885d_c07-238-27211/21/037:38 AM Page248Mac113mac113:1aEDL 248 Part I Structure and Catalysis where it may constitute as much as 7% of the wet weight; it is also present in skeletal muscle. In hepato- cytes glycogen is found in large granules(Fig. 7-14b) which are themselves clusters of smaller granules com- posed of single, highly branched glycogen molecules with an average molecular weight of several million. Such glycogen granules also contain, in tightly bound form, the enzymes responsible for the synthesis and degradation of glycogen. Because each branch in glycogen ends with a nonre- ducing sugar unit, a glycogen molecule has as many nonreducing ends as it has branches, but only one re- ducing end. When glycogen is used as an energy source glucose units are removed one at a time from the nonre- ducing ends. Degradative enzymes that act only at nonreducing ends can work simultaneously on the many branches, speeding the conversion of the polymer to monosaccharides Why not store glucose in its monomeric form? It has been calculated that hepatocytes store glycogen equiv alent to a glucose concentration of 0.4 M. The actual con- centration of glycogen, which is insoluble and con- tributes little to the osmolarity of the cytosol, is about 0.01 uM. If the cytosol contained 0.4 M glucose, the OS- molarity would be threateningly elevated, leading to Os- motic entry of water that might rupture the cell(see Fig. 2-13). Furthermore, with an intracellular glucose concentration of 0.4 m and an external concentration of Glycogen granules about 5 mm(the concentration in the blood of a mam FIGURE 7-14 Electron micrographs of starch and glycogen granules man), the free-energy change for glucose uptake into (a)Large starch granules in a single chloroplast Starch is made in the cells against this very high concentration gradient would hloroplast from D-glucose formed photosynthetically. (b) Glycogen be prohibitively large granules in a hepatocyte. These granules form in the cytosol and are Dextrans are bacterial and yeast polysaccharides much smaller(-0.1 um)than starch granules(-1.0 um). made up of (al-6)-linked poly-D-glucose, all have (a1→3) branches, and some also have(al→2)or (al-4)branches. Dental plaque, formed by bacteria especially abundant in tubers, such as potatoes, and in growing on the surface of teeth, is rich in dextrans. Syn- thetic dextrans are used in several commercial products seeds (for example, Sephadex) that serve in the fractionation Starch contains two types of glucose polymer, amy- of proteins by size-exclusion chromatography(see Fig of long, unbranched chains of D-glucose residues con- 3-18b). The dextrans in these products are chemically nected by (al-4 linkages. Such chains vary in me lecular weight from a few thousand to more than a mil- porosities, admitting macromolecules of various sizes lion. Amylopectin also has a high molecular weight (u to 100 million) but unlike amylose is highly branched. Some Homopolysaccharide Serve Structural Roles The glycosidic linkages joining successive glucose Cellulose, a fibrous, tough, water-insoluble substance, is residues in amylopectin chains are(al-4); the branch found in the cell walls of plants, particularly in stalks points (occurring every 24 to 30 residues) are(al-6) stems, trunks, and all the woody portions of the plant body. Cellulose constitutes much of the mass of wood Glycogen is the main storage polysaccharide of an- and cotton is almost pure cellulose. Like amylose and imal cells. Like an mylopectin, glycogen is a polymer of the main chains of amylopectin and glycogen, the cel- (al-4-linked subunits of glucose, with(al-6)-linked lulose molecule is a linear, unbranched homopolysac- branches, but glycogen is more extensively branched charide, consisting of 10,000 to 15, 000 D-glucose units (on average, every 8 to 12 residues) and more compact But there is a very important difference: in cellulose the than starch. Glycogen is especially abundant in the liver, glucose residues have the B configuration(Fig. 7-16)
