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Introduction

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High-Yield Terms

  • Carbohydrate: any organic molecule composed exclusively of carbon, hydrogen, and oxygen where the hydrogen-to-oxygen ratio is usually 2:1, biological synonym is saccharide, commonly called sugars

  • Saccharide: synonym for carbohydrate in biological systems, lay terminology is sugar

  • Aldose: a monosaccharide that contains only one aldehyde (–CH=O) group per molecule

  • Ketose: a monosaccharide that contains only one ketone (–C=O) group per molecule

  • Enantiomer: one of 2 stereoisomers that are mirror images of each other, which cannot be superimposed

  • Anomeric carbon: the carbon of a carbohydrate bearing the reactive carbonyl about which free rotation into 2 distinct configurations (termed α and β) can occur when in the cyclic form

  • Glycosidic bond: any of the type of covalent bond that joins a carbohydrate molecule to another group

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Simple carbohydrates are biological compounds composed solely of carbon, oxygen, and hydrogen that generally contain large quantities of hydroxyl groups (–OH). In biochemistry, carbohydrate is synonymous with saccharide and the more common term, sugar. The simplest carbohydrates also contain either an aldehyde moiety and are termed polyhydroxyaldehydes, commonly called aldoses (Figure 2-1), or a ketone moiety and are termed polyhydroxyketones, commonly called ketoses (Figure 2-2).

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FIGURE 2-1:

Examples of aldoses of physiologic significance. Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper's Illustrated Biochemistry, 29th ed. New York, NY: McGraw-Hill; 2012.

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FIGURE 2-2:

Examples of ketoses of physiologic significance. Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper's Illustrated Biochemistry, 29th ed. New York, NY: McGraw-Hill; 2012.

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All carbohydrates can be classified as either monosaccharides, oligosaccharides, or polysaccharides. Anywhere from 2 to 10 monosaccharide units, linked by glycosidic bonds, make up an oligosaccharide. Polysaccharides are much larger, generally 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. Derivatives of the carbohydrates can contain nitrogen, phosphates, and sulfur compounds. Carbohydrates can also combine with lipid to form glycolipids (see Chapter 21) or with protein to form glycoproteins (see Chapter 38).

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Carbohydrate Structure and Nomenclature

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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 the 2 conformations, as determined by the orientation of the hydroxyl group about the asymmetric carbon farthest from the carbonyl. 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 ...

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