PPT Carbohydrates

CARBOHYDRATE

CARBOHYDRATE •

Carbohydrates are essential to all living organisms and are the most abundant class of biological molecules.

•

The metabolic breakdown of monosaccharides provides most of  the energy used to power biological processes.

•

Chemically, they are polyhydroxyl aldehydes or ketones.

•

Contain three elements - C, H, O, many according accord ing to the formula (CH2O)n; where n ≥ 3.

•

Greek saccharon means sugar 

•

Carbohydrates function: Energy sources (glucose/glycogen) Structural elements: –

cell wall (plants, bacteria)

–

connective tissues

Monosaccharides •

Monosaccharide is another term for a simple sugar, which is not  linked to any other sugars.

•

Common monosaccharides include: glucose, mannose, fructose, ribose, and galactose

•

Monosaccharides can be classified as or  depending upon the number of carbon atoms.

Some Examples of Monosaccharides

•

It is commonly found in fruits.

•

It is known as dextrose , a name that derives from the fact that the predominant natural form of the sugar is dextrorotatory.

•

It is made by the hydrolysis of starch.

•

It is only 75% as sweet as sucrose.

•

It is found in fruits and honey.

•

It also referred to as levulose because it has an optical rotation that is strongly levorotary (- 92 0)

•

It exists in two forms, pyranose (free state) or  furanose (combined form) .

•

It is the sweetest sugar.

•

It is more commonly found in the disaccharide, lactose or milk sugar.

•

It does not occur in nature in the uncombined state.

•

It is formed by the hydrolysis of lactose.

•

It is needed by the human body for the synthesis of lactose (in the mammary  glands) is obtained by the conversion of Dglucose into D-galactose.

•

It is an important constituent of the glycolipids that occur in the brain and the myelin sheath of nerve cells.

•

Monosaccharides can be classified as as or  depending upon they have an aldehyde or ketone group. Example:

Glyceraldehyde and dihydroxyacetone have the same atomic composition, but differ only in the position of  the hydrogens and double bonds.

 Aldose-ketose intercoversion via an enediol intermediate

When the structures of molecules are related in those ways, the molecules are called

Fischer Projection of Monosaccharide •

The structure of monosaccharide can be drawn by the Fischer projection formula. The carbon chain was composed vertical and the aldehyde carbon at the top.

•

If the position of hydroxyl group of the highest numbered stereogenic center was at:

 Right  D  Left  L

D- and L-isomerism of glyceraldehyde and glucose

The OH group is on the left

L-sugars are mu ch  l es s a b u n d a n t i n   nature.

The OH    H    O group is on the right

Haworth Projection of  Monosaccharide •

Haworth projection is a conventional planar representation of a cyclized monosaccharide molecule. It is useful to show whether the OH groups on the ring are cis or trans to each other.

•

The OH groups that are represented on the right in a Fischer projection are down in a Haworth projection, and vice versa .

Epimers •

Epimers are diastereomers that contain more than one chiral center but differ from each other in the absolute configuration at only one chiral center.

•

For a carbon with 'm' chiral carbons, the number of possible stereoisomers is . Example: An aldohexose (D-glucose) has 4 chiral carbons, thus it has 16 stereoisomers.

How many are the of D-glucose? How many are the of  D-glucose?

•

Naturally occuring ketoses have the ketone group in the 2position. A ketose has one fewer chiral carbon than does an aldose with the same number of carbon atom.

•

Therefore, with the same number of carbon atoms. Example: A ketohexose (D-fructose) has 3 chiral carbons, thus it  has 8 stereoisomers.

How many are the of D-fructose? How many are the of  D-fructose?

Monosaccharide Ring Structures  Alcohol groups can react with the aldehydes or ketones to form hemiacetals and hemiketals.

A new chiral center is born!

Formation of the two cyclic forms of D-glucose

Sugars with ≥ 5 C mostly  exist in their cyclized form (intramolecular hemiacetal  formation) 

Monosaccharide can form the cyclic form as: 

=> five-membered ring 



=> six-membered ring 

The Chair and Boat Conformation of Monosaccharide Both conformations (chair and boat) exist, though the is thermodynamically more stable .

Mutarotation •

Mutarotation is a process whereby the configuration of an anomeric carbon converts from α to β and vice-versa (the gradual change of optical rotation), which continues until equilibrium is established.

Monosaccharide as reducing agents • The reducing end: the sugar with the free anomeric carbon that  can be oxidized. • Oxidation occurs only with the linear form. • Maltose and lactose are reducing sugars, while sucrose is not.

Bond between two anomeric carbons

• Monosaccharides act as reducing agents, because the aldehyde group that is present can be oxidized by oxidizing agents such as Fe3+ or Cu2+ ions to form a carboxylic acid group, or in the presence of a base, a carboxylate ion group.

Disaccharides •

Disaccharides are compounds consisting of two monosaccharide subunits linked together by an glycosidic linkage.

•

Disaccharides are taken apart by hydrolysis and put together  by condensation

•

Common disaccharides include:  Sucrose

=> glucose + fructose

 Maltose

=> glucose + glucose

 Lactose

=> glucose + galactose

 Cellobiose => glucose + glucose

Disaccharides arise through the formation of O-glycosidic bonds: condensation of anomeric carbon hydroxyl group with an alcohol

Glu( 1

4)Glu

Some Examples of Disaccharides

•

It is known as beet sugar, cane sugar, tablet sugar, or simply as sugar.

•

It is incapable of mutarotation and said to be a nonreducing sugar . This is because of the presence of the 1,2-glycosidic linkage makes it  impossible for it to exist in the α- or β- configuration or in the openchain form.

•

It occurs in animals as the principal sugar formed by the enzymic (ptyalin) hydrolysis of starch.

•

It is a reducing sugar, and it exhibits mutarotation. This is because of  the presence of the 1,4-glycosidic linkage makes it possible for it to exist in the α- or β- configuration or in the open-chain form.

•

It is a stereoisomers of maltose. It is also composed of two glucose units, but in this case the two sugar moieties are joined by  β-1,4glycosidic linkage.

•

It is a reducing sugar, and it undergoes mutarotation. This is because of the presence of the 1,4-glycosidic linkage makes it possible for it to exist in the α- or β- configuration or in the open-chain form.

•

It known as milk sugar because it occurs in the milk of humans, cows, and other mammals.

•

It is a reducing sugar, and it exhibits mutarotation.

Polysaccharides •

The polysaccharides are the most abundant of the carbohydrates found in nature.

•

They serve as reserve food substances and as structural components of plant cell.

•

They are high molar mass (25,000-15,000,000) polymers of  monosaccharides joined together by glycosidic linkage.

•

There are two types of polysaccharides: 

: when all the monosaccharide units of a polysaccharide are the same. •

Storage: starch (storage in plants) and glycogen (storage in animals).

•

Structural elements: cellulose (plants cell wall) and chitin (animal exoskeleton).



: when the repeating monosaccharide units of the polysaccharide are different. ex: hyaluronic acid, keratan sulfate and agarase

Some Examples of Homopolysaccharides •

It is a polymer of glucose used for energy storage in plants such as potatoes, rice, beans, and corn.

•

Starch is a mixture of two polymers, amylose (10-30%) and amylopectin  (70-90%).

 Amylose is a straight-chain polysaccharide composed entirely of  D-glucose units join by an α-1,4-glycosidic linkage.

amylose occurs every  24 to 30 residues 

 Amylopectin is a branchedchain polysaccharide composed of glucose units join by  α-1,4glycosidic linkage with α-1,6glycosidic linkage. amylopectin

•

It is specially abundant in the liver, 4-8% (per weight of tissue), and in muscle cell, 0.5-1.0%

•

It is quite similar to amylopectin, but it is more highly branched and its branches are shorter (8-12 glucose units in length).

•

It is the major structural component of the shells of crustaceans (e.g., lobsters, crabs, and shrimps) and the exoskeletons of insects.

•

It is structurally similar to cellulose, but in that it has an N acetylamino group instead of an OH group at the C-2 position.

•

It is the major structural polysaccharide in woody and fibrous plants and is the most  abundant single polymer in the biosphere.

•

Many microorganisms and herbivorous animals can digest cellulose because their  digestive tracts contain enzymes ( cellulase ) that can hydrolyze (14) linkage.

Cellulose breakdown  by wood fungi 

Some Examples of Heteropolysaccharides •

It is a type of polysaccharide called a glycosaminoglycan. Also known as hyaluronan or hyaluronate.

•

It occurs naturally in the human body and is central to regulating cell growth and renewal. In fact, it is found extensively in connective, epithelial, and neural cells.

•

It is any of several sulfated glycosaminoglycans that have been found especially in the cornea, cartilage, and bone.

•

It is also synthesized in the central nervous system where it  participates both in development and in the glial scar formation following an injury.

•

It is an enzyme found in agarolytic bacteria and is the first  enzyme in the agar catabolic pathway 

 Agarases are classified as either α-agarases or β-agarases based upon whether they degrade α or β linkages in agarose, breaking  them into oligosaccharide.

•

Derivatives of Monosaccharides •

Monosaccharides can be chemically altered in several ways, in which a hydroxyl group in the parent compound is replaced with another substituent (–NH-(C=O)-CH3, –NH2,  ‒(C=O)H   ‒COO-, –O-H  –O-PO32- ) or a carbon atom is oxidized to a carboxyl group to provide new classes of compounds.

•

Derivatives of monosaccharides include:   Alditols, ex: erythritol, sorbitol, and D-mannitol   Amino sugars, ex:

β-D-glucosamine, N-acetyl-β-D-

glucosamine, muramic acid, and N-acetylmuramic acid

Some Examples of Derivatives of Monosaccharides

• They are made by reducing  the carbonyl group of a sugar to a hydroxyl. • They are used in chewable tablets as sweetening agents  to mask the unpleasant taste  of vitamins and minerals and to improve texture. • These natural sweeteners are extracted and purified from plant sources, particularly  from fruits.

It is made by reducing a hydroxyl of sugar with an amine group.