s es ca n s ometi mes a dvers el y a ffect thi s ba l a nce by i na dvertentl y reduci ng the number of hel pful gut fl ora . The i ncrea s i ng us e of broa d-s pectrum a nti bi oti cs tha t el i mi na te mul ti pl e ba cteri a l s peci es , es peci a l l y the fa mi l y of fluoroquinolones, ha s a ugmented thi s probl em. One common ba cteri um tha t grows i n the l a rge i ntes ti ne i n the wa ke of overa ggres s i ve a nti bi oti c us e i s C. difficile. Overgrowth by thi s orga ni s m a nd i ts rel ea s e of ha rmful toxi ns l ea ds to chroni c di a rrhea , bl oa ti ng, a nd a bdomi na l pa i n. Thes e toxi ns a re bel i eved to i na cti va te a fa mi l y of G-protei ns /GTPa s e receptors by a l teri ng es s enti a l recogni ti on a nd bi ndi ng s uga r res i dues on thei r s urfa ce. Conti nued growth of C. difficile ca n l ea d to the s eri ous condi ti on of pseudomembranous colitis, a s evere i nfecti on of the col on. Thi s di s ea s e res ul ts from ma rked i nfl a mma ti on of the i ntes ti na l l i ni ng a nd a res ul ti ng membra ne-l i ke s tructure compos ed of fi bri n, l ymphocytes a nd monocytes , a nd dea d a nd dyi ng l i ni ng cel l s . Pa ti ents who ha ve been i n the hos pi ta l or nurs i ng homes a re i ncrea s i ngl y s us cepti bl e to thi s probl em due to i ncrea s ed numbers of ba ckground Clostridium i n thei r i ntes ti nes . Trea tment of the condi ti on i s ei ther by ces s a ti on of a nti bi oti c us e or wi th the ora l us e of the a nti bi oti cs metronidazole or vancomycin.
REVIEW QUESTIONS 1. Wha t a re the genera l rol es of the mouth, s toma ch, l i ver, ga l l bl a dder, pa ncrea s , a nd s ma l l i ntes ti ne i n di ges ti on? 2. Wha t a re the ma jor di ges ti ve enzymes of s a l i va a nd the s peci fi c functi on of ea ch? 3. Wha t i s the rol e of mucus ? 4. Wha t a re the functi ons of the pa ri eta l (oxynti c) a nd Chi ef cel l s i n the s toma ch? 5. Wha t a re the functi ons of the ma jor di ges ti ve hormones a nd where i s ea ch produced? 6. Wha t a re the pri ma ry meta bol i c rol es of l i ver? 7. Wha t fa ctors rel a ted to cl otti ng a nd cl ot di s s ol uti on a re s ecreted by the l i ver a nd wha t a re thei r functi ons ? 8. Wha t tra ns port/ca rri er protei ns a re s ecreted by l i ver? 9. Wha t rol es does the l i ver pl a y i n l i pi d tra ns port a nd meta bol i s m, es peci a l l y wi th rega rd to chol es terol a nd a s s oci a ted l i poprotei ns ? 10. Wha t a re the functi ons of ea ch of the exocri ne pa ncrea ti c hormones ? 11. Wha t a re the functi ons of ea ch of the endocri ne pa ncrea ti c hormones a nd i n whi ch cel l type i s ea ch produced? 12. Wha t a re the ma i n functi ona l mol ecul es /gl a nds of ea ch s ecti on of the s ma l l i ntes ti ne? 13. Wha t i s i ntri ns i c fa ctor a nd the cons equence of i ts defi ci ency? 14. Wha t a re the ma i n functi ons of the l a rge i ntes ti ne?
CHAPTER 12 MUSCLES AND MOTILITY Co-author/Editor: Darren Campbell Di vi s i on of Sports Medi ci ne, U.S. Ai r Force Aca demy
The Ba s i c Components of Mus cl e Exci ta ti on–Contra cti on Coupl i ng Skel eta l Mus cl e Ca rdi a c Mus cl e Smooth Mus cl e Energy Producti on a nd Us e i n Mus cl es Mi crotubul e-Ba s ed Moti l i ty Intermedi a te Fi l a ments Nonmus cl e Cel l s Revi ew Ques ti ons
OVERVIEW Movement a nd i ts regul a ti on, on a mi cros copi c a nd/or ma cros copi c s ca l e, a re es s enti a l for huma n l i fe. Mus cl e, compos ed of a cti n, myos i n, a nd a va ri ety of s tructura l a nd regul a tory protei ns , i s one of the ma i n cel l /ti s s ue types i nvol ved i n thi s movement. Skel eta l , ca rdi a c, a nd s mooth mus cl es offer a coordi na ted a nd regul a ted mea ns to move the huma n body from pl a ce to pl a ce; i ntera ct wi th i ts s urroundi ngs ; keep nutri ents fl owi ng to a nd wa s te products fl owi ng from va ri ous cel l s ; or move nutri ents , bl ood, l ymph, a nd other mol ecul es . Speci a l i zed protei ns a nd the mea ns to provi de energy for thes e proces s es ha ve evol ved for pa rti cul a r ti s s ues a nd functi ons . Mi crotubul es wi th tubul i n, dynei n, a nd ki nes i n mol ecul es a nd a s s oci a ted s tructures s uch a s ci l i a , fl a gel l a , centri ol es , ba s a l bodi es , centromeres , a nd mi toti c s pi ndl es provi de a nother i mporta nt mecha ni s m for a va ri ety of cel l movements a nd i nterna l cel l functi ons tha t a ffect a l l types of cel l s a nd a l l ow the di vi s i on of cel l s . Intermedi a te fi l a ments (IFs ) a l s o s erve s evera l es s enti a l rol es of cel l moti l i ty. Nonmus cl e cel l s uti l i ze a l l of thes e mecha ni s ms —a cti n/myos i n, mi crotubul e/dynei n/ki nes i n, a nd IFs —to a chi eve a wi de a rra y of functi ons throughout the huma n body.
THE BASIC COMPONENTS OF MUSCLE Mus cl e i s a n orga n tha t s peci a l i zes i n tra ns formi ng chemi ca l energy i nto mecha ni ca l work or movement. The mus cul a r s ys tem compri s es a l l the i ndi vi dua l a na tomi c mus cl es . Mus cl e ti s s ue i s deri ved from the mes oderma l l a yer of embryol ogi c germ cel l s a nd i s di vi ded i nto three ma i n types : s kel eta l mus cl e, ca rdi a c mus cl e, a nd s mooth mus cl e (Fi gure 12-1). The s tructure of thes e di fferent types of mus cl es i s s i mi l a r but the a rchi tecture a nd regul a ti on a re often very di fferent, ma ki ng thei r functi ons uni que. Nonmus cl e cel l s a re a fourth cel l type tha t uti l i zes ma ny of the s a me protei ns a nd proces s es for moti l i ty.
Figure 12-1. A–C. The Three Types of Muscle. Li ght mi crogra phs of ea ch type, a ccompa ni ed by l a bel ed dra wi ngs . A. Skel eta l mus cl e i s compos ed of l a rge, el onga ted, mul ti nucl ea ted fi bers tha t s how s trong, qui ck, vol unta ry contra cti ons . B. Ca rdi a c mus cl e i s compos ed of i rregul a r bra nched cel l s bound together l ongi tudi na l l y by i nterca l a ted di s ks a nd s hows s trong, i nvol unta ry contra cti ons . C. Smooth mus cl e i s compos ed of grouped, s pi ndl e-s ha ped cel l s wi th wea k, i nvol unta ry contra cti ons . The dens i ty of i ntercel l ul a r pa cki ng s een refl ects the s ma l l a mount of extra cel l ul a r connecti ve ti s s ue pres ent. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] There a re two ma jor protei ns i nvol ved i n contra cti on i n a l l types of mus cl e: actin a nd myosin. Thes e protei ns convert chemi ca l energy i nto mecha ni ca l work through a n i ntera cti on wi th adenosine triphosphate (ATP). The tri ggeri ng a nd control of thi s proces s a re di fferent for ea ch mus cl e type. Skel eta l , ca rdi a c, a nd s mooth mus cl es ha ve a repea ti ng uni t ca l l ed the sarcomere, conta i ni ng the mus cl e fi bers compos ed of a cti n a nd myos i n a nd a cces s ory protei ns (s ee bel ow). Surroundi ng thes e mus cl e fi bers i s the equi va l ent of a pl a s ma membra ne ca l l ed the sarcolemma. Wi thi n the s a rcol emma i s the s a rcopl a s m conta i ni ng mi tochondri a a nd a sarcoplasmic reticulum (SR), the equi va l ent of a s mooth endopl a s mi c reti cul um (SER). Li ke the SER i n other cel l types , the SR conta i ns ca l ci um i ons (Ca 2+) es s enti a l for the i ni ti a ti on of mus cl e a cti va ti on. More s peci fi ca l l y, s peci a l i zed i nva gi na ti ons of the SR, ca l l ed transverse tubules (T-tubules), i nterdi gi ta te between the mus cl e fi bers
to provi de effi ci ent del i very of ca l ci um. ACTIN The term actin des cri bes both the gl obul a r, s i ngl e a mi no a ci d (monomer) cha i n (G-actin) a nd the thin filament (F-actin) s tructure formed from two pa ra l l el s tra nds of mul ti pl e G-a cti n mol ecul es , whi ch wi nd to form a doubl e hel i x s tructure (Fi gure 12-2A). Three ma jor cl a s s es of a cti n exi s t, i ncl udi ng α-a cti n (found i n s kel eta l , ca rdi a c, a nd s mooth mus cl es ) a nd β- a nd γ-a cti ns (found i n nonmus cl e cel l s ). Thes e F-a cti n fi l a ments provi de s tructure a s pa rt of the cel l ’s cytoskeleton. The cytos kel eton i s a l s o di rectl y l i nked to the pl a s ma membra ne vi a i ntegra l membra ne protei ns a nd i ntra cel l ul a r juncti ons (Cha pter 8) a nd, a s s uch, i s i mporta nt i n ma ny s i gna l i ng functi ons . Acti n thi n fi l a ments i ntera ct wi th myos i n thi ck fi l a ments (Fi gure 12-2C a nd s ee bel ow) a nd a re i ntegra l i n a mul ti tude of cel l moti l i ty functi ons , i ncl udi ng mus cl e contra cti on, the movement of i ntra cel l ul a r ves i cl es , cel l di vi s i on a nd cytoki nes i s , i mmune res pons e, pha gocytos i s , wound hea l i ng, a nd others . Acti n i s a l s o requi red i n the nucl eus for RNA pol ymera s e I, II, a nd III compl ex forma ti on a nd functi on, export of RNA a nd protei ns from the nucl eus , a nd s ome a s pects of chroma ti n remodel i ng.
Figure 12-2. A–C. Molecules Composing Thin and Thick Filaments. The contra cti l e protei ns a re the thi n a nd thi ck myofi l a ments wi thi n myofi bri l s . A. Ea ch thi n fi l a ment i s compos ed of F-a cti n, tropomyos i n, a nd troponi n compl exes . B. Ea ch thi ck fi l a ment cons i s ts of ma ny myos i n hea vy cha i n mol ecul es bundl ed together a l ong thei r rod-l i ke ta i l s , wi th thei r hea ds expos ed a nd di rected towa rd nei ghbori ng thi n fi l a ments . C. Bes i des i ntera cti ng wi th the nei ghbori ng thi n fi l a ments , thi ck myofi l a ment bundl es a re hel d i n pl a ce by l es s wel l -cha ra cteri zed, myos i n-bi ndi ng protei ns wi thi n the M-l i ne. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-
Hi l l , 2010.] TROPOMYOSIN–TROPONINS Tropomyosin i s a regul a tory protei n tha t bi nds to F-a cti n thi n fi l a ments a nd revers i bl y bl ocks myos i n hea d bi ndi ng s i tes (Fi gure 12-2A–C). In noncontra cti ng mus cl e, troponin T and troponin I mol ecul es (pa rt of a troponin compl ex, Fi gure 12-2A) hol d tropomyos i n over the myos i n-bi ndi ng s i te. Upon expos ure to Ca 2+ rel ea s ed from the SR, troponin C, the thi rd protei n of the troponi n compl ex, crea tes a conforma ti ona l cha nge of the tropomyos i n–troponi n compl ex expos i ng the s i te. When ca l ci um concentra ti ons fa l l , the cha nge revers es a nd the bl ock i s a ga i n pres ent. Smooth mus cl e a nd nonmus cl e cel l s do not conta i n troponi n a nd rel y on other regul a tory proces s es to control a cti n–myos i n contra cti on/force genera ti on. Troponi n C conta i ns a s peci fi c a mi no a ci d s equence na med a n EF ha nd. In thi s terti a ry s tructure moti f (Cha pter 1), a n α-hel i x (termed E for i ts s equence i n a s eri es of α-hel i ces ) i s joi ned to a n F α-hel i x by a l oop of a pproxi ma tel y 12 a mi no a ci ds . The hel i x–l oop–hel i x forms a s tructure remi ni s cent of a n extended forefi nger (E hel i x) a nd thumb (F hel i x) wi th the curve between repres enti ng the l oop. Among the 12 a mi no a ci ds of the l oop a re pos i ti ons , 1, 3, 5, 7, 9, a nd 12, whi ch bi nd the Ca 2+. Ami no a ci d 12 i s a l wa ys gl uta ma te or a s pa rta te whos e Rgroup provi des two pa rti a l l y nega ti vel y cha rged oxygen mol ecul es (Cha pter 1) for bi ndi ng the pos i ti vel y cha rged ca l ci um. The s ma l l s i ze of a hi ghl y cons erved gl yci ne a mi no a ci d a t res i due 6 provi des s pa ce for thi s s tructure to form. The other a mi no a ci ds a re ma i nl y hydrophobi c tha t s ta bi l i ze the E a nd F hel i ces a nd, therefore, the hel i x– l oop–hel i x s tructure. MYOSIN The huma n genome hol ds more tha n 40 di fferent types of myosin genes . Thes e di fferent genes produce va ri a ti ons of myos i n s ha pes , whi ch i n turn a ffect the s peed a t whi ch the fi l a ments move. To da te, there a re 12 cl a s s es of myos i n defi ned i n the huma n genome. Al l myos i n mol ecul es ha ve three doma i ns , i ncl udi ng a head, neck, a nd tail porti on of the a mi no a ci d s equence (Fi gure 12-3A). The myos i n hea d bi nds a cti n thi n fi l a ments a nd us es hydrol ys i s of ATP to a denos i ne di phos pha te (ADP) to genera te force. The ta i l doma i n va ri es dependi ng on the type of myos i n but, i n genera l , ca n bi nd to tra ns port ves i cl es or combi ne wi th other myos i n mol ecul es . Some myos i n ta i l doma i ns ca n a l s o regul a te the protei ns ’ a cti vi ty by fol di ng over a nd effecti vel y bl ocki ng the myos i n hea d doma i n. The neck doma i n s erves a s a l i nk or l ever a rm for the tra ns fer of thi s force between the hea d a nd ta i l . The va ryi ng types of myos i n protei ns produce di fferent s peeds of movement, dependi ng on the l ength of the neck regi on. Four myosin light chains: two essential light chains, a nd two regulatory light chains a re a l s o bound to the neck regi ons (Fi gure 12-3A). The functi on of thes e l i ght cha i ns i s di s cus s ed bel ow.
Figure 12-3. A–C. Myosin I and II. A. Basic Myosin Molecule. The hea vy a nd l i ght cha i ns (es s enti a l –red, regul a tory–da rk bl ue) form the ba s i c s tructure of a l l myos i n mol ecul es , i ncl udi ng the l ong ta i l compos ed of two coi l ed α-hel i ces a nd the gl obul a r hea d regi on. B. Myosin I. Ves i cl es for tra ns port a re a tta ched by the myos i n I ta i l s ecti on. The s i ngl e hea d of myos i n I moves down the a cti n fi l a ment i n a s i mi l a r ma nner to other myos i n hea ds . Myos i n I mol ecul es ca n a l s o l i nk between F-a cti n fi l a ments , res ul ti ng i n l i mi ted moti l i ty s i mi l a r to myos i n II mol ecul es . [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] C. Myosin II. The ta i l s ecti on of the two hea vy cha i ns of myos i n II forms a coi l ed-coi l a rra ngement. Thi s s tructure ca n i ntera ct wi th s evera l other myos i n II ta i l s to form thi ck fi l a ments . The two hea d regi ons of ea ch myos i n mol ecul e conta i n a n a cti n-bi ndi ng s i te (red ci rcl e) a nd a n ATP-bi ndi ng s i te (bl ue l i ne). Coordi na ted movement of the myos i n hea ds produces force a nd, therefore, movement when i ntera cti ng wi th a cti n fi l a ments . See text for further deta i l s . [Ada pted wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] Myosin I i s the s i mpl es t form cons i s ti ng of a ba s i c myos i n hea d, neck, a nd a ta i l of va ryi ng l engths (Fi gure 12-3A–B). The ba s i c functi on of myos i n I i s mos t often des cri bed a s ves i cl e tra ns port, a l though other functi ons a re emergi ng. Myosin II i s compos ed of two i denti ca l a mi no a ci d cha i ns , termed the “hea vy cha i ns ” (Fi gure 12-3A). The ta i l s ecti on of ea ch cha i n coi l s a round the other, ma ki ng a di mer myos i n fi l a ment wi th two hea ds . Mul ti pl e myos i n II di mers bi nd a t the ta i l doma i n, formi ng a s tructure ca l l ed a thick filament (Fi gures 12-2B–C a nd 12-3C). Addi ti ona l myos i n s ubtypes ha ve s tructures s i mi l a r to ei ther myos i n I or myos i n II a nd a re res pons i bl e for di vers e types of cel l moti l i ty. Thes e functi ons i ncl ude i ntra cel l ul a r tra ns port (both to a nd from the nucl eus to the cel l peri phery), ma i ntena nce a nd movement of ves i cl es a nd orga nel l es i n s ubregi ons of the cytopl a s m, a nd, pos s i bl y, the percepti on of l i ght i n the reti na a nd s ound wa ves i n the i nner ea r. MYOSIN LIGHT CHAINS Ea ch myos i n hea d conta i ns two essential myosin light chains a nd two regulatory myosin light chains bound to the neck regi on (Fi gure 12-3A). Di fferent forms of myos i n l i ght cha i n a re found on di fferent myos i n types a nd i nfl uence thei r a cti vi ty a nd regul a ti on. Myos i n l i ght cha i ns s erve a n i mporta nt regul a tory rol e for myos i n a cti vi ty i n s mooth a nd nonmus cl e cel l contra cti on where the tropomyos i n–troponi n mecha ni s m i s a bs ent. In s kel eta l mus cl e, the myos i n l i ght cha i ns i nfl uence the s peed of contra cti on but a re not es s enti a l for myos i n a cti vi ty. In ca rdi a c mus cl e, myos i n l i ght cha i ns a ffect myos i n hea d ATPa s e a nd a re bel i eved to ha ve s ome rol e i n the devel opment of hea rt fa i l ure. Acti vi ty of myos i n i n s mooth/nonmus cl e cel l s rel i es on phos phoryl a ti on of s eri ne 19 on the regul a tory cha i ns by myosin light chain kinase (MLCK). MLCK a cti vi ty i s regul a ted by Ca 2+ tha t bi nds to the regul a tory protei n ca l modul i n, a l s o by a n EF ha nd-bi ndi ng moti f (s ee a bove). Ca l modul i n s ubs equentl y a cti va tes MLCK. When ca l ci um concentra ti ons fa l l , myosin light chain phosphatase (MLCP) removes the phos pha te group a nd
i na cti va tes the regul a tory l i ght cha i ns a nd, therefore, myos i n. Ca l ci um a l s o i nhi bi ts MLCP by the a cti ons of a nother s eri ne/ threoni ne enzyme ca l l ed Rho kinase; other s i gna l i ng pa thwa ys a nd enzymes ha ve a l s o been i mpl i ca ted i n myos i n l i ght cha i n phos phoryl a ti on a nd dephos phoryl a ti on. One enzyme, s phi ngos i ne-1-phos pha ta s e, ma y pl a y a n i mporta nt rol e i n modul a ti ng contra cti on a nd rel a xa ti on of bl ood ves s el s a nd, therefore, ma i ntena nce of bl ood fl ow i n res pons e to cha nges i n bl ood pres s ure. ACTIN-BINDING PROTEINS Actin-binding proteins (ABPs) a re a va ri ed group of protei ns tha t regul a te a cti n fi l a ment forma ti on a nd l ength a s wel l a s a cti n–myos i n i ntera cti ons . Al though more promi nent i n non-mus cl e cel l a nd s mooth mus cl e contra cti on, s ome, i ncl udi ng tropomyos i n a nd the troponi ns (s ee a bove), a re found i n s kel eta l a nd ca rdi a c mus cl es . One s uch ABP, α-actinin bi nds a cti n thi n fi l a ments to s kel eta l mus cl e Z-l i nes a nd s mooth mus cl e dens e bodi es (Fi gures 12-4 a nd 12-8A). α-Acti ni n i s a l s o bel i eved to bi nd to a cti n fi l a ment bundl es a nd s epa ra tes ea ch thi n fi l a ment by a pproxi ma tel y 35 nm. Thi s s epa ra ti on a l l ows myos i n thi ck fi l a ments the proper s pa ci ng for opti ma l contra cti on. Addi ti ona l ABPs cros s -l i nk thi n fi l a ments (filamin), s ever thi n fi l a ments (gelsolin, tropomodulin, a nd villin) a nd ca p them to res tri ct thei r l ength (capZ), regul a te a cti n–tropomyos i n a cti vi ty i n s mooth mus cl e cel l s where troponi ns a re a bs ent (caldesmon), regul a te myos i n ATPa s e a cti vi ty (calponin), a nd connect mi crofi l a ments to the cel l membra ne (dystrophin, vinculin, a nd integrins). Ma ny other ABPs a l s o exi s t a nd perform other functi ons i n a va ri ety of cel l types .
Figure 12-4. Excitation–Contraction Coupling. Acti n a nd myos i n form s a rcomere s tructures i n s kel eta l mus cl e a s s hown. The i nfl ux of ca l ci um i ons (Ca 2+) a l l ows myos i n hea ds from thi ck fi l a ments to bi nd to the a cti n fi l a ments . Ca 2+ a l s o a cti va tes myos i n hea d cros s -bri dge cycl i ng, l ea di ng to force genera ti on a nd, ul ti ma tel y, s horteni ng of the s a rcomere. Shorteni ng of mul ti pl e s a rcomeres l ea ds to mus cl e contra cti on. The proces s i s s i mi l a r i n ca rdi a c a nd s mooth mus cl e contra cti on, a l though s tructura l orga ni za ti on a nd regul a ti on di ffer. ADP, a denos i ne di phos pha te; ATP,
a denos i ne tri phos pha te. [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra wHi l l , 2009.]
EXCITATION–CONTRACTION COUPLING Mus cl e a chi eves i ts ma jor functi on of genera ti ng movement by contra cti on through a s eri es of bi ndi ng events a nd enzyma ti c rea cti ons . Thi s proces s i s ca l l ed excitation–contraction coupling (Fi gure 12-4). Certa i n pa rts of thi s mecha ni s m a re cons erved i n a l l types of a cti n–myos i n contra cti on/moti l i ty, wherea s a l terna ti ve forms of i ni ti a ti on a nd/or regul a ti on a re found i n pa rti cul a r mus cl e types . Ma ny of the components a re s i mi l a r i n nonmus cl e cel l s , but ma ny di fferences exi s t beca us e of the va ri a ti ons i n orga ni za ti on a nd a ddi ti ona l protei ns a nd thei r functi ons . Nonmus cl e cel l contra cti on wi l l be di s cus s ed s epa ra tel y bel ow. Two ba s i c proces s es mus t ha ppen i n a l l mus cl e types for contra cti on to occur: (1) bi ndi ng of myos i n hea ds to the a cti n thi n fi l a ment a nd (2) s ti mul a ti on of myos i n to genera te force. Both of thes e proces s es rel y, a t l ea s t i n pa rt, on Ca 2+. In a l l mus cl e types , a s i gna l from a nerve or pa cema ker cel l s ca us es a n i ni ti a l i ncrea s e i n Ca 2+ concentra ti ons . Thi s rel a ti vel y s ma l l i nfl ux of Ca 2+ l ea ds to a l a rger rel ea s e from the SR. Thi s concept of a s ma l l ca l ci um s i gna l l ea di ng to hi gher ca l ci um concentra ti ons a nd contra cti on i s termed calcium-induced calcium release (CICR). The rel ea s e from the SR va ri es dependi ng on the type of mus cl e a nd i s di s cus s ed bel ow. T-tubul es a l l ow del i very of thi s ca l ci um to a l l a rea s of a mus cl e uni t to permi t s ynchronous contra cti on. The rel ea s ed ca l ci um i ntera cts wi th a n ABP—troponi n/tropomyos i n i n s kel eta l a nd ca rdi a c mus cl e a nd ca l des mon i n s mooth mus cl e—to a l l ow myos i n hea d bi ndi ng to a cti n thi n fi l a ments . Myos i n force genera ti on i s the s econd ba s i c functi on requi red for contra cti on. In a l l mus cl e types , ATP bi nds to the myos i n mol ecul e hea d. An ATPa s e on the myos i n hea d cl ea ves ATP i nto ADP a nd a phos pha te mol ecul e (PO4 3–), produci ng a cha rged form of the myos i n protei n tha t bi nds to the now open F-a cti n fi l a ments a t a n a ngl e. Rel ea s e of the phos pha te mol ecul e from the myos i n hea d ca us es i t to s wi vel to a more a cute a ngl e, res ul ti ng i n a ra tcheti ng movement between i t a nd a cti n. Thi s ra tcheti ng movement s l i des the thi ck a nd thi n fi l a ments pa s t ea ch other, converti ng the chemi ca l energy of the ATP i nto the mecha ni ca l energy movi ng the fi l a ments . Rel ea s e of the ADP mol ecul e a nd bi ndi ng of a new ATP mol ecul e brea ks the bond between myos i n hea d a nd F-a cti n thi n fi l a ment. As thi s proces s repea ts ma ny ti mes , the overa l l l ength of the s a rcomere s hortens . Ca l ci um a l s o pl a ys a promi nent rol e i n s mooth mus cl e myos i n a cti vi ty vi a a cti va ti on of regul a tory myos i n l i ght cha i ns (s ee a bove). The cycl e conti nues a s l ong a s the concentra ti on of ca l ci um i n the s a rcopl a s m of the cel l rema i ns el eva ted. At the end of thi s proces s , the ca l ci um entry i nto the cel l s s l ows a nd begi ns to be col l ected a ga i n by the SR by a sarco/endoplasmic reticulum Ca2+-ATPase pump, a l s o powered by ATP. The pump ca n produce a n a pproxi ma tel y 10,000-fol d hi gher concentra ti on i ns i de the SR vers us the s a rcopl a s m. An a ddi ti ona l protei n calsequestrin a l s o bi nds Ca 2+ i ns i de the SR to reduce the effecti ve, free Ca 2+ concentra ti on tha t the pump mus t work a ga i ns t. Ca l s eques tri n ca n bi nd over 40 Ca 2+ per protei n mol ecul e not by a n EF ha nd s tructure but, i ns tea d, by us i ng cha rged a mi no a ci ds res i dues a nd cha nges of s econda ry s tructure to α-hel i ces . Once the ca l ci um concentra ti on l owers , ca l ci um i s rel ea s ed from troponi n C, the previ ous conforma ti ona l cha nge i s revers ed a nd troponi n T a nd I a ga i n bl ock the bi ndi ng of myos i n to the a cti n bi ndi ng s i te. As the cycl e ends , a new mol ecul e of ATP then bi nds to the myos i n hea d, whi ch di s pl a ces the ADP a nd the i ni ti a l s a rcomere l ength i s res tored.
SKELETAL MUSCLE STRUCTURE AND GENERAL OVERVIEW Skel eta l mus cl e compri s es the bul k of the mus cl e ma s s found i n our bodi es (Fi gure 12-5). The a vera ge a dul t ma l e i s ma de up of a pproxi ma tel y 42% s kel eta l mus cl e a nd a n a vera ge a dul t fema l e cons i s ts of 36% s kel eta l mus cl e. Ea ch mus cl e ha s a n ori gi n a nd i ns erti on on the s kel eton, wi th the i ndi vi dua l mus cl e fi bers runni ng pa ra l l el or obl i que to the l ong a xi s of the mus cl e. Skel eta l mus cl es a re ca pa bl e of produci ng a forceful contra cti on a nd movi ng the el ements of the mus cl e over a rel a ti vel y l a rge di s ta nce. Wi th the s horteni ng of l ength duri ng the contra cti on, thi s mus cl e wi l l often cha nge i n di a meter, l i ke the s wel l produced when contra cti ng the bi ceps mus cl e. In s kel eta l mus cl e, the Fa cti n thi n fi l a ments a re bound end to end a t a s tructure ca l l ed the Z-l i ne (Fi gure 12-4). The repea ted Z-l i nes a nd a l terna ti ng thi n a nd thi ck fi l a ments gi ve the mus cl e the s tri a ted a ppea ra nce s een on l i ght mi cros copy.
Figure 12-5. Skeletal Muscle. Skel eta l mus cl e i s hi ghl y orga ni zed. At the mol ecul a r l evel , repeti ti ve s a rcomeres (s ee text a bove) form a myofi bri l . Mul ti pl e myofi bri l s form mus cl e cel l s , col umns , a nd, ul ti ma tel y, mus cl e. [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Centronuclear/Myotubular Myopathies: Centronuclear/ Myotubular myopathies (CNMs) a re a ra re s eri es of i nheri ted di s ea s e s ta tes i n whi ch s kel eta l mus cl e nucl ei a re l oca ted i n the center of the mus cl e cel l s i ns tea d of a t the peri phera l edges . The exa ct ca us es ha ve not been found but errors i n the devel opment of embryol ogi ca l mus cl e cel l s a ppea r to be common. A s us pect enzyme ca l l ed myotubularin, whi ch ha s dephos phoryl a ti ng a cti vi ty, ma y be pa rt of the X-l i nked form, properl y termed myotubul a r myopa thy. X-l i nked CNM pres ents a t bi rth wi th s evere l os s of mus cl e tone (hypotoni a ), a ffects mus cl es requi red for brea thi ng, a nd often res ul ts i n dea th before a ca us e ca n be determi ned. Pa ti ents a l s o often ha ve a n a s s oci a ted na rrow a nd l ong hea d wi th pa l a te, fi nger, a nd ches t s tructura l a bnorma l i ti es . Autos oma l domi na nt a nd reces s i ve forms a l s o exi s t, properl y termed centronucl ea r myopa thi es . The a utos oma l domi na nt vers i on ma y be beca us e of muta ti ons a ffecti ng the protei n dynamin, a gua nos i ne tri phos pha te-dependent tra ns port protei n. Symptoms of CNM ma y never pres ent (e.g., a utos oma l reces s i ve form) or ma y i ncl ude va ri a bl e ra nges of i ncrea s i ng mus cul a r wea knes s , whi ch often pres ents i n the teens or 20s . There i s no cure for CNM di s ea s es a nd trea tment i s onl y s upporti ve. Growth, regenera ti on, a nd a cti vi ty of s kel eta l mus cl e i s under the control of s evera l s i gna l i ng pa thwa ys , whi ch a cti va te s peci fi c gene tra ns cri pti on/tra ns l a ti on, l ea di ng to the producti on of mus cl e protei ns a s wel l a s regul a tory protei ns , i ncl udi ng thos e i nvol ved i n energy producti on for mus cl e contra cti on. Some a re dependent on motor neuron a cti vi ty a nd/or contra cti on [e.g., more gl ucos e tra ns porter type 4 (GLUT4) receptors a re brought to the s urfa ce i n hi ghl y contra cti ng mus cl es to i ncrea s e gl ucos e upta ke] a nd, therefore, a re di rectl y i nfl uenced by s peci fi c mus cl e a cti vi ty. Skel eta l mus cl e i s under vol unta ry control a nd the contra cti on i s dri ven by motor neurons i n the s pi na l cord a nd the neurotra ns mi tter acetylcholine (Cha pter 19). Skel eta l mus cl e neurons a re di rectl y a s s oci a ted wi th rya nodi ne receptors i n the SR, whi ch recei ve the nerve s i gna l a nd depol a ri ze L-type vol ta ge-dependent ca l ci um cha nnel s i n the T-tubul es . Thi s depol a ri za ti on ca us es a cha nge i n the foot s tructures tha t bri dge the T-tubul es a nd SR, a l l owi ng rel ea s e of ca l ci um i nto the s a rcopl a s m. The free ca l ci um bi nds wi th the regul a tory mol ecul e calmodulin a nd a cti va tes troponi n C, unma s ki ng the myos i n-bi ndi ng s i te by di s pl a ci ng troponi n-I a nd tropomyos i n. Acetylcholine and Skeletal Muscle Disease: Skel eta l mus cl e rel i es on the neurotra ns mi tter acetylcholine to i ni ti a te mus cl e contra cti on. Di s rupti on of a cetyl chol i ne’s a cti vi ty l ea ds to di s ea s e s ta tes i ncl udi ng mya s theni a gra vi s a nd La mbert–Ea ton s yndrome. The ma jor form of myasthenia gravis (MG) i s ca us ed by a nti bodi es a ga i ns t the body’s own receptor for a cetyl chol i ne a t the pos ts yna pti c juncti on (nicotinic– acetylcholine receptor). The a nti bodi es bl ock or des troy the receptor nega ti ng the a cti on of a cetyl chol i ne. The body’s na tura l cholinesterase a cti vi ty degra des the neurotra ns mi tter before a cti va ti on of mus cl e contra cti on ca n be a chi eved. The cumul a ti ve effect of both l ea ds to wea keni ng of mus cl es a fter extended peri ods of a cti vi ty (fatigability). Mus cl es often a ffected i ncl ude eyes a nd eyel i ds , fa ci a l /neck mus cl es res pons i bl e for expres s i on, s peech a nd s wa l l owi ng, mus cl es i nvol ved i n brea thi ng, a nd a rm/l eg mus cl es . Exa mi na ti on ca n often be norma l unl es s mea s urement to fa ti gue of mus cl es i s i ncl uded. Bl ood tes ts for the a nti bodi es a re not a l wa ys concl us i ve, l ea di ng to other neurol ogi ca l or mus cl e bi ops y tes ts . Al though ra rel y performed, i ntra venous a dmi ni s tra ti on of the chol i nes tera s e i nhi bi tors edrophonium or neostigmine ca n rel i eve s ymptoms a nd a s s i s t i n di a gnos i s . Pa ti ents di a gnos ed wi th MG a re trea ted wi th s i mi l a r chol i nes tera s e i nhi bi tors (neos ti gmi ne or pyridostigmine) a nd/or i mmune s uppres s a nts (s teroi ds a nd other medi ca ti ons ). Wi th proper trea tment, l i fe expecta ncy i s norma l . Lambert– Easton syndrome a ffects pa ti ents i n a s i mi l a r ma nner to MG but i s ca us ed by a nti bodi es a ga i ns t a pres yna pti c ca l ci um cha nnel tha t s tops the rel ea s e of a cetyl chol i ne. Li ke MG, La mbert–Ea ton s yndrome i s ra re a nd often di ffi cul t to di a gnos e. SKELETAL MUSCLE TYPES Skel eta l mus cl e ca n be di vi ded i nto di fferent types , dependi ng on the myos i n fi bers pres ent. There a re two genera l ca tegori es : s l ow twi tch (type I) a nd fa s t twi tch (type II) wi th type II di vi ded i nto three a ddi ti ona l s ubtypes (type IIa , IIb, a nd IIx). The types of mus cl e fi bers ha ve ma ny di fferences i ncl udi ng contra cti on ti me, a bi l i ty to be fa ti gued, a nd the ma xi mum dura ti on of us e. Slow-twitch (type I) fi bers fi re more s l owl y tha n the fa s t-twi tch fi bers . They conta i n the heme-conta i ni ng, oxygen-bi ndi ng protei n myoglobin, whi ch s erves a s a n i mporta nt oxygen s tora ge protei n ca pa bl e of rel ea s i ng oxygen duri ng peri ods of hypoxi a . Myogl obi n i s a l s o us ed cl i ni ca l l y a s a ma rker for da ma ged mus cl e a nd ca n be el eva ted duri ng peri ods of mus cl e cel l da ma ge s uch a s rhabdomyolysis (s ee s i deba r). Type I fi bers a re s ometi mes referred to a s “red” beca us e of the pres ence of heme/Fe2+ i n the myogl obi n a s wel l a s i ncrea s ed numbers of mi tochondri a a nd s ma l l bl ood ves s el s /ca pi l l a ri es . Thes e fi bers , therefore, a re a bl e to pri ma ri l y us e oxygen for more effi ci ent oxidative phosphorylation (Cha pter 6) to produce energy, ma i nl y from fa tty a ci ds mobi l i zed from triglycerides. As a res ul t, thes e fi bers ca n pa rti ci pa te i n s l ow force genera ti on a nd contra cti on for l ong peri ods of ti me wi thout fa ti gue, es peci a l l y when compa red wi th fa s t-twi tch fi bers . Thi s mus cl e fi ber type hel ps endura nce a thl etes , who often ha ve a grea ter percenta ge of s l ow-twi tch mus cl e fi bers tha n nonendura nce a thl etes , compete over l ong di s ta nces for a n extended peri od of ti me. Type I myos i n i s a l s o found i n mus cl es i nvol ved i n pos ture. Rhabdomyolysis: Rhabdomyolysis or “rhabdo” i s the medi ca l term for the a bnorma l brea kdown of s kel eta l mus cl e ca us ed by tra uma (e.g., ma jor crus h i njuri es ), di s ea s e s ta tes , a nd/or medi ca ti on s i de effects . Rha bdo i s ma rked by the a ppea ra nce of a bnorma l l y hi gh l evel s of mus cl e components i n the bl ood s trea m. Myogl obi n i s a s peci fi c l a bora tory ma rker us ed by cl i ni ci a ns who s us pect rha bdomyol ys i s i n thei r pa ti ents . The s udden rel ea s e of protei ns a nd va ri ous i ons ca n ca us e ki dney fa i l ure, neurol ogi ca l probl ems , a nd i rregul a r hea rt rhythms , whi ch ca n be fa ta l . Infl a mma ti on ca us ed by the da ma ge to mus cl e ca n a l s o l ea d to s eri ous condi ti ons s uch a s compa rtment s yndrome where porti ons of the body ca n l os e thei r bl ood s uppl y beca us e of s wel l i ng. The s udden s wel l i ng ca n a l s o l ea d to s hock. Trea tment i s by i ntra venous fl ui ds a nd s upporti ve ca re often i ncl udi ng di a l ys i s to pres erve ki dney functi on a nd to correct i on i mba l a nces . Compa rtment s yndrome ma y requi re s urgi ca l openi ng of mus cl e connecti ve ti s s ue (fa s ci otomy) to rel i eve the pres s ure from s wel l i ng. Prompt a nd profes s i ona l ca re of a nyone i n da nger of or s howi ng s i gns of rha bdomyol ys i s us ua l l y res ul ts i n a good outcome for the pa ti ent. Type IIa fast-twitch fibers a re a l s o known a s i ntermedi a te fa s t-twi tch fi bers a nd conta i n hi gh a mounts of myogl obi n, mi tochondri a , a nd ca pi l l a ri es . To s ome extent, they bri dge the ga p between the s l ow-twi tch a nd fa s t-twi tch fi bers i n tha t they ca n us e both a erobi c a nd a na erobi c meta bol i s m to crea te energy us i ng creatine phosphate a nd glycogen. Thes e fi bers do not genera te the s a me force a s type IIb fi bers nor a re they a s fa ti gue res i s ta nt a s the s l ow-twi tch fi bers . Type IIb fast-twitch fibers a re the cl a s s i c fa s t-twi tch mus cl e fi bers . Thes e fi bers a re ca pa bl e of produci ng qui ck a nd powerful mus cl e contra cti ons neces s a ry for the s udden expl os i ve need for s trength of a power l i fter or the burs t of s peed needed from a s pri nter. Thi s mus cl e fi ber ha s the hi ghes t ra te of fi ri ng of a l l the mus cl e fi ber types . Type IIb fi bers do not conta i n myogl obi n a nd ha ve fewer ca pi l l a ri es tha n type I. Therefore, they pri ma ri l y us e a na erobi c meta bol i s m a l s o us i ng creatine phosphate a nd glycogen. As a res ul t, they a re s ometi mes ca l l ed whi te fi bers a nd a re a l s o more s ens i ti ve to fa ti gue. Les s i s known a bout type IIx fibers, whi ch ma y ha ve s ome of the oxi da ti ve meta bol i s m of s l ow-twi tch fi bers a l ong wi th the bi ophys i ca l properti es of fa s t-twi tch fi bers . In i ndi vi dua l mus cl es , there i s us ua l l y a combi na ti on of di fferent types of fi bers (Fi gure 12-6). Ea ch i ndi vi dua l motor uni t, however, conta i ns a s i ngl e α-motor neuron a nd the mus cl e fi bers i t i nnerva tes wi l l conta i n the s a me type of mus cl e fi bers . Thes e fi bers a l l ha ve the s a me
contra cti l e cha ra cteri s ti cs a nd fi re a s a n a l l or none phenomenon. The di s tri buti on of thes e di fferent types of motor uni ts wi thi n a s i ngl e s kel eta l mus cl e i s determi ned geneti ca l l y a nd by the s peci fi c functi on of the mus cl e. It i s thi s mi x tha t determi nes the functi on a nd fa ti ga bi l i ty of the i ndi vi dua l mus cl e. A mus cl e wi th more fa s t-twi tch motor uni ts wi l l ha ve qui cker a nd more powerful contra cti ons but be more s us cepti bl e to fa ti gue, wherea s a mus cl e wi th fa ti gue-res i s ta nt s l ow-twi tch motor uni ts wi l l genera te l es s powerful contra cti ons but be a bl e to s us ta i n the contra cti ons for l onger peri ods of ti me. There i s s ome evi dence tha t mus cl es a re a bl e to a da pt a nd s wi tch mus cl e fi ber types wi th s peci fi c tra i ni ng.
Figure 12-6. Skeletal Muscle Fiber Types. Cros s s ecti on of s kel eta l mus cl e s ta i ned hi s tochemi ca l l y to detect the dens i ty of myofi bri l l a r myos i n ATPa s e ca n be us ed to demons tra te the di s tri buti on of s l ow (S)-type I fi bers , i ntermedi a te (I)-type IIa fi bers , a nd fa s t (F)-type IIb fi bers . [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] Muscle Mass Loss from Sarcopenia to Astronauts: Severa l ca us a ti ve fa ctors ca n l ea d to the l os s of s kel eta l mus cl e ma s s . One common ca us e i s sarcopenia, the l os s of s kel eta l mus cl e ti s s ue due to a gi ng, es ti ma ted a t 0.5%–1% per yea r from a ge 25 yea rs on. Al though decrea s ed exerci s e a nd us e ma y be pa rt of the rea s on, bi ochemi ca l cha nges a l s o res ul t i n decrea s ed mus cl e fi ber growth, i ncl udi ng s ma l l er mus cl e fi ber ci rcumference a nd repl a cement of mus cl e by fa t or fi brous ti s s ue. Pa rt of the mecha ni s m ma y be the i ncrea s i ng fa i l ure duri ng a gi ng of mus cl e s a tel l i te cel l s tha t hel p i n mus cl e growth a nd repa i r. Skel eta l mus cl e a l s o a ppea rs to exhi bi t decrea s ed res pons e to growth hormone a nd tes tos terone a s pa rt of i ts s l ow decl i ne. Los s of mus cl e ma s s i s a l s o s een i n a s trona uts expos ed to a peri od of mi crogra vi ty. Duri ng s pa ce fl i ght, the wei ghtl es s envi ronment mea ns tha t mus cl es norma l l y us ed on a n a l mos t cons ta nt ba s i s for pos ture a nd movement wi l l no l onger be bea ri ng wei ght, res ul ti ng i n l os s of mus cl e ti s s ue ca l l ed atrophy. Los s of bone a ccompa ni es mus cl e l os s a nd a dds to a genera l s ta te of decondi ti oni ng when the a s trona uts return to the ea rth’s gra vi ty. Often the mos t s i gni fi ca nt bone l os s i s a t s i tes of mus cl e a tta chment. Attempts to countera ct thi s probl em i ncl ude regul a r exerci s e on s peci a l l y cons tructed exerci s e equi pment a nd even a s pa ce s ui t wi th bungee cords a da pted to provi de counterforce for ma jor mus cl e groups . The probl em of mus cl e ma s s l os s for both ground-ba s ed pa ti ents a nd s pa ce tra vel ers i s often s tudi ed us i ng l engthy (da ys to weeks ) res ea rch s tudi es wherei n i ndi vi dua l s a re res tri cted to hori zonta l bed res t. Ani ma l model s i n whi ch mus cl es a re ma de nonwei ght bea ri ng ha ve been devel oped to a l s o s tudy thi s probl em.
CARDIAC MUSCLE Ca rdi a c cel l s , ca l l ed cardiomyocytes, s ha re s ome of the s a me s tructures a s s kel eta l mus cl e, i ncl udi ng ha vi ng s evera l nucl ei i n one cel l a nd a s i mi l a r orga ni za ti on of thi n a nd thi ck fi l a ments , gi vi ng the res ul ti ng mus cl e a s tri a ted a ppea ra nce (Fi gure 12-7). Li ke s ome types of s kel eta l mus cl e, they ha ve a hi gh content of myogl obi n a nd a l a rge number of mi tochondri a to dri ve oxi da ti ve phos phoryl a ti on. However, ca rdi a c cel l s ma y be bra nched i ns tea d of the norma l l y s tra i ght a nd uni form s tructure found i n s kel eta l mus cl e.
Figure 12-7. Cardiac Muscle Structure. Di a gra m of ca rdi a c mus cl e cel l s i ndi ca tes cha ra cteri s ti c fea tures of thi s mus cl e type. The fi bers cons i s t of s epa ra te cel l s wi th i nterdi gi ta ti ng proces s es wherei n they a re hel d together. Thes e regi ons of conta ct a re ca l l ed the i nterca l a ted di s ks (IDs ), whi ch cros s a n enti re fi ber between two cel l s . The tra ns vers e regi ons of the s tep-l i ke ID ha ve a bunda nt des mos omes a nd other a dherent juncti ons , whi ch hol d the cel l s fi rml y together. Longi tudi na l regi ons of thes e di s ks conta i n a bunda nt ga p juncti ons , whi ch form “el ectri ca l s yna ps es ” a l l owi ng contra cti on s i gna l s to pa s s from cel l to cel l a s a s i ngl e wa ve. Ca rdi a c mus cl e cel l s ha ve centra l nucl ei a nd myofi bri l s tha t a re l es s dens e a nd orga ni zed tha n thos e of s kel eta l mus cl e. Al s o the cel l s a re often bra nched, a l l owi ng the mus cl e fi bers to i nterwea ve i n a more compl i ca ted a rra ngement wi thi n fa s ci cl es tha t produces a n effi ci ent contra cti on mecha ni s m for emptyi ng the hea rt. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] Ca rdi a c mus cl e norma l l y deri ves energy a erobi ca l l y from triglycerides/fat (65%), glucose (30%), a nd proteins/ketone bodies (5%) or a l terna ti vel y from lactic acid tra ns ported from s kel eta l mus cl e. In fa ct, ca rdi a c cel l s a re a l mos t tota l l y rel i a nt on a erobi c res pi ra ti on (onl y a bout 10% of the energy needs ca n ever be deri ved from a na erobi c res pi ra ti on) a nd oxygen del i very i s , therefore, key to ca rdi a c cel l functi on a nd s urvi va l . Decrea s ed bl ood s uppl y a nd oxygen a s wel l a s remova l of wa s te a nd ca rbon di oxi de (CO2 ) vi a the corona ry a rteri es l ea d to hea rt-rel a ted ches t pa i n (a ngi na ) a nd hea rt a tta cks . Ca rdi a c mus cl e produces a s trong a nd forceful contra cti on but i s not under vol unta ry control . Ins tea d of nerve i nput, s peci a l i nterna l pa cema ker cel l s contra ct on a regul a r ba s i s a nd then propa ga te thi s contra cti l e i mpul s e to the rema i nder of the hea rt. Contra cti ons a re further control l ed by s peci a l ca rdi a c mus cl e fi bers tha t a re i nnerva ted by the a utonomi c nervous s ys tem. T-tubul es a re fewer but l a rger a nd run onl y a l ong the Z-l i nes or di s cs (Fi gure 12-7). Ca rdi a c cel l s a re a l s o joi ned to ea ch other by i nterca l a ted di s cs (IDs ), whi ch s erve both s tructura l a nd i mpul s e tra ns mi s s i on rol es (Fi gure 12-7). The IDs s erve a s cytos kel eta l a nchors between i ndi vi dua l cel l s to a l l ow the coordi na ti on of the contra cti on i nto a mul ti cel l compos i te. IDs a l s o conta i n ga p juncti ons (Fi gure 12-7; Cha pter 8), whi ch a l l ow the ra pi d propa ga ti on of fl ow of i ons (Cha pter 19) from one cel l to the next. Both functi ons a re es s enti a l for the ra pi d a nd s ynchroni zed contra cti on requi red by the hea rt. CICR i s the predomi na te mecha ni s m i n ca rdi a c mus cl e. Vol ta ge tri ggeri ng of a dihydropyridine receptor i nduces ca l ci um to fl ow i nto the s a rcopl a s m. Once i ns i de the s a rcol emma , the i ni ti a l i nfl ux of ca l ci um bi nds to a ryanodine receptor on the SR a nd rel ea s es much l a rger s tores of Ca 2+ i n a ma nner s i mi l a r to i nos i tol tri phos pha te. Ca 2+ fl ow through L-type calcium channels to a cti va te the ca rdi omyocytes . Thes e L (“l ong”) cha nnel s s ta y open for a n extended peri od of ti me to s us ta i n the i nfl ux of ca l ci um. Thi s ri s e i n ca l ci um concentra ti on ca us es troponi n– tropomyos i n conforma ti on cha nges a s previ ous l y des cri bed, a cti n–myos i n bi ndi ng a nd contra cti on. The rel ea s e of tempora l a nd s pa ti a l ca l ci um wa ve a nd the i ntegra ted a cti ons res ul ti ng from the IDs l ea d to the orga ni zed contra cti on of va ri ous pa rts of the hea rt mus cl e to produce the force to propel bl ood throughout the body. See Cha pter 16 for a ddi ti ona l deta i l s .
SMOOTH MUSCLE Smooth mus cl e not onl y s ha res s evera l qua l i ti es wi th s kel eta l a nd/or ca rdi a c mus cl e but a l s o di ffers i n others . Smooth mus cl e a cti vi ty rel i es on the i ntera cti on of a cti n thi n fi l a ments a nd myos i n thi ck fi l a ments . Al though s mooth mus cl e conta i ns a cti n a nd myos i n fi l a ments a nd ABPs ; i n contra s t to s kel eta l a nd ca rdi a c mus cl e, there i s much l es s s tructura l orga ni za ti on. Smooth mus cl e, therefore, ha s a s mooth a ppea ra nce (i .e., not s tri a ted) under a l i ght mi cros cope. However, cl os er exa mi na ti on does s how l oca l zones of contra cti l e el ements tha t s omewha t mi mi c the s a rcomere s tructure. Thi s s tructure a l l ows s mooth mus cl e cel l s to perform s peci fi c a nd di rected functi ons , both tempora l l y a nd s pa ti a l l y. Unl i ke s kel eta l a nd ca rdi a c mus cl e cel l s , s mooth mus cl e cel l s ha ve onl y one nucl eus a nd l a ck troponi ns to regul a te a cti n– myos i n hea d bi ndi ng (Fi gure 12-8A–B). Smooth mus cl e cel l s a l s o do not conta i n T-tubul es , a nd the SR i s orga ni zed i n onl y a l oos e network a round the cel l . Ins tea d, ca l modul i n, ca l des mon, a nd ca l poni n regul a te contra cti on vi a ca l ci um (s ee Fi gure 12-9 a nd a s s oci a ted text bel ow). Li ke ca rdi a c mus cl e, the cytos kel eta l el ements of i ndi vi dua l s mooth mus cl e cel l s a re a nchored together a s dense bodies (Fi gure 12-8A), whi ch mi mi c the Zl i ne of s kel eta l mus cl e. Dens e bodi es , compos ed of the i ntermedi a te fi l a ment (IF) protei n desmin, a nchor the a cti n thi n fi l a ments . Other IFs (e.g., vi menti n) a nd s tructures known a s adherens junctions a re a l s o promi nent i n the orga ni za ti on of s mooth mus cl e. Thes e cytos kel eta l /membra ne connecti ons a l l ow tra ns ducti on of force genera ti on between i ndi vi dua l s mooth mus cl e cel l s . Smooth mus cl e cel l s a l s o conta i n ga p juncti ons , whi ch a l l ow the qui ck s prea d of Ca 2+ fl uxes between cel l s . Together, a dherens a nd ga p juncti ons a l l ow coordi na ted, mul ti cel l ul a r contra cti ons s i mi l a r to ca rdi a c mus cl e.
Figure 12-8. A–B. Smooth Muscle Contraction. Mos t mol ecul es tha t a l l ow contra cti on a re s i mi l a r i n the three types of mus cl e, but the fi l a ments of s mooth mus cl e a re a rra nged di fferentl y a nd a ppea r l es s orga ni zed. A. The di a gra m s hows thi n fi l a ments a tta ch to dens e bodi es l oca ted i n the cel l membra ne a nd deep i n the cytopl a s m. Dens e bodi es conta i n α-a cti ni n for thi n fi l a ment a tta chment. Dens e bodi es a t the membra ne a re a l s o a tta chment s i tes for i ntermedi a te fi l a ments a nd for a dhes i ve juncti ons between cel l s . Thi s a rra ngement of both the cytos kel eton a nd contra cti l e a ppa ra tus a l l ows the mul ti cel l ul a r ti s s ue to contra ct a s a uni t, provi di ng better effi ci ency a nd force. B. Contra cti on decrea s es the l ength of the cel l , deformi ng the nucl eus a nd promoti ng contra cti on of the whol e mus cl e. The mi crogra ph s hows a regi on of contra cted ti s s ue i n the wa l l of a uri na ry bl a dder. The l ong nucl ei of i ndi vi dua l fi bers a s s ume a cork-s crew s ha pe when the fi bers contra ct, refl ecti ng the reduced cel l l ength a t thi s ti me. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] Li ke ca rdi a c mus cl e, s mooth mus cl e contra cti ons ca n be i ni ti a ted by pa cema ker cel l s , whi ch contra ct on a regul a r ba s i s a nd a re i nnerva ted by the a utonomi c nervous s ys tem. However, nerve i mpul s es , hormone s i gna l s , a nd even s tretchi ng ca n a l s o i ni ti a te contra cti on. Smooth mus cl e cel l s do not produce the forceful contra cti on of s kel eta l mus cl e or ca rdi a c mus cl e but they ha ve the a bi l i ty to undergo ei ther a s l ow contra cti on for l ong peri ods of ti me wi th l i ttl e energy us e (tonic contraction) or a ra pi d contra cti on a nd rel a xa ti on (phasic contraction). Toni c contra cti ons a re s een i n bl ood ves s el wa l l s . Pha s i c contra cti ons a re i mporta nt for porti ons of the ga s troi ntes ti na l tra ct i nvol ved i n peri s ta l s i s (e.g., es opha gus or s ma l l i ntes ti ne). Pa ra mount to a ny contra cti on by s mooth mus cl e i s a n i nfl ux of Ca 2+. Smooth mus cl e SR conta i ns s peci fi c a nd s peci a l i zed a rea s where i on cha nnel s a nd s i gna l i ng pa thwa y receptors a re found together. Exa mpl es of thes e receptors i ncl ude thos e for G protei ns , neurohormones , a cti va ti on of i ndi vi dua l ki na s es , L-type ca l ci um cha nnel s (s ee a bove), a nd ca l ci um-s ens i ti ve a nd i ns ens i ti ve pota s s i um cha nnel s . Area s of the SR a re a l s o nea r the externa l cel l membra ne a l l owi ng extra cel l ul a r s i gna l s (e.g., hormones ) a nd ca l ci um i nfl uxes to a l s o a cti va te contra cti on. Si gna l s for contra cti on (s i gna l from a n a dja cent s mooth mus cl e cel l vi a a ga p juncti on, nerve, hormone, a nd s tretchi ng) res ul t i n rel ea s e of ca l ci um from the SR (vi a the rya nodi ne receptor, s ee a bove) a s wel l a s pos s i bl e i nfl ux of externa l ca l ci um through the L-type cha nnel s . The res ul ti ng i ncrea s e i n Ca 2+ concentra ti on i ni ti a tes a cti n–myos i n i ntera cti ons a nd force genera ti on. Regul a ti on of contra cti on by ca l ci um di ffers i n ma ny wa ys from s kel eta l a nd ca rdi a c mus cl e. Smooth mus cl e cel l s do not conta i n troponi n, a nd tropomyos i n onl y s erves a n a cces s ory rol e i n a cti n–myos i n force genera ti on. Ins tea d, the protei ns ca l des mon a nd ca l poni n i nhi bi t myos i n ATPa s e enzyma ti c functi on a nd, pos s i bl y, a cti n–myos i n bi ndi ng through mecha ni s ms tha t a re s ti l l bei ng s tudi ed. A genera l overvi ew i s tha t a n i nfl ux of ca l ci um a cti va tes the protei n ca l modul i n, whi ch l ea ds to ca l des mon a nd ca l poni n phos phoryl a ti on a nd l os s of i nhi bi tory effects . Ca l modul i n a cti va ti on a l s o a cti va tes MLCK, whi ch l ea ds to the phos phoryl a ti on of the regul a tory myos i n l i ght cha i n (Fi gure 12-9). MLCP removes the phos pha te group when ca l ci um concentra ti ons fa l l , i na cti va ti ng myos i n a nd s toppi ng contra cti on.
Figure 12-9. Regulation of Smooth Muscle Myosin Force Generation by Calcium Ions. The i nfl ux of ca l ci um i ons (Ca 2+) l ea ds to a cti va ti on of ca l modul i n a nd then myos i n l i ght cha i n ki na s e (MLCK). MLCK phos phoryl a tes the regul a tory l i ght cha i ns on the myos i n mol ecul e a nd a cti va tes i ts ATPa s e a nd force-genera ti ng ca pa bi l i ti es . Myos i n l i ght cha i n phos pha ta s e (MLCP) ca n remove the phos pha te group, i na cti va ti ng myos i n a cti vi ty. See text for more deta i l s . [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] The l a ck of s a rcomere s tructure i n s mooth mus cl e a l s o ena bl es polymerization of G-a cti n to F-a cti n thi n fi l a ments to pa rtl y regul a te contra cti on. Thi s proces s ca n form new thi n fi l a ments a t l oca l a rea s i n a s mooth mus cl e where contra cti on or force genera ti on i s requi red. Brea kdown of exi s ti ng F-a cti n thi n fi l a ments ca n s top contra cti on. The exa ct rol e of a cti n pol ymeri za ti on a nd depolymerization i n s mooth a nd i n nonmus cl e cel l s i s s ti l l bei ng determi ned. Severa l ABPs i nvol ved i n thes e cha nges a re a ffected by ca l ci um a s wel l . Smooth mus cl e cel l types a re found throughout the huma n body a nd va ry i n functi on but a l l rel y on the ba s i c a cti n–myos i n contra cti l e mecha ni s m. Exa mpl es a nd thei r a s s oci a ted functi ons a re s hown i n Ta bl e 12-1.
TABLE 12-1. Types a nd Functi ons of Smooth Mus cl e Cel l s
ENERGY PRODUCTION AND USE IN MUSCLES Huma ns us e energy i n the form of ATP to dri ve the neces s a ry mus cl e contra cti ons . Duri ng exerci s e, both a erobi c a nd a na erobi c energyproduci ng s ys tems need to functi on beca us e di fferent types of mus cl e fi bers a re a cti va ted, rel yi ng on one or both energy s ources . The body i s a bl e to move the energy-produci ng mol ecul es ba ck a nd forth between the a erobi c a nd a na erobi c pa thwa ys , dependi ng on the type of exerci s e/mus cl e contra cti on a nd dura ti on of exerci s e. Ada pta ti on between a erobi c a nd a na erobi c meta bol i s m i s a l s o regul a ted by i ncrea s ed tra ns cri pti on a nd tra ns l a ti on of fa ctors s uch a s hypoxia-inducible factor-1 α. Mus cl es ma i nl y us e one of three di fferent energy s ources (Fi gure 1210) to produce thi s ATP—muscle glycogen (glucose), creatine phosphate, or triglycerols/fatty acids from a di pos e ti s s ue.
Figure 12-10. Sources of ATP in Muscles. ADP, a denos i ne di phos pha te; AMP, a denos i ne monophos pha te; ATP, a denos i ne tri phos pha te. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Creatine phosphate, i n es s ence a s tora ge form of energy for mus cl e, ca n us e i ts hi gh energy phos pha te bond to rea di l y produce ATP i n a rea cti on ca ta l yzed by the enzyme creatine kinase. Muscle glycogen i s a ma jor s tora ge depot from whi ch gl ucos e-6-phos pha te for the gl ycol yti c pa thwa y ca n be produced. Some types of mus cl e fi bers rel y pri ma ri l y on fatty acids from tri a cyl gl ycerol s (tri gl yceri des ) a s fuel . Both gl ycogen a nd fa tty a ci ds ca n be meta bol i zed vi a a erobi c res pi ra ti on by types I a nd IIa mus cl e fi bers a nd produce ATP by oxi da ti ve phos phoryl a ti on. Under a erobi c condi ti ons , the meta bol i s m of gl ucos e yi el ds pyruvate a nd energy i n the form of ATP. The pyruva te i s then compl etel y oxi di zed to CO2 wi th the forma ti on of a ddi ti ona l 15 ATP for ea ch pyruva te mol ecul e. Addi ti ona l l y, the ni coti na mi de a deni ne di nucl eoti de (NADH) genera ted from gl ycol ys i s ca n be proces s ed by the ma l a te–a s pa rta te s huttl e to genera te a ddi ti ona l three ATP per NADH. Hence, under a erobi c condi ti ons , ea ch gl ucos e mol ecul e ca n yi el d up to 38 ATP.
In peri ods of l ow oxygen, the ci tri c a ci d cycl e a nd oxi da ti ve phos phoryl a ti on a re l i mi ted a nd a na erobi c res pi ra ti on i s us ed. Types IIb a nd IIx mus cl e fi bers rel y ma i nl y on a na erobi c ATP producti on. When thi s ha ppens , pyruva te a ccumul a tes a nd i s then converted to lactic acid by the a cti on of the enzyme lactate dehydrogenase a nd us e of a n NADH mol ecul e. In thi s wa y, NAD + i s regenera ted to s us ta i n fl ux through gl ycol ys i s . Thi s poi nt when l a cta te producti on i s grea ter tha n l a cta te uti l i za ti on i s known a s the lactate threshold. Thi s thres hol d us ua l l y occurs a t a round 65% of a pers on’s ma xi ma l oxygen cons umpti on. If mus cl e a cti va ti on conti nues , then l a cti c a ci d a ccumul a tes i n the mus cl e ti s s ue a nd ca n a ffect i mporta nt meta bol i c functi ons , gi vi ng the mus cl e pa i n a nd the s ens a ti on of mus cl e fa ti gue. The s urpl us of l a cti c a ci d i s ul ti ma tel y tra ns ported to the l i ver or hea rt mus cl e where i t i s converted ba ck i nto pyruva te a nd i nto the ci tri c a ci d cycl e. McArdle’s disease: Gl ycogen s tora ge di s ea s es a re a s eri es of di s orders tha t i mpa ct on the body’s a bi l i ty to produce, s tore, a nd/or brea kdown thi s i mporta nt energy s ource. One type of gl ycogen s tora ge di s ea s e, type V or McArdle’s disease, res ul ts from the a bs ence of myophosphorylase, the mus cl e form of gl ycogen phos phoryl a s e. Pa ti ents us ua l l y pres ent i n chi l dhood wi th mus cl e pa i n, fa ti gue, cra mps , a nd wea knes s wi th exces s i ve myogl obi n i n the uri ne, i ndi ca ti ve of rha bdomyol ys i s (s ee a bove) duri ng prol onged peri ods of exerci s e. Progres s i ve s ymptoms a nd mus cl e ma s s l os s a nd wea knes s a re us ua l l y evi dent a s the pa ti ent a ges . Di a gnos i s ma y be compl i ca ted by a “second wind” fel t by pa ti ents . Al though once thought to be s i mpl y due to the us e of a l terna ti ve energy s ources s uch a s fa tty a ci ds a nd protei ns , evi dence now s ugges ts a pos s i bl e rol e of i ncrea s ed numbers of GLUT4 receptors on the pl a s ma membra ne, a l l owi ng i ncrea s ed gl ucos e upta ke for gl ycol ys i s i ns tea d of rel yi ng on gl ycogen s tores .
MICROTUBULE-BASED MOTILITY Microtubules functi on a s a s tructura l cytos kel eton but they a l s o s erve a s a “tra ck” s ys tem for tra ns port wi thi n the cel l . Wi th the us e of motor protei ns , i ntra cel l ul a r components s uch a s orga nel l es i ncl udi ng mi tochondri a , ves i cl es , or gra nul es ca n be moved wi thi n the cel l . Chromos omes ca n a l s o be moved wi th uni que a tta chment protei ns . Joi ned together, mi crotubul es form the s tructure of more compl ex cel l ul a r moti l i ty components s uch a s ci l i a or fl a gel l a . Centri ol es , s uch a s ci l i a a nd fl a gel l a , a re a l s o ma de of mi crotubul es . Centri ol es come i n pa i rs tha t a re ori ented a t ri ght a ngl es to ea ch other a nd a re res pons i bl e for s etti ng up the s pi ndl e tha t moves the chromos omes duri ng mi tos i s . Mi crotubul es form the ba ckbone of a l l of thes e s tructures . Mi crotubul es a re ma de of l i nea r pol ymers of a gl obul a r protei n ca l l ed tubulin (Cha pter 1) a nd ha ve a pol a r a rra ngement wi thi n the cel l wi th the pos i ti ve end di rected towa rd the peri phery a nd the nega ti ve end towa rd the center. Li ke a cti n, there a re s l i ghtl y di fferent forms of tubul i n (α to ε), whi ch form di fferent mi crotubul e s tructures i n di fferent pa rts of the cel l a nd body. α- a nd β-tubul i n form cytopl a s mi c mi crotubul es , γ-tubul i n forms centros omes , a nd δ- a nd ε-tubul i n a re i nvol ved i n centri ol es a nd the mi toti c s pi ndl e a ppa ra tus . Microtubule Polymerization and Cancer Therapy: Mi crotubul e functi on i s es s enti a l for a va ri ety of cel l ul a r functi ons i ncl udi ng s evera l a s pects of mi tos i s , whi ch i nvol ves polymerization a nd depolymerization of tubulin i nto mi crotubul e pol ymers . The medi ca ti ons colchicine, nocodazole, vincristine, a nd colcemid a l l i nhi bi t pol ymeri za ti on by i nterferi ng wi th the tubul i n monomer. The taxane fa mi l y of drugs , i ncl udi ng taxol, brea k down exi s ti ng mi crotubul es . Al l l ea d to the di s rupti on of mi tos i s a nd eventua l dea th of the a ffected cel l . Al though thes e effects a re us ed i n the trea tment of gout (i nhi bi ti on of mi crotubul e-dependent, neutrophi l moti l i ty, a s wel l a s uri c a ci d crys ta l l i za ti on) a nd va ri ous other i nfl a mma tory di s ea s es , thes e a gents ha ve a l s o emerged a s i mporta nt cancer therapies. Beca us e ca ncerous cel l s often ra pi dl y di vi de, a gents tha t i nhi bi t mi crotubul es ha ve emerged i n the trea tment of ca ncer a s wel l a s other di s ea s es . The i nhi bi ti on of mi tos i s a ffects ca ncer cel l s more tha n norma l cel l s , l ea di ng to a pa rtl y s el ecti ve trea tment a gent. Unfortuna tel y, s i de effects of thes e mi crotubul e/mi tos i s i nhi bi tors a re s ti l l a ppa rent, l i mi ti ng thei r us e a nd/or ca us i ng a ddi ti ona l probl ems for pa ti ents . The pol a r a l i gnment of mi crotubul es pl a ys a rol e a s pa rt of the tra ns port s ys tem wi thi n a cel l , us i ng a convers i on of chemi ca l energy to ki neti c energy vi a the motor protei ns dynei n a nd ki nes i n, very s i mi l a r i n mecha ni s m to the i ntera cti on of a cti n a nd myos i n. Wi th the a rra ngement of the mi crotubul e wi th the nega ti ve end towa rd the center a nd the pos i ti ve end towa rd the peri phery, the dynei n mol ecul es a re res pons i bl e for tra ns port towa rd the nucl eus a nd ki nes i ns a re res pons i bl e for tra ns port towa rd the pl a s ma membra ne. Dynein, l i ke myos i n, i s a l a rge ATPa s e wi th two hea ds a nd us es the hydrol ys i s of ATP to move the components of a cel l by l i tera l l y wa l ki ng a l ong the mi crotubul e network s tructure (Fi gure 12-11A). The pol a ri ty of the mi crotubul e pl a ys a rol e i n the di recti on of the movement of the dynei n to the nega ti ve end of the mi crotubul e or towa rd the nucl eus . Two forms of dynei n exi s t. Cytoplasmic dynein orga ni zes i ntra cel l ul a r orga nel l es a nd moves ves i cl es wi thi n cel l s a s wel l a s chromos omes a nd the mi toti c s pi ndl e duri ng mi tos i s . Axonemal dynein functi ons i n ci l i a or fl a gel l a moti l i ty. The two hea ds of cytopl a s mi c dynei n a re joi ned together a t a ta i l wi th i ntermedi a te a nd l i ght cha i ns , the l a tter of whi ch a tta ches the dynei n mol ecul e to the orga nel l e or ca rgo bei ng tra ns ported.
Figure 12-11. A–B. Microtubule Motors—Dynein and Kinesin. A. Structure a nd s i ze of cytopl a s mi c dynei n. B. Structure a nd s i ze of ki nes i n, i ncl udi ng i ts a s s oci a ted regul a tory l i ght cha i ns a nd connecti on to i ts ca rgo ves i cl e. [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] Kinesins a re a l s o mi crotubul e-ba s ed motor protei ns tha t functi on vi a the hydrol ys i s of ATP a nd, l i ke dynei n, a re res pons i bl e for movi ng s ubs ta nces a round the i ns i de of a cel l (Fi gure 12-11B). Li ke dynei n, ki nes i n mol ecul es a re a l s o i nvol ved i n mi toti c s pi ndl e forma ti on a nd chromos ome movement duri ng mi tos i s . The fi rs t ki nes i n wa s di s covered i n 1985, wi th ma ny more s ubfa mi l i es of ki nes i ns di s covered s i nce then. Al l huma n ki nes i ns a re res pons i bl e for i ntra cel l ul a r tra ns port towa rd the pos i ti ve end of the mi crotubul e network or towa rd the pl a s ma membra ne. Ki nes i n mol ecul es res embl e myos i n II mol ecul es wi th two hea vy cha i ns a nd two l i ght cha i ns . The hea vy cha i ns ha ve two hea ds a t one end, whi ch bi nd to a nd tra ns port a l ong the mi crotubul es ; the ta i l porti on of the hea vy cha i n a tta ches to the ca rgo orga nel l e or ves i cl e vi a the l i ght cha i ns . In cilia a nd flagella, the mi crotubul es a re a rra nged i n a n orga ni zed s tructure (Fi gure 12-12). In ci l i a a nd fl a gel l a , two mi crotubul es a re joi ned together to form a doubl et. The IF protei n nexin (s ee Ta bl e 12-2) forms l i nks a l ong the mi crotubul es to hol d them together. Ni ne of thes e doubl ets form a ci rcl e a round two mi crotubul es i n the center. Extendi ng from the doubl ets a re “a rms ” tha t connects nei ghbori ng doubl ets , compos ed of a xonema l dynei n. Radial spokes a l s o provi de s tructure between the i nner a nd outer mi crotubul e doubl ets . The orga ni za ti on of the mi crotubul es a l l ows the dynei n to s l i de a l ong one tubul e, whi l e a tta ched wi th i ts ta i l to a nother mi crotubul e crea ti ng a bend i n the ci l i um or fl a gel l a . However, the nexi n l i nka ges a l l ow for onl y l i mi ted l engthwi s e movement a nd, therefore, crea te a bend i n the ci l i um. The dynei n bri dges a re a l s o regul a ted s o the s l i di ng ca n l ea d to s ynchroni zed bendi ng. A centriole/basal body s tructure a t the ba s e of the ci l i a or fl a gel l a a nd compos ed of tri pl ets of tubul i n monomers hel ps to form a nd a nchor thes e l onger s tructures .
Table 12-2. Intermedi a te Fi l a ment Types a nd Functi ons
Figure 12-12. A–B. Structure of Cilia and Flagella. The s tructure of ci l i a a nd fl a gel l a i s compos ed of (A) a centra l mi crotubul e doubl et s urrounded by ni ne outer doubl ets wi th dynei n a rms between them. The dynei n crea tes movement of the ci l i a or fl a gel l a . Ra di a l s pokes a nd nexi n provi de s tructure a nd l i mi ts to thi s movement. The ba s e of the ma i n ci l i a or fl a gel l a s tructure i ncl udes (B) a centri ol e/ba s a l body wi th a tri pl et mi crotubul e s tructure, whi ch s erves a s a n a nchor. See text for further deta i l s . [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.]
Microtubules, Cilia, and Flagella—Roles in Disease Processes: Al though often ra re, defects i n ci l i a /fl a gel l a , known a s ci l i opa thi es , l ea d to s evera l di s ea s es /s yndromes i ncl udi ng the fol l owi ng: Ka rta gener Syndrome/Pri ma ry Ci l i a ry Dys ki nes i a —Defecti ve ci l i a i n the res pi ra tory tra ct, Eus ta chi a n tube, a nd Fa l l opi a n tubes l ea di ng to chroni c l ung i nfecti ons , ea r i nfecti ons a nd hea ri ng l os s , a nd i nferti l i ty. Pos s i bl e a s s oci a ti on wi th “s i tus i nvers us ,” a condi ti on i n whi ch ma jor i nterna l orga ns a re “fl i pped” l eft to ri ght. Seni or–Loken Syndrome/Nephronopthi s i s —Eye di s ea s e a nd forma ti on of cys ts i n the ki dneys , l ea di ng to rena l fa i l ure. Ba rdet–Bi edl Syndrome—Dys functi on of ci l i a throughout the body l ea di ng to obes i ty beca us e of i na bi l i ty to s ens e s a ti a ti on, l os s of eye pi gment/vi s ua l l os s a nd/or bl i ndnes s , extra di gi ts a nd/or webbi ng of fi ngers a nd toes , menta l a nd growth reta rda ti on a nd beha vi ora l /s oci a l probl ems , s ma l l a nd/or mi s s ha ped geni ta l i a (ma l e a nd fema l e), enl a rged a nd da ma ged hea rt mus cl e, a nd ki dney fa i l ure. Al s trom Syndrome—Chi l dhood obes i ty, brea kdown of the reti na l ea di ng to bl i ndnes s , hea ri ng l os s , a nd type 2 di a betes . Meckel –Gruber Syndrome—Forma ti on of cys ts i n ki dneys a nd bra i n l ea di ng to rena l fa i l ure a nd neurol ogi ca l defi ci ts , extra di gi ts , a nd bowi ng/s horteni ng of the l i mbs . Increa s ed Ectopi c (Tuba l ) Pregna nci es /Ma l e Inferti l i ty—Defi ci ent ci l i a i n Fa l l opi a n tubes or fl a gel l a /s perm ta i l moti l i ty. Autos oma l Reces s i ve Pol ycys ti c Ki dney Di s ea s e—Much ra rer tha n the a utos oma l domi na nt form, dys functi on of ba s a l bodi es a nd ci l i a i n rena l cel l s l ea ds to a l tera ti ons of the l ung a nd ki dneys , res ul ti ng i n a va ri ety of s econda ry medi ca l condi ti ons a nd often dea th. Pa rki ns on’s a nd Al zhei mer’s Di s ea s es —Al though work i s s ti l l ongoi ng, res ea rchers now feel tha t s ome forms of Pa rki ns on’s a nd Al zhei mer’s di s ea s es ma y res ul t, i n pa rt, from da ma ge to mi crotubul es a nd a s s oci a ted protei ns . Trea tments a i med a t s ta bi l i zi ng mi crotubul es ma y hel p s ufferers from thes e s peci fi c protei ns .
INTERMEDIATE FILAMENTS The IF i s the na me for a s i ngl e protei n member of a fa mi l y of rel a ted cytos kel eta l l i nea r protei ns (monomer forms ) or the res ul ti ng fi l a ments tha t thes e protei ns form. The fi l a ments forms a re a bout 10 nm i n di a meter a nd, a s the na me i mpl i es , a re between the s i ze of 6 nm a cti n mi crofi l a ments a nd ei ther 15–20 nm myos i n thi ck fi l a ments or 25 nm mi crotubul es (Fi gure 12-13). Al l IF monomer protei ns ha ve two hea d regi ons for bi ndi ng a nd l engthy, connecti ng α-hel i ca l porti on. IFs a re formed from the i ntertwi ni ng of the α-hel i ca l s ecti on of pa i rs of the s a me (homodi mer) or di fferent (heterodi mer) monomers or pa i rs of pa i rs (homo- or heterotetra mer) IFs ca n s tretch a nd compa ct beca us e of thei r s tructure a nd, thus , ca n s erve to tra ns mi t force or a bs orb mecha ni ca l s tres s . There a re s i x ba s i c types ba s ed on s tructura l s i mi l a ri ti es (Ta bl e 12-2).
Figure 12-13. Comparison of Cytoskeletal Proteins. Rel a ti ve s i zes of a cti n mi crofi l a ments , i ntermedi a te fi l a ments , a nd mi crotubul es a re s hown. [Reproduced wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.]
NONMUSCLE CELLS Nonmus cl e cel l s i ncl ude s ki n fi brobl a s ts ; pl a tel ets ; i mmune s ys tem l ymphocytes a nd monocytes /ma cropha ges (Cha pter 15); neurons , i ncl udi ng photoreceptor cel l s of the reti na (Cha pter 19); chondrocytes a nd os teocytes (Cha pter 13); va ri ous cel l s of the ki dney, pa ncrea s , a nd i ntes ti nes (Cha pter 11); a s wel l a s the thyroi d a nd bl ood ves s el s . As s uch, nonmus cl e cel l s a re res pons i bl e for di vers e ta s ks s uch a s wound hea l i ng, i mmune res pons e, s peci a l i zed nerve functi ons , connecti ve ti s s ue a nd bone forma ti on a nd hea l th, di ges ti on, regul a ti on of meta bol i s m, ca rdi ova s cul a r di s ea s e, a nd even embryol ogi ca l devel opment. As noted previ ous l y, the contra cti l e a ppa ra tus i n nonmus cl e cel l s i s fa r l es s s tructured tha n s kel eta l , ca rdi a c, or s mooth mus cl e types but does pres erve ma ny of the s a me functi ona l a nd regul a tory el ements . Acti n thi n fi l a ments a nd myos i n II thi ck fi l a ments a re ma jor pa rts of nonmus cl e cel l moti l i ty a nd i ntera ct i n a n a na l ogous wa y. Other types of myos i n (e.g., myos i n I, myos i n V, a nd others ) a re a l s o promi nent i n nonmus cl e cel l moti l i ty tha t i ncl udes i ntra cel l ul a r tra ns port (Fi gures 12-3B a nd 12-11B). Regul a ti on of contra cti on i s much l i ke s mooth mus cl e where Ca 2+ regul a te a cti n fi l a ment s ta bi l i ty a nd l ength, a s pects of a cti n– myos i n hea d bi ndi ng, a nd, fi na l l y, myos i n force genera ti on.
REVIEW QUESTIONS 1. Wha t a re the rol es of the ba s i c mus cl e components i ncl udi ng how they ma y i ntera ct wi th ea ch other to fa ci l i ta te mus cl e functi on? 2. Wha t a re the three doma i ns of myos i n a nd how i s ea ch i nvol ved i n mus cl e contra cti on? 3. From where a nd how i s ca l ci um rel ea s ed for s kel eta l mus cl e contra cti on a nd wha t i s i ts rol e i n the proces s ? 4. Wha t i s the rol e of myos i n ATPa s e i n mus cl e contra cti on? 5. How i s ca l ci um s eques tered a nd concentra ted i n the s a rcopl a s mi c reti cul um a fter contra cti on cycl es ? 6. Wha t i s the rol e a nd mecha ni s m of a cti on of a cetyl chol i ne i n mus cl e contra cti on? 7. Wha t a re the types a nd meta bol i c cha ra cteri s ti cs of the di fferent mus cl e fi ber types ? 8. How does s tructure a nd contra cti on of ca rdi a c mus cl e di ffer from s kel eta l mus cl e? 9. Wha t a re the types a nd functi ons of s mooth mus cl e cel l s ?
10. How do mus cl es genera te energy for functi on i n a na erobi c vers us a erobi c condi ti ons ? 11. Wha t a re the functi ons of mi crotubul es a nd the a s s oci a ted rol es of dynei n a nd ki nes i n i n moti l i ty? 12. Wha t a re the types a nd functi ons of i ntermedi a te fi l a ments ? 13. How i s nonmus cl e cel l contra cti on a l i ke a nd di fferent from contra cti on by other mus cl e types ?
CHAPTER 13 CONNECTIVE TISSUE AND BONE Editor: Jacques Kerr, BSc, MB, BS, FRCS, FCEM Lea d Cons ul ta nt i n Emergency Medi ci ne, Borders Genera l Hos pi ta l , Mel ros e, Scotl a nd, Uni ted Ki ngdom
Connecti ve Ti s s ue Components of Bone Bone Growth a nd Remodel i ng Regul a ti on of Ca l ci um Level s Ma rkers of Bone Forma ti on a nd Res orpti on Revi ew Ques ti ons
OVERVIEW Connecti ve ti s s ue provi des a fra mework a nd s upport for a l a rge va ri ety of s tructures , i ncl udi ng orga ns , bl ood ves s el wa l l s , a s wel l a s the better known functi ons of connecti ng mus cl e to bone a nd bone to bone. Chondrocytes a re res pons i bl e for the producti on of connecti ve ti s s ue components , i ncl udi ng the pri nci pa l protei n, col l a gen. The three types of connecti ve ti s s ues offer a wi de va ri ety of functi ons to ful fi l l the ma ny rol es needed by the body. A mul ti tude of di s ea s es res ul t from a bnorma l i ti es , defi ci enci es , or overproducti on of connecti ve ti s s ues or thei r components . Bones provi de a mecha ni ca l s tructure for the huma n body a nd, i n tha t rol e, a l s o a l l ow effecti ve mus cl e contra cti on a nd, therefore, movement. Bones offer protecti on for the body’s i nterna l orga ns , es peci a l l y bra i n, hea rt, l ungs , l i ver, s toma ch, a nd s pl een. In the ea r, bones tra ns duct s ound wa ves from the ea r drum to the i nner ea r. Os teobl a s ts , os teocytes , a nd os teocl a s ts produce, brea k down, remodel , a nd repa i r both the orga ni c a nd i norga ni c ma tri x tha t ma ke up bones . In doi ng s o, thes e cel l s , a s wel l a s s evera l regul a tory mol ecul es , hel p to regul a te ca l ci um a nd phos pha te meta bol i s ms a nd l evel s i n the body. Bones a l s o ha ve a s yntheti c functi on. Wi thi n thei r ma rrow, bones produce red a nd whi te bl ood cel l s a s wel l a s growth fa ctors ; a nd s tore fa tty a ci ds a s yel l ow ma rrow. Fi na l l y, bones provi de for the s tora ge of certa i n mi nera l s , i ncl udi ng ca l ci um a nd phos phorus a nd, to a l es s er extent, zi nc, copper, a nd s odi um. In a n a na l ogous rol e, bone ca n tempora ri l y a bs orb a nd s tore toxi c hea vy meta l s to reduce thei r effects on the body.
CONNECTIVE TISSUE Connective tissue i s a fi brous ti s s ue ma de ma i nl y of col l a gen (Cha pter 1) a nd proteogl yca ns (Cha pter 2) tha t forms , s upports , a nd/or connects va ri ous orga ns i n the body, a tta ches mus cl es to bones (e.g., tendons ) a nd bones to bones (e.g., l i ga ments ), forms the s upporti ve ma tri x duri ng bone forma ti on (s ee bel ow), a nd ma kes up va ri ous s tructures s uch a s pa rts of bl ood ves s el s a nd i ntes ti na l wa l l s . One ma jor exa mpl e of connecti ve ti s s ue i s col l a gen, whi ch i s found i n va ri ous forms throughout the body (Fi gure 13-1A–D).
Figure 13-1. A–D. Distribution of Cartilage in Adults. (A) There a re three types of a dul t ca rti l a ge di s tri buted i n ma ny a rea s of the s kel eton, pa rti cul a rl y i n joi nts a nd where pl i a bl e s upport i s us eful , a s i n the ri bs , ea rs , a nd nos e. Ca rti l a ge s upport of other ti s s ues throughout the res pi ra tory s ys tem i s a l s o promi nent. The photomi crogra phs s how the ma i n fea tures of (B) hya l i ne ca rti l a ge, (C) fi broca rti l a ge, a nd (D) el a s ti c ca rti l a ge. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] Forma ti on of connecti ve ti s s ue rel i es on chondrocytes, whi ch both produce a nd ma i nta i n the col l a gen ma tri x. Chondrocytes di fferenti a te from osteochondrogenic cel l s , whi ch ca n a l terna ti vel y devel op i nto os teobl a s ts (s ee bel ow). Al though the di fferent s tructures va ry wi del y, three ma jor fi ber forms of connecti ve ti s s ue fi bers a re promi nent—col l a genous fi bers , el a s ti c fi bers , a nd reti cul a r fi bers .
Col l a gen i s the pri nci pa l component of collagenous fibers, a nd the type of col l a gen determi nes the s tructura l a nd functi ona l qua l i ti es of tha t pa rti cul a r form. Col l a gen i s bel i eved to repres ent a bout a qua rter of the tota l protei n i n huma n body. Al though 29 types of col l a gen ha ve been di s covered, ea ch va ryi ng i n i ts s tructure, l oca ti on i n pa rts of the body, a nd functi ons , four (types I–IV) ma ke up a va s t ma jori ty of connecti ve ti s s ue. Structure a nd forma ti on of a type I col l a gen fi ber i s i l l us tra ted i n Fi gure 13-2A–B.
Figure 13-2. A. Structure of Collagen. Il l us tra ti on of s tructura l a rra ngement of i ndi vi dua l , overl a ppi ng col l a gen mol ecul es (1), col l a gen fi bri l s , col l a gen fi bers (2 a nd 3), a nd col l a gen fi ber bundl es (4 a nd 5). The overl a ppi ng na ture of thi s s tructura l orga ni za ti on produces the requi red s trength a nd fl exi bi l i ty of thi s mol ecul e. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] The fi na l type of connecti ve ti s s ue fi ber i s the elastic fiber (Fi gure 13-3C). El a s ti c fi bers a re formed from s ma l l er microfibrils, ma de ma i nl y of the protei n tropoelastin/elastin a nd the gl ycoprotei n fibrillin, a nd cros s -l i nki ng pol ypepti des (Fi gure 13-4A–B). The mi crofi bri l s a re i n the form of i rregul a r, ra ndom coi l s , wi th gl yci ne, va l i ne, a l a ni ne, prol i ne, a nd l ys i ne a mi no a ci ds contri buti ng to the s ta bl e s tructure. B. Formation of Collagen Fibers. Col l a gen pepti des a re s ynthes i zed by ri bos omes i n the l umen of rough endopl a s mi c reti cul um (RER) a s “preprocol l a gen” mol ecul es (not s hown) wi th N-termi na l s i gna l pepti des . Si gna l pepti des a re removed to produce “procol l a gen” mol ecul es . Prol i ne a nd l ys i ne a mi no a ci d res i dues a re hydroxyl a ted vi a prol yl hydroxyl a s e a nd l ys yl hydroxyl a s e enzymes , whi ch depend on vi ta mi n C a s a cofa ctor. Some a mi no a ci d res i dues a re a l s o gl ycos yl a ted. The procol l a gen mol ecul es form a l eft-ha nded tri pl e hel i x wi thi n the RER l umen. The hel i ca l mol ecul es a re tra ns ported to the Gol gi a ppa ra tus , where they a re proces s ed a nd s ecreted vi a exocytos i s to the cel l exteri or. The a mi no- a nd ca rboxy-termi na l ends a re removed by the enzyme procol l a gen pepti da s e to form col l a gen fi bri l s wi th further cros s -l i nki ng of hydroxyl ys i ne a nd l ys i ne res i dues on di fferent tropocol l a gen mol ecul es by the enzyme l ys yl oxi da s e. Cros s -l i nked col l a gen fi bri l s form col l a gen fi bers . Beca us e there a re ma ny s l i ghtl y di fferent genes for procol l a gen α cha i ns a nd col l a gen producti on depends on s evera l pos ttra ns l a ti ona l events i nvol vi ng s evera l other enzymes , ma ny di s ea s es i nvol vi ng defecti ve col l a gen s ynthes i s ha ve been des cri bed. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.]
Figure 13-3. A–C. Representative Types of Collagen-Based Connective Tissue. (A) Mol ecul es of type I col l a gen, the mos t a bunda nt type, a s s embl e to form much l a rger s tructures . Tra ns mi s s i on el ectron mi cros copy s hows tha t fi bri l s cut l ongi tudi na l l y a nd tra ns vers el y. In l ongi tudi na l s ecti ons , the fi bri l s di s pl a y a l terna ti ng da rk a nd l i ght ba nds , whi ch a re further di vi ded by cros s -s tri a ti ons , a nd i n cros s s ecti on, the cut ends of i ndi vi dua l col l a gen mol ecul es ca n be s een. (B) Si l ver-s ta i ned s ecti ons of a drena l cortex i l l us tra te a network of reti cul a r fi bers , whi ch provi des a fra mework for cel l a tta chment. Reti cul a r fi bers conta i n type III col l a gen, whi ch i s hea vi l y gl ycos yl a ted. (C) El a s ti c fi bers a re compos ed of a thi rd type of connecti ve ti s s ue, formed from mi crofi bri l s of el a s ti n a nd fi bri l l i n a nd a dd res i l i ency to the connecti ve ti s s ue. They ca n be s een between l a yers of s mooth mus cl es i n the wa l l of el a s ti c a rteri es s uch a s the a orta (pi nk, upper pa nel ) a nd i n s tructures s uch a s mes entery (da rk ma genta , l ower pa nel ). [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra wHi l l , 2010.]
Figure 13-4. A–B. Quaternary Structure of Elastic Fibers. El a s ti n mi crofi bri l s ma y form i nterna l a nd externa l (between di fferent el a s ti n fi bers ) cros s l i nks to form the cros s -l i nk s tructure “des mos i ne,” whi ch provi des both s trength a nd el a s ti ci ty. Il l us tra ti on of functi on of el a s ti n fi bers provi des s trength a nd el a s ti ci ty to a number of ti s s ues (s ee text). 13-4A. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] 13-4B. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] Va ryi ng combi na ti ons of the di fferent types of col l a gen ca n a l ter thi s ba s i c s tructure for pa rti cul a r cel l functi ons . Mos t col l a genous fi bers fol l ow the previ ous l y des cri bed, l eft-ha nded, tri pl e-hel i x of type I (Fi gure 13-3A), wi th mul ti pl e gl yci ne, prol i ne/ hydroxyprol i ne, a nd hydroxyl ys i ne a mi no a ci ds contri buti ng to i ts s tructure. The s econd type of connecti ve ti s s ue i s the reticular fiber, compos ed of type III col l a gen a nd formi ng a n ordered, “reti cul um” mes hwork i ns tea d of a l i nea r s tructure (Fi gure 13-3B). The reti cul a r network l i nks thes e protei ns to ca rbohydra tes , i ncl udi ng gl ucos e, ga l a ctos e, ma nnos e, a nd fucos e (Cha pter 3). Reti cul a r fi bers a re found ma i nl y i n l i ver, mus cl e, bone ma rrow, l ymph s ys tem, a nd va ri ous other ti s s ues a nd orga ns , where i t offers a s upporti ve fra mework. El a s ti c fi bers a re found i n s evera l ti s s ue types , but ma i nl y i n s ki n, the outer ea r, l a rynx a nd epi gl otti s , bl ood ves s el s , l ungs , bl a dder, s ome i ntervertebra l di s cs , a nd a s a n a tta chment between teeth a nd the underl yi ng bones . As the na me s ugges ts , el a s ti c fi bers ca n be ea s i l y s tretched (up to 1.5 × thei r ori gi na l l ength). Di s ea s es rel a ted to defects i n el a s ti n i ncl ude Menkes di s ea s e, Hurl er di s ea s e, Wi l l i a m’s s yndrome, cuti s l a xa , Bus chke– Ol l endorf s yndrome, a nd ps eudoxa nthoma el a s ti cum. Cha nges i n the el a s ti c fi bers i n the hea rt a nd/or a rteri es ma y a l s o pl a y a rol e i n hi gh bl ood pres s ure, a s the a bi l i ty to a bs orb the force of hea rt contra cti on wi th a ppropri a te bl ood ves s el s tretch a nd rebound i s di mi ni s hed. Di s ea s es of connecti ve ti s s ues a re us ua l l y a s s oci a ted wi th defects i n the component col l a gen mol ecul e or defi ci enci es i n es s enti a l nutri ents requi red for thei r s ynthes i s (e.g., vi ta mi n C) a nd s ecreti on. A l a rge group of a utoi mmune di s ea s es of connecti ve ti s s ue a l s o exi s ts , i n whi ch the body’s i mmune s ys tem mi s ta kenl y a tta cks pa rts of the body. If too much col l a gen i s produced, s cl eroderma ma y res ul t, a n a utoi mmune di s ea s e cha ra cteri zed by thi ckeni ng a nd ha rdeni ng of the s ki n a nd del eteri ous cha nges i n bl ood ves s el s . Mos t of the connecti ve ti s s ue di s ea s es a re di a gnos ed by phys i ca l exa mi na ti on, l ooki ng for a bnorma l i ti es i n the orga n(s ) i nvol ved tha t refl ect cha nges i n the connecti ve ti s s ue. DNA s tudi es a nd bi ops i es ca n hel p confi rm the condi ti on(s ). Autoi mmune connecti ve ti s s ue di s ea s es a re norma l l y di a gnos ed by s ymptoms a nd by bl ood tes ts revea l i ng di a gnos ti c a nti bodi es or rea cti ve mol ecul es . The col l a gen types , functi ons , a nd rel a ted di s ea s es a re revi ewed i n Ta bl e 13-1.
Table 13-1. Summa ry of Col l a gen Types
COMPONENTS OF BONE Bone i s ma de up of the i norga ni c mi nera l hydroxyapatite, known more forma l l y a s ca l ci um a pa ti te [Ca 5 (PO4 ) 3 (OH) or, i n i ts crys ta l -s tructure form, Ca 10 (PO4 ) 6 (OH) 2 ]. Ca rbona ted hydroxya pa ti te i s a l s o the ma i n component of the ena mel i n teeth, a nd va ri a bl e chemi ca l forms ca n ca us e na tura l va ri a ti ons i n teeth col or. In a ddi ti on, bone i s compos ed of a n orga ni c ma tri x, pri ma ri l y compos ed of type I col l a gen a nd three pri ma ry cel l types —os teobl a s ts , os teocl a s ts , a nd os teocytes (Fi gure 13-5A-B).
Figure 13-5. A–B. Osteoblasts and Osteocytes. (A) The photomi crogra ph of devel opi ng bone s hows the l oca ti on a nd morphol ogi ca l di fferences between os teobl a s ts (OB) a nd os teocytes (O). Rounded os teobl a s ts , deri ved from the mes enchyma l cel l s nea rby, a ppea r a s a s i mpl e row of cel l s a dja cent to a very thi n l a yer of l i ghtl y s ta i ned ma tri x coveri ng the more hea vi l y s ta i ned ma tri x. The l i ghtl y s ta i ned ma tri x i s os teoi d. Os teocytes a re l es s rounded a nd a re l oca ted wi thi n l a cuna e. In thi n s pi cul es of bone s uch a s thos e s een here, ca na l i cul i a re us ua l l y not pres ent. (B) Schema ti c di a gra m s hows the rel a ti ons hi p of os teobl a s ts wi th os teoi d, bone ma tri x, a nd os teocytes . [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] Osteoblasts, a s peci a l i zed form of a fi brobl a s t cel l , whi ch produces bone, a ri s e from osteoprogenitor cells, whos e growth a nd devel opment a re i nfl uenced by fi brobl a s t growth fa ctor (FGF), pl a tel et-deri ved growth fa ctor (PDGF), tra ns formi ng growth fa ctor-β (TGF-β), a nd bone morphogeneti c protei ns (BMPs ). FGF s i gna l i ng mol ecul es bi nd to a receptor wi th tyro-s i ne ki na s e (phos phoryl a ti on) a cti vi ty, whi ch i ni ti a tes a number of s i gna l i ng pa thwa ys i nduci ng the expres s i ons of s el ected genes . TGF-β s i gna l i ng us ua l l y occurs by fi rs t combi ni ng two TGF-β protei ns vi a a uni que “cys tei ne knot” s tructure, i n whi ch ni ne, hi ghl y cons erved, cys tei ne a mi no a ci ds on ea ch monomer form di s ul fi de bonds wi th the a na l ogous cys tei ne res i due. The TGF-β homodi mer s tructure then i ntera cts wi th a di mer of type II receptors (Fi gure 13-6). The bound TGF-β) di mer/type II receptor di mer then recrui ts two type I receptors , crea ti ng a four-protei n (tetra mer) receptor a l ong wi th two-protei n (di mer) s ubs tra te compl ex. Bi ndi ng of thes e s i x protei ns ca us es a conforma ti ona l cha nge of the type II receptor di mer, whi ch l ea ds to serine kinase a cti vi ty a nd phos phoryl a ti on of s eri ne a mi no a ci ds on the type 1 receptor. Thes e phos phoryl a ted s eri ne res i dues ca n then bi nd to a va ri ety of s peci fi c s i gna l i ng protei ns (known a s SMADs a nd SARAs , the l a tter of whi ch uti l i ze a zi nc-fi nger bi ndi ng moti f; s ee Ta bl e 9-1). Thes e s i gna l i ng protei ns combi ne a nd enter the nucl eus , where they a ct a s tra ns cri pti on-a cti va ti ng fa ctors for a va ri ety of gene products . BMP s ubs tra tes s i gna l i n a s i mi l a r ma nner to TGF-β, uti l i zi ng the SMAD protei ns a s tra ns cri pti on fa ctors .
Figure 13-6. Mechanism of TGF-β Signaling. The TGF-β protei n (l i ght bl ue) forms a di mer vi a a s eri es of ei ght di s ul fi de bonds (repres ented by –S • S–). Thi s di mer recrui ts two type I (TβR-1) a nd two type II (TβR-II) TGF-β receptors . Forma ti on of tetra mer receptor s tructure l ea ds to s eri ne a mi no a ci d phos phoryl a ti on on the type I cytopl a s mi c ta i l a nd s ubs equent s i gna l i ng vi a the SARA, RSMAD, a nd SMAD4 protei ns , l ea di ng to i ncrea s ed tra ns cri pti on i n the nucl eus . [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] Osteoarthritis: Osteoarthritis (OA), s ometi mes ca l l ed “degenera ti ve” or “wea r a nd tea r” a rthri ti s , res ul ts from the brea kdown/l os s of ca rti l a ge a nd bones i n joi nts , res ul ti ng i n pa i n, s ti ffnes s , a nd decrea s ed movement of the a ffected a rea s . Al though a number of i nfecti ve, i nfl a mma tory, a nd mi s cel l a neous s econda ry ca us es a re known, mos t ca s es ha ve no i denti fi a bl e pri ma ry ca us e. However, the res ul ti ng effects a re bel i eved to be due to decrea s i ng wa ter content of the ca rti l a ge, ca us i ng a reducti on i n the proteoglycan content. Wi th l es s proteogl yca ns , the ca rti l a ge i s more prone to degra da ti on tha t eventua l l y l ea ds to the gra dua l l os s of the joi nt components , i nfl a mma ti on from thes e brea kdown products , a nd, occa s i ona l l y, rea cti ve growth of bone “s purs .” Trea tment i s often mul ti fa ctori a l , i ncl udi ng pa i n medi ca ti ons , modera te exerci s e, a nd, occa s i ona l l y, s urgery, a l though thes e often ha ve l es s tha n opti ma l effecti venes s . A va ri ety of a l terna ti ve trea tments ha ve thus been devel oped, i ncl udi ng a cupuncture, herbs , s uppl ements , fi s h oi l s , a nd, proba bl y mos t promi nent, us e of gl ucos a mi ne a nd chondroi ti n s ul pha te. Glucosamine, s i mpl y gl ucos e wi th a n a mi no (NH 2 ) group a t the s econd ca rbon, i s a known component of gl ycos a mi nogl yca ns (GAGs ) a nd proteogl yca ns . Chondroitin, a GAG wi th s ul fa te (SO3 ) groups , i s a l s o a pa rt of the proteogl yca n s tructure of ca rti l a ge. As a res ul t, s ome bel i eve tha t the ora l i nta ke of thes e two s uppl ements ca n hel p to res tore the proteogl yca n content of ca rti l a ge, l ea di ng to rel i ef i n OA s ymptoms . To da te, res ea rch s tudi es a nd a na l ys es ha ve not s hown a ny objecti ve i mprovement of OA, a l though they ha ve a l s o s hown tha t no a dvers e effects res ul t from thei r us e a nd, more i mporta ntl y, tha t a s trong pl a cebo effect ma y a ctua l l y hel p ma ny pa ti ents . Further s tudi es a re ongoi ng.
Fibrodysplasia Ossificans Progressiva (FOP) and BMPs: FOP i s a very ra re, but potenti a l l y, extremel y debi l i ta ti ng di s ea s e, i n whi ch the repa i r proces s of fi brous ti s s ues s uch a s mus cl es , tendons , a nd l i ga ments i s cha nged to bone repa i r, res ul ti ng i n mi nera l depos i ti on/bone ma tri x forma ti on i n the norma l l y fl exi bl e ti s s ue. Progres s i on of the di s ea s e l i tera l l y l ocks the pa ti ent’s body pa rts i n pl a ce, a proces s s ometi mes des cri bed by the expres s i on “turned to s tone.” The di s ea s e i s bel i eved to be ca us ed by a n a utos oma l domi na nt muta ti on of BMP4. Recent res ea rch i ndi ca tes tha t BMP4 i s a ctua l l y the protei n activin, a protei n di mer better known for i ts rol e i n the s ecreti on of fol l i cl e-s ti mul a ti ng hormone (Cha pter 20). Muta ted a cti vi n erroneous l y a ffects i ts BMP type I receptor, promoti ng the a cti va ti on of SMAD protei ns , gene tra ns cri pti on, a nd bone ma tri x growth. Lymphocytes , whi ch res pond to the i ni ti a l da ma ge to the fi brous ti s s ue, a re bel i eved to be the ca rri ers of the muta ted protei n. There i s no known cure for FOP, a l though i t i s pos s i bl e tha t s ha rk s qua l a mi ne, a protei n under tri a l , whi ch prevents bone growth i n ca rti l a ge, ma y provi de s ome trea tment. Os teobl a s ts produce the protei n osteoid, ma de pri ma ri l y of type I col l a gen, a nd a re a l s o res pons i bl e for l a yi ng down the ha rd mi nera l ma tri x (Fi gure 13-7). Os teoi d i s fi rs t s ynthes i zed a s a pproxi ma tel y 300-nm l ong, tropocollagen “mi crofi bri l s ,” whi ch i nterdi gi ta te a nd bi nd vi a hydrogen a nd cova l ent bondi ng wi th nei ghbori ng mi crofi bri l s to form a s heet-l i ke col l a gen ma tri x of pa ra l l el fi bri l s . Ga ps between tropocol l a gen mol ecul es offer s i tes for hydroxya pa ti te mi nera l i za ti on.
Figure 13-7. Mineralization in Bone Matrix. From thei r ends a dja cent to the ma tri x, os teobl a s ts s ecrete type I col l a gen, s evera l gl ycoprotei ns , a nd proteogl yca ns . Some of thes e fa ctors , nota bl y os teoca l ci n a nd certa i n gl ycoprotei ns , bi nd Ca 2+ wi th hi gh a ffi ni ty, thus ra i s i ng the l oca l concentra ti on of thes e i ons . Os teobl a s ts a l s o rel ea s e very s ma l l membra ne-encl os ed ma tri x ves i cl es wi th whi ch a l ka l i ne phos pha ta s e a nd other enzymes a re a s s oci a ted. Thes e enzymes hydrol yze i ons from va ri ous ma cromol ecul es , crea ti ng a hi gh concentra ti on of thes e i ons l oca l l y. The hi gh i on concentra ti ons ca us e crys ta l s of Ca PO4 to form on the ma tri x ves i cl es . The crys ta l s grow a nd mi nera l i ze further wi th the forma ti on of s ma l l growi ng ma s s es of hydroxya pa ti te [Ca 10 (PO4 ) 6 (OH) 2 ], whi ch s urround the col l a gen fi bers a nd a l l other ma cromol ecul es . Eventua l l y, the ma s s es of hydroxya pa ti te merge a s a confl uent s ol i d bony ma tri x a s ca l ci fi ca ti on of the ma tri x i s compl eted. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] In a ddi ti on to os teoi d, os teobl a s ts a l s o produce “ground substance,” compos ed pri ma ri l y of the GAG chondroitin sulphate (Cha pter 2) a nd s evera l os teobl a s t-deri ved gl ycoprotei ns , i ncl udi ng osteocalcin (functi on not compl etel y unders tood, but bel i eved to regul a te bone forma ti on), osteonectin (bi nds ca l ci um a nd col l a gen a nd i ni ti a tes mi nera l i za ti on proces s ), osteopontin, a l s o known a s bone sialoprotein [producti on i ncrea s ed by ca l ci tri ol (Vi ta mi n D 3 ), a nd thought to hel p a nchor os teocl a s ts to the bone ma tri x for bone res orpti on, but ma y a l s o s ti mul a te forma ti on of fi rs t hydroxya pa ti te crys ta l s for new mi nera l i za ti on]. Newl y formed os teobl a s ts a l s o s ecrete the cytoki ne (Cha pter 15) ma cropha ge col ony-s ti mul a ti ng fa ctor (M-CSF), a nother di s ul fi de homodi mer mol ecul e, whi ch bi nds to i ts receptor, l ea di ng to tyros i ne phos phoryl a ti on. Thi s a cti vi ty promotes the growth a nd di fferenti a ti on of os teocl a s ts (s ee bel ow) to hel p ma i nta i n the ba l a nce between bone forma ti on a nd res orpti on, a s wel l a s ca l ci um l evel s (s ee bel ow). Fol l owi ng l a yi ng down of the os teoi d a nd ground s ubs ta nce ma tri x, os teobl a s ts s ecrete ves i cl es wi th the enzyme alkaline phosphatase, whi ch removes phos pha te groups from hydroxya pa ti te. The modi fi ed hydroxya pa ti te a nd remna nt ves i cl es then a ct a s centers for depos i t of ca l ci um a nd phos pha te crys ta l s a nd, a s a res ul t, newl y mi nera l i zed bones . Collagen Synthesis and Scurvy: The proper producti on of col l a gen i s es s enti a l for os teoi d/bone forma ti on, a s wel l a s other connecti ve ti s s ues di s cus s ed bel ow. Pa rt of col l a gen’s uni que s tructure i s the pres ence of hydroxyl a ted prol i ne a nd l ys i ne res i dues , whi ch s ta bi l i ze the tri pl ehel i ca l s tructure es s enti a l for col l a gen’s s tructure a nd functi on. Vitamin C (ascorbic acid) i s a requi red cofa ctor for thes e hydroxyl a ti on rea cti ons , a nd the defi ci ency of thi s vi ta mi n ca us es the di s ea s e scurvy. Symptoms of s curvy i ncl ude a bnorma l bl eedi ng (from a ffected ca pi l l a ri es ), s ki n di s col ora ti on, nonhea l i ng wounds a nd gum deteri ora ti on/l os s of teeth (from connecti ve ti s s ue), a nd wea keni ng of bones (from a dvers e effects on bone forma ti on/remodel i ng). Untrea ted s curvy ca n be fa ta l . The trea tment for s curvy i s s i mpl e s uppl ementa ti on of vi ta mi n C i n the di et. In La ti n, “a s corbi c” l i tera l l y mea ns “no s curvy.” Osteoclasts a re modi fi ed forms of monocyte/ma cropha ge cel l s tha t brea k down bones a nd a re regul a ted i n di fferenti a ti on a nd growth by the bi ndi ng of receptor activator of nuclear factor kappa B (RANK) l i ga nd a nd by the a cti vi ty of the cytoki ne M-CSF (noted a bove). Bi ndi ng of the RANK l i ga nd to i ts receptor [a member of the tumor necros i s fa ctor (TNF) receptor fa mi l y] a cti va tes nucl ea r fa ctor ka ppa B (NF-κB), a n i mporta nt tra ns cri pti ona l a cti va tor protei n. M-CSF bi nds to a tyros i ne ki na s e receptor, l ea di ng to a compl i ca ted a nd not yet ful l y unders tood s i gna l i ng pa thwa y tha t l ea ds to os teocl a s t di fferenti a ti on. Thes e os teocl a s t effector mol ecul es a re produced by os teobl a s ts a nd s urroundi ng cel l s , a nd both RANK a nd M-CSF a re requi red for os teocl a s t producti on. Di fferenti a ti on of os teocl a s ts i s i nhi bi ted by the mol ecul e os teoprotegeri n (OPG) (s ee bel ow), whi ch i nhi bi ts RANK l i ga nd bi ndi ng to i ts receptor. Os teocl a s ts perform the oppos i te functi on to os teobl a s ts , na mel y res orpti on a nd/or remodel i ng of the mi nera l ma tri x. Bone remodel i ng (s ee bel ow) a l l ows the body to cha nge exi s ti ng bone s tructure duri ng growth a nd to repa i r mi crofra ctures tha t res ul t from norma l mecha ni ca l s tres s , a s wel l a s from pa thol ogi ca l fra ctures due to i njury. Remodel i ng a l s o provi des a s ource of ca l ci um a s pa rt of the regul a ti on of ca l ci um l evel s i n the body (s ee bel ow). Os teocl a s t res orpti on/bone remodel i ng i s a ccompl i s hed by s evera l mecha ni s ms . Fi rs t, os teocl a s ts a re a bl e to a tta ch to bone vi a a uni que podosome s tructure, i n whi ch i ntra cel l ul a r a cti n fi l a ments a tta ch to membra ne integrin receptor mol ecul es (Fi gure 13-8), whi ch s ubs equentl y bi nd to a s peci fi c a rgi ni ne–gl yci ne–a s pa rta te a mi no a ci d s equence on the os teoponti n protei n (di s cus s ed ea rl i er). Once bound, a n os teocl a s t-
deri ved carbonic anhydrase produces hydrogen i ons vi a the rea cti on a nd pumps them out i nto the bone 2+ ma tri x by a s peci a l i zed ATPa s e pump. Os teocl a s ts a l s o produce a n Fe -conta i ni ng gl ycoprotei n, tartrate resistant acid phosphatase (TRAP), pres ent i n a bunda nce i n os teocl a s t l ys os omes . TRAP’s Fe 2+ bi nds wi th a phos pha te i n hydroxya pa ti te a nd cl ea ves the phos pha te es ter bond by nucl eophi l i c a tta ck of a hydroxyl group. Free phos pha te a nd ca l ci um i ons tha t a re rel ea s ed a re i ni ti a l l y ta ken up by ves i cl es , but then s ubs equentl y rel ea s ed i nto the bl ood s trea m by exocytos i s . TRAP i s a l s o bel i eved to reduce os teoponti n/bone s i a l oprotei n a cti vi ty (s ee a bove) vi a dephos phoryl a ti on a nd a l s o i nfl uences growth, di fferenti a ti on, mi gra ti on, a nd a cti vi ty of os teobl a s ts , a s wel l a s the mi gra ti on of os teocl a s ts . TRAP a l s o genera tes rea cti ve oxygen s peci es , whi ch hel p to res orb bone. Severa l cathepsin enzymes , es peci a l l y the cathepsin K enzyme, a re a l s o produced by os teocl a s ts , whi ch degra de the col l a gen ma tri x vi a s el ecti ve cl ea va ge of col l a gen a nd other protei ns . Thes e ca theps i ns ha ve no a ppa rent s peci fi ci ty for pa rti cul a r pepti de bonds , a l though s ome preference for a mi no a ci ds wi th l a rge hydrophobi c s i de cha i ns ha s been noted. Other enzymes s uch a s aspartate protease a nd matrix metalloprotease a l s o a i d i n the brea kdown of the i norga ni c a nd orga ni c ma tri ces . Os teocl a s t a cti vi ty i s regul a ted by s evera l hormones , whi ch a re di s cus s ed bel ow.
Figure 13-8. Integrin Cell Surface Matrix Receptor. By bi ndi ng to a ma tri x protei n a nd to the a cti n cytos kel eton (vi a ta l i n) i ns i de the cel l , i ntegri ns s erve a s tra ns membra ne l i nks by whi ch cel l s a dhere to components of the extra cel l ul a r ma tri x (ECM). The mol ecul e i s a heterodi mer, wi th α a nd β cha i ns . The hea d porti on ma y protrude s ome 20 nm from the s urfa ce of the cel l membra ne i nto the ECM, where i t i ntera cts wi th fi bronecti n, l a mi ni n, or col l a gens . [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] Osteoid Mineralization and Osteomalacia/Rickets: The proces s of mi nera l i za ti on of os teoi d by os teobl a s ts i s cri ti ca l for the proper devel opment of bones . When os teobl a s ts a re una bl e to form hydroxya pa ti te or when s uffi ci ent ca l ci um a nd/or phos pha te a re not a va i l a bl e, i t res ul ts i n osteomalacia. In chi l dren, thi s condi ti on i s known a s rickets. Os teoma l a ci a , l i tera l l y mea ni ng “bone s oftnes s ,” exhi bi ts norma l a mounts of orga ni c col l a gen ma tri x, but defi ci ent mi nera l i za ti on. Thi s i s di fferent from os teoporos i s (s ee bel ow) i n whi ch the bone ma tri x i s norma l l y
mi nera l i zed but decrea s ed. As a res ul t, pa ti ents s ufferi ng from os teoma l a ci a ha ve wea k a nd ea s i l y fra ctured bones . Mos t often, os teoma l a ci a /ri ckets i s ca us ed by defi ci ency of vi ta mi n D ei ther i n the di et (i .e., poor i nta ke or poor i ntes ti na l a bs orpti on) or s econda ry to l ow s un expos ure/a bs orpti on. Other ca us es ma y i ncl ude ki dney or l i ver di s ea s e (or other di s orders tha t a ffect vi ta mi n D meta bol i s m a nd/or a bs orpti on), decrea s ed phos pha te l evel s , ca ncers , a nd medi ca ti on s i de effects (e.g., a nti convul s a nt medi ca ti ons ). Symptoms a nd s i gns of os teoma l a ci a i ncl ude bone pa i n (often s ta rti ng i n the l umba r regi on of the s pi ne, pel vi s , a nd l egs ) a nd rel a ted mus cl e a nd nerve wea knes s /numbnes s . La bora tory tes ts s how l ow ca l ci um l evel s i n s erum a nd uri ne (often a ccompa ni ed by l ow s erum phos pha te), hi gh a l ka l i ne phos pha ta s e (s ee a bove) a nd pa ra thyroi d hormone (PTH) l evel s (s ee bel ow), a nd ra di ol ogi ca l evi dence of ps eudofra ctures a nd/or bone l os s . Trea tment i nvol ves correcti on of the probl em a nd repl a cement of ca l ci um a nd vi ta mi n D; ful l recovery often ta kes 6 months or l onger. Os teocytes , the thi rd a nd mos t a bunda nt type of cel l conta i ned i n the orga ni c bone ma tri x, a re a ctua l l y a l tered forms of os teobl a s ts . Os teobl a s ts cha nge i nto os teocytes when they become tra pped i ns i de the growi ng bone ma tri x. As pa rt of thi s cha nge, os teocytes l i nk together vi a s peci a l i zed “canalicular” extens i ons conta i ni ng ga p juncti ons (Cha pter 8), whi ch a l l ow the i ntercha nge of nutri ents a nd wa s te products . Li ke os teobl a s ts , os teocytes a re i nvol ved i n bone forma ti on, a l though thei r exa ct rol e a nd functi on(s ) a re s ti l l unknown. Emergi ng res ea rch i ndi ca tes a uni que a bi l i ty of os teocytes to “s ens e” s tra i n a nd fl ow of fl ui d through the ca na l i cul i cha nnel s . The mecha ni ca l a nd fl ui d-deri ved s tra i n a ppea rs to a cti va te os teocytes to regul a te bone remodel i ng a nd growth vi a os teocl a s t-di rected s i gna l i ng to a nd between os teobl a s ts a nd os teocytes . Osteoporosis/Mechanisms of Bisphosphonate Action: Osteoporosis i s a bone condi ti on i n whi ch the a mount of bone mi nera l i s s i gni fi ca ntl y l owered, l ea di ng to a n a l tered a nd wea kened bone ma tri x a nd a ma rkedl y i ncrea s ed ri s k of fra cture. Thes e fra ctures a re often s een i n vertebra e, l ea di ng to the s tooped-over pos ture of lordosis a nd hi p fra ctures . The ca us e of os teoporos i s ca n be mul ti fa ctori a l but ca n be genera l i zed a s a condi ti on i n whi ch bone res orpti on outwei ghs bone forma ti on. Decrea s ed es trogen s ti mul a ti on res ul ti ng i n reduced bone forma ti on a s wel l a s defi ci ency of ca l ci um a nd vi ta mi n D a re often the pri nci pa l ca us es , es peci a l l y i n pos tmenopa us a l women of Europea n or As i a n des cent. Hea vy us e of a l cohol , s moki ng, a nd s ome medi ca l condi ti ons (e.g., endocri ne, hypogona da l , or certa i n bl ood di s orders ) a re a l s o known ri s k fa ctors for the devel opment of os teoporos i s . Chroni c us e of medi ca ti ons i ncl udi ng gl ucocorti coi ds (e.g., a drena l i ns uffi ci ency, s evere a s thma ti cs , tra ns pl a nt reci pi ents ), l evothyroxi ne, l i thi um, certa i n ba rbi tura tes a nd a nti -s ei zure drugs , hepa ri n a nd wa rfa ri n, the newer di a betes medi ca ti on cl a s s of thi a zol i di nedi ones , a nd proton pump i nhi bi tors ca n a l s o contri bute to os teoporos i s by decrea s i ng ca l ci um a nd/or vi ta mi n D meta bol i s m a nd/or os teobl a s t a cti vi ty. Al though i ncrea s ed ca l ci um a nd vi ta mi n D i nta ke a l ong wi th exerci s e a nd es trogen repl a cement ca n hel p promote bone forma ti on, the devel opment of bisphosphonates ha s a l l owed the di rected trea tment of thi s condi ti on (s ee bel ow). Bi s phos phona tes a re ta ken up by a nd ki l l os teocl a s ts by ei ther repl a ci ng the termi na l phos pha te i n a denos i ne tri -phos pha te (ATP), renderi ng the mol ecul e i nopera bl e (nonni trogenous bi s phos phona tes ), or i nhi bi ti ng 3-hydroxy-3-methyl gl uta ryl -coenzyme A reducta s e enzyme fa rnes yl di phos pha te s ynthes i s (ni trogenous bi s phos phona tes ) (Cha pter 7). Al though requi red for chol es terol bi os ynthes i s , thi s enzyme i s a l s o es s enti a l for modi fi ca ti on a nd correct tra ffi cki ng of s ome membra ne protei ns a nd for pa rti cul a r cytos kel eta l functi ons of os teocl a s ts tha t ena bl e them to res orb bones . Al terna ti ve medi ca ti ons i ncl ude calcitonin (i nhi bi ti on of os teocl a s ts ), PTH (s ti mul a ti on of os teocl a s ts ), a nd OPG (RANK receptor i nhi bi ti on) a na l ogues . Emergi ng trea tments , i ncl udi ng sodium fluoride a nd strontium, ma y offer a ddi ti ona l trea tment moda l i ti es .
Reproduced wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.
BONE GROWTH AND REMODELING The growth of bone a nd i ts cons ta nt remodel i ng i n res pons e to norma l growth, mi crofra ctures (due to norma l or a bnorma l mecha ni ca l s tres s es ), a nd fra ctures due to tra uma i s hi ghl y regul a ted a nd ba l a nced between new bone forma ti on vi a os teobl a s ts a nd bone res orpti on vi a
os teocl a s ts . The a mount of “norma l ” remodel i ng decrea s es ma rkedl y from compl ete bone turnover i n i nfa ncy to onl y a pproxi ma tel y 10% of bone remodel i ng i n a dul ts i n a ny gi ven yea r. The ba l a nce of os teobl a s t a nd os teocl a s t a cti vi ty i s dependent on s evera l fa ctors . Apa rt from FGF, PDGF, TGF-β, a nd BMPs noted a bove, os teobl a s t functi on i s s ti mul a ted by the pi tui ta ry-deri ved, pol ypepti de growth hormone, whi ch s erves both to a ugment bone ma tri x forma ti on a nd to i ncrea s e the retenti on of ca l ci um i n the body. Androgens a nd es trogens a l s o promote os teobl a s t a cti vi ty/bone growth. Es trogens functi on vi a promoti ng the i ncrea s ed s ecreti on of the cytoki ne (Cha pter 15) gl ycoprotei n OPG, whi ch competi ti vel y bi nds to the RANK l i g-a nd TNF fa mi l y receptor (di s cus s ed ea rl i er). By bl ocki ng RANK l i ga nd a cti vi ty, OPG/os teocl a s togenes i s i nhi bi tory fa ctor effecti vel y s tops the devel opment of os teocl a s ts from thei r monocyte/ ma cropha ge precurs or cel l s . PTH a nd vi ta mi n D a l s o a ffect bone growth by i ncrea s i ng ca l ci um l evel s (s ee bel ow). Os teobl a s ts ca n a l s o s ti mul a te the a cti vi ty of os teocl a s ts . Increa s ed PTH a nd vi ta mi n D from l ow s erum ca l ci um l evel s a l ong wi th s i gna l s from os teocytes i ncrea s e the producti on of RANK l i ga nd a nd cytoki ne interleukin 6 (IL-6). Al though IL-6 i s better known for i ts rol e i n fever a nd i nfl a mma ti on, when s ecreted by os teobl a s ts , i t a l s o s erves , together wi th RANK receptor a cti va ti on, to promote the devel opment a nd growth of os teocl a s ts . IL-6’s receptor s i gna l s vi a the Ja nus ki na s e–s i gna l tra ns ducer a nd a cti va tor of tra ns cri pti on pa thwa y, l ea di ng to phos phoryl a ti on of s i gna l mol ecul es a nd gene tra ns cri pti on (Cha pter 8 a nd Fi gure 8-9C). Al though es trogen promotes os teobl a s t growth, i t ca n a l s o bl ock IL-6 a cti vi ty, thereby i nhi bi ti ng os teocl a s t growth. As noted a bove, M-CSF i s a l s o i nvol ved i n os teocl a s t a cti va ti on by promoti ng the devel opment a nd growth of os teocl a s ts from the monocytes / ma cropha ge precurs or cel l s . M-CSF a l s o decrea s es the s ecreti on of the i nhi bi tory hormone, os teoprotegeri n (s ee a bove). Paget’s Disease: Paget’s disease of the bone, a l s o known a s osteitis deformans, i s a condi ti on of exces s i ve bone turnover (brea kdown a nd reforma ti on), res ul ti ng i n bone deformi ti es , pa i n, decrea s ed s trength, a nd a rthri ti s a nd fra ctures . A geneti c l i nka ge ha s been s ugges ted a s the pos s i bl e rol e of pa ra myxovi rus , a l though no convi nci ng evi dence ha s been found. An emergi ng bel i ef s ugges ts i nvol vement of a n a bnorma l res pons e to vitamin D a nd a n i ncrea s ed res pons e of the RANK ligand, pos s i bl y to the a cti ons of IL-6. The di s ea s e s ta rts wi th i ncrea s ed, l oca l i zed res orpti on by os teocl a s ts i n l ong bones a nd the s kul l . A res ul ti ng i ncrea s e of os teobl a s t a cti vi ty res ul ts i n a qui ckl y l a i d-down a nd di s orga ni zed bone ma tri x, fi brous connecti ve ti s s ue, a nd new bl ood ves s el s ra ther tha n the orga ni zed a nd s turdy s tructure of regul a r bones . The i ncrea s ed os teocl a s t a nd os teobl a s t a cti vi ty eventua l l y di mi ni s hes , l ea vi ng perma nent a nd detri menta l cha nges i n the bone. Al though no l onger us ed cl i ni ca l l y, s kul l X-ra ys s how thes e cha nges a s di a gnos ti c “cotton wool s pots .” Trea tment i s norma l l y vi a ca l ci um a nd vi ta mi n D s uppl ementa ti on a l ong wi th bi s phos phona tes a nd, occa s i ona l l y, ca l ci toni n. Pa ti ents a re moni tored by fol l owi ng a l ka l i ne phos pha ta s e, ca l ci um, a nd phos pha te l evel s . Bone growth i n a fetus a nd a chi l d va ri es s l i ghtl y from a n a dul t a nd occurs vi a two s l i ghtl y di fferent proces s es . Intramembranous ossification occurs i n the s kul l a nd requi res the devel opment of l oca l i zed a rea s of bone growth, known a s os s i fi ca ti on centers . Ini ti a l l y, newl y di fferenti a ted os teobl a s ts a ggrega te a nd s ecrete bone ma tri x a round thems el ves to form bone spicules. As more bone ma tri x i s s ecreted, the s pi cul es joi n together to form a n open bone network ca l l ed trabeculae. The tra becul a e joi n together, centers of bone ma rrow a nd a peri os teum devel ops , a nd a more compa ct bone i s l a i d down. Intra membra nous os s i fi ca ti on i s a l s o es s enti a l duri ng the repa i r of bone fra ctures (s ee bel ow). The a l terna ti ve method of ea rl y bone forma ti on i s endochondral ossification, res pons i bl e for l ong bone forma ti on, a s wel l a s s ome fl a t a nd i rregul a r bones . Endochondra l os s i fi ca ti on i s a l s o res pons i bl e for conti nued growth duri ng chi l dhood a nd pa rts of fra cture hea l i ng. Thi s mode of bone forma ti on s ta rts wi th the forma ti on of a ca rti l a ge ma tri x, fol l owed by the devel opment of os s i fi ca ti on centers where the mi nera l ma tri x i s l a i d down. A pri ma ry os s i fi ca ti on center occurs i n the center of the newl y formi ng bone to form the diaphysis; a s econda ry os s i fi ca ti on center i s es ta bl i s hed nea r the ends of the growi ng bone a s pa rt of the epiphyseal plate, whi ch i s compl etel y repl a ced by i norga ni c bone ma tri x nea r the end of the s econd deca de of l i fe (epiphyseal closure). Bone Fractures and Healing: A fracture of a bone i ni ti a tes a s eri es of three ma i n pha s es of hea l i ng-reactive, reparative, a nd remodeling, whi ch a re res pons i bl e for both qui ck s ta bi l i za ti on of the fra cture a nd a s l ower res tora ti on of the da ma ged bone to i ts nea r-ori gi na l s ta te. Immedi a tel y fol l owi ng a fra cture, the reactive phase begi ns , i n whi ch bl ood ves s el s i ni ti a l l y cons tri ct a nd a cl ot forms , whi ch, a l ong wi th fi brobl a s ts , form a n i ni ti a l ma tri x of fi brous connecti ve ti s s ue wi th a bl ood ves s el s uppl y. A few da ys a fter the fra cture, the reparative phase begi ns , wi th devel opment of chondrobl a s ts a nd os teobl a s ts from fi brobl a s t cel l s wi thi n the peri os teum. Thes e cel l s produce a n i ni ti a l woven bone “callus,” compos ed of qui ckl y a nd ha pha za rdl y orga ni zed hya l i ne ca rti l a ge (type II col l a gen a nd chondroi ti n s ul pha te), whi ch uni tes the broken bone fra gments a nd provi des s ome s upport for the fra cture. Thi s hya l i ne ca rti l a ge ma tri x i s repl a ced by a regul a r a nd pa ra l l el a rra y of type I col l a gen lamellar bone ma tri x vi a endochondra l os s i fi ca ti on. The growth of a more perma nent bl ood ves s el s uppl y modi fi es the l a mel l a r bone to trabecular bone, wi th a l mos t a l l of the ori gi na l s trength of the bone res tored. The fi na l pha s e, remodeling, ca n ta ke from s evera l weeks to months to compl ete, wi th s el ected res orpti on by os teocl a s ts a nd redepos i ti on by os teobl a s ts unti l a fi na l compact bone ma tri x i s produced, cl os el y mi mi cki ng the ori gi na l bone.
Ada pted wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.
REGULATION OF CALCIUM LEVELS Beca us e bone repres ents a l a rge s tora ge depot of rea di l y a va i l a bl e ca l ci um, the regul a ti on of ca l ci um l evel s i n the body i nfl uence the
ba l a nce of bone forma ti on a nd brea kdown. Thus , a mul ti orga n, regul a ted proces s i nvol vi ng the i ntes ti nes , ki dneys , a nd bones works i n s ynchrony to a bs orb new ca l ci um, s tore new a nd ol d ca l ci um, a nd, when neces s a ry, free s tored ca l ci um for the body’s us e. The norma l di et a l l ows a n a ddi ti on of a pproxi ma tel y 5 mmol of ca l ci um per da y. The ki dneys excrete a net of 5 mmol of ca l ci um, thereby nega ti ng a ny di eta ry ga i ns . Bone, whi ch conta i ns a pproxi ma tel y 99% of the body’s tota l ca l ci um, therefore, s erves a s both a s tora ge depot a nd a s ource for ca l ci um. Intes ti na l ca l ci um a bs orpti on i s s trongl y i nfl uenced by vi ta mi n D, whi ch ha s the i mporta nt rol e of i ncrea s i ng the number of ca l ci um-bi ndi ng protei ns , known a s calbindin, on the cel l s of the s ma l l i ntes ti ne a s wel l a s the ki dney. Increa s ed a mounts of ca l bi ndi n pres ent i n the i ntes ti ne di rectl y i ncrea s e the a mount of ca l ci um a bs orbed a nd ma y a l s o s ti mul a te a n ATP-dependent ca l ci um pump, whi ch tra ns ports ca l ci um i nto the bl ood s trea m. Ki dney rea bs orpti on of ca l ci um i s a l s o i ncrea s ed by vi ta mi n D; the ki dney i s a l s o res pons i bl e for converti ng 25(OH) vi ta mi n D 3 i nto the a cti ve form of ca l ci tri ol , 1,25(OH) 2 vi ta mi n D 3 (Cha pter 3). In a ddi ti on, PTH a nd calcitonin pl a y promi nent rol es i n the regul a ti on of i ntes ti na l a bs orpti on, ki dney excreti on/ rea bs orpti on, a nd bone turnover of ca l ci um. PTH i s s ecreted by the pa ra thyroi d gl a nds i n res pons e to l ow s erum ca l ci um l evel s (s ens ed by s peci a l receptors on the pa ra thyroi d cel l s ) a nd/or i ncrea s ed s erum phos pha te, whi ch bi nds to a nd decrea s es the l evel of free ca l ci um i ons . Increa s ed ca l ci um i ons l ea d to a reducti on i n PTH s ecreti on. PTH i s res pons i bl e for i ncrea s i ng Ca 2+ concentra ti on vi a the a cti on of a G q protei n, whi ch s ti mul a tes phos phol i pa s e C to produce i nos i tol 1,4,5-tri s phos pha te (IP3 ) a nd, therefore, i ncrea s e ca l ci um rel ea s e (Cha pter 8). Addi ti ona l l y PTH a cts vi a G s to i ncrea s e protei n ki na s e A a cti vi ty. Calcitonin i s a pol ypepti de hormone produced by the pa ra fol l i cul a r C-cel l s of the thyroi d gl a nd a nd i s res pons i bl e for decrea s i ng ca l ci um l evel s . Ca l ci toni n l evel s a re i ncrea s ed when ca l ci um l evel s a re hi gh, a s wel l a s by ga s tri n s ecreti on (Cha pter 11). Al though the exa ct mecha ni s m of ca l ci toni n functi on i s s ti l l bei ng determi ned, i t i s known to a ct not onl y vi a a G q-protei n receptor mecha ni s m but a l s o vi a a G s protei n receptor (a denyl cycl a s e/cycl i c a denos i ne monophos pha te) mecha ni s m. Intera cti on wi th thes e G protei ns i s known to i nhi bi t os teocl a s t res orpti on by qui es cence (Q-effect) a nd retra cti on (R-effect) of the os teocyte membra ne. Ca l ci toni n receptors a re ma i nl y found on os teocl a s ts a nd a l s o on ova ri es a nd tes tes . The a ctua l i mporta nce of ca l ci toni n i n the regul a ti on of ca l ci um i n huma ns i s s ti l l i n ques ti on. The effects of PTH a nd ca l ci toni n a re i l l us tra ted i n Ta bl e 13-2.
TABLE 13-2. Summa ry of PTH a nd Ca l ci toni n Effects on Ta rget Orga ns PTH-Related Peptide and Cancer: A common cons equence of s ome ca ncers ca n be i ncrea s ed l evel s of ca l ci um (hyperca l cemi a ) wi th s ecreti on of PTH-related peptide (PTH-rP). PTH-rP ha s s i gni fi ca nt homol ogy wi th PTH, but onl y i n the a mi no-termi na l 13 a mi no a ci ds , whi ch ma ke up the receptor-bi ndi ng regi on. Beca us e PTH-rP i s not s ecreted by the pa ra thyroi d gl a nds , hyperca l cemi a produces no feedba ck regul a ti on of i ts producti on. The ri s e i n ca l ci um l evel s due to PTH-rP i s termed humoral hypercalcemia of malignancy (HHM). Ca ncers us ua l l y a s s oci a ted wi th HHM i ncl ude brea s t a nd l ung ca ncers a nd mul ti pl e myel oma . A s umma ry of ca l ci um regul a ti on i n the body i s s hown i n Fi gure 13-9.
Figure 13-9. Control of Blood Calcium and Phosphate Levels. Overvi ew of the regul a ti on of ca l ci um (Ca 2+) a nd phos pha te (P) l evel s by the coordi na ted a cti ons of a cti ve vi ta mi n D [1,25(OH) 2 D 3 ], pa ra thyroi d hormone (PTH), ca l ci toni n, a nd es tra di ol (E2 ). See the text a bove a nd Ta bl e 13-2 for further deta i l s . [Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.]
MARKERS OF BONE FORMATION AND RESORPTION On the ba s i s of va ri ous mecha ni s ms i nvol ved i n the forma ti on of bone ma tri x, a number of chemi ca l ma rkers ha ve been cha ra cteri zed, whi ch a l l ow cl i ni ci a ns to mea s ure a nd moni tor bone forma ti on a nd brea kdown. Ca l ci um ca n be mea s ured a s ei ther total calcium or ionized calcium, wi th ca l ci um bound to s erum protei ns (e.g., s erum a l bumi n) a ffecti ng the mea s urement of ca l ci um freel y a va i l a bl e to the body. Abnorma l l evel s of a l bumi n (hypoa l bumi nemi a or hypera l bumi nemi a ) wi l l a l ter the a mount of bound ca l ci um, a nd therefore, a l bumi n l evel s a re often tes ted a s wel l , s o a corrected calcium l evel ca n be ca l cul a ted. Vi ta mi n D l evel s , s peci fi ca l l y the a cti ve 1,25(OH) 2 D 3 (ca l ci tri ol ) form, wi l l a l s o i mpa ct on ca l ci um meta bol i s m a nd ma y provi de i ns i ght i nto a number of rel a ted di s ea s e s ta tes . Phos pha te l evel s a re a l s o i mporta nt i n rel a ti on to the bi ndi ng a nd excreti on of ca l ci um a nd a re, thus , often mea s ured. The i mporta nt rol e of PTH i n ca l ci um, phos pha te, a nd vi ta mi n D regul a ti on a nd pa rti cul a r di s ea s es ma y a l s o prompt a cl i ni ci a n to order mea s urement of i ts l evel . Ca l ci toni n’s i mporta nce i n ca l ci um regul a ti on us ua l l y precl udes i ts mea s urement, except a s a tumor ma rker for medul l a ry thyroi d a denoca rci noma . Fi na l l y, a l ka l i ne phos pha ta s e, the enzyme tha t promotes mi nera l i za ti on of newl y formi ng bone, i s s ometi mes mea s ured i n pa ti ents s ufferi ng from bone di s orders . As a l ka l i ne phos pha ta s e ha s three di fferent i s oforms [i ntes ti na l , pl a centa l , a nd nons peci fi c (e.g., l i ver/bone/ki dney)] a nd i s found i n a l l ti s s ues i n the huma n body, pa rti cul a r i s oenzymes need to be i s ol a ted a nd thei r s epa ra ted l evel (s ) ca reful l y cons i dered. Al ka l i ne phos pha ta s e i s often mea s ured i n condi ti ons s uch a s Pa get’s di s ea s e of bone, os teos a rcoma , ca ncers tha t ha ve meta s ta s i zed to bone, bone fra ctures , os teoma l a ci a (ri ckets ), a chondropl a s i a , congeni ta l hypothyroi di s m (previ ous l y known a s creti ni s m), rena l os teodys trophy, hypophos pha ta s i a , os teoporos i s /es trogen us e, a nd/or vi ta mi n D defi ci ency. However, a l ka l i ne phos pha ta s e l evel s a re more often us ed to mea s ure bl ocka ge of l i ver bi l e ducts tha n for genera l bone a nd ca l ci um s tudi es .
REVIEW QUESTIONS 1. How woul d you des cri be the three ma i n types of connecti ve ti s s ue a nd how do they di ffer? 2. Wha t a re the functi ons of chondrocytes ? 3. Wha t i s the ba s i c pa thwa y for the forma ti on of col l a gen? 4. Wha t a re the ma jor components of bone (i norga ni c a nd orga ni c)? 5. How woul d you des cri be the three types of cel l s i n the orga ni c ma tri x, i ncl udi ng thei r functi on(s ) a nd regul a ti on? 6. How woul d you des cri be the proces s es of bone forma ti on a nd res orpti on, i ncl udi ng thei r regul a ti on? 7. How woul d you des cri be the mecha ni s ms regul a ti ng ca l ci um l evel s a nd bone forma ti on, i ncl udi ng the rol es of ca l ci um, vi ta mi n D, pa ra thyroi d hormone, a nd ca l ci toni n? 8. Wha t a re the ma jor ma rkers of bone forma ti on/res orpti on?
CHAPTER 14 BLOOD Co-Authors/Editors: Matthew Porteus, MD, PhD Di vi s i ons of Ca ncer Bi ol ogy, Hema tol ogy/Oncol ogy, a nd Huma n Gene Thera py, Sta nford Uni vers i ty, Sta nford CA
Tina Mantanona Uni vers i ty of Texa s Southwes tern Medi ca l Center, Da l l a s , TX
Ba s i c Components of Bl ood Red Bl ood Cel l (RBC) Functi ons Di s ea s es As s oci a ted wi th Ina dequa te Synthes i s of Hemogl obi n Components Oxygen Bi ndi ng Phys i ol ogi c Res pons e to Ina dequa te O2 Del i very Si ckl e Cel l Di s ea s e (SCD) Iron Cl otti ng Revi ew Ques ti ons
OVERVIEW Bl ood, i ncl udi ng red cel l s , whi te cel l s , pl a tel ets , a nd a col l ecti on of s peci a l i zed protei ns , s erves a n es s enti a l phys i ol ogi c functi on a s i t ca rri es mol ecul es from one pa rt of the body to a nother. Thi s cha pter di s cus s es the bi ochemi ca l underpi nni ngs of how bl ood tra ns ports oxygen from the l ungs a nd i ron to the ti s s ues tha t need them wi thout ca us i ng da ma ge to the ti s s ues tha t do not. Thi s tra ns port s ys tem a l s o i ns ures a dequa te wa s te remova l from the huma n body. In a ddi ti on, the bi ochemi ca l properti es of cl otti ng, the s ys tem tha t ma i nta i ns the i ntegri ty of the bl ood va s cul a ture, a re di s cus s ed. In des cri bi ng the norma l wa y i n whi ch bl ood ca rri es out thes e functi ons , the di s cus s i on wi l l i l l us tra te, from a bi ochemi ca l pers pecti ve, the wa ys di s ea s es ma y res ul t when thes e proces s es go a wry. Thes e three i mporta nt rol es , however, a re jus t a s ma l l fra cti on of the cri ti ca l functi ons tha t bl ood ca rri es out to ma i nta i n the proper performa nce of the huma n body. The core bi ochemi ca l pri nci pl es of thes e three s ys tems —the rol e of a l l os tery i n regul a ti ng l i ga nd bi ndi ng i n the oxygen–hemogl obi n s ys tem, the s peci a l i zed functi ons of protei ns for i ron tra ns port a nd s tora ge, a nd the i mporta nce of bi ochemi ca l ca s ca des a s i l l us tra ted by the cl otti ng s ys tem—a ppl y broa dl y to other functi ons of the bl ood a nd other orga n s ys tems i n genera l . Thus , by unders ta ndi ng thes e funda menta l bi ochemi ca l mecha ni s ms , a genera l founda ti on for unders ta ndi ng the norma l phys i ol ogy a nd pa thophys i ol ogy of other s ys tems ca n be es ta bl i s hed.
BASIC COMPONENTS OF BLOOD Hema tol ogy, the s tudy of bl ood, ha s been of centra l i mporta nce i n medi ci ne throughout hi s tory. For much of hi s tory, hea l th a nd i l l nes s were cons i dered a refl ecti on of di fferent “humors ” i n the body: bl a ck bi l e, yel l ow bi l e, phl egm, a nd bl ood. Good hea l th wa s a tta i ned when a l l the humors were i n ha rmoni c ba l a nce a nd i l l nes s wa s cons i dered a s the res ul t of a n i mba l a nce i n the humors . Thus , the goa l of hea l ers wa s to res tore the ba l a nce of the humors i n thos e who were i l l . To ma ni pul a te bl ood, hea l ers empl oyed methods s uch a s a ppl yi ng l eeches , cuppi ng, a nd bl oodl etti ng i n hope of a chi evi ng hea l th through crea ti ng proper ba l a nce of humors . A huma n a dul t conta i ns a pproxi ma tel y 5 l i ters (l ) of bl ood, wi th the enti re vol ume ci rcul a ti ng through the body every 1–2 mi n. Centri fuga ti on s epa ra tes bl ood i nto a cel l ul a r l a yer, whi ch col l ects a t the bottom end of the col l ecti on tube, a nd a noncel l ul a r l a yer, l oca ted a t the top of the tube (Fi gure 14-1). If whol e bl ood cl ots before centri fuga ti on, the rema i ni ng noncel l ul a r component i s ca l l ed serum. On the other ha nd, i f a nti cl otti ng a ddi ti ves a re a dded to the tes t tube, the noncel l ul a r l a yer i s ca l l ed plasma. The cel l ul a r a nd noncel l ul a r l a yer ea ch a ccount for a pproxi ma tel y 50% of the tota l bl ood vol ume. The cel l ul a r compa rtment cons i s ts of red bl ood cel l s (RBCs ), the mos t a bunda nt cel l i n the body, whi te bl ood cel l s , a nd pl a tel ets . The genera ti on of new RBCs (erythrocytes) from bone ma rrow i s termed erythropoiesis, a nd i t rel i es on the ki dney-s ecreted hormone erythropoietin (Epo) (s ee bel ow). Whi te bl ood cel l s , whi ch pri ma ri l y s erve to fi ght i nfecti ons , ca n be further di vi ded i nto s peci fi c s ubtypes a nd a re di s cus s ed i n Cha pter 15. Pl a tel ets a re s ma l l cel l s tha t pri ma ri l y pa rti ci pa te i n cl otti ng. The cl otti ng s ys tem cons i s ts of both cel l ul a r a nd noncel l ul a r components a nd wi l l be di s cus s ed bel ow. Si mi l a r to the cel l ul a r l a yer, the noncel l ul a r l a yer ca n be di vi ded i nto va ri ous components , ea ch of whi ch ca rri es out es s enti a l functi ons for ma i nta i ni ng hea l th. For i ns ta nce, wi thi n the noncel l ul a r component a re ca rri er protei ns , whi ch tra ns port other protei ns or s ma l l mol ecul es . Importa nt exa mpl es i ncl ude hemoglobin (Hgb), whi ch ca rri es oxygen (O2 ) a nd ca rbon di oxi de (CO2 ), a nd tra ns ferri n, whi ch tra ns ports i ron a nd regul a tes i ron meta bol i s m. Other ca rri er protei ns tra ns port s ma l l pepti des , l i pophi l i c hormones (Cha pter 3), a nd even drugs .
Figure 14-1. Separation of Blood into its Basic Components. Upon centri fuga ti on, bl ood s epa ra tes i nto a noncel l ul a r “pl a s ma ” l a yer a t the top of the tube; a mi ddl e “buffy coa t,” compos ed of pl a tel ets a nd whi te bl ood cel l s ; a nd a bottom l a yer of mos tl y red bl ood cel l s . Beca us e thi s s a mpl e conta i ns a nti cl otti ng a ddi ti ves , the noncel l ul a r l a yer i s ca l l ed pl a s ma . [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] The mos t a bunda nt noncel l ul a r protei n i n the bl ood i s albumin (3.5–5 g/dl ). Al bumi n i s s ecreted by l i ver cel l s (hepa tocytes , di s cus s ed i n Cha pter 11), s erves a s a ca rri er protei n, a nd, mos t i mporta ntl y, s erves to ma i nta i n oncotic pressure. Oncoti c pres s ure i s the term us ed to des cri be the force tha t keeps fl ui d from l ea ki ng out of a conta i ner through a di ffus i bl e ba rri er. When a l bumi n l evel s become l ow (hypoalbuminemia), s uch a s wi th l i ver fa i l ure (i na dequa te a l bumi n i s ma de) or i n protei n-l os i ng di s ea s es (where too much a l bumi n i s l os t through ei ther uri ne or s tool ), the oncoti c pres s ure of the bl ood fa l l s . Thi s cha nge i n pres s ure ca us es noncel l ul a r fl ui d to l ea k from the bl ood i nto the ti s s ues , res ul ti ng i n edema (s wel l i ng). Edema ca n be l i fe-threa teni ng, pa rti cul a rl y pul mona ry edema , where l ungs become fl ui d fi l l ed a nd O2 excha nge i s i mpa i red. Pul mona ry edema from pure hypoa l bumi nemi a i s extremel y ra re, a nd the mos t common ca us es of pul mona ry edema a re hea rt fa i l ure a nd i nfl a mma ti on. Immunoglobulins (a nti bodi es ) form the s econd mos t common type of protei n i n the bl ood (2.3–3.5 g/dl ). Al though i mmunogl obul i ns pl a y a mi nor rol e i n ma i nta i ni ng bl ood oncoti c pres s ure, the mos t s eri ous compl i ca ti on from l ow i mmunogl obul i n l evel s i s a n i ncrea s ed ri s k of i nfecti on (Cha pter 15). The bl ood a l s o tra ns ports hormones, s ma l l bi oa cti ve mol ecul es tha t a ffect the functi on of one or mul ti pl e ta rget orga ns a nd ti s s ues (Cha pters 1 a nd 3). In a ddi ti on to protei n components , s erum conta i ns a n i ncredi bl e va ri ety of nonprotei n components , i ncl udi ng mi nera l s , el ectrol ytes , l i poprotei n pa rti cl es , a nd es s enti a l nutri ents (s uch a s gl ucos e a nd gl uta mi ne).
RED BLOOD CELL (RBC) FUNCTIONS RBCs a re s ma l l cel l s (6–8 μm i n di a meter) whos e pri ma ry functi on i s to tra ns port O2 from the l ungs to the peri phera l ti s s ues . A s econda ry functi on of RBCs i s to ca rry CO2 , genera ted by the peri phera l ti s s ues , ba ck to the l ungs for el i mi na ti on by exha l a ti on (Cha pter 17) through the mouth a nd nos e. Hgb i s the O2 -ca rryi ng mol ecul e i n RBCs . Hgb cons i s ts of four heme mol ecul es wi th four globin cha i ns , gi vi ng the mol ecul e i ts na me. Adul t Hgb i s a tetra mer of four gl obi n cha i ns : two α-globin s ubuni ts a nd two β-globin s ubuni ts (Fi gure 14-2A–C a nd 14-3A–B).
Figure 14-2. A–C. Basic Components of a Red Blood Cell. (A) Ea ch erythrocyte (red bl ood cel l ) conta i ns ma ny mol ecul es of (B) hemogl obi n (Hgb), a four-s ubuni t mol ecul e. Ea ch s ubuni t of Hgb conta i ns (C) one heme mol ecul e. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Ea ch heme mol ecul e cons i s ts of two pa rts : a protoporphyrin molecule a nd iron. The l oca l envi ronment crea ted by ea ch gl obi n s ubuni t ma i nta i ns i ron i n the ferrous (Fe 2+) s ta te. In thi s form, i ron ha s s i x “coordi na ti on s i tes ” tha t i ntera ct wi th the protoporphyri n ri ng a nd other mol ecul es . The fi rs t four of the coordi na ti on s i tes bi nd to the protoporphyri n ri ng a l ong i ts pl a ne, s us pendi ng the i ron i n the mi ddl e of the ri ng (Fi gure 14-4A). The fi fth s i te forms a s trong, cova l ent l i nka ge to Hgb through hi s ti di ne (ca l l ed the “proxi ma l hi s ti di ne”) of the gl obi n cha i n (Fi gure 14-4A). Si tes 1 through 5 a re fi xed. The s i xth s i te, however, i s not fi xed a nd a ccounts for the vers a ti l i ty of Hgb bi ndi ng. It i s the s i xth s i te tha t bi nds a nd rel ea s es O2 a s wel l a s other ga s es . The deoxygena ted protoporphyri n ri ng ha s a dome-l i ke s tructure beca us e of s teri c hi ndra nces a nd other forces from the s urroundi ng a mi no a ci ds (Fi gure 14-4B). Anemia: Anemia occurs when the qua nti ty of RBCs i s l ower tha n norma l . In s evere forms of a nemi a , the ca pa ci ty of the bl ood to ca rry a dequa te O2 to the peri phera l ti s s ues i s compromi s ed. Anemi a ca n be ca us ed by ei ther the fa i l ure to produce enough RBCs (e.g., iron
deficiency or aplastic anemia), a l os s of RBCs (bleeding), a n i ncrea s ed des tructi on of RBCs tha t ca nnot be compens a ted for by the bone ma rrow (e.g., sickle cell anemia or autoimmune hemolytic anemia), or by s eques tra ti on, i n whi ch RBCs a re hi dden a wa y (us ua l l y i n the s pl een). The mos t common ca us e of a nemi a , a nd the mos t common nutri ti ona l defi ci ency i n the worl d, i s i ron defi ci ency a nemi a . In thi s condi ti on, there i s i ns uffi ci ent di eta ry i ron for erythropoi es i s . The fa i l ure to meet ongoi ng, pers i s tent dema nds of the bone ma rrow to crea te new RBCs res ul ts i n va gue s ymptoms of wea knes s a nd fa ti gue. Seri ous ca s es ca n ma ni fes t a s s hortnes s of brea th a nd even dea th.
Figure 14-3. A–B. Synthesis and Structure of Hemoglobin (Hgb). (A) Heme i s s ynthes i zed vi a mul ti pl e enzyma ti c s teps (i ndi ca ted i n bl ue), whi ch occur i n both the mi tochondri a a nd cytos ol . Increa s ed l evel s of heme nega ti vel y regul a te ALA s yntha s e, the fi rs t enzyme i n the s yntheti c pa thwa y. The fi na l s tep, ca ta l yzed by ferrochel a ta s e, i s res pons i bl e for the a ddi ti on of Fe 2+ to the fi ni s hed heme mol ecul e. Defecti ve enzymes i n a l l of the rema i ni ng s yntheti c s teps l ea d to a s eri es of di s ea s es noted i n red. (B) A heme mol ecul e cons i s ts of a protoporphyri n ri ng wi th a centra l l y s us pended i ron mol ecul e (s ee the text bel ow). Thi s i ron a tom bi nds oxygen (O2 ); therefore, one Hgb mol ecul e ca n bi nd a ma xi mum of four O2 mol ecul es . CoA, coenzyme A; PLP, pyri doxa l phos pha te. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Carbon Monoxide (CO) Poisoning: CO i s a da ngerous ga s not onl y beca us e i t i s undetected by our s ens e of s mel l but a l s o beca us e i t ha s a n a ffi ni ty 200 ti mes grea ter tha n tha t of O2 for Hgb’s s i xth coordi na ti on pos i ti on. As a res ul t, CO wi l l di s pl a ce O2 for bi ndi ng to Hgb. The di s pl a cement of O2 by CO s everel y decrea s es O2 del i very to the peri phera l ti s s ues a nd decrea s es the cel l ’s a bi l i ty to ca rry out oxi da ti ve phos phoryl a ti on. The detecti on of CO poi s oni ng ca n be di ffi cul t. Nota bl y, a pa ti ent does not ha ve to a ppea r bl ue to be i n di re need of O2 ; pa ti ents a re jus t a s pi nk when Hgb i s s a tura ted wi th ei ther O2 or CO. The trea tment for CO poi s oni ng i s to fi rs t remove the pa ti ent from the s ource of CO expos ure a nd, i n extreme ca s es , to put the pa ti ent i n a hyperba ri c O2 cha mber. In a hyperba ri c O2 cha mber, the equi l i bri um between Hgb bi ndi ng O2 a nd CO i s cha nged a nd the hi gh-pres s ure O2 di s pl a ces the CO mol ecul e from the s i xth coordi na ti on pos i ti on of the Hgb mol ecul e.
Figure 14-4. A–B. Binding of Oxygen (O2 ) to Hemoglobin (Hgb) Molecule. (A) Bi ndi ng of O2 by the Fe 2+ a toms ’ fi fth (5) coordi na ti on s i te hi s ti di ne wi th a ddi ti ona l regul a ti on by the s i xth coordi na te s i te (6) hi s ti di ne. Coordi na ti on s i tes 1–4 a re s ymbol i ca l l y i ndi ca ted i n the pl a ne of the porphyri n ri ng. (B) When the heme mol ecul e’s s i xth coordi na ti on s i te i s empty, the i ron mol ecul e projects outwa rd from the center of the protoporphyri n ri ng, ca us i ng the mol ecul e to a s s ume a domed forma ti on. Upon O2 bi ndi ng, the heme mol ecul e a s s umes a pl a na r conforma ti on (ri ght). [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
DISEASES ASSOCIATED WITH INADEQUATE SYNTHESIS OF HEMOGLOBIN COMPONENTS Severa l di s ea s e s ta tes a re a s s oci a ted wi th the i na dequa te s ynthes i s of a ny of the fi na l Hgb components . Of the four components of Hgb (the protoporphyrin ring, α-gl obi n, β-gl obi n, a nd i ron), cel l s mus t s ynthes i ze a l l but i ron, whi ch i t a bs orbs from the i ntes ti na l tra ct. Thes e di s ea s es a re s umma ri zed i n Ta bl e 14-1.
TABLE 14-1. Summa ry of Hemogl obi n Di s ea s es
OXYGEN BINDING TENSE AND RELAXED HGB Ea ch Hgb uni t i s i n equi l i bri um between two di fferent s tructura l s ta tes : tense (T) or relaxed (R) (Fi gure 14-5). Hgb i n the T s ta te ha s a l ow a ffi ni ty for O2 a nd i s more l i kel y to s urrender O2 mol ecul es i t i s hol di ng. Hgb i n the “R” s ta te ha s a n a pproxi ma tel y 100-fol d i ncrea s e i n a ffi ni ty for O2 a s compa red wi th the T s ta te; R s ta te Hgb i s l es s l i kel y to s urrender O2 to ti s s ues . A muta ti on i n a gl obi n gene or a s ma l l mol ecul e tha t s ta bi l i zes the T s ta te (or des ta bi l i zes the R s ta te) wi l l res ul t i n a hi gher proporti on of O2 -bi ndi ng uni ts bei ng i n the T s ta te. An overa l l s hi ft to the ri ght i n the O2 di s s oci a ti on curve wi l l occur (l ower a ffi ni ty for O2 ) (Fi gure 14-6). Convers el y, muta ti ons or s ma l l mol ecul es tha t des ta bi l i ze the T s ta te (or s ta bi l i ze the R s ta te) wi l l res ul t i n a hi gher proporti on of O2 -bi ndi ng uni ts bei ng i n the R s ta te. An overa l l s hi ft i n the O2 di s s oci a ti on curve to the l eft (hi gher a ffi ni ty) wi l l occur.
Figure 14-5. Hemoglobin Binding of Oxygen (O2 ) Causes a Change in the Shape of the Hemoglobin (Hgb) Molecule. Hgb mol ecul es exi s t i n a n equi l i bri um of deoxy/tens e a nd oxy/rel a xed s ta tes . Deoxy Hgb (l eft) ha s a l ow a ffi ni ty for O2 a nd i s a s s oci a ted wi th the tens e (T) s ta te a nd the rel ea s e of O2 to the ti s s ues . Bi ndi ng of O2 ca us es a n a pproxi ma tel y 15° rota ti ona l cha nge i n the qua terna ry s tructure to form oxy Hgb (ri ght). Oxy Hgb ha s a hi gh a ffi ni ty for O2 a nd i s a s s oci a ted wi th the R s ta te. Oxy Hgb wi l l bi nd O2 more tha n rel ea s e O2 . [Reproduced wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.]
Figure 14-6. The Oxygen (O2 ) –Hemoglobin (Hgb) Dissociation Curve. The O2 –Hgb di s s oci a ti on curve depi cts the s a tura ti on of Hgb wi th O2 a s a functi on of the pa rti a l pres s ure of O2 (p O2 ). p O2 i s a mea s ure of the concentra ti on of O2 i n the ti s s ues . The pO2 of meta bol i ca l l y a cti ve ti s s ues i s a pproxi ma tel y 40 mmHg. At mmHg, the a ffi ni ty for O2 s teepl y decl i nes a nd Hgb s urrenders O2 to the ti s s ues . [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] ALLOSTERIC BINDING OF O2 BY Hgb Ea ch of the four s ubuni ts of Hgb conta i ns one protoporphyri n ri ng, thereby a l l owi ng a tota l of four O2 mol ecul es to bi nd per Hgb mol ecul e. However, O2 bi ndi ng i s not a s i mpl e, l i nea r proces s . The R a nd T s ta tes exi s t i n equi l i bri um wi th ea ch other a nd thi s equi l i bri um i s wha t determi nes the overa l l sigmoid or “S” shape of the O2 di s s oci a ti on curve (Fi gure 14-6). The bi ndi ng of the fi rs t O2 to a s i ngl e Hgb s i te s ets i n moti on a s eri es of conforma ti ona l cha nges of the gl obi n, pol ypepti de cha i ns tha t i ncrea s e the proba bi l i ty tha t the other three s i tes wi l l a dopt
the “R” s ta te (the hi gh a ffi ni ty s ta te). Bi ndi ng of s ubs equent O2 mol ecul es a l s o i ncrea s es the bi ndi ng of the other s i tes (a l though much l es s tha n the bi ndi ng of the fi rs t O2 ). In a ddi ti on, more rel a ti ve R s ta te tha n T s ta te s hi fts the curve to the l eft. Convers el y, more T tha n R wi l l s hi ft the curve to the ri ght. The rol e of Hgb i s to tra ns port O2 from hi gher pa rti a l pres s ure of O2 (pO2 ) envi ronments (s uch a s l ungs ) to l ower pO2 envi ronments (s uch a s ti s s ues ). Beca us e the l i ga nd a ffi ni ty of Hgb i s i ncrea s ed by s ucces s i ve bi ndi ng, O2 i s a positive allosteric effector (Cha pter 5). The S s ha pe of the O2 –Hgb di s s oci a ti on curve depi cts a positive cooperativity rel a ti ons hi p between the percent s a tura ti on of the Hgb mol ecul e a nd pO2 (Fi gure 146). The pos i ti ve coopera ti vi ty of O2 bi ndi ng i s a n es s enti a l cha ra cteri s ti c of Hgb tha t ma kes i t both a n effi ci ent O2 ca rryi ng a nd del i very mol ecul e. Thi s cha ra cteri s ti c a l s o a l l ows more O2 del i very to ti s s ues wi th hi gh meta bol i c dema nds (l ow pO2 ). Corres pondi ngl y, ti s s ues tha t need l es s O2 beca us e of fewer meta bol i c dema nds ha ve hi gher pO2 a nd extra ct, a ppropri a tel y, l es s O2 . REGULATION OF O2 BINDING An i mporta nt concept i n unders ta ndi ng Hgb functi on i s tha t Hgb mus t be a bl e to ca rry out two s eemi ngl y oppos i ng functi ons : (1) to become ful l y s a tura ted wi th O2 i n the l ungs a nd (2) to be a bl e to effi ci entl y unl oa d O2 i n the ti s s ues . That is, O2 binding does not guarantee O2 delivery. In fa ct, i f Hgb bi nds O2 too ti ghtl y, ti s s ues woul d not be a bl e to extra ct O2 from the Hgb mol ecul e, a nd functi ona l l y i t woul d be equi va l ent to there bei ng no O2 a t a l l . It i s the release of O2 from Hgb tha t dri ves ti s s ue oxygena ti on. Negative allosteric regulators s uch a s i ncrea s ed temperature, i ncrea s ed CO2 , a nd decrea s ed pH (i ncrea s ed H + concentra ti on) wi l l ca us e the curve to s hi ft to the ri ght a nd hel p Hgb unl oa d O2 i n the ti s s ue (Fi gure 14-7).
Figure 14-7. Effects of Temperature and pH on Hemoglobin (Hgb) Binding of Oxygen (O2 ). (A) Increa s i ng tempera ture (e.g., 10, 20, 38, a nd 43°C) ca us es reduced bi ndi ng of O2 by Hgb, refl ecti ng reduced a ffi ni ty for O2 (T s ta te > R s ta te). In thi s hi gher tempera ture envi ronment, Hgb wi l l more rea di l y rel ea s e O2 a s noted by the ri ght s hi ft of the curves . (B) Increa s i ng pH (e.g., 7.2, 7.4, a nd 7.6) ca us es i ncrea s ed bi ndi ng of O2 by Hgb. As the curve s hi fts l eft, Hgb ha s a n i ncrea s ed a ffi ni ty for O2 (R s ta te > T s ta te). [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] 2,3-bisphosphoglycerate (2,3-BPG) (a l s o known a s 2,3-di phos phogl ycera te) i s a s ma l l mol ecul e crea ted a s a s i de product of gl ycol ys i s from 1,2BPG (Cha pter 6). It i s pres ent i n the s a me concentra ti on a s Hgb i n the RBC (~2 mM) a nd i s the key mol ecul e i n a s s uri ng tha t Hgb does not bi nd O2 too ti ghtl y (i ns uri ng del i very of O2 to the ti s s ues ). The bi ochemi ca l mecha ni s m of 2,3-BPG a cti on i s tha t i t s peci fi ca l l y fi ts i nto a cl eft pres ent i n the T s ta te, whi ch i s not pres ent i n the R s ta te (Fi gure 14-8A). Thi s s ta bi l i zes the T s ta te through a s eri es of noncova l ent i ntera cti ons i n the gl obi n cha i n, thus s hi fti ng the O2 di s s oci a ti on curve to the ri ght (Fi gure 14-8B). Beca us e 2,3-BPG bi nds a t a s i te s epa ra te from the l i ga nd-bi ndi ng s i te (i n thi s ca s e, the l i ga nd i s O2 ) a nd through tha t bi ndi ng a ffects the a ffi ni ty of l i ga nd (O2 ) bi ndi ng, 2,3-BPG i s a n allosteric effector (Cha pter 5) of Hgb functi on. Aci d–ba s e ba l a nce i n the body i s regul a ted by a n i ntegra ted combi na ti on of mecha ni s ms found i n the l ungs (Cha pter 17), ki dney (Cha pter 18), a nd bl ood (Fi gure 14-9). The regul a ti on of Hgb bi ndi ng of O2 by a ci d–ba s e s ta tus , i n pa rti cul a r H + a nd CO2 (both rega rded a s a ci ds ), i s ca l l ed the Bohr effect. H + di rectl y fa ci l i ta tes the forma ti on of s a l t bri dges tha t s ta bi l i ze the T s ta te (the l ow-a ffi ni ty s ta te). CO2 di rectl y s ta bi l i zes the T s ta te by bi ndi ng to the a mi no termi na l ends of the gl obi n cha i ns to genera te a T s ta te s ta bi l i zi ng carbamate moi ety. In a ddi ti on, CO2 i ndi rectl y s ta bi l i zes the T s ta te when i t i s converted by carbonic anhydrase i nto carbonic acid. Ca rboni c a ci d genera tes extra H +, whi ch then a ct a s des cri bed a bove. Meta bol i s m crea tes H + a nd CO2 , a nd the more dema ndi ng the ti s s ue, the more H + a nd CO2 a re crea ted. When more a ci d i s crea ted (a decrea s e i n pH), thi s ca us es a ri ght-s hi ft i n the O2 –Hgb di s s oci a ti on curve. A ri ght-s hi fted O2 bi ndi ng curve mea ns tha t, for a gi ven pO2 , the a ffi ni ty of Hgb for O2 wi l l be decrea s ed a nd the proba bi l i ty tha t O2 wi l l be del i vered to the ti s s ue wi l l be i ncrea s ed.
Figure 14-8. A–B. Binding of 2,3-BPG to Deoxyhemoglobin. (A) 2,3-BPG (i n red) fi ts i nto a cl eft a nd i ntera cts wi th the a mi no termi na l va l i ne (Va l )’s NH 3 + group pl us the pos i ti vel y cha rged R-group s i decha i ns of l ys i ne (Lys ) a nd hi s ti di ne (Hi s ) on both β-gl obi n cha i ns . Thes e s a me i ntera cti ons do not occur i n the rel a xed (R) s ta te of oxy Hgb. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] (B) Thes e noncova l ent bonds s ta bi l i ze the tens e (T) [l ow oxygen (O2 ) a ffi ni ty] s ta te a nd cha nge the norma l bi ndi ng (bl ue, s ol i d l i ne), promoti ng the rel ea s e of a ny bound O2 mol ecul es to ti s s ues . Thus , i ncrea s i ng BPG (red, dotted l i ne) res ul ts i n a ri ght s hi ft (l ower bi ndi ng of O2 ) a nd decrea s i ng BPG (green l i ne) res ul ts i n a l eft s hi ft (hi gher bi ndi ng of O2 ) of the bi ndi ng curve. 2,3-BPG, 2,3bi s phos phogl ycera te. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Hypoxia, Altitude Sickness, and Acetazolamide: Altitude sickness ca n occur when peopl e move from l ow a l ti tudes to hi gh a l ti tudes , res ul ti ng i n i na dequa te O2 to the peri phera l ti s s ues , es peci a l l y the bra i n. The ma i n s ymptom of a l ti tude s i cknes s i s hea da che a nd na us ea tha t us ua l l y res ol ve wi thout a ny i nterventi on. In certa i n ci rcums ta nces , a l ti tude s i cknes s ca n ca us e l i fe-threa teni ng pulmonary edema (fl ui d fi l l i ng the l ungs , ca us i ng s evere s hortnes s of brea th a nd di ffi cul ty i n brea thi ng) a nd cerebra l edema (s wel l i ng of the bra i n, ca us i ng s evere hea da ches , di s ori enta ti on, confus i on, coma , a nd pos s i bl y dea th). The onl y trea tment i s to move the pa ti ent to l ower el eva ti ons a s ra pi dl y a s pos s i bl e. To prevent the ons et of s ymptoms , acetazolamide, a carbonic anhydrase inhibitor, i s pres cri bed prophyl l a cti ca l l y to thos e prone to a l ti tude s i cknes s . Inhi bi ti ng ca rboni c a nhydra s e before movi ng to hi gher a l ti tude ca us es a n a l ka l i ni za ti on of the uri ne a nd cons equent a ci di fi ca ti on of the bl ood. The i ncrea s ed H + i n the bl ood s hi fts the O2 di s s oci a ti on curve to the ri ght through the Bohr effect a nd i ncrea s es O2 unl oa di ng i n the peri phera l ti s s ues , i ncl udi ng the bra i n.
Figure 14-9. Fate of Carbon Dioxide (CO2 ) in the Red Blood Cell (RBC). Upon enteri ng the RBC, CO2 i s ra pi dl y hydra ted to H 2 CO3 by ca rboni c a nhydra s e. H 2 CO3 i s i n equi l i bri um wi th H + a nd i ts conjuga te ba s e HCO3 –. H + ca n i ntera ct wi th deoxy hemogl obi n, wherea s HCO3 – ca n be tra ns ported outs i de of the cel l vi a ba nd 3, a n HCO3 –/ Cl – a ni on excha nger i n the RBC membra ne (green ci rcl e). In effect, for ea ch CO2 mol ecul e tha t enters the RBC, there i s a n a ddi ti ona l HCO3 – or Cl – i n the cel l . [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] In s umma ry, the rel a ti ve s ta bi l i za ti on of the T s ta te by three fa ctors (H +, CO2 , a nd ca rboni c a ci d) i n the ti s s ue i mmedi a tel y s hi fts the O2 di s s oci a ti on curve to the ri ght l oca l l y, thus i ncrea s i ng the a mount of O2 del i vered. A l ower a ffi ni ty mea ns tha t Hgb wi l l more rea di l y release O2 to ti s s ues wi th hi gh H + (l ow pH) a nd/or hi gh CO2 l evel s . Meta bol i ca l l y a cti ve ti s s ues (e.g., mus cl e cel l s ) produce hi gh a mounts of thes e mol ecul es . By decrea s i ng the a ffi ni ty of Hgb for O2 , the Bohr effect i ns ures the a bi l i ty of Hgb to s urrender O2 to the ti s s ues where i t i s needed mos t. In a ddi ti on, meta bol i ca l l y a cti ve ti s s ues genera te hea t, ra i s i ng the tempera ture of the ti s s ue l oca l l y. Thi s i ncrea s e i n tempera ture a l s o decrea s es the a ffi ni ty of Hgb for O2 a nd fa ci l i ta tes the unl oa di ng of O2 i n ti s s ues tha t a re genera ti ng hea t through thei r meta bol i c a cti vi ti es . When the RBCs return to the norma l pH a nd tempera ture of the l ung ca pi l l a ry bed, the exces s CO2 i s exha l ed (Cha pter 17) a nd the norma l equi l i bri um between the R a nd T s ta tes i s res tored.
How Does the Fetus Get Enough O2 ? The s i gmoi d s ha pe of the O2 di s s oci a ti on curve expl a i ns how Hgb ca n bi nd O2 i n the l ungs a nd del i ver O2 to the ti s s ues . But how does the devel opi ng fetus , a peri od of hi gh ma cromol ecul a r s ynthes i s , cel l growth, a nd cons equent O2 requi rement, get enough O2 to grow? The fetus does not brea the a i r. Ins tea d, i t mus t extra ct O2 from the mother’s RBCs a cros s the placental vasculature a nd then del i ver s uffi ci ent O2 to i ts own devel opi ng ti s s ues . The fetus s ol ves thi s probl em by us i ng a di fferent s et of gl obi n genes a s pa rt of i ts Hgb mol ecul e. In contra s t to the mother, whos e Hgb mol ecul e cons i s ts of two α- a nd two β-gl obi n cha i ns crea ti ng HgbA, the fetus us es two α- a nd two β-gl obi n cha i ns crea ti ng fetal Hgb (HgbF). HgbF ha s a l ower a ffi ni ty for 2,3-BPG tha n HgbA beca us e of a s eri ne a t pos i ti on 143 i n the γ-gl obi n gene i ns tea d of hi s ti di ne (s ee Fi gure 14-8A for hi s ti di ne pos i ti on). Thi s decrea s ed a ffi ni ty for 2,3-BPG res ul ts i n HgbF ha vi ng a n i ncrea s ed a ffi ni ty for O2 a nd a s hi ft i n the O2 di s s oci a ti on curve to the l eft. Thus , HgbF i s a bl e to extra ct O2 from HgbA i n the pl a centa a nd then del i ver the O2 to the feta l ti s s ues . The s hi ft to the l eft i s preci s e, however, beca us e a l though i t a l l ows the feta l Hgb to “s tea l ” O2 from the ma terna l Hgb, i t i s not s o s i gni fi ca nt tha t the feta l ti s s ues ca nnot remove the O2 for thei r own us e.
PHYSIOLOGIC RESPONSE TO INADEQUATE O2 DELIVERY The i na dequa te del i very of O2 to ti s s ues (hypoxia) i nduces a compl ex va ri ety of norma l a nd pa thophys i ol ogi c compens a tory mecha ni s ms . Cel l s mus t turn to a na erobi c res pi ra ti on (Cha pter 6), whi ch produces lactic acid a nd l owers the pH. As des cri bed a bove, a l ow pH ha s a n i mmedi a te effect on O2 del i very a nd extra cti on through the Bohr effect. The next l evel of compens a ti on to decrea s ed O2 del i very i s to i ncrea s e the concentra ti on of the a l l os teri c mol ecul e, 2,3-BPG. The pres ence of 2,3-BPG l owers the O2 a ffi ni ty of Hgb, res ul ti ng i n a ri ghtwa rd s hi ft of the O2 bi ndi ng curve a nd i ncrea s ed O2 del i very to ti s s ues i n l ow O2 condi ti ons . The compens a tory i ncrea s e i n 2,3-BPG concentra ti on ta kes a bout 24 hrs to occur. A s l ow compens a tory mecha ni s m of i na dequa te O2 del i very i s vi a the producti on a nd s ecreti on of erythropoi eti n (Epo) to crea te more RBCs . Ki dney cel l s ha ve hi gh O2 dema nds beca us e they need a bunda nt a denos i ne tri phos pha te to power mul ti pl e cha nnel s a nd pumps tha t regul a te el ectrol yte ba l a nce. Thus , thes e cel l s a re i dea l l y s ui ted to be s ens ors of O2 del i very. When there i s i na dequa te del i very of O2 to ki dney cel l s , a tra ns cri pti on fa ctor ca l l ed hypoxia-inducible factor i s not degra ded a nd i s a bl e to a cti va te the tra ns cri pti on of mul ti pl e genes , i ncl udi ng the bl ood hormone Epo, l ea di ng to a n i ncrea s ed l evel of Epo i n the bl ood s trea m. Epo mi gra tes through the bl ood to the bone ma rrow (where RBCs a re ma de), bi nds to the Epo receptor, a nd a cti va tes a Janus kinase (JAK) and Signal Transducer and Activator of Transcription (STAT) i ntra cel l ul a r s i gna l i ng ca s ca de (Cha pter 8). Acti va ti on of thi s ca s ca de does not s ti mul a te RBC precurs or prol i fera ti on; ra ther, the JAK/STAT pa thwa y protects RBC precurs ors from thei r norma l l y progra mmed cel l dea th (a poptos i s ). Increa s ed RBC s urvi va l a ccounts for a n i ncrea s e i n hema tocri t (the a mount of RBCs i n the bl ood, s ee Appendi x II). Ul ti ma tel y, the i ncrea s ed numbers of ma ture RBCs a ugment the O2 ca rryi ng ca pa ci ty of the bl ood. It ta kes a pproxi ma tel y 1 week for a n RBC to be ma de a nd, thus , the ful l effect of thi s s l ow res pons e requi res a bout a month to ma ni fes t i ts el f; however, benefi ci a l effects ca n be found i n jus t a few da ys . Pa ti ents wi th ki dney di s ea s e do not s ecrete enough Epo to ma i nta i n a n a dequa te qua nti ty of RBCs a nd, thus , a re gi ven s uppl ementa l Epo a s trea tment to prevent s evere a nemi a .
