chapter
CARBOHYDRATES AND
GLYCOBIOLOGY
7.1. Monosaccharides and Disaccharides 239 attached to proteins or lipids act as signals that deter-
mine the intracellular location or metabolic fate of these
7.2 Polysaccharides 247
hybrid molecules, called glycoconjugates. This chap-
7.3 Glycoconjugates: Proteoglycans, Glycoproteins, ter introduces the major classes of carbohydrates and
and Glycolipids 255 glycoconjugates and provides a few examples of their
7.4 Carbohydrates as Informational Molecules: many structural and functional roles.
The Sugar Code 261 Carbohydrates are polyhydroxy aldehydes or ke-
7.5 Working with Carbohydrates 267 tones, or substances that yield such compounds on hy-
drolysis. Many, but not all, carbohydrates have the em-
pirical formula (CH:O),,; some also contain nitrogen,
Ah! sweet mystery of life... phosphorus, or sulfur.
—Rida Johnson Young (lyrics) and Victor Herbert (music), There are three major size classes of carbohydrates:
‘Ah! Sweet Mystery of Life,” 1910 monosaccharides, oligosaccharides, and polysaccha-
rides (the word “saccharide” is derived from the Greek
sakcharon, meaning “sugar”). Monosaccharides, or
| would feel more optimistic about a bright future for man simple sugars, consist of a single polyhydroxy aldehyde
if he spent less time proving that he can outwit Nature or ketone unit. The most abundant monosaccharide in
and more time tasting her sweetness and respecting her nature is the six-carbon sugar D-glucose, sometimes re-
seniority. ferred to as dextrose. Monosaccharides of more than
—E, B, White, “Coon Tree,” 1977 four carbons tend to have cyclic structures.
Oligosaccharides consist of short chains of mono-
saccharide units, or residues, joined by characteristic
C2 enrales are the most abundant biomolecules linkages called glycosidic bonds. The most abundant are
on Earth. Each year, photosynthesis converts more the disaccharides, with two monosaccharide units.
than 100 billion metric tons of CO, and HeO into cellu- Typical is sucrose (cane sugar), which consists of the
lose and other plant products. Certain carbohydrates six-carbon sugars b-glucose and b-fructose. All common
(sugar and starch) are a dietary staple in most parts of monosaccharides and disaccharides have names ending
the world, and the oxidation of carbohydrates is the cen- with the suffix “-ose.” In cells, most oligosaccharides
tral energy-yielding pathway in most nonphotosynthetic consisting of three or more units do not occur as free
cells. Insoluble carbohydrate polymers serve as struc- entities but are joined to nonsugar molecules (lipids or
tural and protective elements in the cell walls of bacte- proteins) in glycoconjugates.
ria and plants and in the connective tissues of animals. The polysaccharides are sugar polymers contain-
Other carbohydrate polymers lubricate skeletal joints ing more than 20 or so monosaccharide units, and some
and participate in recognition and adhesion between have hundreds or thousands of units. Some polysac-
cells. More complex carbohydrate polymers covalently charides, such as cellulose, are linear chains; others,
238
, Chapter 7 Carbohydrates and Glycobiology 239
such as glycogen, are branched. Both glycogen and cel- H Oo H
lulose consist of recurring units of p-glucose, but they \ 4 |
. H—C—OH
differ in the type of glycosidic linkage and consequently
have strikingly different properties and biological roles. H—C—OH =
HOH H—(—0H
H H
7.1 Monosaccharides and Disaccharides Glyceraldehyde, Dihydroxyacetone,
an aldotriose a ketotriose
The simplest of the carbohydrates, the monosaccha- (a)
rides, are either aldehydes or ketones with two or more
hydroxyl groups; the six-carbon monosaccharides glu- H
cose and fructose have five hydroxyl groups. Many of H ? |
C H—C—OH
the carbon atoms to which hydroxyl groups are attached
are chiral centers, which give rise to the many sugar H—C—OH C=O
stereoisomers found in nature. We begin by describing HOCH HO-C—H
the families of monosaccharides with backbones of three
H—C—OH H—C—OH
to seven carbons—their structure and stereoisomeric
forms, and the means of representing their three- H—C—OH H—C—OH
dimensional structures on paper. We then discuss sev- CH,OH CH,OH
eral chemical reactions of the carbonyl groups of mono- p-Glucose, p-Fructose,
saccharides. One such reaction, the addition of a an aldohexose a ketohexose
hydroxyl group from within the same molecule, gener- (b)
ates the cyclic forms of five- and six-carbon sugars (the
forms that predominate in aqueous solution) and cre- H O H O
\ 4 \ 4
ates a new chiral center, adding further stereochemical
complexity to this class of compounds. The nomencla-
H—@—OH He
ture for unambiguously specifying the configuration
about each carbon atom in a cyclic form and the means H—C—OH H—C—OH
of representing these structures on paper are therefore H—C—OH H—C—OH
described in some detail; this information will be useful
CH,OH CH,OH
as we discuss the metabolism of monosaccharides in
D-Ribose, 2-Deoxy-pD-ribose,
Part II. We also introduce here some important mono- an aldopentose an aldopentose
saccharide derivatives encountered in later chapters. (c)
The Two Families of Monosaccharides Are Aldoses FIGURE 7-1 Representative monosaccharides. (a) Two trioses, an
aldose and a ketose. The carbonyl group in each is shaded. (b) Two
and Ketoses
common hexoses. (c) The pentose components of nucleic acids.
Monosaccharides are colorless, crystalline solids that p-Ribose is a component of ribonucleic acid (RNA), and 2-deoxy-p-
are freely soluble in water but insoluble in nonpolar sol- ribose is a component of deoxyribonucleic acid (DNA).
vents. Most have a sweet taste. The backbones of com-
mon monosaccharide molecules are unbranched carbon
aldotetroses and ketotetroses, aldopentoses and ke-
chains in which all the carbon atoms are linked by sin-
topentoses, and so on. The hexoses, which include the
gle bonds. In the open-chain form, one of the carbon
aldohexose pb-glucose and the ketohexose pb-fructose
atoms is double-bonded to an oxygen atom to form a
(Fig. 7-1b), are the most common monosaccharides in
carbonyl group; each of the other carbon atoms has a
nature. The aldopentoses p-ribose and 2-deoxy-pb-ribose
hydroxyl group. If the carbonyl group is at an end of the
(Fig. 7-1c) are components of nucleotides and nucleic
carbon chain (that is, in an aldehyde group) the mono-
acids (Chapter 8).
saccharide is an aldose; if the carbonyl group is at any
other position Gn a ketone group) the monosaccharide
Monosaccharides Have Asymmetric Centers
is a ketose. The simplest monosaccharides are the two
three-carbon trioses: glyceraldehyde, an aldotriose, and All the monosaccharides except dihydroxyacetone con-
dihydroxyacetone, a ketotriose (Fig. 7—la). tain one or more asymmetric (chiral) carbon atoms and
Monosaccharides with four, five, six, and seven car- thus occur in optically active isomeric forms (pp. 17-
bon atoms in their backbones are called, respectively, 19). The simplest aldose, glyceraldehyde, contains one
tetroses, pentoses, hexoses, and heptoses. There are chiral center (the middle carbon atom) and therefore has
aldoses and ketoses of each of these chain lengths: two different optical isomers, or enantiomers (Fig. 7-2).
CARBOHYDRATES AND
GLYCOBIOLOGY
7.1. Monosaccharides and Disaccharides 239 attached to proteins or lipids act as signals that deter-
mine the intracellular location or metabolic fate of these
7.2 Polysaccharides 247
hybrid molecules, called glycoconjugates. This chap-
7.3 Glycoconjugates: Proteoglycans, Glycoproteins, ter introduces the major classes of carbohydrates and
and Glycolipids 255 glycoconjugates and provides a few examples of their
7.4 Carbohydrates as Informational Molecules: many structural and functional roles.
The Sugar Code 261 Carbohydrates are polyhydroxy aldehydes or ke-
7.5 Working with Carbohydrates 267 tones, or substances that yield such compounds on hy-
drolysis. Many, but not all, carbohydrates have the em-
pirical formula (CH:O),,; some also contain nitrogen,
Ah! sweet mystery of life... phosphorus, or sulfur.
—Rida Johnson Young (lyrics) and Victor Herbert (music), There are three major size classes of carbohydrates:
‘Ah! Sweet Mystery of Life,” 1910 monosaccharides, oligosaccharides, and polysaccha-
rides (the word “saccharide” is derived from the Greek
sakcharon, meaning “sugar”). Monosaccharides, or
| would feel more optimistic about a bright future for man simple sugars, consist of a single polyhydroxy aldehyde
if he spent less time proving that he can outwit Nature or ketone unit. The most abundant monosaccharide in
and more time tasting her sweetness and respecting her nature is the six-carbon sugar D-glucose, sometimes re-
seniority. ferred to as dextrose. Monosaccharides of more than
—E, B, White, “Coon Tree,” 1977 four carbons tend to have cyclic structures.
Oligosaccharides consist of short chains of mono-
saccharide units, or residues, joined by characteristic
C2 enrales are the most abundant biomolecules linkages called glycosidic bonds. The most abundant are
on Earth. Each year, photosynthesis converts more the disaccharides, with two monosaccharide units.
than 100 billion metric tons of CO, and HeO into cellu- Typical is sucrose (cane sugar), which consists of the
lose and other plant products. Certain carbohydrates six-carbon sugars b-glucose and b-fructose. All common
(sugar and starch) are a dietary staple in most parts of monosaccharides and disaccharides have names ending
the world, and the oxidation of carbohydrates is the cen- with the suffix “-ose.” In cells, most oligosaccharides
tral energy-yielding pathway in most nonphotosynthetic consisting of three or more units do not occur as free
cells. Insoluble carbohydrate polymers serve as struc- entities but are joined to nonsugar molecules (lipids or
tural and protective elements in the cell walls of bacte- proteins) in glycoconjugates.
