13 The Structure and Metabolism of Carbohydrates
13.1
13.2
13.3
13.3.1
13.3.2
13.3.3
13.3.4
13.3.5
13.4
Sugars and Sugar Derivatives
The Carbohydrate Spectrum of Cells
and Extracellular Fluids
Metabolism of Low Molecular Weight
Carbohydrates
The Main Metabolic Reactions of Sugars
Formation of Pentoses and NADPH
in the Pentose Phosphate Pathway
Biosynthesis of Fructose, Sugar Alcohols
and Glycerol
Ethanol Metabolism
Biosynthesis of Ascorbic Acid
Polysaccharides and Proteoglycans
13.1 Sugars and Sugar Derivatives
Compared with the variety of carbohydrates in
plants, relatively few sugars or sugar derivatives
are regularly found in animals either free or as
components of more complex compounds. However, it is possible that sugars originating from
plants in the diet are transiently retained in animals and distort the normal sugar spectrum.
Approximately a dozen sugars and sugar derivatives are regularly found in animals: the pentoses
D-ribose and 2-deoxY-D-ribose; the hexoses Dglucose, D-galactose, D-mannose, D-fructose and
L-fucose; the uronic acids D-glucuronic acid and
L-iduronic acid; and the hexosamines Dglucosamine and D-galactosamine. In addition,
D-erythrose, D-ribulose, D-xylulose and Dsedoheptulose in the form of their phosphoric
acid esters are intermediates of the pentose
phosphate pathway.
Other sugars are found occasionally. In contrast to the occurrence in plants, D-galacturonic
acid is very rare in animals; it has been detected,
for example, in mammalian brain and in secretions of the elasmobranchs and gastropods. The
pentoses L-arabinose and D-xylose and the deoxyhexose L-rhamnose are found in several inverte13.4.1
13.4.2
13.4.3
13.4.4
13.5
13.6
13.6.1
13.6.2
13.6.3
13.6.4
13.6.5
Glycosaminoglycans
Glycan Sulphates and Glycan Phosphates
Chitin and Cellulose
Glycogen, Galactan and Other Reserve
Polysaccharides
Glycoproteins
Carbohydrases
Glycosidases
a-Amylases
Digestion of Cellulose, Lichenin and Laminarin
Chitinolysis
Lysozymes
References
brate polysaccharides; D-xylose binds the polysaccharide chain to the protein component in
various mucopolysaccharides. D-arabinose has
been found in unusual nucleosides of the sponge
Thetya (Cryptothetya) crypta (p. 741). Whereas Dgalactose is ubiquitously distributed, e.g. as part
of lactose and many glycoproteins, L-galactose
has so far been found only in the galactans of several snails. 6-Desoxyglucose is present in the
toxin of the holothurian Actinopyga agassizi, and
free D-glucuronic acid or its lactone is found in
the defence secretions of the cockroach Eurycotis
decipiens. The distribution of ascarylose (3,6dideoxY-L-mannose) (Fig. 13.1) is quite unique;
apart from in the bacteria, it is found only in
Ascaris and its relatives, and occurs in particular
in a lipid material that was previously known as
ascaryl alcohol. This material is in fact a mixture
of so-called ascarosides in which the ascarylose is
bound glycosidically to a mono- or divalent fatty
alcohol with a chain length of 22-37 carbon
atoms; the divalent alcohols may also be esterified with a short-chain fatty acid. Such ascarosides are found in all organs, but especially in the
uterus and the ovary. The presence of ascarosides
in the egg membrane is responsible for the
unusual resistance of Ascaris eggs to aggressive
13.1
13.2
13.3
13.3.1
13.3.2
13.3.3
13.3.4
13.3.5
13.4
Sugars and Sugar Derivatives
The Carbohydrate Spectrum of Cells
and Extracellular Fluids
Metabolism of Low Molecular Weight
Carbohydrates
The Main Metabolic Reactions of Sugars
Formation of Pentoses and NADPH
in the Pentose Phosphate Pathway
Biosynthesis of Fructose, Sugar Alcohols
and Glycerol
Ethanol Metabolism
Biosynthesis of Ascorbic Acid
Polysaccharides and Proteoglycans
13.1 Sugars and Sugar Derivatives
Compared with the variety of carbohydrates in
plants, relatively few sugars or sugar derivatives
are regularly found in animals either free or as
components of more complex compounds. However, it is possible that sugars originating from
plants in the diet are transiently retained in animals and distort the normal sugar spectrum.
Approximately a dozen sugars and sugar derivatives are regularly found in animals: the pentoses
D-ribose and 2-deoxY-D-ribose; the hexoses Dglucose, D-galactose, D-mannose, D-fructose and
L-fucose; the uronic acids D-glucuronic acid and
L-iduronic acid; and the hexosamines Dglucosamine and D-galactosamine. In addition,
D-erythrose, D-ribulose, D-xylulose and Dsedoheptulose in the form of their phosphoric
acid esters are intermediates of the pentose
phosphate pathway.
Other sugars are found occasionally. In contrast to the occurrence in plants, D-galacturonic
acid is very rare in animals; it has been detected,
for example, in mammalian brain and in secretions of the elasmobranchs and gastropods. The
pentoses L-arabinose and D-xylose and the deoxyhexose L-rhamnose are found in several inverte13.4.1
13.4.2
13.4.3
13.4.4
13.5
13.6
13.6.1
13.6.2
13.6.3
13.6.4
13.6.5
Glycosaminoglycans
Glycan Sulphates and Glycan Phosphates
Chitin and Cellulose
Glycogen, Galactan and Other Reserve
Polysaccharides
Glycoproteins
Carbohydrases
Glycosidases
a-Amylases
Digestion of Cellulose, Lichenin and Laminarin
Chitinolysis
Lysozymes
References
brate polysaccharides; D-xylose binds the polysaccharide chain to the protein component in
various mucopolysaccharides. D-arabinose has
been found in unusual nucleosides of the sponge
Thetya (Cryptothetya) crypta (p. 741). Whereas Dgalactose is ubiquitously distributed, e.g. as part
of lactose and many glycoproteins, L-galactose
has so far been found only in the galactans of several snails. 6-Desoxyglucose is present in the
toxin of the holothurian Actinopyga agassizi, and
free D-glucuronic acid or its lactone is found in
the defence secretions of the cockroach Eurycotis
decipiens. The distribution of ascarylose (3,6dideoxY-L-mannose) (Fig. 13.1) is quite unique;
apart from in the bacteria, it is found only in
Ascaris and its relatives, and occurs in particular
in a lipid material that was previously known as
ascaryl alcohol. This material is in fact a mixture
of so-called ascarosides in which the ascarylose is
bound glycosidically to a mono- or divalent fatty
alcohol with a chain length of 22-37 carbon
atoms; the divalent alcohols may also be esterified with a short-chain fatty acid. Such ascarosides are found in all organs, but especially in the
uterus and the ovary. The presence of ascarosides
in the egg membrane is responsible for the
unusual resistance of Ascaris eggs to aggressive