SICKLE CELL DISEASE (SCD) Norma l RBCs ha ve a bi conca ve s ha pe tha t ma xi mi zes thei r a bi l i ty to pa ck Hgb a nd del i ver O2 . RBCs a re a l s o fl exi bl e a nd deforma bl e s o tha t they ca n s queeze through the ca pi l l a ri es . SCD i s ca us ed by a reces s i ve, s i ngl e-nucl eoti de muta ti on i n the β-gl obi n gene of Hgb, i nheri ted from both pa rents (i .e., homozygous ). Thi s muta ti on res ul ts i n the s ubs ti tuti on of the norma l gl uta mi c a ci d (nega ti ve cha rge) wi th a va l i ne (hydrophobi c) a t a mi no a ci d i n pos i ti on 6. SCD i s a s s oci a ted wi th ea rl y morta l i ty (the a vera ge l i fes pa n i s 40–45 yea rs i n the Uni ted Sta tes a nd much s horter i n devel opi ng na ti ons ) a nd l i fel ong medi ca l probl ems . Si ckl e RBCs bi nd O2 norma l l y (Fi gure 14-10A, l eft pa nel ), a nd the oxygena ted s i ckl e RBCs ha ve a norma l bi conca ve s ha pe. However, when the s i ckl e Hgb unl oa ds i ts O2 , the norma l conforma ti ona l cha nge expos es the va l i ne a t pos i ti on 6 to the s urfa ce crea ti ng a “hydrophobi c (s ti cky) pa tch” (Fi gure 14-10A, mi ddl e pa nel ). The hydrophobi c pa tch on one deoxygena ted Hgb mol ecul e ca n i ntera ct wi th the hydrophobi c pa tch on a s econd Hgb mol ecul e (Fi gure 14-10A, ri ght pa nel ), crea ti ng s ti ff Hgb pol ymers (Fi gure 14-10B). Thes e i nterna l pol ymers ca us e the RBC to s ti ffen a nd a dopt a bnorma l s ha pes , i ncl udi ng a cres cent or “s i ckl e” s ha pe tha t gi ves the di s ea s e i ts na me (Fi gure 14-10C). Thes e s ti ffened RBCs l os e thei r pl i a bi l i ty a nd ca nnot ea s i l y s queeze through norma l ca pi l l a ri es , thereby formi ng bl ocka ges . Dehydra ti on ca n wors en thi s effect beca us e s ma l l cha nges i n the hydra ti on s ta tus of the RBC ca n ha ve dra ma ti c effects on s i ckl e cel l Hgb pol ymeri za ti on. When Hgb rebi nds O2 , the res ul ti ng conforma ti ona l cha nge hi des the hydrophobi c pa tch, a l l owi ng s epa ra ti on i nto monomers a nd res umpti on of a norma l , bi conca ve form. After mul ti pl e rounds of exces s i ve Hgb pol ymeri za ti on (s i ckl i ng) a nd recovery, erythrocyte membra nes l os e thei r el a s ti ci ty, a dopt a perma nentl y s ti ffened form, a nd a re ul ti ma tel y des troyed a nd broken down (hemol ys i s ).
Figure 14-10. Sickle Cell Disease. (A) A s i ngl e nucl ei c a ci d cha nge i n the β-gl obi n s ubuni t gene (oxy HbA, l eft pa nel ) ca us es a s ubs ti tuti on of the a mi no a ci d va l i ne for the norma l gl uta ma te, crea ti ng a hydrophobi c “s ti cky pa tch” on the exteri or (Oxy HbS, mi ddl e pa nel ). Rel ea s e of O2 from the s i ckl e cel l hemogl obi n (Hgb) (deoxy HbS, ri ght pa nel ) crea tes a compl i menta ry conforma ti ona l cha nge on the α-s ubuni t tha t ca n bi nd to the s ti cky pa tch a nd l ea d to (B) pol ymeri za ti on. [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] (C) Pol ymers of s i ckl e Hgb deform the na tura l bi conca ve s ha pe of the red bl ood cel l i nto a s i ckl e s ha pe. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] Inheri ta nce of the muta ti on from onl y one pa rent (heterozygous ) i s termed sickle cell trait. Pa ti ents wi th s i ckl e cel l tra i t a re typi ca l l y a s ymptoma ti c, unl es s pl a ced under extreme condi ti ons (l ow O2 or s evere dehydra ti on). Si ckl e cel l tra i t a l s o provi des rel a ti ve res i s ta nce to di s ea s es s uch a s ma l a ri a a nd i s bel i eved to ha ve been a pos i ti ve evol uti ona ry tra i t i n a rea s of the worl d where ma l a ri a i s endemi c. For exa mpl e, i n Sub-Sa ha ra n Afri ca , there i s s uch s evere s el ecti ve pres s ure tha t nucl eoti de cha nge ha s been s el ected for mul ti pl e ti mes . Other l es s common va ri a nts of SCD a l l ha ve a t l ea s t one muta ted copy of the β-gl obi n gene. SCD and Hydroxyurea: SCD l ea ds to a number of l i fel ong, cl i ni ca l cons equences due to a l tera ti ons i n the bi ophys i ca l properti es of the RBC. Veno-occl us i ve cri s es (a l s o known a s “pa i nful cri s es ”) res ul t from obs tructi ons of bl ood ves s el s due to the i na bi l i ty of tough, i nfl exi bl e RBCs to s queeze through ca pi l l a ri es . Thi s obs tructi on l ea ds to orga n i s chemi a (di mi ni s hed bl ood fl ow) a nd ma y res ul t i n i rrevers i bl e da ma ge a nd necros i s of vi ta l bra i n regi ons (s troke) a nd chroni c i njury to orga ns s uch a s the s pl een a nd ki dney. The l i fes pa n of a s i ckl e RBC i s onl y a bout 10 da ys , s i gni fi ca ntl y s horter tha n the a pproxi ma tel y 120-da y l i fes pa n of a norma l RBC, beca us e they a re des troyed by the s pl een a nd l i ver beca us e of thei r a bnorma l s ha pes (hemol ys i s ). Thi s ma rkedl y s hortened l i fes pa n l ea ds to s i gni fi ca nt a nemi a (the Hgb concentra ti on i n the bl ood i s ~50% of norma l ). The bone ma rrow i s una bl e to compens a te for the i ncrea s ed RBC des tructi on (hemol ys i s ). The pers i s tent s evere a nemi a a nd the pers i s tent mi crova s cul a ture bl ocka ges ca n ul ti ma tel y l ea d to chroni c endorga n da ma ge i n es s enti a l l y every orga n of the body over ti me. Increa s ed ri s k of i nfecti ons by enca ps ul a ted orga ni s ms s uch a s Streptococcus (ca us i ng s eps i s ) or Salmonella (ca us i ng os teomyel i ti s ) a re a l s o a common probl em for SCD pa ti ents . One form of trea tment i s the us e of hydroxyurea, whi ch ca us es a n i ncrea s e i n the expres s i on of γ-gl obi n, a va ri a nt gl obi n gene tha t ca n repl a ce β-gl obi n i n the Hgb mol ecul e. Hgb mol ecul es tha t conta i n the γ-gl obi n gene do not ha ve a va l i ne a t pos i ti on 6 a nd they bl ock the forma ti on of Hgb pol ymers , reduci ng the RBC deformi ty probl ems .
IRON Bl ood us es ca rri er protei ns to tra ns fer es s enti a l nutri ents a s pa rt of ea ch pers on’s meta bol i s m. One i mporta nt exa mpl e i s i ron tha t i s i nvol ved i n a va s t a rra y of i mporta nt bi ol ogi c rea cti ons : 1. Bi nds O2 a s pa rt of Hgb mol ecul e. An a dul t huma n ha s a pproxi ma tel y 4 g of i ron, of whi ch a bout two-thi rds i s empl oyed i n the O2 -ca rryi ng rol e of Hgb. 2. Medi a tes a wi de va ri ety of oxi da ti ve–reducti ve rea cti ons by s ervi ng a s a n es s enti a l cofa ctor for ma ny protei ns vi a oxi da ti on between ferri c (Fe 3+) a nd reduced Fe 2+ s ta tes . 3. Iron i s i mporta nt for ma ny mi croorga ni s ms . Keepi ng i ron s eques tered from thes e i nva ders i s a n i mporta nt pa rt of the i mmune s ys tem. IRON METABOLISM In mos t wel l -rounded Wes tern di ets , mea ts a nd green, l ea fy vegeta bl es provi de a dequa te i ron to prevent i ron defi ci ency. Certa i n i ron-poor di ets , s uch a s vega n di ets , however, ca n res ul t i n i ron defi ci ency i f not s uppl emented. Once i ron i s i nges ted, i t i s converted from the Fe 3+ s ta te to the Fe 2+ s ta te by i ntes ti na l ferric reductase. Fe 2+, not Fe 3+, i s tra ns ported i nto the i ntes ti na l epi thel i a l cel l through the divalent metal transporter (DMT1) protei n. Fe 2+ i s tra ns ported out of the i ntes ti na l epi thel i a l cel l s i nto bl ood through a s econd tra ns porter, ferroportin (Fi gure 14-11).
Figure 14-11. Overview of Iron Transport. Intes ti na l l umen ferri c (Fe 3+) reducta s e reduces Fe 3+ to Fe 2+. Fe 2+ i s tra ns ported from the l umen i nto the i ntes ti na l epi thel i a l cel l through heme tra ns porter (HT), endos omes , a nd/or di va l ent meta l tra ns porter 1 (DMT1). Fe 2+ ca n be converted ba ck to Fe 3+ a nd bound to tra ns ferri n wi thi n the i ntes ti na l cel l or ca n be tra ns ported i nto the bl ood by ferroporti n (FP) a nd hepha es ti n (HP). The Fe 2+ oxi di zed to Fe 3+, whi ch bi nds to pl a s ma tra ns ferri n, i s ca rri ed through the ci rcul a ti on to the ti s s ues . [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] TRANSFERRIN 2+
3+
Once i n the bl ood, the Fe i s qui ckl y oxi di zed ba ck to Fe a nd bound by the i ron ca rri er protei n, transferrin. The a ffi ni ty of tra ns ferri n for Fe 3+ a t pH 7.4 i s 10–23 , whi ch mea ns tha t tra ns ferri n wi l l bi nd Fe 3+ even when i ts concentra ti on i s 10–23 (10 yoctomol a r or 0.01 zeptomol a r). Thi s a ffi ni ty s ugges ts tha t i n the enti re 5 l bl ood vol ume of a n a dul t, there woul d be onl y a pproxi ma tel y fi ve free mol ecul es of Fe 3+ a t a ti me. Thi s exceedi ngl y hi gh a ffi ni ty i s the bi ochemi ca l mecha ni s m tha t the huma n body ha s a da pted to prevent a ny free i ron from exi s ti ng i n the bl ood s trea m (Fi gure 14-12).
Figure 14-12. Iron Transportation by Transferrin. Tra ns ferri n tra ns ports i ron i n the bl ood to the bone ma rrow to ma ke hemogl obi n a nd red bl ood cel l s (erythropoi es i s ). Tra ns ferri n a l s o ca rri es i ron to the l i ver a nd hea rt for s tora ge i n ferri ti n mol ecul es , a s wel l a s to other pa rts of the body for va ri ous enzyma ti c a nd other functi ons . The i ron bound tra ns ferri n ci rcul a tes through the bl ood s trea m unti l i t bi nds to a tra ns ferri n receptor on the s urfa ce of a cel l . Cel l s tha t expres s tra ns ferri n receptors ha ve hi gh i ron dema nds . Thes e i ncl ude devel opi ng RBCs , di vi di ng cel l s , a nd mi croorga ni s ms . The tra ns ferri n– i ron–tra ns ferri n–receptor compl ex i s endocytos ed through the cl a s s i c cl a thri n-coa ted pi t pa thwa y. Fol l owi ng endocytos i s , pH of the endocyti c ves i cl e i s reduced to 5, l i bera ti ng i ron from i ts ca rri er protei n a nd ma ki ng i ron a va i l a bl e for bi ol ogi c rea cti ons . FERRITIN
Al though Hgb i s the mos t a bunda nt protei n tha t us es i ron, ferri ti n i s the mos t i mporta nt protei n for i ron s tora ge. Ferri ti n cons i s ts of a 24-uni t mul ti mer of heavy (H) a nd light (L) cha i ns tha t crea te a hol l ow s hel l . Iron i s i mported i nto the s hel l a s Fe 2+ but i s converted to Fe 3+ wi thi n the ferri ti n core by the H cha i n. A ful l y l oa ded mol ecul e of ferri ti n conta i ns 4500 a toms of i ron. It i s for thi s rea s on tha t the ferri ti n mol ecul e i s meta phori ca l l y referred to a s a “ba g of rus t.” A pa thol ogi c form of i ron depos i ti on i s ca l l ed hemosiderin. Thi s i s a nons tructured congl omera te of i ntra cel l ul a r i ron a nd i s often a pa thol ogi c cons equence of prol onged i nfl a mma ti on. Prol onged depos i ti on of hemos i deri n ca n ca us e fi bros i s (s ca rri ng) of ti s s ues . The i mporta nce of i ron i s hi ghl i ghted by the fa ct tha t the huma n body ha s no na tura l wa y to excrete i t. Unl i ke other i ons , s uch a s s odi um, pota s s i um, a nd ca l ci um, ki dneys do not excrete exces s i ron i n the uri ne. Indeed, the phys i ol ogi ca l defa ul t i s to cons erve i ron. Iron defi ci ency i s much ea s i er to trea t tha n i ron overl oa d. Accumul a ted i ron i s toxi c to a va ri ety of ti s s ues , es peci a l l y the hea rt a nd l i ver. Infus i on of a n i ron chel a tor, s uch a s deferoxamine or deferasirox, provi des the onl y mea ns of reduci ng s ome of the i ron a ccumul a ti on. Chel a ti on works by the drugbi ndi ng free i ron i n the bl ood fol l owed by excreti on of the i ron–drug compl ex. The body does l os e a s ma l l a mount of i ron ea ch da y by the s l oughi ng of s ki n a nd epi thel i a l cel l s a nd i n women through thei r mens trua l fl ow. However, the s a me rea cti vi ty tha t ma kes i t a va l ua bl e cofa ctor for protei ns a l s o mea ns tha t free i ron ca n be da ngerous beca us e i t ca n ea s i l y genera te da ma gi ng O2 free ra di ca l s . Thus , the body ha s a powerful s ys tem to control the meta bol i s m of thi s preci ous a nd peri l ous meta l . REGULATION OF IRON AVAILABILITY BY HEPCIDIN Al though the huma n body ha s no na tura l wa y of excreti ng exces s i ron, i t i s a bl e to regul a te the upta ke a nd a va i l a bi l i ty of i ron through the protei n hepcidin. Hepci di n i s s ynthes i zed a s a n 84-a mi no a ci d precurs or, whi ch i s then proces s ed to the 25-a mi no a ci d a cti ve form. Hepci di n a cts a s a nega ti ve regul a tor of ferroportin, bl ocki ng the a bi l i ty of cel l s to export i ron from the cytopl a s m i nto the bl ood. In the i ntes ti na l cel l , thi s i nhi bi ti on res ul ts i n decrea s ed i ron a bs orpti on from the di et. In reticuloendothelial cells, the pri ma ry s tora ge depot for i ron, thi s res ul ts i n a decrea s ed a bi l i ty of i ron to be mobi l i zed from s tora ge pool s to cel l s tha t need i t. Hepci di n i s ma de i n the l i ver, a nd the l evel s of hepci di n i n the bl ood a re control l ed by a va ri ety of di fferent s ti mul i , i ncl udi ng the tota l i ron s tores i n the body, the erythropoi eti c dema nds of ma ki ng RBCs , hypoxi a , a nd i nfl a mma ti on. When i ron s tores a re hi gh, the l evel of hepci di n i s i ncrea s ed a nd the a mount of i ron a bs orbed i s decrea s ed. When the dema nd for ma ki ng RBCs i s i ncrea s ed, for exa mpl e, i n res pons e to a cute bl ood l os s , the l evel of hepci di n decrea s es a nd the a mount of bi oa va i l a bl e i ron i s i ncrea s ed. Si mi l a rl y, when ti s s ues do not recei ve s uffi ci ent O2 (hypoxi a ), i t s i gna l s tha t more RBCs a re needed, a nd hepci di n producti on i s decrea s ed. Fi na l l y, i nfl a mma ti on, s uch a s s i gna l ed through the i nfl a mma tory cytoki ne i nterl euki n-6 (IL-6), ca us es a n i ncrea s e i n hepci di n producti on (Fi gure 14-13). Thi s i nfl a mma tory regul a ti on i s thought to refl ect a hos t defens e mecha ni s m beca us e i t s eques ters i ron a wa y from i nfecti ous orga ni s ms , l i mi ti ng thei r growth. If the i nfl a mma tory s ta te pers i s ts , however, hepci di n s eques ters i ron a wa y from both the mi croorga ni s m a nd the huma n cel l s . There i s i ns uffi ci ent i ron to s upport the dema nds for RBC producti on, a nd a s ta te of anemia of inflammation or anemia of chronic disease devel ops .
Figure 14-13. Action of Hepcidin in an Inflammatory Response. Res pons e to i nfl a mma ti on (e.g., a n i nfecti on) l ea ds to a cti va ti on of monocytes a nd l ymphocytes , often by IL-6. The res ul ti ng i mmunol ogi ca l s i gna l s el i ci t res pons es from s evera l orga ns , i ncl udi ng i ncrea s ed hepci di n s ynthes i s i n the l i ver, i nhi bi ti on of erythropoi eti n from the ki dneys , i nhi bi ti on of red bl ood cel l (RBC) producti on i n the bone ma rrow, a nd i ncrea s ed ma cropha ge pha gocytos i s of RBCs . Increa s ed hepci di n a l s o decrea s es i ron a bs orpti on from di eta ry s ources . Al l of thes e effects decrea s e the a va i l a bi l i ty of i ron for i nfecti ve orga ni s ms , whi ch a i ds i n thei r des tructi on a nd cl ea ra nce. IL-6, i nterl euki n-6. Iron Deficiency Anemia: Iron deficiency anemia i s the mos t common nutri ti ona l di s order i n the worl d, bel i eved to a ffect 1 bi l l i on peopl e. In chi l dren, i n the devel oped worl d, the mos t common ca us e of i ron defi ci ency a nemi a i s the exces s cons umpti on of cow’s milk. Exces s cow’s mi l k ca n ca us e i nfl a mma ti on da ma gi ng the i ntes ti na l l i ni ng, res ul ti ng i n bl ood l os s a s wel l a s a di mi ni s hed ca pa ci ty to a bs orb i ron. The fi rs t ma ni fes ta ti on of i ron defi ci ency i s a n a nemi a tha t s tems from a n i na dequa te i ron s uppl y to s us ta i n erythropoi es i s . The s ymptoms of i ron defi ci ency a nemi a i ncl ude pa l l or, wea knes s , a nd l etha rgy. Severe a nd prol onged i ron defi ci ency ca n res ul t i n neurops ychol ogi ca l probl ems . The di a gnos i s of i ron defi ci ency i s us ua l l y ma de by l a bora tory s tudi es tha t demons tra te a mi crocyti c a nemi a (s ma l l er, pa l e RBCs ), refl ecti ve of poor Hgb producti on. Bl ood s tudi es a l s o s how l ow ferritin l evel a nd l ow transferrin saturation (onl y 10% conta i n i ron a s compa red wi th the norma l 30–40%). Beca us e ferri ti n i s a n a cute pha s e rea cta nt tha t i ncrea s es i n ti mes of s tres s a nd i l l nes s , s ometi mes i t ca n be pa ra doxi ca l l y hi gh even i n i ron-defi ci ent s ta tes . The trea tment for i ron defi ci ency i s to gi ve i ron. The mos t bi oa va i l a bl e di eta ry i ron i s i n red mea t, but often the pa ti ent i s una bl e or unwi l l i ng to purs ue thi s method. In thi s s i tua ti on, ora l el ementa l i ron i s gi ven. Beca us e i ron i s tra ns ported by DMT1 a s Fe 2+, not Fe 3+, s ome phys i ci a ns wi l l a dvi s e thei r pa ti ents to s i mul ta neous l y dri nk ora nge jui ce or ta ke ascorbic acid (vitamin C). As corbi c a ci d reduces Fe 3+ i ron to the Fe 2+ s ta te, fa ci l i ta ti ng the a bs orpti on of el ementa l i ron. In trea ti ng i ron defi ci ency a nemi a i n chi l dren, ca us ed by exces s i ve mi l k cons umpti on, i t i s i mporta nt to both a dmi ni s ter el ementa l i ron a nd dra ma ti ca l l y decrea s e mi l k cons umpti on. In ra re ca s es of a dul t a nd
chi l d a nemi a , s uch a s poor compl i a nce or a na tomi c or geneti c defects i n i ron a bs orpti on, i ron i s a dmi ni s tered i ntra venous l y. Suppl ementa l i ron i s gi ven unti l the mi crocyti c a nemi a i s res ol ved a nd norma l l evel s of ferri ti n a nd tra ns ferri n s a tura ti on a re a chi eved.
CLOTTING Bl ood ves s el s a re s ubjected to occa s i ona l tra uma (e.g., cuts a nd l a cera ti ons ) a nd a l s o to routi ne mi crotra uma s by s eemi ngl y beni gn a cti vi ti es s uch a s runni ng or knocki ng on a door. Wi thout cl otti ng, thes e events woul d ca us e hemorrhage (bleeding), whi ch i s the conti nuous fl ow of RBCs from the i ntra va s cul a r s pa ce i nto the extra va s cul a r s pa ce a nd ti s s ues . Runni ng woul d res ul t i n bl eedi ng i nto the knee joint (hemarthroses), a nd knocki ng on a door woul d l ea ve ecchymoses (brui s es ) over the knuckl es . To prevent hemorrha ge, i t i s es s enti a l to form cl ots a t tra uma s i tes i n the ves s el s . The cl ots mus t be formed qui ckl y to mi ni mi ze bl ood l os s . Cl otti ng a l s o needs to occur i n a control l ed a nd l oca l i zed fa s hi on, nei ther occl udi ng the ves s el nor ca us i ng cl ots to form a t remote s i tes tha t ha ve no need for cl ot forma ti on. The key pri nci pl es of cl otti ng a re a s fol l ows : 1. Cl ots cons i s t of pl a tel ets , the protei n fibrin, a nd RBCs . Fi bri n a s s umes a netl i ke ma tri x tha t s pa ns the ful l thi cknes s of the cl ot, ens na ri ng RBCs throughout the thi cknes s of the mes h-work. Ens na ri ng the RBCs i n the fi bri n network crea tes the cl ot tha t s tops hemorrha ge. 2. The forma ti on of a cl ot occurs i n two pha s es : a. Platelet plug formation—Temporary repa i r unti l a proper cl ot ca n form. b. Clot formation—Longer term a nd s tronger tha n a pl a tel et pl ug. Cl ot forma ti on s tops hemorrha ge l ong enough for ti s s ues to repa i r. PLATELET PLUG FORMATION CONSISTS OF ADHESION, AGGREGATION, AND ACTIVATION OF PLATELETS Da ma ge to the endothel i a l l i ni ng of bl ood ves s el s expos es col l a gen on the ba s ement membra ne (Fi gure 14-14A). Da ma ge a l s o expos es von Willebrand Factor (vWF), norma l l y l oca ted between the endothel i um a nd the ba s ement membra ne (Fi gure 14-14B).
Figure 14-14. A. Formation of the Platelet Plug. Pl a tel et pl ug forma ti on res ul ts from the i ni ti a l res pons e of pl a tel ets to a s i te of bl ood ves s el wa l l i njury. Control of forma ti on of the i ni ti a l pl ug i s vi a ni trous oxi de a nd pros ta cycl i ns , a s noted i n the text. NO, ni tri c oxi de. [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] B. Mechanism of Clot Formation by Platelets. Fol l owi ng the i ni ti a l forma ti on of a pl a tel et pl ug, pa rtl y a i ded by expos ed von Wi l l ebra nd fa ctor (vWF) a t the i njury s i te (s ee the fi gure), pl a tel ets conti nue to a ggrega te vi a fi bri n l i nki ng of thei r Gp IIb/IIIa receptors . Adhes i ons by thes e receptors l ea d to a cti va ti on of the
pl a tel ets a nd degra nul a ti on, whi ch rel ea s es a denos i ne di phos pha te (ADP), thromboxa ne A2 (TXA2 ), a nd other fa ctors , a l l of whi ch further i ncrea s e the forma ti on of i ni ti a l pl a tel et pl ug/cl ot. As noted i n Fi gure 14-14A, ni tri c oxi de a nd a nti pl a tel et pros ta cycl i n (PGI 2 ) l i mi t pl ug a nd cl ot forma ti on beyond the i njury. The cl otti ng ca s ca de noted i n the fi gure i s des cri bed further bel ow. EC, endothel i a l cel l . [Reproduced wi th permi s s i on from Si mmons ML a nd Decker JW: Br Hea rt J, McGra w-Hi l l , 1995.] 1. Adhesion of pl a tel ets to the endothel i um: Col l a gen i s hi ghl y thrombogeni c a nd pl a tel ets wi l l a dhere to i t. vWF, conta i ned wi thi n pl a tel ets a nd endothel i a l cel l s , enha nces pl a tel et a dhes i on by i ncrea s i ng the number of l i nks between pl a tel ets a nd col l a gen fi bri l s . 2. Aggregation of pl a tel ets to one a nother: Pl a tel ets s ti ck to one a nother by fi bri n l i nki ng the glycoprotein IIb/IIIa (Gp IIb/IIIa) of one pl a tel et to the Gp IIb/IIIa of a nother. 3. Activation: The a dhes i on s tep ca us es pl a tel ets to rel ea s e thei r s tored gra nul es (activation) tha t conta i n the fol l owi ng, whi ch a l l a ct to enha nce pl ug forma ti on a nd l i mi t bl eedi ng: a. Adenosine diphosphate (ADP)—i ncrea s es expres s i on of Gp IIb/IIIa on pl a tel ets a nd ca us es them to s wel l . b. Prostaglandin Thromboxane A2 (TXA2 ) (Cha pter 3) a cti va tes a G-coupl ed protei n receptor (G q). TXA2 i ncrea s es va s ocons tri cti on (to decrea s e bl ood fl ow a nd l i mi t hemorrha ge) a nd the a ggrega ti on of pl a tel ets . c. PLA2 i ncrea s es pl a tel et a dhes i on to fi bri n vi a Gp IIb/IIIa . d. ADP a nd Ca 2+ l ea d to a ddi ti ona l fi bri n depos i ti on. THE CLOTTING CASCADE The Cl otti ng ca s ca de cons i s ts of a s eri es of ordered enzyma ti c s teps whos e end res ul t i s the forma ti on of a s turdy cl ot. There a re two l i mbs of the cl otti ng ca s ca de: the extrinsic pathway a nd the intrinsic pathway (Fi gure 14-15). Factors II–XII (na med i n order of di s covery, not i n order of a cti va ti on) a re i na cti ve serine proteases tha t a re s equenti a l l y a cti va ted by enzyma ti c cl ea va ge of the s eri ne protea s e tha t i mmedi a tel y precedes i t (the “a ” fol l owi ng a fa ctor number des i gna tes the a cti ve form). Aspirin (Acetylsalicylic Acid) Prevents Platelet Aggregation: Nonsteroidal anti-inflammatory drugs (NSAIDs) a ct by i na cti va ti ng or i nhi bi ti ng cyclooxygenase (COX) a cti vi ty (Cha pter 3). As pi ri n noncompeti ti vel y a nd i rrevers i bl y i nhi bi ts the COX enzyme vi a a cetyl a ti on. Other NSAIDs s uch a s i ndometha ci n a nd i buprofen a re revers i bl e, competi ti ve i nhi bi tors of COX. In genera l , NSAIDs i nhi bi t the s ynthes i s of mol ecul es res pons i bl e for va ri ous a s pects of pa i n a nd i nfl a mma ti on: pros ta gl a ndi ns , pros ta cycl i ns , a nd thromboxa nes . As pi ri n i s the onl y NSAID tha t bl ocks TXA2 producti on, l i mi ti ng platelet plug forma ti on (s ee the text). Thus , l ow-dos e a s pi ri n i s us ed to trea t potenti a l prothromboti c di s ea s es a nd to reduce the ri s k of hea rt a tta ck a nd s troke (whi ch a re i n pa rt ca us ed by thrombus forma ti on i n the wrong ves s el s ) i n s us cepti bl e pa ti ents . Beca us e a s pi ri n i nhi bi ts pl a tel et pl ug forma ti on, peopl e ta ki ng a s pi ri n a re more prone to brui s i ng a nd prol onged bl eedi ng a fter i njury. The i ntri ns i c pa thwa y i s tri ggered when Fa ctor XII, a n inactive s eri ne protea s e, encounters expos ed collagen and other polyanions from a n i njured bl ood ves s el . A protei n compl ex i s formed, whi ch tra ns forms Fa ctor XII i nto a n a cti va ted s eri ne protea s e, XIIa . XIIa cl ea ves i na cti ve Fa ctor XI i nto a cti va ted Fa ctor XIa . Fa ctor XIa i s a s eri ne protea s e tha t cl ea ves the i na cti ve form of Fa ctor IX i nto IXa . Fa ctor IXa cl ea ves i na cti ve Fa ctor VIII to VIIIa . High-molecular-weight kininogen a nd prekallikrein a l s o tri gger the i ntri ns i c pa thwa y by a cti va ti ng Fa ctor XII. Injury tri ggers the a cti va ti on of Fa ctor III a nd s ets off the extri ns i c pa thwa y. Fa ctor III cl ea ves i na cti ve Fa ctor VII (the mos t a bunda nt of the cl otti ng fa ctors ) to a cti ve VIIa .
Figure 14-15. Extrinsic and Intrinsic Pathways of the Clotting Cascade. See the text for deta i l s of both extri ns i c a nd i ntri ns i c pa thwa ys . [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] The extri ns i c a nd i ntri ns i c l i mbs of the cl otti ng ca s ca de opera te i ndependent of one a nother a nd converge i nto a common pa thwa y, begi nni ng wi th the cl ea va ge of Fa ctor X i nto Xa . Subs equent a cti vi ty of Fa ctor Xa a nd Va convert prothrombin i nto thrombin. (Fa ctor Xa a l s o pos i ti vel y regul a tes Fa ctor VII of the extri ns i c pa thwa y.) Al though i t i s the a ggrega ti on a nd cros s -l i nki ng of fi bri n mol ecul es tha t di rectl y res ul t i n cl ot forma ti on (s ee bel ow), thrombin forma ti on i s the commi tted s tep i n cl ot forma ti on. Thrombi n pos i ti vel y regul a tes the i ntri ns i c pa thwa y fa ctors Fa ctor XI a s wel l a s Fa ctor VIII, the termi na l s tep of the i ntri ns i c l i mb of the cl otti ng ca s ca de. The a cti va ti on of Fa ctor VIII perpetua tes coa gul a ti on, ca us i ng more a nd more cl ot to form. It i s thi s wi des prea d a va i l a bi l i ty of fi bri nogen a nd the thrombogeni c vers a ti l i ty of thrombi n tha t a l l ow for a prompt res pons e to hemorrha ge, no ma tter where i t occurs . THE FIBRIN MESHWORK Unl i ke other cl otti ng fa ctors , proteol ys i s does not tra ns form fibrinogen i nto a n a cti ve s eri ne protea s e. Ra ther, the cl ea va ge of fi bri nogen res ul ts i n the forma ti on of fibrin monomers . Fi bri n monomers cros s -l i nk wi th one a nother, formi ng a netl i ke fi bri n mes h tha t ens na res RBCs , putti ng a n end to the s pi l l i ng of RBCs by formi ng a cl ot. Fi bri nogen, a l a rge (340 kD), s ol ubl e hexa mer (s i x s ubuni ts ), i s formed by two tri mers ma de of α, β, a nd γ, cha i ns (Fi gure 14-16A). The two heterotri mers a re l i nked through di s ul fi de bonds . The s econda ry a nd terti a ry s tructures (Cha pter 1) form three gl obul a r doma i ns : one between the two heterotri mers a nd one on ea ch end of the fi bri nogen mol ecul es . The N termi ni of the α a nd β cha i ns project from the center gl obul a r doma i n a nd a re the s i tes where thrombi n cl ea ves off cys tei nes (Fi gure 14-16B).
Figure 14-16. A–B. Conversion of Fibrinogen to Fibrin. (A) Fi bri nogen, a s ol ubl e hexa mer cons i s ti ng of two α, two β, a nd two γ cha i ns , whi ch a re cros s -l i nked by di s ul fi de bonds (note l i nes between s i ngl e pepti de cha i ns a bove), undergoes cl ea va ge a t α (FPA) a nd β (FPB) cha i ns to form fi bri n. (B) Thrombi n a cti on on FPA or FPB to produce the α- or β-fi bri n cha i ns . FP, fi bri nopepti de. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Fol l owi ng proteol ys i s , fi bri n a ggrega tes to form a s oft cl ot, whi ch connects the centra l gl obul a r doma i ns to the C-termi na l doma i ns of other (di fferent) fi bri n mol ecul es . Fa ctor XIIIa s ta bi l i zes the s oft cl ot i nto a ha rd cl ot. Fa ctor XIIIa ca ta l yzes the forma ti on of a cros s -l i nk between one end of a gl obul a r doma i n a nd a di fferent fi bri n’s gl obul a r doma i n, s ta bi l i zi ng the s oft cl ot i nto a ha rd cl ot (Fi gure 14-17A–B).
Figure 14-17. A–B. Formation of a Hard Clot. (A) Fi bri n mol ecul es a ggrega te to form a s oft cl ot. Fa ctor XIII ca ta l yzes cros s -l i nk forma ti on (red l i nes ) a mong fi bri n mol ecul es , tra ns formi ng the s oft a ggrega te i nto a ha rd cl ot. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] (B) Sca nni ng el ectron mi cros copy i ma ge of a cl ot, i l l us tra ti ng the mes hwork of fi bri n, erythrocytes , a nd pl a tel ets i n va ri ous s ta tes of degra nul a ti on. [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.] DIFFERENCE BETWEEN PLATELET PLUG FORMATION AND CLOT FORMATION Hemophilia refers to a group of di s ea s es cha ra cteri zed by frequent hemorrha ges , both s ponta neous a nd tra uma ti c. Thi s ma y ma ni fes t a s hema rthros es , ea s y brui s i ng, a nd, i n genera l , a n i ncrea s ed a mount of ti me needed to form a cl ot. Hemophi l i a cs requi re more ti me to form cl ots due to clotting factor defects. Hemophilia A (80% of hemophi l i a pa ti ents ) res ul ts from Factor VIII deficiency a nd Hemophilia B (20% of hemophi l i a pa ti ents ) from Factor IX deficiency. Pa ti ents wi th hemophi l i a ha ve l es s tha n 5% of the norma l fa ctor l evel s a s compa red wi th non-hemophi l i a pa ti ents , a nd pa ti ents wi th s evere hemophi l i a ha ve l es s tha n 1% of norma l l evel s (us ua l l y ha vi ng no functi ona l Fa ctor VIII or Fa ctor IX). Pa ti ents wi th hemophi l i a requi re regul a r i ntra venous i nfus i ons of puri fi ed Fa ctor VIII or Fa ctor IX protei n to prevent s ma l l bl eeds from turni ng i nto l i fe-threa teni ng hemorrha ges . von Willebrand’s Disease (vWD) i s a mi l d, i nheri ted bl eedi ng di s order tha t ma ni fes ts a s ea s y brui s i ng, bl eedi ng gums , a nd frequent nos ebl eeds . Des pi te mul ti pl e, heredi ta ry, a nd a cqui red forms , the condi ti on s tems from a defi ci ency, i n qua nti ty a nd/or qua l i ty, of vWF. Bound to Fa ctor VIII, vWF i s res pons i bl e not onl y for pl a tel et a dhes i on, but a l s o for the s urvi va l of Fa ctor VIII. Wi thout vWF, Fa ctor VIII i s qui ckl y degra ded a nd pa ti ents experi ence i ncrea s ed bl eedi ng ti mes . Pa ti ents wi th vWD, therefore, ha ve a defi ci ency of both pl a tel et pl ug forma ti on a nd cl ot forma ti on. Trea tment i s often not neces s a ry unl es s control of bl eedi ng i s requi red (e.g., exces s i vel y l ong mens es , s urgery), a nd cons i s ts of gi vi ng ei ther a ddi ti ona l Fa ctor VIII or by the medi ca ti on desmopressin, whi ch hel ps to ra i s e the vWF l evel . Pl a tel et pl ug forma ti on i s a temporary repa i r unti l a proper cl ot forms . Defi ci ent pl a tel et pl ug forma ti on res ul ts i n epi s ta xi s (bl oody nos e) a nd purpura e (ti ny red/purpl e s pots of bl eedi ng) on the mucos a (es peci a l l y l i ps ) a nd undernea th the s ki n. Defi ci ent cl ot forma ti on, on the
contra ry, res ul ts i n rebleeding. For i ns ta nce, were one to ha ve a mol a r tooth removed, a pl a tel et pl ug woul d s top the bl eedi ng tempora ri l y. A few hours l a ter, however, the gums wi l l rebl eed a s the pl ug i s onl y a tempora ry fi x a nd the i ndi vi dua l l a cks the perma nent fi x of a cl ot. REGULATION OF CLOT FORMATION Fi bri nogen a nd other cl otti ng components freel y exi s t i n the ci rcul a ti on. Wi thout regul a tory mecha ni s ms , thrombi n woul d conti nue to a cti va te Fa ctors V, VIII, a nd fi bri nogen, a nd cl ots woul d form i ndi s cri mi na tel y a nd a t i na ppropri a te ti mes . Three ma jor regul a tors s erve to control cl ot forma ti on: a nti -thrombi n III (ATIII), protei n C, a nd protei n S. A defi ci ency i n ATIII, protei n C, a nd/or protei n S l ea ds to a hypercoa gul a bl e s ta te, ma ki ng pa ti ents vul nera bl e to i na ppropri a te a nd potenti a l l y da ngerous cl ot forma ti on. Vitamin K, Clotting, and Warfarin: Synthes i zed by gut ba cteri a , vi ta mi n K (Cha pter 10) i s a cofa ctor for γ-gl uta myl ca rboxyl a s e, the enzyme tha t ca rboxyl a tes (a cti va tes ) prothromboti c Fa ctors II, VII, IX, a nd X a nd a nti thromboti c regul a tory protei ns C a nd S. Vitamin K epoxide reductase i s requi red to return oxi di zed vi ta mi n KO ba ck to i ts a cti ve form, vi ta mi n KH 2 (s ee fi gure bel ow). Wa rfa ri n, a medi ca ti on us ed to l i mi t cl otti ng, bl ocks epoxi de reducta s e, l ea di ng to l owered l evel s of a cti ve vi ta mi n KH 2 a nd, therefore, a cti ve cl otti ng fa ctors .
Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009. The ba l a nce between the body’s norma l cl otti ng l evel a nd i ts i nhi bi ti on by wa rfa ri n i s not s i mpl e, a nd pa ti ents uti l i zi ng thi s drug mus t be cons ta ntl y moni tored to i ns ure both effi ca cy a nd s a fety of i ts us e. If vi ta mi n K i s defi ci ent, cl ot forma ti on ma y be hi ndered a nd chroni c bl eedi ng ma y res ul t. Vi ta mi n K defi ci ency ca n a l s o be ca us ed by a nti bi oti cs tha t des troy the gut fl ora , thereby decrea s i ng the a mount of vi ta mi n K a va i l a bl e for a bs orpti on. Vi ta mi n K defi ci ency i s trea ted by gi vi ng s uppl ementa l vi ta mi n K ei ther ora l l y or i ntra venous l y. ATIII i na cti va tes Thrombi n, IXa , Xa , a nd XIa . ATIII’s a cti vi ty i s a l s o enha nced ma ny thous a nds of ti mes by the mol ecul e heparin (Cha pter 2). Hepa ri n exi s ts i n a wi de ra nge of mol ecul a r s i zes , determi ned by i ts va ri a bl e number of ol i gos a ccha ri des . The number of ol i gos a ccha ri de uni ts determi nes hepa ri n’s a nti coa gul a ti on effects on ATIII. For i ns ta nce, i n order to i nhi bi t thrombi n, ATIII requi res a hepa ri n wi th more tha n 18 s a ccha ri de uni ts . If ATIII encounters the penta s a ccha ri de form (fi ve s a ccha ri de uni ts ) of hepa ri n, then ATIII i nhi bi ts Fa ctor X.
Protein C a nd protein S work together to degra de a nd i na cti va te Fa ctors Va a nd VIIIa . Protei n C i s a cti va ted a s a membrane-bound compl ex cons i s ti ng of the protei ns thrombomodulin, thrombin, a nd calcium (Ca2+) pl us phospholipids (s ee Fi gure 14-18). As protei n C requi res protei n S a s a cofa ctor, both protei n C a nd protei n S a re rega rded a s coa gul a ti on i nhi bi tors .
Figure 14-18. Regulation of the Fibrinolytic System by Protein C. u-PA, uroki na s e-type pl a s mi nogen a cti va tor. [Reproduced wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] PLASMIN AND CLOT DISSOLUTION Brea kdown of the fi bri n-ba s ed mes hwork a nd di s s ol uti on of cl ots rel i es on the s eri ne protea s e plasmin, whi ch i s formed vi a a cti va ti on of i ts zymogen plasminogen. The convers i on of pl a s mi nogen to pl a s mi n occurs vi a tissue plasminogen activator (tPA), urokinase plasminogen activator (uPA), the pepti da s e kallikrein, or coa gul a ti on Factor XII (a l s o known a s Hageman factor). Acti va ti on occurs vi a cl ea va ge of pl a s mi nogen between a rgi ni ne 560 a nd va l i ne 561 (Fi gure 14-19). Further cl ea va ge of pl a s mi n ca n produce the mol ecul e angiostatin, whi ch l i mi ts the new growth of bl ood ves s el s . Res ea rch i s currentl y a ttempti ng to us e thi s mol ecul e for ca ncer a nd other medi ca l trea tments .
Figure 14-19. Activation of Plasmin. Pl a s mi n i s formed from pl a s mi nogen by the cl ea va ge of a n a rgi ni ne (Arg)–va l i ne (Va l ) bond vi a a ny one of s evera l pl a s mi nogen a cti va tors . The two cha i ns of pl a s mi n a re hel d together by a di s ul fi de bri dge. [Reproduced wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Factor V Leiden: Factor V Leiden pol ymorphi s m cha nges a rgi ni ne to gl uta mi ne i n the Fa ctor V cl otti ng fa ctor. The a rgi ni ne cha nge i s pres ent i n the cl ea va ge s i te tha t protei n C us es to degra de a nd regul a te Fa ctor V. Thus , i n the Fa ctor V Lei den va ri a nt, protei n C i s l es s effi ci entl y a bl e to regul a te Fa ctor V, l ea di ng to hi gher tha n norma l l evel s of a cti ve Fa ctor Va . Hi gh l evel s of a cti ve Fa ctor Va l ea d to exces s i ve thrombin producti on, fibrinogen a cti va ti on, a nd cl ot forma ti on. Thi s hi gher l evel of Fa ctor Va a cti vi ty crea tes a hypercoa gul a bl e s ta te. Peopl e wi th the Fa ctor V Lei den va ri a nt a re prone to devel opi ng bl ood cl ots i n the l egs (deep vein thrombosis, DVT), whi ch ca n embol i ze (brea k off a nd tra vel i n the ci rcul a ti on). Thes e embol i ca n l ea d to s ubs equent emergenci es , i ncl udi ng ri ght-s i ded hea rt cl ots , pul mona ry embol i s m, tra ns i ent i s chemi c a tta cks (tempora ry s trokes ), a nd, i n pregna nt women, a s ma l l i ncrea s e i n mi s ca rri a ges . Clot Dissolution and D-dimers: Cl ot forma ti on (embolism) or brea ka ge of a n exi s ti ng cl ot (thrombus) i s one ma jor ca us e of myocardial infarction (hea rt a tta ck) a nd cerebral vascular accidents (s trokes ). Trea tment of thes e “thromboembol i c events ” i s vi a a number of “cl ot-bus ti ng” medi ca ti ons , whi ch i ncl ude streptokinase, uPA, a nd a nyone of s evera l geneti ca l l y produced tPAs (s ee fi gure). Al l cl ot bus ters l ea d to the brea kdown of cl ots (thrombol ys i s ) by a cti va ti ng pl a s mi nogen a nd ha ve a l l owed cl i ni ci a ns to effecti vel y trea t ea rl y pres enta ti ons of hea rt a tta cks , s trokes , a nd other medi ca l di s orders . Free pl a s mi n, rel ea s ed from a di s s ol ved cl ot, i s s ubs equentl y bound to α2 -a nti pl a s mi n to s top i ts a cti ons .
Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009. Fi bri n brea kdown products (s ee the fi gure), of whi ch one ma jor type i s “D-dimers” (na med a s s uch beca us e thei r mol ecul a r s tructure a ppea rs a s two connected “Ds ”), offer a method to di rectl y mea s ure the a cti vi ty of pl a s mi n. In medi ca l condi ti ons s uch a s DVT or pulmonary embolism, mea s urement of the l evel of D-di mers a l l ows cl i ni ci a ns to both confi rm a nd gra de the extent of a n exi s ti ng cl ot by rel yi ng on thi s s i de product of the body’s efforts to brea kdown cl ots .
REVIEW QUESTIONS 1. Wha t a re the ba s i c components of the bl ood a nd thei r functi ons ? 2. Wha t a re the functi ons of red bl ood cel l s ? 3. Wha t i s the genera l s tructure of hemogl obi n? 4. How do defects i n the α-gl obi n or β-gl obi n genes l ea d to the va ri ous types of tha l a s s emi a s ? 5. Wha t a re the functi ons of proxi ma l hi s ti di ne, i ron, a nd the heme group i n hemogl obi n i n rel a ti on to oxygen bi ndi ng? 6. How does ca rbon monoxi de poi s oni ng res ul t i n a n a na erobi c-l i ke s ta te? 7. How does the oxygen–hemogl obi n bi ndi ng curve expl a i n how hemogl obi n ca n be s a tura ted by O2 i n the l ungs , but onl y pa rti a l l y s a tura ted i n peri phera l ti s s ues ? 8. Wha t i s the Bohr effect a nd wha t i s the mecha ni s m by whi ch i t promotes O2 di s s oci a ti on a t ti s s ues tha t requi re O2 ? 9. Wha t i s the rol e of hemogl obi n i n a ci d–ba s e ba l a nce? 10. Wha t a re the rol es of tra ns ferri n, ferri ti n, a nd hepci di n i n i ron homeos ta s i s ? 11. Wha t i s the mos t common ca us e of nutri ti ona l a nemi a a nd why ma y thi s be pa rti cul a rl y preva l ent i n chi l dren a nd fema l es of chi l dbea ri ng a ge? 12. Wha t i s the rol e of von Wi l l ebra nd Fa ctor i n bl ood cl ot forma ti on? 13. Wha t a re the three pa rts of pl a tel et pl ug forma ti on? 14. Wha t a re the di fferences between the i ntri ns i c a nd extri ns i c pa thwa ys of the cl otti ng ca s ca de? 15. Wha t a re the events a s s oci a ted wi th the common pa thwa y of the cl otti ng ca s ca de? 16. Wha t i s the rol e of proteol ys i s i n the cl otti ng ca s ca de? 17. Wha t a re the rol es of protei n C, protei n S, a nd a nti thrombi n III i n l i mi ti ng cl ot forma ti on (a nti coa gul a ti on) a nd how does a defi ci ency of a ny of thes e components l ea d to a hypercoa gul a bl e s ta te?
CHAPTER 15 THE IMMUNE SYSTEM Editor: Eric L. Greidinger, MD Sta ff Phys i ci a n, Mi a mi VAMC Rheuma tol ogy Secti on a nd As s oci a te Profes s or, Di vi s i on of Rheuma tol ogy a nd Immunol ogy, Leona rd M. Mi l l er School of Medi ci ne, Uni vers i ty of Mi a mi , Mi a mi , FL
Overvi ew of the Immune Sys tem Anti gen Anti body Cel l s As s oci a ted wi th the Immune Sys tem Cytoki nes Inna te Immuni ty Compl ement Sys tem Hypers ens i ti vi ty Rea cti ons Revi ew Ques ti ons
OVERVIEW The i mmune s ys tem, compos ed of a nti gens , a nti bodi es , a nd s evera l s peci a l i zed l eukocyte (whi te bl ood cel l ) types , offers the huma n body a da pta bl e protecti on a ga i ns t i nfecti ons a nd i nva s i on by forei gn mol ecul es a nd cel l s . Bes i des res pons es by a nti bodi es a nd cel l s , the i mmune s ys tem i s a l s o compos ed of a compl ement-a cti va ted pa thwa y tha t a l l ows a n a l terna ti ve to res pond to i nfecti ous orga ni s ms . Al though a bl e to a tta ck thes e pa thogens , the i mmune s ys tem i s a l s o a bl e to recogni ze the hos t huma n cel l s a nd mol ecul es a nd ca n s el ecti vel y not res pond to “s el f.” Uncontrol l ed i mmune res pons es ca n l ea d to ti s s ue da ma ge or dea th.
OVERVIEW OF THE IMMUNE SYSTEM The huma n i mmune s ys tem i s res pons i bl e for genera ti ng a protecti ve res pons e to i nfecti ve orga ni s ms (e.g., ba cteri a , vi rus es , a nd pa ra s i tes ) a nd forei gn cel l s (e.g., tumor a nd tra ns pl a nt) whi l e both recogni zi ng the hos t body a nd l i mi ti ng da ma ge to i ts el f (s el f–nons el f theory). As a tta cks a re a l wa ys evol vi ng, the i mmune s ys tem mus t be ca pa bl e of a da pti ng to ea ch new cha l l enge. The i mmune s ys tem a l s o provi des a n i nfl a mma tory res pons e to tra uma . When ti s s ue or cel l s a re da ma ged, mol ecul es tha t a ct a s endogenous danger signals ca n be rel ea s ed. Thes e da nger s i gna l s ma y i ncl ude mol ecul es s uch a s uri c a ci d (produced by puri ne meta bol i s m, Cha pter 4), whi ch, i f pres ent a t hi gh concentra ti ons , ca n form crys ta l s tha t i nna te i mmune s ens ors recogni ze a nd thus a cti va te i mmune a nd i nfl a mma tory res pons es . Innate immunity refers to preformed hos t defens e s ys tems tha t ca n provi de i mmedi a te hos t defens e a cti vi ti es wi thout fi rs t bei ng tra i ned to di s ti ngui s h s el f from i nva der. El ements of the i nna te i mmune s ys tem often conta i n s tructura l recogni ti on moti fs tha t a l l ow them to i denti fy l i kel y pa thogens to ta rget. Mol ecul es tha t a re found i n mi crobes wi thout s tructura l homol ogs i n huma n cel l s , s uch a s fl a gel l i n or unmethyl a ted deoxyri bonucl ei c a ci d (DNA), a re other exa mpl es of danger signals tha t i nduce i nna te i mmune a cti va ti on when thei r s tructure i s recogni zed by i nna te i mmune s ens ors . The adaptive immune system i s ca pa bl e of genera ti ng a res pons e very s peci fi c to the s tructure of a n i nva di ng orga ni s m or mol ecul e (antigen, s ee bel ow) a s wel l a s the a bi l i ty to remember tha t s tructure vi a memory cel l s (immunological memory). The repea t a cti va ti on of the i mmune s ys tem by a n a nti gen tha t ha s previ ous l y i nduced a n i mmune res pons e i s norma l l y much qui cker a nd s tronger. The i nva di ng pa thogen s erves a s a ta rget, whi ch el i ci ts a res pons e s peci fi c for tha t a nti gen from i mmune cel l s . Thi s res pons e ma y i ncl ude the producti on of a s peci fi c antibody a ga i ns t tha t pa thogen (humoral immune system) or the a cti va ti on of i mmune cel l s tha t ei ther a tta ck a nd ki l l the offendi ng orga ni s m or orches tra te thi s a cti vi ty, the cell-mediated immune system. Thes e cel l types i ncl ude l ymphocytes (i ncl udi ng T a nd B l ymphocyte types ), monocytes , ma cropha ges , dendri ti c cel l s (DCs ), neutrophi l s , eos i nophi l s , a nd ba s ophi l s . Passive immunity i nvol ves a nti body mol ecul es tha t a re tra ns ferred to the ba by from the mother’s a cti ve i mmune s ys tem through the pl a centa . Immunoglobulin (Ig) G (s ee bel ow) i s the s i ngl e a nti body type tha t bi nds to a neonatal Fc receptor a nd i s endocytos ed vi a pinocytosis. Newborns a l s o recei ve IgA a nti bodi es (s ee bel ow) vi a brea s t mi l k, whi ch provi des i ni ti a l protecti on a ga i ns t pa thogeni c mi croorga ni s ms . Pa s s i ve i mmuni ty i s cons i dered to be a s hort-term s ys tem, whi ch i s ta ken over i n the fi rs t yea r or two of l i fe by the a da pti ve i mmune s ys tem. Immunodeficiencies: Medi ca l condi ti ons known a s immunodeficiencies a re cha ra cteri zed by the a bs ence or ma l functi on of a component of the i mmune s ys tem, l ea di ng to the i na bi l i ty to fi ght i nfecti ous di s ea s e. More tha n 120 congeni ta l i mmunodefi ci enci es ha ve been recogni zed. Externa l ca us es of i mmunodefi ci ency i ncl ude poor di et, the effects of i mmunomodul a ti ng drugs , a nd i nfecti ous di s ea s es i ncl udi ng huma n i mmunodefi ci ency vi rus (HIV)/a cqui red i mmune defi ci ency s yndrome.
ANTIGEN Antigen, ori gi na l l y termed from the phra s e “a nti body genera tor,” i s a mol ecul e (norma l l y protei n or ca rbohydra te i n s ource) tha t ca n s ti mul a te the i mmune s ys tem to ma ke a nti bodi es a nd/ or i ni ti a te a cel l -medi a ted res pons e. Sel f-a nti gens a re pres ent on a l l cel l s of the hos t orga ni s m. Sel f-rea cti ve i mmune cel l s a re typi ca l l y del eted or ma i nta i ned i n s ta tes of i mpa i red rea cti vi ty to prevent the devel opment of a utoi mmuni ty. Ma ny pa thogeni c mi crobes a nd ca ncer cel l s ca n us e s ome of thes e s a me mecha ni s ms to eva de i mmune a tta ck. Other “opportuni s ti c” orga ni s ms ma y i nduce a ggres s i ve res pons es from i nta ct i mmune s ys tems (tha t effecti vel y ki l l the orga ni s m) but ca n s ti l l l ea d to di s ea s e i n i mmunocompromi s ed hos ts . The s peci fi c s tructura l s urfa ce recogni zed by cel l s of the a da pti ve i mmune s ys tem i s known a s a n epitope. Severa l a nti bodi es ma y recogni ze va ri ous pa rts of one a nti gen (va ri ous epi topes ). To be recogni zed by T cel l s i n the cel l -medi a ted i mmune s ys tem, a nti gens mus t be pres ented on the cel l s urfa ce of pa rti cul a r i mmune cel l s (s ee bel ow).
ANTIBODY An antibody i s a l a rge, Y-s ha ped protei n (Fi gure 15-1) res pons i bl e for i mmunol ogi ca l i denti fi ca ti on a nd bi ndi ng of a nti gens a nd, ul ti ma tel y, for protecti on a ga i ns t i nva di ng pa thogens . Another na me for a n a nti body i s immunoglobulin, norma l l y a bbrevi a ted a s “Ig.” The functi ons of a nti bodi es a re s umma ri zed i n Fi gure 15-2.
Figure 15-1. Basic Structure of an Antibody Molecule. The a nti body mol ecul e i s a Y-s ha ped, tetra mer protei n, compos ed of two hea vy cha i ns a nd two l i ght cha i ns , hel d together by hydrophi l i c/hydrophobi c forces a nd di s ul fi de bonds . The body of “Y” i s compos ed of a cons ta nt protei n s tructure a nd a hi ghl y va ri a bl e end, whi ch defi nes the a ffi ni ty of a pa rti cul a r a nti body for one a nti gen epi tope. The Fc regi on of the mol ecul e ma y bi nd to the s urfa ce of receptors of s evera l cel l types . [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.]
Figure 15-2. Summary of Antibacterial Antibody Functions. Anti bodi es ca n el i ci t a number of rea cti ons res ul ti ng i n the el i mi na ti on of ba cteri a , i ncl udi ng (A) recogni ti on a nd bi ndi ng to ba cteri a , whi ch a l l ows recogni ti on by other i mmune s ys tem components to i ncl ude (B) ops oni za ti on, l ea di ng to recogni ti on a nd pha goyctys i s by ma cropha ges , a nd (C) bi ndi ng l ea di ng to compl ement a cti va ti on (s ee bel ow), l ea di ng to ba cteri a l l ys i s . [Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, Mc Gra w-Hi l l , 2009.] Anti bodi es exi s t i n fi ve types : IgA, IgD, IgE, IgG, a nd IgM. The ba s i c s tructure, s ource, a nd functi on of ea ch a re s umma ri zed i n Ta bl e 15-1.
TABLE 15-1. Summa ry of Anti body Mol ecul es
CELLS ASSOCIATED WITH THE IMMUNE SYSTEM Cel l s of the i mmune s ys tem, col l ecti vel y ca l l ed l eukocytes or whi te bl ood cel l s , ca n be ca tegori zed a s l ymphocytes , monocytes , neutrophi l s , eos i nophi l s , a nd ba s ophi l s (Fi gure 15-3).
Figure 15-3. White Blood Cells. Overvi ew of whi te bl ood cel l s , i ncl udi ng thos e deri ved from gra nul ocytes a nd a gra nul ocytes . A l a bora tory ful l bl ood count us ua l l y i ncl udes tota l whi te bl ood cel l s a nd a di fferenti a l mea s urement of the fi ve ma jor types of whi te bl ood cel l s . Norma l va l ues a re a s fol l ows : neutrophi l s (45%–60% of tota l ), l ymphocytes (25%–35% of tota l ), monocytes (3%–7% of tota l ), eos i nophi l s (1%–3% of tota l ), a nd ba s ophi l s (3.5 g/ da y) of protei n, termed nephrotic syndrome. Both a re cl i ni ca l i ndi ca tors of a rena l probl em. Gl omerul a r di s order di s ea s es a re norma l l y di vi ded i nto prol i fera ti ve—i nvol vi ng the i ncrea s ed growth or number of gl omerul a r cel l s or ma tri x ma teri a l —a nd nonprol i fera ti ve— no i ncrea s ed cel l ul a ri ty or ma tri x. Thes e va ryi ng di s ea s es a nd the bi ochemi ca l effect on gl omerul a r s tructure a re l i s ted i n Ta bl e 18-1.
Table 18-1. Summa ry of Gl omerul a r Di s orders The fi na l components of the uri na ry s ys tem, the ureters , bl a dder, a nd urethra , a re l i ned by a tra ns i ti ona l epi thel i um wi th s evera l l a yers of cel l s tha t a l l ow the bl a dder to s tretch whi l e s ti l l ma i nta i ni ng bl a dder wa l l i ntegri ty. A thi ck a nd el a s ti c l a mi na propri a l a yer, compos ed of a mes h network of col l a gen, el a s ti c fi bers , a nd proteogl yca ns (Cha pters 1, 2, a nd 13), hol ds a nd a tta ches the epi thel i a l cel l l a yers together to provi de a ddi ti ona l fl exi bi l i ty a nd s trength to the fl ui d-proof but s tretcha bl e s tructure. The two to three s mooth mus cl e l a yers benea th the epi thel i um i ncrea s e movement of uri ne from the ki dneys to the bl a dder a nd, ul ti ma tel y, for excreti on. When uri na ti on occurs , a n internal urethral sphincter, compos ed of s mooth mus cl e (Cha pter 12), i s control l ed by the a utonomi c nervous s ys tem (Cha pter 19), wherea s a n external sphincter, compos ed of s kel eta l mus cl e a nd i nnerva ted by the pudendal nerve, offers vol unta ry control . Urinary Tract Infections (UTIs) and Cranberry Juice: Ba cteri a l UTIs of the bl a dder (cystitis) or ki dney (pyelonephritis) a ffect mi l l i ons of pa ti ents . Al though a nti bi oti cs a re often needed for trea tment, the us e of cra nberry jui ce ma y a l s o hel p. Cra nberry jui ce, s peci fi ca l l y, conta i ns the pl a nt fl a venoi d mol ecul e A type proanthocynidin, whi ch di rectl y i nhi bi ts a tta chment of p-type fimbriae of Escherichia coli, the ca us e of up to 90% of UTIs , to the uri na ry tra ct wa l l . As a res ul t of the us e of cra nberry jui ce, ba cteri a a re s wept a wa y by uri na ry fl ow, a ccel era ti ng recovery from or even preventi ng a UTI.
Diuretics and the Nephron: Diuretics a re a ny s ubs ta nce, i ncl udi ng medi ca ti ons , whi ch i ncrea s e the l os s of wa ter vi a excreti on i n uri ne. Di ureti c
medi ca ti ons a re funda menta l to ma ny trea tments of hi gh bl ood pres s ure a nd hea rt fa i l ure, a nd work vi a s peci fi c effects on the cha nnel s a nd pumps of the nephron. Loop diuretics, na med s o beca us e thei r a cti on a ffects the loop of Henle, i ncl ude furos emi de, bumeta ni de, tora s emi de, a nd etha cryni c a ci d (s ee fi gure). Thes e medi ca ti ons a cti vel y compete (Cha pter 5) for the Cl –-bi ndi ng s i te, bl ocki ng Na+–K +–2Cl– transport a nd reduci ng the Na + a nd K+ i on concentra ti on i n the rena l medul l a . The l ower i on concentra ti on a nd, therefore, osmotic pressure gradient decrea s es the a mount of wa ter rea bs orbed from the uri ne i n the col l ecti ng duct, l owers the tota l fl ui d vol ume of the bl ood pl a s ma , a nd, cons equentl y, l owers bl ood pres s ure. Ma gnes i um i on (Mg2+) a nd ca l ci um i on (Ca 2+) rea bs orpti on i s a l s o decrea s ed, a ddi ng to thi s effect. Beca us e of thi s mecha ni s m, a pos s i bl e s i de effect of l oop di ureti cs i s l ow K+ (hypoka l emi a ), l ow Mg2+ (hypoma gnes emi a ), l ow Ca 2+ (hypoca l cemi a ), a nd l ow Cl – (hypochl oremi a ). Beca us e of the effect on ca l ci um excreti on, l oop di ureti cs a re a l s o us ed i n cl i ni ca l s i tua ti ons a s s oci a ted wi th el eva ted s erum l evel s of ca l ci um (hyperca l cemi a ). Neverthel es s , el eva ted l evel s of uri na ry ca l ci um ma y l ea d to forma ti on of ki dney s tones , a phenomenon s ometi mes s een i n prema ture ba bi es who a re expos ed to hi gh dos es ea rl y i n l i fe. Nonsteroidal anti-inflammatory molecules bl ock the a cti on of l oop di ureti cs by decrea s i ng gl omerul a r fi l tra ti on of the bl ood a nd pa s s a ge of the di ureti cs to the l oop of Henl e.
Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009. Thiazide diuretics, i ncl udi ng hydrochl orothi a zi de, chl orothi a zi de, metol a zone, i nda pa mi de, a nd the pa rent mol ecul e benzothi a di a zi ne, work by bl ocki ng the Na+–Cl– cotrans-porter i n the distal tubule. Bl ocka ge of the membra ne cha nnel i n thi s pa rt of the nephron decrea s es the rea bs orpti on of thes e two i ons i nto the i nters ti ti um a nd the bl ood s trea m, decrea s i ng wa ter i nfl ux, tota l bl ood vol ume, a nd bl ood pres s ure. The res ul ti ng i ncrea s ed Na + concentra ti on a nd wa ter content a l s o a cti va te a n a l dos terone-regul a ted (s ee bel ow) K+ s ecreti on cha nnel i n the col l ecti ng ducts , l ea di ng to l os s of K+. As a res ul t, thi a zi de di ureti cs ca n l ea d to cl i ni ca l l y s i gni fi ca nt l ow K+ l evel s i n the bl ood. Thi a zi de di ureti cs a l s o decrea s e calcium excreti on a nd, therefore, l evel s i n the uri ne, l oweri ng the cha nce of ca l ci um-deri ved ki dney s tone forma ti on a nd other di s ea s es i nvol vi ng hi gh l evel s of s erum ca l ci um. The effects of thi a zi de di ureti cs a re mi mi cked by the di s ea s es of Bartter a nd Gitelman syndromes wherei n ra re geneti c defects l ea d to dys functi on of the a bove-menti oned cotra ns porters i n the a s cendi ng pa rt of the l oop of Henl e or the di s ta l convol uted tubul e, res pecti vel y. NEPHRON The producti on of uri ne by the ki dneys i nvol ves a compl ex, wel l -regul a ted s eri es of membra ne-bound protei n pumps a nd cha nnel s , whi ch s el ecti vel y devel op gra di ents of i ons a nd mol ecul es to determi ne the fi na l content of uri ne. The body ha s mecha ni s ms to s ens e the ma keup of the fi l tra te vi a os moti c mea s urement (e.g., ma cul a dens a ) a nd hormona l res pons e, a s i t progres s es from the rena l corpus cl e to the ureters . Fi l teri ng of the bl ood by the rena l corpus cl e removes wa s te products , but thi s proces s i s not perfect by a ny mea ns . The compl ex ta s k of returni ng pa rti cul a r mol ecul es to the ci rcul a tory s ys tem whi l e progres s i vel y concentra ti ng the res ul ti ng fl ui d tha t wi l l become uri ne i s the job of s evera l protei n cha nnel s (a cti ve a nd pa s s i ve) a nd pumps (Cha pter 8) found i n the membra nes of the nephron (Fi gure 18-3).
Figure 18-3. Overview of Nephron and Urine Production. Il l us tra ti on of components of the nephron a nd s umma ry of functi ons found i n ea ch s egment, i ncl udi ng proxi ma l convol uted tubul e, proxi ma l s tra i ght tubul e, thi n des cendi ng l i mb, l oop of Henl e, thi n a nd thi ck a s cendi ng l i mbs , di s ta l convol uted tubul e, a nd col l ecti ng duct. Mol ecul es l ea vi ng (rea bs orpti on) a nd enteri ng (s ecreti on) a s wel l a s medi ca ti ons a nd/or hormones a ffecti ng ea ch s egment (numbered) a re i ndi ca ted. See text for further deta i l s . ADH, a nti di ureti c hormone; PTH, pa ra thyroi d hormone. [Reproduced wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.] The nephron i s the tubul a r s tructure, whi ch conti nues from the rena l corpus cl e, l ea di ng to the col l ecti ng s ys tem a nd s ubs equentl y ureters , bl a dder, a nd urethra . It i s s urrounded by i nters ti ti a l ti s s ue (the connecti ve ti s s ue of the ki dney found outs i de the tubul e a nd s urroundi ng the nephrons ), whi ch conta i ns “peri tubul a r” ca pi l l a ri es . Wa ter a nd s el ected mol ecul es a re tra ns ferred a cros s the nephron members by thes e cha nnel s a nd pumps a nd returned to the body vi a thes e ca pi l l a ri es , a proces s ca l l ed reabsorption (the i ni ti a l a bs orpti on i s cons i dered to ha ve ta ken pl a ce i n the i ntes ti nes ). Wa ter a nd mol ecul es movi ng from the i nters ti ti um i nto the nephron for el i mi na ti on i n uri ne undergo a proces s termed secretion. The tubul e s ys tem beyond the gl omerul us i s di vi ded functi ona l l y i nto four di s ti nct s ecti ons (Fi gure 18-3): (1) the proximal convoluted tubule, (2) the loop of Henle (des cendi ng a nd a s cendi ng pa rts ), (3) the distal convoluted tubule (proxi ma l a nd di s ta l pa rts ), a nd (4) the collecting duct. Ea ch s ecti on i s cha ra cteri zed by s peci fi c cha nnel s / pumps a nd os moti c condi ti ons requi red for i ts pa rti cul a r functi on i n uri ne producti on. A s umma ry of the a cti ons on certa i n mol ecul es by the va ri ous s egments of the nephron i s gi ven i n Fi gure 18-3 a nd Ta bl e 18-2.
Table 18-2. Summa ry of Nephron Acti ons on Va ri ous Mol ecul es The col l ecti ve functi ons of a l l the s egments of the nephron, i n tota l , a l l ow the ki dney to not onl y regul a te wa ter but a l s o the cons erva ti on or el i mi na ti on of el ectrol ytes ; the proper ma i ntena nce of a ci d–ba s e ba l a nce; the excreti on of wa s te products ; a nd the pres erva ti on of i mporta nt protei ns , ca rbohydra tes , a nd even medi ca ti ons . INULIN/CREATININE CLEARANCE The mea s urement of the glomerular filtration rate (GFR) (vol ume of fl ui d fi l tered from the a fferent a rteri ol e i nto the rena l corpus cl e per uni t ti me) a l l ows cl i ni ci a ns to determi ne the hea l th of the ki dneys a nd to qua nti fy a ny degree of ki dney or rena l fa i l ure. GFR i s mos t a ccura tel y mea s ured by i njecti on of inulin i nto the bl oods trea m, a pol ys a ccha ri de mol ecul e from pl a nts , whi ch i s nei ther rea bs orbed nor s ecreted by the ki dney. A more pra cti ca l method of es ti ma ti ng GFR, though, i s to mea s ure bl ood creatinine, a mol ecul e deri ved i n mus cl e from the brea kdown of creatine phosphate, a ra pi dl y a va i l a bl e s ource of a denos i ne tri phos pha te (ATP) energy for mus cl e a nd bra i n. Crea ti ni ne i s excl us i vel y excreted by the ki dneys , compl etel y fi l tered a t the rena l corpus cl e, a nd onl y s ma l l a mounts a re s ecreted i nto the peri tubul a r ca pi l l a ri es . The na tura l occurrence of s tea dy l evel s of crea ti ni ne i n the bl ood a nd uri ne offers a n ea s y wa y to es ta bl i s h GFR a nd, therefore, ki dney functi on. Va ryi ng ma thema ti ca l formul a s a l l ow for correcti on of known va ri a ti ons dependi ng on the pa ti ent’s mus cl e ma s s , a ge, gender, ra ce, a nd/or s i ze. The norma l ra nge of GFR i s (100–130) ml /mi n/1.73 m 2 but thi s va ri es i n chi l dren a nd ol der a dul ts . A GFR of over 60 ml /mi n/1.73 m 2 i s us ua l l y s uffi ci ent for norma l hea l th, a l though hi gh bl ood pres s ure, di a betes , a nd other chroni c di s ea s es ca n decrea s e ki dney functi on a nd res ul t i n chronic kidney disease (CKD). CKD i s enumera ted i n s i x s ta ges , dependi ng on the pa ti ent’s GFR, a s noted bel ow (Ta bl e 18-3).
Table 18-3. Sta ges of Chroni c Ki dney Di s ea s e a nd GFR
RENIN–ANGIOTENSIN–ALDOSTERONE SYSTEM (RAAS) The RAAS i s a compl ex, regul a tory pa thwa y i nvol vi ng the ki dney, l i ver, l ung, a drena l gl a nds , pi tui ta ry gl a nd, a nd s el ected a rteri ol es found i n the ki dney. It i ncl udes precurs or a nd a cti va ted pepti des a s s i gna l s a mong thes e va ri ous orga ns , the enzymes tha t a cti va te the pepti des , a nd regul a ti on by two ma jor hormones of the rena l a nd ca rdi ova s cul a r s ys tems (Fi gure 18-4). The JAG a ppa ra tus i s l oca ted between the a fferent a rteri ol e a nd the rena l corpus cl e a nd di s ta l convol uted tubul e of the s a me nephron. Thi s s peci a l i zed col l ecti on of cel l s i s a bl e to s ens e a nd regul a te bl ood fl ow vol ume i nto a nd out of the rena l corpus cl e/gl omerul us by the producti on of the s i gna l i ng pepti de hormone/enzyme renin a nd by two, s i mpl e bi ochemi ca l mecha ni s ms of the macula densa, i nvol vi ng Na + a nd the ga s eous mol ecul e nitric oxide (NO).
Figure 18-4. Renin–Angiotensin–Aldosterone System. Overvi ew of effects of reni n, a ngi otens i n II a nd a l dos terone on orga ns control l i ng vol ume s ta tus a nd bl ood pres s ure. a a , a mi no a ci d. See text for further deta i l s . [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] RENIN AND BLOOD PRESSURE Reni n i s produced by s peci a l i zed cel l s (gra nul a r cel l s of the JAG a ppa ra tus ) i n res pons e to l ow bl ood pres s ure s ens ed i n the JAG a nd by decrea s ed Na Cl l evel s detected by ma cul a dens a cel l s (s ee bel ow). Bl ood pres s ure i n the JAG i s detected by s peci a l i zed nerve cel l s , whi ch a re a cti va ted (Cha pter 19) by cha nges i n the a rteri ol e bl ood pres s ure, l ea di ng to a l oca l i zed nerve s i gna l a nd s ecreti on of the hormone reni n. As wi l l be di s cus s ed further bel ow, s ympa theti c nerve a cti vi ty (Cha pter 19) a l s o l ea ds to i ncrea s ed reni n s ecreti on. Secreted reni n enzyma ti ca l l y converts the precurs or protei n a ngi otens i nogen, produced i n the l i ver, i nto a ngi otens i n I, whi ch, when converted to a ngi otens i n II (s ee bel ow), l ea ds to i ncrea s ed bl ood pres s ure a nd, therefore, the pres s ure of perfus i on i n the rena l corpus cl es . Renin Inhibitors: Recentl y devel oped i nhi bi tors of renin ha ve i ntroduced a novel cl a s s of medi ca ti ons to hel p trea t hi gh bl ood pres s ure. Two s uch medi ca ti ons , aliskiren a nd remikiren, bi nd di rectl y to reni n a nd competi ti vel y i nhi bi t bi ndi ng of angiotensinogen a nd, therefore, producti on of angiotensin I. The effi ca cy a nd s a fety of thes e new drugs i s s ti l l bei ng determi ned but ma y offer a novel method of pa ti ent ca re. MACULA DENSA AND BLOOD FLOW/OSMOLARITY Wi th hi gh bl ood fl ow through the a rteri ol es of the rena l corpus cl e, i ncrea s ed Na + a re fi l tered i nto the gl omerul us . Cel l s found i n a pa rt of the JAG, termed the macula densa, conta i n a Na+–Cl– cotransporter channel, whi ch tra ns ports Na + i nto thes e cel l s . A l i mi ted number of Na+–K +-ATPase pumps a ttempt to tra ns port the Na + ba ck out of the cel l but, i f Na + l evel s a re hi gh (i .e., os mol a ri ty i s hi gh), they a re s oon overwhel med. The
i nterna l Na + concentra ti on i ncrea s es a nd crea tes a n os moti c gra di ent, whi ch bri ngs wa ter mol ecul es i nto the ma cul a dens a cel l s . Thes e cel l s , s wol l en wi th Na + a nd wa ter, conta i n a “stretch-activated” channel, whi ch a l l ows ATP to es ca pe fol l owed by convers i on to s i mpl e a denos i ne. Thes e a denos i ne mol ecul es a re bound by membra ne-bound protei na ceous receptors on the a dja cent a fferent a nd efferent a rteri ol es of the gl omerul us , l ea di ng to cons tri cti on of the a fferent a rteri ol e a nd di l a ti on of the efferent a rteri ol e. The res ul t i s decrea s ed bl ood fl ow through a nd fi l tra ti on by the rena l corpus cl e. As a res ul t, l es s Na + get rea bs orbed a nd the l oca l concentra ti on decrea s es . Al terna ti vel y, l ow fl ow through the a rteri ol es l ea ds to a l ower Na + concentra ti on i n the ma cul a dens a cel l s . The l ower Na + concentra ti on (i .e., l ow os mol a ri ty) a cti va tes the enzyme nitric oxide synthetase (NOS) i n thes e cel l s . NOS ca ta l yzes the rea cti on, whi ch converts the a mi no a ci d a rgi ni ne a nd a n oxygen mol ecul e to ci trul l i ne a nd NO, uti l i zi ng reduced nicotinamide adenine dinucleotide phosphate (NADPH) a s a cofa ctor. NO then a cti va tes s peci fi c G s protei ns , whi ch produce cyclic guanosine monophosphate (cGMP) a nd cyclic adenosine monophosphate (cAMP). cGMP a cti va tes a protein kinase G, whi ch, i n turn, l ea ds to i na cti va ti on of myos i n mol ecul es vi a phos phoryl a ti on of thei r regul a tory l i ght cha i n. Ina cti va ted myos i n mol ecul es i n the bl ood ves s el wa l l s l ea ds to ves s el rel a xa ti on a nd di l a ti on. cAMP l ea ds to the i ncrea s ed rel ea s e of reni n. Erectile Dysfunction (ED), NO, cGMP, and Phosphodiesterase Inhibitors: Unders ta ndi ng the rol e of NO a nd cGMP i n the rel a xa ti on of a rteri ol e wa l l s mooth mus cl e ha s l ed to the devel opment of phosphodiesterase inhibitor medi ca ti ons , whi ch ha ve revol uti oni zed the trea tment of ED. As i n the ki dney, NO-medi a ted producti on of cGMP ca us es di l a ti on of a rteri es i n the corpus ca vernos um, i ncrea s i ng bl ood fl ow a nd l ea di ng to erecti on. Va ri ous fa ctors i ncl udi ng a ge, chroni c di s ea s es s uch a s hi gh bl ood pres s ure a nd di a betes , a s wel l a s s i de effects of certa i n drugs decrea s e the bl ood fl ow a nd l ea d to ED. Speci fi c phos phodi es tera s e i nhi bi tors [e.g., s i l dena fi l (Vi a gra ), va rdena fi l (Levi tra ), a nd ta da l a fi l (Ci a l i s )], whi ch ha ve s tructures mi mi cki ng the cGMP mol ecul e, competi ti vel y bl ock the enzyme cGMP-specific phosphodiesterase type 5, whi ch brea ks down cGMP (s ee fi gure bel ow). By i nhi bi ti ng thi s enzyme, the cGMP s i gna l rema i ns , promoti ng va s odi l a ti on a nd l ea di ng to better a nd s us ta i ned erecti on. The mecha ni s m of cGMP rel a xa ti on of a rteri ol e wa l l s a nd the a cti ons of the phos phodi es tera s e i nhi bi tors a re a l s o us ed for certa i n types of pulmonary hypertension a nd pulmonary edema due to altitude sickness, i n whi ch pul mona ry a rtery di l a ti on l owers pres s ure i n the l ungs reduci ng di s ea s e da ma ge to l ung ti s s ue.
Reproduced wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009. Both cons ta nt moni tori ng of bl ood pres s ure (reni n) a nd bl ood fl ow/os mol a ri ty (ma cul a dens a ) through the a rteri ol es a nd, therefore, the rena l corpus cl es a nd nephrons , provi de feedba ck to porti ons of the nephron (to a l ter el ectrol yte a nd wa ter rea bs orpti on a nd s ecreti on) a nd
the res t of the body. The rema i nder of the RAAS i s res pons i bl e for thes e a cti ons . ANGIOTENSINOGEN/ANGIOTENSIN I AND II Angiotensinogen i s a 452-a mi no-a ci d-l ong protei n, whi ch i s produced on a cons ta nt ba s i s by the l i ver (Fi gure 18-5). Angiotensin I, compri s i ng the fi rs t 10 a mi no a ci ds of a ngi otens i nogen, i s produced by cl ea va ge of a l euci ne–va l i ne bond by reni n. The 10-a mi no-a ci d a ngi otens i n I i s compl etel y i na cti ve but i s s ubs equentl y converted to the ei ght-a mi no-a ci d pepti de hormone, angiotensin II. Thi s rea cti on i s ca ta l yzed by angiotensin-converting enzyme (ACE), a n enzyme found throughout the body but es peci a l l y concentra ted i n the l ung’s ca pi l l a ry beds . Angi otens i n II ha s s evera l a cti vi ti es throughout the body, i ncl udi ng on the ki dney, a drena l gl a nds , hea rt a nd bl ood ves s el s , s ympa theti c nervous s ys tem, a nd hypotha l a mus . Al l of thes e effects a re propa ga ted vi a G q or G i protei n-coupl ed a ngi otens i n receptors , whi ch a cti va te phospholipase C, l ea di ng to phos phoryl a ti on vi a protein kinase C (PKC) a nd Ca 2+ rel ea s e (Cha pter 8). Both PKC a nd a n i nfl ux of ca l ci um ca n l ea d to myos i n l i ghtcha i n phos phoryl a ti on a nd, therefore, contra cti l e a cti vi ty. Acti va ti on of the G i-coupl ed receptor a l s o i nhi bi ts adenyl cyclase a nd, therefore, the producti on of cAMP, whi ch i nhi bi ts s mooth mus cl e rel a xa ti on (whi ch a l s o promotes contra cti l e a cti vi ty). Fi na l l y a ngi otens i n II a cti va tes certa i n tyrosine kinases, l ea di ng to other effects i n the body. Thes e effects a re i l l us tra ted i n Fi gure 18-4 a nd l i s ted i n Ta bl e 18-4.
Table 18-4. Angi otens i n II Effects on the Body
Figure 18-5. Conversion of Angiotensinogen. Angi otens i nogen i s converted to a ngi otens i n I by the a cti on of reni n from the ki dney by the remova l of more tha n 400 a mi no a ci ds by proteol yti c cl ea va ge (s i te i ndi ca ted by a rrow). Angi otens i n I i s s ubs equentl y converted to a ngi otens i n II by the a cti on of a ngi otens i n-converti ng enzyme (ACE), found ma i nl y i n the l ung, by the remova l of a n a ddi ti ona l two a mi no a ci ds (cl ea va ge s i te i ndi ca ted by a rrow). The mol ecul e i s further proces s ed a s s hown, a l though the ma jori ty of functi ons i n the body a re vi a a ngi otens i n II a s s umma ri zed i n Ta bl e 18-4. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] The s um effect of a ngi otens i n II i s the i ncrea s e i n body bl ood pres s ure wi th muti ng of ma ny of the body’s compens a ti ng mecha ni s ms . Angi otens i n II ca n a l s o be converted to a s even-a mi no-a ci d a ngi otens i n III wi th decrea s ed effect on bl ood pres s ure but a s i mi l a r effect on a l dos terone s ecreti on a nd a s i x-a mi no-a ci d a ngi otens i n IV, whi ch ha s even l es s a cti vi ty. ACE Inhibitors: The ma rked effect of a ngi otens i n II on bl ood pres s ure, i ncl udi ng i ncrea s ed pres s ure a nd, therefore, potenti a l ves s el da ma ge i n the ki dney, l ed to the devel opment of medi ca ti ons ca l l ed ACE inhibitors. Thi s drug cl a s s , a l l of whi ch end i n the s uffi x “-pri l ,” s peci fi ca l l y bl ock the enzyme, whi ch converts i na cti ve a ngi otens i n I i nto i ts a cti ve a ngi otens i n II counterpa rt. The effecti venes s i n l oweri ng bl ood pres s ure a nd provi di ng protecti on to the ki dney a ga i ns t da ma ge ha s l ed to wi de us e of ACE Inhi bi tors i n hi gh bl ood pres s ure, di a betes , ki dney di s ea s e wi th protei n l os s i n the uri ne (protei nuri a ), a fter hea rt a tta ck, conges ti ve hea rt fa i l ure, a nd ki dney di s ea s es . Of note, ACE Inhi bi tors ma y a l s o a ffect the kinin–kallikrein system by i ncrea s i ng the producti on of bradykinin, a ni ne-a mi no-a ci d pepti de i nvol ved i n pa i n, i nfl a mma ti on, a nd bl ood pres s ure control . Bra dyki ni n ma y be res pons i bl e for s ome of the ACE i nhi bi tors ’ a dvers e effects (s uch a s cough a nd the potenti a l l y fa ta l s i de effect of a ngi oedema ). Thes e s i de effects ca n be a voi ded by us e of the rel a ted angiotensin receptor blocker cl a s s of medi ca ti ons , whos e na mes a l l end i n the s uffi x “-s a rta n,” whi ch competi ti vel y bl ock the effects of a ngi otens i n II a t i ts G-protei n receptors .
Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010. ALDOSTERONE Aldosterone, the fi na l pl a yer of the RAAS, i s the mi nera l ocorti coi d s teroi d hormone (Cha pter 3), whi ch a cts to i ncrea s e Na + a nd wa ter rea bs orpti on a nd, a s a res ul t, K+ s ecreti on. The overa l l effect i s a n i ncrea s e i n bl ood vol ume a nd, therefore, bl ood pres s ure. Al dos terone producti on/excreti on i s ma i nl y rel a ted to the concentra ti on of K+ i n the s erum, proba bl y mea s ured by ca roti d a rtery s ens ors . Sympa theti c nerve a cti vi ty ca n a l s o i nfl uence a l dos terone producti on but on a much l es s er s ca l e. Al dos terone’s a cti vi ty i s predomi na tel y s een i n the di s ta l convol uted tubul e a nd col l ecti ng duct. Aldosterone and Disease—Conn Syndrome and Addison’s Disease: Increa s ed a mounts of a l dos terone l ea d to primary aldosteronism (Conn syndrome), wherea s decrea s ed l evel s l ea d to adrenal insufficiency (Addison’s disease). Conn syndrome res ul ts from overproducti on of aldosterone, us ua l l y ca us ed by uncontrol l ed growth (hyperpl a s i a ) of the a drena l gl a nd(s ) or a n a l dos terone-s ecreti ng tumor. The exces s i ve a l dos terone l ea ds to a n i ncrea s e i n s odi um a nd wa ter rea bs orpti on wi th i ncrea s ed s ecreti on/ excreti on of pota s s i um tha t a l s o produces exces s hydrogen i on (H +) s ecreti on (meta bol i c a l ka l os i s ). The s um effect i s hi gh bl ood pres s ure, whi ch ca n onl y be trea ted by remova l of the hyperpl a s ti c a drena l gl a nd or tumor a nd/or us e of the pota s s i um-s pa ri ng di ureti cs s pi ronol a c-tone or epl erenone (s ee bel ow). Addison’s disease, a l terna ti vel y, res ul ts from underproducti on of a l dos terone from ei ther geneti c or di s ea s e/ tra uma ca us es or from i nhi bi ti on of a l dos terone producti on by the a drena l gl a nds . Effects on the nephron l ea d to i ncrea s ed l os s of s odi um a nd wa ter a nd
i ncrea s ed pota s s i um a nd H + (meta bol i c a ci dos i s ) l evel s i n the bl ood. Thi s l ea ds to l ow bl ood pres s ure, whi ch ma y be s evere enough to ca us e pa ti ents to fa i nt, es peci a l l y when s ta ndi ng up. Severe s ymptoms ma y be s een duri ng a n “Addisonian crisis” i n whi ch l os s of cons ci ous nes s a nd dea th ma y res ul t. Al dos terone s peci fi ca l l y i ncrea s es the rea bs orpti on of Na + a nd wa ter out of a nd the s ecreti on of K+ ba ck i nto the tubul e by bi ndi ng to s peci fi c membra ne-bound receptors a nd i ncrea s i ng the membra ne permea bi l i ty of cel l s l i ni ng the tubul e. In a ddi ti on, a l dos terone i ncrea s es the number of a nd s ti mul a tes the ATP-dependent phos phoryl a ti on of Na+–K + pumps, l ea di ng to a conforma ti ona l cha nge of the pump, whi ch expos es Na + to the i nters ti ti um a nd decrea s es Na + bi ndi ng. As a res ul t, i ncrea s ed Na + a re rea bs orbed a l ong wi th Cl – (to ma i nta i n el ectri ca l neutra l i ty) a nd wa ter (to ma i nta i n os moti c ba l a nce). The res ul t i s a n i ncrea s e i n Na + a nd wa ter i n the i nters ti ti um, l ea vi ng them to be ta ken up by i nters ti ti a l ca pi l l a ri es ba ck to the bl ood s trea m. As a res ul t of the i na cti va ted Na +–K+ pumps but s econda ry to s ecreti on vi a i ncrea s ed membra ne permea bi l i ty, K+ a ccumul a te i n the tubul es a nd, s ubs equentl y, i n the uri ne. Fa l l i ng K+ concentra ti ons , s ubs equentl y, decrea s e a l dos terone s ecreti on. Al dos terone s ecreti on i s a ddi ti ona l l y a ffected by condi ti ons i n whi ch bl ood vol ume a nd/or el ectrol yte concentra ti ons a re cha nged s uch a s s i gni fi ca nt bl ood l os s , pregna ncy, burns , s hock, a nd prol onged phys i ca l exerci s e. Pseudohyperaldosteronism and Licorice: Pseudohyperaldosteronism, i n whi ch a l dos terone l evel s a re norma l , ca n a l s o occur a s a res ul t of di et. Exces s i ve l i cori ce i n the di et, conta i ni ng the s weetener mol ecul e glycyrrhizin, i nhi bi ts 11-β-hydroxys teroi d dehydrogena s e, whi ch reduces the convers i on of exces s cortisol to i na cti ve cortisone i n a l dos terone-s el ecti ve epi thel i a l cel l s . Inhi bi ti on of thi s enzyme a l l ows the extra corti s ol to produce a l dos terone-l i ke effects , i ncl udi ng hi gh Na + a nd bl ood pres s ure a s wel l a s l ow K+ l evel s i n the bl ood. Ti s s ue res i s ta nce to the effects of a l dos terone (pos s i bl y beca us e of decrea s ed a cti on on the receptors ) a nd/or Li ddl e’s s yndrome (a geneti c defect l ea di ng to overa cti vi ty of rea bs orbi ng s odi um cha nnel s ) ca n a l s o l ea d to ps eudohypera l dos teroni s m. Both ca n be trea ted us i ng pota s s i um-s pa ri ng di ureti cs (s ee bel ow).
Potassium-Sparing Diuretics and Aldosterone Regulation: Spi ronol a ctone a nd epl erenone a re members of a group of di ureti cs known a s “potassium-sparing.” Thes e two medi ca ti ons , us ed for hi gh bl ood pres s ure a nd hea rt fa i l ure, mi mi c the s teroi d s tructure of aldosterone (s ee fi gure bel ow) a nd functi on a s a competi ti ve i nhi bi tor for a l dos terone receptors i n the col l ecti ng duct. Thus , the effects of a l dos terone on membra ne permea bi l i ty a nd number/a cti vi ty of Na +–K+ pumps a re not s een, decrea s i ng rea bs orpti on of Na + a nd wa ter but decrea s i ng (i .e., s pa ri ng) the s ecreti on a nd l os s of K+.
Al dos terone a l s o i nfl uences pH by s ti mul a ti ng the s ecreti on of H +. Fi na l l y, a l dos terone i nfl uences rea bs orpti on of wa ter i n the ki dneys a nd, therefore, bl ood pres s ure, by i ncrea s i ng the rel ea s e of the pepti de hormone va s opres s i n (a l s o known a s a nti di ureti c hormone, ADH) from the pi tui ta ry gl a nd. Va s opres s i n’s a cti ons wi l l be di s cus s ed i n grea ter deta i l bel ow. VASOPRESSIN Al though not forma l l y pa rt of the RAAS, vasopressin, a l s o known a s ADH, i s i nti ma tel y i nvol ved i n s i mi l a r proces s a nd, i n fa ct, compl etes the proces s of determi ni ng the fi na l wa ter content of uri ne a nd, therefore, fl ui d ba l a nce i n the body (Ta bl e 18-4). Va s opres s i n, a pepti de hormone produced a s a prohormone i n the hypotha l a mus , i s es s enti a l for enha nced rea bs orpti on of wa ter by i ncrea s i ng the pres ence of ki dneys peci fi c, membra ne-bound wa ter cha nnel s ca l l ed aquaporin-2. Va s opres s i n ra i s es the number of thes e cha nnel s by enha nci ng the gene expres s i on for the protei n a nd i ncrea s i ng i ts del i very to the a ppropri a te pl a s ma membra ne. Thes e functi ons occur vi a s peci fi c va s opres s i n receptors coupl ed to a G s protei n (Cha pter 8) found on cel l s of the di s ta l convol uted tubul e a nd col l ecti ng ducts . Bi ndi ng of va s opres s i n to the receptor a cti va tes a n adenyl cyclase, whi ch converts ATP to cAMP. The res ul ti ng s i gna l i ng pa thwa y a cti va tes protein kinase A, whi ch phos phoryl a tes a qua pori n-2 protei n, s peci fi ca l l y on s eri ne 262 whi l e i n the Gol gi a ppa ra tus , promoti ng del i very of the ves i cl es to a nd membra ne fus i on wi th col l ecti ng duct membra nes . Vasopressin, Aquaporin-2, and Water Balance: The proper ba l a nce of wa ter a nd, therefore, pl a s ma vol ume i s es s enti a l for the proper functi oni ng of the body a nd di rectl y i nfl uences bl ood pres s ure a nd the hea rt. Aquaporin-2 pl a ys a ma jor rol e i n the regul a ti on of thi s fl ui d s ta tus a nd a l tera ti ons or muta ti ons of thi s protei n l ea d qui ckl y to di s ea s e s ta tes . Nephrogenic (kidney-associated) diabetes insipidus, a condi ti on of poor fl ui d ba l a nce regul a ti on a nd nota bl e for l a rge output of very di l ute uri ne, i s cl os el y a s s oci a ted wi th muta ti ons of the a qua pori n-2 cha nnel . Cha nges i n the expres s i on of the a qua pori n-2 gene a re l i nked to the unwa nted wa ter retenti on s een i n conges ti ve hea rt fa i l ure, l i thi um toxi ci ty, a nd duri ng pregna ncy. Neurogenic (brain-associated) or “central” diabetes insipidus occurs when vasopressin i s not s ynthes i zed beca us e of hypotha l a mus or pi tui ta ry gl a nd fa i l ure. Common ca us es i ncl ude hea d tra uma , tumors , or unknown rea s ons . Fi na l l y, etha nol i nta ke i nhi bi ts the a cti on of va s opres s i n a nd, therefore, a qua pori n-2, l ea di ng to decrea s ed wa ter rea bs orpti on a nd a s s oci a ted thi rs t/mi l d dehydra ti on.
Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010. Therefore, the coopera ti ve a cti ons of a l dos terone, whi ch ma i nl y i nfl uences el ectrol ytes , a nd va s opres s i n (ADH), whi ch i nfl uences wa ter, control the fi na l wa ter content of the uri ne, i ncl udi ng how di l ute i t i s . In fa ct, i n ti mes of extreme dehydra ti on, the col l ecti ng duct a l one ca n rea bs orb a pproxi ma tel y one qua rter of the wa ter i n the fi l tra te. ATRIAL NATRIURETIC PEPTIDE (ANP) ANP (Fi gure 18-6) i s a nother i mporta nt pepti de hormone tha t control s body wa ter a s wel l a s Na + a nd K+ l evel s . ANP i s rel ea s ed i n ti mes of body fl ui d overl oa d a nd/or hi gh bl ood pres s ure from mus cl e cel l s of the hea rt. ANP bi nds to s peci fi c receptors , whi ch a cti va te a guanyl cyclase to ma ke cGMP. The effects of ANP on the ki dney i ncl ude the fol l owi ng:
Figure 18-6. Natriuretic Peptides. Atri a l na tri ureti c pepti de (ANP) a nd bra i n na tri ureti c pepti de (BNP) a mi no a ci d s equences a nd s tructures a re i l l us tra ted. See text for further i nforma ti on. [Reproduced wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.] Increa s ed gl omerul a r fi l tra ti on by • di l a ti on of the a fferent a rteri ol e, • cons tri cti on of the efferent a rteri ol e, • rel a xa ti on of the mes a ngi a l cel l s . Increa s ed rea bs orpti on of Na Cl by • i ncrea s ed bl ood fl ow through the va s a recta , • i ncrea s ed os moti c dri ve for further wa ter a nd Na Cl excreti on. cGMP-dri ven phos phoryl a ti on/dea cti va ti on of s odi um cha nnel s (l oca ted on the di s ta l convol uted tubul e a nd col l ecti ng duct). Inhi bi ti on of reni n a nd a l dos terone producti on. The s um effect of ANP’s a cti ons i s i ncrea s ed excreti on of wa ter, s odi um, a nd pota s s i um by the ki dneys to hel p compens a te for exces s fl ui d a nd hi gh bl ood pres s ure. Beca us e of thes e qua l i ti es , ANP a nd brain natriuretic peptide (Fi gure 18-6), whi ch, des pi te the l a tter’s na me a re both s ecreted by hea rt cel l s , ha ve s hown uti l i ty i n fol l owi ng the progres s i on a nd res ol uti on of conges ti ve hea rt fa i l ure.
ACID–BASE BALANCE The ma i ntena nce of the proper acid–base balance between pH 7.35 a nd 7.45 i s es s enti a l to ma i nta i n the huma n body. Imba l a nces , i ncl udi ng a ci dos i s a nd a l ka l os i s , not onl y a dvers el y a ffect mul ti pl e protei n a nd enzyma ti c functi ons but, a s a res ul t, ca n a l s o l ea d ra pi dl y to dea th. Va ri ous mecha ni s ms found i n the l ungs , ki dneys , a nd bl ood s trea m cons ta ntl y a djus t thi s equi l i bri um a s per the needs of the body. Other buffers , i ncl udi ng a mmoni a (NH 3 ) (s ee bel ow), protei ns , a nd phos pha te a l s o contri bute to thi s i mporta nt functi ona l a nd protecti ve mecha ni s m by combi ni ng wi th H + to remove them from s ol uti on. The ki dney pl a ys a n es s enti a l rol e i n regul a ti ng the body’s a ci d–ba s e ba l a nce mecha ni s m a nd rel i es on the s el ecti ve a nd regul a ted rea bs orpti on a nd s ecreti on of H + a nd bicarbonate ions (HCO3 –), two pri ma ry mol ecul es of the bi ca rbona te bufferi ng s ys tem, i nvol vi ng the rea cti on
. Thi s i s done by two types of cel l s , whi ch s ecrete or rea bs orb H + or HCO3 – vi a
H +-ATPase proton pumps, a nd H +–K+ a nd Cl ––HCO3 – cha nnel s . Aci d–ba s e regul a ti on i s a l s o found i n the col l ecti ng duct. Increa s ed a ci di ty i n the body a ugments rea bs orpti on of bi ca rbona te from the proxi ma l convol uted tubul e a nd l oop of Henl e, a nd col l ecti ng duct cel l s whi l e a l s o ca us i ng i ncrea s ed s ecreti on of hydrogen from the col l ecti ng duct. Increa s ed ba s e ca us es the ki dney to decrea s e H + s ecreti on a nd ca us es l es s rea bs orpti on/more s ecreti on of bi ca rbona te from the nephron. Ta bl e 18-2 provi des s peci fi cs rega rdi ng the s peci fi c mecha ni s ms i nvol ved wi th thes e two i ons . NH 3 AND ACID–BASE BALANCE Another a ci d–ba s e ba l a nce mecha ni s m (Fi gure 18-7) i nherent i n the ki dney’s functi ons i s the ha ndl i ng of ammonia (NH 3 ) a s pa rt of a s econda ry bufferi ng s ys tem. Al though the pri ma ry ma na gement of a mmoni a l evel s i s vi a the urea cycle i n the l i ver (Cha pter 11), a ny neces s a ry el i mi na ti on of urea a nd/or exces s , toxi c ammonium ions (NH 4 +) occurs i n the ki dney’s nephrons .
Figure 18-7. Overview of Ammonia (NH 3 ) Balance in the Body. In pa rti cul a r, NH 3 ca n freel y cros s the membra nes of the nephron a nd, a fter combi ni ng wi th a H + to form NH 4 +, be excreted i n the uri ne (Fi gure 18-8A) wi th a net a ci d l os s . Fi na l l y, NH 3 i s produced by the enzyma ti c degra da ti on of the a mi no a ci d glutamine, converted fi rs t to glutamate a nd then to α-ketoglutarate, whi ch conti nues through the Krebs ’ cycl e (Fi gure 18-8B). If a ba s e exces s exi s ts , the convers i on of gl uta mi ne a nd, therefore, the producti on of NH 3 i s i nhi bi ted to hel p the body re-es ta bl i s h the proper a ci d–ba s e ba l a nce.
Figure 18-8. A–B. The Ammonia (NH 3 ) Buffering System. (A) Interconvers i on of NH 3 to a mmoni um (NH 4 +), whi ch ca ptures one H +. Thi s rea cti on occurs freel y i n the huma n body, dependi ng on concentra ti ons of ea ch res pecti ve mol ecul e. (B) Convers i on of gl uta mi ne to gl uta ma te, vi a the enzyme gl uta mi na s e, wi th producti on of one NH 3 mol ecul e (upper) a nd gl uta ma te to α-ketogl uta ra te vi a the enzyme gl uta ma te dehydrogena s e wi th producti on of a s econd NH 3 a nd NADH (l ower). Inges ted s ources of NH 3 s uch a s protei n a nd a mi no a ci ds a re proces s ed vi a the ga s troi ntes ti na l s ys tem a nd el i mi na ted i n feces or tra ns ferred through the porta l ci rcul a ti on to the l i ver or to the body vi a s ys temi c ci rcul a ti on. In the l i ver, NH 3 i s converted to urea a nd returned to the s ys temi c ci rcul a ti on where i t ca n be el i mi na ted a s uri ne by the ki dneys or s ent ba ck to the ga s troi ntes ti na l s ys tem. The ba l a nce of NH 3 depends on va ryi ng contri buti ons of l i ver, ki dney, a nd i ntes ti na l meta bol i s m a nd/or el i mi na ti on. [Reproduced wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] Aci d–ba s e ba l a nce ca n a l s o ha ve a ma rked effect on the effi ca cy of medi ca ti ons by i nfl uenci ng how they a re el i mi na ted from the body. Ma ny medi ca ti ons a re excreted vi a the ki dneys but, once i n the nephron, they ca n be ei ther rea bs orbed a nd returned to the ci rcul a ti on (Fi gure 18-3) or s ecreted/ el i mi na ted i n the uri ne. Medi ca ti ons s ecreted i n the proxi ma l tubul e a re i ndi ca ted bel ow a l ong wi th predomi na te form (a ci d, ba s e, or nei ther) a t s ecreti on (Ta bl e 18-5). The form of a medi ca ti on ca n i nfl uence s ecreti on, a s a ci di c medi ca ti ons a re s ecreted more when the uri ne i s ba s i c a nd vi ce vers a . In cri ti ca l ca re a nd other cl i ni ca l s etti ngs , cha ngi ng uri ne pH, ei ther a s the res ul t of a n i l l nes s or s el ecti ve medi ca l i nterventi on, ca n a ffect the ul ti ma te el i mi na ti on of a ci di c a nd ba s i c drugs from the bl ood s trea m. In very s peci fi c ci rcums ta nces , thi s a bi l i ty ca n be uti l i zed to a i d i n pa ti ent ca re.
Table 18-5. Medi ca ti ons Secreted i n Proxi ma l Tubul e Hyperammonemia and NH 3 Toxicity: Hi gh l evel s of free NH 3 i n the body ca us e ha rm to va ryi ng pa rts of the body. Mos t promi nent a re the effects on the bra i n a nd nervous s ys tem, whi ch s ubs equentl y l ea d to other cl i ni ca l ma ni fes ta ti ons . Menta l reta rda ti on, s ei zures , a nd unus ua l a nd uncontrol l ed mus cl e movements a s wel l a s brea thi ng i rregul a ri ti es a nd poor control of body tempera ture a re common. Coma a nd dea th ma y res ul t wi thout trea tment. There a re mul ti pl e enzyme defi ci enci es , mos t due to a utos oma l reces s i ve muta ti ons , whi ch res ul t i n exces s a mmoni um i n the body. Thes e va ri ous di s ea s e s ta tes a re s epa ra ted i nto pri ma ry—i nvol vi ng enzymes from the urea cycl e (s ee Fi gure 5.9a ) a nd s econda ry—i nvol vi ng enzymes tha t a re not pa rt of the urea cycl e. Exa mpl es a re l i s ted bel ow.
SYNTHETIC FUNCTIONS SYNTHESIS OF ERYTHROPOIETIN Erythropoietin, a l s o known a s hema topoi eti n, i s a ma jor gl ycoprotei n cytoki ne a nd hormone tha t regul a tes the producti on of red bl ood cel l s (Fi gure 18-9). Erythropoi eti n i s produced by endothel i a l cel l s tha t a re pa rt of the ki dney’s peri tubul a r ca pi l l a ri es . The a mount of erythropoi eti n produced i s bel i eved to be regul a ted by a tra ns cri pti on fa ctor tha t becomes hydroxyl a ted (OH –) a nd then gets degra ded when oxygen i s a bunda nt. When oxygen l evel s a re l ow, the tra ns cri pti on fa ctor bi nds to erythropoi eti n receptors on red bl ood cel l membra nes a nd bone ma rrow cel l s a nd a cti va tes a n a s s oci a ted Janus kinase 2 res ul ti ng i n phos phoryl a ti on of tyros i ne res i dues (Cha pter 8). Acti va ti on by erythropoi eti n protects the red bl ood cel l s from norma l l y progra mmed cel l dea th (a poptos i s ) a nd s ti mul a tes the producti on of new red bl ood cel l s from thei r s peci fi c precurs or cel l s .
Figure 18-9. Regulation of Red Blood Cell Production by Erythropoietin. If the a bi l i ty of bl ood to ca rry oxygen decrea s es beca us e of a fa l l i n numbers of red bl ood cel l s (e.g., norma l cel l dea th, pa thol ogi ca l des tructi on of red bl ood cel l s , bl eedi ng, etc.), the ki dney s ens es l ower pO2 l evel s a nd i ncrea s es the l evel s of erythropoi eti n (EPO). EPO then s i gna l s the bone ma rrow to i ncrea s e producti on of red bl ood cel l s . See text for further deta i l s . [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Rena l di s ea s e l ea ds to a drop i n erythropoi eti n producti on a nd, therefore, a l ow red bl ood cel l count or a nemi a . Arti fi ci a l l y produced erythropoi eti n ca n be us ed to i ncrea s e thes e pa ti ents ’ red bl ood cel l s count a s wel l a s thos e s ufferi ng from certa i n chroni c di s ea s es s uch a s hea rt fa i l ure, undergoi ng chemothera py or ra di a ti on thera py for ca ncer, a nd/or cri ti ca l l y i l l a nd i n need of the a ddi ti ona l oxygen ca rryi ng ca pa ci ty. Erythropoi eti n ha s a l s o been s hown to be a cti ve on bra i n cel l s a nd ha s been us eful i n the trea tment of s chi zophreni a a nd rel a ted di s ea s es . It ha s a l s o been a bus ed by a thl etes who uti l i ze thi s cytoki ne to i ncrea s e red bl ood cel l counts a nd, therefore, oxygen ca rryi ng/performa nce ca pa ci ti es . ROLE IN VITAMIN D SYNTHESIS The ki dney a l s o pl a ys a vi ta l rol e i n the s ynthes i s of a cti ve vitamin D a nd, s econda ri l y, the regul a ti on of Ca 2+ concentra ti on i n the body. Vitamin D 3 i s i ni ti a l l y hydroxyl a ted i n the l i ver to produce 25(OH)D 3 a nd s tored there unti l requi red (Cha pter 3, Fi gure 3-10). A s econd hydroxyl group i s a dded when parathyroid hormone (PTH) a cti va tes 1-α-hydroxyl a s e i n the proxi ma l convol uted tubul e of the ki dney to produce the a cti ve form 1,25(OH)2 D 3 or calcitriol. Vi ta mi n D a nd the vi ta mi n D receptor found ma i nl y i n the di s ta l convol uted tubul e a l s o a ct to i ncrea s e Ca 2+ a nd phos pha te i on (PO4 3–) rea bs orpti on by i ncrea s i ng the number of thes e i on cha nnel s . PTH a l s o i ncrea s es rea bs orpti on of Ca 2+ a nd Mg2+ i n the proxi ma l porti on of the di s ta l convol uted tubul e a nd the a s cendi ng l i mb of the l oop of Henl e i n oppos i ti on to the a cti on of the hormone calcitonin (for more deta i l , s ee Cha pter 13). Kidney Stones: Kidney stones (nephrolithiasis) ca n form beca us e of a number of di fferent bi ochemi ca l mea ns often s econda ry to a di s ea s e s ta te. Types of ki dney s tones a nd potenti a l ca us es a re l i s ted bel ow:
REVIEW QUESTIONS 1. Wha t a re the rol es of the rena l corpus cl e, gl omerul us , nephron, tubul es , a nd col l ecti ng s ys tems ? 2. How do rea bs orpti on, s ecreti on, a nd excreti on di ffer? 3. Wha t a re the functi ons of reni n, a ngi otens i n I a nd II, a ngi otens i n-converti ng enzyme, a l dos terone, va s opres s i n, a nd a tri a l na tri ureti c pepti de a nd how does ea ch rel a te to the reni n–a ngi otens i n s ys tem? 4. Wha t i s the functi on of erythropoi eti n? 5. How do the components of the gl omerul us a ct together to crea te a mol ecul a r fi l teri ng mecha ni s m? 6. Wha t receptors a re a cti va ted a nd by wha t s i gna l i ng pa thwa ys to i ncrea s e or decrea s e rena l rea bs orpti on or s ecreti on of mol ecul es ? 7. Wha t i s the rol e of the reni n–a ngi otens i n s ys tem i n fl ui d a nd el ectrol yte homeos ta s i s a nd wha t i s the i mpa ct of a bnorma l i ti es i n tri ggeri ng s econda ry hypertens i on? 8. Wha t i s the rol e of the ki dney i n the s ynthes i s of erythropoi eti n a nd the a cti ve form of vi ta mi n D? 9. Wha t a re the ba s i c concepts , rea cti ons , a nd rol e of the ki dney i n the reni n–a ngi otens i n–a l dos terone s ys tem?
CHAPTER 19 THE NERVOUS SYSTEM Editor: Kathryn Beck-Yoo, MD Depa rtment of Anes thes i ol ogy, Swedi s h Medi ca l Center a t Ba l l a rd, Sea ttl e, Sea ttl e, Wa s hi ngton, USA
Components of the Nervous Sys tem Nerve Impul s e Conducti on Autonomi c Nervous Sys tem Neurotra ns mi tters Bi ochemi s try of Vi s i on Anes thes i a Revi ew Ques ti ons
OVERVIEW The nervous s ys tem provi des a network of coordi na ted s i gna l i ng for bodi l y functi ons . Soma ti c nerves promote s kel eta l mus cl e a cti vi ty a nd, therefore, gros s movements . Autonomi c nerves provi de s i gna l s for i nterna l orga ns , i ncl udi ng regul a ti on of thei r functi on. Thes e s i gna l s a re tra ns mi tted by nerves , whi ch rel y on membra ne-bound protei n pumps for conducti on of the i mpul s e. Speci a l i zed l i pi ds form myel i n s hea ths , whi ch a i d i n the fa s t a nd effi ci ent tra ns mi s s i on of thes e s i gna l s . Neurotra ns mi tters , often s ma l l pepti des , a l l ow conti nua ti on of the nerve s i gna l between neurons a nd from neurons to ta rget ti s s ues . The brea kdown of thi s network of s i gna l i ng a nd regul a ti on l ea ds to s evera l neurol ogi ca l di s ea s es .
COMPONENTS OF THE NERVOUS SYSTEM The nervous s ys tem i s a na tomi ca l l y a nd functi ona l l y compos ed of two ma i n pa rts : the central nervous system (CNS) a nd the peripheral nervous system (PNS) (Fi gure 19-1). The CNS cons i s ts of the bra i n a nd s pi na l cord a nd the PNS i s compos ed of a l l nerves outs i de of the CNS, i ncl udi ng a l l s pi na l a nd cra ni a l nerves . In a ddi ti on, there a re further cl a s s i fi ca ti ons for the nerves of the PNS. Fi rs t, nerves a re di s ti ngui s hed by the di recti on of nerve propa ga ti on. Afferent (sensory) neurons conduct a cti on potenti a l s towa rd the CNS. Efferent (motor) neurons tra ns mi t i mpul s es a wa y from the CNS to effectors s uch a s mus cl es or gl a nds . Second, nerves of the PNS ca n be further di vi ded i nto the somatic (skeletal muscle) a nd autonomic (organs, glands, and smooth muscle) nervous systems. The a utonomi c nervous s ys tem (ANS) i s di vi ded i nto the sympathetic (SNS) a nd parasym-pathetic nervous system (PSNS). Genera l l y, both i ntera ct wi th the s a me effectors , but often pa ra doxi ca l l y. The SNS i s i nvol ved i n the a cti vi ti es a s s oci a ted wi th the fi ght-or-fl i ght res pons e, hel pi ng the body to cope wi th s tres s . The PSNS promotes a cti vi ti es tha t s upport the body whi l e a t res t, i ncl udi ng di ges ti on.
Figure 19-1. Overview of the Nervous System. Schema ti c di a gra m compa ri ng s ome a na tomi c a nd neurotra ns mi tter fea tures of a utonomi c a nd s oma ti c motor nerves . Onl y the pri ma ry tra ns mi tter s ubs ta nces a re s hown; pa ra s ympa theti c ga ngl i a a re not i ndi ca ted. See text for more deta i l s . Ach, a cetyl chol i ne; D, dopa mi ne; Epi , epi nephri ne; M, mus ca ri ni c receptors ; N, ni coti ni c receptors ; NE, norepi nephri ne. [Reproduced
wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.] Neurons a re the funda menta l components of the nervous s ys tem. The ma i n components of a neuron i ncl ude the s oma (or cel l body), dendri tes , a xon, a nd the a xon termi na l s (Fi gure 19-2). The cel l body, or soma, i s the centra l pa rt of the neuron tha t conta i ns the nucl eus a nd other cel l orga nel l es . The dendrites a re the bra nchi ng s tructures of the neuron tha t recei ves s ti mul i or mes s a ges vi a chemoreceptors . The axon i s a s l ender, ca bl e-l i ke extens i on of a neuron tha t ca rri es nerve i mpul s es a wa y from the s oma towa rd the a xon termi na l . The axon terminals a re the ha i r-l i ke termi na l s of the a xon where neurotra ns mi tters a re rel ea s ed from the s yna pti c knobs . Neurons communi ca te wi th one a nother vi a s yna pti c tra ns mi s s i on, where the a xon termi na l of one neuron comes i nto cl os e conta ct wi th the dendri tes of a nother neuron. Neurons ca n communi ca te chemi ca l l y, vi a neurotra ns mi tters , or el ectri ca l l y wi th el ectri ca l l y conducti ve juncti ons between the cel l s . The myelin sheath i s a phos phol i pi d membra ne tha t s urrounds a nd protects the a xons of s ome nerves (Fi gure 19-2). The pri ma ry phos phol i pi d of the membra ne i s ga l a ctocerebro-s i de, a s phi ngol i pi d (Cha pters 3 a nd 8). Such phos phol i pi ds i n nerve membra nes s trengthen the s hea th. Myel i n i s produced by Schwa nn cel l i n the PNS a nd by ol i godendrocytes i n the CNS. The neurofibrillar nodes (a l s o known a s nodes of Ranvier) a re the ga ps i n the myel i n s hea th, where the a cti on potenti a l occurs duri ng conducti on a l ong the a xon.
Figure 19-2. General Structure of a Nerve Cell. The genera l s tructure of a nerve cel l i ncl udes the cel l body wi th a nucl eus a nd nucl eol us , mul ti pl e dendri tes , a nd a n a xon. Further deta i l s a re provi ded i n the text. The i ni ti a l s egment, i mmedi a tel y a fter the a xon hi l l ock, conta i ns no myel i n s hea th but does conta i n a very hi gh dens i ty of vol ta ge-ga ted Na + cha nnel s for conti nued propa ga ti on of the nerve i mpul s e. In the myel i na ted s ecti on, the i mpul s e i s conducted from node to node (s ee bel ow) unti l rea chi ng the termi na l boutons where neurotra ns mi tter mol ecul es a re s tored for rel ea s e a nd nerve s i gna l propa ga ti on (s ee bel ow). [Reproduced wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.]
NERVE IMPULSE CONDUCTION Succes s ful conducti on of nerve i mpul s es rel i es on three el ements : a n el ectri ca l l y i ns ul a ted membra ne, energy-dri ven pumps to es ta bl i s h a nd ma i nta i n i on gra di ents , a nd i oni c s el ecti ve cha nnel s . Demyelinating Disorders: Demyel i na ti ng di s orders refer to a ny condi ti on tha t di s rupts myelin i n ei ther the CNS or PNS res ul ti ng i n a n i nterrupti on of norma l nerve tra ns mi s s i on. The proces s of demyel i na ti on ca n be beca us e of a n i s chemi c, meta bol i c, geneti c, or i nfecti ous i ns ul t. However, beca us e a demyel i na ti ng di s order ca n fol l ow a n i nfecti on or a n i nocul a ti on, i t ha s been hypothes i zed tha t a n a utoi mmune mecha ni s m i s to bl a me. In s ome di s orders , remyel i -na ti on ca n occur wi th a n a s s oci a ted degree of neura l recovery. However, ma ny ti mes , neurol ogi c defi ci ts a re perma nent. Beca us e myel i n i s formed i n the CNS by ol i godendrocytes a nd i n the PNS by Schwa nn cel l s , demyel i na ti ng di s orders ca n ta rget di fferent l oca ti ons a nd ha ve a va ri a bl e pres enta ti on a s i l l us tra ted bel ow.
NEURON AT REST Neurons ma i nta i n a res ti ng membra ne potenti a l of –70 mV by a denos i ne tri phos pha te (ATP)-medi a ted a cti ve tra ns port a nd pa s s i ve di ffus i on of i ons . The Na+–K +-ATPase pump of the cel l membra ne tra ns fers three Na + out of the cel l whi l e tra ns ferri ng two K+ i ntra cel l ul a rl y (Cha pter 8 a nd Fi gure 19-3). The s tea dy-s ta te condi ti on res ul ts i n a n extra cel l ul a r Na + concentra ti on of 150 mmol /L a nd a n i ntra cel l ul a r Na + concentra ti on of 5–10 mmol /L. The extra cel l ul a r K+ concentra ti on i s 3–4 mmol /L a nd i ntra cel l ul a r i s 120–135 mmol /L. Thi s gi ves ri s e to the nega ti ve s ta te wi thi n the cel l , a nd i t crea tes a concentra ti on gra di ent tha t wi l l fa vor the extra cel l ul a r di ffus i on of pota s s i um a nd the i ntra cel l ul a r di ffus i on of s odi um upon depol a ri za ti on.
Figure 19-3. The Na+–K +-ATPase Pump. The pump a nd a s s oci a ted s odi um (Na +)- a nd pota s s i um (K+)-ga ted cha nnel s a re s hown. Thes e ga ted cha nnel s a l l ow the s l ow res tora ti on of Na + a nd K+ to thei r res pecti ve oppos i te s i de (s ee Fi gure 19-4 a nd a s s oci a ted text). ADP, a denos i ne di phos pha te; ATP, a denos i ne tri phos pha te. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
Figure 19-4. A–B. The Nerve Impulse. (A) The res ti ng membra ne i s ma i nta i ned a t a membra ne potenti a l of –70 mV. The ons et of a n a cti on potenti a l opens the ga ted Na + cha nnel s (s ee Fi gure 19-3 a nd Fi gures bel ow upper gra ph), l ea di ng to the effl ux of Na +. Once the thres hol d l evel i s rea ched a t –55 mV (da s hed red l i ne), the depol a ri za ti on of the membra ne ca us ed by Na + i s ra pi d unti l a ma xi mum membra ne potenti a l of
+30–35 mV i s rea ched. At thi s ma xi mum vol ta ge, the ga ted Na + cha nnel s cl os e a nd the ga ted K+ cha nnel s open (s ee Fi gure 19-3 a nd Fi gures bel ow upper gra ph) a nd a n i nfl ux of K+ l ea ds to a drop i n the membra ne potenti a l . The Na +–K+-ATPa s e pump (Fi gure 19-3) a l s o contri butes to res tora ti on of the res ti ng membra ne potenti a l (not s hown). (B) Il l us tra ti on of rel a ti ve membra ne permea bi l i ty of Na + (PNa+) a nd K+ (PK+) duri ng propa ga ti on of the nerve i mpul s e. [Reproduced wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] NERVE IMPULSE Severa l s ti mul i ca n a cti va te a neuron l ea di ng to the genera ti on of a nerve impulse (Fi gure 19-4). Thes e i ncl ude chemi ca l , mecha ni ca l , or el ectri ca l exci ta ti on. Impul s e propa ga ti on i s a ccompa ni ed by s ma l l membra ne depol a ri za ti ons . When the depol a ri za ti on exceeds the threshold l evel of -55 mV, a cti va ted s odi um cha nnel s ca us e a fl ood of Na + i ons i nto the cel l whi ch dri ves further depol a ri za ti on. Thi s ca us es a rel a ti ve exces s of ca ti ons i n the cel l , whi ch eventua l l y l ea ds to a membra ne potenti a l of +35 mV. If the i ni ti a l s ti mul us i s l es s tha n –55 mV, few Na + cha nnel s a re opened a nd the thres hol d l evel i s never rea ched. As a res ul t, nerve conducti on i s a n a l l -or-none res pons e. Niemann–Pick C Disease: Niemann–Pick C Disease i s a n a utos oma l reces s i ve, l i fe-threa teni ng l ys os oma l s tora ge di s ea s e, s tri ki ng ma i nl y i n mi d-to-l a te chi l dhood but a l s o i n i nfa nts a nd a dul ts . A ma jori ty of ca s es i nvol ve a muta ti on on the NPC1 gene on chromos ome 18 res ul ti ng i n l ys os oma l chol es terol a nd gl ycos phi ngol i pi d a ccumul a ti on i n neurons a nd gl i a l cel l s i n the CNS. The l i pi d bui l d-up res ul ts i n a di s torti on of the neuron ma i nl y by erroneous forma ti on of dendri tes , whi ch s ubs equentl y a l ters neurotra ns mi s s i on. The cl i ni ca l pres enta ti on i s hi ghl y va ri a bl e but us ua l l y pres ents wi th the ons et of movement a nd l ea rni ng di s a bi l i ti es a nd ea rl y i mpa i rment of verti ca l eye movement (verti ca l s upra nucl ea r ga ze pa l s y). In a ddi ti on, pa ti ents ca n experi ence l os s of mus cl e tone (dys toni a ), s ei zures , di s a bl i ng joi nt probl ems , probl ems s wa l l owi ng (dys pha gi a ), a nd, i n a dul ts , va ryi ng cogni ti ve i mpa i rment a s wel l a s ps ychos i s a nd/or depres s i on. Mos t pa ti ents di e due to compl i ca ti ons of thi s di s ea s e before the a ge of 20 yea rs . As the neuron’s a cti on potenti a l depol a ri zes , vol ta ge-ga ted ca l ci um cha nnel s ca us e a n i nfl ux of ca l ci um i ons (Ca 2+). Thi s Ca 2+ i nfl ux tri ggers the fus i on of s tora ge ves i cl es wi th the termi na l bud membra ne, rel ea s i ng a neurotra ns mi tter i nto the s yna pti c cl eft (Fi gure 19-5). Receptors on the a djoi ni ng neuron bi nd wi th the neurotra ns mi tter ca us i ng a n a l tera ti on i n the receptor’s confi gura ti on. Thes e neurotra ns mi tter-ga ted i on cha nnel s a ct a s a n i nterna l pore a l l owi ng Na + to fl ow down thei r el ectrochemi ca l gra di ent i nto the cel l . As the i ntra cel l ul a r concentra ti on of Na + ri s es , the cha rge wi thi n the cel l becomes pos i ti ve ca us i ng a depol a ri za ti on of the membra ne. The res ul t i s a propa ga ti ng el ectri ca l s i gna l known a s a n action potential. Thi s proces s repea ts unti l the ta rget ti s s ue ha s been rea ched.
Figure 19-5. The Synaptic Cleft. After the nerve i mpul s e rea ches a nd depol a ri zes the pres yna pti c membra ne, the i nfl ux of Ca 2+ res ul ts i n the fus i on of pres yna pti c ves i cl es to the s urfa ce of the membra ne. The neurotra ns mi tter rel ea s ed i nto the cl eft bi nds to the receptor s i te on the pos ts yna pti c membra ne, whi ch wi l l propa ga te the a cti on potenti a l onto the a dja cent neuron. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Alzheimer’s Disease (AD): AD i s a progres s i ve a nd fa ta l degenera ti ve di s ea s e of the bra i n a nd the l ea di ng ca us e of dementi a . Al zhei mer’s i s due to the a ccumul a ti on of amyloid plaques between neurons a nd neurofibrillary tangles (NFTs) wi thi n the neurons . The exa ct mecha ni s m of how the pl a ques a nd ta ngl es a ffect bra i n functi on i s not ful l y unders tood. Pa ti ents va ry i n both a mount a nd ra ti o of pl a ques vers us ta ngl es . However, pa ti ents who ha ve pri ma ri l y onl y a myl oi d pl a ques deteri ora te s l ower tha n thos e wi th pri ma ri l y ta ngl es . Amyl oi d pl a ques cons i s t of β-amyloid protein, whi ch i s a protei n fra gment of amyloid precursor protein (APP). Norma l l y, thes e protei n fra gments a re degra ded a nd el i mi na ted vi a one of two pa thwa ys . In the ca s e of AD, APP i s cl ea ved by β-secretase a nd γ-secretase to form a myl oi d-β-deri ved di ffus i bl e l i ga nds . The a ccumul a ti on of the a myl oi d forms ha rd, dens e pl a ques . Thes e pl a ques phys i ca l l y a l ter the a rchi tecture of the s urroundi ng ti s s ue, a nd therefore a l ter norma l neura l functi oni ng. In a ddi ti on, a myl oi d depos i ts ca us e mi tochondri a l dys functi on l ea di ng to prema ture, s chedul ed cel l dea th (a poptos i s ). The s econd ma jor fi ndi ng i n AD i s NFT. Thes e ta ngl es a re ma de of s i x i s oforms of tau proteins, hi ghl y s ol ubl e mi crotubul e-a s s oci a ted protei ns found norma l l y i n hea l thy bra i ns . In a hea l thy pers on, phos phoryl a ti on of ta u protei n des ta bi l i zes a xona l mi crotubul es , a l l owi ng norma l neuron functi on. In AD, the proces s i s a l tered when hyperphosphorylation of tau res ul ts i n the forma ti on of NFTs . Al l s i x ta u i s oforms a re pres ent i n the hyper-phos phoryl a ted s ta te. As they a re a l tered, they become i ns ol ubl e a ggrega tes wi thi n the neuron l ea di ng to i ns ta bi l i ty i n the neura l tubes . Thi s i mpa cts norma l nerve conducti on wi thi n the bra i n. Myel i n s hea ths ena bl e a cti on potenti a l s to tra vel fa s ter tha n i n unmyel i na ted a xons a nd a t decrea s ed energy expendi ture (Fi gure 19-6). The uns hea thed nodes of Ra nvi er conta i n a hi gh dens i ty of vol ta ge-ga ted i on cha nnel s . The a cti on potenti a l i s conducted a l ong the a xon vi a
conducti on jumpi ng from one node of Ra nvi er to the next.
Figure 19-6. Effect of the Myelin Sheath on Nerve Impulse Propagation. Pos i ti ve cha rges from the membra ne a hea d of a nd behi nd the a cti on potenti a l fl ow i n the a rea of nega ti ve cha rge a s the potenti a l tra vel s down the a xon (s ee di recti on of propa ga ti on). Unmyel i na ted nerves propa ga te the nerve i mpul s e by s l ow, conti nuous conducti on. Nerves wi th a myel i n s hea th a re a bl e to conduct the nerve i mpul s e much qui cker vi a s a l ta tory conducti on from one node of Ra nvi er to the next. [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] REPOLARIZATION Repolarization i s dependent upon s evera l fa ctors . The s odi um cha nnel s a re i na cti va ted a nd ca nnot reopen unti l membra ne pol a ri za ti on i s res et to the res ti ng membra ne potenti a l of -70mV. Ina cti vi ty of the s odi um cha nnel s ca us es a cons equent drop i n s odi um permea bi l i ty a nd a n i ncrea s e i n pota s s i um conducta nce. Duri ng thi s ti me, the membra ne i s una bl e to repea t exci ta ti on (refractory period). Eventua l l y, the s odi um– pota s s i um pump re-es ta bl i s hes the ori gi na l ba s el i ne gra di ents a nd the cel l i s returned to i ts res ti ng potenti a l .
AUTONOMIC NERVOUS SYSTEM The PNS i s di vi ded i nto the somatic nervous s ys tem a nd ANS. The s oma ti c nervous s ys tem i ncl udes vol unta ry movements of s kel eta l mus cl es a s wel l a s touch s ens a ti on, vi s i on, a nd hea ri ng. The ANS coordi na tes a nd ma i nta i ns a s tea dy s ta te a mong i nterna l orga ns , s uch a s regul a ti on of ca rdi a c mus cl e, s mooth mus cl e, a nd gl a nds . It i s cons i dered to be a n i nvol unta ry s ys tem. The ANS i s further s ubdi vi ded i nto the sympathetic a nd parasym-pathetic nervous s ys tems , the a cti vi ty of whi ch i s i ntegra ted by the hypotha l a mus . An a utonomi c nerve pa thwa y from the CNS to a vi s cera l effector cons i s ts of two ma jor neurons tha t s yna ps e i n a ga ngl i on outs i de of the CNS: the preganglionic a nd postganglionic neurons . Thi s di ffers from the s oma ti c nervous s ys tem, wherei n a s i ngl e motor neuron tra vel s from the CNS to the i nnerva ted ti s s ue. SYMPATHETIC NERVOUS SYSTEM Thi s s ys tem i s domi na nt i n a s tres s s ta te or the “fi ght-or-fl i ght” res pons e. It s erves to mobi l i ze energy s tores i n ti mes of need. Acti va ti on of the SNS res ul ts i n ta chyca rdi a (i ncrea s ed hea rt ra te), di l a ti on of l a rge bl ood ves s el s i n s kel eta l mus cl es , va s ocons tri cti on of s ki n a nd vi s cera l bl ood ves s el s , di l a ti on of bronchi ol es , mobi l i za ti on of fa tty a ci ds from tri gl yceri des i n a di pos e ti s s ue, a nd mobi l i za ti on of l i ver gl ycogen to gl ucos e. The l a tter cha nges a re des i gned to s uppl y energy to the body a nd to provi de mus cl e ti s s ue wi th di rect energy vi a a na erobi c gl ycol ys i s . In a ddi ti on, di ges ti ve s ecreti ons a nd i ntes ti na l peri s ta l s i s decrea s e. The SNS’s prega ngl i oni c neurons ori gi na te i n the thora col umba r s pi na l cord (from T1 to L2). The s ympa theti c ga ngl i a a re l oca ted i n two cha i ns jus t outs i de of the s pi na l col umn, i n the pa ra vertebra l ga ngl i a . The prega ngl i oni c, unl i ke the pos tga ngl i oni c, neurons a re myel i na ted. PARASYMPATHETIC NERVOUS SYSTEM The rol e of the pa ra s ympa theti c nervous s ys tem i s to cons erve a nd res tore energy or, i n oppos i ti on to the s ympa theti c s ys tem, to “res t a nd di ges t.” It i s the pri ma ry functi oni ng di vi s i on i n rel a xed s i tua ti ons to promote the norma l functi oni ng of s evera l orga n s ys tems . The res ul t i n the body i s decrea s ed hea rt ra te (bra dyca rdi a ), bronchi ol a r cons tri cti on, pupi l cons tri cti on, a nd i ncrea s ed peri s ta l s i s a nd s ecreti ons . The prega ngl i oni c neurons ori gi na te i n s evera l of the cra ni a l nerve nucl ei (CN 3, 7, 9, a nd 10) a nd s a cra l s egments (S2, S3, a nd S4) of the s pi na l cord, gi vi ng ri s e to the term cra ni os a cra l di vi s i on. In contra s t to the SNS, the ga ngl i a a re l oca ted cl os e to the ta rget orga ns , a nd the pos tga ngl i oni c neurons a re rel a ti vel y s hort. The effects of both the s ympa theti c a nd pa ra s ympa theti c s ys tems on the body a re s umma ri zed i n Ta bl e 19-1.
Table 19-1. Summa ry of the Autonomi c Nervous Sys tem Guillain–Barré Syndrome (GBS, Acute Idiopathic Polyneuropathy): GBS i s a n a cute i nfl a mma tory di s ea s e l ea di ng to demyel i na ti on a ffecti ng the PNS. It i s the mos t commonl y a cqui red i nfl a mma tory neuropa thy. GBS i s a medi ca l emergency, but a ma jori ty of pa ti ents ful l y recover over a peri od of months . As ma ny a s one-thi rd of pa ti ents wi l l exhi bi t res i dua l wea knes s 3 yea rs a fter the event, neces s i ta ti ng retra i ni ng, orthoti c a ppl i a nces , a nd reha bi l i ta ti ons . Fa ta l i ti es occur i n more tha n 2% of a l l pa ti ents . GBS i s typi ca l l y cha ra cteri zed by a progres s i ve s ymmetri c mus cl e wea knes s , l os s of s ens a ti on, pa ra l ys i s , a nd decrea s ed refl exes (hyporefl exi a ) s ta rti ng i n the l ower extremi ti es a nd progres s i ng ra pi dl y to the a rms a nd fa ce. As the s ymptoms progres s , the res pi ra tory mus cl es ca n a l s o be a ffected neces s i ta ti ng mecha ni ca l venti l a ti on. In a ddi ti on, the ANS ca n be di s rupted res ul ti ng i n s evere ta chyca rdi a , a rrhythmi a s , a nd hypo- or hypertens i on. The di a gnos i s of GBS i s a cl i ni ca l one. Al though the preci s e mecha ni s m of GBS i s unknown, there i s proba bl y a n i mmunol ogi c ba s i s to the di s order res ul ti ng i n da ma ge to the myel i n s hea th of the PNS. GBS s ometi mes fol l ows a n i nfecti ve i l l nes s , i nocul a ti on, or s urgi ca l procedure. The mos t common a ntecedent i nfecti on i s the ba cteri a Campylobacter jejuni. Other known pa thogens i ncl ude enteri c vi rus es , cytomega l ovi rus , Eps tei n–Ba rr vi rus , a nd Mycomplasma. Lymphocyti c i nfi l tra ti on a nd ma cropha ge-medi a ted demyel i na ti on of peri phera l nerves i s the underl yi ng pa thol ogy, a nd s ymptoms genera l l y res ol ve wi th remyel i na ti on. Symptoms us ua l l y a ppea r 1–3 weeks a fter the a ntecedent event.
NEUROTRANSMITTERS Neurotransmitters a re mol ecul es (often a mi no a ci ds /s ma l l pepti des a nd/or thei r deri va ti ves ) whi ch ca rry a nd often a mpl i fy a s i gna l from one nerve to a nother nerve or to a nother cel l type. Cl a s s i c exa mpl es of neurotra ns mi tters i ncl ude gl uta mi ne, γ-a mi nobutyri c a ci d (GABA), a cetyl chol i ne (Ach), dopa mi ne, norepi nephri ne (NE), a nd s erotoni n a mong ma ny others . Neurotra ns mi tters ca n a ffect thei r ta rget nerve or cel l ei ther vi a exci ta ti on or i nhi bi ti on a s i s s umma ri zed i n Ta bl e 19-2. Des cri pti on of i ndi vi dua l neurotra ns mi tters a nd thei r a cti ons fol l ow.
Table 19-2. Summa ry of Neurotra ns mi tters DOPAMINE Dopamine i s a monoa mi ne a nd i s s ynthes i zed from the a mi no a ci d tyros i ne i n a s eri es of enzyma ti c rea cti ons tha t fi rs t produces L-DOPA, then dopa mi ne, then NE, a nd fi na l l y epi nephri ne (Fi gure 19-7A). Dopa mi ne i s ei ther exci ta tory or i nhi bi tory dependi ng upon whi ch of i ts receptor s ubtypes i s a cti va ted. In the bra i n, dopa mi ne functi ons i n the rol e of beha vi or, cogni ti on, vol unta ry movement, s l eep, mood, a nd l ea rni ng. The a cti on of dopa mi ne i s mos tl y i n the s ubs ta nti a ni gra , a rcua te nucl eus of the hypotha l a mus , a nd the ventra l tegmenta l a rea of the mi dbra i n. Dopa mi ne i s a l s o very i mporta nt i n the rewa rd s ys tem. Dopa mi ne i s i na cti va ted by reupta ke vi a the dopa mi ne tra ns porter fol l owed by enzyma ti c brea kdown vi a the combi ned effects of monoamine oxidase (MAO) a nd catechol-O-methyltransferase (COMT) (Fi gure 19-7B, upper).
Figure 19-7. A–B. Synthesis and Degradation of Dopamine, Epinephrine, and Norepinephrine. (A) The forma ti on of the ca techol a mi nes , dopa mi ne, epi nephri ne, a nd norepi nephri ne i s s hown, ea ch s epa ra ted by a s i ngul a r enzyma ti c s tep. (B) Brea kdown of the ca techol a mi nes , i ncl udi ng (upper) dopa mi ne to homova ni l l i c a ci d (HVA), a nd (l ower) epi nephri ne a nd norepi nephri ne to va ni l l yl ma ndel i c a ci d (VMA). Both HVA a nd VMA a re s ubs equentl y el i mi na ted by the ki dneys . COMT, ca techol -O-methyl tra ns fera s e; MAO, monoa mi ne oxi da s e. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Vi a the dopa mi ne receptors (D 1–5 ), dopa mi ne i ncrea s es the a cti ons of the di rect pa thwa y wi thi n the ba s a l ga ngl i a . Receptors D 1 (regul a ti on of growth/devel opment of neurons , beha vi ora l res pons e, a nd D 2 -receptor s i gna l i ng) a nd D 5 (bel i eved to pl a y a rol e i n control of bl ood pres s ure) a cti va te a G s protei n to a cti va te a denyl cycl a s e a nd cycl i c a denos i ne monophos pha te (cAMP) s i gna l i ng (Cha pter 8). Receptors D 2 (proba bl e rol e i n regul a ti on of mus cl e tone; a s s oci a ti on wi th s chi zophreni a ), D 3 [bel i eved to pl a y a rol e i n emoti ons (e.g., depres s i on) a nd cogni ti ve thought; a l s o ma y pl a y a rol e i n drug a ddi cti on a nd s chi zophreni a ], a nd D 4 (pos s i bl e rol e i n “thri l l s eeki ng” beha vi or a s wel l a s a n a s s oci a ti on wi th s chi zophreni a ) a cti va te a G i protei n to i nhi bi t cAMP producti on (Cha pter 8). Norma l l y, dopa mi ne promotes s mooth, coordi na ted mus cl e movement wi thi n the body. Ins uffi ci ent dopa mi ne bi os ynthes i s i n the dopa mi nergi c neurons ca n ca us e Parkinson’s disease (s ee bel ow); receptor types D 3 a nd D 4 a re pos s i bl y i mpl i ca ted. In the fronta l l obes , dopa mi ne control s the fl ow of i nforma ti on from other a rea s of the bra i n. Dopa mi ne di s orders i n thi s regi on of the bra i n ca n ca us e a decl i ne i n neurocogni ti ve functi ons , es peci a l l y memory, a ttenti on, a nd probl em-s ol vi ng. NE/EPINEPHRINE NE i s s ynthes i zed from tyros i ne (Fi gure 19-7A) i n the cytopl a s m a nd pa cka ged i nto ves i cl es of pos tga ngl i oni c fi bers . It i s rel ea s ed vi a exocytos i s . NE i s termi na ted by reupta ke i nto the pos tga ngl i oni c nerve endi ng, di ffus i on from receptor s i tes , or meta bol i s m by the enzymes monoamine oxidase (MAO), a nd catechol-O-methyltransferase (COMT) (Fi gure 19-7B, l ower). NE i s uti l i zed i n s ympa theti c pos tga ngl i oni c neurons . Epinephrine, s ometi mes referred to a s adrenaline, i s the fi na l product of the tyros i ne pa thwa y i n a drena l medul l a chroma ffi n cel l s tha t a l s o produces dopa mi ne a nd NE. Epi nephri ne ha s a wi de va ri ety of effects on s evera l orga n s ys tems a s wel l a s meta bol i c functi ons , whi ch a re revi ewed i n Cha pter 10. Epi nephri ne fol l ows a s i mi l a r degra da ti on pa thwa y a s NE (Fi gure 19-7B, l ower). Both NE a nd epi nephri ne a ct a s hormones a nd neurotra ns mi tters vi a a drenergi c receptors . Adrenergi c receptors a re s ubdi vi ded i nto α a nd β types a s i l l us tra ted bel ow. Both NE a nd epi nephri ne bi nd to a l l a drenergi c receptor types but wi th di fferi ng a ffi ni ty. 1. α1 -receptors a re pos ts yna pti c a drenoreceptors l oca ted i n s mooth mus cl e, eye, l ung, bl ood ves s el s , gut, a nd the geni touri na ry s ys tem. Sti mul a ti on of thes e receptors a cti va tes a G q protei n res ul ti ng i n i ncrea s ed phos phol i pa s e a cti vi ty tha t rel ea s es di a cyl gl ycerol a nd i nos i tol tri s phos pha te wi th the l a tter promoti ng i ncrea s ed ca l ci um (Cha pter 8). Thes e s i gna l s l ea d to exci ta ti on, whi ch i s exhi bi ted by di l a ti on of the pupi l s (mydri a s i s ), bronchodi l a ti on, cons tri cti on of bl ood ves s el s , i ncrea s ed s trength of hea rt contra cti on (pos i ti ve inotropy), a nd decrea s ed hea rt ra te (nega ti ve chronotropy). NE bi nds thi s receptor better tha n epi nephri ne. 2. α2 -receptors a re l oca ted chi efl y on the pres yna pti c nerve termi na l s . G i protei n a cti va ti on i nhi bi ts a denyl cycl a s e a cti vi ty/cAMP producti on a nd decrea s es the entry of Ca 2+ i nto the neurona l termi na l . Thi s reduces the a mount of rel ea s ed NE. In a ddi ti on, i t produces va s ocons tri cti on a nd reduces s ympa theti c outfl ow i n the CNS. Epi nephri ne bi nds thes e receptors better tha n NE. 3. β 1 -receptor s ti mul a ti on a cti va tes a G s protei n, whi ch i ncrea s es a denyl cycl a s e a cti vi ty, ATP convers i on to cAMP, a nd protei n ki na s e phos phoryl a ti on of a va ri ety of protei ns . Thi s a cti on res ul ts i n pos i ti ve chronotropy (hea rt ra te i ncrea s e), dromotropy [conducti on of the i mpul s e through the hea rt’s AV node (Cha pter 16)], a nd inotropy (force of hea rt contra cti on). Whether NE a nd epi nephri ne bi nd equa l l y to thi s receptor or epi nephri ne bi ndi ng i s preferred over NE, rema i ns controvers i a l . 4. β 2 -receptors a re mos tl y pos ts yna pti c a drenoceptors l oca ted i n s mooth mus cl e a nd gl a nds . They not onl y a cti va te a denyl cycl a s e vi a a G s protei n but a l s o i nhi bi t the s a me enzyme by a G i protei n. The di rectl y oppos i te effects a re bel i eved to a l l ow di fferent functi ons i n s peci fi c cel l a nd ti s s ue l oca ti ons . The β2 s ti mul a ti on rel a xes s mooth mus cl e res ul ti ng i n va s odi l a ti on a nd bronchodi l a ti on a s wel l a s rel ea s e of i ns ul i n a nd the i nducti on of gluconeogenesis. Epi nephri ne bi nds β2 -receptors much more s trongl y tha n NE. 5. β 3 -receptors a re mos tl y found i n fa t/a di pos e ti s s ue. They a l s o a cti va te a denyl cycl a s e vi a a G s protei n. The β 3 -receptors i nduce the brea kdown of fa ts /l i pi ds (lipolysis) a nd a l s o hel p to regul a te hea t producti on (thermogenesis), es peci a l l y i n s kel eta l mus cl e. NE bi nds thi s receptor better tha n epi nephri ne.
Figure 19-8. Synthesis of Serotonin. The forma ti on of s erotoni n (5-hydroxytrypa tmi ne) from tryptopha n i s i l l us tra ted. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] SEROTONIN Serotonin (5-HT) i s a l s o a monoa mi ne neurotra ns mi tter i nvol ved i n s l eep, ea ti ng, a rous a l , drea mi ng, a nd the regul a ti on of mood, tempera ture, a nd pa i n tra ns mi s s i on. Serotoni n i s s ynthes i zed from the a mi no a ci d tryptopha n by a s hort meta bol i c pa thwa y (Fi gure 19-8) cons i s ti ng of two enzymes : tryptophan hydroxylase a nd amino acid decarboxylase. At l ea s t s even s erotoni n or 5-HT receptors ha ve been cha ra cteri zed uti l i zi ng G s , G q, or G i protei ns tha t a cti va te a n i ntra cel l ul a r s econd mes s enger ca s ca de; one receptor functi ons a s a Na +–K+ cha nnel l ea di ng to membra ne depol a ri za ti on. Serotoni n i s termi na ted vi a upta ke a t the s yna ps e on the pres yna pti c neuron. Va ri ous a gents ca n i nhi bi t 5-HT reupta ke, i ncl udi ng MDMA (ecstasy), amphetamine, cocaine, dextromethorphan, tricyclic antidepressants (TCAs), a nd selective serotonin reuptake inhibitors (SSRIs). ACETYLCHOLINE (Ach) Ach i s the neurotra ns mi tter uti l i zed i n s oma ti c efferent neurons , s ympa theti c prega ngl i oni c neurons (i ncl udi ng the a drena l medul l a a nd s wea t gl a nds ), a nd the enti re pa ra s ympa theti c nervous s ys tem. Ach i s s ynthes i zed i n the nerve termi na l by choline acetyltransferase, whi ch ca ta l yzes the rea cti on between acetyl coenzyme A a nd choline (Fi gure 19-9). In the s yna ps e, Ach i s degra ded by acetylcholinesterase. Cholinergic receptors refer to thos e tha t bi nd to Ach. They a re further s ubdi vi ded i nto nicotinic a nd muscarinic. Ni coti ni c receptors a re l oca ted i n the somatic system on the motor end pl a tes of s kel eta l mus cl e cel l s , a l l postganglionic neurons, a nd the adrenal medulla. When Ach bi nds to a nicotinic receptor, the res ul t i s exci ta tory. Muscarinic receptors a cti va te end-orga n effector cel l s i n bronchi a l s mooth mus cl e, s a l i va ry gl a nds , a nd the s i noa tri a l node. Thes e effects a re revi ewed i n Ta bl e 19-3.
Figure 19-9. Synthesis of Acetylcholine. The forma ti on of a cetyl cho-l i ne from a cetyl -CoA a nd chol i ne i s i l l us tra ted. CoA, coenzyme A. REGULATION OF CATECHOLAMINES
Table 19-3. Summa ry of Effects of Acetyl chol i ne on Ni coti ni c a nd Mus ca ri ni c Receptors The group of neurotra ns mi tters ca l l ed ca techol a mi nes i ncl ude dopamine, NE, a nd epinephrine a nd a re s ecreted by the nervous s ys tem i n ei ther a n a cute or chroni c res pons e to s tres s . Acutel y, the neura l s i gna l i s del i vered from the hypotha l a mus a nd bra i n s tem to the a drena l medul l a vi a rel ea s e of Ach from prega ngl i oni c neurons (Fi gure 19-10). Neura l s ti mul a ti on of the a drena l medul l a ca us es depol a ri za ti on of the a xona l membra ne a nd thus promotes the i nfl ux of ca l ci um. Ca l ci um, i n turn, s ti mul a tes the rel ea s e of ca techol a mi nes from the neurosecretory granules. Thes e s tora ge gra nul es fus e wi th the pl a s ma membra ne, l ea di ng to the exocytoti c rel ea s e of NE (mi nor component) a nd epi nephri ne (ma jor component). Nerve s ti mul a ti on a l s o promotes a chroni c res pons e res ul ti ng i n the s ynthes i s of ca techol a mi nes . Prol onged s tres s a nd s ympa theti c nerve a cti vi ty res ul ts i n a n i nducti on of phenylethanolamine N-methyltransferase (PNMT) by gl ucocorti coi ds a s a mea ns of a da pti ng to phys i ol ogi c s tres s (Fi gure 19-10). Cortisol i s tra ns ported from the a drena l cortex to the medul l a vi a the i ntra -a drena l porta l s ys tem. Thi s offers a n a dva nta ge beca us e the corti s ol concentra ti on i n thi s s ys tem i s 100-fol d enha nced rel a ti ve to the a rteri a l bl ood. Thi s res pons e pa ra l l el s s ynthes i s a nd rel ea s e of i ns ul i n i n res pons e to Ach.
Figure 19-10. Regulation of the Release of Catecholamines and Synthesis of Norepinephrine/Epinephrine in the Adrenal Medulla Chromaffi Cell. Immedi a te rel ea s e of norepi nephri ne/epi nephri ne res ul ts from neurona l s i gna l s vi a the neurotra ns mi tter a cetyl chol i ne a nd a res ul ti ng i nfl ux of ca l ci um i ons . Thi s ca l ci um pul s e l ea ds to the rel ea s e of the neurotra ns mi tters vi a exocytos i s . Longer term rel ea s e i n res pons e to s tres s or other fa ctors rel i es on hormones from the hypotha l a mus a nd the res ul ti ng i nducti on of PNMT. See text for deta i l s . ACTH, a drenocorti cotropi c hormone; CRH, corti cotropi n-rel ea s i ng hormone; DD, dopa deca rboxyl a s e; DH, dopa mi ne hydroxyl a s e; DPN, dopa mi ne; Epi , epi nephri ne; NE, norepi nephri ne; PNMT, phenyl etha nol a mi ne N-methyl tra ns fera s e; TH, tyros i ne hydroxyl a s e. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.]
Myasthenia Gravis (MG)–Lambert–Easton Syndrome: MG i s a di s order cha ra cteri zed by fl uctua ti ng wea knes s a nd ea s y fa ti ga bi l i ty of commonl y us ed s kel eta l mus cl es . Thi s di s order produces s ymptoms s uch a s doubl e vi s i on (di pl opi a ), eyel i d droopi ng (ptos i s ), a nd probl em s wa l l owi ng (dys pha gi a ). MG typi ca l l y s tri kes not onl y young women i n thei r 30s but a l s o men i n thei r 60–70s . MG ca n occur i n a s s oci a ti on wi th tumors of the thymus a s wel l a s other a utoi mmune di s orders (e.g., l upus , rheuma toi d a rthri ti s ). MG i s due to the a utoi mmune des tructi on or i na cti va ti on of pos ts yna pti c Ach receptors a t the neuromus cul a r juncti on, functi ona l l y reduci ng the number of Ach receptors . MG ca n be rel a ps i ng–remi tti ng i n na ture, but us ua l l y i s s l owl y progres s i ve tha t ca n l ea d to s evere res pi ra tory wea knes s a nd compl i ca ti ons . Infecti ons , s tres s , s urgery, mens es , a nd pregna ncy ca n often wors en s ymptoms tempora ri l y. Trea tment i s wi th a nti chol i nes tera s e drugs , s teroi ds , pl a s ma excha nge (pl a s ma pheres i s ), a nd s urgi ca l remova l of the thymus (thymectomy). Anti chol i nes tera s e drugs (s uch a s pyri dos ti gmi ne) trea t the s ymptoms of MG by i ncrea s i ng the a mount of a va i l a bl e Ach i n the neuromus cul a r juncti on vi a i nhi bi ti on of a cetyl chol i nes tera s e. In contra s t, Lambert–Eaton Syndrome (or Myasthenic syndrome) fea tures proxi ma l mus cl e wea knes s ca us ed by a pres yna pti c defect of neuromus cul a r tra ns mi s s i on. Anti bodi es to ca l ci um cha nnel s on the nerve termi na l ca us e the dra ma ti c decrea s e of Ach rel ea s e a t the motor end-pl a te. Unl i ke MG, s trength i mproves wi th s us ta i ned contra cti on. Trea tment i s us ua l l y provi ded by s teroi d thera py a nd pl a s ma pheres i s . Anti chol i nes tera s e thera py gi ves onl y a modes t i mprovement.
Parkinson’s Disease (PD): PD i s a bra i n di s order ca us ed by the des tructi on of dopa mi ne-produci ng neurons i n the substantia nigra. Thi s l ea ds to a chemi ca l i mba l a nce between dopamine a nd Ach i n the corpus s tri a tum. The four ca rdi na l s ymptoms of PD a re tremor (often a t res t), ri gi di ty, s l ownes s of movement (bradykinesia), a nd pos tura l i ns ta bi l i ty a nd refl ect the l os s of dopa mi ne’s i mpa ct on s mooth mus cl e movement. As PD progres s es , pa ti ents begi n to ha ve di ffi cul ty i n wa l ki ng (s l ow, s huffl i ng wa l k) a nd muffl ed s peech, depres s i on, a nd di ffi cul ty i n s wa l l owi ng (dys pha gi a ). Ma ki ng a n a ccura te di a gnos i s of PD ca n be di ffi cul t gi ven tha t ma ny other neurodegenera ti ve di s orders ma ni fes t wi th pa rki ns oni a n-type s ymptoms . No bl ood tes t exi s ts tha t ca n confi rm or refute a di a gnos i s of PD, but s evera l a re us ed i n conjuncti on wi th ma gneti c res ona nce i ma ges (MRIs ) more to rul e out other pos s i bl e di s orders . The di a gnos i s i s , therefore, ba s ed on hi s tory, a thorough phys i ca l exa mi na ti on a nd excl us i on of other ca us es . There i s no cure for PD. Trea tment i s di rected a t reduci ng the chemi ca l i mba l a nce by bl ocki ng Ach a nd i ncrea s i ng dopa mi ne wi th levodopa a nd carbidopa. Levodopa i s the precurs or to dopa mi ne but ha s s evera l neuromus cul a r/ movement s i de effects (dys ki nes i a s ) i ts el f. Ca rbi dopa i nhi bi ts the enzyme tha t brea ks down l evodopa to dopa mi ne a l l owi ng a reducti on of the a mount of l evodopa tha t needs to be ta ken a nd, therefore, the number of s i de effects from l evodopa . Al though l evodopa ca n dra ma ti ca l l y i mprove s ymptoms , es peci a l l y bra dyki nes i a , i t does not ha l t the progres s i on of the di s order. Other drugs us ed for PD i ncl ude bromocriptine, whi ch mi mi cs the rol e of dopa mi ne i n the bra i n. An a nti vi ra l drug, amantadine, a l s o a ppea rs to reduce s ymptoms . Wherea s l evodopa pri ma ri l y rel i eves bra dyki nes i a , a nti chol i nergi c drugs hel p to control tremor a nd ri gi di ty by decrea s i ng the a va i l a bl e a mount of Ach. Drug-res i s ta nt pa ti ents ca n benefi t from a thera py ca l l ed deep brain stimulation (DBS), whi ch us es el ectrodes i mpl a nted i nto the bra i n a nd connected to a s ma l l el ectri ca l devi ce ca l l ed a pul s e genera tor tha t ca n be externa l l y progra mmed. DBS ca n reduce the need for l evodopa a nd rel a ted drugs , whi ch i n turn decrea s es the l i kel i hood of dys ki nes i a s tha t a re a common s i de effect of l evodopa . GLYCINE, GLUTAMATE, AND GABA Glycine (Fi gure 19-11) i s found wi del y di s tri buted i n the s pi na l cord, bra i ns tem, a nd reti na a nd s erves a n i nhi bi tory functi on. The receptor i s a ga ted cha nnel (Cha pter 8), compos ed of fi ve protei n s ubuni ts , whi ch a l l ows a n i nfl ux of chl ori de i ons . Thi s i nfl ux a cts a s a n inhibitory postsynaptic potential a nd l es s ens the a bi l i ty for the occurrence of future pos ts yna pti c or motorneuron a cti on potenti a l s . The poi s on strychnine i s a competi ti ve i nhi bi tor of gl yci ne receptor a cti vi ty. Caffeine i s a nother gl yci ne receptor i nhi bi tor.
Figure 19-11. Glycine Structure. Glutamic acid (Fi gure 19-12A) i s the mos t a bunda nt exci ta tory neurotra ns mi tter i n the CNS a nd the mos t common neurotra ns mi tter i n the bra i n. It i s a l wa ys exci ta tory vi a glutaminergic receptor openi ng of nons el ecti ve i on cha nnel s . Gl yci ne s erves a n i mporta nt coa goni s t for the receptor. Gl uta mi c a ci d s ti mul a ti on i s termi na ted by a membra ne tra ns port s ys tem tha t i s us ed for rea bs orbi ng gl uta ma te a nd a s pa r-ta te a cros s the pres yna pti c membra ne. Gl uta mi c a nd a s pa rti c a ci d re-enter the cel l a s s odi um enters the cel l a nd pota s s i um exi ts . Thus , gl uta mi c a ci d/a s pa rti c a ci d entry i s i ndi rectl y powered by the ATP-dri ven s odi um pump. Both neurotra ns mi tters work together to control ma ny proces s es , i ncl udi ng the bra i n’s overa l l l evel of exci ta ti on. Ma ny of the drugs of a bus e a ffect ei ther gl uta mi c a ci d or GABA or both to exert tra nqui l i zi ng or s ti mul a ti ng effects on the bra i n.
Figure 19-12. A. Structure of Glutamic Acid (Glutamate and Synthesis of GABA). The forma ti on of GABA (γ-a mi nobutyri c a ci d) from gl uta mi c a ci d i s i l l us tra ted. B. GABA Receptors. The functi ona l a s pects of GABAA- a nd GABAB-type receptors a re i l l us tra ted. See text for a ddi ti ona l deta i l s . GABA, γ-a mi nobutyri c a ci d; AC, a denyl cycl a s e. [(Left cha nnel ) Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.] [(Rema i nder pa rt) Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] GABA (Fi gure 19-12A) i s a n a mi no a ci d neurotra ns mi tter a nd i s the mos t wi des prea d i nhi bi tory neurotra ns mi tter i n the bra i n. It i s mos t hi ghl y concentra ted i n the s ubs ta nti a ni gra , hypotha l a mus , a nd the hi ppoca mpus . GABA reduces the exci ta bi l i ty of neurons by hyperpol a ri zi ng them. There a re two types of GABA receptors (Fi gure 19-12B). GABAA receptor a cti va ti on opens chl ori de cha nnel s , a nd GABAB receptors a ct vi a a s econd mes s enger to ei ther open pota s s i um cha nnel s or to cl os e ca l ci um cha nnel s . GABA i s s ynthes i zed from gl uta mi c a ci d a nd i s i na cti va ted by a cti ve tra ns port i nto the a s trocyte gl i a l cel l s nea r the s yna ps es . Huntington’s Chorea (HD): HD i s a n a utos oma l domi na nt, progres s i ve degenera ti ve di s order ca us ed by a geneti ca l l y progra mmed des tructi on of neurons i n the ba s a l ga ngl i a a s wel l a s the cortex. It us ua l l y ma ni fes ts i ts el f i n mi ddl e a ge wi th progres s i ve deteri ora ti on unti l dea th 10–30 yea rs l a ter. As i ts na me i mpl i es , HD i s cha ra cteri zed a s a n a bnorma l i nvol unta ry movement di s order (chorea) ca us ed by overa cti vi ty of dopamine. Thes e movements a re bri ef, i rregul a r, nonrepea ti ng contra cti ons tha t a ppea r to move from one mus cl e group to the next a nd i t i s often a ccompa ni ed by wri thi ng movements . HD ca n a l s o ca us e dementi a , cl ums i nes s , i na bi l i ty to s wa l l ow (dys pha gi a ), a nd pers ona l i ty cha nges . Bi ochemi ca l l y, there i s a n undera cti vi ty of GABA a nd Ach a s wel l a s a rel a ti ve overa cti vi ty of dopa mi ne. HD ha s no cure, a nd the progres s i on of the di s ea s e ca nnot be ha l ted. Therefore, a l l thera pi es a re di rected purel y a t the s ymptoms , i ncl udi ng the us e of phenothiazines to bl ock dopa mi ne receptors a nd control a bnorma l movements a nd the drug reserpine to depl ete monoa mi nes .
Eating Disorders: The s a ti ety center of the bra i n i s a col l ecti on of neurons i n the l a tera l a s pect of the ventromedi a l hypotha l a mus . Sti mul a ti on of thi s a rea i ncrea s es a ppeti te a nd des tructi on of thi s center ca us es s uppres s i on of food i nta ke. The medi a l a rea of the ventromedi a l hypotha l a mus demons tra tes a regul a tory rol e over the a forementi oned l a tera l a s pect. Des tructi on of thi s a rea ha s been found to ca us e gros s overea ti ng (hyperpha gi a ) wi th res ul ta nt obes i ty i n a ni ma l model s . The protei n leptin ta rgets the hypotha l a mus a nd ca us es decrea s ed food i nta ke by a cti va ti ng melanocyte-stimulating hormone, a s a ti ety fa ctor, whi l e s uppres s i ng the hunger s i gna l from AGRP (Agouti -rel a ted pepti de). Addi ti ona l hormones s uch a s glucagon a nd cholecystokinin a l s o i nhi bi t a ppeti te. The rol e of thes e centers i n ea ti ng di s orders s uch a s bul i mi a a nd a norexi a i s i ncompl etel y unders tood. There i s s ome evi dence to s ugges t tha t neurotra ns mi tter i mba l a nces exi s t wi thi n the hypotha l a mus of pa ti ents who s uffer from a norexi a nervos a . The pa thol ogy behi nd s uch ea ti ng di s orders i s mul ti fa ctori a l , i ncl udi ng geneti c, hormona l a bnorma l i ti es wi th decrea s ed l evel s of s erotoni n, a nd ps ychos oci a l . NEUROPEPTIDES Neuropeptides a re cha i ns of l i nked a mi no a ci ds produced i n the bra i n. They a re ma de from l a rger pol ypepti des tha t a re cl ea ved. The endogenous opi ods (endorphins) or enkephalins produce a na l ges i a a nd euphori a . Thei r receptors a re ca ta gori zed a s del ta (δ), ka ppa (κ), mu (μ), or noci cepti n-type receptors a nd functi on vi a a va ri ety of G protei ns . Substance P i s found i n the s yna pti c ves i cl e of unmyel i na ted C fi bers , whi ch enha nce the tra ns mi s s i on of pa i n s i gna l s . The s ubs ta nce P receptor i s a G q protei n, whi ch a cti va tes phos phol i pa s e C a nd i nos i tol tri phos pha te/Ca 2+ producti on.
BIOCHEMISTRY OF VISION Nutri ti ona l vitamin A (reti nol es ters ) i s converted i n the reti na to 11-cis-retinal, a n i s omer of a l l -trans-reti na l forms by retinal isomerase. 11-cisreti na l then cova l entl y a tta ches to a l ys i ne res i due on the vi s ua l protei n opsin. The res ul ti ng rhodopsin mol ecul e becomes a G-protei n-coupl ed receptor wi th the “l i ga nd” for a cti va ti on bei ng l i ght. A defi ci ency of vi ta mi n A i n the reti na wi l l , therefore, decrea s e the s ens i ti vi ty of rods , res ul ti ng i n ni ght bl i ndnes s . Norma l l y, cycl i c gua nos i ne monophos pha te (cGMP) keeps i nwa rd Na +–Ca 2+ cha nnel s open to ma i nta i n membra ne depol a ri za ti on (Fi gure 19-
13). When l i ght s ti kes the reti na , photoexci ted rhodops i n bi nds to a mul ti s ubuni t membra ne protei n ca l l ed transducin, whi ch i n turn a cti va tes a cGMP phosphodiesterase by ca us i ng di s s oci a ti on of i nhi bi tory s ubuni ts . Sti mul a ti on of the cGMP phos phodi es tera s e l ea ds to the excha nge of a bound gua nos i ne di phos pha te for gua nos i ne tri -phos pha te (GTP). Thi s proces s rel ea s es the α s ubuni t of tra ns duci n wi th bound GTP, remi ni s cent of the mecha ni s m by whi ch cAMP a cti va tes protei n ki na s e A. The phos phodi es tera s e ca ta l yzes the des tructi on of cGMP (cGMP → GMP) nea r the membra ne of the rod cel l . When cGMP concentra ti ons fa l l , i nwa rd Na +–Ca 2+ cha nnel s cl os e, thereby l oweri ng both i ntra cel l ul a r Na + a nd Ca 2+ concentra ti ons . The fa l l i n Na + el i ci ts hyperpol a ri za ti on ca us i ng the rod cel l to rel ea s e l es s gl uta ma te neuro-tra ns mi tter. The reduced a mount of thi s i nhi bi tory neurotra ns mi tter genera tes a n el ectri ca l i mpul s e to the occi pi ta l l obe of the bra i n tha t tri ggers the percepti on of l i ght.
Figure 19-13. Biochemistry of Vision. Photoreceptors of the eye a re norma l l y depol a ri zed beca us e of the i nfl uence of cGMP on Na +–Ca 2+ cha nnel s . When l i ght s tri kes the receptor, tra ns duci n a cti va tes a cGMP phos phodi es tera s e, whi ch decrea s es cGMP concentra ti on, l ea di ng to cl os ure of the cha nnel s . In thi s hyperpol a ri zed envi ronment, the neurotra ns mi tter gl uta ma te i s rel ea s ed to provi de a n i mpul s e to a l l ow the percepti on of l i ght. cGMP, cycl i c gua nos i ne monophos pha te. [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Beca us e of i ts dyna mi c na ture a nd the need to ra pi dl y percei ve ever-cha ngi ng i ntens i ti es of l i ght, bui l t-i n “ces s a ti on” a nd “recovery” fa cets of the cycl e exi s t. When the l i ght a cti va ti on s i gna l i s termi na ted (“ces s a ti on”), the a cti ve rhodops i n mol ecul e i s phos phoryl a ted by rhodopsin kinase. Arrestin, a s ol ubl e protei n, then bi nds the phos phoryl a ted rhodops i n to i na cti va te i t. Bi ndi ng to a rres ti n a l s o s l ows rhodops i n’s a bi l i ty to propa ga te a nother l i ght s i gna l . The “recovery” pha s e, duri ng whi ch the cycl e returns to a s ta te tha t i s prepa ra tory for the next fl a s h of l i ght, i nvol ves repl eni s hment of the cGMP. Producti on of cGMP i s tri ggered by the low intracellular Ca2+ tha t wa s i nduced by the decl i ne i n the concentra ti on of cGMP (s ee a bove). The l ow Ca 2+ l ea ds to cGMP producti on from GTP vi a guanyl cyclase, whi ch i s s ti mul a ted by l ow i ntra cel l ul a r ca l ci um vi a a “recoverin” medi a tor protei n. Further brea kdown of cGMP i s a ccompl i s hed through di rect i nhi bi ti on of phos phodi es tera s e.
ANESTHESIA Genera l anesthesia i s a n a l tered phys i ol ogi c s ta te cha ra cteri zed by a tempora ry l os s of cons ci ous nes s , rel i ef of pa i n (a na l ges i a ), a mnes i a , a nd va ri a bl e l evel s of mus cl e rel a xa ti on. Des pi te the l os s of a wa renes s , the vi ta l phys i ol ogi c functi ons a re una ffected. Broa dl y s pea ki ng, genera l a nes theti cs ca n be ca tegori zed a s ei ther i nha l a ti on (brea thed-i n) or i ntra venous (by vei n). Inhalation anesthetics (e.g., ether, chl oroform, ha l otha ne, a nd s evofl ura ne) do not a ppea r to ha ve a s i ngl e s i te of a cti on. However, known a rea s of the bra i n tha t a re a ffected i ncl ude the reti cul a r a cti va ti ng s ys tem, cerebra l cortex (uncons ci ous nes s a nd a mnes i a ), cunea te nucl eus , the ol fa ctory cortex, a nd the hi ppoca mpus . Al l i nha l a ti on a nes theti cs work the s a me a t the mol ecul a r l evel , but thei r exa ct mecha ni s m of a cti on rema i ns onl y pa rti a l l y unders tood. The hydrophobi c s i tes on a neuron’s membra ne bi nd to the l i pophi l i c a nes theti c, expa ndi ng the phos phol i pi d bi l a yer a nd a l teri ng membra ne functi on. Another pos tul a ted theory i s tha t a nes theti cs decrea s e membra ne conducta nce. Ma ny a nes theti cs a l s o enha nce GABA i nhi bi ti on of the CNS. Acti on a t the bra i ns tem l evel depres s es the wi thdra wa l from pa i n (mos t nota bl y a t the dors a l horn i nterneurons ). In a ddi ti on, i nha l a ti on a nes theti cs depres s exci ta tory tra ns mi s s i on i n the s pi na l cord. Des pi te not unders ta ndi ng the exa ct mecha ni s m, the end res ul t i s the i nhi bi ti on of s yna pti c functi on. Intravenous anesthetics (e.g., ba rbi tura tes , benzodi a zepi nes , propofol , a nd opi a tes ) depres s the reti cul a r a cti va ti ng s ys tem, l oca ted i n the bra i ns tem control l i ng cons ci ous nes s . It works by depres s i ng tra ns mi s s i on of exci ta tory neurotra ns mi tters s uch a s Ach a nd enha nci ng i nhi bi tory neurotra ns mi tters s uch a s GABA.
REVIEW QUESTIONS 1. Wha t a re the centra l nervous s ys tem a nd peri phera l nervous s ys tem, a nd wha t rol es do they perform i n the huma n body? 2. Wha t a re the ba s i c pa rts a nd the functi on(s ) of unmyel i na ted a nd myel i na ted neurons ? 3. Wha t a re the ma i n s teps i n nerve i mpul s e conducti on, i ncl udi ng the rol e of membra ne-bound cha nnel s ? 4. Wha t a re the ba s i c rol es of the s ympa theti c a nd pa ra s ympa theti c nervous s ys tems ? 5. Wha t a re the ma jor neurotra ns mi tters a nd wha t a re thei r ma jor functi ons ? 6. How i s the producti on a nd rel ea s e of ca techol a mi ne neuro-tra ns mi tters regul a ted a cutel y a nd chroni ca l l y? 7. Wha t i s the bi ochemi ca l proces s behi nd vi s i on?
CHAPTER 20 THE REPRODUCTIVE SYSTEM Editor: Catrina Bubier, MD Women’s Hea l th Ca re As s oci a tes , Engl ewood, Col ora do, USA
Ba s i c Ana tomy a nd Devel opment Fema l e Reproducti ve Sys tem The Mens trua l Cycl e Ferti l i za ti on Brea s t Devel opment a nd La cta ti on Ma l e Reproducti ve Sys tem Revi ew Ques ti ons
OVERVIEW The fema l e a nd ma l e reproducti ve s ys tems devel op s el ecti vel y a s a res ul t of s peci fi c hormona l s i gna l s (s ex-determi ni ng regi on on the Y chromos ome, a nti -Mul l eri a n hormone, tes tos terone/5α-di hydrotes tos terone, a nd es trogens ) tha t l ea d to further devel opment or regres s i on of embryol ogi ca l s tructures . Addi ti ona l hormones (gona dotropi n-rel ea s i ng hormone, fol l i cl e-s ti mul a ti ng hormone, l utei ni zi ng hormone, proges terone, a nd huma n chori oni c gona dotropi n) i nfl uence further devel opment a nd s ubs equent a dul t functi ons , i ncl udi ng the mens trua l cycl e, ferti l i za ti on a nd pregna ncy, l a cta ti on, a nd oogenes i s / s perma togenes i s . Thes e hormones work vi a s i gna l i ng protei ns , i ncl udi ng s evera l va ri a ti ons of G protei ns , to s el ecti vel y a cti va te or i nhi bi t thes e devel opmenta l a nd functi ona l events .
BASIC ANATOMY AND DEVELOPMENT The reproducti ve s ys tem i s deri ved from the i ntermedi a te mes oderm a nd i ncl udes the reproducti ve orga ns of both ma l es a nd fema l es deri ved from the Wolffian ducts (ma l e), Mullerian ducts (fema l e), a nd the gonads (ma l e a nd fema l e), i ncl udi ng the tes tes a nd ova ri es . The i nfl uence of hormones a nd bi ochemi ca l s i gna l s i n the forma ti on a nd a cti vi ty of the reproducti ve s ys tem i s va s tl y i mporta nt a nd, when thes e s i gna l s go a wry, di s ea s e ens ues . Duri ng the i ni ti a l s ta ges of ges ta ti on, a l l huma ns begi n wi th both Wol ffi a n a nd Mul l eri a n ducts a nd the devel opment of ma l e a nd fema l e embryos i s i ndi s ti ngui s ha bl e. At a bout ges ta ti ona l da y 56, further growth a nd devel opment i nto ma l e or fema l e s exua l orga ns i s dependent on the effects of the sex-determining region of the Y chromosome (sry). In the geneti c ma l e, the s ry product bi nds to deoxyri bonucl ei c a ci d (DNA) a nd di s torts i t dra ma ti ca l l y out of s ha pe. Thi s a l ters the properti es of the DNA a nd l i kel y a l ters the expres s i on of a number of genes . One of thes e genes produces anti-Mullerian hormone (AMH), a di meri c, gl ycoprotei n hormone a l s o known a s Mullerian inhibiting factor (MIF). AMH i s produced by Sertol i cel l s i n the tes tes a nd s i gna l s , vi a i ts receptor, a member of the transforming growth factor (TGF)-β I and II receptor fa mi l y. Bi ndi ng of AMH to TGF-βtype II receptor a l l ows i t to bi nd to the type I receptor. The type I receptor i s then a bl e to phos phoryl a te s eri ne a nd/or threoni ne a mi no a ci ds to a cti va te tra ns cri pti on fa ctors i n the nucl eus , whi ch regul a te gene expres s i on. The pres ence of AMH l ea ds to ful l devel opment of the Wol ffi a n ducts a nd ma l e s tructures ; onl y a few remna nts of the Mul l eri a n ducts s urvi ve i n ma l es .
Figure 20-1. Summary of Hormonal Involvement in Sexual Differentiation. DHT, di hydrotes tos terone; MIF, Mul l eri a n i nhi bi ti ng fa ctor; s ry, s exdetermi ni ng regi on of Y chromos ome; T, tes tos terone; 5αR, 5α-reducta s e-2. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] In ma l es , the Leydig cells a l s o a ppea r a nd testosterone s ynthes i s begi ns , l ea di ng to ma l e s exua l cha ra cteri s ti cs , i ncl udi ng tes ti s forma ti on. Tes tos terone’s effect on the s emi ni ferous tubul es (s ee bel ow) i s a l s o cri ti ca l for modul a ti ng s i gna l i ng a nd gene expres s i on a nd, therefore, ma l e devel opment (Fi gure 20-1). In the a bs ence of s ry, embryos s ponta neous l y devel op i nto phenotypi c fema l es . Wi th no s ry product, AMH a nd tes tos terone a re not produced a nd the Wol ffi a n ducts regres s wi th onl y a few remna nts rema i ni ng. The mecha ni s m of AMH s uppres s i on of Mul l eri a n duct forma ti on i s unknown. Further devel opment of the Mul l eri a n ducts crea tes the fema l e s exua l orga ns a nd s tructures . In both s exes , the Wol ffi a n duct i s res pons i bl e for devel opment of the bl a dder tri gone. Androgen Insensitivity Syndrome (AIS): AIS, a l s o previ ous l y known a s “testicular feminization,” res ul ts from a muta ti on on the X chromos ome, whi ch yi el ds a defective androgen receptor. Beca us e thi s i s a n a utos oma l reces s i ve condi ti on, a l l pa ti ents a re geneti c ma l es a nd, therefore, produce s ry, AMH, a nd tes tos terone/5α-di hydrotes tos terone (DHT) norma l l y. However, the defecti ve a ndrogen receptors do not a l l ow the norma l a ndrogeni c functi ons to be expres s ed, l ea di ng to fa i l ure of Wol ffi a n duct devel opment a nd compl ete femi ni za ti on of the externa l geni ta l i a a nd bl i nd ended va gi na . Thes e pa ti ents a re, therefore, phenotypi ca l l y fema l e wi th a ma l e ka ryotype. A s i mi l a r medi ca l condi ti on, 17α-hydroxylase deficiency, res ul ts i n the i na bi l i ty to form a ny a ndrogens (Cha pter 3) a nd, therefore, es trogens a s wel l . The condi ti on, a l s o referred to a s “sexual infantilism,” l ea ds to underdevel opment of the gona ds (hypogona di s m).
FEMALE REPRODUCTIVE SYSTEM As noted a bove, the devel opment a nd functi on of the fema l e reproducti ve s ys tem i s under the i nfl uence of s evera l hormone s i gna l s . Fol l owi ng devel opment, pri ma ry hormones i n the fema l e s ys tem a re follicle-stimulating hormone (FSH), luteinizing hormone (LH), a nd the hormone tha t regul a tes thei r expres s i on—gonadotropin-releasing hormone (GnRH). Thes e hormones a re a l s o es s enti a l i n the ma l e reproducti ve s ys tem (s ee bel ow). GnRH producti on a nd rel ea s e begi ns a t puberty a nd rema i ns a cti ve duri ng the ma l e a nd fema l e reproducti ve yea rs . GnRH GnRH i s a s ma l l pepti de produced i n the hypotha l a mus by s peci a l i zed nerve cel l s ; a s s uch, GnRH i s ca l l ed a neurohormone, a cl a s s of hormones tha t i ncl ude thyrotropi n-rel ea s i ng hormone, oxytoci n (s ee bel ow), a nti di ureti c hormone (Cha pter 18), a nd corti cotropi n-rel ea s i ng hormone. Rel ea s e of GnRH res ul ts i n a cti va ti on of a s peci fi c GnRH receptor, l oca ted i n the gona dotropes of the pi tui ta ry gl a nd. Thi s receptor i s a membra ne-bound G-protei n-coupl ed s ti mul a tor of phos phol i pa s e C, whi ch res ul ts i n ca l ci um rel ea s e a nd protei n ki na s e C a cti va ti on vi a convers i on of pl a s ma membra ne phos pha ti dyl i nos i tol i nto i nos i tol tri phos pha te a nd di a cyl gl ycerol (Cha pter 8). Thes e s i gna l s res ul t i n producti on a nd rel ea s e of FSH a nd LH, a s wi l l be des cri bed bel ow. Regul a ti on of thi s i mporta nt s i gna l i s mul ti fol d. GnRH i s degra ded wi thi n mi nutes s o i t i s cons ta ntl y produced i n pul s es , a nd the s i ze a nd frequency of thes e pul s es i s i mporta nt i n s i gna l i ng. Thes e GnRH pul s es a re cons ta nt i n ma l es but va ry i n fema l es , dependi ng on the mens trua l cycl e. Interes ti ngl y, the frequency of the GnRH pul s es res ul t i n di fferent expres s i on of FSH (l ow frequency) a nd LH (hi gh frequency). As a res ul t FSH a nd LH a re va ri a bl y expres s ed duri ng the fema l e mens trua l peri od (Fi gure 20-2). Level s of tes tos terone, es trogen, a nd prol a cti n (i ncrea s ed duri ng pregna ncy) a s wel l a s i ncrea s ed concentra ti on of FSH a nd LH crea te a nega ti ve feedba ck l oop, whi ch ca n decrea s e GnRH pul s es .
Figure 20-2. Variable Expression of Reproductive Hormones During the Menstrual Cycle (Follicle-Stimulating Hormone, Luteinizing Hormone, Estradiol, and Progesterone). Peri ods of the mens trua l cycl e, i ncl udi ng mens trua ti on, fol l i cul a r pha s e, ovul a ti on, a nd l utea l pha s e a re i ndi ca ted. [Ada pted
wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] FSH FSH i s a di meri c gl ycoprotei n hormone produced a nd s ecreted by the a nteri or pi tui ta ry gl a nd. FSH ha s a n α- a nd β-s ubuni t, wi th the α-s ubuni t bei ng i denti ca l i n a mi no a ci d s equence to the α-s ubuni t of LH, thyroi d-s ti mul a ti ng hormone (TSH), a nd huma n chori oni c gona dotropi n (hCG) (s ee bel ow). The β-s ubuni t bi nds to a nd a cti va tes the FSH receptor. The FSH receptor i s bel i eved to exi s t i n two conforma ti ona l s ta tes (Cha pter 1)—one a cti ve a nd the other i na cti ve. Bi ndi ng of FSH to i ts receptor l ocks the receptor’s conforma ti on i nto the a cti ve form, whi ch a cti va tes a G s -coupl ed protei n ca us i ng rel ea s e of a n αs -s ubuni t. Thi s αs s ubuni t then a cti va tes a denyl cycl a s e to i ncrea s e the cycl i c a denos i ne monophos pha te (cAMP) s i gna l a nd protei n ki na s e A a cti vi ty (Cha pter 8). Phos phoryl a ted s i gna l i ng protei ns bi nd to cAMP res pons e el ements on DNA, l ea di ng to a cti va ti on of pa rti cul a r genes . FSH ca n a l s o a cti va te the fa mi l y of extracellular signal-related kinases. Thi s fa mi l y of receptor-a cti va ted ki na s e, a l s o known a s mi togen-a cti va ted protei n (MAP) ki na s es , res pond to a va ri ety of s i gna l mol ecul es , i ncl udi ng FSH, whi ch regul a te mei os i s a nd mi tos i s of s el ected cel l s . Al though ma ny mecha ni s ms ma y be i n pl a y, the a cti va ti on of tyros i ne ki na s es vi a the ras GTPa s e protei n i s cons i dered i mporta nt i n the s ubs equent a cti va ti on of tra ns cri pti on fa ctors . FSH performs ma ny functi ons i n the devel opment of fema l e a nd ma l e a nd i s a l s o es s enti a l for the proper ti mi ng a nd pha s i ng of the mens trua l cycl e. FSH, a s i ts na me i mpl i es , s ti mul a tes gra nul os a cel l s i n the fol l i cl es i n the ova ry to i ni ti a te egg growth a nd devel opment (oogenesis). FSH s peci fi ca l l y bl ocks progra mmed dea th or atresia of ea rl y devel opi ng egg fol l i cl es . As one egg fol l i cl e becomes domi na nt, rea chi ng a bout 10 μm i n di a meter, i t begi ns to s ecrete estradiol, whi ch nega ti vel y feedba cks on GnRH a nd, therefore, FSH producti on. The bl ock of a tres i a i n a l l fol l i cl es except the domi na nt one l ea ds to the s urvi va l of onl y one egg. The a cti vi ty of FSH i s regul a ted by s evera l mea ns . FSH ha s a bi ol ogi ca l ha l f-l i fe of onl y a bout 3–4 hrs . Unl es s GnRH pul s es conti nue to promote FSH s ynthes i s a nd s ecreti on, i ts bi ochemi ca l effects wi l l begi n to di mi ni s h. Increa s ed es trogen wi l l a l s o decrea s e FSH s ynthes i s vi a i ts effect on GnRH. Activin (Fi gure 20-3, ri ght), a protei n heterodi mer (compos ed of two di fferent s ubuni ts ) or homodi mer (compos ed of two i denti ca l s ubuni ts ), s erves to i ncrea s e FSH producti on a nd s ecreti on vi a a mecha ni s m extremel y s i mi l a r to the FSH receptor, l ea di ng to protei n ki na s e A a cti vi ty a nd upregul a ti on of gene expres s i on. However, the s a me s ma l l a ntra l fol l i cl es tha t FSH s ti mul a tes a l s o produce the protei n inhibin (Fi gure 20-3, l eft), a protei n heterodi mer wi th one s ubuni t i denti ca l to the a cti vi n s ubuni t a nd one di fferent. Inhi bi n decrea s es FSH s ynthes i s a nd s ecreti on a nd, therefore, works i n a s i mi l a r ma nner to the domi na nt fol l i cl e’s es trogen producti on to decrea s e FSH a nd i ncrea s e non-domi na nt fol l i cl e a tres i a . As the l utea l pha s e ends , a s ma l l ri s e i n FSH ca n be detected tha t hel ps to s i gna l the s ta rt of the next mens trua l cycl e. In a ddi ti on, es trogen a nd proges terone l evel s fa l l a nd decrea s e thei r nega ti ve effect on GnRH, a l l owi ng FSH l evel s to ri s e wi th a n i ni ti a l pea k a t da y 3 (Fi gure 20-2). In a di rectl y oppos i te s i gna l i ng mecha ni s m, es trogen i ncrea s es GnRH a nd, therefore, FSH s ecreti on a t the ti me of ovul a ti on (s ee bel ow). Fi na l l y, a s a woma n a pproa ches menopa us e, the number of i ni ti a l fol l i cl es recrui ted to egg producti on decrea s es a nd, a s a res ul t, the l evel of i nhi bi n i s a l s o l es s ened. Thi s l ea ds to a n overa l l ri s e i n s erum FSH l evel s tha t i s one s i gn of peri menopa us e/menopa us e.
Figure 20-3. Inhibins and Activins. Il l us tra ti on of s tructures of i nhi bi ns (l eft), heterodi mers of three s epa ra te protei ns (repres ented a s three di fferent col ors ), a nd a cti vi ns (ri ght), homodi mers or heterodi mers of the green a nd purpl e protei ns . [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] LH LH i s a di meri c gl ycoprotei n, compos ed of a n α- a nd β- s ubuni t, produced by the a nteri or pi tui ta ry gl a nd. The α-s ubuni t i s i denti ca l i n a mi no a ci d s equence to the α-s ubuni t of FSH, TSH, a nd hCG (s ee bel ow). The β-s ubuni t i s s i mi l a r but not i denti ca l to tha t of hCG a nd i s res pons i bl e for bi ndi ng to the LH receptor. LH i s not produced unti l puberty a nd then i s genera ted i n res pons e to GnRH s i gna l s , regul a ted a ccordi ng to reproducti ve need. Li ke FSH, LH l evel s a l s o ri s e i n fema l es a fter menopa us e. Li ke the FSH receptor, the LH receptor i s bel i eved to exi s t i n two conforma ti ona l s ta tes (Cha pter 1)—one a cti ve a nd the other i na cti ve. Bi ndi ng of LH to i ts receptor l ocks the receptor’s conforma ti on i nto the a cti ve form, whi ch a cti va tes a G s -coupl ed protei n ca us i ng rel ea s e of a n α-s -s ubuni t. Thi s αs -s ubuni t then a cti va tes a denyl cycl a s e to produce the cAMP s i gna l a nd i ncrea s ed protei n ki na s e A a cti vi ty (Cha pter 8). Phos phoryl a ted s i gna l i ng protei ns bi nd to cAMP res pons e el ements on DNA, l ea di ng to a cti va ti on of pa rti cul a r genes . LH ha s a bi ol ogi ca l ha l fl i fe of a bout 20 mi n, s o gene regul a ti on vi a i ts receptor ma y s top unl es s a ddi ti ona l LH or other s i gna l i ng mol ecul es conti nue the nucl ea r s i gna l . In fema l es , LH provi des the hormone s i gna l for rel ea s e of a n egg from the ova ry. Increa s i ng es trogen, whi ch res ul ts from FSH s ti mul a ti on of ova ri a n granulosa cells, i ncrea s es the pres ence of LH receptors . In a ddi ti on, es trogen a cti va tes the pi tui ta ry to i ncrea s e LH s ecreti on. Both of thes e functi ons a ct to produce a n LH “surge” over a 24–48 hr peri od (Fi gure 20-2). Unl i ke FSH, a cti vi n, i nhi bi n, a nd the s ex hormones do not a ffect LH producti on a t the DNA l evel . The LH s urge produces ovul a ti on a nd a l s o s i gna l s the corpus l uteum, a tempora ry s tructure crea ted from the remna nt ova ri a n fol l i cl e a fter egg rel ea s e. The a cti va ted corpus l uteum produces the hormone progesterone, whi ch, i n turn, s i gna l s the uteri ne wa l l to prepa re for egg i mpl a nta ti on i f ferti l i za ti on s houl d occur. The pres ence of LH a l s o s ta rts a nd, i n the ca s e of ferti l i za ti on a nd i mpl a nta ti on of the egg, ma i nta i ns the luteal phase (Fi gure 20-2) for 8 weeks . LH promotes a ndrogen a nd es trogen producti on vi a s ti mul a ti on of theca l cel l s i n the ova ry. The LH s urge a l s o i ni ti a tes the conti nua ti on of mei os i s i n the oocyte, the compl eti on of whi ch occurs a fter the s perm enters the ovum. Fi na l l y, LH i s i mporta nt to l utei ni ze the gra nul os a cel l s to produce proges terone for ma i ntena nce of the l utea l pha s e. Ovulation Prediction by LH Measurement: The 24–48 hr LH s urge tha t l ea ds to ovul a ti on a nd, therefore, predi cts a n opti ma l ti me for concepti on ha s l ed to the devel opment of “ovul a ti on predi ctor ki ts ” to a s s i s t coupl es wa nti ng chi l dren. Thes e ki ts ha ve a nti -LH a nti bodi es , whi ch bi nd to LH excreted i n the uri ne. Bi ndi ng of the LH a nd a pos i ti ve col or cha nge vi a a chemi ca l rea cti on i ndi ca tes when the l evel i s a bove the norma l 1–20IU/L—l evel s i ndi ca ti ng ma xi mum ferti l i ty ti mes . Di gi ta l mea s urements of LH l evel a re a l s o a va i l a bl e. ESTROGENS Estrogen i s the i ncl us i ve term for a group of s teroi ds tha t pri ma ri l y i mpa ct the fema l e reproducti ve s ys tem. Es trogens a re ma i nl y s ynthes i zed when FSH a nd LH s ti mul a te theca l a nd gra nul os a cel l s i n the ova ri a n fol l i cl es , the corpus l uteum a fter rel ea s e of a domi na nt egg, a nd, i n the
ca s e of pregna ncy, the pl a centa . The l i ver, a drena l gl a nds , a di pos e ti s s ue, a nd brea s t ti s s ue ca n a l s o produce s teroi da l es trogen. The hi ghes t l evel of es trogen occurs i mmedi a tel y pri or to ovul a ti on. As a cl a s s of s teroi d hormones , a l l es trogens functi on by cros s i ng the cel l membra ne a nd a cti va ti ng es trogen receptors (ERs ). Unbound ERs a re ma i nl y found i n the cytos ol but, upon bi ndi ng of a n es trogen mol ecul e, the receptors move i nto the nucl eus , form di mers , a nd bi nd to s peci fi c s equences of the DNA mol ecul e known a s hormone responsive elements. The bound ER–DNA a cti va tes pa rti cul a r protei ns tha t s ta rt tra ns cri pti on of DNA to res ul t i n s ynthes i s of s peci fi c protei ns . ERs ca n a l s o be found i n the nucl eus where they s erve to regul a te the tra ns cri pti on of other protei ns a nd ma y a l s o a s s oci a te wi th pl a s ma membra ne G protei ns , whi ch a cti va te tyros i ne ki na s es . Three ma jor forms of es trogen mol ecul es occur i n huma ns (Fi gure 20-4 a nd Cha pter 3).
Figure 20-4. The Three Major Classes of Estrogen Steroid Hormones in Humans. [Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.] Estrone (E1 ), produced by the enzyme aromatase from a ndros tenedi one i n a di pos e cel l s , i s found predomi na tel y i n menopa us a l women a s wel l a s i n men. 17β-Estradiol (E2 ) i s formed from tes tos terone by a roma ta s e a nd i s the ma i n es trogen i n nonpregna nt, ferti l e fema l es . Aroma ta s e i n the gra nul os a cel l s of the ova ri es i s a cti va ted by FSH. Es tra di ol ca n a l s o be produced i n s ma l l er a mounts by a roma ta s e convers i on i n the l i ver a nd fa t cel l s (Cha pter 3). Estriol (E3 ) i s the mos t a bunda nt es trogen duri ng pregna ncy a nd i s formed i n the pl a centa a nd feta l a drena l gl a nds a nd l i ver a s i l l us tra ted i n Fi gure 20-5. Other nons teroi da l mol ecul es , found i n na ture a nd a rti fi ci a l l y produced, ca n exhi bi t es trogen a cti vi ty.
Figure 20-5. Steroid Production by Mother, Placenta, and Fetus During Pregnancy. Overvi ew of producti on of s teroi ds from chol es terol duri ng pregna ncy, s howi ng va ri ous s ources for the producti on of s teroi ds i n the pl a centa a nd feta l compa rtment (e.g., feta l a drena l gl a nds a nd l i ver) a s wel l a s tra ns port of ma terna l , pl a centa l , a nd feta l mol ecul es . See text for further di s cus s i on. DHEA, dehydroepi a ndros terone; DHEAS, dehydroepi a ndros terone s ul fa te; LDL, l ow-dens i ty l i poprotei n. [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Es trogens ha ve a wi de-ra ngi ng s et of functi ons i n both men a nd women a nd onl y the ma jor functi ons wi l l be covered here. In fema l es , es trogens a re es s enti a l for the i ni ti a l growth of the col umna r epi thel i a l cel l s of the endometri um. From a devel opmenta l s ta ndpoi nt, es trogen hel ps to i ni ti a te a nd propa ga te the devel opment of s econda ry s exua l cha ra cteri s ti cs to i ncl ude brea s t forma ti on, growth of the uterus a nd va gi na , growth of pubi c a nd undera rm ha i r, ons et of a dul t body cha nges (e.g., hi p wi deni ng a nd decel era ti on of teena ge growth), a nd the ons et a nd regul a ti on of mens trua ti on (s ee bel ow). Es trogen a l s o i nfl uences a number of bi ochemi ca l a nd meta bol i c proces s es to i ncl ude bone rea bs orpti on; i ncrea s ed cl otti ng of bl ood vi a modul a ti on of cl otti ng fa ctors II, VII, IX, a nd X a s wel l a s pl a s mi nogen a nd a nti thrombi n III (Cha pter 14); i ncrea s es i n hi gh-dens i ty l i poprotei n a nd tri gl yceri des wi th decrea s es i n LDL a nd fa t depos i ts (Cha pter 7); a nd
a l tera ti ons i n fl ui d ba l a nce a nd hormone l evel s . The rol e of es trogens i n the fema l e mens trua l cycl e wi l l be exa mi ned i n ful l er deta i l bel ow. PROGESTERONE Progesterone i s a s teroi d hormone from the hormone cl a s s ca l l ed progestogens, i mporta nt i n mens trua ti on, pregna ncy, a nd embryo devel opment. Proges terone i s ma de from pregnenol one, deri ved from chol es terol , i n the corpus l uteum a fter ovul a ti on. If ferti l i za ti on occurs , the pl a centa becomes the pri ma ry s ource by week 8, uti l i zi ng ci rcul a ti ng chol es terol from ma terna l bl ood. Proges terone i s a l s o produced i n the a drena l gl a nds . Level s of proges terone a re l ow i n the fol l i cul a r/ prol i fera ti ve pha s e a nd hi gher i n the l utea l /s ecretory pha s e (Fi gure 20-2 a nd bel ow) beca us e of the contri buti on of the corpus l uteum. Proges terone, l i ke other s teroi d hormones , pa s s es through the pl a s ma membra ne a nd bi nds to a n i ntra cel l ul a r proges terone receptor. Al s o, l i ke other s teroi d hormones , bi ndi ng of proges terone l ea ds to receptor di meri za ti on, entra nce i nto the nucl eus , a nd a cti va ti on of s el ected genes vi a DNA bi ndi ng. Tra ns cri pti ona l a cti va ti on by the proges terone receptor i s norma l l y i nhi bi ted by the receptor’s ca rboxytermi na l end. Bi ndi ng of the s teroi d hormone es trogen to the proges terone receptor ca us es a conforma ti on cha nge tha t rel ea s es thi s i nhi bi ti on a nd i s , therefore, es s enti a l for proges terone/proges terone receptor functi ons . Es trogen a l s o a cts to i ncrea s e the tota l number of proges terone receptors , thereby a mpl i fyi ng proges terone effects . However, two di fferent i s oforms of the proges terone receptor exi s t: type A a nd type B, the l a tter of whi ch ha s a n a mi no-termi na l , tra ns cri pti ona l a cti va ti on doma i n. The two receptor types a ppea r to ha ve s i mi l a r but uni que a cti vi ti es tha t a re s ti l l bei ng i nves ti ga ted. Interes ti ngl y, proges terone a l s o bi nds very s trongl y to the receptor for a l dos terone, competi ti vel y i nhi bi ti ng the mi nera l corti coi d’s functi on. Increa s ed l evel s of proges terone, therefore, bl ock a l dos terone’s effects (Cha pter 18) a nd l ea d to i ncrea s ed l os s of s odi um a nd wa ter by the ki dneys . A s udden decrea s e i n proges terone s ubs equentl y ca us es s odi um a nd wa ter retenti on. Progesterone Antagonists: The a cti ons of es trogen a nd proges terone on proges terone receptor a cti vi ty ha ve l ed to the devel opment of a cl a s s of medi ca ti ons ca l l ed selective progesterone receptor modulators (SPRMs). SPRMs a ffect the two i s oforms of the proges terone receptor di fferentl y, dependi ng on bi ndi ng s trength a nd a cti va ti on/i nhi bi ti on a cti vi ty. SPRMs ma y, therefore, a l s o offer the opti on of s el ecti ve receptor-type effects . Exa mpl es i ncl ude trea tment of uterine leiomyoma, endometriosis, a nd hormone-responsive breast cancers. One SPRM, RU486, i s a l rea dy i n s el ected us e a s a medi ca l a borta nt. Proges terone’s ma i n rol e i s to prepa re the endometri a l l i ni ng of the uterus for pos s i bl e i mpl a nta ti on of a ferti l i zed egg a nd ma i ntena nce of the uteri ne l i ni ng to s upport the fetus duri ng pregna ncy. Proges terone’s i nfl uence on the endometri um i s di fferent from es trogen, though, a s i t turns the endometri um from a prol i fera ti ve to a s ecretory pha s e, i ncl udi ng the ma rked growth of s pi ra l a rteri es (Fi gure 20-6). In thi s rol e, proges terone i s often us ed to s upport i n vi tro ferti l i za ti on trea tments a nd/or i rregul a r uteri ne bl eedi ng. Duri ng pregna ncy, proges terone a l s o i nhi bi ts l a cta ti on (s ee bel ow) a nd s mooth mus cl e contra cti on; decrea s ed proges terone l evel s a re one potenti a l tri gger for l a bor a nd a l s o i ni ti a te mi l k producti on. As a res ul t, recent l i tera ture s ugges ts gi vi ng weekl y i ntra mus cul a r i njecti ons of proges terone to hel p a voi d preterm l a bor i n a t-ri s k mothers . If pregna ncy does not occur, there i s a regres s i on of the corpus l uteum a nd, therefore, decrea s ed producti on of proges terone, whi ch i ni ti a tes mens trua ti on. Apa rt from the reproducti ve s ys tem, proges terone a l s o i mpa cts nerve functi on vi a bi ndi ng to a s eri ne/ threoni ne ki na s e ca l l ed glycogen synthase kinase 3. Proges terone’s protecti ve effect on myel i n s hea ths (Cha pter 19) of nerves ha s ra i s ed i ts potenti a l to be us ed i n pa ti ents s ufferi ng from mul ti pl e s cl eros i s . Proges terone ma y a l s o s erve a s a protecti ve fa ctor a ga i ns t endometri a l ca ncer by oppos i ng the effects of es trogen.
Figure 20-6. Human Menstrual Cycle. Cha ngi ng l evel s of hormones a nd thei r i nfl uence on fol l i cl e/egg devel opment; corpus l uteum functi ons ; a nd degenera ti on, growth, a nd thi ckeni ng of the endometri um duri ng the two pha s es of the cycl e. The fi gure a s s umes a n a vera ge 28-da y cycl e a nd s ta rts a t the ons et of mens trua ti on. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] hCG hCG i s a heterodi meri c (two di fferent s ubuni ts ) gl ycoprotei n hormone produced by newl y ferti l i zed embryos a nd, s ubs equentl y by the pl a centa . hCG ha s a n α-s ubuni t tha t i s i denti ca l to the α-s ubuni t of FSH, LH, a nd TSH a nd a uni que β-s ubuni t tha t bi nds to the luteinizing hormone/choriogonadotropin receptor (LHCGR). LHCGR i s a G s -coupl ed protei n receptor found predomi na tel y i n the ova ry a nd tes tes , whi ch s pa ns the membra ne. LHCGR a l s o bi nds wi th LH a nd, a s a res ul t, hCG a nd LH s ha re s evera l functi ons . Jus t l i ke the LH receptor (s ee a bove), upon bi ndi ng of hCG or LH, the receptor undergoes a conforma ti ona l cha nge to i ts a cti ve s ta te, l ea di ng to cAMP producti on, a cti va ti on of protei n ki na s e A, a nd i ncrea s es i n expres s i on of s el ected genes . Li ke LH, hCG a l s o ma i nta i ns the corpus l uteum a nd ca us es i ncrea s i ng producti on of proges terone (s ee a bove). Pregnancy Testing and hCG: Beca us e hCG i s produced by the newl y ferti l i zed egg, the a bi l i ty to mea s ure i ts l evel s i n bl ood a nd even uri ne ha s l ed to the a dvent of ea s i l y performed pregnancy tests. Mos t of thes e tes ts uti l i ze a monocl ona l a nti body to the β-s ubuni t of hCG, thereby
a voi di ng i nterference from the s i mi l a r FSH a nd LH. Uri ne tes ts do not provi de a ctua l numeri ca l l evel s of hCG a nd a re, therefore, referred to a s a qua l i ta ti ve pregna ncy tes t. Bl ood s erum tes ts uti l i ze detecti on methods tha t ca n determi ne a ctua l β hCG l evel s ; therefore, thi s tes t i s referred to a s a qua nti ta ti ve tes t. Qua nti ta ti ve mea s urement of hCG i s a l s o i mporta nt i n the di a gnos i s a nd trea tment of a va ri ety of ova ri a n, tes ti cul a r-, a nd/or pl a centa l -deri ved ca ncers (e.g., germi noma s , chori o-ca rci noma , hyda ti di form mol e, a nd tera toma /dermoi d cys t) a s wel l a s ectopi c pregna ncy a nd mi s ca rri a ge. Level s of hCG a re a l s o one of four components of a cl i ni ca l quad screen test, us ed i n ea rl y pregna ncy to determi ne the ri s k of Down s yndrome, Edwa rd’s s yndrome, Pa ta u’s s yndrome, a nd neura l tube defects i n the fetus .
THE MENSTRUAL CYCLE The fema l e mens trua l cycl e i s control l ed by a s eri es of hormones tha t i ni ti a te a nd regul a te growth of the endometri a l l i ni ng, devel opment of a n egg, rel ea s e a nd pos s i bl e i mpl a nta ti on of the egg, a nd, i f pregna ncy does not ens ue, compl ete purgi ng of the uteri ne l i ni ng to a l l ow a repea t of the s a me proces s (Fi gure 20-6). MENSTRUATION (DAYS 1–4) The mens trua l cycl e’s s ta rt i s cons i dered a t the ons et of mens trua ti on (da y 1), the eva cua ti on of the endometri a l l i ni ng i n the nonpregna nt fema l e, whi ch ma y rel y, i n pa rt, on a s ma l l s urge of FSH a nd then decl i ne i n the proges terone l evel (s ee a bove). Subs equentl y, i ncrea s i ng es trogen s tops l os s of the endometri a l l i ni ng a nd begi ns a nd i ni ti a tes new thi ckeni ng of the endometri um on a pproxi ma tel y da y 4. FOLLICULAR/PROLIFERATIVE PHASE (DAYS 5–13) Da y 5 i s the a pproxi ma te s ta rt of the follicular/proliferative phase beca us e of the growth a nd devel opment of the uteri ne endometri a l l i ni ng a nd of the ova ri a n fol l i cl es . Speci fi ca l l y, a fter da ys 1–7 of the mens trua l cycl e, a cti vi n a s wel l a s l ow l evel s of es trogen (es tra di ol ) a nd proges terone a l l ow GnRH pul s es to s ecrete i ncrea s i ng a mounts of FSH, whi ch, then, i ni ti a tes devel opment of s evera l ova ri a n fol l i cl es . As a dominate follicle emerges , es tra di ol i s i ncrea s i ngl y s ynthes i zed, whi ch modul a tes FSH vi a i nhi bi ti on of GnRH. Inhi bi n, produced by the s a me fol l i cl es , a l s o decrea s es FSH a cti vi ty. Es tra di ol a l s o i nhi bi ts s ynthes i s a nd s ecreti on of LH unti l a pproxi ma tel y da y 12. At thi s poi nt, a cri ti ca l l evel of es tra di ol i s rea ched a nd promoti on of LH producti on s ta rts , l ea di ng to the LH s urge. The mecha ni s m behi nd the di rectl y oppos i te effects of es tra di ol on LH i s not compl etel y unders tood but one pos s i bi l i ty i s the di fferenti a l a cti va ti on of ei ther of the two ERs . ERα i s known to oppos e LH s ecreti on, wherea s ERβ i ncrea s es LH l evel s a nd i nhi bi ts α-receptor a cti vi ty. How preferenti a l a cti va ti on of the α- or β-receptors occurs i s unknown, but di fferenti a l expres s i on, di fferent es tra di ol bi ndi ng, or s ome other mecha ni s m ma y expl a i n thi s obs erva ti on. Rega rdl es s of the mecha ni s m, the i ncrea s i ng LH l evel l ea ds to the brea kdown of the fol l i cul a r wa l l a nd rel ea s e of the egg, known a s ovulation on a pproxi ma tel y da y 14. THE LUTEAL/SECRETORY PHASE (DAYS 15–28) The luteal/secretory phase i s na med s o beca us e of the a cti vi ty centers on the corpus l uteum, the remna nt of the ova ri a n fol l i cl e, a nd i ts s ecreti on of proges terone. If the egg i s not ferti l i zed wi thi n a bout 24 hrs , i t di es a nd i s rea bs orbed by the body. Mea nwhi l e, the rema i ni ng ova ri a n fol l i cl e converts i nto a corpus luteum a nd begi ns to produce proges terone i n i ncrea s i ng a mounts (Fi gure 20-6). Increa s ed l evel s of FSH a nd LH a l s o hel p to devel op the corpus l uteum. Proges terone a cts on the endometri um for pos s i bl e ferti l i zed egg i mpl a na ti on. If the egg does not i mpl a nt, hCG i s not s ecreted a nd the corpus l uteum di s i ntegra tes , ca us i ng ma rkedl y l ower l evel s of proges terone, whi ch i ni ti a tes the s heddi ng of the endometri a l l i ni ng a s mens trua ti on. Es trogen a nd proges terone s ynthes i zed by the growi ng corpus l uteum i nhi bi t GnRH a nd, therefore, FSH a nd LH producti on, l ea di ng to a ra pi d fa l l from ovul a ti on l evel s . The fa l l of FSH a nd LH l evel s a dds to the a trophy of the corpus l uteum a nd a s ubs equent decrea s e i n proges terone s ynthes i s , a ga i n l ea di ng to mens trua ti on. In the ca s e of ferti l i za ti on a nd pregna ncy (s ee bel ow), the new embryo begi ns to produce hCG, whi ch a cts l i ke LH i n pres ervi ng the corpus l uteum duri ng the fi rs t 6–8 weeks of pregna ncy unti l pl a centa l s ynthes i s ca n ta ke over.
FERTILIZATION The proces s of ferti l i za ti on, the entra nce of a s perm cel l or, i n ra re ca s es , s perm cel l s i nto a n egg, rel i es on s evera l funda menta l bi ochemi ca l proces s es , i ncl udi ng cel l moti l i ty a nd pl a s ma membra ne fus i on, i nvol vi ng cha ngi ng fl ui di ty a nd s tructure of the s perm a nd egg membra nes (Cha pter 8, Fi gure 20-7).
Figure 20-7. Mechanism of Fertilization. Steps of ferti l i za ti on a re i ndi ca ted, i ncl udi ng the a cros oma l rea cti on (s ee text). [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] The s perm ta i l i s a cl a s s i c fl a gel l um (Cha pter 12), produci ng whi ppi ng or l a s hi ng movements vi a a denos i ne tri phos pha te hydrol ys i s tha t propel s the s perm through the fema l e reproducti ve orga ns towa rd the egg. As di s cus s ed i n Cha pters 1 a nd 12, thi s proces s rel i es on the s tructura l protei ns tubul i n–mi crotubul es a nd thei r i ntera cti on wi th the motor protei n, dynei n. Once a s perm rea ches the egg, i t bi nds to the outer l a yer a nd then a ttempts to penetra te the ha rd zona pellucida. Upon rea chi ng the corona radiata l a yer of the egg, the acrosome, a ca p-l i ke s tructure a t the ti p of ea ch s perm, rel ea s es the enzyme hyaluronidase, whi ch brea ks down hya l uroni c a ci d conta i ned i n the pres enti ng egg l a yer. Next, the enzyme acrosin di ges ts the zona pel l uci da a nd promotes cha nges i n the s tructure a nd fl ui di ty of the membra nes of the s perm ti p a nd the egg to ca us e them to fus e. Fol l owi ng thi s i ni ti a l fus i on proces s , a protei n mol ecul e on the s perm a cros ome bi nds to a di rectl y i nterl ocki ng protei n, ZP3, on the egg. Thi s bi ndi ng i s bel i eved to i ni ti a te a s eri es of rea cti ons , i ncl udi ng the acrosome reaction, a rel ea s e of enzymes by the a cros ome, whi ch compl ete the membra ne fus i on; the cortical reaction (s ee bel ow); a nd a cti va ti on of the egg to undergo a s econd mei oti c di vi s i on. Fus i on of the ferti l i zi ng s perm a nd egg res ul ts i n ha pl oi d nucl ei . The corti ca l rea cti on i s cons i dered the a na l ogous proces s to the a cros ome rea cti on. The bi ndi ng of s perm to egg res ul ts i n a rel ea s e of ca l ci um i ons , whi ch promote fus i on of corti ca l gra nul e membra nes wi th the egg’s pl a s ma membra ne. Fus i on expres s es fa ctors tha t ca us e the rel ea s e of a n externa l l y bound, membra ne protei n on the outer vitelline layer of the egg. Rel ea s e of thi s protei n forces the vi tel l i ne l a yer a wa y from the zona pel l uci da , movi ng a ny other s perm a wa y perma nentl y a nd i nhi bi ti ng a ny other s perm from ferti l i zi ng the egg. The pol ys a ccha ri de mol ecul e hyalin i s a l s o rel ea s ed from the corti ca l gra nul es to crea te a l a yer a round the egg tha t a l s o i mpedes a ny further ferti l i za ti on. Oral Contraceptives: The i nfl uences of es trogen a nd proges terone on the mens trua l cycl e, ovul a ti on, a nd i mpl a nta ti on ha ve been uti l i zed to a l l ow the devel opment of va ri ous forms of contra cepti on. Es trogen a nd proges ti n, the generi c na me for a ny s yntheti c proges terone compound, mol ecul es i nhi bi t GnRH pul s es , whi ch then decrea s es FSH a nd LH s ecreti on, i nhi bi ti ng devel opment of ova ri a n fol l i cl es a nd ovul a ti on. The es trogen component i n the ora l contra cepti ve pi l l pri ma ri l y s uppres s es FSH s ecreti on a nd the proges terone component s uppres s es LH s ecreti on, thereby preventi ng fol l i cul a r recrui tment a nd ovul a ti on, res pecti vel y. The combi na ti on of es trogen a nd proges terone i s a l s o us ed to trea t mens trua l di s orders by hel pi ng to decrea s e the endometri a l l i ni ng a nd regul a te or even prevent mens es . The control l ed effect on the mens trua l cycl e i s often us ed for fema l e pa ti ents wi th pa i nful , i rregul a r, hea vy, or tempora ri l y a bs ent mens trua l peri ods .
BREAST DEVELOPMENT AND LACTATION Brea s t ti s s ue devel opment or thelarche i s a pa rt of s econda ry s exua l cha nges tha t occur a t puberty beca us e of i ncrea s ed es tra di ol , proges terone, a nd prol a cti n (s ee bel ow). Bes i des other cha nges i n body s ha pe, the a cti vi ty of thi s es trogen vi a i ts receptor l ea ds to a va ri a bl e i ncrea s e of the a di pos e cel l s , s upporti ng l i ga ments a nd gl a nds tha t ma ke up the fema l e brea s t ti s s ue. La cta ti on requi res the a cti on of oxytocin a nd human placental lactogen hormones , whi ch, a l ong wi th prol a cti n, a re di s cus s ed i mmedi a tel y bel ow. OXYTOCIN Oxytocin i s a ni ne-a mi no-a ci d pepti de, i ncl udi ng a di s ul fi de bri dge formed by two cys tei ne res i dues , whi ch i s rel ea s ed by the pos teri or pi tui ta ry gl a nd. Va s opres s i n, a cti ve i n the regul a ti on of tota l body wa ter, di ffers from oxytoci n by onl y two a mi no a ci ds a nd i s a l s o rel ea s ed from the pos teri or pi tui ta ry gl a nd (Fi gure 20-8, Cha pter 18).
Figure 20-8. Comparison of Oxytocin and Antidiuretic Hormone (ADH, Vasopressin) Structures. The ni ne-a mi no-a ci d pepti des oxytoci n a nd ADH (va s opres s i n) wi th di s ul fi de bri dge a s s hown. Di fferi ng a mi no a ci ds a re s een a t pos i ti ons 3 a nd 8. [Ada pted wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009.] Bi ndi ng of oxytoci n to i ts receptor a cti va tes a G q protei n, whi ch a cti va tes phos phol i pa s e C, l ea di ng to ca l ci um rel ea s e a nd protei n ki na s e C functi on. Interes ti ngl y, the oxytoci n receptor requi res both Mg2+ a nd chol es terol , whi ch a re thought to a l l os teri ca l l y a cti va te the receptor. Oxytoci n i s i nvol ved i n the ejecti on of mi l k from the brea s ts ’ mi l k gl a nds a s wel l a s contra cti on of the uterus duri ng bi rth. Both functi ons res ul t from the a cti va ti on of s mooth mus cl e. Uteri ne contra cti on res ul ts i n a nota bl e i ncrea s e i n the number of oxytoci n receptors i n the s mooth mus cl e l a yer a nd rel ea s e of es trogen a nd pros ta gl a ndi n F 2α. Thi s pros ta gl a ndi n, whi ch bi nds to the corpus l uteum a nd hel ps to l ea d to i ts degra da ti on, a cti va tes the G s-coupl ed pa thwa y, l ea di ng to cAMP producti on vi a a denyl cycl a s e. Beca us e of oxytoci n’s ma rked s i mi l a ri ty to va s opres s i n, i t ca n a l s o i ncrea s e s odi um a nd wa ter excreti on i n the ki dneys . PROLACTIN Prolactin i s a pepti de hormone tha t i s produced i n the a nteri or pi tui ta ry gl a nd vi a the a cti on of a s peci fi c tra ns cri pti on fa ctor ca l l ed Pit-1. Increa s ed producti on of prol a cti n i s due to es trogen, whi ch s ti mul a tes Pi t-1 a nd i ncrea s es the number of prol a cti n-produci ng cel l s . Prol a cti n ca n a l s o be produced i n brea s t ti s s ue a s wel l a s nerve a nd i mmune s ys tem cel l s . Prol a cti n bi nds i ts receptor on the ma mmi l l a ry gl a nds a nd ova ri es where di meri za ti on of the receptor a cti va tes a Ja nus 2 tyros i ne ki na s e. Thi s l ea ds to s ubs equent tyros i ne phos phoryl a ti on, di meri za ti on, a nd a cti va ti on of a cl a s s of protei ns known a s signal transducers and activators of transcription (STATs). Thes e a cti va ted STAT di mers tra vel to the nucl eus a nd a ugment gene expres s i on of certa i n protei ns . Prol a cti n bi ndi ng to i ts receptor a l s o a cti va tes MAP ki na s es a nd Src ki na s e, i nvol ved i n the regul a ti on of cel l growth a nd devel opment. The a cti vi ty of both of thes e a ddi ti ona l ki na s es a l s o l ea ds to regul a ted gene expres s i on. Prol a cti n receptors a re a l s o found on va ri ous other orga ns a nd ti s s ues throughout the body. The ma jor functi on of prol a cti n i s to s ti mul a te the producti on of mi l k i n the ma mma ry gl a nds , whos e enl a rgement duri ng pregna ncy i s promoted by es trogen a nd proges terone. As noted a bove, the drop i n i nhi bi tory proges terone l evel s a t the end of a pregna ncy i s the fi na l s i gna l for the begi nni ng of mi l k producti on. The pres ence of prol a cti n a fter pregna ncy a l s o s tops the mens trua l cycl e through i nhi bi ti on of the pul s a ti l e rel ea s e of GnRH a nd, therefore, FSH a nd LH. Prol a cti n conti nues thi s effect throughout the brea s tfeedi ng s ta ge. The a ct of s uckl i ng s ti mul a tes the hypotha l a mus to conti nue promoti ng pi tui ta ry gl a nd s ecreti on of prol a cti n. Prol a cti n a l s o functi ons i n the l ungs to promote the producti on of surfactants i n feta l l ungs (Cha pter 17). In nerve cel l s , prol a cti n promotes the forma ti on of myelin sheaths (Cha pter 19). Fi na l l y, prol a cti n i s thought to countera ct the s exua l a rous a l effects of dopa mi ne a nd provi de pos tcoi ta l s exua l gra ti fi ca ti on. As a res ul t, the refractory period a fter s ex ma y be beca us e of the functi on of prol a cti n. In a s i mi l a r mecha ni s m, overproducti on of prol a cti n ma y pl a y a rol e i n erecti l e di s order a nd i mpotence. The effects of hormones on the producti on of mi l k or l a cta ti on a re l i s ted i n Ta bl e 20-1.
Table 20-1. Hormones Invol ved i n La cta ti on Prolactinoma: The pres ence of otherwi s e beni gn growth of prol a cti n-produci ng cel l s i n the a nteri or pi tui ta ry gl a nd, known a s a prolactinoma, ca n l ea d to s evera l concerni ng a nd even da ngerous s i gns a nd s ymptoms . The i ncrea s ed number of thes e cel l s ca n l ea d to the worryi ng producti on of mi l k (galactorrhea) i n a nonpregna nt woma n or even a ma n. Women ca n a l s o experi ence i rregul a r or mi s s ed mens trua l cycl es a nd/or i nferti l i ty beca us e of the i nhi bi tory effects of exces s prol a cti n on GnRH functi ons . More concerni ng, though, i s the pres ence of hea da ches a nd l os s of peri phera l vi s i on (bi tempora l hemi a nops i a ) beca us e of the pres s ure pl a ced on the opti c chi a s m by the growi ng tumor. Peopl e ma y ha ve a prol a cti noma but wi l l be una wa re unti l thes e ma s s effect s ymptoms a ppea r. Trea tment of a prol a cti noma i s us ua l l y vi a medi ca ti ons (e.g., bromocri pti ne or ca bergol i ne) or neuros urgery.
MALE REPRODUCTIVE SYSTEM The devel opment of the ma l e reproducti ve s ys tem rel i es on the ea rl y a cti vi ty of the sex-determining region (sry) found on the Y chromos ome a nd i ts s ubs equent effects on the Wol ffi a n a nd Mul l eri a n ducts (s ee a bove). In ma l es , s ry a nd AMH a cti vi ti es a l ong wi th the cri ti ca l i nfl uence of tes tos terone res ul t i n the forma ti on of ma l e s exua l cha ra cteri s ti cs a nd the regres s i on of the Mul l eri a n ducts . TESTOSTERONE Testosterone i s the pri ma ry ma l e s teroi d hormone a nd i s produced i n the tes tes by Leydi g cel l s . Tes tos terone i s produced from chol es terol vi a the a ndrogen s yntheti c pa thwa y noted i n Cha pter 3. The enzyme 5α-reductase converts tes tos terone i nto the much more a cti ve DHT (Fi gure 20-9). Tes tos terone ca n a l s o be converted to es tra di ol by the a cti on of the a roma ta s e enzyme. Es tra di ol i s bel i eved to be i mporta nt i n ma l e reproducti ve functi ons but i ts exa ct rol e i s s ti l l bei ng el uci da ted. Women a l s o produce s ma l l er a mounts of tes tos terone from the a drena l gl a nds , ova ri es , a nd, i n ti me of pregna ncy, the pl a centa .
Figure 20-9. Conversion of Testosterone to 5α-Dihydrotestosterone by the Enzymatic Action of 5α-Reductase (s ee text for further di s cus s i on). [Ada pted wi th permi s s i on from Ba rrett KE, et a l .: Ga nong’s Revi ew of Medi ca l Phys i ol ogy, 23rd edi ti on, McGra w-Hi l l , 2010.] Defect of the 5α-Reductase-2 Enzyme: The pres ence of ma l e or fema l e externa l geni ta l s tructures i s determi ned by the pres ence or a bs ence of the tes ti cul a r hormone, testosterone, a nd i ts deri va ti ve, DHT. Unti l the di s covery of DHT, i t wa s a s s umed tha t the devel opment of the ma l e reproducti ve tra ct depended enti rel y on tes tos terone. For exa mpl e, geneti c ma l es l a cki ng 5α-reductase-2 a cti vi ty a nd, therefore DHT, ha ve norma l Wol ffi a n s tructures beca us e thi s a s pect of devel opment i s control l ed by tes tos terone. However, the l a ck of DHT a l s o res ul ts i n a n externa l fema l e phenotype, except tha t the va gi na i s i ncompl etel y devel oped beca us e of the a cti ons of AMH. FSH a nd LH l evel s a re a l s o norma l beca us e of norma l s i gna l i ng by tes tos terone. As a res ul t, thes e geneti c ma l es ca n ha ve ma l e, fema l e, or a bnorma l geni ta l i a wi th s ubs equent gender i denti ty becomi ng a choi ce for the pa ti ent. Furthermore, puberty wi l l i ncrea s e tes tos terone l evel s , whi ch ca n be converted to DHT i n s uffi ci ent qua nti ty by the i s oenzyme 5α-reductase-1, potenti a l l y i ni ti a ti ng the devel opment of a dul t ma l e cha ra cteri s ti cs , i ncl udi ng a peni s a nd s crotum but s ti l l l a cki ng the pros ta te. For the pa ti ent who ha s chos en a fema l e gender i denti ty, s urgi ca l gender convers i on i s requi red. Tes tos terone a nd the res ul ti ng DHT functi on i n a va ri ety of di fferent wa ys throughout the huma n body. The effects of tes tos terone on ea rl y devel opment ha ve been noted a bove. Sta rti ng i n puberty, tes tos terone a nd other a ndrogens ri s e i n concentra ti on a t the s ta rt of puberty a nd, a s s teroi d hormones , enter i nto cel l s vi a pa s s i ve di ffus i on through the pl a s ma membra ne wi th convers i on to DHT vi a 5α-reducta s e-2. Severa l s econda ry s exua l cha ra cteri s ti cs i n ma l es a nd fema l es a re the res ul t of tes tos terone/DHT, i ncl udi ng pubi c, a xi l l a ry a nd a ddi ti ona l fa ci a l , ches t a nd l eg ha i r, i ncrea s ed body oi l producti on a nd body odor, i ncrea s ed growth (e.g., bone a nd mus cl e), enl a rgement of the peni s a nd cl i tori s a l ong wi th i ncrea s ed s exua l dri ve a nd erecti l e frequency, cha nges i n fa ci a l bone s tructure, voi ce cha nges , a nd the ons et of s perma togenes i s (ma l e) a nd oogenes i s (fema l e). Tes tos terone a l s o contri butes to the regul a ti on of pl a tel et a ggrega ti on by i nfl uence on pl a tel et thromboxa ne A2 . Benign Prostatic Hyperplasia (BPH): BPH cl i ni ca l l y a ffects a l mos t 50% of men i n thei r 50s a nd over 75% of men i n thei r 60s , occurri ng more often i n men from wes tern l i fes tyl e a rea s . Al though the exa ct mecha ni s ti c deta i l s of the beni gn growth of the pros ta ti c cel l s (s troma l a nd epi thel i a l ) a re s ti l l unknown, the effect of a ndrogens a s wel l a s es trogen a re wel l es ta bl i s hed. DHT i s bel i eved to pl a y a promi nent rol e vi a bi ndi ng a s a homodi mer to a ndrogen receptors i n the s troma l cel l s wi th res ul ti ng a cti va ti on of DNA tra ns cri pti on a nd i ncrea s ed expres s i on of growth fa ctors for thes e cel l s . Indeed, DHT bi nds more ti ghtl y to the a ndrogen receptor tha n does tes tos terone. 5α-Reductase l evel s , res pons i bl e for converti ng tes tos terone to DHT, a re very hi gh i n thes e a ffected cel l s , wi th DHT ea s i l y a ffecti ng nea rby epi thel i a l cel l s . Es trogen (es tra di ol ) a l s o a ppea rs to functi on i n l a ter l i fe, perha ps by s ens i ti zi ng thes e cel l s to the effects of DHT, a l though thi s mecha ni s m i s even l es s wel l unders tood. Ini ti a l trea tment i s often by “androgen receptor blockers” (na me often ends i n “os i n”), whi ch bl ock the a ndrogen/DHT receptor, or by di rect inhibitors of 5α-reductase (na me us ua l l y ends i n “s teri de”). Surgi ca l or other more a ggres s i ve trea tment moda l i ti es ma y be needed i n s ome pa ti ents .
Reproduced wi th permi s s i on from Ka tzung BG, et a l .: Ba s i c a nd Cl i ni ca l Pha rma col ogy, 11th edi ti on, McGra w-Hi l l , 2009. FSH AND LH FSH a nd LH ha ve i mporta nt rol es i n the ma l e reproducti ve s ys tem (Fi gure 20-10) a s wel l a s i n the fema l e. FSH i s cri ti ca l i n the devel opment of the s emi ni ferous tubul es a s wel l a s s perm (s perma togenes i s ). FSH promotes spermatogenesis by bi ndi ng to i ts receptor l oca ted on Sertoli cells, a l s o ca l l ed “nurs e cel l s ,” whi ch l i ne the tubul es .
Figure 20-10. Effects of Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH) on the Testes. LH s ti mul a tes the Leydi g cel l s to s ecrete tes tos terone. FSH s ti mul a tes the s ecreti on of a ndrogen-bi ndi ng protei n by the Sertol i cel l s i nto the l umen, whi ch bi nds a nd concentra tes tes tos terone i n the s emi ni ferous tubul es a t the s i ght of s perma togenes i s . See text for further deta i l s . [Reproduced wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] Sertol i cel l s a re pri ma ri l y res pons i bl e for the s tructura l a nd nutri ent s upport of devel opi ng s perm, i ncl udi ng the fol l owi ng: Induces mei os i s of s perma togeni c cel l s . Secretes a ndrogen-bi ndi ng protei n, a gl ycoprotei n tha t bi nds a nd concentra tes requi red tes tos terone nea r to the devel opi ng s perm cel l s . Sertol i cel l a roma ta s e converts tes tos terone to 17β-es tra di ol , whos e functi on i n the ma l e reproducti ve s ys tem i s s ti l l bei ng el uci da ted. Secretes a cti vi n a nd i nhi bi n, thereby regul a ti ng FSH s ecreti on (s ee a bove). Produces a nd s ecretes AMH (di s cus s ed a bove). LH i ncrea s es the a cti vi ty i n Leydi g cel l s (Fi gure 20-10) of the cholesterol sidechain cleaving enzyme (Fi gure 3-9), whi ch converts chol es terol to pregnenol one (Cha pter 4). As a res ul t, Leydi g cel l s i ncrea s e s ynthes i s a nd s ecreti on of tes tos terone a s wel l a s a ndros tenedi one. FSH a l s o pl a ys a rol e by i ncrea s i ng the number of receptors for LH on Leydi g cel l s .
REVIEW QUESTIONS 1. How woul d you des cri be (a ) Wol ffi a n ducts , (b) Mul l eri a n ducts , (c) s ex-determi ni ng regi on of the Y chromos ome (s ry), a nd (d) a nti -Mul l eri a n hormone? 2. Wha t i s the functi on of ea ch of the fol l owi ng hormones : (a ) fol l i cl e-s ti mul a ti ng hormone (FSH), (b) l utei ni zi ng hormone (LH), (c) es trogen, (d) proges terone, (e) huma n chori oni c gona dotropi n (hCG), (f) oxytoci n, (g) prol a cti n, (h) tes tos terone, a nd (i ) 5α-di hydrotes tos terone (DHT)? 3. Wha t i s the ba s i c pa thwa y of a cti on for the fol l owi ng proces s es : (a ) fema l e devel opment, (b) ma l e devel opment, (c) oogenes i s , (d) s perma togenes i s , (e) fema l e mens trua l cycl e (i ncl udi ng the a cti ons a nd effects of the va ri ous hormones i nvol ved), a nd (f) ferti l i za ti on? 4. Wha t a re the rol es a nd i mporta nce of G protei ns i n the receptor-medi a ted functi ons of the s ex hormones ?
SECTION III INTEGRATED USMLE-STYLE QUESTIONS AND ANSWERS QUESTIONS III-1. A mi ddl e-Ea s tern fa mi l y pres ents for eva l ua ti on beca us e thei r i nfa nt s on di ed i n the nurs ery wi th s evere hemol ys i s a nd ja undi ce. The coupl e ha s two pri or fema l e i nfa nts who a re a l i ve a nd wel l , a nd the wi fe rel a tes tha t s he l os t a brother i n i nfa ncy wi th s evere hemol ys i s i nduced a fter a vi ra l i nfecti on. The phys i ci a n s us pects gl ucos e-6-phos pha te dehydrogena s e (G6PD) defi ci ency, i mpl yi ng defecti ve s ynthes i s of whi ch of the fol l owi ng compounds ? A. Deoxyri bos e a nd ni coti na mi de a deni ne di nucl eoti de phos pha te (NADP) B. Gl ucos e a nd l a cta te C. La ctos e a nd NADPH D. Ri bos e a nd NADPH E. Sucros e a nd NAD III-2. Rel ea s e of whi ch of fol l owi ng pepti des to the bl ood expl a i ns the enha nced i ns ul i n s ecreti on a fter a mea l ri ch i n ca rbohydra te? A. Chol ecys toki ni n (CCK) B. Ga s tri n C. Gl ucos e-dependent i ns ul i notropi c pepti de (GIP) D. Soma tos ta ti n E. Va s oa cti ve i ntes ti na l pepti de (VIP) III-3. You a re a s ked to revi ew the res ul ts for two pa ti ents who a re s i s ters , both of whom ha ve been di a gnos ed wi th a tha l a s s emi a . Ma ry ha s a s evere ca s e tha t requi res frequent bl ood tra ns fus i ons . Al i ce ha s mi l d mi crocyti c, hypochromi c a nemi a . One pa rent wa s born i n Greece a nd the other i n the Phi l i ppi nes . Both chi l dren expres s the β-globin gene a t 30% of the expected norma l l evel . Ma ry expres s es α-gl obi n i n norma l a mounts . However, Al i ce expres s es the α-gl obi n a t onl y 50% of norma l . Thi s di fference i n the α-gl obi n expres s i on between the s i s ters i n s ome wa y l ea ds to the di fference i n s everi ty of the di s ea s es i n the s i s ters . Whi ch of the fol l owi ng expl a na ti ons a ccount for Ma ry’s more s evere tha l a s s emi a ? A. Al i ce ha s hi gher i ron l evel s tha n does Ma ry s o tha t i ron defi ci ency a ccounts for the grea ter s everi ty. B. Ma ry ha s hemogl obi n (Hgb) H di s ea s e, whi ch i s s i gni fi ca ntl y more s evere tha n Al i ce’s β-tha l a s s emi a i ntermedi a . C. Muta ti on i n the α-gl obi n gene i s expres s ed more i n Ma ry’s devel opi ng red bl ood cel l s . D. Preci pi ta ti on of α-gl obi n tetra mers i n Ma ry’s red bl ood cel l s ki l l s the devel opi ng cel l s . E. Synthes i s of ε-gl obi n i n Al i ce’s devel opi ng red bl ood cel l s compens a tes for her β-gl obi n gene muta ti on. III-4. A 10-yea r-ol d fema l e pres ents wi th ches t pa i n a nd xa nthoma s over her el bows a nd knees . Her fa ther di ed of a hea rt a tta ck a t a ge 35. Her mother ha s hi gh chol es terol . Her phys i ci a n s us pects heterozygous fa mi l i a l hyperchol es terol emi a i n the pa rents wi th homozygous di s ea s e i n the da ughter. Thi s di s ea s e res ul ts from muta ti ons i n the receptor for l ow-dens i ty l i poprotei n (LDL) or the l i ga nd porti on of i ts a poprotei n coa t. Whi ch of the fol l owi ng a poprotei ns mi ght be muta ted? A. AI B. B100 C. B48 D. CII E. E III-5. Infa nts born prema turel y a re a t ri s k for res pi ra tory di s tres s s yndrome. In s uch ca s es , i t i s common to a dmi ni s ter s urfa cta nt, the purpos e of whi ch i s to a l ter whi ch of the fol l owi ng properti es of wa ter a t the a l veol a r i nterfa ce wi th a i r? A. Di el ectri c cons ta nt B. Eva pora ti on C. Hea t of va pori za ti on D. Ioni za ti on E. Surfa ce tens i on III-6. Ful l y a cti va ted pyruva te ca rboxyl a s e depends on the pres ence of whi ch of the fol l owi ng s ubs ta nces ? A. Acetyl -coenzyme (CoA) a nd bi oti n B. Acetyl -CoA a nd thi a mi ne pyrophos pha te C. Ma l a te a nd ni a ci n D. Oxa l oa ceta te a nd bi oti n E. Oxa l oa ceta te a nd ni a ci n III-7. The s tra tegy for thera py for dopa mi ne defi ci ency i n the s ubs ta nti a ni gra of i ndi vi dua l s wi th Pa rki ns on di s ea s e i s i ndi ca ted by whi ch of the fol l owi ng? A. Competi ti ve i nhi bi ti on of bi os ynthes i s from hi s ti -di ne B. Feedba ck i nhi bi ti on of dopa mi ne oxi da ti on C. Provi s i on of meta bol i tes i n the a l a ni ne pa thwa y D. Provi s i on of meta bol i tes i n the tyros i ne pa thwa y E. Sti mul a ti on of monoa mi ne oxi da s e (MAO) III-8. A teena ge fema l e pres enti ng wi th l a ck of mens trua ti on (a menorrhea ) i s found to ha ve a 46XY ka ryotype, a nd deoxyri bonucl ei c a ci d (DNA) tes ti ng s hows a dys functi ona l tes tos terone receptor cha ra cteri s ti c of a ndrogen i ns ens i ti vi ty s yndrome. Whi ch of the fol l owi ng s ta tements a ccura tel y des cri bes s ex hormones s uch a s tes tos terone? A. They bi nd s peci fi c membra ne receptors B. They ca us e rel ea s e of a protei na ceous s econd mes s enger from the cel l membra ne C. They enha nce tra ns cri pti on when bound to receptors D. They i nhi bi t tra ns l a ti on through s peci fi c cytopl a s mi c protei ns E. They i ntera ct wi th DNA di rectl y III-9. A pa ti ent ha s compl a i ned of mus cl e wea knes s tha t ha s gra dua l l y wors ened. After runni ng a n exerci s e tol era nce tes t a nd other a na l ys es , you concl ude tha t pa ti ent ha s McArdl es di s ea s e. Whi ch of the fol l owi ng i s a res ul t from thi s pa ti ent’s exerci s e tol era nce tes t tha t woul d ha ve thi s s peci fi c di a gnos i s a s oppos ed to other mus cl e ca rbohydra te di s orders ? A. Decrea s ed bl ood gl ucos e due to i ncrea s ed mus cl e gl ucos e upta ke
B. Decrea s ed bl ood l a cta te C. Decrea s ed fa t oxi da ti on i n mus cl e D. Decrea s ed mus cl e gl ycogen E. Decrea s ed us e of crea ti ne phos pha te III-10. An el derl y pa ti ent ha s been mos tl y hous ebound for the wi nter a nd ha s ha d a poor di et. In the s pri ng, s he pres ents to the emergency room (ER) wi th mul ti pl e vi ta mi n defi ci enci es . As a res ul t of her defi ci ency i n vi ta mi n C, s he wi l l ha ve a bi ochemi ca l defect i n col l a gen ma tura ti on. Whi ch of the fol l owi ng cha ra cteri s ti cs a ppl i es to thi s vi ta mi n C defi ci ency? A. Defi ci ent a cti vi ty of l ys yl oxi da s e B. Defi ci ent di s ul fi de bond forma ti on, both i ntra cha i n a nd i ntercha i n C. Defi ci ent proteol yti c cl ea va ge of di s ul fi de-l i nked C-a nd N-termi na l extens i ons D. Ina dequa te col l a gen mes s enger ri bonucl ei c a ci d (mRNA) tra ns l a ti on E. Ina dequa te hydroxyl a ti on of prol i ne a nd l ys i ne res i dues III-11. Neura l tube defects s uch a s a nencepha l y a nd s pi na bi fi da ha ve hi gher frequenci es i n certa i n popul a ti ons s uch a s thos e of Cel ti c ori gi n a nd i n certa i n regi ons s uch a s South Texa s . Thi s s ugges ti on of envi ronmenta l ca us e produced res ea rch s howi ng tha t defi ci ency of whi ch of the fol l owi ng vi ta mi ns i s a s s oci a ted wi th the occurrence of neura l tube defects (a nencepha l y a nd s pi na bi fi da )? A. As corbi c a ci d (vi ta mi n C) B. Fol i c a ci d C. Ni a ci n (vi ta mi n B 3 ) D. Ri bofl a vi n (vi ta mi n B 2 ) E. Thi a mi ne (vi ta mi n B 1 ) III-12. Whi ch of the fol l owi ng s ubs ta nces i s s ecreted by ga s tri c pa ri eta l (oxynti c) cel l s ? A. CCK B. Ga s tri n C. Intri ns i c fa ctor D. Moti l i n E. Soma tos ta ti n III-13. A pa ti ent pres ents wi th a hemol yti c a nemi a due to a drug trea tment. Whi ch of the fol l owi ng woul d be a cons equence of thi s di s order? A. Hi gher percent of erythrocytes tha t ha ve reta i ned thei r mi tochondri a B. Hi gher percent of reti cul ocytes i n the bl ood C. Increa s e i n the a mount of feta l Hgb (HgbF) to compens a te for the drug effect D. Lower percent of whi te bl ood cel l s (WBCs ) E. More reti cul ocytes tha t ha ve l os t thei r mi tochondri a a nd nucl ei III-14. A 45-yea r-ol d ma n i s found to ha ve a n el eva ted s erum chol es terol of 300 mg percent mea s ured by s ta nda rd condi ti ons a fter a 12 hr fa s t. Whi ch of the fol l owi ng l i poprotei ns woul d contri bute to a mea s urement of pl a s ma chol es terol i n a norma l pers on fol l owi ng a 12 hr fa s t? A. Chyl omi cron remna nts a nd very-l ow-dens i ty-l i po-protei ns (VLDLs ) B. Chyl omi crons a nd VLDLs C. Hi gh-dens i ty l i poprotei ns (HDLs ) a nd LDLs D. LDLs a nd a di pocyte l i pi d dropl ets E. VLDLs a nd LDLs III-15. A coma tos e i nfa nt i s brought to the ER. In the cours e of the exa mi na ti on, pl a s ma a mmoni a wa s found to be el eva ted 20-fol d over norma l va l ues . Uri ne oroti c a ci d a nd ura ci l were both grea ter tha n norma l . A defect of whi ch of the fol l owi ng enzymes i s the mos t l i kel y di a gnos i s ? A. Argi na s e B. Argi ni nos ucci na te l ya s e C. Ca rba moyl phos pha te s yntheta s e I (a mmoni a ) D. Ca rba moyl phos pha te s yntheta s e II (gl uta mi ne) E. Orni thi ne tra ns ca rba moyl a s e III-16. Duri ng a n overni ght fa s t, whi ch of the fol l owi ng i s the ma jor s ource of bl ood gl ucos e? A. Di eta ry gl ucos e from the i ntes ti ne B. Gl uconeogenes i s C. Gl ycerol from l i pol ys i s D. Hepa ti c gl ycogenol ys i s E. Mus cl e gl ycogenol ys i s III-17. One of the mecha ni s ms of hypers ens i ti vi ty rea cti ons tha t i s a s s oci a ted wi th thrombocytopeni a i nvol ves a nti gens promoti ng the producti on of a nti bodi es , whi ch a tta ch to the pa ti ent’s cel l membra nes . Thes e cel l s become recogni zed a s forei gn a nd a re then ta gged wi th i mmunogl obul i n (Ig) G or IgM a nti bodi es . Thi s ma kes thes e cel l s s us cepti bl e to a tta ck by na tura l ki l l er (NK) or ma cropha ge cel l s . Whi ch of the fol l owi ng types of hypers ens i ti vi ty rea cti ons does thi s mecha ni s m des cri be? A. Ana phyl a cti c B. Cel l medi a ted C. Cytotoxi c D. Del a yed E. Immune compl ex III-18. The a rteri a l bl ood ga s (ABG) tes t res ul ts for your pa ti ent s how pH 7.49 (N: 7.35–7.45), pCO2 = 25 mmHg (N: 35–45), a nd HCO3 − = 19 mEq/L (N: 24–28). On the ba s i s of thes e res ul ts , whi ch of the fol l owi ng condi ti ons do you predi ct exi s ts i n your pa ti ent? A. Meta bol i c a ci dos i s wi th i ncrea s ed rena l rea bs orpti on of bi ca rbona te B. Meta bol i c a l ka l os i s wi th i ncrea s ed rena l excreti on of bi ca rbona te C. Res pi ra tory a ci dos i s wi th i ncrea s ed rena l rea bs orpti on of bi ca rbona te D. Res pi ra tory a l ka l os i s wi th i ncrea s ed rena l excreti on of bi ca rbona te III-19. A pa ti ent i s found to ha ve a defect i n cycl i c gua nos i ne monophos pha te (cGMP) phos phodi es tera s e. In whi ch of the fol l owi ng wa ys
woul d thi s mos t di rectl y a ffect the vi s ua l cycl e i n thi s pa ti ent? A. Fa i l ure of Na +–K+-ATPa s e to functi on B. Inwa rd Na +–Ca 2+ cha nnel s of the rod cel l membra ne rema i n cl os ed C. Membra ne of the rod cel l rema i ns depol a ri zed D. Requi rement for reti nol becomes i ncrea s ed E. Tra ns duci n ca nnot become a cti va ted III-20. Whi ch of the fol l owi ng des cri bes a mecha ni s m of a cti on of a hypotha l a mi c fa ctor or pi tui ta ry hormone? A. Anti di ureti c hormone (ADH) a cts on ki dney vi a G q, l ea di ng to i ncrea s ed i ntra cel l ul a r ca l ci um. B. Corti cotropi n-rel ea s i ng hormone (CRH) a cts on corti cotropi c cel l s by i ncrea s i ng i ntra cel l ul a r ca l ci um. C. Gona dotropi n-rel ea s i ng hormone (GnRH) a cts on gona dotropi c cel l s by i ncrea s i ng i ntra cel l ul a r ca l ci um. D. Soma tos ta ti n (growth hormone-i nhi bi ti ng hormone) a cts on s oma totrophs vi a Gs , l ea di ng to i ncrea s ed i ntra cel l ul a r cycl i c a denos i ne monophos pha te (cAMP). E. Thyrotropi n-rel ea s i ng hormone a cts on thyrotrophs vi a G s , l ea di ng to i ncrea s ed i ntra cel l ul a r cAMP. III-21. A 22-yea r-ol d woma n enga gi ng i n a pol i ti ca l protes t goes on a hunger s tri ke on a promi nent corner i n a ci ty pa rk. Al though food i s offered to her s evera l ti mes ea ch da y by s oci a l workers a nd the pol i ce, s he refus es a l l offers except for wa ter through the fi rs t 2 weeks . An exa mi na ti on of a s a mpl e of thi s woma n’s bra i n ti s s ue woul d revea l tha t her bra i n ha d a da pted to us i ng whi ch of the fol l owi ng a s fuel ? A. Ami no a ci ds B. Free fa tty a ci ds C. Gl ucos e D. Gl ycerol E. Ketone bodi es III-22. Whi ch of the fol l owi ng des cri bes a fea ture of i ntes ti na l a bs orpti on of a mi no a ci ds from the di et? A. Dri ven by a Na + gra di ent from the i ntes ti na l l umen to the i ns i de of the cel l B. Occurs by s i mpl e di ffus i on C. Occurs vi a cotra ns port wi th K+ D. Upta ke of i ndi vi dua l a mi no a ci ds i s not a fea ture E. Us es the s a me Na +-dependent tra ns porter a s gl ucos e III-23. Whi ch of the fol l owi ng s ta tements a ccura tel y cha ra cteri zes feta l hemogl obi n (HgbF)? A. Compl etel y repl a ced pri or to bi rth B. Exhi bi ts no Bohr effect C. Incl udes two β-s ubuni ts a nd two γ-s ubuni ts D. Lower a ffi ni ty for 2,3-bi s phos phogl ycera te (BPG) rel a ti ve to a dul t Hgb E. Lower a ffi ni ty for oxygen (O2 ) rel a ti ve to a dul t Hgb III-24. Corona ry a rtery di s ea s e i s a mul ti fa ctori a l di s order i nvol vi ng occl us i on of the corona ry a rtery wi th a theros cl eroti c pl a ques . Severa l Mendel i a n di s orders a ffecti ng l i pi d meta bol i s m i ncrea s e s us cepti bi l i ty for hea rt a tta cks , wherea s envi ronmenta l fa ctors i ncl ude s moki ng a nd hi gh-fa t di ets . A 45-yea r-ol d ma n ha s a mi l d hea rt a tta ck a nd i s pl a ced on di et a nd s ta ti n thera py. Whi ch of the fol l owi ng wi l l be the mos t l i kel y res ul t of thi s thera py? A. Hi gh bl ood chol es terol B. Hi gh bl ood gl ucos e C. Low bl ood gl ucos e D. Low bl ood LDLs E. Low oxi da ti on of fa tty a ci ds III-25. A col l ege s tudent wi th a norma l medi ca l hi s tory col l a ps es duri ng a n i ntra mura l ba s ketba l l ga me. Hi s fri ends thi nk he i s joki ng a nd then noti ce hi s bl ue col or a nd ca l l a n a mbul a nce. Res us ci ta ti on i s uns ucces s ful a nd the a utops y revea l s di l a ted ca rdi a c cha mbers wi th i ncrea s ed thi cknes s of the ventri cul a r wa l l s (hyper-trophi c, di l a ted ca rdi omyopa thy). El ectron mi cros copy of the hea rt mus cl e s hows a bnorma l thi ck fi l a ments , a nd the pa thol ogi s t s us pects a geneti c di s order. Genes encodi ng whi ch of the fol l owi ng protei ns woul d be mos t l i kel y to revea l the ca us a ti ve muta ti on? A. α-Acti ni n B. Acti n C. Myos i n D. Tropomyos i n E. Troponi n III-26. A 14-yea r-ol d gi rl i s brought to the cl i ni c by her fa ther wi th a compl a i nt of l i ghthea dednes s experi enced on the s occer fi el d ea rl i er i n the a fternoon. She s ta ted tha t s he fel t col d a nd nea rl y fa i nted s evera l ti mes , a nd tha t the s ymptoms di d not res ol ve even a fter s he dra nk a power bevera ge. On further ques ti oni ng, her fa ther s ta ted tha t s he ha d been very thi rs ty recentl y, whi ch bothered hi m beca us e i t mea nt ha vi ng to ma ke frequent ba throom s tops whi l e dri vi ng on tri ps . She a l s o “ea ts l i ke a hors e” a nd never s eems to ga i n a ny wei ght or grow ta l l er. Phys i ca l exa mi na ti on revea l s a thi n gi rl who i s a t the 30th percenti l e for hei ght a nd wei ght. A ra pi d di ps ti ck tes t revea l s gl ucos e i n her uri ne. Eva l ua ti on of thi s gi rl ’s l i ver woul d revea l a n i ncrea s ed ra te of whi ch of the fol l owi ng proces s es ? A. Fa tty a ci d s ynthes i s B. Gl ycogenes i s C. Gl ycol ys i s D. Ketogenes i s E. Protei n s ynthes i s III-27. In a pa ti ent wi th a type I col l a gen muta ti on, whi ch of the fol l owi ng groups of di s ea s es woul d you cons i der i n a di a gnos i s ? A. Al port s yndrome, Goodpa s ture s yndrome, beni gn fa mi l i a l hema turi a B. Col l a genopa thi es types II a nd XI, hypochondrogenes i s C. Dys trophi c epi dermol ys i s bul l os a , epi dermol ys i s bul l os a a cqui s i ta D. Ehl ers –Da nl os s yndromes types I, II, VII; os teogenes i s i mperfecta E. Ehl ers –Da nl os s yndromes types III, IV; a neurys m
III-28. Whi ch of the fol l owi ng cha ra cteri zes nephrogeni c di a betes i ns i pi dus ? A. Defect i n a qua pori n-2 B. Defect i n the G q protei n C. Defect i n the a ngi otens i n II receptor D. Increa s ed os mol a l i ty of uri ne E. Muta nt (i na cti ve) ci rcul a ti ng va s opres s i n (ADH) III-29. Some cytoki nes , s uch a s i nterl euki n (IL)-1, a cti va te phos phol i pa s e C a nd s ubs equentl y a cti va te the nucl ea r fa ctor-ka ppa B (NF-κB) gene. Whi ch of the fol l owi ng receptor cl a s s es i s a s s oci a ted wi th thi s mecha ni s m? A. Chemoki ne receptors B. Ig receptors C. Tra ns formi ng growth fa ctor-β receptors D. Tumor necros i s fa ctor (TNF) receptors E. Type I receptors III-30. A 35-yea r-ol d ma l e pres ents to your offi ce wi th a 1-yea r hi s tory of s hortnes s of brea th. Hi s s hortnes s of brea th ha s i ncrea s ed i n s everi ty over the pa s t yea r. Ini ti a l l y, hi s s hortnes s of brea th wa s noti cea bl e onl y on exerti on wi th modera te a cti vi ty, but over the pa s t 6 months he becomes s hort of brea th wi th mi l d a cti vi ty. Further hi s tory revea l s tha t the pa ti ent ha s been a two pa ck per da y s moker s i nce the a ge of 14 yea rs when “he got hooked on ci ga rettes i n hi gh s chool .” He cons umes a l cohol very modera tel y. On phys i ca l exa mi na ti on, he i s s l i ghtl y ta chypnei c a t res t wi th us e of a cces s ory neck mus cl es duri ng i ns pi ra ti on. Hi s ches t i s ba rrel s ha ped a nd there i s cl ubbi ng of the di s ta l fi ngers . You obta i n a ches t X-ra y tha t revea l s hyperl ucency of the l ungs a nd a number of vi s i bl e bl ebs . Pul mona ry functi on tes ti ng revea l s cha nges of chroni c obs tructi ve l ung di s ea s e cons i s tent wi th emphys ema . Beca us e of hi s a ge, you order a pl a s ma protei n el ectrophores i s , whi ch revea l s a decrea s ed pea k i n the α-1 regi on. You ma ke a di a gnos i s . Whi ch of the fol l owi ng s ta tements mos t l i kel y expl a i ns the emphys ema i n thi s pa ti ent? A. α-1-Anti tryps i n defi ci ency l ea di ng to el eva ted a cti vi ty of el a s ta s e tha t des troys l ung ti s s ue. B. El a s ta s e defi ci ency tha t ca us es exces s i ve a ccumul a ti on of el a s ti n a nd cl ogs the l ungs . C. Oxi da nts from s moki ng genera te rea cti ve oxygen s peci es tha t a cti va te el a s ta s e tha t des troys l ung ti s s ue. D. Oxi da nts from s moki ng genera te rea cti ve oxygen s peci es tha t a cti va te l ung-s peci fi c col l a gena s e tha t des troys l ung ti s s ue. E. The pa ti ent ha s devel oped a nona l cohol i c hepa ti ti s tha t bl ocks α-1-a nti tryps i n s ecreti on, l ea di ng to i ncrea s ed el a s ta s e a cti vi ty a nd des tructi on of l ung ti s s ue. III-31. A pa ti ent i s found to ha ve a ra re di s ea s e i n whi ch the s ecretory functi on of the α-cel l s of the pa ncrea s i s i mpa i red. Di rect s ti mul a ti on of whi ch of the fol l owi ng pa thwa ys i n l i ver wi l l be i mpa i red? A. Ci tri c a ci d cycl e B. Gl ycogenes i s C. Gl uconeogenes i s D. Gl ycol ys i s E. Pentos e phos pha te pa thwa y III-32. In pa ti ents wi th Ni ema nn–Pi ck C di s ea s e, whi ch of the fol l owi ng cel l ul a r proces s es woul d be defecti ve? A. Chol es terol es teri fi ca ti on i n the cytopl a s m B. Chol es terol hydroxyl a ti on for bi l e s a l t forma ti on C. Chol es terol tra ffi cki ng to the Gol gi D. Interna l i za ti on of LDL pa rti cl es E. Lyos oma l hydrol ys i s of chol es terol es ter III-33. Whi ch of the fol l owi ng des cri bes the correct functi on for l i ngua l l i pa s e? A. De-es teri fi ca ti on of chol es terol es ter l i pi d to chol es terol a nd free fa tty a ci d. B. Hydrol ys i s of phos phol i pi ds to free fa tty a ci d a nd di a cyl gl ycerol . C. Pri ma ri l y works i n the mouth where the pH i s cl os er to neutra l . D. Produces gl ycerol a nd three free fa tty a ci ds from tri gl yceri des conta i ni ng medi um-cha i n fa tty a ci ds . E. Produces l ong-cha i n free fa tty a ci ds from the i ni ti a l hydrol ys i s of tri gl yceri des . III-34. How i s the rel a xed (R) s ta te of Hgb s tructura l l y defi ned? A. Bi ndi ng of 2,3-bi s phos phogl ycera te (BPG) to a cl eft between the Hgb s ubuni ts B. Bi ndi ng of ca rbon di oxi de (CO2 ) to the N-termi nus C. Ioni za bl e hi s ti di nes bei ng protona ted D. Subuni ts bei ng di s s oci a ted E. The pos i ti on of i ron i n the pl a ne of the heme III-35. Whi ch of the fol l owi ng i s the ma jor s ource of extra cel l ul a r chol es terol for huma n ti s s ues ? A. Al bumi n B. γ-Gl obul i n C. HDLs D. LDLs E. VLDLs III-36. Whi ch of the fol l owi ng i s a fea ture of the enzyme tha t produces the hormone tha t pea ks s hortl y before ovul a ti on i s i ni ti a ted i n the mens trua l cycl e? A. Acti va ted by fol l i cl e-s ti mul a ti ng hormone (FSH) B. Acti va ted by l utei ni zi ng hormone (LH) C. Cons ti tuti vel y a cti ve to produce es tri ol (E3 ) from a ndros tenedi one D. Feedba ck i nhi bi ted by es tra di ol -17β (E2 ) E. Forms es trone (E1 ) from tes tos terone III-37. A 4-month-ol d pres ents wi th hypogl ycemi a a nd hepa tomega l y. Injecti on of gl uca gon produces no el eva ti on of bl ood gl ucos e or bl ood l a cta te, yet a l a ni ne ca n be converted to gl ucos e. Whi ch of the fol l owi ng l i ver enzymes woul d mos t l i kel y be defecti ve i n thi s pa ti ent? A. Gl ucos e-6-phos pha ta s e B. G6PD
C. Gl ycogen phos phoryl a s e D. Pyruva te ca rboxyl a s e E. Pyruva te ki na s e III-38. Whi ch of the fol l owi ng di s ea s es wi l l be found i n a n a dul t wi th i ns uffi ci ent a mounts of ca l ci um a nd phos pha te i n the bl ood? A. Hyperpa ra thyroi di s m B. Os teoma l a ci a C. Os teoporos i s D. Pa get’s di s ea s e E. Ri ckets III-39. Whi ch of the fol l owi ng condi ti ons woul d ca us e reni n l evel s to ri s e? A. A chroni c hi gh-s a l t di et B. Acute trea tment wi th a tri a l na tri ureti c pepti de (ANP) C. Chroni c trea tment wi th a n a l dos terone receptor a nta goni s t D. Trea tment wi th a mi nera l ocorti coi d receptor a goni s t E. Trea tment wi th a ngi otens i n II III-40. Whi ch of the fol l owi ng i s the correct chronol ogi ca l order of the ma jor bi ochemi ca l events occurri ng duri ng one cycl e of s kel eta l mus cl e contra cti on a nd rel a xa ti on? 1. Acti n i s rel ea s ed from the compl ex 2. ATP bi nds the hea d of myos i n 3. Power s troke 4. Hea d of myos i n bi nds a cces s i bl e a cti n 5. Hea d of myos i n hydrol yzes ATP to ADP a nd Pi A. 1, 2, 3, 4, 5 B. 4, 2, 5, 3, 1 C. 2, 4, 5, 3, 1 D. 2, 5, 4, 3, 1 E. 5, 4, 3, 2, 1 III-41. Fol l owi ng tra uma ti c i njury, epi nephri ne i s s ecreted a nd s ubs equentl y decrea s es i ns ul i n s ecreti on by the β-cel l s of the pa ncrea s . Thi s i nhi bi ti on occurs vi a a cti va ti on of α2 -a drenergi c receptors l ea di ng to decrea s ed producti on of cAMP. Al though pa ncrea ti c β-cel l s a l s o ha ve β2 -a drenergi c receptors tha t i ncrea s e cAMP, the α2 -receptors a re more a bunda nt, a nd wi th el eva ted epi nephri ne i n tra uma , thi s l a tter effect predomi na tes . Gi ven thi s i nforma ti on, why woul d a chi l d i nfected wi th pertus s i s toxi n potenti a l l y fa i l to decrea s e i ns ul i n s ecreti on fol l owi ng a tra uma ti c i njury? A. Toxi n a cti va tes G s -protei n a cti vi ty s o tha t the effect of epi nephri ne i n tra uma i s offs et. B. Toxi n a cts l i ke a tumor promoter to a cti va te protei n ki na s e C tha t i n turn i ncrea s es i ns ul i n s ynthes i s . C. Toxi n di rectl y fa ci l i ta tes the openi ng of ca l ci um cha nnel s s o tha t i nwa rd ca l ci um movement ca n promote i ns ul i n s ecreti on. D. Toxi n i nhi bi ts cAMP phos phodi es tera s e to ca us e a n i ncrea s e i n the cAMP concentra ti on. E. Toxi n i nhi bi ts G i-protei n a cti vi ty s o tha t the effect of epi nephri ne vi a α2 -receptors i s bl ocked. III-42. Whi ch of the fol l owi ng s ta tements des cri be the protei ns tha t a ccumul a te i n cl a s s i c Creutzfel dt–Ja kob di s ea s e? A. Ba cteri a l endotoxi n protei n tha t i s i nfecti ous B. Deri ved from a n i nfecti ous vi rus C. Infecti ous protei n tha t ca n ca us e ca ncer D. Infecti ous protei n tha t i s s i mi l a r to a norma l protei n i n i ts s econda ry s tructure E. Intra cel l ul a r protei n tha t reverts to a n i nfecti ous form III-43. Whi ch of the fol l owi ng s ta tements expl a i n why fructos e 2,6-bi s phos pha te i s i mporta nt i n regul a ti ng gl ycol ys i s i n the l i ver? A. Ha s a phos pha te group wi th a hi gh nega ti ve free energy of hydrol ys i s B. Is a n a l l os teri c i nhi bi tor of phos phofructoki na s e-1 C. Is cl ea ved to tri os e phos pha tes i n the gl ycol yti c pa thwa y D. Its forma ti on i s ca ta l yzed by a gl ycol yti c enzyme E. Provi des a n i ntra cel l ul a r s i gna l tha t i s s ens i ti ve to cha nges i n bl ood gl ucos e l evel s III-44. Whi ch of the fol l owi ng enzymes i nvol ved i n ca rbohydra te di ges ti on i s rel ea s ed by the exocri ne pa ncrea s ? A. Amyl a s e B. Dextri na s e C. Gl ucoa myl a s e D. La cta s e E. Sucra s e III-45. Whi ch of the fol l owi ng cha ra cteri zes s i ckl e cel l di s ea s e? A. Aggrega ti on of Hgb tetra mers due to a hydrophobi c effect B. Ari s es from a poi nt muta ti on cha ngi ng gl uta ma te to l ys i ne on the β-s ubuni t of Hgb C. Bi ndi ng of BPG to Hgb i s di mi ni s hed D. Expres s i on of the s i ckl ed phenotype i nvol ves a hydrophi l i c i ntera cti on E. Feta l Hgb (HgbF) i s s i gni fi ca ntl y a ffected III-46. A 28-yea r-ol d woma n pres ents to her obs tetri ci a n a t week 23 of pregna ncy, compl a i ni ng of extreme fa ti gue. Eva l ua ti ons of s erum i ron l evel a nd feta l wel l bei ng a re norma l , but the nurs e noti ces a wea k a nd i rregul a r pul s e. Ches t X-ra y revea l s a n enl a rged hea rt a nd the el ectroca rdi ogra m (ECG) revea l s a s hort PR i nterva l a nd prol onged QRS, i ncl udi ng a s l urred-up s troke of the R-wa ve ca l l ed a δ-wa ve. The ECG i s rea d a s s howi ng Wol ff–Pa rki ns on–Whi te s yndrome, a condi ti on wi th ri s ks for pa roxys ma l s upra ventri cul a r ta chyca rdi a . The phys i ci a n cons i ders trea tment wi th ca l ci um cha nnel regul a tors , ba l a nci ng thei r ri s ks to mother a nd fetus . Contra cti on of ca rdi a c a nd s kel eta l mus cl e i s i ni ti a ted by the bi ndi ng of ca l ci um to whi ch of the fol l owi ng s ubs ta nces ? A. Acti n B. Actomyos i n C. Myos i n
D. Tropomyos i n E. Troponi n III-47. Whi ch of the fol l owi ng i s a n a cti on of di hydrotes tos terone? A. Ana bol i c effects on s kel eta l mus cl e B. Devel opment of the pros ta te C. Devel opment of the tes tes D. Preventi ng uteri ne devel opment E. Promoti ng s perma togenes i s III-48. Whi ch of the fol l owi ng woul d pa ti ents wi th pri ma ry hyperpa ra thyroi di s m be una bl e to s uppres s (vi a feedba ck i nhi bi ti on)? A. 1-Hydroxyl a s e enzyme i n vi ta mi n D meta bol i s m by ca l ci toni n B. 25-Hydroxyl a s e enzyme i n vi ta mi n D meta bol i s m by a hi gh bl ood concentra ti on of a cti ve vi ta mi n D 3 C. Acti on of pa ra thyroi d hormone (PTH) on os teo-bl a s ts by s ex s teroi ds D. Producti on of ca l ci toni n i n the thyroi d by a hi gh bl ood concentra ti on of ca l ci um E. Producti on of PTH by a tumor by a hi gh bl ood concentra ti on of ca l ci um III-49. A pa ti ent pres ents wi th a defi ci ency of thi a mi ne. If thi s pa ti ent’s l i ver cel l s a re compa red wi th norma l cel l s , whi ch of the fol l owi ng s ubs ta nces woul d be produced i n the thi a mi ne-defi ci ent cel l i n l es s er a mounts i f the cel l s a re onl y gi ven gl ucos e a s a fuel ? A. Al a ni ne B. CO2 C. La cta te D. NADP+ E. Pyruva te III-50. The ABG tes t res ul ts for your pa ti ent s how pH 7.31 (N: 7.35–7.45), pCO2 = 25 mmHg (N: 35–45), a nd HCO3 − = 20 mEq/L (N: 24–28). On the ba s i s of thes e res ul ts , whi ch of the fol l owi ng condi ti ons do you predi ct exi s ts i n your pa ti ent? A. Anxi ety B. Chroni c obs tructi ve pul mona ry di s ea s e (COPD) C. Di a beti c ketoa ci dos i s D. Hypera mmonemi a III-51. Whi ch of the fol l owi ng i s a fea ture of the reni n–a ngi otens i n s ys tem? A. Angi otens i nogen s ynthes i zed i n, a nd rel ea s ed by, the juxta gl omerul a r a ppa ra tus B. Reni n proteol yti ca l l y proces s i ng a ngi otens i n I to a ngi otens i n II C. Angi otens i n I bi ndi ng to the a ngi otens i n receptor D. Angi otens i n II a cti ng a s a va s ocons tri ctor E. Angi otens i n-converti ng enzyme cl ea vi ng a ngi otens i n II to a ngi otens i n III III-52. A pos tmortem s a mpl e of a dementi a pa ti ent s howed evi dence of A-β pepti de a nd hyperphos phoryl a ted ta u protei n depos i ted i n pl a ques . Whi ch of the fol l owi ng obs erva ti ons i s cons i s tent wi th thi s pos tmortem fi ndi ng? A. Al zhei mer dementi a B. Col chi ci ne poi s oni ng C. New va ri a nt Creutzfel dt–Ja kob di s ea s e D. Ni ema nn–Pi ck type A E. Pa rki ns on’s di s ea s e III-53. Crea ti ne phos pha te i s a hi gh energy phos pha te compound tha t i s found i n s kel eta l mus cl e. Whi ch of the fol l owi ng s ta tements mos t l i kel y expl a i ns why there woul d be l i ttl e benefi t to a norma l i ndi vi dua l to ta ke crea ti ne s uppl ements to boos t mus cl e crea ti ne phos pha te concentra ti on? A. Norma l crea ti ne phos pha te concentra ti on i s s uffi ci ent to i ni ti a te exerci s e unti l gl ycogenol ys i s begi ns B. Exces s crea ti ne wi l l a l l os teri ca l l y i nhi bi t the crea ti ne ki na s e rea cti on C. Exces s crea ti ne i nta ke wi l l ca us e ki dney da ma ge i n mos t i ndi vi dua l s D. Crea ti ne phos pha te norma l l y i s s uffi ci ent to i ni ti a te exerci s e unti l fa tty a ci d oxi da ti on begi ns E. Crea ti ne ca nnot cros s the mus cl e cel l membra ne III-54. An i ncrea s ed concentra ti on of whi ch of the fol l owi ng hormones prevents l a cta ti on from occurri ng i n pregna ncy by bl ocki ng the a cti on of prol a cti n? A. Andros tenedi one a nd proges terone B. Es tra di ol a nd es trone C. Growth hormone a nd es tri ol D. Huma n chori oni c gona dotropi n (hCG) hormone a nd proges terone E. Proges terone a nd es tri ol III-55. Two pa ti ents pres ent, one of whom ha s a defect i n gl ucos e-6-phos pha ta s e a nd the other exhi bi ts a defect of fructos e 1,6bi s phos pha ta s e. Whi ch of the fol l owi ng i s mos t l i kel y the condi ti on tha t woul d onl y be found i n the pa ti ent wi th the defect i n gl ucos e6-phos pha ta s e? A. Al a ni ne a ccumul a ti on i n the bl ood fol l owi ng food depri va ti on B. Al tered mus cl e meta bol i s m of gl ucos e C. Gl ycogen a ccumul a ti on i n the l i ver D. La cti c a ci dos i s E. Pentos e phos pha te pa thwa y i s not functi ona l III-56. A 7-yea r-ol d gi rl ha s a 1-month hi s tory of foul -s mel l i ng di a rrhea . Upon further i nqui ry, the frequency s eems to be 4–6 s tool s per da y. She ha s a l s o ha d troubl e s eei ng a t ni ght i n the pa s t 2 weeks . Her WBC count i s norma l . Phys i ca l exa mi na ti on i s enti rel y norma l . Exa mi na ti on of a s tool s a mpl e revea l s tha t i t i s bul ky a nd grea s y. Ana l ys i s does not revea l a ny pa thogeni c mi croorga ni s ms or pa ra s i tes but confi rms the pres ence of fa ts . Further eva l ua ti on of thi s pa ti ent woul d l i kel y revea l whi ch of the fol l owi ng condi ti ons ? A. Di a betes B. Ga s troi ntes ti na l (GI) i nfecti on C. Il ea l di s ea s e
D. Ins uffi ci ent bi l e producti on E. La ctos e i ntol era nce III-57. Whi ch of the fol l owi ng woul d be the ea rl i es t event i n the cl otti ng ca s ca de? A. Bi ndi ng of pl a tel ets to red bl ood cel l s B. Bi ndi ng of vi ta mi n K to endothel i a l cel l s urfa ces C. Forma ti on of fi bri n D-di mers D. Rel ea s e of ti s s ue fa ctor (thrombopl a s ti n) by da ma ged ves s el s E. Secreti on of von Wi l l ebra nd fa ctor by pl a tel ets III-58. A pa ti ent wi th myoca rdi a l i nfa rcti on i s trea ted wi th ni trogl yceri n to di l a te hi s corona ry a rteri es . Whi ch of the fol l owi ng bes t des cri bes the a cti on of ni trogl yceri n? A. Acetyl -CoA a nd chol i ne a re condens ed to form a neurotra ns mi tter B. Argi ni ne i s converted to a neurotra ns mi tter tha t a cti va tes gua nyl yl cycl a s e C. Gua nos i ne tri phos pha te hydrol ys i s a ccompl i s hes oxi da ti on of LDL protei ns D. Methyl a ti on occurs to produce S-a denos yl methi o-ni ne E. Tyros i ne i s converted to s erotoni n III-59. Whi ch of the fol l owi ng cytoki nes i s produced by hel per T (Th) cel l s ? A. Interferon-α B. IL-1 C. IL-2 D. IL-3 E. IL-5 F. IL-10 G. Neutrophi l chemota cti c fa ctor III-60. Whi ch of the fol l owi ng s ta tements a ccura tel y des cri bes functi ons of α1 -a drenergi c receptors ? A. Bronchodi l a ti on a nd decrea s ed s trength of hea rt contra cti on B. Cons tri cti on of bl ood ves s el s a nd decrea s ed hea rt ra te C. Increa s ed gl ycogenol ys i s a nd l i pol ys i s D. Va s ocons tri cti on a nd reduced s ympa theti c outfl ow i n the centra l nervous s ys tem (CNS) E. Va s odi l a ti on a nd i ncrea s ed hea rt ra te III-61. Whi ch of the fol l owi ng i s a n i nhi bi tory neurotra ns mi tter found excl us i vel y i n the centra l nervous s ys tem? A. Acetyl chol i ne B. Gl uta ma te C. Norepi nephri ne D. Serotoni n E. Subs ta nce P ANSWERS III-1. The answer is D. Gl ucos e-6-phos pha te dehydrogena s e (G6PD) i s the fi rs t enzyme of the pentos e phos pha te pa thwa y, a s i de pa thwa y for gl ucos e meta bol i s m whos e pri ma ry purpos e i s to produce ri bos e a nd NADPH (Fi gure 6-7A). Its defi ci ency i s the mos t common enzymopa thy. It contra s ts wi th gl ycol ys i s i n i ts us e of NADP ra ther tha n NAD for oxi da ti on, i ts producti on of CO2 , a nd i ts producti on of pentos es (ri bos e, ri bul os e, a nd xyl ul os e). Producti on of NADPH by the pentos e phos pha te pa thwa y i s cruci a l for reducti on of gl uta thi one, whi ch i n turn removes hydrogen peroxi de. Erythrocytes a re pa rti cul a rl y s us cepti bl e to hydrogen peroxi de a ccumul a ti on, whi ch oxi di zes red bl ood cel l membra nes a nd produces hemol ys i s . Stres s es s uch a s newborn a djus tment, i nfecti on, or certa i n drugs ca n i ncrea s e red bl ood cel l hemol ys i s i n G6PD-defi ci ent i ndi vi dua l s , l ea di ng to s evere a nemi a , ja undi ce, pl uggi ng of rena l tubul es wi th rel ea s ed Hgb, rena l fa i l ure, hea rt fa i l ure, a nd dea th. Beca us e the l ocus encodi ng G6PD i s on the X chromos ome, the defi ci ency exhi bi ts X-l i nked reces s i ve i nheri ta nce wi th s evere a ffl i cti on i n ma l es a nd tra ns mi s s i on through a s ymptoma ti c fema l e ca rri ers . Ri bos e5-phos pha te produced by the pentos e phos pha te pa thwa y i s a n i mporta nt precurs or for ri bo-nucl eoti de s ynthes i s (Fi gure 6-7C). III-2. The answer is C. CCK i s a pepti de hormone whos e ma i n effect i s contra cti on of s mooth mus cl e of the ga l l bl a dder a nd s i mul ta neous s ecreti on of pa ncrea ti c s ol uti ons to i ncrea s e di ges ti on. However, emptyi ng of the s toma ch a nd ga s tri c a ci d s ecreti on i s a l s o decrea s ed by CCK a s di ges ti on progres s es beyond the s toma ch. Ga s tri n s ti mul a tes HCl , peps i nogen, a nd i ntri ns i c fa ctor s ecreti on from pa ri eta l cel l s , a nd peps i nogen/reni n by chi ef cel l s of the s toma ch. Smooth mus cl e contra cti on (i .e., moti l i ty) of the s toma ch i s a l s o enha nced by ga s tri n. Soma tos ta ti n decrea s es rel ea s e of ga s tri n, CCK, s ecreti n, moti l i n, VIP, GIP, a nd enterogl uca gon, l ea di ng to decrea s ed s toma ch s ecreti on a nd contra cti on. VIP s ti mul a tes peps i nogen/peps i n s ecreti on, di l utes bi l e a nd pa ncrea ti c jui ce, i ncrea s es bi ca rbona te producti on i n the pa ncrea s , decrea s es ga s tri n-i nduced ga s tri c a ci d s ecreti on, a nd i ncrea s es wa ter s ecreti on i n i ntes ti ne. GIP bes i des i ncrea s i ng i ns ul i n s ecreti on decrea s es the rel ea s e of ga s tri c a ci d by pa ri eta l cel l s a s wel l a s s mooth mus cl e contra cti on (moti l i ty) of the s toma ch a nd i ncrea s es fa t meta bol i s m by a cti va ti ng l i poprotei n l i pa s e (s ee Cha pter 11). III-3. The answer is D. The key to thi s a ns wer i s the rel a ti ve proporti on of α-gl obi n a nd β-gl obi n tha t i s functi ona l i n ea ch s i s ter. Norma l a dul t Hgb conta i ns two α-gl obi n a nd two β-gl obi n cha i ns . Both s i s ters ha ve a s i gni fi ca nt defi ci ency of the β-gl obi n gene to the s a me extent. Hence, thi s effect woul d be the s a me i n both s i s ters . Al i ce, unl i ke Ma ry, a l s o ha s reduced expres s i on of her α-gl obi n. Cons equentl y, i n cons i deri ng the proporti on of α-gl obi n to β-gl obi n i n ea ch s i s ter, Al i ce ha s a ra ti o of a pproxi ma tel y 50:30, wherea s Ma ry ha s a hi gher proporti on of 100:30, tha t i s , her α-gl obi n exceeds the a mount of her β-gl obi n by cons i dera bl y more. Therei n l i es the probl em for Ma ry. Beca us e the α-gl obi n i s more tha n three ti mes the a mount of β-gl obi n, i t i s pos s i bl e for α-gl obi n tetra mers to form. Thes e tetra mers a re not s ol ubl e a nd preci pi ta te i n the red bl ood cel l s , l ea di ng to cel l des tructi on a nd a nemi a . In terms of the other pos s i bl e a ns wers , there i s no evi dence of a di fference i n i ron l evel s i n the s i s ters . Hgb H di s ea s e ca nnot ha ppen i n Ma ry beca us e thi s i s a condi ti on i n whi ch Hgb ha s four β-gl obi n s ubuni ts . Ma ry ha s no muta ti on of α-gl obi n gene a s expres s i on i s norma l . Fi na l l y, ε-gl obi n i s a n embryoni c form (s ee Cha pter 14). III-4. The answer is B. The s hel l of a poprotei ns coa ti ng bl ood tra ns port l i poprotei ns i s i mporta nt i n the phys i ol ogi c functi on of the l i poprotei ns . Some of the a poprotei ns conta i n s i gna l s tha t ta rget the movement of the l i poprotei ns i n a nd out of s peci fi c ti s s ues . B48 a nd E s eem to be i mporta nt i n ta rgeti ng chyl omi cron remna nts to be ta ken up by l i ver. B100 i s s ynthes i zed a s the coa t protei n of VLDLs a nd ma rks thei r end product, LDLs , for upta ke by peri phera l ti s s ues . Other a poprotei ns a re i mporta nt for the s ol ubi l i za ti on a nd movement of l i pi ds a nd chol es terol i n a nd out of the pa rti cl es . CII i s a l i poprotei n l i pa s e a cti va tor tha t VLDLs a nd chyl omi crons recei ve from HDLs . The “A” a poprotei ns a re found i n HDLs a nd a re i nvol ved i n l eci thi n–chol es terol a cyl tra ns fera s e regul a ti on. Fa mi l i a l hyperchol es terol emi a ca us es ea rl y hea rt a tta cks i n heterozygotes , pa rti cul a rl y i n ma l es , a nd chi l dhood di s ea s e i n ra re homozygotes .
