《生物化学原理》（英文版）chapter 7 CARBOHYDRATES AND GLYCOBIOLOGY

88507-238-27211/21/037:38 AM Page243ac113mac11:4EDL Chapter 7 Carbohydrates and Glycobiology CHOH the hydroxyl group at C-6 of L-galactose or L-mannose produces L-fucose or L-rhamnose, respectively; these HOCH2O、1CH2OH deoxy sugars are found in plant polysaccharides and in OH H H HO the complex oligosaccharide components of glycopro- Oxidation of the carbonyl (aldehyde) carbon of glu- a-D-Fructofuranose cose to the carboxyl level produces gluconic acid; other aldoses yield other aldonic acids. Oxidation of the car- bon at the other end of the carbon chain--C-6 of glucose galactose, or mannose--forms the corresponding uronic acid: glucuronic, galacturonic, or mannuronic acid. Both HO\OH H aldonic and uronic acids form stable intramolecular es- CH2OH ters called lactones(Fig. 7-9, lower left). In addition to H OH OH H these acidic hexose derivatives. one nine-carbon acidic B-D-Glucopyranose B-D-Fructofuranose ugar deserves mention: N-acetylneuraminic acid (a sialic acid, but often referred to simply as"sialic acid), a de- HC-0 rivative of N-acetylmannosamine, is a component of many glycoproteins and glycolipids in animals. The carboxylic CH acid groups of the acidic sugar derivatives are ionized at pH 7, and the compounds are therefore correctly named as the carboxylates-glucuronate, galacturonate, and so forth FIGURE 7-7 Pyranoses and furanoses. The pyranose forms of D- glucose and the furanose forms of D-fructose are shown here as Haworth perspective formulas. The edges of the ring nearest the reader are represented by bold lines. Hydroxyl groups below the plane of the ring in these Haworth perspectives would appear at the right side of a Fischer projection(compare with Fig. 7-6). Pyran and furan are whereas two configurations can be interconverted only Two possible chair forms by breaking a covalent bond--for example, in the case of c and B configurations, the bond involving the ring oxygen atom. The specific three-dimensional confor mations of the monosaccharide units are important in determining the biological properties and functions of some polysaccharides, as we shall see Organisms Contain a Variety of Hexose Derivatives H In addition to simple hexoses such as glucose, galactose, nd mannose, there are a number of sugar derivatives in which a hydroxyl group in the parent compound is FIGURE 7-8 Conformational formulas of pyranoses. (a)Two chair replaced with another substituent, or a carbon atom is forms of the pyranose ring Substituents on the ring carbons may be oxidized to a carboxyl group(Fig. 7-9).In glucosamine, either axial (ax), projecting parallel to the vertical axis through the galactosamine, and mannosamine, the hydroxyl at c f the parent compound is replaced with an amino ing, or equatorial (eq), projecting roughly perpendicular to this axis. Two conformers such are these are not readily interconvertible with- group. The amino group is nearly always condensed with out breaking the ring. However, when the molecule is"stretched"( acetic acid, as inN-acetylglucosamine. This glucosamine atomic force microscopy), an input of about 46 k) of energy per mole derivative is part of many structural polymers, includ- of sugar can force the interconversion of chair forms.Generally, sub- ng those of the bacterial cell wall. Bacterial cell walls stituents in the equatorial positions are less sterically hindered by Iso contain a derivative of glucosamine, N-acetylmu neighboring substituents, and conformers with bulky substituents in ramic acid, in which lactic acid (a three-carbon car equatorial positions are favored. Another conformation, the"boat"(not boxylic acid) is ether-linked to the oxygen at C-3 of shown), is seen only in derivatives with very bulky substituents.(b)A N-acetylglucosamine. The substitution of a hydrogen for chair conformation of a-D-glucopyranose

Chapter 7 Carbohydrates and Glycobiology 243 5 2 3 1 4 6 HOCH2 HO O CH2OH OH H -D-Fructofuranose H OH H HOCH2 HO O CH2OH OH H -D-Fructofuranose H OH H -D-Glucopyranose H OH H H H CH2OH O OH H HO OH -D-Glucopyranose 1 3 2 4 H OH H H H O OH H HO OH 5 6CH2OH H2C CH HC O CH Pyran HC HC O CH C H Furan C H FIGURE 7–7 Pyranoses and furanoses. The pyranose forms of D￾glucose and the furanose forms of D-fructose are shown here as Haworth perspective formulas. The edges of the ring nearest the reader are represented by bold lines. Hydroxyl groups below the plane of the ring in these Haworth perspectives would appear at the right side of a Fischer projection (compare with Fig. 7–6). Pyran and furan are shown for comparison. ax ax ax eq O O eq eq eq eq eq eq eq eq ax ax ax ax ax ax ax Axis Axis Two possible chair forms (a) eq H H2COH HO OH H HO H H H OH (b) O Axis -D-Glucopyranose FIGURE 7–8 Conformational formulas of pyranoses. (a) Two chair forms of the pyranose ring. Substituents on the ring carbons may be either axial (ax), projecting parallel to the vertical axis through the ring, or equatorial (eq), projecting roughly perpendicular to this axis. Two conformers such are these are not readily interconvertible with￾out breaking the ring. However, when the molecule is “stretched” (by atomic force microscopy), an input of about 46 kJ of energy per mole of sugar can force the interconversion of chair forms. Generally, sub￾stituents in the equatorial positions are less sterically hindered by neighboring substituents, and conformers with bulky substituents in equatorial positions are favored. Another conformation, the “boat” (not shown), is seen only in derivatives with very bulky substituents. (b) A chair conformation of
-D-glucopyranose. whereas two configurations can be interconverted only by breaking a covalent bond—for example, in the case of
and configurations, the bond involving the ring oxygen atom. The specific three-dimensional confor￾mations of the monosaccharide units are important in determining the biological properties and functions of some polysaccharides, as we shall see. Organisms Contain a Variety of Hexose Derivatives In addition to simple hexoses such as glucose, galactose, and mannose, there are a number of sugar derivatives in which a hydroxyl group in the parent compound is replaced with another substituent, or a carbon atom is oxidized to a carboxyl group (Fig. 7–9). In glucosamine, galactosamine, and mannosamine, the hydroxyl at C-2 of the parent compound is replaced with an amino group. The amino group is nearly always condensed with acetic acid, as in N-acetylglucosamine. This glucosamine derivative is part of many structural polymers, includ￾ing those of the bacterial cell wall. Bacterial cell walls also contain a derivative of glucosamine, N-acetylmu￾ramic acid, in which lactic acid (a three-carbon car￾boxylic acid) is ether-linked to the oxygen at C-3 of N-acetylglucosamine. The substitution of a hydrogen for the hydroxyl group at C-6 of L-galactose or L-mannose produces L-fucose or L-rhamnose, respectively; these deoxy sugars are found in plant polysaccharides and in the complex oligosaccharide components of glycopro￾teins and glycolipids. Oxidation of the carbonyl (aldehyde) carbon of glu￾cose to the carboxyl level produces gluconic acid; other aldoses yield other aldonic acids. Oxidation of the car￾bon at the other end of the carbon chain—C-6 of glucose, galactose, or mannose—forms the corresponding uronic acid: glucuronic, galacturonic, or mannuronic acid. Both aldonic and uronic acids form stable intramolecular es￾ters called lactones (Fig. 7–9, lower left). In addition to these acidic hexose derivatives, one nine-carbon acidic sugar deserves mention: N-acetylneuraminic acid (a sialic acid, but often referred to simply as “sialic acid”), a de￾rivative of N-acetylmannosamine, is a component of many glycoproteins and glycolipids in animals. The carboxylic acid groups of the acidic sugar derivatives are ionized at pH 7, and the compounds are therefore correctly named as the carboxylates—glucuronate, galacturonate, and so forth. 8885d_c07_238-272 11/21/03 7:38 AM Page 243 Mac113 mac113:122_EDL:

88607238-2721/21/037:38 AM Page244ac113ac11:aEDL 244 Part I Structure and Catalysis Glucose family Amino sugars CHoO CH。OH CHOOH CHOH CHoOHI OH H H NH2 H NH HH B-D-Galactosamine B-D-Mannosamine B-D-Glucosamine N-Acetyl-B-D-glucosamine Deoxy sugars CH2-0--POa CHOH CHOO OH H Hg XOH R=-0-C-H H HH HO 00 H NHo OH OH C=0 B-pD-Glucose 6-phosphate Muramic acid N-Acetylmuramie acid B-L-Fi g-L-Rhamnose CH3 CHoO H-C-OH CHoO B-D-Glucuronate D-Gluconate D-Glucono-s-lactone N-Acetylneuraminic acid (a sialic acid) FIGURE 7-9 Some hexose derivatives important in biology. In amino mers. The acidic sugars contain a carboxylate group, which confers a sugars,an-NH2 group replaces one of the -OH groups in the par- negative charge at neutral pH. D-Glucono-8-lactone results from for- ent hexose. Substitution of -H for -OH produces a deoxy sugar; mation of an ester linkage between the C-1 carboxylate group and the note that the deoxy sugars shown here occur in nature as the L iso- C-5 (also known as the 8 carbon) hydroxyl group of D-gluconate In the synthesis and metabolism of carbohydrates, Monosaccharides Are Reducing Agents e intermediates are very often not the sugars them selves but their phosphorylated derivatives. Condensation Monosaccharides can be oxidized by relative of phosphoric acid with one of the hydroxyl groups of a mild oxidizing agents such as ferric (Fe +)or sugar forms a phosphate ester, as in glucose 6-phosphate cupric(Cu-v ion(Fig. 7-10a). The carbonyl carbon is (Fig. 7-9). Sugar phosphates are relatively stable at neu- oxidized to a carboxyl group. Glucose and other sugars tral pH and bear a negative charge. One effect of sugar capable of reducing ferric or cupric ion are called re phosphorylation within cells is to trap the sugar inside the ducing sugars. This property is the basis of fehlings cell; most cells do not have plasma membrane trans- reaction, a qualitative test for the presence of reducing porters for phosphorylated sugars. Phosphorylation also sugar. By measuring the amount of oxidizing agent re- activates sugars for subsequent chemical transformation. duced by a solution of a sugar, it is also possible to es- Several important phosphorylated derivatives of sugars timate the concentration of that sugar. For many years are components of nucleotides(discussed in the next this test was used to detect and measure elevated glu- chapter) cose levels in blood and urine in the diagnosis of dia

In the synthesis and metabolism of carbohydrates, the intermediates are very often not the sugars them￾selves but their phosphorylated derivatives. Condensation of phosphoric acid with one of the hydroxyl groups of a sugar forms a phosphate ester, as in glucose 6-phosphate (Fig. 7–9). Sugar phosphates are relatively stable at neu￾tral pH and bear a negative charge. One effect of sugar phosphorylation within cells is to trap the sugar inside the cell; most cells do not have plasma membrane trans￾porters for phosphorylated sugars. Phosphorylation also activates sugars for subsequent chemical transformation. Several important phosphorylated derivatives of sugars are components of nucleotides (discussed in the next chapter). Monosaccharides Are Reducing Agents Monosaccharides can be oxidized by relatively mild oxidizing agents such as ferric (Fe3) or cupric (Cu2) ion (Fig. 7–10a). The carbonyl carbon is oxidized to a carboxyl group. Glucose and other sugars capable of reducing ferric or cupric ion are called re￾ducing sugars. This property is the basis of Fehling’s reaction, a qualitative test for the presence of reducing sugar. By measuring the amount of oxidizing agent re￾duced by a solution of a sugar, it is also possible to es￾timate the concentration of that sugar. For many years this test was used to detect and measure elevated glu￾cose levels in blood and urine in the diagnosis of dia- 244 Part I Structure and Catalysis CH2OH H O HO NH C PO3 N-Acetylmuramic acid R H OH H H H CH2 OH H HO D-Glucono--lactone OH H OH H H CH2OH H O HO NH2 -D-Mannosamine H OH H H H CH2OH H O OH HO OH H OH H H H H2N H O OH H OH H H H O CH3 -D-Glucose Muramic acid CH2OH H O OH HO NH C glucosamine H OH H H H O CH3 R Glucose family H OH HO -L-Rhamnose OH H OH H H O C O O C OH H HO -D-Glucuronate OH H OH H H H O O CH2OH CH2OH H N-Acetylneuraminic acid (a sialic acid) OH H O O -D-Glucosamine CH2OH H O OH HO NH2 H OH H H H -D-Galactosamine CH2OH H O OH HO CH3 H OH H H H CH2OH HO R O O O H H HO -D-Glucose 6-phosphate OH H OH H OH H O NH2 OH H HO -L-Fucose OH H OH H H H O CH3 H Amino sugars Acidic sugars Deoxy sugars O OH CH2OH H HO D-Gluconate OH H H H C OH H HN CH3 C O R H H 2 N-Acetyl--D-
C OH H H OH C O COO O C H CH3  R
FIGURE 7–9 Some hexose derivatives important in biology. In amino sugars, an ONH2 group replaces one of the OOH groups in the par￾ent hexose. Substitution of OH for OOH produces a deoxy sugar; note that the deoxy sugars shown here occur in nature as the L iso￾mers. The acidic sugars contain a carboxylate group, which confers a negative charge at neutral pH. D-Glucono--lactone results from for￾mation of an ester linkage between the C-1 carboxylate group and the C-5 (also known as the carbon) hydroxyl group of D-gluconate. 8885d_c07_238-272 11/21/03 7:38 AM Page 244 Mac113 mac113:122_EDL:

885c07-238-27211/21/037:38 AM Page245Mac113mac11:4EDL FIGURE 7-10 Sugars as reducing agents. (a)Oxidation of the H carbon of glucose and other sugars is the basis for 1*) produced under alkalin conditions forms a red cuprous oxide precipitate. In the hem HoH H-C-OH acetal (ring)form, C-1 of glucose cannot be oxidized by Cu2+. owever, the open-chain form is in equilibrium with the ring H-c-oh 2Cu 2CuH-c-oh m,and eventually the oxidation reaction goes to completion The reaction with Cu2+ is not as simple as the equation here H-C-OH implies; in addition to D-gluconate, a number of shorter-chain H OH CH,OH CH,OH acids are produced by the fragmentation of glucose. (b) D-Gl glucose concentration is commonly determined by measuring the amount of H2O2 produced in the reaction catalyzed by glucose oxidase. In the reaction mixture, a second enzyme, peroxidase, talyzes reaction of the H2O2 with a colorless compound to oxidase, D- Glucono-& -Lactone+Ho produce a colored compound, the amount of which is then measured spectrophotometrically. betes mellitus. Now. more sensitive methods for meas- To name reducing disaccharides such as maltose un uring blood glucose employ an enzyme, glucose oxidase ambiguously, and especially to name more complex Fig.7-10b).■ oligosaccharides, several rules are followed By conven- tion, the name describes the compound with its nonre- Disaccharides Contain a Glycosidic Bond ducing end to the left, and we can"build up"the name Disaccharides(such as maltose, lactose, and sucrose) in the following order. (1) Give the configuration (a or consist of two monosaccharides joined covalently by an B) at the anomeric carbon joining the first O-glycosidie bond, which is formed when a hydroxy charide unit (on the left) to the second. (2) Name the group of one sugar reacts with the anomeric carbon of the other(Fig. 7-11). This reaction represents the for- mation of an acetal from a hemiacetal(such as glu- CH,OH CH,OH copyranose) and an alcohol (a hydroxyl group of the second sugar molecule)(Fig. 7-5). Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base H、oHH Thus disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid OH H OH N-glycosyl bonds join the anomeric carbon of a sugar to aD-Glucose a nitrogen atom in glycoproteins(see Fig 7-31) and nu- cleotides(see Fig 8-1) The oxidation of a sugar's anomeric carbon by cupric or ferric ion(the reaction that defines a reduc- HOOH 6CH2OH ng sugar) occurs only with the linear form, which ex- ists in equilibrium with the cyclic form(S). When the anomeric carbon is involved in a glycosidic bond, that OH H OH H sugar residue cannot take the linear form and therefore becomes a nonreducing sugar. In describing disaccha rides or polysaccharides, the end of a chain with a free Maltose anomeric carbon (one not involved in a glycosidic bond) ae-D-glucopyranosyl-(1-4)-D-glucopyranose is commonly called the reducing end. FIGURE 7-11 Formation of maltose. a disaccharide is formed from The disaccharide maltose (ig. 7-11)contains two two monosaccharides there, two molecules of D-glucose) when an D-glucose residues joined by a glycosidic linkage be--OH (alcohol) of one glucose molecule (right)condenses with the tween C-1(the anomeric carbon) of one glucose residue intramolecular hemiacetal of the other glucose molecule (left), with and C-4 of the other. Because the disaccharide retains elimination of H,o and formation of an O-glycosidic bond. The re- a free anomeric carbon(C-1 of the glucose residue on versal of this reaction is hydrolysis-attack by H2O on the glycoside the right in Fig. 7-11), maltose is a reducing sugar. The bond. The maltose molecule retains a reducing hemiacetal at the configuration of the anomeric carbon atom in the gly- C-1 not involved in the glycosidic bond. Because mutarotation inter- cosidic linkage is a. The glucose residue with the free converts the a and B forms of the hemiacetal, the bonds at this posi anomeric carbon is capable of existing in a-and B-pyra- tion are sometimes depicted with wavy lines, as shown here, to indi- nose forms cate that the structure may be either a or B

betes mellitus. Now, more sensitive methods for meas￾uring blood glucose employ an enzyme, glucose oxidase (Fig. 7–10b). ■ Disaccharides Contain a Glycosidic Bond Disaccharides (such as maltose, lactose, and sucrose) consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other (Fig. 7–11). This reaction represents the for￾mation of an acetal from a hemiacetal (such as glu￾copyranose) and an alcohol (a hydroxyl group of the second sugar molecule) (Fig. 7–5). Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base. Thus disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid. N-glycosyl bonds join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins (see Fig. 7–31) and nu￾cleotides (see Fig. 8–1). The oxidation of a sugar’s anomeric carbon by cupric or ferric ion (the reaction that defines a reduc￾ing sugar) occurs only with the linear form, which ex￾ists in equilibrium with the cyclic form(s). When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a nonreducing sugar. In describing disaccha￾rides or polysaccharides, the end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end. The disaccharide maltose (Fig. 7–11) contains two D-glucose residues joined by a glycosidic linkage be￾tween C-1 (the anomeric carbon) of one glucose residue and C-4 of the other. Because the disaccharide retains a free anomeric carbon (C-1 of the glucose residue on the right in Fig. 7–11), maltose is a reducing sugar. The configuration of the anomeric carbon atom in the gly￾cosidic linkage is
. The glucose residue with the free anomeric carbon is capable of existing in
- and -pyra￾nose forms. To name reducing disaccharides such as maltose un￾ambiguously, and especially to name more complex oligosaccharides, several rules are followed. By conven￾tion, the name describes the compound with its nonre￾ducing end to the left, and we can “build up” the name in the following order. (1) Give the configuration (
or ) at the anomeric carbon joining the first monosac￾charide unit (on the left) to the second. (2) Name the Chapter 7 Carbohydrates and Glycobiology 245 D-Glucose  O2 glucose oxidase D-Glucono--lactone H2O2 CH2OH HO C O H OH 6 1 A G J A O O A A A O O O O O O O C C C C OH OH H H H 2 3 4 5 CH2OH HO C O H OH A G J A O O A A A O O O O O O H C C C C OH OH H H H D-Glucose (linear form) D-Gluconate (a) -D-Glucose 3 5 6 4 1 2 H OH OH H H H CH2OH O H OH HO 2Cu 2Cu2 H OH OH H H H  CH2OH O H OH alcohol H2O Maltose -D-glucopyranosyl-(1n4)-D-glucopyranose H OH OH H H H CH2OH O H HO OH -D-Glucose condensation acetal hydrolysis H2O 3 5 6 4 1 2 H OH OH H CH2OH O H OH HO 3 5 6 4 1 2 H OH OH H H CH2OH O H H H H O -D-Glucose hemiaceta hemiacetal O H FIGURE 7–10 Sugars as reducing agents. (a) Oxidation of the anomeric carbon of glucose and other sugars is the basis for Fehling’s reaction. The cuprous ion (Cu) produced under alkaline conditions forms a red cuprous oxide precipitate. In the hemi￾acetal (ring) form, C-1 of glucose cannot be oxidized by Cu2. However, the open-chain form is in equilibrium with the ring form, and eventually the oxidation reaction goes to completion. The reaction with Cu2 is not as simple as the equation here implies; in addition to D-gluconate, a number of shorter-chain acids are produced by the fragmentation of glucose. (b) Blood glucose concentration is commonly determined by measuring the amount of H2O2 produced in the reaction catalyzed by glucose oxidase. In the reaction mixture, a second enzyme, peroxidase, catalyzes reaction of the H2O2 with a colorless compound to produce a colored compound, the amount of which is then measured spectrophotometrically. FIGURE 7–11 Formation of maltose. A disaccharide is formed from two monosaccharides (here, two molecules of D-glucose) when an OOH (alcohol) of one glucose molecule (right) condenses with the intramolecular hemiacetal of the other glucose molecule (left), with elimination of H2O and formation of an O-glycosidic bond. The re￾versal of this reaction is hydrolysis—attack by H2O on the glycosidic bond. The maltose molecule retains a reducing hemiacetal at the C-1 not involved in the glycosidic bond. Because mutarotation inter￾converts the
and forms of the hemiacetal, the bonds at this posi￾tion are sometimes depicted with wavy lines, as shown here, to indi￾cate that the structure may be either
or . (a) (b) 8885d_c07_238-272 11/21/03 7:38 AM Page 245 Mac113 mac113:122_EDL:

885d_c07-238-27211/21/037:38 AM Page246Mac113mac113:1aEDL 246 Part I Structure and Catalysis nonreducing residue; to distinguish five- and six-mem- many plants it is the principal form in which sugar is bered ring structures, insert"furano"or "pyrano"into transported from the leaves to other parts of the plant the name. B)Indicate in parentheses the two carbon body. Trehalose, Glc(alelaGlc(Fig 7-12)-a disac atoms joined by the glycosidic bond, with an arrow con- charide of D-glucose that, like sucrose, is a nonreducing necting the two numbers; for example, (1-4)shows sugar--is a major constituent of the circulating fluid that C-1 of the first-named sugar residue is joined to (hemolymph) of insects, serving as an energy-storage C-4 of the second. (4) Name the second residue. If there compound is a third residue, describe the second glycosidic bond by the same conventions. (To shorten the description CH2O CH2OH of complex polysaccharides, three-letter abbreviations for the monosaccharides are often used, as given in Table 7-1. Following this convention for naming oligosaccharides, maltose is a-D-glucopyranosyl-(1-4) D-glucopyranose. Because most sugars encountered in this book are the d enantiomers and the pyranose form Lactose(B form) of hexoses predominates, we generally use a shortened B-D-galactopyranosyl-(1-4)-B-D-glucopyranose version of the formal name of such compounds, giving Ga(B1→4)Glc the configuration of the anomeric carbon and naming the carbons joined by the glycosidic bond. In this ab- CH.OHI breviated nomenclature, maltose is Glc(a1-4)Glc. The disaccharide lactose (Fig. 7-12), which yields BRH HO D-galactose and D-glucose on hydrolysis, occurs natu- loHI rally only in milk. The anomeric carbon of the glucose residue is available for oxidation. and thus lactose is a reducing disaccharide. Its abbreviated name is a-D-glucopyranosyl B-D-fructofuranoside Gal(Bl-4)Glc. Sucrose(table sugar)is a disaccharide Glc(a1+2B)Fru of glucose and fructose. It is formed by plants but not by animals. In contrast to maltose and lactose, sucrose CH2OH contains no free anomeric carbon atom. the anomeric carbons of both monosaccharide units are involved in H the glycosidic bond(Fig. 7-12). Sucrose is therefore a HO nonreducing sugar. Nonreducing disaccharides are named as glycosides; in this case, the positions joined are the anomeric carbons. In the abbreviated nomen- clature, a double-headed arrow connects the symbols ucopyra Glda1-1aGlc specifying the anomeric carbons and their configura- tions. For example, the abbreviated name of sucrose FIGURE 7-12 Some common disaccharides. Like maltose in Figure is either Gle(al+2B)Fru or Fru(B26laGlc Sucrose 7-11, these are shown as Haworth perspectives. The common name, is a major intermediate product of photosynthesis; in full systematic name, and abbreviation are given for each disaccharide TABLE 7-1 Abbreviations for common monosaccharides and some of their derivatives Abequose Glucuronic acid GIcA Galactosamine Glucosamine N-Acetylgalactosamine N-Acetylglucosamine GICNAc Iduronic acid Mannose Muramic acid Rhamnose N-Acetylmuramic acid Mur2Ac N-Acetylneuraminic acid Neu5Ac Xylose (a sialic acid)