especially abundant in tubers, such as potatoes, and in seeds. Starch contains two types of glucose polymer, amylose and amylopectin (Fig. 7–15). The former consists of long, unbranched chains of D-glucose residues connected by (1n4) linkages. Such chains vary in molecular weight from a few thousand to more than a million. Amylopectin also has a high molecular weight (up to 100 million) but unlike amylose is highly branched. The glycosidic linkages joining successive glucose residues in amylopectin chains are (1n4); the branch points (occurring every 24 to 30 residues) are (1n6) linkages. Glycogen is the main storage polysaccharide of animal cells. Like amylopectin, glycogen is a polymer of (1n4)-linked subunits of glucose, with (1n6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch. Glycogen is especially abundant in the liver, where it may constitute as much as 7% of the wet weight; it is also present in skeletal muscle. In hepatocytes glycogen is found in large granules (Fig. 7–14b), which are themselves clusters of smaller granules composed of single, highly branched glycogen molecules with an average molecular weight of several million. Such glycogen granules also contain, in tightly bound form, the enzymes responsible for the synthesis and degradation of glycogen. Because each branch in glycogen ends with a nonreducing sugar unit, a glycogen molecule has as many nonreducing ends as it has branches, but only one reducing end. When glycogen is used as an energy source, glucose units are removed one at a time from the nonreducing ends. Degradative enzymes that act only at nonreducing ends can work simultaneously on the many branches, speeding the conversion of the polymer to monosaccharides. Why not store glucose in its monomeric form? It has been calculated that hepatocytes store glycogen equivalent to a glucose concentration of 0.4 M. The actual concentration of glycogen, which is insoluble and contributes little to the osmolarity of the cytosol, is about 0.01 M. If the cytosol contained 0.4 M glucose, the osmolarity would be threateningly elevated, leading to osmotic entry of water that might rupture the cell (see Fig. 2–13). Furthermore, with an intracellular glucose concentration of 0.4 M and an external concentration of about 5 mM (the concentration in the blood of a mammal), the free-energy change for glucose uptake into cells against this very high concentration gradient would be prohibitively large. Dextrans are bacterial and yeast polysaccharides made up of (1n6)-linked poly-D-glucose; all have (1n3) branches, and some also have (1n2) or (1n4) branches. Dental plaque, formed by bacteria growing on the surface of teeth, is rich in dextrans. Synthetic dextrans are used in several commercial products (for example, Sephadex) that serve in the fractionation of proteins by size-exclusion chromatography (see Fig. 3–18b). The dextrans in these products are chemically cross-linked to form insoluble materials of various porosities, admitting macromolecules of various sizes. Some Homopolysaccharides Serve Structural Roles Cellulose, a fibrous, tough, water-insoluble substance, is found in the cell walls of plants, particularly in stalks, stems, trunks, and all the woody portions of the plant body. Cellulose constitutes much of the mass of wood, and cotton is almost pure cellulose. Like amylose and the main chains of amylopectin and glycogen, the cellulose molecule is a linear, unbranched homopolysaccharide, consisting of 10,000 to 15,000 D-glucose units. But there is a very important difference: in cellulose the glucose residues have the configuration (Fig. 7–16), 248 Part I Structure and Catalysis Starch granules (a) Glycogen granules (b) FIGURE 7–14 Electron micrographs of starch and glycogen granules. (a) Large starch granules in a single chloroplast. Starch is made in the chloroplast from D-glucose formed photosynthetically. (b) Glycogen granules in a hepatocyte. These granules form in the cytosol and are much smaller (~0.1 m) than starch granules (~1.0 m). 8885d_c07_238-272 11/21/03 7:38 AM Page 248 Mac113 mac113:122_EDL:
885d_c07-238-27211/21/037:38 AM Page249Mac113mac113:1aEDL Chapter 7 Carbohydrates and Glycobiology ChOH CH,OH CH,OH CH,OH OH F H OH (a)amylose CHOH (a1→6) branch Amylose Branch H OH Reducing CH FIGURE 7-15 Amylose and amylopectin, the polysaccharides of to occur in starch granules. Strands of amylopectin (red) form double- starch (a)A short segment of amylose, a linear polymer of D-glucose helical structures with each other or with amylose strands(blue) residues in(a1-4)linkage. A single chain can contain several thou- Glucose residues at the nonreducing ends of the outer branches a sand glucose residues. Amylopectin has stretches of similarly linked removed enzymatically during the mobilization of starch for energy residues between branch points. (b) An(a1-6)branch point of amy. production. Glycogen has a similar structure but is more highly lopectin. (c)A cluster of amylose and amylopectin like that believed branched and more compact. whereas in amylose, amylopectin, and glycogen the glu cose is in the a configuration. The glucose residues in OHIIIIIII IOH cellulose are linked by (B1-4 glycosidic bonds, in con- trast to the(al-4)bonds of amylose, starch, and glyco- gen. This difference gives cellulose and amylose very OIII I IHO different structures and physical properties Glycogen and starch ingested in the diet are hy- (B1-+4)-linked D-glucose units drolyzed by a-amylases, enzymes in saliva and intestinal glucose units. Most animals cannot use cellulose dg ecretions that break(al-4) glycosidic bonds betwe fuel source, because they lack an enzyme to hydrolyze the (B1-4)linkages. Termites readily digest cellulose FIGURE 7-16 The structure of cellulose. (a)Two units of a cellulose chain; the D-glucose residues are in(B1-4)linkage. The rigid chai = structures can rotate relative to one another.(b) Scale drawing of seg. ments of two parallel cellulose chains, showing the conformation of Q the D-glucose residues and the hydrogen-bond cross-links In the hex- ose unit at the lower left, all hydrogen atoms are shown; in the other three hexose units, the hydrogens attached to carbon have been omit- ted for clarity as they do not participate in hydrogen bonding
whereas in amylose, amylopectin, and glycogen the glucose is in the configuration. The glucose residues in cellulose are linked by (1n4) glycosidic bonds, in contrast to the (1n4) bonds of amylose, starch, and glycogen. This difference gives cellulose and amylose very different structures and physical properties. Glycogen and starch ingested in the diet are hydrolyzed by -amylases, enzymes in saliva and intestinal secretions that break (1n4) glycosidic bonds between glucose units. Most animals cannot use cellulose as a fuel source, because they lack an enzyme to hydrolyze the (1n4) linkages. Termites readily digest cellulose Chapter 7 Carbohydrates and Glycobiology 249 Reducing end 4 1 H OH OH H CH2OH O H 4 H OH H OH CH2OH H H H O 1 H H H 4 H OH H OH CH2OH O H H O 1 O 3 5 2 O 6 H 4 H OH H OH CH2OH O H H O 1 Nonreducing end (a) amylose Amylose Amylopectin (c) Nonreducing ends Reducing ends 6 O H 4 H OH H OH CH2OH O H H O 1 Branch 6 H 4 H OH H OH CH2 O H H O 1 branch point O A Main chain (b) (1n6) FIGURE 7–15 Amylose and amylopectin, the polysaccharides of starch. (a) A short segment of amylose, a linear polymer of D-glucose residues in (1n4) linkage. A single chain can contain several thousand glucose residues. Amylopectin has stretches of similarly linked residues between branch points. (b) An (1n6) branch point of amylopectin. (c) A cluster of amylose and amylopectin like that believed to occur in starch granules. Strands of amylopectin (red) form doublehelical structures with each other or with amylose strands (blue). Glucose residues at the nonreducing ends of the outer branches are removed enzymatically during the mobilization of starch for energy production. Glycogen has a similar structure but is more highly branched and more compact. (b) OH HO (1n4)-linked D-glucose units (a) O O O HO OH O OH OH 4 6 5 2 3 1 O FIGURE 7–16 The structure of cellulose. (a) Two units of a cellulose chain; the D-glucose residues are in (1n4) linkage. The rigid chair structures can rotate relative to one another. (b) Scale drawing of segments of two parallel cellulose chains, showing the conformation of the D-glucose residues and the hydrogen-bond cross-links. In the hexose unit at the lower left, all hydrogen atoms are shown; in the other three hexose units, the hydrogens attached to carbon have been omitted for clarity as they do not participate in hydrogen bonding. 8885d_c07_238-272 11/21/03 7:38 AM Page 249 Mac113 mac113:122_EDL: