ABSTRACT
Carbohydrates are carbon compounds that contain large quantities of hydroxyl groups. The simplest carbohydrates also contain either an aldehyde moiety (these are termed polyhydroxyaldehydes) or a ketone moiety (polyhydroxyketones). They are the most abundant biological molecules, and fill numerous roles in living organisms, such as the storage and transport of energy (starch, glycogen and sucrose) and structural components (cellulose in plants, chitin in animals).1,2 All carbohydrates can be classified as monosaccharides, oligosaccharides, or polysaccharides. Anywhere from two to ten monosaccharide units, linked by glycosidic linkage, make up an oligosaccharide. Polysaccharides are much larger, containing hundreds of monosaccharide units. The presence of the hydroxyl groups allows carbohydrates to interact with the aqueous environment and to participate in hydrogen bonding, both within and between chains. Additionally, carbohydrates and their derivatives play major roles in the functioning of the immune system, fertilization, pathogenesis, blood clotting, and development. Derivatives of the carbohydrates can contain nitrogens, phosphates, and sulfur groups. Some carbohydrates can attach by C-, Oor N-glycosidic bonds, giving rise to complex structures namely glycoconjugates. When the sugar moiety is combined with lipid it forms a glycolipid and when it is combined with protein it forms another glycoconjugate molecule, a glycoprotein. The predominant carbohydrates encountered in the body are structurally related to the aldotriose glyceraldehyde and to the ketotriose dihydroxyacetone. All carbohydrates contain at least one asymmetrical (chiral) carbon and are, therefore, optically active. In addition, carbohydrates can exist in either of two conformations D or L, as determined by the orientation of the hydroxyl group about the farthest asymmetric carbon from the carbonyl group. With a few exceptions, those carbohydrates that are of physiological significance exist in the D-conformation. The mirror-image conformations, called enantiomers, are in the L-conformation. The carbohydrates found in plants are monosaccharides, disaccharides, higher oligosaccharides, polysaccharides, and their derivatives: glycosides and glycoproteins. However, plants have many polysaccharides and glycoconjugates structures containing L-arabinose and L-fucose, which have been found as complex heteropolysaccharides, namely arabinogalactans and gums, respectively.1-3
Monosaccharides are classified according to three different characteristics: the placement and function of its carbonyl group, the number of carbon atoms, and its chiral center. If the carbonyl group is an aldehyde, the monosaccharide is an aldose; if the carbonyl group is a ketone, the monosaccharide is a ketose. The smallest possible monosaccharides, those with three carbon atoms, are called trioses, just as those with four, five, and six are tetroses, pentoses, hexoses, and so on.1,2 Combining the function and number of carbons there is, for example, an aldohexose, aldopentose, and ketohexose-glucose (six-carbon aldose), arabinose (five-carbon aldose) and fructose (six-carbon ketone), respectively (Figure 11.1). The carbon atoms of sugars linked to a hydroxyl group (-OH), with the exception of the carbonylic groups and
the last carbon, are asymmetric, making them stereocenters with two possible configurations. D-glucose is one isomer for an aldohexose (noncyclic form) with the formula (CH2O)6, four of its six carbons atoms are chiral centers, making D-glucose one of the 24¼ 16 possible stereoisomers in straight-chain form. Sugars belonging to the D-series are the most common isomers found in nature.
