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
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).
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.
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.
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).
Carbohydrate Structure and Nomenclature
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 (Figure 2-3).
FIGURE 2-3: D– and L-isomerism of glycerose and glucose. Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper’s Illustrated Biochemistry, 29th ed. New York, NY: McGraw-Hill; 2012.
The aldehyde and ketone moieties of the carbohydrates with 5 and 6 carbons will spontaneously react with alcohol groups present in the neighboring carbons to produce intramolecular hemiacetals or hemiketals, respectively. This results in the formation of 5- or 6-membered rings. Because the 5-membered ring structure resembles the organic molecule furan, derivatives with this structure are termed furanoses. Those with 6-membered rings resemble the organic molecule pyran and are termed pyranoses (Figure 2-4).
FIGURE 2-4: Pyranose and furanose forms of glucose. Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper’s Illustrated Biochemistry, 29th ed. New York, NY: McGraw-Hill; 2012.
The ring structures of the carbohydrates can be depicted by either Fischer- or Haworth- (also called the chair form) style diagrams (Figure 2-5). The numbering of the carbons in carbohydrates proceeds from the carbonyl carbon, for aldoses, or the carbon nearest the carbonyl, for ketoses. The rings can open and reclose, allowing rotation to occur about the carbon bearing the reactive carbonyl yielding 2 distinct configurations (α and β) of the hemiacetals and hemiketals (Figure 2-6). The carbon about which this rotation occurs is the anomeric carbon and the 2 forms are termed anomers. Carbohydrates can change spontaneously between the α- and β-configurations: a process known as mutarotation. When drawn in the Fischer projection, the α-configuration places the hydroxyl attached to the anomeric carbon to the right, toward the ring. When drawn in the Haworth projection, the α-configuration places the hydroxyl downward. Constituents of the ring that project above or below the plane of the ring are axial and those that project parallel to the plane are equatorial. In the chair conformation, the orientation of the hydroxyl group about the anomeric carbon of α-D-glucose is axial and equatorial in β-D-glucose.
FIGURE 2-5: D-Glucose. (A) Straight-chain form. (B) α-D-glucose; Haworth projection. (C) α-D-glucose; chair form. Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper’s Illustrated Biochemistry, 29th ed. New York, NY: McGraw-Hill; 2012.
FIGURE 2-6: Pyranose and furanose forms of fructose. Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper’s Illustrated Biochemistry, 29th ed. New York, NY: McGraw-Hill; 2012.
Monosaccharides
The monosaccharides commonly found in humans are classified according to the number of carbons they contain in their backbone structures. The major monosaccharides, used either for energy production, or as components in complex macromolecules, contain 3 to 6 carbon atoms (Table 2-1). The 5-carbon sugars (the pentoses, Table 2-2) and the 6-carbon sugars (the hexoses, Table 2-3) represent the largest groups of physiologically important carbohydrates in human metabolism. Sedoheptulose is an additional biologically important carbohydrate that contains 7-carbon atoms (see Figure 2-2). The amino sugars are another class of biologically significant carbohydrates that contain nitrogen. These include N-acetylglucosamine (GlcNAc: Figure 2-7), N-acetylgalactosamine (GalNAc), and N-acetylneuraminic acid (NANA; also known sialic acid, Sia: Figure 2-8).
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