88607238-2721/21/037:38 AM Page247Mac113ac11:aEDL SUMMARY 7. 1 Monosaccharides and Disaccharides Homopolysaccharides Heteropolysaccharides Unbranched Branched Multiple a Sugars(also called saccharides) are compounds monol monomer containing an aldehyde or ketone group and types two or more hydroxyl groups a Monosaccharides generally contain several chiral carbons and therefore exist in a variety of stereochemical forms, which may be represented on paper as Fischer projections Epimers are sugars that differ in configuration at only one carbon atom. a Monosaccharides commonly form internal hemiacetals or hemiketals. in which the aldehyde or ketone group joins with a hydroxyl group of the same molecule, creating a cyclic QrrrQ structure; this can be represented as a Haworth perspective formula. The carbon atom originally found in the aldehyde or ketone group(the anomeric carbon) can assume either of two configurations, a and B, which are FIGURE 7-13 Homo- and heteropolysaccharides. Polysaccharides form,which is in equilibrium with the cyclized in straight or branched chains of varying lengen t monosaccharides interconvertible by mutarotation. In the linear ay be composed of one, two, or several differe forms, the anomeric carbon is easily oxidized I A hydroxyl group of one monosaccharide can for example) serve as structural elements in plant cell add to the anomeric carbon of a second walls and animal exoskeletons. Heteropolysaccharides monosaccharide to form an acetal. in this provide extracellular support for organisms of all king disaccharide, the glycosidic bond protects the doms. For example, the rigid layer of the bacterial cell anomeric carbon from oxidation envelope(the peptidoglycan) is composed in part of a a Oligosaccharides are short polymers of several heteropolysaccharide built from two alternating mond monosaccharides joined by glycosidic bonds. At saccharide units. In animal tissues, the extracellular one end of the chain, the reducing end, is a space is occupied by several types of heteropolysac- monosaccharide unit whose anomeric carbon is charides, which form a matrix that holds individual cells not involved in a glycosidic bond together and provides protection, shape, and support to cells, tissues, and organs a The common nomenclature for di-or Unlike proteins, polysaccharides generally do not oligosaccharides specifies the order of have definite molecular weights. This difference is a con- monosaccharide units, the configuration at sequence of the mechanisms of assembly of the two each anomeric carbon and the carbon atoms types of polymers. As we shall see in Chapter 27, pro involved in the glycosidic linkage(s) teins are synthesized on a template(messenger RNA) of defined sequence and length, by enzymes that follow the template exactly For polysaccharide synthesis there 7.2 Polysaccharides is no template; rather, the program for polysaccharide Most carbohydrates found in nature occur as polysac- synthesis is intrinsic to the enzymes that catalyze the charides, polymers of medium to high molecular weight. polymerization of the monomeric units, and there is no Polysaccharides, also called glycans, differ from each pecific stopping point in the synthetic process other in the identity of their recurring monosaccharide units, in the length of their chains, in the types of bonds Some Homopolysaccharide Are Stored Forms of Fuel linking the units, and in the degree of branching. Homo- The most important storage polysaccharides are starch polysaccharides contain only a single type of monomer; in plant cells and glycogen in animal cells. Both poly heteropolysaccharides contain two or more different saccharides occur intracellularly as large clusters or kinds(Fig. 7-13). Some homopolysaccharide serve as granules (Fig. 7-14). Starch and glycogen molecules are storage forms of monosaccharides that are used as fuels; heavily hydrated, because they have many exposed hy- starch and glycogen are homopolysaccharide of this droxyl groups available to hydrogen-bond with water. type. Other homopolysaccharide(cellulose and chitin, Most plant cells have the ability to form starch, but it isSUMMARY 7.1 Monosaccharides and Disaccharides ■ Sugars (also called saccharides) are compounds containing an aldehyde or ketone group and two or more hydroxyl groups. ■ Monosaccharides generally contain several chiral carbons and therefore exist in a variety of stereochemical forms, which may be represented on paper as Fischer projections. Epimers are sugars that differ in configuration at only one carbon atom. ■ Monosaccharides commonly form internal hemiacetals or hemiketals, in which the aldehyde or ketone group joins with a hydroxyl group of the same molecule, creating a cyclic structure; this can be represented as a Haworth perspective formula. The carbon atom originally found in the aldehyde or ketone group (the anomeric carbon) can assume either of two configurations,
and , which are interconvertible by mutarotation. In the linear form, which is in equilibrium with the cyclized forms, the anomeric carbon is easily oxidized. ■ A hydroxyl group of one monosaccharide can add to the anomeric carbon of a second monosaccharide to form an acetal. In this disaccharide, the glycosidic bond protects the anomeric carbon from oxidation. ■ Oligosaccharides are short polymers of several monosaccharides joined by glycosidic bonds. At one end of the chain, the reducing end, is a monosaccharide unit whose anomeric carbon is not involved in a glycosidic bond. ■ The common nomenclature for di- or oligosaccharides specifies the order of monosaccharide units, the configuration at each anomeric carbon, and the carbon atoms involved in the glycosidic linkage(s). 7.2 Polysaccharides Most carbohydrates found in nature occur as polysac￾charides, polymers of medium to high molecular weight. Polysaccharides, also called glycans, differ from each other in the identity of their recurring monosaccharide units, in the length of their chains, in the types of bonds linking the units, and in the degree of branching. Homo￾polysaccharides contain only a single type of monomer; heteropolysaccharides contain two or more different kinds (Fig. 7–13). Some homopolysaccharides serve as storage forms of monosaccharides that are used as fuels; starch and glycogen are homopolysaccharides of this type. Other homopolysaccharides (cellulose and chitin, for example) serve as structural elements in plant cell walls and animal exoskeletons. Heteropolysaccharides provide extracellular support for organisms of all king￾doms. For example, the rigid layer of the bacterial cell envelope (the peptidoglycan) is composed in part of a heteropolysaccharide built from two alternating mono￾saccharide units. In animal tissues, the extracellular space is occupied by several types of heteropolysac￾charides, which form a matrix that holds individual cells together and provides protection, shape, and support to cells, tissues, and organs. Unlike proteins, polysaccharides generally do not have definite molecular weights. This difference is a con￾sequence of the mechanisms of assembly of the two types of polymers. As we shall see in Chapter 27, pro￾teins are synthesized on a template (messenger RNA) of defined sequence and length, by enzymes that follow the template exactly. For polysaccharide synthesis there is no template; rather, the program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the polymerization of the monomeric units, and there is no specific stopping point in the synthetic process. Some Homopolysaccharides Are Stored Forms of Fuel The most important storage polysaccharides are starch in plant cells and glycogen in animal cells. Both poly￾saccharides occur intracellularly as large clusters or granules (Fig. 7–14). Starch and glycogen molecules are heavily hydrated, because they have many exposed hy￾droxyl groups available to hydrogen-bond with water. Most plant cells have the ability to form starch, but it is Chapter 7 Carbohydrates and Glycobiology 247 Homopolysaccharides Unbranched Branched Heteropolysaccharides Two monomer types, unbranched Multiple monomer types, branched FIGURE 7–13 Homo- and heteropolysaccharides. Polysaccharides may be composed of one, two, or several different monosaccharides, in straight or branched chains of varying length. 8885d_c07_238-272 11/21/03 7:38 AM Page 247 Mac113 mac113:122_EDL: