ElectronTransportChain

The Electron Transport Chain

The electron transport chain is a series of biochemical reactions in which electrons and hydrogen ions from NADH and FADH2 are passed to intermediate carriers and then ultimately react with molecular oxygen to produce water. The electron transport chain is also frequently called the respiratory chain.

The enymes and electron carriers needed for the !T" are located along the inner mitochondrial membrane. #ithin this membrane are four distinct protein complexes$ each containing some of the molecules needed for the !T" process to occur. These four protein complexes$ which are tightly bound to the membrane$ are "omplex %& NADH'coenyme ( reductase "omplex %%& )uccinate'coenyme ( reductase "omplex %%%& "oenyme ('cytochrome c reductase "omplex %*& "ytochrome c oxidase

Complex I: NADH–Coenzyme Q Reductase

"omplex % contains o+er ,- subunits$ including a+in mononucleotide /F0N1 and se+eral iron'sulfur proteins /Fe)1. The 3rst electron transfer step that occurs in complex % in+ol+es the interaction of NADH with a+in mononucleotide /F0N1. The NADH is oxidied to NAD /which can again participate in the citric acid cycle1 as it passes two hydrogen ions and two electrons to F0N$ which is reduced to F0NH2.

The next steps in+ol+e transfer of electrons from the reduced F0NH2 through a series of iron4sulfur proteins /Fe)s1.

%n the 3nal complex % reaction$ Fe/%%1) is recon+erted into Fe/%%%1) as each of two Fe/%%1) units passes an electron to "o($ changing it from its oxidied form /"o(1 to its reduced form /"o(H21.

"oenyme ($ in both its oxidied and reduced forms$ is lipid soluble and can mo+e laterally within the mitochondrial membrane. %ts function is to shuttle its newly acquired electrons to complex %%%$ where it becomes the initial substrate for reactions at this complex. %n its most common form$ coenyme ( has a long carbon chain containing 5isoprene units attached to its quinone unit.

Complex II: uccinate–Coenzyme uccinate–Coenzym e Q Reductase

"omplex %%$ which is much smaller than complex %$ contains only , subunits$ including two Fe)s. This complex is used to process the FADH2 that is generated in the citric acid cycle when succinate is con+erted to fumarate.

Complex III: Coenzyme Q–Cytochrome c Reductase

"omplex %%% contains 55 different subunits. !lectron carriers present include se+eral iron' sulfur proteins as well as se+eral s e+eral cytochromes. A cytochrome is a heme6conta heme6containing ining protein in which re+ersible oxidation and reduction of an iron atom occur.

*arious cytochromes$ abbre+iated cyt a$ cyt b$ cyt c$ and so on$ differ from each other in /51their protein constituents$ /21the manner in which the heme is bound to the protein$ and /71attachments to the heme ring.

The initial substrate for complex %%% is "o(H2 molecules carrying the electrons that ha+e been processed through complex % /from NADH1 and also those processed through complex %% /from FADH21. The electron transfer process proceeds from "o(H2 to an Fe)$ then to cyt b$ then to another Fe)$ then to cyt c5$ c5 $ and 3nally to cyt c. "yt c can mo+e laterally in the intermembrane space8 it deli+ers its electrons to complex %*.

Complex I!: Cytochrome c "xidase

"omplex %* contains 57 subunits$ including two cytochromes. The electron mo+ement ows from cyt c to cyt a to cyt a7.