ria and plants and in the connective tissues of animals. The polysaccharides are sugar polymers contain-
Other carbohydrate polymers lubricate skeletal joints ing more than 20 or so monosaccharide units, and some
and participate in recognition and adhesion between have hundreds or thousands of units. Some polysac-
cells. More complex carbohydrate polymers covalently charides, such as cellulose, are linear chains; others,
238
, Chapter 7 Carbohydrates and Glycobiology 239
such as glycogen, are branched. Both glycogen and cel- H Oo H
lulose consist of recurring units of p-glucose, but they \ 4 |
. H—C—OH
differ in the type of glycosidic linkage and consequently
have strikingly different properties and biological roles. H—C—OH =
HOH H—(—0H
H H
7.1 Monosaccharides and Disaccharides Glyceraldehyde, Dihydroxyacetone,
an aldotriose a ketotriose
The simplest of the carbohydrates, the monosaccha- (a)
rides, are either aldehydes or ketones with two or more
hydroxyl groups; the six-carbon monosaccharides glu- H
cose and fructose have five hydroxyl groups. Many of H ? |
C H—C—OH
the carbon atoms to which hydroxyl groups are attached
are chiral centers, which give rise to the many sugar H—C—OH C=O
stereoisomers found in nature. We begin by describing HOCH HO-C—H
the families of monosaccharides with backbones of three
H—C—OH H—C—OH
to seven carbons—their structure and stereoisomeric
forms, and the means of representing their three- H—C—OH H—C—OH
dimensional structures on paper. We then discuss sev- CH,OH CH,OH
eral chemical reactions of the carbonyl groups of mono- p-Glucose, p-Fructose,
saccharides. One such reaction, the addition of a an aldohexose a ketohexose
hydroxyl group from within the same molecule, gener- (b)
ates the cyclic forms of five- and six-carbon sugars (the
forms that predominate in aqueous solution) and cre- H O H O
\ 4 \ 4
ates a new chiral center, adding further stereochemical
complexity to this class of compounds. The nomencla-
H—@—OH He
ture for unambiguously specifying the configuration
about each carbon atom in a cyclic form and the means H—C—OH H—C—OH
of representing these structures on paper are therefore H—C—OH H—C—OH
described in some detail; this information will be useful
CH,OH CH,OH
as we discuss the metabolism of monosaccharides in
D-Ribose, 2-Deoxy-pD-ribose,
Part II. We also introduce here some important mono- an aldopentose an aldopentose
saccharide derivatives encountered in later chapters. (c)
The Two Families of Monosaccharides Are Aldoses FIGURE 7-1 Representative monosaccharides. (a) Two trioses, an
aldose and a ketose. The carbonyl group in each is shaded. (b) Two
and Ketoses
common hexoses. (c) The pentose components of nucleic acids.
Monosaccharides are colorless, crystalline solids that p-Ribose is a component of ribonucleic acid (RNA), and 2-deoxy-p-
are freely soluble in water but insoluble in nonpolar sol- ribose is a component of deoxyribonucleic acid (DNA).
vents. Most have a sweet taste. The backbones of com-
mon monosaccharide molecules are unbranched carbon
aldotetroses and ketotetroses, aldopentoses and ke-
chains in which all the carbon atoms are linked by sin-
topentoses, and so on. The hexoses, which include the
gle bonds. In the open-chain form, one of the carbon
aldohexose pb-glucose and the ketohexose pb-fructose
atoms is double-bonded to an oxygen atom to form a
(Fig. 7-1b), are the most common monosaccharides in
carbonyl group; each of the other carbon atoms has a
nature. The aldopentoses p-ribose and 2-deoxy-pb-ribose
hydroxyl group. If the carbonyl group is at an end of the
(Fig. 7-1c) are components of nucleotides and nucleic
carbon chain (that is, in an aldehyde group) the mono-
acids (Chapter 8).
saccharide is an aldose; if the carbonyl group is at any
other position Gn a ketone group) the monosaccharide
Monosaccharides Have Asymmetric Centers
is a ketose. The simplest monosaccharides are the two
three-carbon trioses: glyceraldehyde, an aldotriose, and All the monosaccharides except dihydroxyacetone con-
dihydroxyacetone, a ketotriose (Fig. 7—la). tain one or more asymmetric (chiral) carbon atoms and
Monosaccharides with four, five, six, and seven car- thus occur in optically active isomeric forms (pp. 17-
bon atoms in their backbones are called, respectively, 19). The simplest aldose, glyceraldehyde, contains one
tetroses, pentoses, hexoses, and heptoses. There are chiral center (the middle carbon atom) and therefore has
aldoses and ketoses of each of these chain lengths: two different optical isomers, or enantiomers (Fig. 7-2).