The da ughter’s ches t pa i n wa s l i kel y a ngi na due to corona ry a rtery occl us i on a nd her s ki n pa tches were fa tty depos i ts known a s xa nthoma ta (Fi gure 16-8). III-5. The answer is E. Pul mona ry s urfa cta nt, compos ed of l eci thi n a nd myel i n, i s s ecreted on a conti nuous ba s i s by type II a l veol a r cel l s a nd Cl a ra cel l s begi nni ng a t a pproxi ma tel y 20 weeks of ges ta ti on. Pul mona ry s urfa cta nt ha s both a l i pi d (~90% of tota l ) a nd a protei n (~10% of tota l ) component. About ha l f of the l i pi ds a re di pa l mi toyl phos pha ti dyl chol i ne wi th a cha rged a mi ne group on i ts hea d group. The rema i ni ng l i pi ds i ncl ude phos pha ti dyl gl ycerol , whi ch modul a tes the fl ui di ty of the s urfa cta nt a s wel l a s chol es terol a nd other l i pi ds . The l i pi ds a nd protei ns i n s urfa cta nt a l l ha ve the ca pa bi l i ty of i ntera cti ng wi th a queous (hydrophi l i c) or nona queous (hydrophobi c) envi ronments a nd i t i s thi s qua l i ty tha t l ea ds to thei r s peci a l i zed functi on i n the l ung (Fi gure 17-2). By di rectl y i ntera cti ng wi th a l veol a r wa ter vi a thei r hydrophi l i c regi ons whi l e the hydrophobi c regi ons rema i n i n the a i r, pul mona ry s urfa cta nt crea tes a myel i n mes hwork tha t l i nes the a l veol i —a s trong, i ntertwi ned l i poprotei n s ys tem tha t i s a na l ogous to the myel i n s hea th of nerve cel l s . Thi s uni que a l veol a r l i ni ng grea tl y reduces s urfa ce tens i on, a l l owi ng ea s i er expa ns i on/s tretchi ng a nd col l a ps e of a l veol i duri ng res pi ra ti on a nd the res ul ti ng cha nges i n pres s ure. Thi s reducti on i n s urfa ce tens i on ma kes the work of res pi ra ti on much l es s a nd reduces the tota l a mount of pres s ure tha t mus t be genera ted for effi ci ent a nd effecti ve i ns pi ra ti on a nd expi ra ti on. The pul mona ry s urfa cta nt a l s o hel ps a l l the l ung a l veol i to expa nd (i ns pi ra ti on) a nd s hri nk (expi ra ti on) a t the s a me ra te, thereby reduci ng the cha nce for i s ol a ted overexpa ns i on a nd the tota l col l a ps e of the a l veol a r s a cs . Al though a l l the other opti ons repres ent properti es of wa ter or s ol uti ons , they ha ve nothi ng to do wi th the properti es of s urfa cta nt. III-6. The answer is A. Pyruva te ca rboxyl a s e ca ta l yzes the convers i on of pyruva te to oxa l oa ceta te i n gl uconeogenes i s : pyruva te + HCO3 − + ATP → oxa l oa ceta te + ADP + Pi. For pyruva te ca rboxyl a s e to be rea dy to functi on, i t requi res bi oti n, Mg2+, a nd Mn 2+. It i s a l l os teri ca l l y a cti va ted by a cetyl -CoA. The bi oti n i s not ca rboxyl a ted unti l a cetyl -CoA bi nds the enzyme. By thi s mea ns , hi gh l evel s of a cetyl -CoA s i gna l the need for more oxa l oa ceta te. When ATP l evel s a re hi gh, the oxa l oa ceta te i s cons umed i n gl uconeogenes i s . When ATP l evel s a re l ow, the oxa l oa ceta te enters the ci tri c a ci d cycl e. Gl uconeogenes i s onl y occurs i n the l i ver a nd ki dneys . III-7. The answer is D. Dopa mi ne i s produced from L-3,4-di hydroxyphenyl a l a ni ne ( L-DOPA), whi ch i n turn i s ma de from tyros i ne. Thera py wi th the L-DOPA precurs or i ncrea s es dopa mi ne concentra ti ons a nd i mproves the ri gi di ty a nd i mmobi l i ty tha t occur i n Pa rki ns on di s ea s e. Dopa mi ne i s degra ded i n the s yna pti c cl eft by MAO-A a nd MAO-B, produci ng 3,4-di hydroxyphenyl -a ceta l dehyde (DOPAC). DOPAC i s i n turn broken down to homova ni l l i c a ci d, whi ch ca n be mea s ured i n s pi na l fl ui d to a s s es s dopa mi ne meta bol i s m. Inhi bi tors of MAO-A a nd MAO-B ha ve s ome us e i n trea ti ng Pa rki ns on di s ea s e. The meta bol i s m of hi s ti -di ne or a l a ni ne i s not rel a ted to tha t of dopa mi ne, but phenyl a l a ni ne i s a precurs or of tyros i ne a nd L-DOPA (s ee Cha pter 19; Fi gure 19-7A). III-8. The answer is C. Al l s teroi d hormones , i ncl udi ng the s ex hormones es trogen, tes tos terone, a nd proges terone, ca n be cl a s s i fi ed a s group I hormones , mea ni ng tha t they a ct by bi ndi ng s peci fi c cytopl a s mi c receptors tha t enter the nucl eus a nd s ti mul a te tra ns cri pti on by s peci fi c DNA bi ndi ng. Mos t nons teroi da l hormones , for exa mpl e, epi nephri ne, a re group II hormones tha t i ntera ct wi th the cel l membra ne a nd produce a s econd-mes s enger effect. The group II hormones , i n contra s t to s teroi ds , a ct i n mi nutes , wherea s s teroi d hormones requi re hours for a bi ol ogi c effect. Recent s tudi es ha ve i ndi ca ted tha t s peci fi c cytopl a s mi c receptors for s teroi d hormones ha ve a n extra ordi na ri l y hi gh a ffi ni ty for the hormones . In a ddi ti on, the receptors conta i n a DNA-bi ndi ng regi on tha t i s ri ch i n a mi no a ci d res i dues tha t form meta l -bi ndi ng fi ngers . Li kewi s e, thyroi d hormone receptors conta i n DNA-bi ndi ng doma i ns wi th meta l -bi ndi ng fi ngers . Li ke s teroi d hormones , thyroi d hormones a re tra ns cri pti ona l enha ncers . III-9. The answer is A. Pa ti ents wi th McArdl es di s ea s e a re defi ci ent i n myophos phoryl a s e, the mus cl e form of gl ycogen phos phoryl a s e. Pa ti ents us ua l l y pres ent i n chi l dhood wi th mus cl e pa i n, fa ti gue, cra mps , a nd wea knes s wi th exces s i ve myogl obi n i n the uri ne, i ndi ca ti ve of rha bdomyol ys i s duri ng prol onged peri ods of exerci s e. Progres s i ve s ymptoms a nd mus cl e ma s s l os s a nd wea knes s a re us ua l l y evi dent a s the pa ti ent a ges . If i ni ti a l exerci s e i ntens i ty i s too grea t, crea ti ne phos pha te i s qui ckl y depl eted. Beca us e mus cl e gl ycogen ca nnot be hydrol yzed, i t tends to be a t grea ter concentra ti ons i n mus cl e a nd exerci s e ca nnot el i ci t a n i ncrea s e i n bl ood l a cta te. Di a gnos i s ma y be compl i ca ted by a “s econd wi nd” phenomenon exhi bi ted by the pa ti ents . Evi dence s ugges ts tha t a s l ong a s the pa ti ent’s i ni ti a l exerci s e a ttempts a re of very l ow i ntens i ty, wi thi n a pproxi ma tel y 15 mi n, exerci s e ca n i ncrea s e the mobi l i za ti on of gl ucos e tra ns porter type 4 to the pl a s ma membra ne. Thi s fa ci l i ta tes i ncrea s ed upta ke of gl ucos e s o the pa ti ent ca n us e gl ycol ys i s to provi de energy for s us ta i ni ng the exerci s e. Indeed pa ti ents often s how a concomi ta nt decrea s e i n bl ood gl ucos e tha t i s often s l i ghtl y el eva ted i n McArdl es ’ pa ti ents beca us e of mi l d i ns ul i n res i s ta nce. The a bi l i ty to a chi eve s econd wi nd a l s o a l l ows the mus cl e to begi n oxi di zi ng fa tty a ci ds . III-10. The answer is E. Prol yl a nd l ys yl hydroxyl a s es both requi re vi ta mi n C a s a cofa ctor. Thes e a re i ron-conta i ni ng enzymes , a nd vi ta mi n C (a s corbi c a ci d) i s needed to ma i nta i n i ron i n i ts reduced (Fe 2+) s ta te. Al though l ys yl oxi da s e us es the hydroxyl ys i ne product of l ys yl hydroxyl a s e a s a s ubs tra te, i ts a cti vi ty wi l l not be defi ci ent. The l ys yl oxi da s e enzyme requi res copper a nd vi ta mi n B6 a s cofa ctors . There wi l l be l es s a l l ys i ne product onl y beca us e of a reduced a mount of s ubs tra te. Di s ul fi de bond forma ti on occurs a fter the hydroxyl a ti on s tep a nd a l though i t wi l l be i ndi rectl y a ffected by vi ta mi n C defi ci ency, thi s proces s i f not defi ci ent. mRNA tra ns l a ti on of cours e occurs pri or to the hydroxyl a ti on s teps (s ee Fi gure 13-2B). III-11. The answer is B. Spi na bi fi da , or myel omeni ngocel e, i s a defect of the l ower neura l tube tha t produces a n expos ed s pi na l cord i n the thora ci c or s a cra l regi ons . Expos ure of the s pi na l cord us ua l l y ca us es nerve da ma ge tha t res ul ts i n pa ra l ys i s of the l ower l i mbs a nd uri na ry bl a dder. Anencepha l y i s a defect of the a nteri or neura l tube tha t res ul ts i n l etha l bra i n a noma l i es a nd s kul l defects . Fol i c a ci d i s neces s a ry for the devel opment of the neura l tube i n the fi rs t few weeks of embryoni c l i fe, a nd the chi l dren of women wi th nutri ti ona l defi ci enci es ha ve hi gher ra tes of neura l tube defects . Beca us e neura l tube cl os ure occurs a t a ti me when ma ny women a re not a wa re tha t they a re pregna nt, i t i s es s enti a l tha t a l l women of chi l dbea ri ng a ge ta ke a fol i c a ci d s uppl ement of a pproxi ma tel y 0.4 mg/da y. Fra nk fol i c a ci d defi ci ency ca n a l s o ca us e mega l obl a s ti c a nemi a beca us e of a decrea s ed s ynthes i s of the puri nes a nd pyri mi di nes needed for cel l s to ma ke DNA a nd di vi de. Defi ci enci es of thi a mi ne i n chroni c a l cohol i cs a re rel a ted to Werni cke–Kors a koff s yndrome, whi ch i s cha ra cteri zed by l os s of memory, l a cka da i s i ca l beha vi or, a nd a conti nuous rhythmi c movement of the eyeba l l s . Thi a mi ne di eta ry defi ci ency from exces s of pol i s hed ri ce ca n ca us e beri beri . Ni a ci n defi ci ency l ea ds to pel l a gra , a di s order tha t produces s ki n ra s h (derma ti ti s ), wei ght l os s , a nd neurol ogi c cha nges i ncl udi ng depres s i on a nd dementi a . Ri bofl a vi n defi ci ency l ea ds to mouth ul cers (s toma ti ti s ), chei l os i s (dry, s ca l y l i ps ), s ca l y s ki n (s eborrhea ), a nd photophobi a . Beca us e bi oti n i s wi del y di s tri buted i n foods a nd i s s ynthes i zed by i ntes ti na l ba cteri a , bi oti n defi ci ency i s ra re. However, the hea t-l a bi l e mol ecul e a vi di n, found i n ra w egg whi tes , bi nds bi oti n ti ghtl y a nd bl ocks i ts a bs orpti on, ca us i ng derma ti ti s , dehydra ti on, a nd l etha rgy. La cti c a ci dos i s res ul ts a s a bui l dup of l a cta te due to the l a ck of functi ona l pyruva te ca rboxyl a s e when bi oti n i s mi s s i ng. Vi ta mi n C defi ci ency l ea ds to s curvy, whi ch ca us es bl eedi ng gums a nd bone di s ea s e. III-12. The answer is C. Intri ns i c fa ctor i s norma l l y produced by the pa ri eta l cel l s of the s toma ch a nd i s es s enti a l i n a l l owi ng the a bs orpti on of vi ta mi n B 12 i n the i l eum. CCK i s a pepti de hormone produced by cel l s i n the duodenum. Its ma i n effect i s contra cti on of s mooth mus cl e of the ga l l bl a dder a nd s i mul ta neous s ecreti on of pa ncrea ti c s ol uti ons to i ncrea s e di ges ti on. Ga s tri n i s produced by G-cel l s i n res pons e to pres ence of undi ges ted protei ns a nd/or di s tens i on of the a ntrum of the s toma ch. Moti l i n i s a pepti de hormone ma de ma i nl y i n the duodenum a nd jejunum tha t i ncrea s es s mooth mus cl e contra cti on (fundus , a ntrum, a nd ga l l bl a dder). Soma tos ta ti n i s produced i n the s toma ch, i ntes ti nes , a nd pa ncrea s . III-13. The answer is B. The a nemi a i s a res ul t of l os s of red bl ood cel l s . To compens a te, there i s a n i ncrea s ed producti on of new cel l s . New cel l forma ti on coupl ed to l os s of ma ture cel l s crea tes a hi gher percent of reti cul ocytes . Erythrocytes a l wa ys ha ve no mi tochondri a a nd the reti cul ocytes a l wa ys l os e thei r mi tochondri a a nd nucl ei . HgbF i s gone wi thi n 6 months a fter bi rth. WBCs count i s unrel a ted to a drug-
i nduced a nemi a . III-14. The answer is C. In the pos ta bs orpti ona l (pos tpra ndi a l ) s ta te, pl a s ma conta i ns a l l the l i poprotei ns : chyl omi crons deri ved from di eta ry l i pi ds pa cka ged i n the i ntes ti na l epi thel i a l cel l s a nd thei r remna nts ; VLDLs , whi ch conta i n endogenous l i pi ds a nd chol es terol pa cka ged i n the l i ver; LDLs , whi ch a re end products of del i pi da ti on of VLDLs ; a nd HDLs , whi ch a re s ynthes i zed i n the l i ver. HDLs a re i n pa rt ca ta l yti c beca us e tra ns fer of thei r CII a pol i poprotei n to VLDLs or chyl omi crons a cti va tes l i poprotei n l i pa s e. In norma l pa ti ents , onl y LDLs a nd HDLs rema i n i n pl a s ma fol l owi ng a 12-h fa s t beca us e both chyl omi crons a nd VLDLs ha ve been del i pi da ted. Mos t of the chol es terol mea s ured i n bl ood pl a s ma a t thi s ti me i s pres ent i n the chol es terol -ri ch LDLs . However, HDL-chol es terol a l s o contri butes to the mea s urement. In a ddi ti on to tota l pl a s ma chol es terol , the ra ti o of HDL (good) to LDL (ba d) chol es terol i s a l s o us eful for predi cti ng hea rt a tta ck ri s ks (s ee Cha pter 16). III-15. The answer is E. Argi na s e ca ta l yzes the l a s t rea cti on of the urea cycl e by cl ea vi ng a rgi ni ne i nto urea a nd orni thi ne. Urea i s s ecreted from the l i ver i nto the bl ood to be cl ea red by the ki dney for excreti on. The orni thi ne i s regenera ted for a nother turn of the cycl e. Argi ni nos ucci na te l ya s e cl ea ves a rgi ni nos ucci na te i nto fuma ra te a nd a rgi ni ne i n the urea cycl e rea cti on tha t precedes a rgi na s e. Ca rba moyl phos pha te s yntheta s e I i s a n i ni ti a l rea cti on a s s oci a ted wi th the urea cycl e s tep tha t ca ta l yzes the producti on of ca rba moyl phos pha te from ATP, NH 3 , a nd CO2 . Thi s ca rba moyl phos pha te i s then condens ed wi th orni thi ne to produce ci trul l i ne i n a rea cti on ca ta l yzed by orni thi ne tra ns ca rba moyl a s e. In the pyri mi di ne s yntheti c pa thwa y, ca rba moyl phos pha te i s a l s o produced i n a rea cti on ca ta l yzed by ca rba moyl phos pha te s yntheta s e II tha t us es gl uta mi ne i ns tea d of NH 3 a s a ni trogen s ource. When orni thi ne tra ns ca rba moyl a s e i s defi ci ent i n the l i ver, ca rba moyl phos pha te a ccumul a tes a nd moves from the mi tochondri a l ma tri x i nto the cytopl a s m. In the cytopl a s m, thi s ca rba moyl phos pha te i s us ed to produce pyri mi di ne ba s es (oroti c a ci d a nd ura ci l ) tha t a re produced i n a n uncontrol l ed ma nner wi th the exces s bei ng excreted i n the uri ne (Fi gure 5-9A; Cha pter 18). III-16. The answer is D. In the a bs orpti ve pha s e fol l owi ng a mea l , the ma jor s ource of gl ucos e i s gl ucos e ta ken di rectl y from the i ntes ti ne i nto the bl ood s ys tem. Much of thi s gl ucos e i s a bs orbed i nto cel l s a nd, i n pa rti cul a r, i nto the l i ver vi a the a cti on of i ns ul i n, where i t i s s tored a s gl ycogen. Once the effects of da yti me ea ti ng ha ve s ubs i ded a nd a l l the gl ucos e from a bs orpti on ha s been s tored, the norma l overni ght fa s t begi ns . Duri ng thi s peri od, the ma jor s ource of bl ood gl ucos e i s hepa ti c gl ycogen. Through the effects of gl ycogenol ys i s , whi ch a re medi a ted by gl uca gon, hepa ti c gl ycogen i s s l owl y pa rcel ed out a s gl ucos e to the bl oods trea m, keepi ng bl ood gl ucos e l evel s norma l . In contra s t, mus cl e gl ycogenol ys i s ha s no effect on bl ood gl ucos e l evel s beca us e no gl ucos e-6-phos pha ta s e exi s ts i n mus cl e a nd hence phos phoryl a ted gl ucos e ca nnot be rel ea s ed from mus cl e i nto the bl oods trea m. Fol l owi ng a more prol onged fa s t or i n the ea rl y s ta ges of s ta rva ti on, gl uconeogenes i s i s needed to produce gl ucos e from gl ucogeni c a mi no a ci ds a nd the gl ycerol rel ea s ed by l i pol ys i s of tri gl yceri des i n a di pocytes . Thi s i s beca us e the l i ver gl ycogen i s depl eted a nd the l i ver i s forced to turn to gl uconeogenes i s to produce the a mount of gl ucos e neces s a ry to ma i nta i n bl ood l evel s (Cha pter 10). III-17. The answer is C. The mecha ni s m provi ded des cri bes type II tha t i s a l s o known a s a nti body dependent. The a na phyl a cti c or a l l ergy (type I) rea cti on i nvol ves repea t expos ure to certa i n a l l ergens tha t l ea ds to bi ndi ng of IgE to i ts receptor on ma s t cel l s or ba s ophi l s . Bi ndi ng rel ea s es hi s ta mi ne; l eukotri enes B4, C4, a nd D4; pros -ta gl a ndi n D2; pl a tel et; eos i nophi l ; a nd neutrophi l fa ctors . Immune compl ex (type III) rea cti on occurs when a nti gen bi nds a nti bodi es to produce i mmune compl exes wi th compl ement protei ns . Compl exes depos i t i n s ma l l bl ood ves s el s , joi nts , a nd gl omerul i el i ci ti ng i nfl a mma tory res pons e. Neutrophi l s a nd pl a tel ets ca us e ti s s ue da ma ge. Cel l medi a ted or del a yed (type IV) rea cti on i s a s s oci a ted wi th cytotoxi c T cel l s recogni zi ng a n a nti gen a nd a tta cki ng the ti s s ue, wherea s Th1 cel l s rel ea s e cytoki nes , whi ch a ttra ct monocytes a nd ma cropha ges tha t ca us e mos t of the cel l ul a r da ma ge (s ee Ta bl e 15-3). III-18. The answer is D. The pa ti ent’s pH i s a bove norma l s o thi s ha s to be a n a l ka l os i s . The l ow pCO2 i s i ndi ca ti ve of i ncrea s ed expi ra ti on of CO2 . On the ba s i s of ca rboni c a nhydra s e rea cti on , thi s i s a n exa mpl e of res pi ra tory a l ka l os i s . Increa s ed expi ra ti on of CO2 (i .e., hyperventi l a ti on) dri ves the rea cti on towa rd the l eft, thereby decrea s i ng the forma ti on of both protons (H +) a nd bi ca rbona te. The bi ca rbona te l evel s a re decrea s ed, i ndi ca ti ng tha t the ki dney ha s ma rkedl y decrea s ed rea bs orpti on (i ncrea s ed excreti on) of bi ca rbona te. Thi s decrea s ed rea bs orpti on of bi ca rbona te, a ba s e, by the ki dney compens a tes for the a l rea dy ba s i c pH (Cha pters 17 a nd 18). III-19. The answer is C. Nutri ti ona l vi ta mi n A (reti nol es ters ) i s converted i n the reti na to 11-cis-reti na l , a n i s omer of a l l -trans-reti na l forms by reti na l i s omera s e. 11-cis-Reti na l then cova l entl y a tta ches to a l ys i ne res i due on the vi s ua l protei n ops i n. The res ul ti ng rhodops i n mol ecul e becomes a G-protei n-coupl ed receptor wi th the “l i ga nd” for a cti va ti on bei ng l i ght. When l i ght s ti kes the reti na , photoexci ted rhodops i n bi nds to a mul ti s ubuni t membra ne protei n ca l l ed tra ns duci n whi ch, i n turn, a cti va tes a cGMP phos phodi es tera s e by ca us i ng di s s oci a ti on of i nhi bi tory s ubuni ts . The phos phodi es tera s e ca ta l yzes the des tructi on of cGMP (cGMP → GMP) nea r the membra ne of the rod cel l . Norma l l y, cGMP keeps i nwa rd Na +–Ca 2+ cha nnel s open to ma i nta i n membra ne depol a ri za ti on. When cGMP l evel s fa l l , i nwa rd Na +–Ca 2+ cha nnel s cl os e, thereby l oweri ng both i ntra cel l ul a r Na + a nd Ca 2+. The fa l l i n Na + el i ci ts hyper-pol a ri za ti on ca us i ng the rod cel l to rel ea s e l es s gl uta ma te neurotra ns mi tter. The reduced a mount of thi s i nhi bi tory neurotra ns mi tter genera tes a n el ectri ca l i mpul s e to the occi pi ta l l obe of the bra i n tha t tri ggers the percepti on of l i ght (Fi gure 19-13). III-20. The answer is C. Rel ea s e of GnRH by the hypotha l a mus res ul ts i n a cti va ti on of a s peci fi c GnRH receptor (GnRHR) l oca ted i n the pi tui ta ry gl a nd. Thi s receptor i s a membra ne-bound G q-protei n-coupl ed s ti mul a tor of phos phol i pa s e C, whi ch res ul ts i n ca l ci um rel ea s e a nd protei n ki na s e C a cti va ti on vi a convers i on of pl a s ma membra ne phos pha ti dyl i nos i tol i nto i nos i tol tri phos pha te a nd di a cyl gl ycerol . CRH i s rel ea s ed from the hypotha l a mus a nd bi nds to a receptor on corti cotrophs tha t a cts vi a G s -protei n-coupl ed s ti mul a ti on of a denyl cycl a s e l ea di ng to producti on of cAMP. ADH works vi a a s i mi l a r mecha ni s m a s CRH. TRH, l i ke GnRH, works vi a G q protei n. Soma tos ta ti n works vi a G i-protei n-coupl ed i nhi bi ti on of a denyl cycl a s e (Cha pter 20). III-21. The answer is E. Thi s woma n ha s crea ted a s el f-i mpos ed s ta rva ti on through her hunger s tri ke. Duri ng s ta rva ti on, ma ny fuel s ources a re recrui ted to s upport bodi l y functi ons , i ncl udi ng protei n degra da ti on, whi ch s uppl i es a mi no a ci ds a s gl uconeogeni c precurs ors , a nd tri gl yceri de degra da ti on, whi ch yi el ds gl ycerol , free fa tty a ci ds , a nd, eventua l l y, ketone bodi es . The bra i n norma l l y prefers gl ucos e a s i ts ma i n fuel , s o no a da pta ti on i s needed. Duri ng s ta rva ti on, cha nges i n bra i n gene expres s i on upregul a te s evera l enzymes to ena bl e us e of ketone bodi es a s fuel . No ma tter how l ong the fa s t l a s ts , the bra i n ca nnot us e gl ycerol , a mi no a ci ds , or free fa tty a ci ds a s di rect fuel s ources (s ee Cha pter 10). III-22. The answer is A. After di ges ti on, a mi no a ci ds a nd very s ma l l pepti des a re coa bs orbed wi th s odi um vi a group-s peci fi c a mi no a ci d or pepti de a cti ve tra ns port s ys tems i n the a pi ca l membra ne. At l ea s t fi ve di s ti nct brus h border tra ns port s ys tems exi s t tha t a re cl a s s i fi ed a s fol l ows : (a ) neutra l a mi no a ci ds (uncha rged a l i pha ti c a nd a roma ti c), (b) ba s i c a mi no a ci ds a nd cys ti ne (Cys –Cys ), (c) a ci di c a mi no a ci ds (As p, Gl u), (d) i mi no a ci ds (Pro), a nd (e) di pepti des a nd tri pepti des . The mecha ni s ms for concentra ti ve tra ns epi thel i a l tra ns port of L-a mi no a ci ds a nd di pepti des a re a na l ogous to tha t for Na -dependent gl ucos e a bs orpti on. The dri vi ng force for the Na +-dependent tra ns port i s deri ved from the ma i ntena nce of l ow i ntra cel l ul a r l evel s of Na + by the a cti on of the Na +–K+-ATPa s e. Hydrol ys i s of ATP provi des energy to export three Na + i n excha nge for two K+. Thus , the hi gh gra di ent of Na + between the i ntes ti na l l umen a nd the cytopl a s m provi des the dri vi ng force for a cti ve tra ns port of a mi no a ci ds , di pepti des , a nd tri pepti des (s ee Cha pter 11). III-23. The answer is D. HgbF i s compos ed of two α-gl obi n a nd two γ-gl obi n protei ns . It ha s a l ower a ffi ni ty for 2,3-BPG tha n HgbA beca us e of a s eri ne a t pos i ti on 143 i n the γ-globin gene i ns tea d of a hi s ti di ne res i due. Thi s decrea s ed a ffi ni ty for 2,3-BPG res ul ts i n HgbF ha vi ng a n i ncrea s ed a ffi ni ty for O2 rel a ti ve to a dul t a nd a s hi ft i n the O2 di s s oci a ti on curve to the l eft. Thus , HgbF i s a bl e to extra ct O2 from HgbA
i n the pl a centa a nd then del i ver the O2 to the feta l ti s s ues . The feta l ti s s ues s ti l l produce protons a nd CO2 to fa ci l i ta te the rel ea s e of O2 a t the ti s s ues (Bohr effect). The s hi ft to the l eft i s preci s e, however, beca us e though i t a l l ows the HgbF to “s tea l ” O2 from the ma terna l Hgb, i t i s not s o s i gni fi ca nt tha t the feta l ti s s ues ca nnot remove the O2 for thei r own ti s s ues . HgbF pers i s ts for a bout 6 months a fter bi rth. It i s for tha t rea s on tha t defects i n the β-gl obi n ma y go undetected for s evera l months (s ee Cha pter 14). III-24. The answer is D. Sta ti ns a ct a s feedba ck i nhi bi tors of 3′-hydroxy-3′-methyl gl uta ryl -CoA (HMG-CoA) reducta s e, the regul a ted enzyme of chol es terol s ynthes i s . Effecti ve trea tment wi th a s ta ti n, a l ong wi th a l ow-fa t di et, decrea s es l evel s of bl ood chol es terol . The l oweri ng of chol es terol a l s o l owers the a mounts of the l i poprotei n tha t tra ns port chol es terol to the peri phera l ti s s ues , LDL. Beca us e l i pi ds , l i ke chol es terol a nd tri gl yceri des , a re i ns ol ubl e i n wa ter, they mus t be a s s oci a ted wi th l i poprotei ns for tra ns port a nd s a l va ge between thei r ma jor s i te of s ynthes i s (l i ver) a nd the peri phera l ti s s ues . Thos e l i poprotei ns a s s oci a ted wi th more i ns ol ubl e l i pi ds thus ha ve l ower dens i ty duri ng centri fuga ti on, a techni que tha t s epa ra tes the l owes t dens i ty chyl omi crons from VLDLs (VLDLs wi th pre-βl i poprotei ns ), LDLs (LDLs wi th β-l i poprotei ns ), i ntermedi a te-dens i ty l i poprotei ns , a nd HDLs (HDLs wi th α-l i poprotei ns ). Ea ch type of l i poprotei n ha s typi ca l a pol i poprotei ns s uch a s the a po B100 a nd a po B48 (tra ns l a ted from the s a me mRNA) i n LDL. LDL i s i nvol ved i n tra ns porti ng chol es terol from the l i ver to peri phera l ti s s ues , wherea s HDL i s a s ca venger of chol es terol . The ra ti o of HDL to LDL i s thus a predi ctor of chol es terol depos i ti on i n bl ood ves s el s , the ca us e of myoca rdi a l i nfa rcti ons (hea rt a tta cks ). The hi gher the HDL–LDL ra ti o, the l ower the ra te of hea rt a tta cks (s ee Cha pter 16). III-25. The answer is C. Ca rdi a c a nd s kel eta l mus cl es a re s i mi l a r i n tha t both a re s tri a ted a nd conta i n two ki nds of i ntera cti ng protei n fi l a ments . The thi ck fi l a ments conta i n pri ma ri l y myos i n, wherea s the thi n fi l a ments conta i n a cti n, troponi n, a nd tropomyos i n. The thi ck a nd thi n fi l a ments s l i de pa s t one a nother duri ng mus cl e contra cti on (Fi gure 12-2). Myos i ns a re a fa mi l y of protei ns wi th hea vy a nd l i ght cha i ns , a nd mus cl e myos i ns functi on a s ATPa s es tha t bi nd to thi n fi l a ments duri ng contra cti on. Congeni ta l defects i n mus cl e fi l a ments , pota s s i um cha nnel s , a nd the l i ke tha t a ffect ca rdi a c contra cti l i ty a re s ubs ta nti a l contri butors to thes e tra gi c a nd unexpected di s ea s e ca tegori es —i mporta nt rea s ons for a nnua l a nd tra ns i ti ona l (s ports , precol l ege) phys i ca l exa mi na ti ons . III-26. The answer is D. Thi s gi rl ’s s ymptoms a re cons i s tent wi th extreme hypergl ycemi a , whi ch i s cons i s tent wi th her exces s i ve thi rs t (pol ydi ps i a ), uri na ti on ha bi ts (pol yuri a ), a nd a ppeti te (pol ypha gi a ). Her neurol ogi c s ymptoms a re proba bl y s econda ry to ketoa ci dos i s , l i kel y res ul ti ng from type 1 di a betes . The fi ndi ng of gl ucos e s pi l l over i nto her uri ne s trongl y s upports thi s concl us i on. An a cute hypergl ycemi c condi ti on due to type 1 di a betes i s cha ra cteri zed by a nea r a bs ence of i ns ul i n wi th unoppos ed gl uca gon a cti on, pa rti cul a rl y i n the l i ver. So both gl uconeogenes i s a nd ketogenes i s a re el eva ted i n s uch pa ti ents . Al l the other proces s es l i s ted woul d be opera ti ng a t reduced a cti vi ty rel a ti ve to thei r l evel s i n the pres ence of a hi gher i ns ul i n–gl uca gon ra ti o (Cha pter 10). III-27. The answer is D. Al port s yndrome, Goodpa s ture s yndrome, a nd beni gn fa mi l i a l hema turi a a re a s s oci a ted wi th type IV col l a gen muta ti ons . Col l a genopa thi es types II a nd XI a nd hypochondrogenes i s a re a s s oci a ted wi th type II col l a gen muta ti ons . Dys trophi c epi dermol ys i s bul l os a a nd epi dermol ys i s bul l os a a cqui s i ta a re a s s oci a ted wi th type VII col l a gen muta ti ons . Ehl ers –Da nl os s yndromes types III a nd IV a nd a neurys m a re a s s oci a ted wi th type III col l a gen muta ti ons (Cha pter 13). III-28. The answer is A. Nephrogeni c (ki dney-a s s oci a ted) di a betes i ns i pi dus , a condi ti on of poor fl ui d ba l a nce regul a ti on a nd nota bl e for l a rge output of very di l ute uri ne, i s cl os el y a s s oci a ted wi th muta ti ons of the a qua pori n-2 cha nnel . Under norma l condi ti ons , a decrea s e i n bl ood vol ume or a n i ncrea s e i n os mol a l i ty tri ggers rel ea s e of ADH (va s opres s i n) from the pos teri or pi tui ta ry to s i gna l mobi l i za ti on of a qua pori n-2 cha nnel s i n the ki dney to i ncrea s e wa ter rea bs orpti on. A defect i n the G s protei n tha t medi a tes ADH effects i n the ki dney coul d l ea d to nephrogeni c di a betes i ns i pi dus . However, a defect of G q protei n woul d onl y a ffect the va s opres s energi c effect. A defecti ve a ngi otens i n II receptor woul d l ea d to decrea s ed s ecreti on of a l dos terone tha t i n turn woul d ca us e a decrea s ed rea bs orpti on of s odi um wi th a s ubs equent l os s of wa ter. However, thi s i s not nephrogeni c beca us e the pri ma ry defect i s i n the zona gl omerul os a cel l s of the a drena l cortex. A muta ted ADH woul d be cons i dered a neurogeni c (bra i n a s s oci a ted) or “centra l ” di a betes i ns i pi dus (s ee Cha pter 18). III-29. The answer is B. The receptor cl a s s ca l l ed Ig receptors , i ncl udi ng the IL-1 receptor, a cti va tes phos phol i pa s e C a nd s ubs equent NF-kB gene a cti va ti on. Types I a nd II receptors , whi ch i ncl ude ma ny of the ma jor cytoki nes a nd i nterferons , both functi on vi a a Ja nus ki na s e mecha ni s m. TNF receptor group uti l i zes a n i ntermedi a ry protei n termed TNF receptor type 1-a s s oci a ted dea th doma i n, whi ch tra ns mi ts conforma ti ona l cha nges from the receptor to TNF receptor-a s s oci a ted fa ctor 2, whi ch i ntera cts wi th s evera l other s i gna l i ng pepti des to medi a te progra mmed cel l dea th (a poptos i s ) vi a NF-κB gene a cti va ti on or by the a cti va tor protei n 1, a tra ns cri pti on fa ctor rel a ted to cFos a nd c-Jun. The chemoki ne receptor group functi ons vi a G q protei ns , l ea di ng to the rel ea s e of i ntra cel l ul a r ca l ci um tha t el i ci ts di rected chemota xi s . The tumor growth fa ctor-β receptors a re s eri ne–threoni ne ki na s es whos e phos phoryl a ti on ca n l ea d to the producti on of cAMP, cGMP, di a cyl gl ycerol , a nd/or a cti va ted ca l modul i n, whi ch s ubs equentl y l ea d to cel l functi ons a nd DNA expres s i on (s ee Cha pter 15). III-30. The answer is A. Thi s pa ti ent ha s a defi ci ency of α-1-a nti tryps i n. α-1-Anti tryps i n i s a pl a s ma protei n of a pproxi ma tel y 400 a mi no a ci ds . It mi gra tes i n the α-1 regi on on a pl a s ma protei n el ectrophores i s a nd i s the ma jor component of thi s fra cti on. Norma l l y, i t s erves a s the pri ma ry s eri ne protea s e i nhi bi tor i n the ci rcul a ti on, pa rti cul a rl y of el a s ta s e. Beca us e the l a ck of α-1-a nti tryps i n a l l ows for i ncrea s ed el a s ta s e a cti vi ty, thi s condi ti on res ul ts i n a s l ow but s tea dy degra da ti on of extra cel l ul a r fi bri l s , pa rti cul a rl y el a s ti n. Thus , the l ungs undergo proteol yti c da ma ge. Thi s i s pa rti cul a rl y promi nent i n the l ung a nd l ea ds to prema ture ons et of emphys ema . Thi s proces s i s ma rkedl y a ccel era ted by s moki ng s o tha t a voi da nce of s moki ng i s a pri ori ty. On the α-1-a nti tryps i n, a methi oni ne res i due (Met358) i s cri ti ca l for bi ndi ng protea s es to i t. In the l ungs of s mokers , the s i de cha i n of thi s methi oni ne res i due i s oxi di zed to methi oni ne s ul foxi de. Thus , a ny (α-1-a nti tryps i n tha t rema i ns becomes i neffecti ve a s a protea s e i nhi bi tor. Cons equentl y, i ndi vi dua l s wi th α-1a nti tryps i n defi ci ency who s moke ha ve reduced a nti protei na s e functi on from both ca us es a nd therefore s how more s evere s ymptoms tha n nons mokers wi th the di s ea s e. The i ncrea s ed s everi ty i s refl ected i n grea ter proteol yti c des tructi on of l ung ti s s ue l a rgel y by el a s ta s e (s ee Cha pter 17). III-31. The answer is C. Gl uca gon i s a hormone produced a nd s ecreted by the α-cel l s of the pa ncrea s i n res pons e to decrea s es i n bl ood gl ucos e. Thi s hormone s ti mul a tes gl uconeogenes i s i n the l i ver by i ncrea s i ng the l evel of cAMP, whi ch i n turn a cti va tes the cAMP-dependent protei n ki na s e. Thi s enzyme i na cti va tes pyruva te ki na s e by phos phoryl a ti on a nd thus i nhi bi ts gl ycol ys i s . Gl yco-genes i s a nd gl ycol ys i s i n l i ver a re i nhi bi ted by gl uca gon. Pentos e pa thwa y a nd ci tri c a ci d cycl e a re not di rectl y a ffected by gl uca gon (s ee Cha pter 10). III-32. The answer is C. Chol es terol i n cel l s ma y be s ynthes i zed de novo or ta ken up vi a LDL pa rti cl es . A defect i n the l a tter proces s l ea ds to fa mi l i a l hyperchol es terol emi a . Once the LDL pa rti cl es a re endocytos ed, they a re proces s ed i n the l ys os ome i n pa rt by a n a ci d chol es terol es ter hydrol a s e (ACEH) tha t removes the es ter crea ti ng free chol es terol . Pa ti ents wi th Wol ma n di s ea s e ha ve defecti ve ACEH. The free chol es terol i s then tra ns ferred from the l ys os ome to the Gol gi us i ng Ni ema nn-Pi ck C (NPC) protei n. It i s thi s protei n tha t i s defecti ve i n pa ti ents wi th Ni ema nn–Pi ck C di s ea s e. Chol es terol i n the Gol gi i s further proces s ed by es teri fi ca ti on by a cyl -CoA– chol es terol a cyl tra ns fera s e a nd the chol es terol es ter a ccumul a tes i n dropl ets i n the cytopl a s m. Unl i ke other ti s s ues , l i ver ha s the uni que a bi l i ty to get ri d of chol es terol by convers i on to bi l e s a l ts vi a the fi rs t key rea cti on ca ta l yzed by the enzyme chol es terol 7αhydroxyl a s e (s ee Cha pter 19). III-33. The answer is E. Li ngua l l i pa s e i ni ti a tes hydrol ys i s of l ong-cha i n tri gl yceri des i nto gl ycerol a nd free fa tty a ci ds , whi ch conti nues i nto a nd through s toma ch. Opti ma l a cti va ti on requi res a ci di c (~pH 4) envi ronment, s o va s t ma jori ty of a cti vi ty i s i n s toma ch. Li ngua l l i pa s e i s s ecreted by the dors a l s urfa ce of the tongue (Ebner’s gl a nds ). In i nfa nts , brea s t mi l k comes equi pped wi th mi l k l i pa s e tha t i s s ynthes i zed i n the ma mma ry cel l a nd exported by exocytos i s . Mi l k l i pa s e i s uni que beca us e i t i s s ti mul a ted by bi l e a ci ds a nd therefore i s i na cti ve unti l i t rea ches the i nfa nt’s i ntes ti na l l umen, hydrol yzes tri gl yceri des a t a l l three pos i ti ons , a nd conveni entl y prefers to
cl ea ve off the medi um-cha i n fa tty a ci ds tha t a re enri ched i n mi l k (s ee Cha pter 11). III-34. The answer is E. Hgb exi s ts i n a n equi l i bri um between a tens e (l ow O2 a ffi ni ty; gi ves up O2 ea s i l y to ti s s ues ) a nd a rel a xed s ta te (hi gh O2 a ffi ni ty). In the deoxy (tens e) form, the s ubuni ts a re rota ted 15° rel a ti ve to the oxy form (s ee Fi gure 14-4). At a more mol ecul a r l evel , the R- a nd T-s ta tes of Hgb ca n be defi ned by the pos i ti on of the i ron, rel a ti ve to the pl a ne of the heme. In the T-s ta te, the i ron i s pul l ed a wa y from the pl a ne of the heme, ma ki ng i t more di ffi cul t for O2 to bi nd. In the R-s ta te, the i ron i s “pul l ed” (by O2 ) i nto the pl a ne, ma ki ng i t ea s i er for O2 to bi nd. Hence, the R-s ta te of Hgb ha s a hi gher a ffi ni ty for O2 tha n does the T-s ta te, whether or not O2 i s pres ent. O2 s ta bi l i zes the R-s ta te, wherea s , the T-s ta te i s s ta bi l i zed by H +, CO2 , a nd BPG. Al though thes e l a tter fa ctors s ta bi l i ze the Ts ta te, they do not s tructura l l y determi ne the s hi ft to the T-s ta te (Fi gure 14-5). III-35. The answer is D. The upta ke of exogenous chol es terol by cel l s res ul ts i n a ma rked s uppres s i on of endogenous chol es terol s ynthes i s . Huma n LDL not onl y conta i ns the grea tes t ra ti o of bound chol es terol to protei n but a l s o ha s the grea tes t potency i n s uppres s i ng endogenous chol es terogenes i s . LDLs norma l l y s uppres s chol es terol s ynthes i s by bi ndi ng to a s peci fi c membra ne receptor tha t medi a tes i nhi bi ti on of hydroxymethyl gl uta ryl (HMG) coenzyme A reducta s e. In fa mi l i a l hyperchol es terol emi a , the LDL receptor i s dys functi ona l , wi th the res ul t tha t chol es terol s ynthes i s i s l es s res pons i ve to pl a s ma chol es terol l evel s . Suppres s i on of HMG-CoA reducta s e i s a tta i ned us i ng i nhi bi tors (s ta ti ns ) tha t mi mi c the s tructure of meva l oni c a ci d, the na tura l feedba ck i nhi bi tor of the enzyme (s ee Cha pter 16). III-36. The answer is A. E2 pea ks jus t before ovul a ti on begi ns a nd i s formed from tes tos terone by a roma ta s e. Thi s i s the ma i n es trogen i n nonpregna nt, ferti l e fema l es . Aroma ta s e i n the gra nul os a cel l s of the ova ri es i s a cti va ted by FSH. There i s no feedba ck i nhi bi ti on of a roma ta s e. Control of es trogen producti on occurs vi a feedba ck control of rel ea s e of FSH a nd LH from the pi tui ta ry. Al though LH i ncrea s es producti on of E2 , i ts effect i s on the producti on of the tes tos terone precurs or i n the ova ri a n theca cel l s . E3 i s the mos t a bunda nt es trogen duri ng pregna ncy a nd i s formed i n the pl a centa a nd feta l a drena l gl a nds a nd l i ver. E1 , produced by a roma ta s e from a ndros tenedi one i n a di pos e cel l s , i s found predomi na tel y i n menopa us a l women a s wel l a s i n men (s ee Cha pter 20). III-37. The answer is C. The pres enta ti on of hepa tomega l y a nd hypogl ycemi a i s i mmedi a tel y s ugges ti ve of a l i ver gl ycogen s tora ge di s ea s e. Gl uca gon i njecti on norma l l y s houl d i ncrea s e gl ycogenol ys i s , l ea di ng to gl ucos e producti on. However, i n thi s pa ti ent, nei ther gl ucos e nor l a cta te i s produced, s ugges ti ng a n i na bi l i ty to hydrol yze gl ycogen a t a l l . Gl ucos e-6-phos pha ta s e a nd pyruva te ca rboxyl a s e a re both a s s oci a ted wi th s ynthes i s of gl ucos e from a l a ni ne a nd thi s i s norma l i n the pa ti ent. Nei ther G6PD nor pyruva te ki na s e i s i nvol ved i n gl ucos e producti on (s ee Cha pter 10). III-38. The answer is B. When os teobl a s ts a re una bl e to form hydroxya pa ti te or when s uffi ci ent ca l ci um a nd/or phos pha te a re not a va i l a bl e, the di s ea s e of os teoma l a ci a res ul ts . In chi l dren, thi s condi ti on i s known a s ri ckets . Os teoma l a ci a , l i tera l l y mea ni ng “bone s oftnes s ,” ha s norma l a mounts of orga ni c col l a gen ma tri x but defi ci ent mi nera l i za ti on unl i ke os teoporos i s (s ee bel ow), where norma l l y mi nera l i zed but decrea s ed bone ma tri x i s the probl em. As a res ul t, pa ti ents s ufferi ng from os teoma l a ci a ha ve wea k a nd ea s i l y fra ctured bones . Mos t often, os teoma l a ci a /ri ckets i s ca us ed by defi ci ent vi ta mi n D ei ther i n the di et (i .e., poor i nta ke or poor i ntes ti na l a bs orpti on) or s econda ry to l ow s un expos ure/a bs orpti on. Other ca us es i ncl ude ki dney or l i ver di s ea s e (or other di s orders tha t a ffect vi ta mi n D meta bol i s m a nd/or a bs orpti on) decrea s ed phos pha te l evel s , ca ncers , a nd medi ca ti on s i de effects (e.g., a nti convul s a nt medi ca ti ons ). Symptoms of os teoma l a ci a i ncl ude bone pa i n (often s ta rti ng i n the l umba r regi on of the s pi ne, pel vi s , a nd l egs ) a nd rel a ted mus cl e a nd nerve wea knes s /numbnes s . La bora tory tes ts s how l ow ca l ci um l evel s i n s erum a nd uri ne (often a ccompa ni ed by l ow s erum phos pha te). Hyperpa ra thyroi di s m ca n be ca us ed by l ow bl ood ca l ci um but phos pha te woul d be norma l or el eva ted. Os teoporos i s i s a bone condi ti on i n whi ch the a mount of bone mi nera l i s s i gni fi ca ntl y l owered, l ea di ng to a n a l tered a nd wea kened bone ma tri x a nd a ma rkedl y i ncrea s ed ri s k of fra cture. In fema l es , i t occurs a fter menopa us e beca us e of l os s of es trogen a nd ca n occur i n ol der men beca us e of l os s of tes tos terone. Pa get’s di s ea s e of the bone, a l s o known a s os tei ti s deforma ns , i s a condi ti on of exces s i ve bone turnover (brea kdown a nd reforma ti on) res ul ti ng i n bone deformi ti es , pa i n, decrea s ed s trength, a nd res ul ti ng a rthri ti s a nd fra ctures . A geneti c l i nka ge ha s been s ugges ted a s the pos s i bl e rol e of a pa ra myxovi rus , a l though no convi nci ng evi dence ha s been found (s ee Cha pter 13). III-39. The answer is C. Reni n i s a proteol yti c enzyme produced i n the juxta gl omerul a r cel l s of the rena l a fferent a rteri ol e. Reni n rel ea s e i s control l ed i n s evera l wa ys : (1) Ba roreceptors i n the ki dney a re s ti mul a ted by decrea s ed rena l a rteri ol a r pres s ure. (2) Low bl ood pres s ure s ti mul a tes ca rdi a c receptors tha t a cti va te the s ympa theti c nervous s ys tem a nd the res ul ti ng ca techol a mi nes s ti mul a te the juxta gl omerul a r cel l s vi a β1 -a drenergi c receptors . (3) The ma cul a dens a , whi ch i s compos ed of cel l s a dja cent to the juxta gl omerul a r cel l s , i s s ti mul a ted by a decrea s e i n the ci rcul a ti ng concentra ti on of Na + or Cl −. Once the reni n–a ngi otens i n s ys tem i s a cti va ted, a ngi otens i n II tri ggers the rel ea s e of a l dos terone, a mi ner-a l ocorti coi d, tha t bi nds to i ts receptor i n the ki dney ca us i ng i ncrea s ed s odi um rea bs orpti on to correct the probl em be i t a drop i n Na or pres s ure. Hence, a n a goni s t of the mi nera l ocorti coi d receptor woul d decrea s e reni n s ecreti on. Li kewi s e, provi di ng a ngi otens i n II wi l l ca us e a l dos terone s ecreti on a nd hence l ower reni n rel ea s e. In contra s t, bl ocki ng the a l dos terone receptor wi th a n a nta goni s t woul d prevent Na rea bs orpti on, l ea di ng to i ncrea s ed reni n. A chroni c s a l t di et woul d repres s reni n s ecreti on. ANP oppos es the reni n–a ngi otens i n s ys tem a nd woul d hence reduce reni n s ecreti on (s ee Cha pter 18). III-40. The answer is E. In the rel a xa ti on pha s e of s kel eta l mus cl e contra cti on, the hea d of myos i n hydrol yzes ATP to ADP a nd Pi, but the products rema i n bound. When contra cti on i s s ti mul a ted (vi a the regul a tory rol e of ca l ci um i n a cti n a cces s i bi l i ty), a cti n becomes a cces s i bl e a nd the a cti n–myos i n–ADP–Pi compl ex i s formed. Forma ti on of the compl ex res ul ts i n rel ea s e of Pi. Thi s i s fol l owed by rel ea s e of ADP, whi ch i s a ccompa ni ed by a conforma ti ona l cha nge i n the hea d of myos i n. Thi s conforma ti ona l cha nge res ul ts i n the power s troke i n whi ch a cti n fi l a ments a re bei ng pul l ed pa s t the myos i n a bout 10 nm towa rd the center of the s a rcomere. Another mol ecul e of ATP i s now a bl e to bi nd the hea d of myos i n, formi ng a n a cti n–myos i n–ATP compl ex. The ATP-bound myos i n ha s a l ow a ffi ni ty for a cti n a nd thus a cti n di s s oci a tes from the compl ex (s ee Cha pter 12). III-41. The answer is E. Bordetella pertussis produces a nd rel ea s es a n enzyma ti ca l l y a cti ve, protei n toxi n res pons i bl e for the i l l nes s . Rel ea s ed i n i ts i na cti ve form, pertus s i s toxi n bi nds to receptors on a cel l membra ne a nd i s tra ns ported vi a the Gol gi a ppa ra tus to the endopl a s mi c reti cul um. Upon a cti va ti on, the toxi n a dds ADP mol ecul es to the α-s ubuni ts of G i protei ns a nd, bei ng i nhi bi ted by the ADP ri bos yl a ti on, a re una bl e to s top a denyl cycl a s e producti on of cAMP. The res ul ti ng a l tered s i gna l i ng l ea ds to a va ri ety of cl i ni ca l ma ni fes ta ti ons . In contra s t to pertus s i s toxi n effects , epi nephri ne bi ndi ng to α-2-a drenergi c receptors i ncrea s es G i protei n a cti vi ty a nd norma l l y l owers cycl i c AMP producti on. In tra uma , a cti on vi a α-2-a drenergi c receptors cons equentl y l owers the concentra ti on of cAMP, l ea di ng to decrea s ed s ecreti on a nd s ynthes i s of i ns ul i n. Thi s effect i n tra uma i s i mporta nt beca us e s ecreti on of i ns ul i n woul d l ower bl ood gl ucos e l evel s tha t woul d be detri menta l to res us ci ta ti on fol l owi ng tra uma . Al though the other opti ons l i s ted woul d i nterfere wi th the epi nephri ne effect i n tra uma , they a re unrel a ted to how the toxi n functi ons (s ee Cha pter 17). III-42. The answer is E. The “pri on hypothes i s ” a s s erts tha t protei ns , devoi d of nucl ei c a ci ds , ca n thems el ves be i nfecti ous a gents . Infecti ous a gents requi re hori zonta l tra ns mi s s i on i nto a n uni nfected hos t, a nd s ubs equent propa ga ti on of further i nfecti ous a gents wi thi n tha t hos t. Protei n-onl y i nfecti vi ty ha s been expl a i ned wi th s ome fa s ci na ti ng bi ochemi s try. Pri on protei ns (PrPs ) a re res pons i bl e for a hos t of tra ns mi s s i bl e s pongi form encepha l opa thy i ncl udi ng Creutzfel dt–Ja kob di s ea s e, ovi ne s cra pi e, a nd bovi ne s pongi form encepha l opa thy (BSE, a l s o known a s ma d cow di s ea s e). Pri on i nfecti vi ty requi res the endogenous expres s i on of PrPs , whi ch a re norma l cel l ul a r cons ti tuents , a nchored to the s urfa ce of neurons . The i nfecti ous a gent i n thes e di s ea s es i s a n a myl oi dogeni c form of PrP, ca l l ed PrP-res
(for protea s e-res i s ta nt), whi ch i s a va ri a nt of PrP tha t i s i n a di fferent conforma ti on (e.g., a s l i ghtl y s ol ubl e protei n wi th β-s tra nd cha ra cter). PrP-res i nduces a conforma ti ona l cha nge i n the norma l cel l ul a r PrP protei n, turni ng i t i nto a PrP-res . Thi s convers i on of otherwi s e wel l -beha ved protei ns i nvol ves a conforma ti ona l cha nge from α-hel i x to β-s tra nd. Nota bl y, PrP-res i s s ta bl e to hea t, s o BSEta i nted mea t i s s ti l l i nfecti ous a fter cooki ng. Tra ns mi s s i on of s pongi form encepha l opa thi es between s peci es ca n occur, a s evi denced by tra ns mi s s i on of BSE to huma ns . Tra ns mi s s i on depends upon s equence homol ogy between the tra ns mi tted PrP-res a nd the endogenous PrP a nd hence occurs wi th very l ow frequency. However, once the na ti ve hos t form of PrP i s converted to the PrP-res conformer, i t s eeds further ol i gomer forma ti on. The ol i gomeri c or “protofi bri l ” s ta ge ma y be the mos t toxi c to neura l cel l s . La rge a myl oi d fi bri l s a nd i ncl us i on bodi es or pl a ques ma y be l es s toxi c or even protecti ve. The bes t thera peuti c s tra tegy ma y therefore be one tha t i s focus ed on ea rl y preventi on of ol i gomer forma ti on (s ee Cha pter 9). III-43. The answer is E. Fructos e 2,6-bi s phos pha te i s a n a l l os teri c a cti va tor of l i ver gl ycol ys i s a t the phos phofructoki na s e-1 rea cti on a nd a n i nhi bi tor of gl uconeogenes i s a t the fructos e 1,6-bi s phos pha ta s e rea cti on. Fructos e 2,6-bi s phos pha te i s ma de by a bi functi ona l enzyme tha t conta i ns both phos phofructoki na s e-2 a nd fructos e 2,6-bi s phos pha ta s e a cti vi ti es dependi ng on the nutri ti ona l s ta te. In the fed s ta te, i ns ul i n promotes forma ti on of thi s regul a tor by a cti va ti ng the ki na s e a cti vi ty of the bi functi ona l enzyme, wherea s gl uca gon promotes remova l of the regul a tor by a cti va ti ng the protei n’s phos pha ta s e a cti vi ty. The fructos e 2,6-bi s phos pha te ca n onl y be converted to fructos e-6-phos pha te a nd i s not cl ea ved (s ee Cha pter 10). III-44. The answer is A. Amyl a s e i s rel ea s ed by the exocri ne pa ncrea s to i ni ti a te ra ndom di ges ti on of a myl os e a nd a myl opecti n cha i ns produci ng ma l totri os e, ma l tos e, a myl os e, gl ucos e, a nd ol i gos a ccha ri des . Dextri na s e, gl ucoa myl a s e, l a cta s e, a nd s ucra s e a re a l l l oca l i zed on the s urfa ce of the s ma l l i ntes ti na l epi thel i a l cel l s a nd i nvol ved i n very s peci fi c hydrol ys i s rea cti ons of products deri ved from a myl a s e a cti on (s ee Cha pter 11). III-45. The answer is A. Si ckl e cel l di s ea s e i s ca us ed by a reces s i ve, s i ngl e-nucl eoti de muta ti on i n the β-gl obi n gene of Hgb (Fi gure 14-7A), i nheri ted from both pa rents (i .e., homozygous ). Thi s muta ti on res ul ts i n the s ubs ti tuti on of the norma l gl uta mi c a ci d (nega ti ve cha rge) wi th a va l i ne (hydrophobi c) a t a mi no a ci d 6. Si ckl e red bl ood cel l s bi nd O2 norma l l y a nd the oxygena ted red bl ood cel l s ha ve a norma l bi conca ve s ha pe. However, when the s i ckl e Hgb unl oa ds i ts O2 , the norma l conforma ti ona l cha nge expos es the va l i ne a t pos i ti on 6 to the s urfa ce crea ti ng a “hydrophobi c pa tch.” The hydrophobi c pa tch on one deoxygena ted Hgb mol ecul e ca n i ntera ct wi th the hydrophobi c pa tch on a s econd Hgb mol ecul e crea ti ng s ti ff Hgb pol ymers (Fi gure 14-10A–B). Thes e i nterna l pol ymers ca us e the red bl ood cel l to s ti ffen a nd a dopt a bnorma l s ha pes i ncl udi ng a cres centi c or “s i ckl e” s ha pe tha t gi ves the di s ea s e i ts na me (Fi gure 1410C). Beca us e the i nterna l s tructure of the Hgb i s uncha nged, effects of H +, CO2 , a nd 2,3-BPG a re reta i ned. III-46. The answer is E. Ca l ci um i ons a re the regul a tors of contra cti on of s kel eta l mus cl e. Ca l ci um i s a cti vel y s eques tered i n s a rcopl a s mi c reti cul um by a n ATP pump duri ng rel a xa ti on of mus cl e. Nervous s ti mul a ti on l ea ds to the rel ea s e of ca l ci um i nto the cytos ol a nd ra i s es the concentra ti on from l es s tha n 1 mM to a bout 10 mM. The ca l ci um bi nds to troponi n C. The ca l ci um–troponi n compl ex undergoes a conforma ti ona l cha nge, whi ch i s tra ns mi tted to tropomyos i n a nd ca us es tropomyos i n to s hi ft pos i ti on. The s hi ft of tropomyos i n a l l ows a cti n to i ntera ct wi th myos i n a nd contra cti on to proceed. Muta ti ons a ffecti ng protei ns i nvol ved i n mus cl e contra cti on ca n pres ent wi th l ow mus cl e tone a nd devel opmenta l del a y i n chi l dhood, a s chroni c mus cl e cra mps or fa ti gue, or wi th ca rdi omyopa thi es due to wea kened hea rt mus cl e. As ca rdi a c mus cl e contra cti on i s coordi na ted by el ectri ca l conducti on from the s i nus a nd a tri oventri cul a r (AV) nodes , mus cl e protei n a bnorma l i ti es ca n a l s o i nterfere wi th ca rdi a c rhythm. A s peci fi c muta ti on i n troponi n I ca n ca us e ca rdi omyopa thy a nd the i rregul a r ca rdi a c rhythm known a s Wol ff–Pa rki ns on–Whi te s yndrome (s ee Cha pter 12; Fi gure 12-4). III-47. The answer is B. The rol es of di hydrotes tos terone i ncl ude devel opment of the pros ta te, peni s , s crotum, a nd geni ta l s ki n. In contra s t, tes tos terone duri ng devel opment promotes forma ti on of the epi di dymi s , va s deferens , eja cul a tory duct, a nd s emi na l ves i cl es . Duri ng puberty, i t i s res pons i bl e for s econda ry s ex cha ra cteri s ti cs , i ncl udi ng mus cl e growth, a nd s perma togenes i s . Devel opment of the tes tes i s determi ned by s ry (s ex-determi ni ng regi on of the Y-chromos ome). Mul l eri a n i nhi bi tory fa ctor a ccounts for duct regres s i on i n ma l es s o tha t the uterus does not devel op (s ee Cha pter 20). III-48. The answer is E. Pri ma ry hyperpa ra thyroi di s m refers to the uncontrol l ed overproducti on of PTH. The 1-hydroxyl a s e enzyme i n vi ta mi n D meta bol i s m i s l oca ted i n the ki dney a nd i s a cti va ted by PTH not by ca l ci toni n. The 25-hydroxyl a s e enzyme i s not regul a ted by vi ta mi n D 3 . Al though s ex s teroi ds (es trogen/ tes tos terone) ca n bl ock the a cti on of PTH, thi s effect i s not l os t i n thes e pa ti ents . Ins tea d the a mount of PTH tha t i s produced overwhel ms thes e effects of s ex s teroi ds . Producti on of ca l ci toni n woul d be hi gh beca us e of el eva ted bl ood ca l ci um a nd woul d not be s uppres s ed (s ee Cha pter 13). III-49. The answer is B. In the mi tochondri a , thi a mi ne i s requi red for functi oni ng of both the pyruva te dehydrogena s e rea cti on (pyruva te + CoA + NAD + → a cetyl CoA + ATP + CO2 ) a nd the ci tri c a ci d cycl e α-ketogl uta -ra te dehydrogena s e rea cti on (α-ketogl uta ra te + CoA + NAD + → s ucci nyl -CoA + ATP + CO2 ). La cta te, a l a ni ne, a nd pyruva te woul d a l l a ctua l l y be i ncrea s ed beca us e of the i na bi l i ty of the cel l s to oxi di ze pyruva te. NADPH producti on i s unrel a ted to meta bol i s m of pyruva te. Argua bl y decrea s ed oxi da ti on of pyruva te woul d i ncrea s e NAD + concentra ti on tha t, i f a nythi ng, woul d ra i s e the NADP+ l evel (s ee Cha pter 10). III-50. The answer is C. The pa ti ent’s pH i s bel ow norma l s o thi s ha s to be a condi ti on a s s oci a ted wi th a n a ci dos i s . The rema i ni ng i s s ue i s whether i t i s meta bol i c or res pi ra tory. On the ba s i s of the ca rboni c a nhydra s e rea cti on ,a meta bol i c a ci dos i s wi l l overproduce protons (H +) s uch a s i n a l a cti c a ci dos i s or a di a beti c ketoti c cri s i s . Thi s l ower pH dri ves the rea cti on towa rd the l eft, both l oweri ng bi ca rbona te a nd i ncrea s i ng producti on of CO2 . To compens a te for the meta bol i c a ci dos i s , the l ungs wi l l compens a te by i ncrea s i ng res pi ra ti on to bl ow off the exces s CO2 , hence a ccounti ng for the decrea s ed pCO2 . In a res pi ra tory a ci dos i s , pCO2 wi l l be el eva ted beca us e of hypoventi l a ti on a s i n COPD. Anxi ety ca us es hyperventi l a ti on tha t wi l l decrea s e pCO2 a nd by the ca rboni c a nhydra s e rea cti on a l s o l ower the proton concentra ti on, thereby ra i s i ng the pH (i .e., res pi ra tory a l ka l os i s ). In the ca s e of hypera mmonemi a , a ba s e a mmoni a wi l l ca us e a meta bol i c a l ka l os i s wi th a ri s e i n pCO2 to reta i n a ci d. III-51. The answer is D. When the reni n–a ngi otens i n s ys tem i s a cti va ted, reni n i s rel ea s ed. Reni n rel ea s e i s control l ed i n s evera l wa ys : (1) Ba roreceptors i n the ki dney a re s ti mul a ted by decrea s ed rena l a rteri ol a r pres s ure. (2) Low bl ood pres s ure s ti mul a tes ca rdi a c receptors tha t a cti va te the s ympa theti c nervous s ys tem a nd the res ul ti ng ca techol a mi nes s ti mul a te the juxta gl omerul a r cel l s vi a β1 -a drenergi c receptors . (3) The ma cul a dens a , whi ch i s compos ed of cel l s a dja cent to the juxta gl omerul a r cel l s , i s s ti mul a ted by a decrea s e i n the ci rcul a ti ng concentra ti on of Na + or Cl −. Reni n i s a proteol yti c enzyme tha t cl ea ves a ngi otens i nogen to a ngi otens i n I. Angi otens i nogen i s s ecreted by the l i ver. Convers i on of a ngi otens i n I to a ngi otens i n II i s ca ta l yzed by a ngi otens i n-converti ng enzyme. Angi otens i n II i s the a cti ve form tha t bi nds to a receptor i n the a drena l cortex tri ggeri ng a l dos terone s ynthes i s a nd s ecreti on. Addi ti ona l l y, to a cutel y ra i s e bl ood pres s ure, a ngi otens i n II bi nds to receptors i n the va s cul a r s ys tem ca us i ng cons tri cti on (s ee Cha pter 18). III-52. The answer is A. Bra i ns from Al zhei mer di s ea s e (AD) pa ti ents conta i n i nterneurona l a myl oi d pl a ques a nd neurofi bri l l a r ta ngl es . Amyl oi d pl a ques cons i s t predomi na ntl y of a myl oi d-β (Aβ) protei ns , a l ong wi th s ma l l er a mounts of other protei ns , s uch a s α-s ynucl ei n a nd pres eni l l i n. The predomi na nt a myl oi dogeni c form of Aβ i s 42 a mi no a ci ds l ong (des i gna ted Aβ42 ) a nd i s a n extra cel l ul a r proteol yti c product of a myl oi d precurs or protei n (APP), pres ent i n the pl a s ma membra nes of neurons . APP i s cl ea ved by protea s es ca l l ed s ecreta s es . Cl ea va ge by s ecreta s es α a nd γ rel ea s es two s ol ubl e fra gments of APP, nei ther of whi ch i s a myl oi dgeni c. However, cl ea va ge by s ecreta s es β a nd γ rel ea s es the Aβ42 fra gment. The membra ne-s pa nni ng regi on of Aβ42 i s pri ma ri l y α-hel i ca l , a fter cl ea va ge a nd rel ea s e from the membra ne, i t undergoes a tra ns i ti on to a β-s tra nd conforma ti on. Once i n thi s conforma ti on, Aβ42 i s norma l l y cl ea red
ra pi dl y. If the Aβ42 i s not cl ea red, i t ca n nucl ea te pl a ques from vi rtua l l y a l l other Aβ42 protei ns tha t i t conta cts . As thes e pl a ques form, they res i s t proteol ys i s a nd tra ns port a nd s erve to tra p future Aβ42 protei ns a s they a re formed. Ea rl y ons et fa mi l i a l AD i s ca us ed by muta ti ons i n certa i n protei ns pa rt of the a cti ve s ecreta s e γ compl ex, res ul ti ng i n exces s i vel y hi gh producti on of Aβ42 . APP muta ti ons a re us ua l l y i n or nea r the Aβ42 porti on of APP a nd pres uma bl y ma ke APP a better s ubs tra te for cl ea va ge by s ecreta s es β or γ, or i t ma kes Aβ42 more l i kel y to undergo the α-hel i x to β-s tra nd convers i on (s ee Cha pter 19). III-53. The answer is A. Crea ti ne phos pha te i s us ed i n the fi rs t a pproxi ma tel y 5 s ec of exerci s e to i mmedi a tel y repl a ce ATP tha t i s bei ng us ed. The rea cti on crea ti ne phos pha te + ADP → crea ti ne + ATP i s ca ta l yzed by the enzyme crea ti ne ki na s e. Thi s enzyme l i es a t equi l i bri um s o tha t i t i ns ta nta neous l y res ponds when ATP concentra ti on begi ns to fa l l . There i s no other regul a ti on of thi s rea cti on except by ma s s a cti on effect ca us ed by cha nges i n concentra ti ons of rea cta nts a nd products . The 5 s ec of crea ti ne phos pha te us e a re s uffi ci ent to a l l ow the mus cl e to a cti va te gl ycogenol ys i s s o tha t meta bol i s m of gl ucos e uni ts ca n begi n to provi de the energy to s us ta i n exerci s e. In a n a erobi c mus cl e, i ni ti a l us e of gl ycogen ca n a l s o provi de ti me to mobi l i ze fa tty a ci ds a s the mus cl e fuel but crea ti ne phos pha te a l one ca nnot “buy” enough ti me. Crea ti ne i s produced i n s uffi ci ent a mounts by l i ver tha t s ecretes to the ci rcul a ti on for upta ke a nd s ubs equent phos phoryl a ti on by mus cl e. Crea ti ne mus t be repl a ced by l i ver on a regul a r ba s i s beca us e mus cl e converts crea ti ne to crea ti ni ne for excreti on. One concern wi th ta ki ng crea ti ne s uppl ements i s tha t i t ca n ca us e dehydra ti on s o tha t extra wa ter mus t be cons umed when ta ki ng i t. There a re i ns ta nces where crea ti ne s uppl ements ma y be es peci a l l y benefi ci a l —i n pa ti ents wi th a defi ci ency of crea ti ne due to l i ver enzyme defect or i n McArdl es di s ea s e pa ti ents who ca nnot mobi l i ze gl ycogen a nd hence ca n benefi t from the a ddi ti ona l a va i l a bi l i ty of crea ti ne phos pha te unti l i ncrea s ed gl ucos e upta ke a nd fa t mobi l i za ti on ca n occur (s ee Cha pter 12). III-54. The answer is E. Duri ng pregna ncy, the pri ma ry es trogen produced i s E3 . Bi ndi ng of es trogen to the proges terone receptor ca us es a conforma ti on cha nge tha t hel ps a cti va te the receptor s o tha t es trogen i s es s enti a l for proges terone/proges terone receptor functi ons . Es trogen a l s o a cts to i ncrea s e the tota l number of proges terone receptors , thereby a mpl i fyi ng proges terone effects . Duri ng pregna ncy, proges terone, together wi th E3 , i nhi bi ts l a cta ti on a nd s mooth mus cl e contra cti on; decrea s ed proges terone l evel s a re one potenti a l tri gger for l a bor a nd a l s o i ni ti a te mi l k producti on. E2 i s the ma i n es trogen i n nonpregna nt, ferti l e fema l es . Andros tenedi one i s a n a drena l a ndrogen tha t i n fema l es a nd ma l es ca n s erve a s precurs or to the s ynthes i s of E1 i n fa t cel l s . Growth hormone ha s no l i nk to l a cta ti on i nhi bi ti on or ons et. hCG i s a heterodi meri c gl ycoprotei n hormone produced by newl y ferti l i zed embryos a nd, s ubs equentl y by the pl a centa . hCG ma i nta i ns the corpus l uteum a nd ca us es i ncrea s i ng producti on of proges terone but i s not di rectl y rel a ted to i nhi bi ti on of l a cta ti on (s ee Cha pter 20). III-55. The answer is C. Both gl ucos e-6-phos pha ta s e a nd fructos e 1,6-bi s phos pha ta s e a re enzymes i n the gl uconeogeni c pa thwa y. However, onl y gl ucos e-6-phos pha ta s e i s a l s o requi red for the convers i on of gl ycogen to gl ucos e. Hence, onl y the pa ti ent wi th the gl ucos e-6phos pha ta s e defect wi l l s how gl ycogen a ccumul a ti on. In ea rl y s ta rva ti on, l a cta te provi des a pproxi ma tel y 50% of the ca rbons for gl ucos e s ynthes i s . Its decrea s ed us e for thi s purpos e i n both pa ti ents ca n l ea d to l a cti c a ci dos i s . Si mi l a rl y, a l a ni ne i s the key a mi no a ci d precurs or for gl uconeogenes i s a nd hence i ts us e to ma ke gl ucos e wi l l be a ffected i n both pa ti ents . Beca us e gl ucos e-6phos pha ta s e i s not found i n mus cl e, onl y the pa ti ent wi th the fructos e 1,6-bi s phos pha ta s e defect coul d s how a l tered mus cl e gl ucos e meta bol i s m. Fi na l l y, thes e defects woul d not a ffect functi on of the pentos e phos pha te pa thwa y (s ee Cha pter 10). III-56. The answer is D. Thi s pa ti ent’s grea s y, foul -s mel l i ng s tool s i ndi ca te s tea torrhea . Her vi s i on probl ems ma y be a ma ni fes ta ti on of vi ta mi n A defi ci ency due to fa t ma l a bs orpti on. The mos t l i kel y expl a na ti on i s bi l i a ry i ns uffi ci ency, tha t i s , decrea s ed bi l e s a l t producti on l ea di ng to poor emul s i fi ca ti on of di eta ry fa ts . Acti ve i l ea l di s ea s e i s a pos s i bi l i ty, but the WBC count woul d l i kel y be el eva ted unl es s her condi ti on wa s i n remi s s i on. GI i nfecti on i s l es s l i kel y due to the a bs ence of pa thogeni c orga ni s ms i n her s tool . La ctos e i ntol era nce ca n produce di a rrhea but not s tea torrhea (s ee Cha pter 11). III-57. The answer is D. The cl otti ng ca s ca de cons i s ts of a s eri es of ordered enzyma ti c s teps whos e end res ul t i s the forma ti on of a cl ot. There a re two l i mbs of the cl otti ng ca s ca de: the extri ns i c pa thwa y a nd the i ntri ns i c pa thwa y. The extri ns i c pa thwa y rel i es on the expos ure of ti s s ue fa ctor by a n i njury, whi ch then a cti va tes Fa ctor VII, the mos t a bunda nt of the cl otti ng fa ctors . The i ntri ns i c pa thwa y s ta rts when Fa ctor XII, a n i na cti ve s eri ne protea s e encounters expos ed col l a gen from a n i njured bl ood ves s el . Fa ctor XII i s tra ns formed i nto a n a cti va ted s eri ne protea s e. Acti va ted Fa ctor XII cl ea ves i na cti ve Fa ctor XI i nto a cti va ted Fa ctor XI. Thi s i n turn cl ea ves the i na cti ve form of Fa ctor IX, a nd the a cti va ted Fa ctor IX tra ns forms i na cti ve Fa ctor VII to i ts a cti ve form. La ter events i n the a ctua l forma ti on of the cl ot i nvol ve pl a tel ets , von Wi l l ebra nd fa ctor, ca l ci um i ntera cti ng wi th endothel i a l s urfa ces , a nd forma ti on of D -di mers (s ee Cha pter 14; Fi gure 14-15). III-58. The answer is B. Ni trogl yceri n ca us es rel ea s e of ni tri c oxi de (NO), whi ch a cti va tes gua nyl cycl a s e, produces cGMP, a nd ca us es va s odi l a ti on. NO i s formed from one of the gua ni di no ni trogens of the a rgi ni ne s i de cha i n by the enzyme NO s yntha s e. NO ha s a s hort ha l f-l i fe, rea cti ng wi th O2 to form ni tri te a nd then ni tra tes tha t a re excreted i n uri ne. Corona ry va s odi l a ti on ca us ed by ni trogl yceri n i s thus s hort l i ved, ma ki ng other mea s ures neces s a ry for l ong-term rel i ef of corona ry occl us i on. The neurotra ns mi tter formed by condens a ti on of a cetyl -CoA a nd chol i ne i s a cetyl chol i ne, whi ch does not pl a y a rol e i n di l a ti on of corona ry a rteri es (s ee Cha pter 16). III-59. The answer is D. IL-3 i s produced by Th cel l s to promote growth a nd di fferenti a ti on of precurs ors of neutrophi l s , eos i nophi l s a nd ba s ophi l s , ma s t cel l s , pl a tel ets , red bl ood cel l s , a s wel l a s monocytes , ma cropha ges , a nd dendri ti c cel l s . IL-3 a l s o promotes hi s ta mi ne rel ea s e from ma s t cel l s . IL-1 cos ti mul a tes Th l ymphocytes but i s produced by monocytes , ma cropha ges , B l ymphocytes , dendri ti c cel l s , a nd fi brobl a s ts . IL-2 i s produced by Th1 cel l s a nd i s i nvol ved i n growth, prol i fera ti on, a nd further a cti va ti on of T a nd B l ymphocytes a nd NK cel l s . IL-5 produced by Th2 a nd ma s t cel l s s ti mul a tes growth a nd di fferenti a ti on of a cti va ted B cel l s a nd eos i nophi l s to i ncrea s e Ig s ecreti on, es peci a l l y IgG a nd IgA. Neutrophi l chemota cti c fa ctor (IL-8) produced by ma cropha ges , epi thel i a l , a nd endothel i a l cel l s promotes chemota cti c movement of neutrophi l s to a s i te of i nfecti on a nd i nfl a mma ti on a s wel l a s i ntra cel l ul a r s i gna l i ng, a nd i ncrea s ed meta bol i s m a nd hi s ta mi ne rel ea s e by neutrophi l s . IL-10 i s produced by cytotoxi c T cel l s a mong s evera l others a nd i ts functi ons i ncl ude i nhi bi ti on of a nti gen pres enta ti on a nd cytoki ne producti on by ma cropha ges a nd Th1 cel l s . Fi na l l y, i nter-feron-α ma de by WBCs (l eukocytes ) promotes the devel opment of fever by rel ea s e of pros ta gl a ndi n-E2 , reduces pa i n by i ntera cti ng wi th μopi oi d receptor, a nd s ti mul a tes ma cropha ges a nd NK cel l s to ki l l vi rus es (Ta bl e 15-2). III-60. The answer is B. α1 -Receptors a re pos ts yna pti c a drenoreceptors l oca ted i n s mooth mus cl e, eye, l ung, bl ood ves s el s , gut, a nd the geni touri na ry s ys tem. Sti mul a ti on of thes e receptors a cti va tes a G q protei n. Thes e s i gna l s l ea d to exci ta ti on, whi ch i s exhi bi ted by mydri a s i s , bronchodi l a ti on, cons tri cti on of bl ood ves s el s , i ncrea s ed s trength of hea rt contra cti on (pos i ti ve i notropy), a nd decrea s ed hea rt ra te (nega ti ve chronotropy). α2 -Receptors a re l oca ted chi efl y on the pres yna pti c nerve termi na l s a nd G i-protei n a cti va ti on, whi ch i nhi bi ts a denyl cycl a s e a cti vi ty. It produces va s ocons tri cti on a nd reduces s ympa theti c outfl ow i n the CNS. β1 -Receptor s ti mul a ti on a cti va tes G s protei n, whi ch i ncrea s es a denyl cycl a s e a cti vi ty. Thi s a cti on res ul ts i n pos i ti ve chronotropy (hea rt ra te i ncrea s e), dromotropy (conducti on of the i mpul s e through the hea rt’s AV node), a nd i notropy (force of hea rt contra cti on). β2 -Receptors a re mos tl y pos ts yna pti c a drenoceptors l oca ted i n s mooth mus cl e a nd gl a nds . They a l s o a cti va te a denyl cycl a s e vi a a G s protei n but a l s o i nhi bi ti on of the s a me enzyme by a G i protei n. The di rectl y oppos i te effects a re bel i eved to a l l ow di fferent functi ons i n s peci fi c cel l a nd ti s s ue l oca ti ons . The β2 s ti mul a ti on rel a xes s mooth mus cl e res ul ti ng i n va s odi l a ti on a nd bronchodi l a ti on a s wel l a s rel ea s e of i ns ul i n a nd the i nducti on of gl uconeogenes i s (Cha pters 8 a nd 19). III-61. The answer is D. Serotoni n i s a genera l l y i nhi bi tory mona mi ne type neurotra ns mi tter found i n the CNS. Acetyl chol i ne ca n be exci ta tory or i nhi bi tory a nd i s found i n the CNS, neuromus cul a r juncti on, a nd ma ny a utonomi c nervous s ys tem (ANS) s yna ps es . Gl uta ma te i s found i n
the bra i n a nd s pi na l cord a nd i s exci ta tory. Norepi nephri ne, s uch a s s erotoni n, i s a monoa mi ne neurotra ns mi tter but ca n be exci ta tory or i nhi bi tory found i n the CNS, s ympa theti c ANS s yna ps es , a nd nea rl y a l l ti s s ues . Subs ta nce P i s a neuropepti de tha t i s genera l l y exci ta tory a nd found i n the s pi na l cord, bra i n, s ens ory neurons , a nd GI tra ct (s ee Cha pter 19).
