480
13 The Structure and Metabolism of Carbohydrates
in the regulation of melanization and sclerotization of the cuticala. In crustaceans of the genus
Penaeus, an ascorbic acid deficiency results in a
lethal condition knows as "black death". The biosynthesis of ascorbic acid has been claimed to
occur in, for example, various cockroach species
but has never been unambiguously demonstrated. In contrast to those fed on a normal diet,
caterpillars of the lepidopteran Manduca sexta
fed on an ascorbic acid-free diet have no tissue
ascorbic acid; gulonolactone oxidase is as undetectable in this species as it is in Plodia interpunctella or Periplaneta americanus. Searches for this
enzyme were also unsuccessful in annelids, crustaceans, spiders, chilopods and pulmonate snails.
The only reliable report of gulonolactone oxidase
in an invertebrate is that from Limulus polyphemus. The heart and several other organs of this
species showed specific activities of up to 5 mU/g,
relatively low compared with the values for rat
liver (195 mU/g) and chicken kidney (570 mU/g)
[285].
13.4 Polysaccharides and Proteoglycans
Polysaccharides in living cells are always more-orless tightly bound to proteins. The so-called
protein-free sugar polymers are really artefacts,
resulting in part from the use of drastic isolation
procedures. This is also true for storage polysaccharides such as glycogen, which in the cell is very
closely associated with the enzymes responsible
for its synthesis and catabolism. It is especially
true for the structural polysaccharides such as cellulose, chitin or the glycosaminoglycans. Covalent bonding of carbohydrates and proteins
occurs, for example, by N-glycosidic binding to
asparagine, or O-glycosidic binding to serine,
theronine, hydroxylysine or other hydroyamino
acids. However, structural details are known
almost only for the proteoglycans and glycoproteins of the mammals.
Two types of carbohydrate-protein complexes
can be clearly distinguished in the vertebrates. In
the proteoglycans, the carbohydrate is the dominant component responsible for the properties of
the complex. These are always linear glycosaminoglycans built up repetitively of disaccharide
subunits. They all contain hexosamine and,
except for keratosulphate, also uronic acids. With
the exception of hyaluronic acid, they are all
sulphated, and the sulphation gives the polysaccharides their polyanionic character. Less is
known about the protein component. The large
proteoglycan aggregate from rat cartilage is the
only proteoglycan for which the complete amino
acid sequence is known. In contrast to proteoglycans, the properties of the glycoproteins are
determined by the protein component. In this
case, the protein carries one or more oligosaccharides which may be identical or variable. The carbohydrate chains consist of up to 20 sugar residues and are often branched. They all contain
glucosamine or galactosamine residues, the
majority of which are acetylated, as well as mannose, galactose and fucose, but seldom glucose.
Up to seven different sugars may be found in one
carbohydrate chain. There is often a terminal
sialic acid which gives the molecule its negative
charge; sulphate residues are rare and there are
no uronic acids [214].
The distinction between proteoglycans and glycoproteins is not so clear in the invertebrates.
Sialic acid is not present in the glycoproteins of
many invertebrate groups, but uronic acids are
sometimes found. The proteoglycans are much
more variable than the glycosaminoglycans of the
vertebrates; their polysaccharide components are
often made up only of sulphated or phosphorylated neutral sugars and lack hexosamines and
uronic acids. In this case, as for other classes of
substances in the organism, there is a clear tendency in the vertebrates towards specialization
and simplification [123].
13.4.1 Glycosaminoglycans
The glycosaminoglycans or mucopolysaccharides
are typical components of intercellular matrices.
More recently, they have been recognized as components of the cell surface, where they probably
play a role in cell-cell and cell-substrate interactions [115]. Several basic types are recognizable in
the vertebrates (Fig. 13.13). The glycosaminoglycans are typically very variable in chain length,
the number and position of the sulphate groups
and, in derivatives of chondroitin and heparin,
also in the ratio of glucuronic to iduronic acids;
thus, the fractions isolated from tissues are also
very heterogeneous. The relative proportions of
individual glycosaminoglycans are species specific
and dependent upon the developmental stage
(Table 13.3); this is true for all classes of vertebrates [97].
