Biomedical Importance of Carbohydrates

BIOMEDICAL IMPORTANCE Carbohydrates are widely distributed in plants and animals; they have important structural and metabolic roles. In plants, glucose is synthesized from carbon dioxide and water by photosynthesis and stored as starch or used to synthesize the cellulose of the plant cell walls. Animals can synthesize carbohydrates from amino acids, but most are derived ultimately from plants. Glucose is the most important carbohydrate; most dietary carbohydrate is absorbed into the bloodstream as glucose formed by hydrolysis of dietary starch and disaccharides, and other sugars are converted to glucose in the liver. Glucose is the major metabolic fuel of mammals (except ruminants) and a universal fuel of the fetus. It is the precursor for synthesis of all the other carbohydrates in the body, including glycogen for storage, ribose and deoxyribose in nucleic acids, galactose for synthesis of lactose in milk, in glycolipids, and in combination with protein in glycoproteins (see Chapter 46) and proteoglycans. Diseases associated with carbohydrate metabolism include diabetes mellitus, galactosemia, glycogen storage diseases, and lactose intolerance. Glycobiology is the study of the roles of sugars in health and disease. The glycome is the entire complement of sugars of an organism, whether free or in more complex molecules. Glycomics, an analogous term to genomics and proteomics, is the comprehensive study of glycomes, including genetic, physiological, pathological, and other aspects. A very large number of glycoside links can be formed between sugars. For example, three different hexoses may be linked to each other to form over 1000 different trisaccharides. The conformations of the sugars in oligosaccharide chains vary depending on their linkages and proximity to other molecules with which the oligosaccharides may interact. Oligosaccharide chains encode biological information that depends on their constituent sugars, sequences, and linkages. CARBOHYDRATES ARE ALDEHYDE OR KETONE DERIVATIVES OF POLYHYDRIC ALCOHOLS Carbohydrates are classified as follows: 1. Monosaccharides are those sugars that cannot be hydrolyzed into simpler carbohydrates. They may be classified as trioses, tetroses, pentoses, hexoses, or heptoses, depending on the number of carbon atoms (3-7), and as aldoses or ketoses, depending on whether they have an aldehyde or ketone group. Examples are listed in Table 15-1. In addition to aldehydes and ketones, the polyhydric alcohols (sugar alcohols or polyols), in which the aldehyde or ketone group has been reduced to an alcohol group, also occur naturally in foods. They are synthesized by reduction of monosaccharides for use in the manufacture of foods for weight reduction and for diabetics. They are poorly absorbed, and have about half the energy yield of sugars. TABLE 15-1 Classification of Important Sugars Aldoses Ketoses Trioses (CHO) Glycerose (glyceraldehyde) Dihydroxyacetone Tetroses (CHO) Erythrose Erythrulose Pentoses (CHO) Ribose Ribulose Hexoses (CH₁₂O) Glucose, galactose, mannose Fructose Heptoses (C,H,O,) Sedoheptulose 2. Disaccharides are condensation products of two monosaccharide units, for example, lactose, maltose, isomaltose, sucrose, and trehalose. 3. Oligosaccharides are condensation products of 3 to 10 monosaccharides. Most are not digested by human enzymes. 4. Polysaccharides are condensation products of more than 10 monosaccharide units; examples are the starches and dextrins, which may be linear or branched polymers. Polysaccharides are sometimes classified as hexosans or pentosans, depending on the constituent monosaccharides (hexoses or pentoses, respectively). In addition to starches and dextrins (which are hexosans), foods contain a wide variety of other polysaccharides that are collectively known as nonstarch polysaccharides; they are not digested by human enzymes, and are the major component of dietary fiber. Examples are cellulose from plant cell walls (a glucose polymer; see Figure 15-13) and inulin, the storage carbohydrate in some plants (a fructose polymer; see Figure 15-13). BIOMEDICALLY, GLUCOSE IS THE MOST IMPORTANT MONOSACCHARIDE The Structure of Glucose Can Be Represented in Three Ways The straight-chain structural formula (aldohexose; Figure 15-1A) can account for some of the properties of glucose, but a cyclic structure (a hemiacetal formed by reaction between the aldehyde group and a hydroxyl group) is thermodynamically favored and accounts for other properties. The cyclic structure is normally drawn as shown in Figure 15-1B, the Haworth projection, in which the molecule is viewed from the side and above the plane of the ring; the bonds nearest to the viewer are bold and thickened, and the hydroxyl groups are above or below the plane of the ring. The hydrogen atoms attached to each carbon are not shown in this figure. The ring is actually in the form of a chair (Figure 15-1C). A HC=0 21 12 HC-OH 31 HO-CH 41 HC-OH 51 HC-OH 61 CH2OH B 6 CH2OH 5 4 OH 2 OH OH 3 OH C H 6 HO H H 2 H HO 3 CH2OH OH H OH FIGURE 15-1 D-Glucose. (A) Straight-chain form. (B) a-D-glucose; Haworth projection. (C) a-D-glucose; chair form. Sugars Exhibit Various Forms of Isomerism Glucose, with four asymmetric carbon atoms, can form 16 isomers. The more important types of isomerism found with glucose are as follows. 1. D- and L-isomerism: The designation of a sugar isomer as the D form or its mirror image as the L form is determined by its spatial relationship to the parent compound of the carbohydrates, the three-carbon sugar glycerose (glyceraldehyde). The L and D forms of this sugar, and of glucose, are shown in Figure 15-2. The orientation of the-Hand-OH groups around the carbon atom adjacent to the terminal alcohol carbon (carbon 5 in glucose) determines whether the sugar belongs to the D or L series. When the -OH group on this carbon is on the right (as seen in Figure 15-2), the sugar is the D-isomer; when it is on the left, it is the L-isomer. Most of the naturally occurring monosaccharides are D sugars, and the enzymes responsible for their metabolism are specific for this configuration. HC=0 HO-CH 31 CH2OH L-glycerose (L-glyceraldehyde) HC=0 21 HC-OH 31 CH2OH D-glycerose (D-glyceraldehyde) 1 HC=0 HC-OH 31 HO-CH 41 HC-OH 51 HO-CH 61 CH2OH L-Glucose 1 HC=0 21 HC-OH HO-CH 41 HC-OH 51 HC-OH 61 CH2OH D-Glucose FIGURE 15-2 D- and L-isomerism of glycerose and glucose. 2. The presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarized light is passed through a solution of an optical isomer, it rotates either to the right, dextrorotatory (+), or to the left, levorotatory (-). The direction of