nonreducing residue; to distinguish five- and six-mem￾bered ring structures, insert “furano” or “pyrano” into the name. (3) Indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow con￾necting the two numbers; for example, (1n4) shows that C-1 of the first-named sugar residue is joined to C-4 of the second. (4) Name the second residue. If there is a third residue, describe the second glycosidic bond by the same conventions. (To shorten the description of complex polysaccharides, three-letter abbreviations for the monosaccharides are often used, as given in Table 7–1.) Following this convention for naming oligosaccharides, maltose is
-D-glucopyranosyl-(1n4)- D-glucopyranose. Because most sugars encountered in this book are the D enantiomers and the pyranose form of hexoses predominates, we generally use a shortened version of the formal name of such compounds, giving the configuration of the anomeric carbon and naming the carbons joined by the glycosidic bond. In this ab￾breviated nomenclature, maltose is Glc(
1n4)Glc. The disaccharide lactose (Fig. 7–12), which yields D-galactose and D-glucose on hydrolysis, occurs natu￾rally only in milk. The anomeric carbon of the glucose residue is available for oxidation, and thus lactose is a reducing disaccharide. Its abbreviated name is Gal(1n4)Glc. Sucrose (table sugar) is a disaccharide of glucose and fructose. It is formed by plants but not by animals. In contrast to maltose and lactose, sucrose contains no free anomeric carbon atom; the anomeric carbons of both monosaccharide units are involved in the glycosidic bond (Fig. 7–12). Sucrose is therefore a nonreducing sugar. Nonreducing disaccharides are named as glycosides; in this case, the positions joined are the anomeric carbons. In the abbreviated nomen￾clature, a double-headed arrow connects the symbols specifying the anomeric carbons and their configura￾tions. For example, the abbreviated name of sucrose is either Glc(
1mn2)Fru or Fru(2mn1
)Glc. Sucrose is a major intermediate product of photosynthesis; in many plants it is the principal form in which sugar is transported from the leaves to other parts of the plant body. Trehalose, Glc(
1mn1
)Glc (Fig. 7–12)—a disac￾charide of D-glucose that, like sucrose, is a nonreducing sugar—is a major constituent of the circulating fluid (hemolymph) of insects, serving as an energy-storage compound. 246 Part I Structure and Catalysis Sucrose 4 5 1 2 H HOCH2 H HO HO 3 5 6 4 1 2 H OH OH H H CH2OH O H H O 6 Trehalose 3 5 1 4 2 H OH OH H O H OH HO 3 5 6 4 1 2 H OH OH H H CH2OH O H H H H O 6 -D-glucopyranosyl -D-glucopyranoside O OH H CH2OH 3 Glc( 1nn1 )Glc -D-glucopyranosyl - D-fructofuranoside Lactose ( form) 3 5 4 1 2 H OH OH H CH2OH O H HO OH 3 5 6 4 1 2 H OH OH H H CH2OH O H H H H O 6 -D-galactopyranosyl-(1n4)--D-glucopyranose Gal(1n4)Glc HOCH2 Glc(1nn 2)Fru FIGURE 7–12 Some common disaccharides. Like maltose in Figure 7–11, these are shown as Haworth perspectives. The common name, full systematic name, and abbreviation are given for each disaccharide. Abequose Abe Glucuronic acid GlcA Arabinose Ara Galactosamine GalN Fructose Fru Glucosamine GlcN Fucose Fuc N-Acetylgalactosamine GalNAc Galactose Gal N-Acetylglucosamine GlcNAc Glucose Glc Iduronic acid IdoA Mannose Man Muramic acid Mur Rhamnose Rha N-Acetylmuramic acid Mur2Ac Ribose Rib N-Acetylneuraminic acid Neu5Ac Xylose Xyl (a sialic acid) TABLE 7–1 Abbreviations for Common Monosaccharides and Some of Their Derivatives 8885d_c07_238-272 11/21/03 7:38 AM Page 246 Mac113 mac113:122_EDL:

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 is

SUMMARY 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: