Carbohydrates are vastly important in human biology, including roles as a major energy source, structural molecules when combined with other carbohydrates, proteins, and other molecules, and binding and signaling between molecules and cells. As a result of all these important functions, carbohydrate biochemistry is involved in a large number of disease states. Although multiple carbohydrates exist, only a few sugar molecules and polysaccharides are important to human physiology (e.g. only eight different carbohydrates are found as constituents of glycoproteins and glycolipids). However, a number of additional molecules created by linkages of carbohydrates to proteins play various roles in cell–cell interactions and biological structures.
Carbohydrates, whose names end in “-ose,” have a formula of (CH2O)x where x is a number from three to seven (giving the names of triose, tetrose, pentose, hexose, and heptose). All carbohydrates contain a ketone or an aldehyde group, as well as one or more hydroxyl groups (Figure 2-1A–B; Appendix III). The oxygen atoms of the ketone and aldehyde groups have similar reactive qualities to that of the carboxylic acid group seen in amino acids and are the sites of chemical reactions within the carbohydrate molecule, as well as with other carbohydrate, protein, or lipid molecules. Often, the ketone or aldehyde reacts with a hydroxyl group from the same sugar molecule to form a carbohydrate ring structure as shown.
A–B. Basic Carbohydrate Structures. A. The reactive ketone group of carbon 2 (green carbon group) from the hexose fructose reacts with the hydroxyl group of carbon 5 to form a new bond and a five-sided (pentose) ring structure. All carbon atoms are numbered for clarity. This reaction is fully reversible as indicated by the bidirectional arrows. As a result, the linear and ring structures are constantly changing in solution. B. The reactive aldehyde group of carbon 1 (green carbon group) from the hexose glucose reacts with the hydroxyl group of carbon 5 to form a new bond and a six-sided (hexose) ring structure. All carbon atoms are numbered for clarity. This reaction is fully reversible as indicated by the bidirectional arrows. As a result, the linear and ring structures are constantly changing in solution. [Adapted with permission from Naik P: Biochemistry, 3rd edition, Jaypee Brothers Medical Publishers (P) Ltd., 2009.]
Carbohydrates play a major role in humans as energy sources and storage, and their role in diet and nutrition, although sometimes controversial, is always one of supreme importance. However, carbohydrates play other roles as noted in Table 2-1.
Table 2-1. Biochemical Roles of Carbohydrates