The protozoans apparently have no glycosaminoglycans. In contrast, all examined metazoans
have been found to have one or more mucopoly-
13 The Structure and Metabolism of Carbohydrates
in the regulation of melanization and sclerotization of the cuticala. In crustaceans of the genus
Penaeus, an ascorbic acid deficiency results in a
lethal condition knows as "black death". The biosynthesis of ascorbic acid has been claimed to
occur in, for example, various cockroach species
but has never been unambiguously demonstrated. In contrast to those fed on a normal diet,
caterpillars of the lepidopteran Manduca sexta
fed on an ascorbic acid-free diet have no tissue
ascorbic acid; gulonolactone oxidase is as undetectable in this species as it is in Plodia interpunctella or Periplaneta americanus. Searches for this
enzyme were also unsuccessful in annelids, crustaceans, spiders, chilopods and pulmonate snails.
The only reliable report of gulonolactone oxidase
in an invertebrate is that from Limulus polyphemus. The heart and several other organs of this
species showed specific activities of up to 5 mU/g,
relatively low compared with the values for rat
liver (195 mU/g) and chicken kidney (570 mU/g)
[285].
13.4 Polysaccharides and Proteoglycans
Polysaccharides in living cells are always more-orless tightly bound to proteins. The so-called
protein-free sugar polymers are really artefacts,
resulting in part from the use of drastic isolation
procedures. This is also true for storage polysaccharides such as glycogen, which in the cell is very
closely associated with the enzymes responsible
for its synthesis and catabolism. It is especially
true for the structural polysaccharides such as cellulose, chitin or the glycosaminoglycans. Covalent bonding of carbohydrates and proteins
occurs, for example, by N-glycosidic binding to
asparagine, or O-glycosidic binding to serine,
theronine, hydroxylysine or other hydroyamino
acids. However, structural details are known
almost only for the proteoglycans and glycoproteins of the mammals.
Two types of carbohydrate-protein complexes
can be clearly distinguished in the vertebrates. In
the proteoglycans, the carbohydrate is the dominant component responsible for the properties of
the complex. These are always linear glycosaminoglycans built up repetitively of disaccharide
subunits. They all contain hexosamine and,
except for keratosulphate, also uronic acids. With
the exception of hyaluronic acid, they are all
sulphated, and the sulphation gives the polysaccharides their polyanionic character. Less is
known about the protein component. The large
proteoglycan aggregate from rat cartilage is the
only proteoglycan for which the complete amino
acid sequence is known. In contrast to proteoglycans, the properties of the glycoproteins are
determined by the protein component. In this
case, the protein carries one or more oligosaccharides which may be identical or variable. The carbohydrate chains consist of up to 20 sugar residues and are often branched. They all contain
glucosamine or galactosamine residues, the
majority of which are acetylated, as well as mannose, galactose and fucose, but seldom glucose.
Up to seven different sugars may be found in one
carbohydrate chain. There is often a terminal
sialic acid which gives the molecule its negative
charge; sulphate residues are rare and there are
no uronic acids [214].
The distinction between proteoglycans and glycoproteins is not so clear in the invertebrates.
Sialic acid is not present in the glycoproteins of
many invertebrate groups, but uronic acids are
sometimes found. The proteoglycans are much
more variable than the glycosaminoglycans of the
vertebrates; their polysaccharide components are
often made up only of sulphated or phosphorylated neutral sugars and lack hexosamines and
uronic acids. In this case, as for other classes of
substances in the organism, there is a clear tendency in the vertebrates towards specialization
and simplification [123].
13.4.1 Glycosaminoglycans
The glycosaminoglycans or mucopolysaccharides
are typical components of intercellular matrices.
More recently, they have been recognized as components of the cell surface, where they probably
play a role in cell-cell and cell-substrate interactions [115]. Several basic types are recognizable in
the vertebrates (Fig. 13.13). The glycosaminoglycans are typically very variable in chain length,
the number and position of the sulphate groups
and, in derivatives of chondroitin and heparin,
also in the ratio of glucuronic to iduronic acids;
thus, the fractions isolated from tissues are also
very heterogeneous. The relative proportions of
individual glycosaminoglycans are species specific
and dependent upon the developmental stage
(Table 13.3); this is true for all classes of vertebrates [97].
The protozoans apparently have no glycosaminoglycans. In contrast, all examined metazoans
have been found to have one or more mucopoly-