Carbohydrates are carbon compounds that contain largequantities of hydroxyl groups. The simplest carbohydrates also contain eitheran aldehyde moiety (these are termed polyhydroxyaldehydes) or a ketone moiety (polyhydroxyketones). Allcarbohydrates can be classified as either monosaccharides, oligosaccharides or polysaccharides.Anywhere from two to ten monosaccharide units, linked by glycosidic bonds, make up an oligosaccharide.Polysaccharides are much larger, containing hundreds of monosaccharide units.The presence of the hydroxyl groups allows carbohydrates to interact with theaqueous environment and to participate in hydrogen bonding, both within andbetween chains. Derivatives of the carbohydrates can contain nitrogens,phosphates and sulfur compounds. Carbohydrates also can combine with lipid toform glycolipids or withprotein to form glycoproteins.

The predominant carbohydrates encountered in the bodyare structurally related to the aldotriose glyceraldehyde and to the ketotriose dihydroxyacetone. All carbohydratescontain at least one asymmetrical (chiral) carbon and are, therefore, opticallyactive. In addition, carbohydrates can exist in either of two conformations, asdetermined by the orientation of the hydroxyl group about the asymmetric carbonfarthest from the carbonyl. With a few exceptions, those carbohydrates that areof physiological significance exist in the D-conformation. The mirror-image conformations, called enantiomers, are in the L-conformation.

The monosaccharides commonly found in humans areclassified according to the number of carbons they contain in their backbonestructures. The major monosaccharides contain four to six carbon atoms.

The aldehyde and ketone moieties of the carbohydrateswith five and six carbons will spontaneously react with alcohol groups presentin neighboring carbons to produce intramolecular hemiacetals or hemiketals, respectively. This results in the formation of five-or six-membered rings. Because the five-membered ring structure resembles theorganic molecule furan, derivatives with this structure are termed furanoses. Those with six-membered rings resemble the organicmolecule pyran and are termed pyranoses

Such structures can be depicted byeither Fischer or Haworthstyle diagrams. The numbering of thecarbons in carbohydrates proceeds from the carbonyl carbon, for aldoses, or thecarbon nearest the carbonyl, for ketoses.

Structural models of glucose. Glucose can exist in the - and -enantiomeric forms in solution. The structure of either form of glucose is commonly depicted using cyclic Fischer projection or the cyclic Haworth projection.

The rings can open and re-close, allowing rotation tooccur about the carbon bearing the reactive carbonyl yielding two distinctconfigurations ( and ) of the hemiacetals and hemiketals. The carbon aboutwhich this rotation occurs is the anomeric carbon and the twoforms are termed anomers. Carbohydrates can change spontaneously between the and configurations: a process known asmutarotation. When drawn in the Fischer projection, the configuration places the hydroxyl attached to the anomeric carbon to the right,towards the ring. When drawn in the Haworth projection, the configuration places the hydroxyl downward.

The spatial relationships of the atoms of the furanose and pyranose ring structures are more correctly describedby the two conformations identified as the chair form and the boatform. The chair form is the more stable of the two. Constituents of thering that project above or below the plane of the ring are axial and those that project parallelto the plane are equatorial. In the chair conformation, the orientationof the hydroxyl group about the anomeric carbon of -D-glucose is axial and equatorial in -D-glucose.

Covalent bonds between the anomeric hydroxyl of acyclic sugar and the hydroxyl of a second sugar (or another alcohol containingcompound) are termed glycosidic bonds, and the resultant molecules are glycosides. The linkage of two monosaccharides to formdisaccharides involves a glycosidic bond. Several physiogically importantdisaccharides are sucrose, lactose and maltose.

Sucrose: prevalent in sugar cane and sugar beets, iscomposed of glucose and fructose through an (1,2)-glycosidic bond.

Lactose: is found exclusively in the milk of mammals andconsists of galactose and glucose in a (1,4)glycosidic bond.

Maltose: the major degradation product of starch, iscomposed of 2 glucose monomers in an (1,4)glycosidic bond.

Most of the carbohydrates found in nature occur inthe form of high molecular weight polymers called polysaccharides. The monomeric building blocks usedto generate polysaccharides can be varied; in all cases, however, thepredominant monosaccharide found in polysaccharides is D-glucose. Whenpolysaccharides are composed of a single monosaccharide building block, theyare termed homopolysaccharides.Polysaccharides composed of more than one type of monosaccharide are termed heteropolysaccharides.

Glycogen is the major form of stored carbohydrate inanimals. This crucial molecule is a homopolymer of glucose in (1,4) linkage; it is also highly branched, with (1,6) branch linkages occurring every 8-10 residues.Glycogen is a very compact structure that results from the coiling of thepolymer chains. This compactness allows large amounts of carbon energy to bestored in a small volume, with little effect on cellular osmolarity.

Glycogen Structure. Section of a glycogen polymer depicting glucose monomers as colored balls. The blue balls represent glucose linked by 1,4 glycosidic bonds. The red balls represent glucose at branch points where there are both 1,4 and 1,6 glycosidic bonds. The orange balls represent the reducing ends of the polymeric chains of 1,4-linked glucoses. The area in the box is expanded to show the actual structure of the glucose monomers in both -1,4- and -1,6 glycosidic linkages.

Starch is the major form of stored carbohydrate inplant cells. Its structure is identical to glycogen, except for a much lowerdegree of branching (about every 2030 residues). Unbranched starch is called amylose; branched starch is called amylopectin.

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Biochemistry of Carbohydrates