SECTION IV APPENDICES
APPENDIX I BIOCHEMICAL BASIS OF DISEASES Contributing Editor: Harold Cross, MD, PhD Cofounder, Wi ndows of Hope, (www.wohproject.org) Profes s or a nd Di rector, Medi ca l Student Tea chi ng, Depa rtment of Ophtha l mol ogy a nd Vi s i on Sci ences , Col l ege of Medi ci ne, Uni vers i ty of Ari zona , Tucs on, Ari zona , USA
Thi s a ppendi x pres ents s el ected exa mpl es of i nheri ted di s ea s es a ffecti ng ba s i c bi ochemi ca l proces s es . Di s ea s es a re ca tegori zed a s per thei r i nvol vement wi th the four ba s i c ca tegori es of bi ochemi ca l mol ecul es (a mi no a ci ds /protei ns , ca rbohydra tes / gl ycoprotei ns , l i pi ds /gl ycol i pi ds , a nd nucl ei c a ci ds /deoxyri bonucl ei c a ci d). Addi ti ona l ca tegori es of mi tochondri a l enzymes a nd di s ea s es a ffecti ng bi l i rubi n, bl ood cl otti ng, s teroi d hormones , a nd vi ta mi ns /mi nera l s /el ectrol ytes a re a l s o provi ded. Inheri ta nce i s predomi na tel y a utos oma l reces s i ve unl es s otherwi s e s ta ted. Mi nor va ri a ti ons of thes e geneti c di s ea s es ma y not be noted. For further i nforma ti on, the rea der i s referred to the Onl i ne Mendel i a n Inheri ta nce i n Ma n ® (www.ncbi .nl m.ni h.gov/omi m/), “a comprehens i ve compendi um of more tha n 12,000 huma n genes a nd geneti c phenotypes ” a nd Wi ndows of Hope (www.wohproject.org), “a popul a ti on-ba s ed medi ca l project dedi ca ted to the detecti on, cha ra cteri za ti on, a nd trea tment of i nheri ted hea l th probl ems .”
AMINO ACID SYNTHESIS/DEGRADATION
AMINO ACID TRANSPORT
UREA CYCLE DISORDERS
STRUCTURAL PROTEINS
CARBOHYDRATES
GLYCOGEN STORAGE
MITOCHONDRIAL ENZYMES (EXCLUDING UREA CYCLE AND FATTY ACID OXIDATION)
LIPIDS AND FATTY ACID OXIDATION ERRORS
NUCLEOTIDE METABOLISM
DEFECTIVE DNA
BILIRUBIN METABOLISM
BLOOD CLOTTING FACTOR DEFECTS
STEROID HORMONE SYNTHESIS
VITAMINS/MINERALS AND ELECTROLYTES
APPENDIX II BIOCHEMICAL METHODS POLYACRYLAMIDE GEL ELECTROPHORESIS (PAGE) [SODIUM DODECYL SULFATE (SDS)/NON-SDS] PAGE i s a l a bora tory techni que des i gned to s epa ra te protei ns a ccordi ng to thei r s i ze, s ha pe, a nd a l s o cha rge. Protei ns for PAGE a re us ua l l y prepa red by pl a ci ng them i n a n a ni oni c (nega ti ve cha rge) detergent mi xture, whi ch, a l ong wi th hea ti ng to a pproxi ma tel y 60°C, dena tures the protei n, brea ks a ny cys tei ne–cys tei ne di s ul fi de bonds , a nd crea tes a fa i rl y l i nea r protei n s tructure. PAGE ca n a l s o be run wi th a n a ddi ti on of SDS. SDS, i f us ed, i s i ncl uded i n the detergent mi xture a nd a s s i s ts i n di s rupti ng the protei ns ’ s econda ry a nd terti a ry s tructures . In a ddi ti on, SDS bi nds to ea ch pepti de cha i n i n a ra ti o of one SDS per two a mi no a ci d res i dues , thereby a ddi ng a nega ti ve cha rge proporti ona l to the pepti de l ength. The a ddi ti on of SDS to PAGE s a mpl es , therefore, a l s o ma kes the protei ns ’ s ha pes a nd na ti ve cha rges i rrel eva nt; onl y the tota l l ength a nd the res ul ti ng SDS cha rge ma tters .
Pol ya cryl a mi de gel i s compos ed of l i nea r acrylamide mol ecul es , cros s -l i nked by bisacrylamide vi a the ca ta l yti c a cti ons of ammonium persulfate a nd tetramethylethylenediamine (TMED). Cros s -l i nki ng forms a web-l i ke pol ya cryl a mi de l a tti ce wi th pores of a pproxi ma tel y the s a me s i ze throughout. Va ryi ng the a mounts of a cryl a mi de, bi s a cryl a mi de, a nd a queous s ol uti on a l l ows s ci enti s ts to a ccura tel y va ry the pore s i ze a nd, therefore, the rel a ti ve a bi l i ty to s epa ra te di fferentl y s i zed protei n mol ecul es .
After pol ymeri za ti on, the gel i s pl a ced i n a n el ectrophores i s devi ce a nd i mmers ed i n a buffer wi th a ca thode (pos i ti ve cha rge) a t the top a nd a n a node (nega ti ve cha rge) a t the bottom. The protei n s a mpl es a re “l oa ded” onto i ndi vi dua l gel l a nes formed by a remova bl e comb. A tracking dye i s us ua l l y i ncl uded i n the s a mpl es to moni tor the progres s of the protei ns through the gel . The el ectri c fi el d ca us es the nega ti vel y cha rged protei ns (cha rge us ua l l y enha nced by the proporti ona l nega ti ve cha rge of SDS mol ecul es ) to move towa rd the a node proporti ona l to thei r l ength (a nd, therefore, cha rge). Movement through the gel i s not di rectl y proporti ona l to the overa l l cha rge, though, beca us e the l a rger protei ns wi l l encounter much more di ffi cul ty i n movi ng through the pores beca us e of thei r s i ze a nd l i nea r ri gi di ty. Al though the l ength of ea ch pepti de i ncrea s es the el ectri ca l force movi ng i t down the gel , the “fi l teri ng” a cti on of the gel pores wi l l a l l ow s ma l l er mol ecul es to tra vel further down the gel , wherea s l a rger mol ecul es rema i n nea r the top.
(A) The PAGE system employs the electric charge deployed between a positive electrode (anode) and negative electrode (cathode).(B) Proteins are loaded (origin) onto polyacrylamide gel, which is exposed to this electric field. Negatively charged proteins are driven toward the anode, whereas positively charged proteins will remain nearer to the cathode. The relative movement of each protein depends on its specific charge determined by its amino acid composition. When SDS is not used (“native PAGE”), the secondary-to-quaternary structure of each protein also influences the movement through the acrylamide gel matrix. When SDS is used, higher order structure is eliminated, separation by charge predominates. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] When the tra cki ng dye a pproa ches the bottom of the gel , the el ectri ca l fi el d i s s topped a nd the gel i s s ta i ned us i ng a va ri ety of chemi ca l s or s ol uti ons , whi ch bi nd to protei ns (e.g., Coomassie blue, silver) to a l l ow vi s ua l i za ti on i n the gel . Protei ns of known mol ecul a r wei ght, referred to a s “molecular markers,” a re norma l l y run i n a s epa ra te l a ne to a l l ow di rect compa ri s on wi th the protei ns from the experi menta l s a mpl e. PAGE gel s ca n be us ed further for wes tern bl otti ng (s ee bel ow) a nd other bi ochemi ca l techni ques . SDS or non-SDS PAGE i s often us ed duri ng the bi ochemi ca l i s ol a ti on of a s i ngl e protei n from ti s s ue or protei n mi xtures , a l though i t ca n a l s o be us ed cl i ni ca l l y for di a gnos ti c a nd/or trea tment purpos es . Beca us e of i ts a bi l i ty to di s s ol ve mol ecul es , SDS i s s ometi mes us ed i n enema s
a s a l a xa ti ve.
Markers of known molecule weight (kDa) are loaded in one gel lane (left) to allow characterization of proteins separated on the other lanes of the gel. [Reproduced wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Two-dimensional (2D) PAGE i s performed i n two di recti ons on the s a me s a mpl e, offeri ng i mproved s epa ra ti on for compl ex mi xtures of mol ecul es . In thi s techni que, s a mpl es a re expos ed to a n el ectri c current for a fi rs t s epa ra ti on. The gel i s then turned 90° a nd a current i s a ga i n a ppl i ed wi th a n a l terna ti ve buffer, whi ch a ffects the movement of the s a mpl es di fferentl y. As a res ul t, a s econd s epa ra ti on i s a chi eved. Exa mpl es i ncl ude mol ecul a r wei ght s epa ra ti on fol l owed by s epa ra ti on ba s ed on the mol ecul es ’ overa l l pH, known a s the “i s oel ectri c poi nt.” 2D el ectrophores i s i s us ed i n ma ny cl i ni ca l condi ti ons a s a prepa ra tory s tep before wes tern bl otti ng (s ee bel ow) for tes ts s uch a s confi rma ti on of huma n i mmunodefi ci ency vi rus (HIV) a nd hepa ti ti s B i nfecti on, the detecti on of pri ons i n Creutzfel dt–Ja kob di s ea s e, a l s o known a s “ma d cow di s ea s e,” a nd di a gnos i s of Lyme di s ea s e, a mong others .
Samples are run via normal SDS–PAGE (shown in vertical direction), which separates the proteins based on size, turned 90°, and run a second time via isoelectric focusing (shown in horizontal direction) technique with a pH gradient, which separates the proteins based on total pH of the protein (see text). Running the same sample by these two different techniques allows increased separation of complex mixtures of proteins and/ or separation of proteins of the same size, but differing isoelectric points. [Reproduced wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.]
IMMUNOASSAYS Immunoassay i s a generi c term for the a ppl i ca ti on of antibodies to bi ochemi ca l tes ti ng. Thes e techni ques pa ved the wa y for a wi de va ri ety of techni ques of s epa ra ti on, mea s urement, a nd pos i ti ve i denti fi ca ti on of bi ol ogi ca l mol ecul es , a nd i ncl ude radioimmunoassays (RIA), enzymelinked immunosorbant assay (ELISA or EIA), a nd western blotting. Al l i mmunoa s s a ys i nvol ve the a ddi ti on of a n a nti body s peci fi c for a known mol ecul e (antigen) to a s ol uti on pres uma bl y conta i ni ng, a t l ea s t i n pa rt, tha t mol ecul e. If the a nti gen i s pres ent, the a nti body wi l l bi nd s peci fi ca l l y a nd, i f properl y chos en a nd prepa red, s trongl y. Monoclonal antibodies wi l l norma l l y bi nd the ti ghtes t a nd wi th the grea tes t s peci fi ci ty; polyclonal antibodies a re us ua l l y wea ker a nd l es s s peci fi c i n thei r bi ndi ng pa tterns . Sel ecti ve preci pi ta ti on or chroma togra phy (s ee bel ow) ca n be us ed to i s ol a te a nd puri fy the a nti body–a nti gen compl ex a s needed. If a chemi ca l or a nother l a bel ha s been previ ous l y a tta ched to the a nti body, i t ca n be us ed to detect the pres ence a nd, i n ma ny ca s es , the qua nti ty of the a nti gen. Exa mpl es of thes e detecti on techni ques i ncl ude fl uores cent mol ecul es , enzymes tha t produce a pa rti cul a r col or when
provi ded a n a ppropri a te s ubs tra te, ra di oa cti ve or ma gneti c l a bel s , a nd even gol d pa rti cl es , whi ch coa l es ce to form a vi s ua l preci pi ta te. Al terna ti vel y, a nother a nti body wi th a ra di oa cti ve detector mol ecul e, whi ch s el ecti vel y bi nds to the fi rs t a nti body, ca n be a dded l a ter i n the tes t. Immunoa s s a ys a re us ed i n a mul ti tude of cl i ni ca l tes ts , es peci a l l y for the detecti on of i nfecti ons i n bl ood, uri ne, cerebra l s pi na l fl ui d, a nd s o on. RIA The fi rs t i mmunoa s s a y devel oped wa s the RIA. In thi s tes t, a mol ecul e of i nteres t, s uch a s a hormone, ha s a ra di oa cti ve mol ecul e (e.g., i odi ne125, ca rbon-14, or hydrogen-3) a tta ched to i t. The res ea rcher a l s o ha s a n a nti body (monocl ona l or pol ycl ona l ), whi ch s peci fi ca l l y recogni zes a nd bi nds to thi s hormone. A mea s ured a mount of the a nti body a nd a mea s ured a mount of the pure hormone (both ra di ol a bel ed a nd unl a bel ed forms ) a re a dded to a s s a y tubes wi th concentra ti on of the unl a bel ed hormone grea tl y exceedi ng tha t of the ra di ol a bel ed hormone. Beca us e onl y a s ma l l a mount of a nti body i s a dded a nd i ts ca pa ci ty to bi nd the hormone i s l i mi ted, the a nti body-bi ndi ng s i tes wi l l be s a tura ted (i .e., a l l occupi ed; s ee fi gure bel ow) As a res ul t, onl y a fra cti on of the tota l a mount of hormone wi l l be a ctua l l y bound to the a nti body. Gi ven tha t the a nti body ca nnot di fferenti a te between l a bel ed a nd unl a bel ed hormones , both forms of the hormone wi l l compete for the l i mi ted number of bi ndi ng s i tes on the a nti body. When a s ma l l a mount of unl a bel ed hormone (i .e., from s ta nda rd or pa ti ent s a mpl e) i s a dded to the a s s a y, then onl y a s ma l l a mount of the bound-l a bel ed hormone i s di s pl a ced from the a nti body. As the a mount of unl a bel ed hormone i ncrea s es , the a mount of ra di ol a bel ed hormone bound to the a nti body decrea s es cons i dera bl y. A va ri ety of techni ques ca n be us ed to s epa ra te unbound hormone from the a nti body-bound hormone. Therefore, di rect mea s urement of bound-l a bel ed hormone ca n be ea s i l y ma de. Us i ng known qua nti ti es of unl a bel ed hormone, i t i s pos s i bl e to cons truct a s ta nda rd curve whi ch compa res the a mount of ra di ol a bel ed hormone bound to the a nti body wi th the a mount of pure unl a bel ed hormone a dded to the tube. By repl a ci ng pure hormone wi th a pa ti ent s a mpl e, the concentra ti on of the hormone i n the s a mpl e ca n then be di rectl y determi ned.
A molecule of interest (A) can be detected by placing a radioactive label (yellow starburst) onto a protein (B), which is known to bind to (A). Important examples of protein (B) include hormones and monocolonal and polyclonal antibodies. [Ada pted wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.]
A specific antibody to the hormone is incubated with the radiolabeled hormone. A standard curve is created by introducing known concentrations of unlabeled hormone to displace the radiolabeled hormone. The amount of radioactivity remaining is a function of the unlabeled hormone concentration. When unknown samples are used, the measurement of the radioactivity remaining allows the hormone concentration to be determined from the standard curve. [Ada pted wi th permi s s i on from Ki bbl e JD a nd Ha l s ey CR: The Bi g Pi cture: Medi ca l Phys i ol ogy, 1s t edi ti on, McGra w-Hi l l , 2009.] One dra wba ck of thi s method i s tha t RIA mea s ures the i mmunol ogi c a cti vi ty, not the bi ol ogi ca l a cti vi ty, of a hormone. Idea l l y, both a bi oa s s a y a nd RIA a re eva l ua ted. However, i n a cl i ni ca l s i tua ti on, wi th a few ra re excepti ons , us i ng the RIA provi des s uffi ci entl y a ccura te i nforma ti on on hormone l evel s i n the pa ti ent s a mpl e. The a dva nta ges of RIA a re s ens i ti vi ty a nd s peci fi ci ty a s wel l a s ra pi d a nd economi c eva l ua ti on of pa ti ent bl ood or uri ne s a mpl es . ELISA OR EIA The pri nci pl e of RIA i s the ba s i s of newer methods of mea s urement, for exa mpl e, ELISA or EIA. The ELISA techni que us es two di fferent a nti bodi es wi th va ryi ng bi ndi ng a ffi ni ty to a mol ecul e tha t the res ea rcher wi s hes to detect or s tudy. An exa mpl e i s a pol ycl ona l or a monocl ona l a nti body ma de a ga i ns t the s a me mol ecul e. Norma l l y, a mi croti ter pl a te, conta i ni ng mul ti pl e tes t wel l s coa ted wi th the pol ycl ona l a nti body (not s hown i n fi gure), i s us ed. The tes t s a mpl es a re pl a ced i n the wel l s a nd the a nti gen, i f pres ent, i s a l l owed to bi nd to the a nti body. After a s ui ta bl e ti me peri od, the rema i nder of the tes t s a mpl e i s removed a nd the wel l s a re wa s hed to remove nons peci fi ca l l y bound mol ecul es . Next, a monocl ona l a nti body a ga i ns t the s a me a nti gen i s a dded to the wel l s a nd a l l owed to bi nd to the a nti gen mol ecul e. The us e of pol ycl ona l a nd monocl ona l a nti bodi es us ua l l y a l l ows mul ti pl e bi ndi ng to the s a me a nti gen mol ecul e. Detecti on techni ques a re then us ed to determi ne the pres ence a nd qua nti ty of the a nti gen bei ng tes ted. One common method i s to us e a n enzyme a tta ched to the a nti body, whi ch produces a col ored product. After the bi ndi ng of the s econd a nti body, a ddi ti on of the enzyme’s s ubs tra te crea tes a col or rea cti on, whi ch ca n often be qua nti ta ted (s ee the fi gure bel ow). Detecti on us i ng ra di oa cti ve or fl uores cent mol ecul es a tta ched to the s econd a nti body ca n a l s o be uti l i zed. ELISA i s us ed for a wi de va ri ety of cl i ni ca l tes ts (e.g., i denti fi ca ti on of HIV, a l l ergens , a nd/or drugs ), es peci a l l y i f the concentra ti on of a certa i n protei n or other a nti gen mus t be determi ned. The ELISA techni que i s a l s o the ba s i s for home pregna ncy tes ts , where the hormone bi nds fi rs t to a n a nti body–enzyme compl ex wi th detecti on a nd qua nti za ti on vi a a s econd a nti body conta i ni ng a dye tha t i s produced by a n enzyme.
Antibody to an antigen being tested is applied to the sample. Following binding, the antigen can be detected either via an attached enzyme on the primary antibody (direct method, left) or by the addition of a secondary antibody with an attached enzyme (indirect method, right). When substrate is added to the antibody–antigen sample, the enzyme produces either a fluorescent or colored product, allowing easy measurement, which allows both quantitative and qualitative assessment of the original antigen. [Ada pted wi th permi s s i on from Mes cher AL: Junquei ra ’s Ba s i c Hi s tol ogy Text a nd Atl a s , 12th edi ti on, McGra w-Hi l l , 2010.]
CHROMATOGRAPHY
THIN LAYER (PAPER) CHROMATOGRAPHY (TLC) TLC or paper chromatography rel i es on a thi n l a yer of ma teri a l (e.g., pa per, cel l ul os e, s i l i ca , a nd others ) wi th va ryi ng a bs orba nce for mol ecul es . Al though often us ed for nonbi ol ogi ca l mol ecul es , TLC ca n be uti l i zed for protei n a nd l i pi d s epa ra ti on a nd/or i denti fi ca ti on. A s ma l l vol ume of ea ch s a mpl e for a na l ys i s i s pl a ced i n verti ca l l a nes cl os e to one end of the TLC l a yer, a nd tha t end i s pa rti a l l y i mmers ed i n a s ol vent, ens uri ng tha t the s a mpl e i s not covered by the s ol vent. Thi s s ol vent i s dra wn through a nd up the TLC ma teri a l by capillary action. As i t tra vel s , i t ca rri es the s a mpl e mol ecul es up wi th i t a t ra tes dependi ng on the mol ecul e a nd s ol vent properti es , i ncl udi ng the l evel of a ds orpti on onto the chroma togra phy l a yer a nd the s ol ubi l i ty of the s a mpl e i n the l i qui d (s ee the fol l owi ng fi gure). As a res ul t of the va ryi ng s ol i d a nd l i qui d l a yer a ffi ni ti es , s epa ra ti on of a mi xture of mol ecul es res ul ts . The TLC l a yer i s then “devel oped” for mol ecul e detecti on vi a chemi ca l or s pectros copi c mea ns . In the ca s e of a s i ngl e, puri fi ed mol ecul e, the di s ta nce tra vel ed on a s peci fi c TLC wi th a s peci fi c s ol vent ca n provi de i denti fi ca ti on by compa ri s on wi th known s ta nda rds . Two-wa y TLC ca n a l s o be us ed, i n whi ch a fter s epa ra ti on by one s ol vent, the s heet i s turned 90° a nd expos ed to the a cti ons of a s econd type of s ol vent. Two-wa y TLC offers i mproved s epa ra ti on for s ome mol ecul es .
(A) Diagram of TLC equipment, illustrating TLC support medium, solvent (light brown), point of application of three samples (black dots), and direction of movement (indicated by arrow) of molecular samples as the solvent moves up the medium. (B) Actual TLC plate showing known sample of G (left lane) and L (middle lane) and separation of a mixture of these two proteins into the component parts (right lane). [Ada pted wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] COLUMN CHROMATOGRAPHY Column chromatography uti l i zes the s a me pri nci pl es of TLC, but i n a three-di mens i ona l col umn s ys tem tha t a l l ows much l a rger a mounts of s a mpl e to be s epa ra ted a nd s tudi ed. A gl a s s tube or “col umn” i s fi l l ed wi th a s el ected ma teri a l , ca l l ed stationary phase, whi ch ha s pa rti cul a r chemi ca l qua l i ti es (e.g., hydrophobi c, hydrophi l i c, a ni oni c (–) cha rges , ca ti oni c (+) cha rges , etc.). Mol ecul es wi l l bi nd to the s ta ti ona ry pha s e, dependi ng on thei r i ndi vi dua l chemi ca l qua l i ti es . For exa mpl e, a protei n wi th a s trong hydrophi l i c externa l cha ra cter wi l l not wa nt to bi nd to a hydrophobi c col umn ma teri a l , but wi l l wa nt to bi nd to a hydrophi l i c one. The va ri a ti on i n bi ndi ng of ea ch mol ecul e l ea ds to s epa ra ti on beca us e mol ecul es tha t bi nd l es s wi l l tra vel more qui ckl y through the col umn a nd vi ce vers a . Often, the mol ecul es wi l l bi nd to the s ta ti ona ry pha s e unti l a nother l i qui d s ol vent, ca l l ed mobile phase or eluent, wi th s tronger hydro-phobi c/hydrophi l i c or a ni oni c/ca ti oni c qua l i ti es i s run through the col umn. Someti mes , s equenti a l el uents , ea ch wi th a di fferent chemi ca l qua l i ty, a re run through the col umn, ea ch ca us i ng s el ected mol ecul es to unbi nd a nd fl ow out of the col umn. The ma teri a l tha t pa s s es through the col umn i s col l ected i n s ma l l s a mpl es of a known vol ume, ca l l ed fra cti ons , for further s tudy. Often, mol ecul es i n the fra cti ons a re moni tored by l i ght [e.g., ul tra vi ol et (UV)] a bs orpti on or other detecti on methods . GEL FILTRATION CHROMATOGRAPHY Gel filtration chromatography, a va ri a ti on of col umn chroma togra phy, i s us ed for the bi ochemi ca l s epa ra ti on of bi ol ogi ca l mol ecul es , ma i nl y protei ns a nd nucl ei c a ci d s tra nds , dependi ng on the s i ze a nd s ha pe of the mol ecul e. The techni que us ua l l y pres erves bi ol ogi ca l functi on. Gel fi l tra ti on us es beads formed from pol ya cryl a mi de (s ee a bove), dextra ns , or a ga ros e (s ee Cha pter 2) wi th pores of a pproxi ma tel y equa l s i ze. The pa rti cul a r pore size of ea ch type of gel fi l tra ti on bea d depends on thei r exa ct ma teri a l s a nd di fferent fa bri ca ti on techni ques whos e cha ra cteri s ti cs a l l ow res ea rchers to opti mi ze col umn chroma togra phy for va ryi ng s a mpl es . Bi ol ogi ca l s a mpl es a re a ppl i ed to the top of the col umn i n a s ma l l vol ume. A s ol uti on i s run through the col umn ca rryi ng the s a mpl e through the gel bea ds . Sma l l a nd/or predomi na tel y compa ct gl obul a r mol ecul es a re a bl e to enter the bea d pores a nd, therefore, ha ve a l a rge vol ume a va i l a bl e to them. Thi s l a rge vol ume res ul ts i n a s l ower tra ns i t ti me through the col umn. La rger pa rti cl es or ones wi th a noncompa ct or l i nea r s tructure a re una bl e to enter the bea d pores a nd tra vel more qui ckl y through the s ma l l er vol ume between the bea ds . The mol ecul a r s i ze a nd s ha pe of ea ch bi ol ogi ca l mol ecul e a ppl i ed to the col umn di rectl y determi ne how fa s t or s l ow i t tra vel s through a nd out (elution) of the col umn (s ee fi gure bel ow). Thi s va ryi ng s peed res ul ts i n s epa ra ti on. ION-EXCHANGE CHROMATOGRAPHY Ion-exchange chromatography, a nother type of col umn chroma togra phy, ta kes a dva nta ge of charged groups (e.g., ions) tha t a re pa rt of the gel bea d ma teri a l . Di fferent bi ol ogi ca l mol ecul es i ntera ct more or l es s wi th thes e chemi ca l groups (e.g., a more nega ti ve mol ecul e wi l l be a ttra cted to pos i ti vel y cha rged gel bea ds ), a l teri ng thei r s peed of el uti on a nd res ul ti ng i n s epa ra ti on. Some mol ecul es wi l l bi nd s o s trongl y to the cha rged gel bea ds tha t a s ol uti on wi th a s tronger i oni c cha rge mus t be us ed to di s pl a ce or “el ute” them.
Small molecules (blue) are able to enter the gel beads, increasing their available volume and slowing down the speed of travel through the column. Larger molecules (red) are unable to enter the gel beads and travel more quickly through and out of the column. As a result, molecules separate and elute from the column at a rate proportional to their molecular weight/size. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
Gel beads with a positive ionic charge attract negatively charged molecules (blue) more than positively charged ones (orange). This different affinity, based on charge, allows separation of various molecules. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] AFFINITY CHROMATOGRAPHY Affinity chromatography works much l i ke i on-excha nge chroma togra phy (a bove), but ta kes a dva nta ge of s peci fi c mol ecul es tha t a re pa rt of or previ ous l y a tta ched to the gel bea d ma teri a l . As a n exa mpl e, antibodies coupl ed to the gel ma teri a l a re s ometi mes us ed for very s peci fi c bi ndi ng to the des i red mol ecul e. Other exa mpl es of a ffi ni ty chroma togra phy i ncl ude the us e of gel -bound substrates or coenzymes. The bi ol ogi ca l mol ecul es rema i n bound to the gel bea d unti l a s econd s ol uti on of cha nged i oni c s trength or pH i s a ppl i ed to a ffect thei r rel ea s e a nd el uti on. Rega rdl es s of the pa rti cul a r type of col umn chroma togra phy us ed, el uted bi ol ogi ca l mol ecul es a re col l ected i n s ma l l , ordered vol ume a l l otments , known a s fra cti ons , a nd the mol ecul es a re detected by s pectros copi c or chemi ca l i denti fi ca ti on methods . HIGH-PERFORMANCE/PRESSURE LIQUID CHROMATOGRAPHY (HPLC) HPLC i s a techni que us ed for enha nced a nd preci s e s epa ra ti on of even s ma l l a mounts of bi ol ogi ca l a nd nonbi ol ogi ca l mol ecul es . HPLC col umns uti l i ze va ri a ti ons of the s epa ra ti on pri nci pl e behi nd col umn chroma togra phy, dependi ng on va ri a bl e a ttra cti ve a nd repul s i ve forces on the gel ma teri a l a nd thei r i ntera cti on wi th the bi ol ogi ca l mol ecul es bei ng s tudi ed. HPLC a l s o uti l i zes s ma l l er col umns tha n regul a r col umn chroma togra phy, wi th more s peci a l i zed gel ma teri a l s . The s ma l l er col umns a nd res ul ti ng ti ghter pa cki ng of the gel ma teri a l i n HPLC combi ned wi th the us e of a pump to propel the col umn s ol uti on (s ometi mes orga ni c s ol vents ) ei ther i n a s i ngl e s ol uti on or a s a gra di ent of di fferent s ol uti ons under hi gh pres s ure l ea d to fa s t, effi ci ent, a nd more s el ecti ve s epa ra ti on of mol ecul es tha t ma y not be pos s i bl e wi th other techni ques . Methods to detect thes e mol ecul es once they a re el uted, i ncl udi ng l i ght a bs orba nce, fl uores cence, el ectrochemi ca l , a nd others , a re s i mi l a r to thos e uti l i zed i n col umn chroma togra phy.
PROTEIN AND DEOXYRIBONUCLEIC ACID (DNA)/ RIBONUCLEIC ACID (RNA) PRECIPITATION The s el ected preci pi ta ti on of bi ol ogi ca l mol ecul es , i ncl udi ng protei ns a nd DNA/RNA, i s often us ed duri ng the prepa ra ti on, a na l ys i s , a nd concentra ti on of bi ochemi ca l s a mpl es . Al l methods rel y on the va ryi ng s ol ubi l i ty of thes e mol ecul es i n di fferent a queous or nona queous s ol uti ons , dependi ng on thei r hydro-phobi c, hydrophi l i c, a nd other bi ochemi ca l properti es . The pa rti cul a r a mi no a ci d R-groups (pri ma ry s tructure) a s wel l a s the res ul ti ng s econda ry, terti a ry, a nd qua terna ry s tructures (Cha pter 1) of a protei n determi ne i ts i ntera cti ons wi th wa ter mol ecul es or orga ni c s ol vents a nd, therefore, how s ol ubl e i t wi l l be i n thos e s ol uti ons . Beca us e protei ns ha ve di fferent pri ma ry s tructures , ea ch wi l l ha ve uni que s ol ubi l i ty i n a pa rti cul a r s ol vent a nd wi l l a l l ow s epa ra ti on by va ryi ng preci pi ta ti on techni ques . One s uch techni que tha t i s often us ed i s referred to a s “salting out” a nd often us es the chemi ca l ammonium sulfate (a l though other s a l ts ca n be us ed). Us i ng thi s techni que, the concentra ti on of a mmoni um s ul fa te i n a mi xture of protei ns i s gra dua l l y i ncrea s ed, res ul ti ng i n grea ter bi ndi ng of wa ter by the i ncrea s i ng i on concentra ti on. As the i ncrea s i ng number of i ons dra w the wa ter a wa y from the protei ns , i ts a bi l i ty to keep them i n s ol uti on decrea s es . Protei ns ma y then s ta rt to a ggrega te together to protect thei r hydrophi l i c a nd/or hydrophobi c porti ons . If the proces s conti nues , the protei ns wi l l come out of the s ol uti on (preci pi ta te). The pa rti cul a r hydrophi l i c/hydrophobi c cha ra cteri s ti cs of ea ch protei n wi l l ma ke i t a nd i ts i denti ca l mol ecul es preci pi ta te a t the s a me ti me, res ul ti ng i n s epa ra ti on. As ea ch protei n ha s a s et s a l t concentra ti on a t whi ch i t preci pi ta tes , thi s techni que ca n be ea s i l y reproduced a s pa rt of a mol ecul a r puri fi ca ti on proces s . Other techni ques us i ng orga ni c, pol ymeri c, a nd meta l s a l t s ol uti ons , i ncl udi ng ones tha t ta ke a dva nta ge of the overa l l pH of a protei n (a s determi ned by i ts a mi no a ci d R-groups ), a re us ed i n a s i mi l a r fa s hi on. Us ua l l y, the protei ns ca n be redi s s ol ved a nd ca n reta i n thei r bi ol ogi ca l functi ons . DNA a nd RNA mol ecul es (Cha pter 4) ca n be s i mi l a rl y preci pi ta ted us i ng ethanol or isopropanol vi a a s i mi l a r mecha ni s m. Wa ter norma l l y s urrounds thes e mol ecul es , i ntera cti ng wi th the cha rged phos pha te ba ckbone a nd keepi ng the DNA or RNA i n s ol uti on. The gra dua l a ddi ti on of etha nol (us ua l l y a bout 64% etha nol /wa ter) bl ocks the wa ter–phos pha te ba ckbone i ntera cti ons a nd l ea ds to preci pi ta ti on of the DNA or RNA. Sma l l er nucl ei c a ci d mol ecul es ma y requi re l onger expos ure to etha nol or other techni ques to ful l y preci pi ta te. After preci pi ta ti on, centri fuga ti on i s us ed to bri ng the i ns ol ubl e nucl ei c a ci d mol ecul es together i n a s ma l l ma s s , ca l l ed a “pel l et,” a nd the rema i ni ng s ol uti on ca n be removed. The DNA or RNA ca n be res us pended i n a n a queous s ol uti on a nd ca n norma l l y reta i n ful l bi ol ogi ca l a cti vi ty. Etha nol preci pi ta ti on ca n a l s o be us ed for pol ys a ccha ri des (Cha pter 2) or metha nol for protei ns (Cha pter 1).
DNA AND RNA SEQUENCING DNA a nd RNA s equenci ng ha ve a dva nced the unders ta ndi ng of genes a nd gene products exponenti a l l y s i nce thei r devel opment. Further a dva nces a nd refi nement of the DNA sequencing technol ogy a l l owed determi na ti on of the compl ete huma n genome i n 2003, a ta s k tha t unti l then wa s deemed a l mos t i mpos s i bl e. Sequenci ng of the nucl eoti des of DNA a nd RNA rel y on ma ny of the s a me bi ochemi ca l techni ques a l rea dy di s cus s ed a bove. The Ma xa m–Gi l bert method of s equenci ng wa s devel oped i n the 1970s a nd i t rel i es on ra di oa cti ve l a bel i ng a nd chemi ca l cl ea va ge of the DNA bei ng exa mi ned. The l a bel ed fra gments a re s epa ra ted by gel el ectrophores i s , a nd the s equence i s i nterpreted from the res ul ti ng X-ra y fi l m. Al though thi s s equenci ng techni que ha s fa l l en out of fa vor i n compa ri s on wi th the Sa nger techni que (s ee bel ow), i t i s s ti l l us ed i n s peci a l ty a ppl i ca ti ons i ncl udi ng DNA footprinting, i n whi ch the i ntera cti on of DNA a nd DNA-bi ndi ng protei ns ca n be determi ned. In 1975, the Sa nger techni que, often referred to a s “cha i n-termi na ti on” method, wa s devel oped, a nd i t i s s ti l l us ed, wi th s ome i mprovements , toda y. The key to thi s method i s the a ppl i ca ti on of dideoxynucleotide triphosphates (ddATP, ddGTP, ddCTP, a nd ddTTP), whi ch l a ck 5′ and 3′-hydroxyl (OH) group a nd, therefore, bl ock the a ddi ti on of further nucl eoti des . Thi s cha i n termi na ti on by the di deoxynucl eoti des res ul ts i n DNA fra gments of va ri ous l engths , whi ch ca n be s epa ra ted by gel el ectrophores i s . More s peci fi ca l l y, the DNA to be s equenced i s reduced to a s i ngl e-s tra nd DNA templ a te by hea t dena tura ti on. The s a mpl e i s di vi ded i nto four pa rts to whi ch DNA pol ymera s e a nd a l l four of the regul a r dATP, dGTP, dCTP, a nd dTTP nucl eoti des a re a dded a l ong wi th one of the di deoxynucl eoti des per s a mpl e. The DNA pol ymera s e us es the s i ngl e-s tra nd templ a te to form a s econd s tra nd. At va ri ous ti mes , the di deoxynucl eoti de ends DNA repl i ca ti on, but onl y a t the poi nt where tha t di deoxynucl eoti de i s a dded to the growi ng s tra nd. As a res ul t, mul ti pl e l ength s tra nds , a l l endi ng i n the s a me di deoxynucl eoti de, a re produced. When the ddATP, ddGTP, ddCTP, a nd ddTTP s a mpl es a re s epa ra ted by gel el ectrophores i s , ea ch on i ts own gel l a ne, a n ea s i l y rea d s equence of fra gments di rectl y denotes the DNA s equence (s ee bel ow).
(A) DNA to be sequenced is reduced to single strands by heat denaturization and used to produce four separate samples, G, A, T and C, with DNA polymerase, deoxynucleotides, and one of the four dideoxynucleotides (ddGTP, ddATP, ddTTP, or ddCTP), respectively. (B) The DNA polymerase produces DNA strands of varying length, each terminated by the specific dideoxynucleotide. (C) These strands are separated by gel electrophoresis and the sequence read from bottom (shortest fragment) to top (longest fragment) from the gel (example sequence shown on gel: AGTCTTGGAGCT). [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] Sta nda rd DNA s equenci ng a l l ows res ea rchers to determi ne the nucl eoti de code for DNA up to 1,000 ba s es l ong. Other techni ques a l l ow s i mpl er, fa s ter, a nd more compl ex s equenci ng. DNA res tri cti on endonucl ea s e enzymes , often ca l l ed “restriction enzymes,” cut DNA a t very s peci fi c nucl eoti de s equences . The us e of di fferent res tri cti on enzymes , ea ch cutti ng the DNA a t very di fferent poi nts , produces di fferent but overl a ppi ng fra gments of the s a me DNA mol ecul e. Sequenci ng of ea ch res tri cti on enzyme fra gment a nd s ubs equent compa ri s on a nd a ppropri a te overl a p of the s equences a l l ow cha ra cteri za ti on of DNA tha t ma y be too l ong for the s ta nda rd techni que. Va ri a ti ons i n l a bel i ng
the fra gments , i ncl udi ng a techni que i n whi ch the di deoxynucl eoti des a re l a bel ed, a nd techni ques for better fra gment prepa ra ti on a nd s epa ra ti on ha ve a l s o a l l owed ea s i er a nd even ma chi ne-a utoma ted DNA s equenci ng of l onger a nd l onger nucl eoti des cha i ns a pproa chi ng 5,000 ba s es . Thes e i mproved techni ques were cri ti ca l i n the a bi l i ty to s equence the huma n genome. RNA sequencing rel i es on the a bi l i ty to us e reverse transcriptase to produce a doubl e-s tra nded DNA mol ecul e (often referred to a s complementary DNA or cDNA) from a n RNA s equence of i nteres t. Fol l owi ng the producti on of the a na l ogous DNA mol ecul e, s equenci ng i s done a s des cri bed a bove a nd then i nterpreted for the RNA mol ecul e. However, RNA s equenci ng i s more probl ema ti c tha n DNA s equenci ng beca us e of the fa ct tha t RNA i s much l es s s ta bl e. Si mi l a r techni ques a re us ed to produce a cDNA l i bra ry—a col l ecti on of DNA tha t codes for a l l mes s enger RNA (mRNA) conta i ned i n a pa rti cul a r cel l type. cDNA l i bra ri es a re us ua l l y es ta bl i s hed for s i mpl e orga ni s ms wherei n the number of mRNA mol ecul es i s l i mi ted. cDNA l i bra ri es a re a l s o us ed i n cloning (s ee bel ow).
SOUTHERN, NORTHERN, AND WESTERN BLOTS DNA Southern a nd RNA northern bl ots a re techni ques combi ni ng PAGE a nd a va ri a ti on of TLC/pa per chroma togra phy (des cri bed a bove). Southern bl otti ng wa s devel oped by Dr. Ed Southern a nd i s na med i n hi s honor. The s ubs equent a da pta ti on of the techni que to RNA a nd protei ns (des cri bed bel ow) res ul ted i n the a na l ogous na mes of northern a nd wes tern bl otti ng. A method wa s a l s o devel oped to detect pos ttra ns l a ti ona l modi fi ca ti ons to protei ns (eastern blotting), a l though the term ha s fa l l en s omewha t out of fa vor. SOUTHERN BLOTTING Southern blotting i s us ed to i denti fy a nd i s ol a te a s peci fi c DNA s equence a nd ca n a l s o qua nti ta ti vel y determi ne the number of ti mes a s peci fi c s equence occurs i n a pa rti cul a r cel l type (e.g., how ma ny repea ts a nd/or copi es of a gene i n a pa rti cul a r orga ni s m or cel l type of i nteres t). The procedure begi ns wi th i s ol a ti on of DNA vi a preci pi ta ti on a nd col umn puri fi ca ti on techni ques fol l owed by expos ure to res tri cti on enzymes , whi ch cut the l ong DNA mol ecul e i nto s ma l l er a nd more ma na gea bl e pi eces . The DNA i s next el ectrophores ed for s i ze s epa ra ti on on a n a ga ros e gel . The gel i s next removed from the gel el ectrophores i s equi pment a nd the DNA i s tra ns ferred from the gel to a pi ece of nylon membrane whos e pos i ti ve cha rge bi nds the nega ti vel y cha rged DNA mol ecul es (e.g., phos pha te groups of the ba ckbone). The tra ns fer ta kes pl a ce vi a s i mpl e ca pi l l a ry a cti on by pl a ci ng the membra ne on top of the gel i n a s ui ta bl e tra ns fer buffer. After tra ns fer, hea t or UV ra di a ti on i s a ppl i ed to the membra ne to cova l entl y a tta ch the DNA mol ecul es to the membra ne, a nd hybri di za ti on probes (DNA, RNA, or s yntheti c ol i gonucl eoti des ) wi th ra di oa cti ve, fl uores cent, or chemi ca l detecti on ca pa bi l i ti es a re us ed to i denti fy a s peci fi c DNA s tra nd s equence.
The basic blotting techniques (Southern, northern, and western) are illustrated, including gel separation, transfer to paper, and detection. The techniques of northern and western blotting are further explained in the text (see below). The specific DNA, RNA, or protein of interest is detected and identified by any one of a variety of techniques discussed above. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.] DNA mol ecul es , s epa ra ted on a gel a nd pri or to tra ns fer to the nyl on membra ne, ca n be vi s ua l i zed us i ng the chemi ca l ethidium bromide (EtBr). EtBr revers i bl y fi ts or “intercalates” between nucl eoti de ba s e pa i rs a nd becomes ca pa bl e of fl uores ci ng i f i l l umi na ted by UV l i ght. Beca us e the i nterca l a ti on of EtBr i s revers i bl e, i denti fi ed a nd functi ona l DNA ca n be col l ected by cutti ng out the pa rti cul a r ba nd of i nteres t from the a ga ros e or pol ya cryl a mi de gel . Interca l a ti on a l ters the s econda ry a nd terti a ry s tructures of the DNA mol ecul e a nd, therefore, DNA repl i ca ti on a nd expres s i on. As a res ul t, i nterca l a ti on of DNA i s i nvol ved i n the a cti on of s evera l chemothera peuti c drugs (e.g., doxorubi ci n, da unorubi ci n, a nd da cti nomyci n). For the s a me rea s on, EtBr ca n be a potent ca rci nogen i f not ha ndl ed correctl y.
NORTHERN BLOTTING Northern blotting a l l ows res ea rchers to determi ne the l evel of producti on (a nd, therefore, control of expres s i on) of a pa rti cul a r mRNA by qua nti ta ti ve determi na ti on of the rel a ti ve a mount of tha t pa rti cul a r mRNA rel a ti ve to tota l mRNA. Va ryi ng i nfl uences on both tota l a nd s peci fi c mRNA expres s i ons s uch a s devel opment, di s ea s es (e.g., ca ncers ), a nd/or experi menta l techni ques (i ncl udi ng drug devel opment) ca n, therefore, be a s certa i ned. Northern bl otti ng s ta rts wi th the i s ol a ti on of RNA from a cel l wi th a ppl i ca ti on of a ti s s ue or cel l s a mpl e to a n oligo (dT) cellulose affinity column, whi ch wi l l bi nd onl y to the polyadenosine nucleotide tails found i n RNAs (s ee Cha pter 9), wherea s other mol ecul es run through the col umn unbound. Other techni ques i ncl udi ng chemi ca l extra cti on a nd ma gneti c bea d i s ol a ti on a re a l s o us ed. The i ni ti a l s epa ra ti on/puri fi ca ti on of the RNA provi des a ma rkedl y puri fi ed s a mpl e of RNA. The RNAs a re then a ppl i ed to a n a ga ros e gel wi th forma l dehyde a dded to promote l i nea ri za ti on of a l l RNA mol ecul es (s ee fi gure a bove). PAGE gel s wi th urea a dded ca n a l s o be us ed for s ma l l er RNA s equences . The RNA on the gel i s then tra ns ferred to a nyl on membra ne whos e pos i ti ve cha rge bi nds the nega ti vel y cha rged RNA mol ecul es (e.g., phos pha te groups of the ba ckbone). Thi s tra ns fer i s a ccompl i s hed i n a ma nner a na l ogous to Southern bl otti ng (s ee a bove) a nd then hea t or UV ra di a ti on i s a ppl i ed to the membra ne to crea te cova l ent l i nka ges between the RNAs a nd the nyl on. Much l i ke Southern bl ot detecti on, s peci fi c “hybridization probes” (DNA, RNA, or a rti fi ci a l l y cons tructed ol i gonucl eoti des ) of known s equence a nd l a bel ed ra di oa cti vel y, fl uores centl y, or wi th enzymes for chemi ca l detecti on a re then us ed to i denti fy pa rti cul a r RNA s tra nds of i nteres t. The us e of northern bl otti ng for the qua nti ta ti on of mRNA a bunda nce ha s been ma i nl y repl a ced to a grea t extent by qua nti ta ti ve revers e-tra ns cri pti on pol ymera s e cha i n rea cti on (RT-PCR) (des cri bed bel ow), whi ch i s much more s ens i ti ve a nd rel i a bl e. WESTERN BLOTTING Western blotting i s a bi ochemi ca l techni que, a da pted from a s i mi l a r techni que for DNA of Southern bl otti ng, for detecti on a nd i denti fi ca ti on of s peci fi c protei ns . Li ke Southern a nd northern bl otti ng, wes tern bl ots ta ke a dva nta ge of the s a me i mmunoa s s a y techni ques des cri bed a bove, coupl ed wi th thos e of PAGE s epa ra ti on. Wes tern bl otti ng begi ns wi th a PAGE gel (one or two di mens i ona l ) to s epa ra te a mi xture of protei ns (s ee the fi gure a bove). The uns ta i ned gel i s removed from the gel el ectrophores i s equi pment a nd pl a ced i n a s peci a l a ppa ra tus tha t a l l ows tra ns fer of the protei ns , a ga i n us i ng a n el ectri c current, from the gel to a pi ece of nitrocellulose or polyvinylidene difluoride membra ne. Thi s membra ne s trongl y bi nds to a nd i mmobi l i zes the protei ns i n ea ch ba nd s o they ca nnot di ffus e, thereby ma i nta i ni ng the ori gi na l s epa ra ti on whi l e bl otti ng a nd i denti fyi ng the s a mpl e. The membra ne ca n then be us ed for detecti on of a s peci fi c protei n us i ng a nti bodi es a s di s cus s ed i n the Immunoassays s ecti on a bove. Detecti on i s a ga i n vi a enzyme-l i nked, fl uores cent ta g, ra di oa cti ve-l a bel , or other methods a s di s cus s ed a bove.
PCR PCR i s a powerful techni que devel oped i n the 1980s to produce mul ti pl e copi es (i .e., thous a nds to mi l l i ons ) of a pa rti cul a r DNA s equence. PCR rel i es on repeti ti ve cycl es of (a ) pa rti a l “mel ti ng” of doubl e-s tra nded DNA, (b) a ppl i ca ti on of s hort DNA primer s equences (two pri mers s peci fi c for one pa rti cul a r gene, but for DNA repl i ca ti on i n both the 5′ → 3′ a nd 3′ → 5′, s ens e a nd a nti s ens e di recti ons ), a nd (c) a ddi ng DNA polymerase a nd a l l four DNA nucl eoti des (dA, dG, dC, a nd dT) to copy tha t gene. The proces s ends wi th cool i ng of the DNA to a l l ow rea nnea l i ng of the doubl e-s tra nded DNA. Subs equent mel ti ng/pri mer, pol ymera s e, a nd nucl eoti des /cool i ng cycl es a l s o a ct on a ny newl y produced DNA, res ul ti ng i n a n exponenti a l a mpl i fi ca ti on of the DNA pri mer’s gene ta rget wi th every cycl e. An a ga ros e gel wi th EtBr i s often us ed to veri fy the a mpl i fi ca ti on. PCR i s us ed i n a va s t number of va ryi ng mol ecul a r bi ol ogi ca l a nd medi ca l techni ques , i ncl udi ng a mpl i fi ca ti on of DNA cl ones (s ee bel ow) a nd/or hybri di za ti on probes for northern or Southern bl otti ng (s ee a bove). In a ddi ti on, PCR ena bl es ea s i er gene a na l ys i s of orga ni s ms a nd ti s s ues /cel l s , geneti c determi na ti ons to i ncl ude the s tudy of geneti c di s ea s es a nd pa terni ty/heredi ta ry tes ti ng, detecti on of l ow l evel s of i nfecti ve ba cteri a l or vi ra l orga ni s ms , a nd forens i c determi na ti on of i ndi vi dua l s a t cri me s cenes (geneti c fi ngerpri nti ng) when onl y tra ce a mounts of a DNA s a mpl e a re a va i l a bl e. Quantitative PCR a l l ows the determi na ti on of rel a ti ve a mounts of DNA s tra nds a nd further extends PCR’s di a gnos ti c a nd s ci enti fi c ca pa bi l i ti es a nd a ppl i ca ti ons . Fi na l l y, the techni que of RT-PCR a l l ows the a mpl i fi ca ti on of DNA from RNA s tra nds by ma ki ng a cDNA copy us i ng revers e tra ns cri pta s e. Thi s ena bl es the qua nti fi ca ti on of mRNA a bunda nce for s peci fi c genes , a nd ha s l a rgel y repl a ced northern bl otti ng a s the method of choi ce for mRNA determi na ti ons .
Each PCR cycle includes DNA denaturization of a segment of double-stranded DNA with a segment to be amplified to produce a mixture of single-stranded DNA, annealing with specific DNA primers, and, finally, polymerization by the addition of polymerase and nucleotides (G, C, A, and T). Prior to the start of the next sample, the replicated DNA is allowed to reanneal to again form double-stranded structures. Further cycles will exponentially reproduce the DNA strand of interest. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.]
CLONING Cloning i s a term us ed i n two s ens es ; fi rs t, the proces s to produce i denti ca l copi es of a n orga ni s m a nd, s econd, for the copyi ng of DNA s equences . Onl y the l a tter wi l l be di s cus s ed here. The a bi l i ty to cl one a DNA fra gment a l l ows res ea rchers to produce mul ti pl e copi es of a s el ected gene or porti on of DNA (e.g., vi a PCR), but a l s o a l l ows them to s tudy nonexpres s ed porti ons of DNA, i ncl udi ng promoter a nd regul a ti on regi ons a nd DNA tha t i s not expres s ed. Indeed, the a bi l i ty to s el ect a nd a mpl i fy a ny pa rti cul a r DNA s equence for s tudy i s a n es s enti a l pa rt of the s ci ence of mol ecul a r bi ol ogy. The ba s i c a pproa ch to cl oni ng of a ny pi ece of DNA i ncl udes (a ) cutti ng the DNA i nto a n a ppropri a tel y s i zed fra gment vi a res tri cti on enzymes ;
(b) i ns erti ng the DNA i nto a nother s peci a l pi ece of DNA ca l l ed a vector, whi ch conta i ns a n a ppropri a te promoter regi on to a l l ow DNA repl i ca ti on a nd/or tra ns cri pti on; (c) i ntroduci ng (transfection) thes e DNA fra gment/vector DNA mol ecul es i nto a n a ppropri a te cel l type for a mpl i fi ca ti on of the DNA; a nd, fi na l l y, (d) s creeni ng the res ul ti ng cel l s to fi nd a nd confi rm the i denti ty of the DNA fra gment of i nteres t vi a a reporter or ma rker gene (e.g., one tha t produces a nti bi oti c res i s ta nce or a n enzyme produci ng a col ored s ubs tra te). Once a cl one i s i s ol a ted a nd confi rmed, i t then offers a powerful tool for the res ea rcher a s noted a bove.
[Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] The s i ze of the DNA fra gment tha t ca n be i ntroduced i nto a cl oni ng vector a nd s ti l l a l l ow the vector to functi on i s l i mi ted. As a res ul t, the fra gmenta ti on of a l a rge DNA to a s ui ta bl e l ength i s a n i mporta nt fi rs t s tep i n cl oni ng. Thi s fra gmenta ti on ca n be done us i ng very s peci fi c or ra ndom res tri cti on enzymes . Cl oni ng vectors a re s peci fi c, ci rcul a r pi eces of DNA (plasmids) tha t ma y ha ve ori gi na ted from ba cteri a or vi rus es or tha t ha ve been a rti fi ci a l l y produced. They ha ve the a bi l i ty to enter i nto cel l s a nd to be expres s ed i n tha t cel l , ei ther s epa ra tel y or by i ncorpora ti ng thems el ves i nto tha t cel l ’s DNA. Cl oni ng vectors a l s o ha ve s peci fi c DNA s equences where known res tri cti on enzymes ca n be uti l i zed to cut open the vector a nd i ns ert DNA fra gments under wel l -control l ed l a bora tory condi ti ons . Therefore, a s peci fi c res tri cti on enzyme i s us ed to cut open the vectors ; newl y fra gmented DNA s equences of i nteres t a re a dded a nd DNA l i ga s e repa i rs the vector, whi ch now i ncl udes the DNA fra gment. Tra ns fecti on of thes e cl oni ng vectors i nto cel l hos ts i s performed by va ri ous techni ques tha t wi l l not be di s cus s ed here.
Vector DNA (left) can be incorporated into human DNA (right) by the use of specific restriction enzymes (e.g., BamH1 or EcoR1) that produce DNA fragments with particular end, base-pair sequences that can subsequently be joined by DNA ligase. The ability to produce specific recombinant DNAs is an important research and diagnostic tool. [Reproduced wi th permi s s i on from Na i k P: Bi ochemi s try, 3rd edi ti on, Ja ypee Brothers Medi ca l Publ i s hers (P) Ltd., 2009.] Beca us e the proces s ma y i nvol ve s omewha t ra ndom fra gmenta ti on of the DNA a nd tra ns fecti on i s often of l ow effi ci ency, a s creeni ng method mus t be us ed to i denti fy thos e cel l s tha t conta i n a functi oni ng cl one. The cl oni ng vectors wi l l often expres s a pa rti cul a r ma rker (e.g., a n enzyme tha t produces a col ored product, a s des cri bed a bove, or confers res i s ta nce to a n a nti bi oti c) tha t a l l ows the res ea rcher to i denti fy the cel l s wi th thes e worki ng cl one vectors . Fi na l l y, cel l s tha t a ppea r to conta i n the DNA fra gment of i nteres t i n a functi ona l vector ma y be us ed for PCR a mpl i fi ca ti on (s ee a bove) a nd DNA s equence a na l ys i s (s ee a bove) of the cl one/DNA fra gment to confi rm tha t a s ucces s ful cl one ha s been produced.
FLOW CYTOMETRY Flow cytometry, devel oped i ni ti a l l y i n 1968, i s a n i mporta nt s ci enti fi c a nd medi ca l res ea rch a ppl i ca ti on tha t ha s a l s o begun to ga i n a rol e i n cl i ni ca l trea tments . Thi s method ta kes a dva nta ge of fl uores cent l a bel i ng of a nti bodi es (a s uti l i zed i n s evera l procedures a bove) a nd the a bi l i ty to exci te thes e fl uores cent mol ecul es vi a l a s ers . Exci ta ti on a l l ows the detecti on (vi a a di gi ta l detector) of i ndi vi dua l cel l s or mol ecul es (e.g., chromos omes ) a t a n extremel y fa s t ra te (up to thous a nds of pa rti cl es / s econd). Pa rti cl es (i .e., cel l s or l a rge mol ecul es ) ca n a l s o be detected by l i ght s ca tteri ng (often us ed to determi ne cel l type) a t the s a me ti me. Mul ti pl e l a s ers a nd mul ti pl e detectors ca n be a rra nged s o a s to a l l ow the opera tors to s tudy cel l s l a bel ed wi th s evera l a nti bodi es /fl uores cent probes . Fl ow cytometers ha ve been us ed to s tudy chromos omes , DNA, RNA, protei ns , cel l a cti va ti on, enzyme a cti vi ty, pH, i ntra cel l ul a r concentra ti on of s el ected i ons , membra ne fl ui di ty a nd a poptos i s , a nd mul ti drug-res i s ta nt ca ncer cel l s . Medi ca l fi el ds s uch a s hema tol ogy, oncol ogy, pa thol ogy, geneti cs , a nd other s peci a l ti es uti l i ze fl ow cytometry extens i vel y. More a dva nced fl ow cytometry even a l l ow s orti ng of ea ch i ndi vi dua l cel l or mol ecul e ba s ed upon thei r s ca tteri ng a nd/or l a bel ed fl uores cence cha ra cteri s ti cs .
A reporter gene containing a particular enzyme [e.g., chloramphenicol transferase (CAT) whose activity can be easily detected by established testing methods] is recombined with a test gene of interest, precipitated, and, finally, transfected into cells. The cells are cultured, then harvested and assayed for CAT activity to identify cells that have received a copy of the test gene. [Ada pted wi th permi s s i on from Murra y RA, et a l .: Ha rper’s Il l us tra ted Bi ochemi s try, 28th edi ti on, McGra w-Hi l l , 2009.]
Principles of Flow Cytometry See text for further details.
LABORATORIES Cl i ni ca l l a bora tory tes ti ng rel i es on s evera l bi ochemi ca l techni ques for the a s s a y of i mporta nt phys i ol ogi ca l mol ecul es found i n the huma n body. The l i s t/ta bl e bel ow revi ews thes e techni ques for s evera l common tes ts . BIOCHEMICAL BASIS FOR CLINICAL LABORATORY TESTS (CHEMISTRIES)
APPENDIX III ORGANIC CHEMISTRY PRIMER OVERVIEW Know a nd be fa mi l i a r wi th the s peci fi c rol es of the s i x i mporta nt el ements i n l i vi ng orga ni s ms . Thes e a re: • Ca rbon • Hydrogen • Ni trogen • Oxygen • Phos phorous • Sul fur Be fa mi l i a r wi th the i mporta nt functi ona l groups tha t a re formed by the s i x ba s i c el ements a nd unders ta nd thei r genera l s tructura l a nd functi ona l rol es i n bi ol ogy a nd medi ci ne. • Hydrogen (pa rti a l l y cha rged a nd i on) • Hydroxyl group • Ca rboxyl i c a ci d group • Ami ne group • Phos pha te group • Sul fur-s ul fur bond • Al dehyde group • Ketone group
INTRODUCTION Al though there a re over 100 el ements known, onl y a few a re regul a rl y s een i n na ture. In fa ct, l es s tha n a thi rd of the el ements found on ea rth a re found i n s ome l i fe form, wi th onl y s i x el ements s een i n a l l l i vi ng orga ni s ms . Thes e el ements a re ca rbon (C), hydrogen (H), ni trogen (N), oxygen (O), phos phorus (P), a nd s ul fur (S). Thes e s i x “bi ol ogi ca l ” el ements ca n be remembered a s C—H—N—O—P—S tha t s ounds l i ke “chi nups .” Thes e s i x el ements ca n bond wi th thems el ves (e.g., C—C a nd O—O) a nd wi th ea ch other (C—H, O—H, a nd C—N). Speci a l bondi ng pa tterns l ea d to cha rged H a toms a s wel l a s hydroxyl , ca rboxyl , a mi ne, hi gh-energy phos phorus , a nd s trong s ul fi de-bonded groups . Thes e mol ecul es s erve s tructura l a nd functi ona l rol es a s wel l a s s peci a l i zed rol es i n very s peci fi c bi ol ogi ca l a cti ons a nd rea cti ons for functi ons i n the huma n body (Ta bl e 1).
TABLE 1. The Si x Ba s i c El ements of Li fe
THE SIX ORGANIC ELEMENTS (C, H, N, O, P, AND S) CARBON (C) Ma i n s tructura l el ement of a l l l i vi ng ti s s ue, C us ua l l y forms four bonds wi th other el ements . C—C bonds (e.g., s uga r or fa t mol ecul es ) conta i n energy us ed i n meta bol i s m. C i s found i n a l mos t a l l bi ol ogi ca l mol ecul es . HYDROGEN (H) Hydrogen ca n exi s t wi th both a pa rti a l cha rge a nd a ful l i oni c cha rge, ma ki ng i ts rol es both va ri ed a nd vers a ti l e. H i s a n i mporta nt s tructura l el ement but i s often a l s o s een i n a cces s ory rol es , hel pi ng to form i mporta nt functi ona l groups a nd mol ecul a r s tructures . Functi ona l l y, H ca n tra ns fer from one mol ecul e to a nother or be by i ts el f a s H + (s ee next s ecti on). H forms one bond wi th a nother el ement. H i s found i n mos t bi ol ogi ca l mol ecul es . NITROGEN (N) Invol ved i n ma ny energy tra ns fers a nd s peci a l i zed s tructures . N us ua l l y forms three or four bonds wi th other el ements . N i s i mporta nt i n bi ol ogi ca l mol ecul es i ncl udi ng a mi no a ci ds / protei ns , compl ex ca rbohydra tes , a nd l i pi ds a nd i n nucl ei c a ci ds . OXYGEN (O) Invol ved i n ma ny energy-rel a ted rea cti ons . O us ua l l y forms two bonds wi th other el ements . O i s found i n a l mos t a l l bi ol ogi ca l mol ecul es i ncl udi ng a mi no a ci ds /protei ns , ca rbohydra tes , l i pi ds a nd nucl ei c a ci ds , a s wel l a s mul ti pl e other chemi ca l a nd bi ol ogi ca l mol ecul es . PHOSPHORUS (P) Invol ved i n energy s tora ge. P us ua l l y forms four bonds wi th other el ements . P i s mos t i mporta ntl y found i n deoxyri bonucl ei c a ci d (DNA) a nd ri bonucl ei c a ci d (RNA) a nd the nucl eoti des tha t form them. SULFUR (S) Invol ved i n s peci a l i zed s tructures . S us ua l l y forms two bonds wi th other el ements . S i s found i n two a mi no a ci ds a nd i s a l s o s een i n compl ex ca rbohydra tes a nd l i pi ds .
BIOCHEMICAL FUNCTIONAL GROUPS (H, OH, COOH, NH3, PO3, S—S, COH, AND
)
From H bondi ng i n protei ns a nd DNA to S—S cova l ent bondi ng, thes e s i x a toms ca n combi ne, ei ther a l one or wi th tra ce i norga ni c el ements a nd cofa ctors , to form s evera l bi ol ogi ca l l y functi ona l groups tha t a re the key to l i fe. Some of the es s enti a l functi ona l groups a re di s cus s ed bel ow a nd s umma ri zed i n Ta bl e 2.
TABLE 2. Bi ochemi ca l Functi ona l Groups HYDROGEN (PARTIALLY CHARGED AND IONIC FORMS, H +) Al though H pl a ys a n i mporta nt rol e i n i ts noni oni zed form, i ts further rol e i n l i vi ng crea tures i s fa r too i mporta nt to not menti on here. H a toms i nvol ved i n bonds often unfa i rl y s ha re the energy of the bond. When thi s ha ppens , the a toms formi ng tha t bond wi l l be ei ther pa rti a l l y nega ti ve or pa rti a l l y pos i ti ve. H often a cqui res a pa rti a l pos i ti ve cha rge, whi ch s erves a s the ba s i s for H bondi ng a nd the s ta bi l i za ti on of protei n s tructure, DNA’s hel i ca l form, a nd even bi ol ogi ca l membra nes . If the H compl etel y l os es i ts el ectron, i t becomes a H i on (H +) wi th a cha rge of +1 a nd goes i nto s ol uti on. H + i s a n ea s i l y tra ns ferra bl e a tom tha t hel ps dri ve a myri a d of bi ol ogi ca l rea cti ons tha t a re l i tera l l y too numerous to menti on. The rol e of H, both pa rti a l l y cha rged a nd a s a n i on, i s expl ored i n va ri ous cha pters . HYDROXYL GROUP (—OH –) The pa i ri ng of a n O a nd a H, referred to a s a hydroxyl group (OH −), forms a n i mporta nt mol ecul a r component tha t pl a ys numerous rol es i n bi ol ogy. Wa ter (H 2 O, a l s o known a s H—O—H) ha s a OH − a nd a s epa ra te H + a nd thes e nega ti ve a nd pos i ti ve cha rges hel p wa ter to di s s ol ve other mol ecul es a nd s erve i n bi ol ogi ca l functi ons . The OH – a l s o combi nes wi th other functi ona l groups to form protei ns , DNA a nd RNA, a nd functi ona l membra nes , a nd often s erves a s the es s enti a l group i n a n enzyme a cti vi ty. When a OH – i s on a ny bi ol ogi ca l mol ecul e, i ts i mporta nce, ei ther s tructura l l y or functi ona l l y, i s us ua l l y hi gh. CARBOXYL GROUP (—COOH)
A C wi th a doubl e-bonded O a nd a s epa ra tel y bonded OH i s a ca rboxyl group (COOH). The H pa rti a l l y di s s oci a tes from thi s group formi ng COO– a nd H + both of whi ch a re rea cti ve i n thei r own ri ght. The COOH, much l i ke the OH – di s cus s ed a bove, i s i nvol ved i n numerous bi ol ogi ca l rea cti ons i ncl udi ng forma ti on of a mi no a ci d to a mi no a ci d bonds ; forma ti on of the DNA a nd RNA ba ckbone s tructure; the meta bol i s m of s uga rs ; a nd the forma ti on of compl ex l i pi d s tructures conta i ned i n membra nes , hormones , a nd other s peci a l i zed l i pi d-ba s ed mol ecul es . If a COOH i s on a mol ecul e, i mporta nt s tructura l or functi ona l rea cti ons often occur. AMINE GROUP (—NH 2) A N a nd two H pl us a n a l kyl group form the i mporta nt bi ol ogi ca l group referred to a s a pri ma ry a mi ne. Bonds to one or two a ddi ti ona l a l kyl groups (e.g., C) i ns tea d of H crea te a s econda ry a nd terti a ry a mi ne, res pecti vel y. When converted to a n a mi no group, the a mi ne wi l l often ha ve a pa rti a l pos i ti ve cha rge (NH 3 +). Wi th a cha nge i n the bondi ng of the N a tom, a n a mi ne group ca n s epa ra te a nd become a mmoni a (NH 3 ), whi ch i s i mporta nt i n protei n a nd a mi no a ci d wa s te el i mi na ti on (i .e., uri ne). Ammoni a , es peci a l l y i n exces s , a l s o s hows up i n ma ny i l l nes s es a s a n i mporta nt cl ue i n di a gnos i s a nd trea tment. Ami ne groups a re a nother cl ue to the l oca ti on of i mporta nt bi ol ogi ca l proces s es . The a ddi ti on of a fourth H a tom to the a mi no group forms the pos i ti vel y cha rged, ca ti on a mmoni um (NH +4 ), a l s o i mporta nt i n ma ny rea cti ons a nd medi ca l condi ti ons i n huma ns . PHOSPHATE GROUP (PO3 AND PO4) The phos pha te group, compos ed of one P a nd ei ther three or four O, i s the energy-ca rryi ng mol ecul e of bi ol ogy, s peci fi ca l l y when l i nked to a nother phos pha te group. The ma jor functi ona l component of a denos i ne tri phos pha te, the phos pha te group i s a l s o i nvol ved i n formi ng the ba ckbone s tructure of DNA a nd RNA, a nd i s s een i n ma ny extra cel l ul a r s i gna l i ng proces s es tha t i ni ti a te DNA s ynthes i s a nd the producti on of s peci fi ed protei ns . SULFUR–SULFUR BONDS (—S—S—) Two S a toms ca n l i nk together wi th one bond to form one of the s tronges t l i nka ges i n bi ol ogy. S—S bonds a re i mporta nt i n ma ny protei n s tructures tha t need to ha ve hi gh res i s ta nce to brea ka ge. Whi l e S—S bonds a re not a s preva l ent a s the other groups l i s ted a bove, they a re very i mporta nt i n certa i n s tructures found i n both hea l thy a nd di s ea s ed pers ons . ALDEHYDE GROUP (—COH) Li ke COOH, the a l dehyde group (COH) conta i ns a doubl e-bonded O, but the rema i ni ng bond i s to a H a tom. Al though not a s rea cti ve a s the COOH, COHs a re s ti l l i mporta nt s tructura l a nd functi ona l groups i n huma n bi ol ogy. KETONE (—
)
The ketone group ( ) conta i ns a doubl e-bonded O l i ke the COOH a nd COH, but i ts C i s bonded onl y to two other C. Al though thi s el i mi na tes the rea cti ve OH – found i n the COOH, i t a l s o ma kes thi s group very rea cti ve beca us e of a n i nequa l i ty of cha rge between the C a nd O a toms . As a res ul t, the C i s very s us cepti bl e to bondi ng wi th a number of other a toms . a re s een i n mul ti pl e bi ol ogi ca l mol ecul es i mporta nt to ma n, i ncl udi ng a cetone, a cetoa ceta te, a mi no a ci ds , ca rbohydra tes , fa tty a ci ds , a nd ketone bodi es .
SUMMARY Si x i mporta nt a tomi c el ements —C, H, O, N, P, a nd S—a re found i n every l i vi ng crea ture a nd form the s tructure of bi ol ogi ca l mol ecul es . In a ddi ti on, thes e el ements form i mporta nt functi ona l groups , i ncl udi ng H (pa rti a l l y cha rged a nd i oni c), OH −, COOH, a mi ne, P, S—S, COH, a nd . Unders ta ndi ng thes e s i mpl e s tructures gi ves better i ns i ght a nd unders ta ndi ng of the funda menta l chemi ca l a nd bi ochemi ca l rea cti ons of the huma n body.
INDEX Note: Pa ge numbers fol l owed by “f” a nd “t” i ndi ca te fi gures a nd ta bl es , res pecti vel y. α–l i nka ges , 18 α-hel i x, 10 α-l i nked hel i ca l s tructure, 18 α1-a drenergi c receptors , 319 α-1-a nti tryps i n, 255 α-1-a nti tryps i n (AAT) protea s e i nhi bi tor defi ci ency, 343t α-1-a nti tryps i n defi ci ency, 255 β2 -a goni s ts , 256–257 β-l i nka ges , 18 β-l i nked l i nea r cha i n, 18 β-ketothi ol a s e/a cetyl -CoA a cyl tra ns fera s e, 84 β-La cta m a nti bi oti cs , 58 β-N-a cetyl hexos a mi ni da s e, 91 β-oxi da ti on cycl e, 84 β-turn s tructure, 50 17β-es tra di ol (E2), 301 ω-3 a ci d [ei cos a penta enoi c a ci d (EPA)], 29 ω-6 a ci ds (di homoga mma -l i nol eni c a ci d (DGLA), 29 10-ca rbon, 15-ca rbon (fa rnes yl pyrophos pha te), 92 11-deoxycorti cos terone, 34 16-ca rbon fa tty a ci d pa l mi ta te, 27, 28f 2, 3-bi s phos phogl ycera te, 59, 207 2,3-di phos phogl ycera te, 207 2, 3-enoyl -CoA hydra s e, 84 2, 4-di enoyl -CoA reducta s e, 86 2′-deoxyri bos e s uga rs , 50 3-ketoa cyl -CoA thi ol a s e, 84 30-ca rbon s qua l ene mol ecul e, 92 3-HMG-CoA, 91–92 11-β-Hydroxyl a s e defi ci ency, 366t 11-β-Hydroxys teroi d dehydrogena s e type 2 defi ci ency, 367t 17-α-hydroxyl a s e defi ci ency, 299, 367t 18-Hydroxyl a s e defi ci ency, 368t 21-Hydroxyl a s e (s a l t-l os i ng type), 368t 3-β-Hydroxys teroi d dehydrogena s e defi ci ency, 366t 3-hydroxy-3-methyl -gl uta ryl -CoA (3-HMG-CoA), 91 3-hydroxya cyl -CoA dehydrogena s e, 84 3-hydroxya cyl -CoA dehydrogena s e defi ci ency, 84 3′-hydroxyl s , 50 11-cis-reti na l , 294 A A (reti nol ), 144t ABO Bl ood Groups , 29 Abs ent β-oxi da ti on of VLCFA, 87 Abeta l i poprotei nemi a (Ba s s en–Kornzwei g s yndrome), 351t ABG tes t, 318 Abs ent A-oxi da ti on of VLCFA, 87 Aca rbos e, 76 Aceta l dehyde dehydrogena s e, 228 Aceta zol a mi de, 208 Acetoa ceta te, 88 Acetone, 88 Acetyl coenzyme A (CoA), 69, 80, 136 Acetyl -CoA ca rboxyl a s e, 80, 136 regul a ti on of, 80f Acetyl -CoA fra gment, 84 Acetyl chol i ne (Ach), 173, 174, 248, 256, 290, 295 Acetyl s a l i cyl i c a ci d, 214 Achl orydi a , 154 Aci d–ba s e ba l a nce, 251, 254–255, 268, 277 NH 3 a nd, 277–278 Aci di c kera ti ns , 183t Aci dos i s , 254 Acros i n, 305 Acros ome rea cti on, 305 Acryl a mi de mol ecul es , 374 Acti n, 168, 169f Acti n-bi ndi ng protei ns (ABP), 171 Acti n fi l a ments , 11
s kel eta l mus cl e, 11 Acti va tors , 56 Acti vi n, 300, 309 Acti ve enzyma ti c pol ymers , 80, 80f Acti ve tra ns port, 98 Acti ve vi ta mi n D (ca l ci trol ), 35f Acute l ymphobl a s ti c l eukemi a , 114b Acute rejecti on, 234 Acute res pi ra tory di s tres s s yndrome (ARDS), 259–260 Acyl -CoA dehydrogena s e defi ci enci es , 354t Acyl -CoA dehydrogena s e enzyme, 84 Ada pti ve i mmune s ys tem, 222 Addi s oni a n cri s i s , 275 Adeni ne (A), 38 Adenos i ne dea mi na s e, 42 defi ci ency a nd gene thera py, 42 Adenos i ne di phos pha te (ADP), 214 mol ecul es , 262 Adenos i ne monophos pha te (AMP), 39 Adenos i ne tri phos pha te (ATP), 39, 65, 132, 152, 168, 209, 236, 270 i n mus cl es , s ources of, 179f Adenyl cycl a s e, 100, 101f, 273 Adrena l a ndrogens , 35 Adrenol eukodys trophy (ALD) (Addi s on– Schi l der Di s ea s e or Si emerl i ng–Creutzfel dt Di s ea s e), 87 Adherens juncti ons , 177 Adhes i on, 213 ADH (va s opres s i n), 306f Adrena l i ns uffi ci ency (Addi s on’s di s ea s e), 275 Adrenergi c receptors , 289 Affi ni ty chroma togra phy, 380 Affi ni ty of the enzyme for the s ubs tra te, 56, 56f Aggrega ti on, 213 Al a ni ne, 5t, 135 cycl e, 135 tra ns a mi na s e, 135 Al dol a s e reducta s e, 140b Al dos terone, 34 Ai rwa ys , 251 Al bi ni s m, 334t Al bumi n, 157t, 202 Al dehyde dehydrogena s e, 151t Al dehyde group (COH), 397, 397t Al dos terone, 275–276, 275f a nd di s ea s e, 275 effect on pH, 276 a nd pota s s i um-s pa ri ng di ureti cs , 275 Al i s ki ren, 270 Al ka l i ne phos pha ta s e, 194, 200 Al ka l os i s , 254 Al ka ptonuri a , 335t Al l os teri c effector, 206, 207 Al l os teri c regul a ti on, 59, 59f Al pers ’ di s ea s e, 350t Al pha -a cti ni n, 171 Al pha -a myl a s e, 151 Al port’s s yndrome, 264 Al s trom s yndrome, 12, 182 Al ti tude s i cknes s , 208, 272 Al veol a r s a cs , 253 Al veol i , 251, 253, 253f col l a ps e duri ng res pi ra ti on, 253 Al zhei mer’s di s ea s es , 94, 182 Amel ogeni n, 150 Ami ne group (–NH 2 ), 397, 397t Ami no a ci d, 1, 3, 130, 156t, 165t, 269t. See also Enzymes ba s i c s tructure of, 6f components of va ri ous bi ol ogi ca l mol ecul es , 11t defi ned, 4 enzyme’s pri ma ry s tructure, 56 i ntes ti na l a bs orpti on of, 314 pepti des a nd protei ns , 3 R-group, 3 s equence, 170, 170f s ources a nd fa tes , 60f
s ynthes i s of, 60f s ynthes i s /degra da ti on of, 334t–339t Ami no a ci d degra da ti on, 61–62 pa thwa ys , 61f Ami no a ci d meta bol i s m, 61–62, 130f di s ea s es of, 62t Ami no a ci d res i due, 7, 7f Ami no a ci d s ynthes i s , 61, 60f Ami nopteri n, 40 Amphoteri ci n B, 95b Ami oda rone, 260 Ammoni a (NH 3 ) toxi ci ty, 278 Ammoni um pers ul fa te, 374 Amyl a s e, 18 Amyl opecti n, 18 Amyl os e α-bond, 18 Ana erobi c res pi ra ti on, 135 Androgens , 35 bi ndi ng protei n, 309 i ns ens i ti vi ty s yndrome (AIS), 299, 312, 369t receptor bl ockers , 308, 308f Andros tenedi one, 35 Anemi a , 203 Aneurys m, 246 Angi os ta ti n, 218 Angi otens i n, 240 Angi otens i n-converti ng enzyme (ACE), 273 i nhi bi tors , 248 Angi otens i nogen, 159t Angi otens i nogen/a ngi otens i n I a nd II, 270, 273 convers i on of a ngi otens i nogen, 273f effects of body, 274t Angi otens i n receptor bl ocker, 273 Anoxi a , 247 Anthra cos i s , 260t Anti ba cteri a l a nti body functi ons , 223f Anti ba cteri a l compounds , 151t Anti bi oti c us e a nd clostridium difficile, 166 Anti body, 222–223, 222f, 223f, 224t s tructure of, 222f Anti body mol ecul es s tructure of, 222f s umma ry of, 224t Anti codons , 108 Anti di ureti c hormone, 34, 306f Anti gen, 222 dendri ti c cel l pres enta ti on of, 229f T-cel l receptor recogni ti on of, 225f Anti gen-pres enti ng cel l s , 256 Anti gl omerul a r ba s ement membra ne di s ea s e, 264 Anti mi crobi a l enzymes , 151t Anti -Mul l eri a n hormone (AMH), 298 Anti thrombi n III, 301 Apl a s ti c a nemi a , 203 Apol i poprotei n, 242 Apoprotei ns , 33, 253 Aqua pori n-2 (AQP-2), 99, 276 Aqua pori ns , 99 Ara chi da te (20-ca rbon), 24f Ara chi doni c a ci d, 29, 51 Argi na s e 1, 278t Argi na s e defi ci ency (a rgi ni nemi a ), 341t Argi ni ne, 5t Argi ni nos ucci na te l ya s e, 278t Argi ni nos ucci na te s yntheta s e (ASS), 278t defi ci ency (ci trul l i nemi a , types I a nd II), 341t Argi ni nos ucci ni c a ci duri a /a ci demi a , 342t Arres ti n, 294 Arteri a l bl ood ga s (ABG) tes t, 314 Arteri os tenos i s , 246 Artery, 241f Arti fi ci a l pul mona ry s urfa cta nts , 261 As bes tos i s , 261t As corbi c a ci d, 194, 213
As pa ra gi ne, 5t As pa ra gi ne a mi no a ci d / “N-gl ycos yl a ti on”, 76 As pa rti c a ci d, 5t As pergi l l os i s funga l i nfecti ons , 262 As pi ri n, 214, 248 i n trea tment of myoca rdi a l i nfa rcti on, 248 As thma , 254, 256 β-a drenergi c theory of, 256 a s thma ti c type I hypers ens i ti vi ty res pons e, 257t envi ronmenta l fa ctors , 256 geneti c component to, 256 a nd i mmunol ogi ca l di s ea s es , 256 s evere ca s es , 256 trea tment, 256–259 Ata xi a tel a ngi ecta s i a (AT), 360t Atel ecta s i s , 253 Atheroma , 246 Atheros cl eros i s , 246, 247f forma ti on a nd i nfa rcti on, 247f trea tment of, 246 Atheros cl eros i s forma ti on a nd i nfa rcti on, 247f Atopi c derma ti ti s , 192t ATP, 66–68. See also Adenos i ne Tri phos pha te ATP s tructure wi th i ts ma gnes i um cofa ctor, 68f ATP s yntha s e, 70 cyl i ndri ca l -s ha ped F 0 protei n, 70 F 1 hea dpi ece s ta l k, 70 F 1 hea dpi ece s ubuni t, 70 ATP7A enzyme, 57 Atres i a , 300 Atri a l na tri ureti c pepti de (ANP), 274 receptors , 239 Atrophi c ga s tri ti s , 163 Atrophy, 175 Attra cta nts , for neutrophi l s a nd eos i nophi l s , 257t A type proa nthocyni di n, 267 Autoi mmune di s ea s es , 231 Autoi mmune hemol yti c a nemi a , 203 Autos oma l reces s i ve pol ycys ti c ki dney di s ea s e, 12, 182 Axonema l dynei n, 180 Azi dothymi di ne (AZT), 107, 107b Azi thromyci n, 110b B B1 (thi a mi ne), 144t B2 (ri bofl a vi n), 144t B3 (ni a ci n), 144t B5 (pa ntotheni c a ci d), 144t B6 (pyri doxi nepyri doxa l , pyri doxa mi ne), 144t B7 (bi oti n), 144t B9 (fol i c a ci d), 144t B12 (cya nocoba l a mi n), 144t Ba cteroi des , 163 Ba rbi tura tes , 295 Ba rdet–Bi edl s yndrome, 12, 182 Ba ri tos i s , 261t Ba rrett’s es opha gus , 154 Ba rters ’s s yndrome (Gi tel ma n va ri a nt), 370t Ba rth s yndrome, 94. See also X-l i nked di s ea s e Ba rtter s yndrome, 267 Ba s a l cel l ca rci noma , 113b Ba s a l l a mi na , 264 di s orders , 264 Ba s i c protei n s tructure, a ffecti ng fa ctors , 7–9 a mi no a ci d compos i ti on, 7 fi na l des ti na ti on, 7 fi na l modi fi ca ti ons , 7 functi ona l s i tes , 7 s peci a l a mi no a ci ds , 7 Ba s ophi l s , 228 Ba uxi te fi bros i s , 261t Beni gn pros ta ti c hyperpl a s i a (BPH), 308 Benzodi a zepi nes , 295
Benzothi a zepi nes , 236, 238f, 238t Berger’s di s ea s e, 266t Beryl l i os i s , 261t Beta -bl ockers , 239 Bi ca rbona te, 269t Bi ca rbona te i ons , movement of, 255 Bi ca rbona te i ons producti on by pa ncrea s , 161f Bi functi ona l enzyme, 134 Bi gua ni de medi ca ti ons , 71 Bi l e s a l ts , 24, 33 s eques tra nts , 33 i n tri gl yceri des meta bol i s m, rol e of, 160f Bi l e s ynthes i s , 157, 157t Bi l i rubi n mea s urement, 155 Bi ndi ng of the effector, 59, 59f Bi ochemi ca l functi ona l groups , 396 Bi oti n (bi oti ni da s e) defi ci ency, 370t Bi s a cryl a mi de, 374 Bi s phos phona tes , 197 Bl a dder, 266 Bl eomyci n, 260 Bl ood ca l ci um a nd phos pha te l evel s , control of, 200f cl otti ng, 213, 214f ca s ca de, 214, 215, 216, 216f fi bri n mes hwork, 216, 217f pl a s mi n a nd cl ot di s s ol uti on, 218–219, 219f pl a tel et pl ug forma ti on, 213–214, 215f pl a tel et pl ug forma ti on a nd cl ot forma ti on, di fference between, 217, 218 regul a ti on of cl ot forma ti on, 218, 218f components of, 202–203, 202f i mpa ct of O2 –CO2 excha nge, 254 i na dequa te O2 del i very, phys i ol ogi c res pons e to, 209 i na dequa te s ynthes i s of hemogl obi n (Hgb) mol ecul e, 205, 206t i ron ferri ti n, 211, 212 hepci di n i n regul a ti on of, 212–213 meta bol i s m, 211, 212f tra ns ferri n, 211, 212f O2 bi ndi ng a l l os teri c bi ndi ng of, 205, 206–207, 207f regul a ti on of, 207–209, 207f, 208f rel a xed hemogl obi n (Hgb), 205, 205f tens e hemogl obi n (Hgb), 205, 205f overvi ew of, 201–202 RBC functi ons , 203, 203f, 204f, 205f s epa ra ti on of, 202f s i ckl e cel l di s ea s e (SCD), 209–211, 210f Bl ood gl ucos e, 313 concentra ti on, 140f Bl ood ves s el s , 179t, 240, 241f, 242f compos i ti on of, 241f permea bi l i ty, 240 Bl oom s yndrome, 361t B l ymphocytes , 227 Bohr effect, 207 Bone, 199t. See also Connecti ve ti s s ue a nd bone components of, 187, 194–197, 194f, 195f, 196f forma ti on, ma rkers of, 200 fra ctures a nd hea l i ng, 198 growth a nd remodel i ng, 197–198 rol e i n huma n body, 185 s i a l oprotei n, 194 Bone ma tri c, mi nera l i za ti on i n, 195 Bone s i a l oprotei n, 194 Bordetella pertussis, 262 Bowma n’s ca ps ul e, 264 2,3-BPG to deoxyhemogl obi n, bi ndi ng of, 208f Bra dyki ni n, 273 Bra i n Na tri ureti c Pepti de (BNP), 239, 276 Brea thi ng mus cl es , 253 Bronchi , 251 Bronchi ol e tubes , 252
Bronchi ti s , 255–256 Bronchodi l a tors , 258f Brunner’s gl a nds , 164t B-type na tri ureti c pepti de (BNP), 239 Bys s i nos i s , 261t C Ca l bi ndi n, 199 Ca l ci ferol , 145t Ca l ci toni n, 198, 199t, 279 effects on ta rget orga ns , 199t Ca l ci tri ol , 279 Ca l ci um, 146t, 269t a cti va ti on of s mooth mus cl e cel l s , 237f Ca l ci um cha nnel bl ockers (CCBs ), 236, 237, 239f cl a s s es a nd a cti ons , 238t Ca l ci um-i nduced ca l ci um rel ea s e (CICR), 171, 176 Ca l ci um l evel regul a ti on, 199, 199t, 200f Ca l ci um oxa l a te, 279 Ca l ci um phos pha te, 279 Ca l ci um rel ea s e cha nnel s , 240 Ca l or (wa rmth), 30 Ca l l us , 198 Ca l modul i n, 173, 240 a cti va ti on, 178 Ca l s eques tri n, 173 Campylobacter jejuni, 287 Ca na l i cul a r, 196 Ca ncer thera pi es , 180 Ca rba ma te, 207 Ca rba moyl phos pha te s yntheta s e I (CPS-I), 62, 278t defi ci ency, 342t Ca rbohydra te di ges ti on, 317 Ca rbohydra te i ntol era nce, 18 Ca rbohydra te meta bol i s m, 156t overvi ew of, 66f Ca rbohydra te mol ecul es brea kdown (ca ta bol i s m), 65 s ynthes i s (a na bol i s m), 65 Ca rbohydra tes , 15, 130 a nd ferti l i za ti on, 21 protei n mol ecul es , 18 bi ochemi ca l Rol es of, 15t defi ni ti on, 15 s tructura l confi gura ti on, 15–16, 16f Ca rbon (C), 396, 396t Ca rbon–ca rbon doubl e bonds , a l tera ti on of, 86f Ca rbon di oxi de (CO2 ), el i mi na ti on of, 253 Ca rbon di oxi de (CO2 ) i n red bl ood cel l s , fa te of, 208f Ca rboni c a ci d, 254 Ca rboni c a nhydra s e, 195, 207, 254, 254f i nhi bi tor, 208 Ca rbon monoxi de (CO) poi s oni ng, 204 Ca rboxyl a te, 269t Ca rboxyl group (COOH), 397, 397t Ca rboxyl i c a ci d group, 24 Ca rdi a c cycl e, 239–240 Ca rdi a c ma rkers , 247 Ca rdi a c mus cl e, 168f, 175–176, 176f s tructure, 176f s tructure a nd functi on, 236, 236f, 237f, 238f, 238t Ca rdi ol i pi n (Di phos pha ti dyl gl ycerol ), 94 Ca rdi omyocytes , 175 Ca rdi ova s cul a r s ys tem, 253 a theros cl eros i s , 246, 247f bl ood ves s el s , 240, 241f, 242f ca rdi a c cycl e, 239–240 ca rdi a c mus cl e s tructure a nd functi on, 236, 236f, 237f, 238f, 238t endogenous chol es terol /l i poprotei n meta bol i s m a nd tra ns port, 241, 243f, 244f, 245f HDL, 244, 245f i ntermedi a te-dens i ty l i poprotei n (IDL) a nd LDL, 243–244 VLDL, 241, 242, 243 hea rt a tta ck, 247–248 overvi ew of, 235
s i noa tri a l a nd tri oventri cul a r nodes , 236, 237 Ca rni ti ne pa l mi toyl tra ns fera s e I (CPT I), 82, 82f Ca rni ti ne pa l mi toyl tra ns fera s e (CPT) I defi ci ency, 83, 352t Ca rni ti ne pa l mi toyl tra ns fera s e (CPT) II defi ci ency, 83, 353t trea tment of, 83 Ca rni ti ne tra ns l oca s e defi ci ency, 352t Ca rri er protei n, 98 Ca rti l a ge di s tri buti on i n a dul ts , 186f Ca s pofungi n, 75 Ca ta ra ct, 264 Ca techol a mi nes , 137–138, 290–292, 291f Ca theps i n enzymes , 195 Ca theps i n K enzyme, 195 Cel l -medi a ted i mmuni ty, 222 a nd humora l i mmuni ty, 226f Cel l cycl e, 114, 114t mi crotubul es , 114b pha s es , 114t Cel l s , 152t Cel l ul os e, 18, 19f Cel l ul os e β-bond, 18 Centra l nervous s ys tem (CNS), 152, 319 Centri ol e/ba s a l body, 182 Centroa ci na r emphys ema , 255 Centronucl ea r/ myotubul a r myopa thi es (CNM), 173 Cepha l i c pha s e of ea ti ng, 150 Cera mi de/s phi ngol i pi ds s ynthes i s , 27, 28f, 90 Cerebros i des , 27, 28f CGMP-s peci fi c phos phodi es tera s e type 5, 272 Cha l i cos i s , 261t Cha perones , 110 Cha rged R-groups , 4 Chemoki ne receptor group, 229 Chemothera py, 260 a gents , 40 Chi ef (zymogeni c) cel l s , 154 Chl ori de, 145t, 269t Chol ecys toki ni n, 293 Chol ecys toki ni n (CCK), 153t, 161, 164t Chol es terol , 24, 30–32, 246 deri ved hormones , 100t effl ux regul a tory protei n (CERP), 244 es ter, 32, 32f, 242 es tera s e, 162t exogenous a nd endogenous pa thwa ys of, 243f functi ons of, 30–32, ga l l s tones , 161 i ntera cti on, 95f meta bol i s m, 32 mol ecul e, 32, 32f regul a ti on of, 92 s i decha i n cl ea vi ng enzyme, 309 s ynthes i s , 91–92, 92f three-di mens i ona l s tructure, 32 Chol i nes tera s e, 174 Chondrocytes , 186 Chondroi ti n, 20, 76, 193 Chondroi ti n s ul pha te, 194 Chromi um, 147t Chromos omes (ri ght) a nd a gene, rel a ti ons hi p between, 45f Chroni c bronchi ti s , 255–254 Chroni c gra nul oma tous di s ea s e, 73 Chroni c ki dney di s ea s e (CKD), 270 Chroni c obs tructi ve pul mona ry di s ea s e (COPD), 254 Chroni c rejecti on, 234 Churg–Stra us s s yndrome, 262 Chyl omi crons , 242 meta bol i c fa te of, 244f Chymotryps i n, 162t Chymotryps i nogen, 162t Ci ga rette s moki ng a nd bronchi ti s , 256 Ci l i a , 181, 181f, 182 Ci l i a /fl a gel l a , defects i n, 12 Ci l i opa thi es , 12
Ci tri c a ci d cycl e, 65, 68–69 regul a ti on of, 69 Cl a ra cel l s , 253 Cl a ri thromyci n, 110b Cl i ni ca l l a bora tory tes ts , 388t–394t Cl ofi bra te, 246 Cl oni ng, 42, 384–386, 385f cons tructi on of recombi na nt DNA, 386f Cl ot di s s ol uti on, 219 Cl ot forma ti on, 213, 215f by pl a tel ets , 215 regul a ti on of, 218, 218f Cl otti ng. See under Bl ood Cl otti ng ca s ca de, extri ns i c a nd i ntri ns i c pa thwa ys of, 216f Cl otti ng ca s ca de event, 318 Cl otti ng fa ctors , 301 Coba l t, 147t Coa l worker’s pneumoconi os i s , 261t Cocka yne s yndrome, 361t Coenzyme A (CoA), 57 Cofa ctors , 56 Col l a gen, 10, 50 fi bers , forma ti on of, 188f pepti des , 189f s tructure of, 187f s ynthes i s a nd s curvy, 194 types , 190t–192t Col l a gen-ba s ed connecti ve ti s s ue, 189f Col l a gen fi bers , 186 forma ti on of, 188f Col l a gen IV, 255 Col l ecti ng duct, 268 Col umn chroma togra phy, 378 Common fa tty a ci ds found i n huma ns , 27f Common phos phol i pi ds found i n huma ns , 26f Common uns a tura ted fa tty a ci ds , 24f Compa ct bone ma tri x, 198 Competi ti ve i nhi bi ti on, 58 Compl ement defi ci ency a nd di s ea s e, 233 Compl ement s ys tem a l terna ti ve pa thwa y, 232, 232f, cl a s s i ca l pa thwa y, 232, 232f Compl ex l i pi ds , meta bol i s m of, 88–92 Congeni ta l l oba r emphys ema , 255 Conjuga ted bi l e a ci ds , 33 Connecti ng ca pi l l a ry, 241f Connecti ve ti s s ue, 179t, 186–187, 186f, 187f–188f, 189f col l a gen-ba s ed, 189f Connecti ve ti s s ue a nd bone. See also Bone bone, components of, 187, 194–197, 194f, 195f, 196f bone forma ti on, ma rkers of, 200 bone growth a nd remodel i ng, 197–198 ca l ci um l evel regul a ti on, 199, 199t, 200f connecti ve ti s s ue, 186–187, 186f, 187f–188f, 189f col l a gen types , 190t–192t overvi ew of, 185 res orpti on, 200 Conn s yndrome, 275 Copper, 146t Copper defi ci ency, di s ea s es of, 57 Cori cycl e, 135, 135f Corona ra di a ta l a yer, 305 Corona ry a rtery di s ea s e, 314 Corrected ca l ci um, 200 Corti ca l rea cti on, 305 Corti cos teroi ds (a drena l gl a nd), 29, 34, 261 for a s thma , 259 for ILDs , 260 Corti s ol , 275 Corti s one, 275 Cow’s mi l k, exces s cons umpti on of, 213 CPT di s ea s es , 83 Cra nberry Jui ce, 267 C-rea cti ve protei n (CRP), 159t
Crea ti ne ki na s e (CK), 247 Crea ti ne phos pha te, 174, 270, 318 Crea ti ni ne, 266 Creutzfel dt–Ja kob di s ea s e, 111b, 317, 375 Cri gl er–Na jja r s yndromes , 364t Crohn’s di s ea s e, 262 Cryptogeni c orga ni zi ng pneumoni a , 261t Cus hi ng’s s yndrome, 90 Cycl i c a denos i ne monophos pha te (cAMP), 100, 101f, 150, 237, 262, 271, 273, 299, 303 Cycl i c gua nos i ne monophos pha te (cGMP), 271, 276, 294 defect, 314 NO-medi a ted producti on of, 272 Cycl i n–CDK heterodi mer compl ex, 115 Cycl i n-dependent ki na s e (CDKs ), 114 Cycl ooxygena s e (COX)-1 or -2, 29, 214, 248 Cys tei ne knot, 187 Cys ti c fi bros i s (CF), 262, 371t Cys ti c fi bros i s tra ns membra ne conducta nce regul a tor (CFTR), 262 Cys tei ne, 4, 6f, 6t, 279 Cys ti ne “di s ul fi de” bond, 4, 7f Cys ti nuri a , 340t Cys ti ti s , 267 Cyti di ne s ynthes i s , 40f Cyti di ne tri phos pha te (CTP), 39, 88 Cytoki nes , 257t Cytoki nes , 229 Cytomega l ovi rus , 287 Cytopl a s mi c dynei n, 180 Cytos kel eta l protei ns , compa ri s on of, 182f Cytos ta ti cs , 231 Cytos i ne (C), 39 Cytotoxi c T cel l s , 224 D D-3-hydroxybutyra te, 88 Da nger s i gna l s , 222 D-di mers , 219 Decrea s ed HMG-CoA reducta s e a cti vi ty, 92 Del eti on muta ti on, 113 Deep vei n thrombos i s (DVT), 219 Defecti ve a ndrogen receptor, 299 Defera s i rox, 211 Degenera ti ve a rthri ti s , 193 Dehydra ti on, 210 Dementi a , 318 Demyel i na ti ng di s orders , 283 Dendri ti c cel l s (DCS), 227, 228–229, 229f, 230t–231t pres enta ti on of a nti gen, 229f Denta l pl a que forma ti on, 150 Deoxy form of UMP (dUMP), 39 Deoxyri bonucl ei c a ci d (DNA), 38 Deoxyhemogl obi n, 208f Deoxyri bos e, 39 Derma tomyos i ti s , 261t Des a tura s e enzyme, 81 Des mi n, 177, 183t Des mopres s i n, 217 Des qua ma ti ve i nters ti ti a l pneumoni a , 261t Detoxi fi ca ti on, 157, 157t DHT, 307–308 Di a betes Mel l i tus (DM), 138–139 Di a beti c neuropa thy, 143b Di a beti c nephropa thy, 266t Di a cyl gl ycerol , 24, 25f Di a phra gm dys functi on, 254 Di a rrhea , 318 Di eta ry mea s ures , 246 Di ges ti ve s ys tem ga l l bl a dder, 160–161 l a rge i ntes ti ne a nd a nus /rectum, 163, 166f l i ver, 155, 156f–159f l i pi d meta bol i s m i n, 159–160, 160f mouth, 150–152, 151t, 152t overvi ew of, 149, 150f
pa ncrea s , 161, 161f, 162t s ma l l i ntes ti ne, 161, 163, 163f, 164t–165t s toma ch, 152–155, 152f, 153t, 155f chi ef (zymogeni c) cel l s , 154 enterochroma ffi n-l i ke cel l (ELC), 154 G cel l s , 154 hormone producti on from, 152f mucus (neck) cel l s , 152 pa ri eta l (oxynti c) cel l s , 153–154 pros ta gl a ndi n E2 , 154, 155f Di ges ti ve tra ct, functi ons of, 179t Di hydropyri di ne, 236, 238t receptor, 176 Di hydrotes tos terone, 317 Di l ti a zem, 238t Di pa l mi toyl phos pha ti dyl chol i ne, 253 Di s a ccha ri des , 50 i n huma n bi ol ogy, 16f Di s ta l convol uted tubul e, 268, 268f Di s ta l tubul e, 265f, 267 Di ureti c medi ca ti ons , 267 Di va l ent meta l tra ns porter (DMT), 211, 212 DNA ba s i c s tructure of, 44t doubl e-hel i x s tructure of, 42 s equences of A’s , G’s , C’s , a nd T’s of genes , 42 DNA a nd RNA s equenci ng techni que, 380–381, 381f DNA bi ndi ng moti fs , 112t DNA hel i ca s e, 106 DNA mol ecul es , 42 DNA pol ymera s e, 106 DNA repl i ca ti on, 106–107 DNA Southern a nd RNA northern bl ots , 382–383, 382f DNA topoi s omera s e, 106 Dol or (pa i n), 30 Dopa mi ne, 236, 237, 287–288 Dopa mi ne defi ci ency, 312 Doubl e hel i x, 42 Drummond’s s yndrome (bl ue di a per s yndrome), 340t Dubi n–Johns on s yndrome, 364t Duodenum, 161 Dyna mi n, 173 Dynei n, 180, 181f Dys trophi n, 9 E Ea ti ng di s orders , 293 Ecchymos es , 213 Edema , 203 Effector mol ecul e, 59 Ehl ers –Da nl os s yndrome (EDS) (cuti s hyperel a s ti ca ), 190t, 343t Ei cos a noi d, 29 overvi ew of, 31t s yntheti c pa thwa y, 30f Ei cos a tri enoi c a ci d, 51 El a s ta s e, 162t El a s ti c fi bers , 187, 188, 189f qua terna ry s tructure of, 189f El a s ti n mi crofi bri l s , 189f El ectroca rdi ogra m (ECG), 239 El ectrol ytes , 151t El l i ptocytos i s , 96, 96b ELISA techni que, 377 El onga ti on fa ctors , 110b Embol ys i s , 248 Emphys ema , 255 centroa ci na r, 255 congeni ta l l oba r, 255 pa na ci na r, 255 Endocri ne pa ncrea s , 161 Endonucl ea s e enzymes , 113 Endocri ne pa ncrea ti c hormones , 162t Endogenous chol es terol /l i poprotei n meta bol i s m a nd tra ns port, 241, 243f, 244f, 245f Endometri os i s , 302
Endothel i a l cel l , 253 Energy producti on a nd us e i n mus cl es , 178, 179f, 180 Enha ncer protei ns , 111 Enoyl -CoA hydra s e, 84 Enoyl -CoA hydra ta s e, 84 Enoyl -CoA i s omera s e, 86 Enta moeba hi s tol yti ca , 95 Enteri c vi rus es , 287 Enterochroma ffi n-l i ke cel l (ELC), 154 Enterogl uca gon, 153t Enzyme a cti vi ty a l l os teri c regul a ti on of, 59, 59f Enzyme-ba s ed di s ea s es , 56 Enzyme compl exes wi th more tha n s ubuni t (“mul ti meri c”), 59 Enzyme defi ci enci es , 18 Enzyme ki neti cs , 56 Enzyme rea cti ons , 11, 56–57, 58 cofa ctors , 57, 57f ma xi mum ra te of the rea cti on, 57 number of mol ecul es of s ubs tra te, 57 Enzyme regul a ti on, 58 competi ti ve, 58 competi ti ve a nd noncompeti ti ve i nhi bi ti on, 58f noncompeti ti ve, 58 uncompeti ti ve, 58 Enzymes , 11, 55, 56–61 ca ta l yza ti on, s peed or “ra te”, 58 di ges ti ve, 150, 151t feedba ck regul a ti on, 58 meta bol i s m, 58 product, 56, 56f regul a ti on, 58 Eos i nophi l s , 227–228 Epi nephri ne, 80, 236, 240, 316 Epi thel i a l cel l , 253 Epi tope, 222 Eps tei n–Ba rr vi rus , 287 ER-bound ri bos omes , 110 Erecti l e dys functi on (ED), 272 Ergos terol , 95b Erythrocytes , 202 Erythromyci n, 110b Erythropoi es i s , 202 Erythropoi eti n (Epo), 202, 278–279 regul a ti on of red bl ood cel l producti on by, 279f Escherichia coli, 27 Es s enti a l a mi no a ci ds , 4, 25, 60 Es s enti a l fa tty a ci ds , 25 Es s enti a l uns a tura ted fa tty a ci ds , 82 Es s enti a l myos i n l i ght cha i ns , 171 Es teri fi ed chol es terol mol ecul e, 32f Es tra di ol , 300 Es tri ol (E3 ), 301 Es trogens , 301, 307t cl a s s es of, 301f functi ons , 301 Es trone (E1 ), 301 Etha mbutol , 75 Even-cha i n fa tty a ci d, 84 Exci ta ti on-contra cti on coupl i ng, 171–173, 172f Exocri ne functi ons , 161 Exocri ne pa ncrea ti c enzymes , 162f, 162t “Expres s ed s equences ” or “exons , 42 Externa l s phi ncter, 266 Extra cel l ul a r chol es terol , 316 Extra cel l ul a r ma tri x (ECM), 196 Extra cel l ul a r ma tri x s tructure, 22f Extra cel l ul a r s i gna l -rel a ted ki na s es , 299 Eye, 179t F Fa bry di s ea s e, 353t Fa ci l i ta ted protei n cha nnel s , 97f, 98 Fa ctor V Lei den, 219, 365t
Fa l l opi a n tubes , 11 Fa mi l i a l hyperchol es terol emi a , 245 Fa nconi a nemi a , 361t Fa rber l i pogra nul oma tos i s , 353t Fa rmer’s l ung, 261t Fa s ci otomy, 174 Fa s t-twi tch fi bers , 174 Fa tty a ci d cha i n doubl e bondi ng, 24f Fa tty a ci d cha i ns , 84 ca rboxyl (CO2 –) group, 84 Fa tty a ci d degra da ti on, 82–88, 85f Fa tty a ci d el onga s e, 81 Fa tty a ci d el onga ti on, 81 Fa tty a ci d meta bol i s m, 80–88 Fa tty a ci d oxi da ti on, 140t Fa tty a ci d pa cks , 25 Fa tty a ci d s yntha s e, 81, 82f Fa tty a ci d s ynthes i s , 80–81, 140t–141t ma l onyl -CoA producti on, 81 Fa tty a ci d-deri ved pros ta gl a ndi ns , 265 Fa tty a ci ds , 24, 24f Fa tty a ci ds , 178 s hort-cha i n, 163 Fa tty a cyl -CoA mol ecul e, 82 Fa tty s trea k, 246 Fema l e reproducti ve s ys tem brea s t ti s s ue devel opment a nd l a cta ti on, 306–307, 307t es trogens , 301 FSH, 299–300 GnRH, 299 hCG, 302–303 LH, 300 mens trua l cycl e, 303, 304f proces s of ferti l i za ti on, 305, 305f proges terone, 301–302 Fenes tra ti ons , 264 Fermenta ti on, 163 Ferri c reducta s e, 211 Ferri ti n, 111b, 211, 212, 213 Ferroporti n, 212 Ferti l i za ti on, 305 Feta l red bl ood cel l s , 159t Fi bra tes , 246 Fi bri l -a s s oci a ted col l a gens , 191t, 192t Fi bri l l i n, 187 Fi bri n, 213 Fi bri n mes hwork, 216, 217f Fi bri nogen, 216 to fi bri n, convers i on of, 217f Fi bri nol yti c s ys tem regul a ti on by protei n C, 218f Fi brodys pl a s i a os s i fi ca ns progres s i va (FOP), 194 Fi brous protei ns , 8 Fi ve-ca rbon i s opentenyl pyrophos pha te, 92 Fl a gel l a , 181, 181f, 182 Fl a vi n a deni ne di nucl eoti de (FAD) cofa ctor mol ecul e, 84 Fl a vi n a deni ne di nucl eoti de, reduced (FADH 2 ), 57 Fl ow cytometry, 386, 387f Fl uoroqui nol ones , 166 Foca l s egmenta l gl omerul os cl eros i s , 266t Fol l i cl e-s ti mul a ti ng hormone (FSH) fema l e, 299–300 ma l e, 308f, 309 Fra gi l e X s yndrome, 362t Fredri cks on cl a s s i fi ca ti on, 245 Fri edrei ch a ta xi a , 362t Fructos e, 165t Fructos e 2, 6-bi s phos pha te, 317 Fructos uri a (hepa ti c fructoki na s e defi ci ency), 344t Fuel homeos ta s i s duri ng s ta rva ti on, 35 Fuma ra s e defi ci ency, 350t G G protei ns , 93, 99, 199, 262, 297
G-protei n receptors , 101t, 262, 273 GABA, 292, 293f, 295 GAG ca rbohydra tes , 76 GAGs , 76 functi ons of, 78t Ga l a ctoki na s e defi ci ency (ga l a ctos emi a II), 345t Ga l a ctorrhea , 306 Ga l a ctos a mi ne, 20 Ga l a ctos e, 16, 17f Ga l a ctos e-ba s ed gl ycos phi ngol i pi d mol ecul es , 27 Ga l a ctos e epi mera s e defi ci ency (ga l a ctos emi a III), 345t Ga l a ctos emi a , di a gnos i s of, 51 Ga l a ctos emi a (cl a s s i c) (ga l a ctos emi a I), 345t Ga l l bl a dder, 160–161 Ga l NAc Ca rbohydra te, 28f Ga ngl i os i des , 29, 90 Ga s eous wa s te products , el i mi na ti on of, 252 Ga s tri c i nhi bi tory pepti de (GIP), 153t Ga s tri c pa ri eta l (oxynti c) cel l s , 313 Ga s tri c ul cers , 154 Ga s tri n, 153t Ga s troenteri ti s , 163 Ga s troes opha gea l refl ux di s ea s e (GERD), 154 Ga ucher’s di s ea s e (GD), 353t G cel l s , 154 Gel fi l tra ti on chroma togra phy, 378–379, 379f Gene expres s i on, 113 Genome, 42 Gi l bert s yndrome, 364t Gi tel ma n s yndrome, 267 Gl i a l fi bri l l a ry a ci di c protei n, 183t Gl obi n cha i ns , 203 Gl obos i de, 28f Gl obul a r protei ns , 8 Gl uca gon, 80, 136–137 Gl ucocorti coi ds , 34–35, 52, 138 Gl ucoki na s e, 66 Gl uconeogenes i s , 65, 71–72, 140t–141t metformi n, 71 pa thwa y of, 72f pri ma ry regul a ti on of, 71 Gl ucos a mi ne, 20, 76 Gl ucos e, 16 Gl ucos e-6-phos pha ta s e, 71 Gl ucos e-6-phos pha te dehydrogena s e (G6PDH) defi ci ency, 73 modera te G6PDH defi ci ency, 73 trea tment of, 73 Gl ucos e-6-phos pha te, 66, 134–135 Gl ucos e–Al a ni ne cycl e, 135f Gl omerul a r di s order di s ea s es , 266 Gl omerul a r fi l tra ti on ra te (GFR), 270 Gl omerul onephri ti s , 264 Gl omerul us , 264 Gl uca gon, 293 Gl ucocorti coi ds , 231 Gl ucos a mi ne, 193 Gl ucos e, 269t Gl ucos e-6-phos pha ta s e, 318 Gl ucos e-6-phos pha te dehydrogena s e defi ci ency (G6PDH), 311, 345t Gl ucos e-6-phos pha te i s omera s e, 151t Gl uta mi c a ci d, 5t, 292, 293f Gl uta mi ne, 5t Gl uta thi one tra ns fera s e, 151t Gl yceryl tri ni tra te, 242f Gl yci ne, 292 Gl yci ne encepha l opa thy, 335t Gl ycoca l yx, 265 Gl ycera l dehyde-3-phos pha te, 66 Gl ycerol , 24 Gl ycerol -3-phos pha te dehydrogena s e, 70–71 Gl yci ne, 5t Gl yco- a nd ta uro-bi l e a ci ds , 33 Gl ycogen, 18, 65 a nd the pl a nt s ta rch forms a myl opecti n a nd a myl os e, 19f
Gl ycogen s tora ge di s ea s e (GSD), 18, 76, 77t, 180, 347t GSD (type Ia ) (Von Gi erke di s ea s e), 347t GSD (type III) (Cori or Forbes di s ea s e), 348t GSD (type II) (Pompe di s ea s e), 348t GSD (type IV) (Anders en di s ea s e; a myl opecti nos i s ), 348t GSD (type VI) (Hers di s ea s e), 349t GSD (type VII) (Ta rui di s ea s e), 349t GSD (type V) (McArdl e di s ea s e), 349t Gl ycogen s tora ge di s ea s e type VII / Ta rui ’s di s ea s e, 66 “cl a s s i c” va ri a nt, 66 “l a te ons et” va ri a nt, 66 i nfa nti l e, 66 Gl ycogen s yntha s e ki na s e 3, 302 Gl ycogen s ynthes i s , 75, 75f, 140t–141t Gl ycogeni n, 75 Gl ycogenol ys i s /gl ycogen brea kdown, 75–76, 76f debra nchi ng enzyme, 75 gl uca n tra ns fera s e enzyme, 75 i nhi bi tors , 76, 140t–141t repeti ti ve remova l of gl ucos e res i dues , 75 Gl ycol i pi ds , 24, 27, 32 Gl ycol ys i s , 65, 66–68, 69, 140t–141t energy producti on, 66 i ncl udes 10 enzyme s teps , pa thwa y of, 67f rea cti on of, 66 phos phoryl a ti on, 66 Gl ycoprotei ns , 18, 76, 152t bi ochemi ca l rol es of, 78t phos phoryl a ted, 150 Gl ycos a mi nogl yca ns (GAGs ), 20, 193, 264 a nd proteogl yca n a s s oci a ted wi th col l a gen, 20f bi ol ogi ca l rol e of, 21t Gl ycos phi ngol i pi d mol ecul es , 27, 29 Gl ycos phi ngol i pi d, 27 Gl ycos yl tra ns fera s es , 75 i nhi bi tors , 75 Gl ycyrrhi zi n, 275 Gona dotropi n-rel ea s i ng hormone (GnRH), 299 Gona ds , 297 Goodpa s ture’s s yndrome, 261t, 264, 266t Gout, 41 Gra m-nega ti ve ba cteri a , 233 Gra m-pos i ti ve ba cteri a , 58 Gua ni ne (G), 38 Gra nul ocyte col ony-s ti mul a ti ng fa ctor, 230t Gra nul ocytema cropha ge col ony-s ti mul a ti ng fa ctor, 230t Gra nul os a cel l s , 300 Gra nul ys i n, 224 Gra nzyme, 227 Gua nyl yl cycl a s e, 239, 276, 294 Gua nos i ne monophos pha te (GMP), 39 Gua nos i ne tri phos pha te (GTP), 69 Gui l l a i n–Ba rré Syndrome (GBS), 287 Gus ti n, 150 Gut fl ora , 163, 166 H H-s ubs ta nce, 29 Ha gema n, 218 Ha i r a nd cys ti ne doubl e bondi ng, 4 Ha rd cl ot forma ti on, 217f Ha rtnup di s ea s e (neutra l a mi noa ci duri a ), 340t H bondi ng, 396 HDL. See Hi gh-dens i ty l i poprotei n (HDL). Hea rt a tta ck, 247–248 s ecti ona l vi ew of, 236f “Hea t s hock” protei ns , 110 Hel i x–l oop–hel i x, 112t Hel i x–turn–hel i x, 112t Hel per T l ymphocytes , 224 Hel per T (Th) cel l s , 319 Hema tol ogy, 202 Heme
brea kdown of, 160f mol ecul e, 203, 204f s ynthes i s of, 204f Hemogl obi n (Hgb) to bi l i rubi n, degra da ti on of, 160f brea kdown of, 157t di s ea s es , 206t i na dequa te s ynthes i s of, 205, 206t O2 bi ndi ng of oxygen to, 205f s umma ry of di s ea s es , 206t s ynthes i s a nd s tructure of, 204, 204f Hemogl obi nopa thi es , 96, 96b Hemol ys i s , 210, 211 Hemol yti c a nemi a , 313 Hemophi l i a , 217 Hemophi l i a A, B, a nd C, 365t Hemorrha ge, 213 Hemos i deri n, 211 Henoch–Schönl ei n purpura , 266t Hepa ri n, 20, 218 Hepa tomega l y, 316 Hepci di n, 212, 213 a cti on of, 214f i n i nfl a mma tory res pons e, 214f i n regul a ti on of i ron, 212–213 Heredi ta ry fructos e i ntol era nce (a l dol a s e B defi ci ency), 346t Heredi ta ry gl omerul onephri ti s , 266t Heterodi meri c gl ycoprotei n hormone (hCG), 302–303 Hetero-ol i gomer, 9 Heterotri mers , 216 Hexa mer, s ol ubl e, 216 Hexoki na s e, 66 Hexos es fructos e, 16 Hgb, rel a xed (R) s ta te of, 316 HgbF, 314 Hi gh-dens i ty l i poprotei n (HDL), 241, 244, 245f Hi gh-performa nce/pres s ure l i qui d chroma togra phy (HPLC), 380 H i on (H +), 396–397, 397t rol e i n bl ood, 254 Hi s ta mi ne (H2), 153, 228, 257t receptors , 98b Hi s ti di ne, 4, 6t, 228 Hi s ti di nemi a , 335t Hi s tocompa ti bi l i ty, 234 Hi s tone a cetyl tra ns fera s es (HATs ), 111 Hi s tone dea cetyl a s es , 111 Hi s tones , 104 HMG-CoA reducta s e, 92 Homocys ti nuri a , 336t Homo-ol i gomer, 9 Hormona l i nvol vement, i n s exua l di fferenti a ti on, 298f Hormone-res pons i ve brea s t ca ncers , 302 Hormone res pons i ve el ements , 301 Hormones a ffecti ng s toma ch, 153t producti on by di ges ti ve s ys tem, 155f producti on from s toma ch, 152f Huma n chori oni c gona dotropi n (hCG), 299 Huma n i mmunodefi ci ency vi rus (HIV), 95 Huma n pa pi l l oma vi rus (HPV), 229 Humora l hyperca l cemi a of ma l i gna ncy (HHM), 199 Humora l i mmuni ty, 222, 226f, 227 Hunti ngton’s Chorea (HD), 293 Hya l i n, 305 Hya l i ne membra ne, 260 Hya l i ne membra ne di s ea s e, 254 Hya l uroni c a ci d, 20 Hydrogen (H), 396, 396t Hydrophi l i c l i pi ds , 23 Hydrophi l i c (wa ter l ovi ng) R-groups , 4 Hydrophobi c (s ti cky) pa tch, 209, 210f Hydrophobi c (wa ter ha ti ng) R-groups , 4 Hydrophobi c “ta i l ,” 36
Hydrophobi c l i pi ds , 23 Hydrophobi c regi ons , 253 Hydrophobi c s ubs tra te, 56 Hydrophobi c, s i gna l i ng mol ecul es , 100t Hydroxya pa ti te, 187, 195f Hydroxyl groups (OH), 16, 399 Hydroxyl ys i ne or hydroxyprol i ne res i dues / “O-gl ycos yl a ti on”, 76 Hydroxyurea , 211 Hypera cute rejecti on, 234 Hypera mmonemi a , 278 Hypera mmonemi a –hyperorni thi nemi a –homoci trul l i nuri a (HHH s yndrome), 340t Hyperbi l i rubi nemi a , 155 Hyperca l cemi a , 267 Hyperca pni a , 254 Hyperchol es terol emi a , 312 Hyperdi ba s i c a mi noa ci duri a type 2, 278t Hypergl ycerol emi a (gl ycerol ki na s e defi ci ency), 346t Hyperpa ra thyroi di s m, 317 Hypers ens i ti vi ty rea cti ons , 233–234, 233t–234t, 313 Hyperva l i nemi a , 336t Hypoa l bumi nemi a , 202 Hypochl oremi a , 267 Hypochl ori di a , 154 Hypogl ycemi a , 247, 316 Hypoka l emi a , 267 Hypoma gnes emi a , 267 Hyporefl exi a , 287 Hypoventi l a ti on, 254 Hypoxa nthi ne, 247 Hypoxi a , 208, 209 Hypoxi a , 254 i nduci bl e fa ctor, 209 I Idi opa thi c pul mona ry fi bros i s , 261t IgA nephropa thy, 266t Immune cytoki nes , 230t–231t Immune s ys tem a nti body, 222–223, 222f, 223f, 224t a nti gen, 222 cel l s a s s oci a ted wi th ba s ophi l s , 228 B l ymphocytes , 227 cytoki nes , 229 dendri ti c cel l s (DCS), 228–229, 229f, 230t–231t eos i nophi l s , 227–228 i nna te i mmune s ys tem, 229, 231 monocytes a nd ma cropha ges , 227 na tura l ki l l er (NK) cel l s , 227 neutrophi l s , 227 T l ymphocytes , 223–227, 225f, 226f compl ement s ys tem, 232–233, 232f hypers ens i ti vi ty rea cti ons , 233–234, 233t–234t overvi ew of, 222 Immunoa s s a ys , 376 Immunodefi ci enci es , 222 Immunogl obul i n (Ig), 256 a nti bodi es , 151t, 203, 222 Immunol ogi ca l memory, 222 Immunophi l i n i nhi bi tors , 231 Immunoreceptor tyros i ne-ba s ed a cti va ti on moti f (ITAM), 223 Immunos uppres s a nts , 231 Increa s ed ectopi c (tuba l ) pregna nci es /ma l e i nferti l i ty, 12 Indi vi dua l a mi no a ci ds , 61 Infa nt res pi ra tory di s tres s s yndrome (IRDS), 254 Infecti ve di s ea s es , of res pi ra tory s ys tem, 262 Infl a mma ti on, 30 Infl a mma tory rubor (rednes s ), 30 Inha l a ti on a nes theti cs , 295 Inhi bi n, 300, 309 Inhi bi tory neurotra ns mi tter, 319 Inhi bi tors , 56 Inna te i mmune s ys tem, 222, 229, 231
Inos i ne (I), 38 Ins erti on muta ti on, 113 Ins ul i n, 81, 159t s ecreti on a nd pepti des , 311 Integri n cel l s urfa ce ma tri x receptor, 196f Integra l protei ns , 96 Integri n receptor, 195 Interl euki n (IL), 197, 226, 230t, 255 Intermedi a te-dens i ty l i poprotei n (IDL), 243–244, 245, 245f Intermedi a te fi l a ments (IF), 167, 182–183, 182f, 183t Interna l urethra l s phi ncter, 266 “Interveni ng s equences ” or “i ntrons , 42, 45f Inters ti ti a l l ung di s ea s es (ILDs ), 254, 261 Intes ti na l ca l ci um a bs orpti on, 199 Intes ti na l l umen ferri c, 212f Intes ti ne, 199t Intra venous a nes theti cs , 295 Iodi ne, 146t Ion cha nnel , 100t Ionophores , 98b Ion-excha nge chroma togra phy, 379, 379f Ioni zed ca l ci um, 200 Ipra tropi um bromi de, 257–258 Iron, 146t. See also under Bl ood defi ci ency, 203 defi ci ency a nemi a , 206t, 213 Iron res pons e el ement (IRE), 111b Iron tra ns port overvi ew of, 212f by tra ns ferri n, 212f Is chemi a , 247 Is ol euci ne, 5t Is ova l eri c a ci demi a , 336t Is ozymes /i s oenzymes , 59 J Ja nus Ki na s e (JAK), 2, 102f, 209, 224, 278 Juxta gl omerul a r (JAG) cel l s , 265 K Ka rta gener Syndrome, 12, 182 Kera ti n, 51, 183t Kera ti nocytes , 229 Kerni cterus , 73, 155 Ketoa ci d deri va ti ves , 51 Ketoa ci dos i s , 140, 141f, 142f Ketone bodi es , 88 Ketone body s ynthes i s /degra da ti on, 88, 91f “Ki nked” cis doubl e bond, 24 Krebs cycl e, 69 Ketone group (CPO), 397t, 398 Ketoti c hypergl yci nemi a , 336t Ki dney, 199t ba s i c a na tomy, 264f a nd bi ca rbona te excreti on, 255 i mpa ct of O2 –CO2 excha nge, 254 rol e i n s ynthes i s of a cti ve vi ta mi n D, 279 Ki dney s tones (nephrol i thi a s i s ), 279 Ki nes i ns , 180, 181f Ki ni n–ka l l i krei n s ys tem, 273 Ki ni nogen, 215 Knobl och s yndrome, 191t, 192t Kra bbe di s ea s e, 354t Kupffer cel l s , 159t L L-3-hydroxyl a cyl -CoA dehydrogena s e, 84 La bra dor l ung, 261t La cta te, 135 La cta te dehydrogena s e, 135, 180 La cta te thres hol d, 180 La cti c a ci d, 135f, 175, 178 La ctoferri n, 151t La ctoperoxi da s e (s a l i va ry), 151t
La ctos e, 52 i ntol era nce, 18 La mbert–Ea ton s yndrome, 174 La mel l a r bone, 198 La mi na , 105 Lamina rara interna, 264 Lamina rare externa, 264 La mi ni n, 264 La mi nopa thi es , 106 La rge i ntes ti ne a nd a nus /rectum, 163, 166f functi ons of, 166f La rynx, 252 Leci thi n, 253 Leci thi n chol es terol a cyl tra ns fera s e (LCAT), 244 Lecti n pa thwa y, 232 Lei gh s yndrome, 350t Lens rupture, 264 Lepti n, 293 Leptomyci ns , 104, 104b Les ch–Nyha n s yndrome (LNS), 41, 52, 358t Leuci ne, 5t zi pper, 112t Leukocyte-s peci fi c tyros i ne ki na s e (Lck), 223 Leukotri ene a nta goni s ts , 258–259 Leukotri ene-a s s oci a ted bronchocons tri cti on, 258 Leukotri ene B4, 257t Leukotri enes , 29, 257t Leydi g cel l s , 298, 309 “Li ck a nd fl i p” a cti vi ty, 61 Li ddl e’s s yndrome, 275 Li ghthea dednes s , 315 Li nea r trans doubl e bond, 24 Li ngua l l i pa s e, 151t, 316 Li nol ei c a ci d, 51 Li pi d meta bol i s m, 79, 156t i n l i ver, 159–160, 160f Li pi d mol ecul es , 23 ba s i c functi ons , 24 doubl e bonds , 25 Li pi d ra fts , 96, 97f Li pi d s ynthes i s i n s el ected ti s s ues , 81 Li pi d-deri ved hormones , 34, 79 Li pi d-deri ved mol ecul es , 23 Li pi ds , 23, 79, 130, 165t Li pi d tra ns port a nd meta bol i s m, 243f Li pol ys i s , 141t Li poprotei n-a s s oci a ted phos phol i pa s e, 246 Li poprotei n di s orders , 244 Li poprotei n l i pa s e (LPL), 242 Li poprotei n s tructure, 32f Li poprotei ns , 32, 139, 141t, 246 ba s i c cha ra cteri s ti cs , 33t meta bol i s m a nd tra ns port, 241, 243f, 244f, 245f 5-l i poxygena s e, 258 Li ver, 155, 156f–159f l i pi d meta bol i s m i n, 159–160, 160f Long-cha i n 3-hydroxya cyl -CoA dehydrogena s e (LCHAD) defi ci ency, 84 Long-cha i n a cyl -CoA dehydrogena s e (LCAD), 87 Loop di ureti cs , 267 Lordos i s , 197 Low-dens i ty l i poprotei n (LDL), 88, 159 L-type ca l ci um cha nnel s , 176, 239 Lungs , 251, 252 a nd a ci d–ba s e ba l a nce, 254–255, 255f a nd O2 –CO2 excha nge, 254–255 s tructure of, 252f Lutea l pha s e, 299f, 300 Lutei ni zi ng hormone/chori ogona dotropi n receptor (LHCGR), 303 Lutei ni zi ng hormone (LH) α- a nd β-s ubuni t, 300 fema l e, 300 ma l e, 308f, 309 ovul a ti on predi cti on by, 300
Lyme di s ea s e, 375 Lymphocyte functi on-a s s oci a ted a nti gen (LFA), 229 Lymphoi d i nters ti ti a l pneumoni a , 261t Lymph ves s el s , 179t Lys i ne, 5t Lys i nuri c protei n i ntol era nce, 278t Lys ophos phol i pa s e, 162t Lys ozyme, 151t M Ma crol i des , 110b Ma cropha ge pha gocyti c functi ons , 262 Ma cropha ges , 227 Ma d cow di s ea s e, 111b, 375 Ma gnes i um, 146t, 269t Ma jor hi s tocompa ti bi l i ty compl ex (MHC), 225f Ma l e i nferti l i ty, 182 Ma l i gna nt mel a noma , 113b Ma l onyl -CoA, 80, 81, 82f, 136 producti on by hormones , 80f Ma nga nes e, 146t Ma nnos e-bi ndi ng l ecti n pa thwa y, 232 MAP ki na s es , 306 Ma pl e s yrup uri ne di s ea s e, 337t Ma rfa n s yndrome, 344t Ma tri x meta l l oprotea s es , 255 Ma xa m–Gi l bert method of s equenci ng, 380 MCAD defi ci ency, 87 McArdl es di s ea s e, 180, 312 Meckel –Gruber Syndrome, 12, 182 Medi um-cha i n a cyl -CoA dehydrogena s e (MCAD), 87 Medi um-cha i n fa tty a ci ds , 83 Mega l obl a s ti c a nemi a s , 163 Mel a nocyte-s ti mul a ti ng hormone, 293 Membra ne, 93–94 cha nnel s , 98 functi ons , 97 s i gna l i ng, 99–102 s tructure, 94–96 Membra ne a tta ck compl ex (MAC), 232 Membra ne cha nnel s , 98, 98b Membra no-prol i fera ti ve gl omerul onephri ti s , 266t Membra ne l i pi d, 24 ba s i c components , 24 fa tty a ci ds , 24 gl ycerol , 24 hea d group, 25, 26f Membra ne receptors , 93 Membra ne s i gna l i ng receptors , 100t Membra ne tra ns port protei ns , s chema ti c pres enta ti on, 97f Menke’s “ki nky ha i r” di s ea s e, 57 ATPa s e, Cu 2+ tra ns porti ng, a l pha pol ypepti de (ATP7A), 57 exces s copper a ccumul a ti on, 57 Mens trua l cycl e, 303, 304f, 316 expres s i on of reproducti ve hormones duri ng, 299f fol l i cul a r/prol i fera ti ve pha s e (da ys 5–13), 303 l utea l /s ecretory pha s e (da ys 15–28), 303 mens trua ti on (da ys 1–4), 303 Mepol i zuma b, 259 Mes a ngi a l cel l s , 265 Mes s enger RNA (mRNA) mol ecul es , 42 Meta bol i c fa te of chyl omi crons , 244f of very-l ow-dens i ty l i poprotei n (VLDL), 245f Meta bol i c s ys tems , 137f Meta bol i s m, 141 epi nephri ne effects , 141t gl uca gon effects , 140t hormona l control , 136–140 i ns ul i n effects , 139t Meta chroma ti c l eukodys trophy (MLD), 355t Metha mpheta mi ne, 152 Methi oni ne, 4, 6t Methi oni ne ma l a bs orpti on s yndrome (Smi th–Stra ng di s ea s e/Oa s thous e uri ne di s ea s e), 341t
Meth mouth, 152 Methotrexa te, 40, 51, 260 Methyl ma l oni c a ci demi a (MMA), 278t, 337t Metoprol ol , 239 Metroni da zol e, 166 Meva l ona te, 92 Mi croca l ci fi ca ti on, 246 Mi crotubul e-ba s ed moti l i ty, 180, 181f, 182 Mi crotubul e motors , 181f Mi crotubul e pol ymeri za ti on a nd ca ncer thera py, 180 Mi crotubul es , 11 Mi gl i tol , 76 Mi nera l i za ti on i n bone ma tri x, 195f Mi nera l ocorti coi ds , 34, 52 Mi nera l s , 130, 143, 145t–147t Mi ni -chromos ome ma i ntena nce compl ex, 106 Mi ni ma l cha nge di s ea s e, 265 Mi ni ma l cha nge gl omerul onephri ti s , 266t Mi nor enzyma ti c pa thwa y, 56 Mi s opros tol , 154 Mi tochondri a , 236 Mi tochondri a l DNA (mtDNA), 42 Mi tochondri a l Tri functi ona l Protei n (MTP), 84 Mi togen-a cti va ted protei n ki na s e (MAPK), 255 Modi fi ed ca rbohydra tes , 76 Mol ecul a r ma rkers , 375 Mol ecul a r “s hock a bs orbers ,” 76 Mol ybdenum, 147t Monocytes , 227 a nd ma cropha ges , 227 Monomeri c enzymes a l l os teri c regul a ti on of, 61 Monomers ” or “s ubuni ts ”, 9 Monos a ccha ri des , 16 i n huma n bi ol ogy, 16f Montel uka s t, 258 Moti l i n, 153t Mothers ca rryi ng a fetus wi th LCHAD defi ci ency, 84 “Motor” protei ns , 11–12 Mouth, 150–152, 151t, 152t mRNA, tRNA, a nd rRNA, cha ra cteri s ti cs of, 43t MTP defi ci ency, 84 Muci ns , 152 Mucopol ys a ccha ri des , 152t Mucus , 152, 152t Mul l eri a n ducts , 297, 299 Mul l eri a n i nhi bi ti ng fa ctor, 298 Mul ti pl e Acyl -CoA Dehydrogena s e Defi ci ency (MADD), 87 Mul ti pl e enzymes , 55 Mul ti pl e protei n s ubuni ts , 9Mus cl e, types of, 168f Mus cl es , 253 fa ti ga bi l i ty, 174 Mus cl e contra cti on, 11 Mus cul a r dys trophi es , 9 Mus cl es a nd moti l i ty. See also Skel eta l mus cl e. ca rdi a c mus cl e, 175–176, 176f components of mus cl e, 168, 168f a cti n, 168, 169f a cti n-bi ndi ng protei ns (ABP), 171 myos i n, 170 myos i n l i ght cha i ns , 171 tropomyos i n, 170, 170f energy producti on a nd us e i n mus cl es , 178, 179f, 180 exci ta ti on–contra cti on coupl i ng, 171–173, 172f i ntermedi a te fi l a ments , 182–183, 182f, 183t mi crotubul e-ba s ed moti l i ty, 180, 181f, 182 nonmus cl e cel l s , 183, 183f overvi ew of, 167 s kel eta l mus cl e mus cl e types , 174, 175f s tructure a nd overvi ew of, 173–174, 173f s mooth mus cl e, 176–178, 177f, 178f, 179t Muta ti ons , 113, 113f types , 113f
Mya s theni a gra vi s (MG), 174 Mycomplasma, 287 Myel i n, 253, 283 Myel i n s hea ths , 253, 306 Myoca rdi a l i nfa rcti on, 319 Myoca rdi a l i nfa rcti on, a s pi ri n i n trea tment of, 248f Myogl obi n, 174 Myos i n, 12, 170 Myos i n I, 12 Myos i n II mol ecul es , 12 Myos i n force genera ti on, 171 Myos i n l i ght cha i n ki na s e (MLCK), 171, 178f, 237f, 240 Myos i n l i ght cha i n phos pha ta s e (MLCP), 171, 178f Myotoni c dys trophy (DM), 362t Myotubul a ri n, 173 Myri s tyl a ted a l a ni ne-ri ch C ki na s e, 152 N N-a cetyl -ga l a ctos a mi ne (a .k.a . Ga l NAc), 27, 28f N-a cetyl gl uta mi c a ci d, 63 N-Acetyl gl uta ma te s yntha s e, 278t N-Acetyl gl uta ma te s yntha s e (NAGS) defi ci ency, 342t N-a cetyl neura mi ni c (NANA), 29, 29f N-a cetyl mura moyl -L-a l a ni ne a mi da s e, 151t Na +–Cl – cha nnel , 270 Na +–Cl – cotra ns porter, 267 Na +–K+-ATPa s e pumps , 270, 285f Na +–K+–2Cl - tra ns port, 267 Na s a l ca vi ty, 252 Na tura l ki l l er (NK) cel l s , 227 NE/epi nephri ne, 288–290 Nephri n, 265 Nephri ti c s yndrome, 266 Nephrogeni c di a betes i ns i pi dus , 50, 315 Nephrogeni c (ki dney-a s s oci a ted) di a betes i ns i pi dus , 276 Nephron, 267–268 di ureti c medi ca ti ons , 267 overvi ew, 268f ta s k of, 268 Nephronopthi s i s , 182 Nephroti c s yndrome, 266 Nervous s ys tem a nd a nes thes i a , 294–295 a utonomi c nervous s ys tem (ANS), 286–287, 287t bi ochemi s try of vi s i on, 294, 294f components of, 281–283 nerve i mpul s e conducti on nerve i mpul s e, 284, 285f, 286f neuron a t res t, 283–284 repol a ri za ti on, 284 s yna pti c cl eft, 286f neurotra ns mi tters a cetyl chol i ne (Ach), 290, 295 ca techol a mi nes , 290–292, 291f dopa mi ne, 287–288 γ-a mi nobutyri c a ci d (GABA), 292, 293f, 295 gl uta mi c a ci d, 292 gl yci ne, 292 neuropepti des , 293–294 norepi nephri ne (NE)/epi nephri ne, 288–290 s erotoni n (5-HT), 290 overvi ew, 281 pa ra s ympa theti c, 287 s oma ti c nervous s ys tem, 286 s ympa theti c, 286 Neura l tube defects , 313 Neurofi l a ments , 183t Neurogeni c (bra i n-a s s oci a ted) di a betes i ns i pi dus , 276 Neurohormone, 299 Neuropepti des , 293–294 Neurotra ns mi tters , 236 Neutra l protea s es , 257t Neutrophi l el a s ta s e, 255
Neutrophi l extra cel l ul a r tra ps (NET), 227 Neutrophi l s , 227 Nexi n, 181, 181f, 183t Ni a ci n, 246 Ni ckel , 147t Ni coti na mi de a deni ne di nucl eoti de (NADH), 57 Ni coti na mi de a deni ne di nucl eoti de phos pha te (NADPH), 57, 73, 271 Ni coti ni c–a cetyl chol i ne receptor, 174 Ni ema nn–Pi ck C di s ea s e, 315 Ni ema nn–Pi ck di s ea s e, 355t Ni fedi pi ne, 238f, 238t Ni tri c oxi de s yntheta s e (NOS), 271 Ni trogen (N), 396, 396t Ni trogl yceri n, 242f, 248, 319 Ni trous oxi de, mecha ni s m of a cti on of, 242f Non-es s enti a l a mi no a ci ds , 4, 61 Non-Hodgki n’s l ymphoma , 114b Noni nfecti ve di s ea s es , of res pi ra tory s ys tem a cute res pi ra tory di s tres s s yndrome (ARDS), 259–260 a s thma , 256 bi ochemi ca l proces s es l ea di ng to, 256–259 bronchi ti s , 255–256 emphys ema , 255 i nters ti ti a l l ung di s ea s es (ILDs ), 261 pneumoconi os i s di s ea s es , 261 res ul t of a n occupa ti on/envi ronmenta l condi ti on, 261 Nonmoti l e ci l i a , 11 Nonmus cl e cel l s , 183, 183f Nons peci fi c i nters ti ti a l pneumoni a , 261t Nons teroi da l a nti -i nfl a mma tory drugs (NSAID), 214, 267 Nos e, 252 Nucl ea r fa ctor-ka ppa b (NF-k b), 255 Nucl ea r export s equence, 104 Nucl ea r l a mi ns , 183t Nucl ea r ma tri x, 105, 106 Nucl ea r ma tri x/s ca ffol d, 104–106 Nucl ea r membra ne, 104 Nucl ea r pores , 104 Nucl eol us , 106 Nucl eopori ns , 104 Nucl eos i des a nd/or nucl eoti des a na l ogues a s chemothera py a gents a nd a nti bi oti cs , 40 ba s i c s tructure of, 38f brea kdown of puri nes a nd pyra mi di nes , 40–41 components of ca rbohydra te, 39 ni trogenous ba s e, 38 Nucl eoti de pol ymers , 50 Nucl eoti des , 38 ni trogenous ba s e, 38 Nucl eus , 104 Nurs e cel l s , 309 Nutri ents , a bs orpti on of, 163f O O2 bi ndi ng. See under Bl ood O2 del i very, phys i ol ogi c res pons e to i na dequa te, 209 Obes i ty, 161 Odd number fa tty a ci d degra da ti on, 86f Oka za ki fra gments , 106 Ol i gomer protei n, 9 Ol i gos a ccha ri de, 18, 76 Oma l i zuma b, 259 Omega -3 (Ω-3) fa tty a ci ds , 25, 246 Omega -6 (Ω-6) fa tty a ci ds , 25 Oncoti c pres s ure, 202 Oogenes i s , 300 Opi a tes , 295 Ops i n, 294 Ops oni za ti on, 227 Ora l contra cepti ve pi l l s , 305 Orni thi ne tra ns ca rba myl a s e (OTC), 278t defi ci ency, 342t Oros ens ory pha s e of ea ti ng, 150 Oroti c a ci duri a (type I), 358t
Os el ta mi vi r / Ta mi fl u, 76 Os moti c pres s ure gra di ent, 267 Os tei ti s deforma ns , 198 Os teoa rthri ti s (OA), 76, 193 Os teobl a s ts , 187, 193f, 197 Os teoca l ci n, 194 Os teochondrogeni c cel l s , 186 Os teocl a s ts , 194, 195 Os teocytes , 193f, 196 Os teogenes i s i mperfecta (OI), 10, 344t Os teoi d, 194 Os teoi d mi nera l i za ti on, 196 Os teoma l a ci a /ri ckets , 196 Os teomyel i ti s , 211 Os teoponti n, 194 Os teoporos i s , 197 Os teoprogeni tor cel l s , 187 Os teoprotegeri n (OPG), 195 Oxa l oa ceta te, 68, 68f concentra ti on, 88 Oxi da ti ve phos phoryl a ti on, 66, 69–71, 69f, 174 compl ex I, 70 compl ex II, 70 compl ex III, 70 compl ex IV, 70 cytochrome c mol ecul es , 70, 69f mi tochondri a l enzyme, 71 mi tochondri a l ma tri x, 70 Q protei n, 70 res pi ra tory cha i n porti on, 70 “s uper” compl ex, 70 Oxygen–hemogl obi n di s s oci a ti on curve, 207f Oxygen (O2 ), 396, 396t Oxygen (O2 )–hemogl obi n (Hgb) di s s oci a ti on curve, 207f Oxygen (O2 ) tra ns porta ti on i n body, 253 Oxytoci n, 306, 307t P Pa get’s di s ea s e, 198 Pa l mi ta te (16-ca rbon), 24f, 81 Pa l mi toyl -CoA, 80 Pa na ci na r emphys ema , 255 Pa ncrea s , 161, 161f, 162t i mpa ct of O2 –CO2 excha nge, 254 i mpa i rment of, 315 Pa ncrea ti c a ci ni , 161 Pa ncrea ti c a myl a s e, 162t Pa ncrea ti c ca rboxypepti da s es , 162t Pa ncrea ti c enzymes , exocri ne, 162f Pa ncrea ti c hormones , endocri ne, 162t Pa ncrea ti c l i pa s e, 162t Pa ra thyroi d hormone (PTH), 196, 197, 199, 199t, 200, 200f, 279 Pa res thes i a , 163 Pa ri eta l (oxynti c) cel l s , 153–154 bi ca rbona te i on, 154 ga s tri c a ci d, 153–154 i ntri ns i c fa ctor, 154 Pa rki ns on’s di s ea s e, 94, 182 Pa s s i ve di ffus i on excha nge mecha ni s m, 253 Pa s s i ve i mmuni ty, 222 Pa thwa ys , 55 Pa ti ents wi th CPT I defi ci ency, 83 Pa ti ents wi th MADD, 87 Pa ti ents wi th OI, 10 Pa ti ents wi th PKU, 56 Pa ti ents wi th Refs um di s ea s e, 87 PCR techni que, 383, 384f PDH defi ci ency, 351t P-di methyl a mi nobenza l dehyde, 155 Pentos e phos pha te pa thwa y, 65, 73–74 nonoxi da ti ve pha s e, 73, 74f oxi da ti ve pha s e, 73, 74f Pentos e ri bos e, 16
Peps i n, 154 Pepti des , 227, 269t Pepti de bond, 109 Pepti dogl yca n cel l wa l l , s yneths i s of, 58 Perfori n, 224 Peri phera l protei ns , 96 Perni ci ous a nemi a , 163 Peroxi n protei ns , 87 Peroxi s omes , 87 di s ea s es of, 87 Pertus s i s toxi n, 262 PGD 2 , 31t PGE1 , 31t PGE2 , 31t PGG 2 , 31t PGH 2 , 31t PGI 2 , 31t PGIs , 29 Pha gocytos i s , 224 Pha s i c contra cti on, 177 Phenothi a zi nes , 293 Phenyl a l a ni ne, 5t, 50, 56 Phenyl a l kyl a mi nes , 238f, 238t Phenyl ketonuri a (PKU), 50–51, 56, 338t Phos pha te, 269t Phos pha te group (PO3 a nd PO4 ), 397, 397t Phos pha te (P), 396, 396t Phos pha ti di c a ci d, 88 Phos phodi es ter bond, 41 found i n RNA a nd DNA, 41f Phos phodi es tera s e i nhi bi tor, 272 Phos pha ti dyl gl ycerol , 253 Phos pha ti dyl etha nol a mi ne from phos pha ti dyl s eri ne, forma ti on, 89f Phos pha ti dyl chol i ne (PC), 94 Phos pha ti dyl etha nol a mi ne (PE), 94 Phos pha ti dyl i nos i tol , 94 Phos pha ti dyl s eri ne (PS), 94 Phos pha ti dyl s eri ne or i nos i tol , forma ti on of, 89 3-Phos phogl ycera te dehydrogena s e defi ci ency, 338t Phos phoenol pyruva te ca rboxyki na s e, 71 Phos phofructoki na s e defi ci ency, 66 Phos phofructoki na s e-1, 66 Phos phol i pa s e, 162t Phos phol i pa s e C, 273 Phos phogl ycera te ki na s e, 66 Phos phogl yceri de s ynthes i s , 88 Phos phogl yceri des , 88, 89f–90f Phos phol i pi d components a nd forma ti on, 26f Phos phol i pi d mol ecul es , 25 Phos phol i pi ds , 24, 25, 30f, 94 Phos phorous , 146t Phos phos eri ne a mi notra ns fera s e defi ci ency, 338t Phyl oqui none, mena qui none, 145t Pi nk puffers , 255 Pi nocytos i s , 222 Pi tui ta ry hormone, 314 Pl a centa l va s cul a ture, 209 Pl a que, 150 Pl a s ma , 202 Pl a s mi n a cti va ti on of, 219f a nd cl ot di s s ol uti on, 218–219, 219f Pl a s mi nogen, 218 Pl a tel et a ggrega ti on, preventi on of, 214 cl ot forma ti on, 217–218 pl ug forma ti on, 213–214, 215f Pl a tel et-a cti va ti ng fa ctor, 257t Pl a tel et pl ug, forma ti on of, 215f Pneumoconi os i s di s ea s es , 261, 261t Podoci n, 265 Podocytes , 264
Podos ome, 195 Poi nt muta ti on, 113 cl a s s i fi ca ti on, 113 Pol ya cryl a mi de gel , 374 Pol ya cryl a mi de gel el ectrophores i s (PAGE) techni que, 373–375 ba s i s , 374f s odi um dodecyl s ul fa te (SDS), 375f two-di mens i ona l (2D), 375, 375f Pol ya denyl a ti on, 107 Pol ycys ti c ki dney di s ea s e (PCKD), 264 Pol ymeri za ti on rea cti on, 50 Pol yol pa thwa y, 140b Pol ymyos i ti s , 261t Pol ypepti de, 7 hormones (Gs receptor), 100t Porphyri a s , 206t Pos teri or pol ymorphous dys trophy, 190t Pos ti nfecti ous gl omerul onephri ti s , 266t Pos t-tra ns l a ti ona l modi fi ca ti on, 110 Pota s s i um, 145t, 269t Pota s s i um-s pa ri ng di ureti cs , 275 Precurs or or proenzymes , 58–59 Pregna ncy tes ti ng a nd hCG, 303 Preka l l i krei n, 215 Preprocol l a gen, 188f Pri ma ry Ci l i a ry Dys ki nes i a , 182 Pri nzmeta l ’s a ngi na , 248 Pri ons , 111b Proges terone, 35, 301–302, 307t i mpa ct of i ncrea s ed l evel s , 301 i nfl uence on endometri um, 302 Proges terone a nta goni s ts , 302 Proges togens , 301 Progra mmed cel l dea th (a poptos i s ), 278 Prol a cti n, 306, 307t, 318 Prol a cti noma , 306 Prol i ne, 4, 6f, 6t Promoter s equence, 107 Propi oni c a ci demi a , 278t Propi onyl -CoA, 81 Propofol , 295 Propra nol ol , 239 Pros ecreti n, 164t Pros ta cycl i ns (PGIs ), 29 Pros ta gl a ndi ns (PGs ), 29 Pros ta gl a ndi n D 2 , 257t Pros ta gl a ndi n E2 , 154, 155f Pros ta gl a ndi n precurs or (PGH 2 ), 29 Pros ta gl a ndi n thromboxa ne, 214 Pros ta noi ds , 29 Prota mi nes , 104 Protea s e-a nti protea s e theory, 255 Protea s e enzymes , 257t Protei n a nd deoxyri bonucl ei c a ci d (DNA)/ri bonucl ei c a ci d (RNA) preci pi ta ti on, 380 Protei n “ba ckbone”, 7 Protei n C, 218 Protei n fol di ng, 9 Protei n ki na s e C (PKC), 101, 273 Protei n ki na s e G, 271 Protei n s tructure, l evel s of, 7–9 pri ma ry (1°) s tructure, 7 qua terna ry (4°) s tructure, 9, 10, 10f s econda ry (2°) s tructure, 8, 8f terti a ry (3°) s tructure, 8, 9f Protei n s ynthes i s , 108–109 el onga ti on (EF) fa ctor, 108 i ni ti a ti on fa ctor (IF), 108 Protei n tra ffi cki ng, 110 Protei n tyros i ne ki na s e (PTK), 255 Protei ns , 96 ca tegori es of, 9–13 cl a s s i fi ca ti on, 96 Proteogl yca ns , 20
Proteol ys i s , 216 Proteos ome, 226 Prothrombi n, 216 Proton pump i nhi bi tors , 98b Protons (H +), 269t Protoporphyri n, 203, 205 Proxi ma l convol uted tubul e, 268 Proxi ma l hi s ti di ne, 203 Ps eudohypera l dos teroni s m, 275 Ps eudomembra nous col i ti s , 166 PTK phos phoryl a ti ons , 255 P-type fi mbri a e of Escherichia coli, 267 Pudenda l nerve, 266 Pul mona ry a rteri es , 252–253, 252f Pul mona ry compl i a nce, 253 Pul mona ry edema , 208, 272 Pul mona ry hypertens i on, 272 Pul mona ry s urfa cta nt, 251 l i pi d component, 253 a nd l ung devel opment, 253 a nd prema ture bi rth, 254 protei n component, 253 Pul mona ry vei ns , 252–253, 252f Puri ne ri ng, cons ti tuents of, 39 Puri nes , 38 Pyel onephri ti s , 267 Pyri dos ti gmi ne, 174 Pyri mi di ne ni trogenous ba s es , 39 Pyri mi di ne ura ci l ri ng, 40f Pyri mi di ne-deri ved nucl eos i des cyti di ne, 40f Pyri mi di ne-deri ved nucl eos i des uri di ne, 40f Pyri mi di nes , 38 Pyruva te, 69, 135, 178 ca rboxyl a s e, 71, 135, 312 dehydrogena s e, 69 dehydrogena s e ki na s e, 135 ki na s e, 66 ki na s e defi ci ency, 346t Q Qui none, 151t R Ra di a l s pokes , 182 Ra di a ti on, 260 Ra ndom coi l , 8 Ra pi dl y progres s i ve (cres centi c) gl omerul onephri ti s , 266t Ra pi dl y progres s i ve gl omerul a r nephri ti s , 264 Ra s GTPa s e protei n, 299 Rea bs orpti on proces s , 268 Rea cti ve oxygen s peci es , 140b Rea cti ve pha s e of bone hea l i ng, 198 Receptor a cti va tor of nucl ea r fa ctor ka ppa B (RANK), 194 Red bl ood cel l s (RBC), 69, 96 brea kdown of, 155 components of, 203f functi ons , 203, 203f, 204f, 205f Refra ctory peri od of repol a ri za ti on, 284 Refs um di s ea s e, 87, 356t Regul a tory myos i n l i ght cha i ns , 171 Regul a tory RNA, 42 Rejecti on, tra ns pl a nt, 234 Rel a xed hemogl obi n (Hgb), 205, 205f Remi ki ren, 270 Rena l corpus cl e, 265 Rena l l i thi a s i s , 359t Reni n, 265, 316 Reni n i nhi bi tors , 270 Reni n–a ngi otens i n s ys tem, 318 Reni n–a ngi otens i n–a l dos terone s ys tem (RAAS), 263, 270–276, 271f a l dos terone, 275–276 a ngi otens i nogen/a ngi otens i n I a nd II, 273 a tri a l na tri ureti c pepti de (ANP), 276, 276f ma cul a dens a a nd bl ood fl ow/os mol a ri ty, 270–272
reni n a nd bl ood pres s ure, 270 va s opres s i n, 276 Renni n, 154 Repea ted pepti de bonds , 7 Reperfus i on i njury, 248 Reproducti ve s ys tem ba s i c a na tomy a nd devel opment, 297–299 fema l e brea s t ti s s ue devel opment a nd l a cta ti on, 306–307, 307t es trogens , 301 FSH, 299–300 GnRH, 299 hCG, 302–303 LH, 300 mens trua l cycl e, 303, 304f proces s of ferti l i za ti on, 305, 305f proges terone, 301–302 ma l e fol l i cl e-s ti mul a ti ng hormone (FSH), 308f, 309 l utei ni zi ng hormone (LH), 308f, 309 tes tos terone, 307 overvi ew, 297 Reproducti ve tra ct, 179t Res erpi ne, 293 Res orpti on, 200 Res pi ra tory a ci dos i s , 254–255 Res pi ra tory a l ka l os i s , 254–255 Res pi ra tory ba cteri a , 255 Res pi ra tory bronchi ol i ti s -a s s oci a ted i nters ti ti a l l ung di s ea s e, 261t Res pi ra tory di s tres s s yndrome, 312 Res pi ra tory mus cl e fa ti gue, 254 Res pi ra tory s ys tem ba s i c a na tomy a nd devel opment, 252–254 rol e i n body, 251–252, 252 Res pi ra tory tra ct, 179t Res us ci ta ti on, 314 Reti cul a r fi ber, 186 Reti cul oendothel i a l cel l s , 212 Reti na l i s omera s e, 294 Reti n-opa thy, 143b Retrovi ra l thera py, 107, 107b Retrovi ra l s , 107 Revers e tra ns cri pta s e, 107b i nhi bi tors , 107, 107b R-Group cl a s s i fi ca ti ons , 5t–6t R-group, 3 cha ra cteri s tcs of, 4 Rha bdo, 174 Rha bdomyol ys i s , 174 Rheuma toi d a rthri ti s , 261t, 262 Rho ki na s e, 171 Rhodops i n, 294 Rhodops i n ki na s e, 294 RIA, 376–377, 376f Ri bonucl ei c a ci d (RNA), 38 Ri bos oma l RNA (rRNA), 42, 106 Ri bos ome s ynthes i s , 106 Ri bozyme, 107 Ri ckets , 196 RNA mol ecul es , 42 RNA pol ymera s e, 107 Rota vi rus e, 110b Rough endopl a s mi c reti cul um (RER), 188f Roxi thromyci n, 110b r-protei ns , 106 RU-486, 302 Rya nodi ne receptor, 176, 240 S Sa l i va , compos i ti on of, 151t–152t Sa l i va ry a ci d phos pha ta s es , 151t Sa ndhoff di s ea s e, 356t Sa rcoi dos i s , 261t, 262 Sa rcol emma , 168 Sa rcoma homol ogy doma i ns , 223
Sa rcomere, 168 Sa rcopeni a , 175 Sa rcopl a s mi c reti cul um (SR), 168, 240 Sa tura ted fa tty a ci ds , 24 mel ti ng poi nts , 25 Sa tura ted l i pi d ta i l s , 94f Schmi d-type meta phys ea l chondrodys pl a s i a (SMCD), 190t Scurvy, 194 Secreti n, 153t, 154 Secreti on, 268 Sel ecti ve proges terone receptor modul a tors (SPRMs ), 302 Sel eni um, 147t Sel f-a nti gens , 222 Sel f-huggi ng, 61 Seni or–Loken s yndrome, 12, 182 Sequence recogni ti on pa rti cl es (SRPs ), 110 Seri ne ki na s e a cti vi ty, 193 Seri ne, 5t Serotoni n (5-HT), 290 Sertol i cel l s , 309 a roma ta s e, 309 Serum, 202 Severe combi ned i mmunodefi ci ency di s ea s e (SCID), 359t Sex-determi ni ng regi on, on the Y chromos ome (SRY), 298 hormona l i nvol vement, i n s exua l di fferenti a ti on, 298f Sexua l i nfa nti l i s m, 299 Short gut s yndrome, 163 Short-cha i n a cyl -CoA dehydrogena s e (SCAD), 87 Si a l i c a ci d mol ecul es , 29 Si ckl e cel l di s ea s e (SCD), 209–211, 210f, 317 Si de cha i ns , 51 Si deros i s , 261t Si gmoi d, 205 Si gna l s equences , 110 Si gna l tra ns ducer, 102f a nd a cti va tor of tra ns cri pti on (STAT), 209, 224, 306 Si l dena fi l , 272 Si l i cos i deros i s , 261t Si l i cos i s , 261t Si mpl e one a nd two s ubs tra te enzyme rea cti on, 56f mol ecul a r s ha pe, 56f Si ngl e s ubuni t (“monomeri c”) enzyme, 59 Si ngl e-s tra nded bi ndi ng protei ns , 106 Si noa tri a l a nd tri oventri cul a r nodes , 236, 237 Si nus i ti s , 262 Skel eta l mus cl e, 11, 168f, 173f. See also under Mus cl es a nd moti l i ty contra cti on a nd rel a xa ti on, cycl e of, 316 fi ber types , 175f Ski n, 179t Sl i t di a phra gms , 265 Sl ow-twi tch fi bers , 174 Sma l l i ntes ti ne, 161, 163, 163f functi ons of, 164t–165t i nfecti ous /i nfl a mma tory di s ea s es of, 163 Sma l l s i gna l i ng mol ecul es , 100t Smi th-Ma geni s s yndrome (SMS), 339t Smooth mus cl e, 168f, 176–178, 177f, 178f, 179t contra cti on, 177f myos i n force genera ti on, regul a ti on of, 178f Smooth mus cl e cel l s , 179t ca l ci um a cti va ti on of, 237f Sodi um, 145t, 269t Sodi um fl uori de, 197 Sodi um–pota s s i um ATPa s e (Na +–K+-ATPa s e) pump, 98 Sol ubl e receptor-a s s oci a ted tyros i ne ki na s es , 101 Soma tos ta ti n, 153t Sorbi tol –a l dol a s e reducta s e pa thwa y, 140b Speci a l R-groups , 4 Sphi ngol i pi d, 27, 28f Sphi ngomyel i n, 27 Sphi ngos i ne, 27, 90 Spi nobul ba r mus cul a r a trophy (SBMA) (Kennedy’s di s ea s e), 363t Spondyl ometa phys ea l dys pl a s i a (SMD), 190t
Squa mous cel l ca rci noma , 113b Src ki na s e, 306 SRP docki ng protei n, 110 Sta rch, 18 Sta ti ns , 92 Stea ra te (18-ca rbon), 24f Steroi d hormone receptor, a cti va ti on, 99f. Steroi d hormones , 34, 99, 111 Steroi d producti on, duri ng pregna ncy, 302f Steroi d res pons e el ements , 111 Stoma ch. See under Di ges ti ve s ys tem Streptococcus pyogenes, 266t Stretch-a cti va ted cha nnel , 270 Structura l protei ns , 11 Struvi te s tones , 279 “Stuck” fa tty a ci d degra da ti on, 87 Subl i ngua l ni trogl yceri n, 240 Subs ta nce P, 293 Subs tra te concentra ti on, 56 Subs tra te-bi ndi ng s i tes /a cti ve s i te, 56 Subs tra tes , 56 Succi na te dehydrogena s e, 70 Suga r ri bos e, 39 Sul fa drugs , 73 Sul fa ti de, 27, 28f Sul fur, 147, 147t Sul fur a tom-conta i ni ng s ul fa te (SO4 –2) group, 27 Sul fur (S), 396, 396t Sul fur–s ul fur bonds (–S–S–), 397, 397t Superoxi de di s muta s e, 151t Surfa cta nts , 306 Syncoi l i n, 183t Synemi n (Des mus l i n), 183t Sys temi c l upus erythema tous (SLE), 261t Sys temi c s cl eros i s , 261t T T l ymphocytes , 223–227, 225f, 226f Ta da l a fi l , 272 Ta ngi er di s ea s e, 357t Ta rtra te res i s ta nt a ci d phos pha ta s e (TRAP), 195 TATA box, 111 Ta xol , 114b Ta y–Sa chs di s ea s e, 91, 357t T-cel l receptor recogni ti on of a nti gen, 225f Tempera ture a nd pH on hemogl obi n bi ndi ng of oxygen, 207f Tendon xa nthoma , 245 Tens e hemogl obi n (Hgb), 205, 205f Tes tos terone, 307, 309 Tetra methyl ethyl enedi a mi ne (TMED), 374 Tha l a s s emi a , 206t, 312 Thi a mi ne-defi ci ent cel l , 317 Thi a zi de di ureti cs , 267 Thi ck fi l a ment, 170f, 171 compos i ti on of, 169f Thi n ba s a l l a mi na di s order, 266t Thi n fi l a ment, 170f Thi n l a yer (pa per) chroma togra phy (TLC), 377, 378f Thi ocya na te, 152t Threoni ne, 5t Thrombi n, 216 Thrombol ys i s , 248 Thrombopoi eti n, 159t Thromboxa nes (TXs ), 29 Thrombus , 246 Thymi di ne deoxyri boncl eoti de (dTMP), 40 s ynthes i s of, 40f Thymi di ne di mer repa i r, 113b Thymi ne (T), 39 Thyroi d-s ti mul a ti ng hormone (TSH), 299 Tocopherol s /tocotri enol s , 145t Toni c contra cti on, 177 Tota l ca l ci um, 200 Toxi c oxygen “ra di ca l s ”, 73
Tra becul a r bone, 198 Tra chea , 251, 252 Tra cki ng dye, 374 Tra ns fa tty a ci ds , 25 Tra ns cri pti on (JAK–STAT) s i gna l i ng, a cti va tor of, 102f Tra ns cri pti on fa ctor, 107–108, 111 Tra ns duci n, 294 Tra ns fecti on, 385 Tra ns fer RNA (tRNA) mol ecul es , 42 Tra ns ferri n, 111, 211, 212f i n i ron tra ns porta ti on, 212f Tra ns i ti on muta ti on, 113 Tra ns pl a nt rejecti on, 234 Tra ns port cha nnel protei ns , 12–13 of fa tty a ci ds us i ng CPT I a nd CPT II, 83f of food from mouth to rectum/a nus , 150f Tra ns porter a s s oci a ted wi th a nti gen proces s i ng (TAP tra ns porters ), 226 Tra ns vers e tubul es (T tubul es ), 168 Tri a cyl gl ycerol , 24, 25f, 29, 81, 88, 242 s ynthes i s , 88 s yntha s e enzyme compl ex, 88 Tri a cyl gl ycerol -ri ch l i poprotei n, 241 Tri ca rboxyl i c a ci d cycl e, 69 Tri gl yceri de, 141t, 159, 160, 174 Tri os e gl ycera l dehydes , 16 Tri oventri cul a r nodes , 236, 237 Tri pl e hel i x, 50 Tropocol l a gen, 194 Tropomyos i n, 170, 170f Troponi n, 170, 247 Tryps i n, 162t Tryps i nogen, 162t Tryptopha n, 5t Tubercul os i s , 262 Tubul e s ys tem, 268 Tubul i n, 11, 180 Tuftel i n, 150 Tumor (s wel l i ng), 30 Tumor necros i s fa ctor (TNF), 231t, 255, 257t TXA2 , 31t Type I col l a gen muta ti on, 315 Type I hypers ens i ti vi ty res pons e, 256 Type I tyros i nemi a (tyros i nos i s ), 339t Type II a l veol a r cel l s , 253, 260 Type II tyros i nemi a , 339t Type III hypers ens i ti vi ty, 262 Type III tyros i nemi a , 339t Type IV col l a gen, 264 Tyros i ne, 5t Tyros i ne ki na s es , 273 U Ul ori c (febuxos ta t), 41 Unes teri fi ed chol es terol mol ecul e, 32f Uns a tura ted fa tty a ci ds , 24, 24f wi th ca rbon–ca rbon doubl e bonds , degra da ti on of, 86, 86f Uns a tura ted l i pi d ta i l s , 94f Ura ci l (U), 38 Urea , 62, 63f, 266, 269t Urea cycl e, 62–63, 63f, 157, 157t regul a ti on of, 62 Ureters , 266 Urethra , 266 Uri c a ci d, 279 Uri di ne monophos pha te (UMP), 39 Uri na ry s ys tem ba s i c a na tomy a nd phys i ol ogy, 263–264 components , 263, 266 i nul i n/crea ti ni ne cl ea ra nce, 270 nephron, 267–268 rena l corpus cl e, 264–266, 265f Uri na ry tra ct, 179t Uri na ry Tra ct Infecti ons (UTIs ), 267
Urobi l i nogen, 154 Uteri ne l ei omyoma , 302 V Va ga l nervectomy, 154 Va gus nerve, 154 Va l i ne, 5t Va ncomyci n, 166 Va rdena fi l , 272 Va s ocons tri cti on, 240 Va s odi l a ti on proces s , 240 Va s opres s i n, 276 Vector, 384 Vei n, 241f Veno-occl us i ve cri s es , 211 Vera pa mi l , 238t Very-l ong-cha i n fa tty a ci ds (VLCFA), 87 Very-l ow-dens i ty l i poprotei n (VLDL), 88, 159, 160, 241, 242, 243, 244f meta bol i c fa te of, 245f Vi menti n, 183 Vi ncri s ti ne, 114b Vi ri ons , 111b Vi s i on, bi ochemi s try of, 294, 294f Vi ta mi n A (reti nol es ters ), 50, 294 defi ci ency, 51 Vi ta mi n B 12 a bs orpti on, 163 Vi ta mi n C (a s corbi c a ci d), 50, 194 defi ci ency, 313 Vi ta mi n D, 35, 50, 198 from chol es terol , producti on of, 35f Vi ta mi n D 3 , 279 Vi ta mi n E, 50 Vi ta mi n K, 50, 218 Vi ta mi ns , 130, 143, 144t–145t Vi tel l i ne l a yer of egg, 305 VLDL. See Very-l ow-dens i ty l i poprotei n (VLDL) Vol ta ge-dependent ca l ci um cha nnel s (VDCC), 239 Von Wi l l ebra nd’s Di s ea s e (vWD), 213, 217, 365t W Wa rfa ri n, 218 Wa rts , 229 “Wa ter ha ti ng” R-groups , 7 Wegener’s gra nul oma tos i s , 262, 266t Werni cke–Kors a koff s yndrome, 130 Wes tern bl otti ng, 383 Whi te bl ood cel l s , 225f Whoopi ng cough, 262 Wi l m’s tumor, 114b Wi l s on’s di s ea s e, 57 ATPa s e, Cu 2+ tra ns porti ng, beta pol ypepti de (ATP7B), 57 Wol ffi a n ducts , 297, 299 Wol ff–Pa rki ns on–Whi te s yndrome, 317 Woma n’s bra i n ti s s ue, a fter hunger s tri ke, 314 X Xa ntha nuri a , 359t Xa nthi ne, 40 Xa nthoma s , 245, 312 Xeroderma pi gmentos um (XP), 113b, 363t X-l i nked di s ea s e, 94. See also Ba rth s yndrome Z Za fi rl uka s t, 258 Za na mi vi r/ Rel enza , 76 Zel l weger s pectrum, 87 Zel l weger s yndrome, 87 Lorenzo’s oi l , 87 trea tment for, 87 Zeta -cha i n-a s s oci a ted protei n ki na s e-70 (ZAP-70), 223 Zi dovudi ne, 107, 107b Zi l euton, 258 Zi nc, 146t Zi nc fi nger, 112t
Zona pel l uci da , 305 ZP3, 305
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