(Helix). It is rather puzzling that the nutrient
reserve for gastropod embryos is not glycogen, as
it is in all other metazoans. The galactans of the
gastropods are all polysaccharides of D-galactose
residues with species-specific degrees of branching. Depending upon the origin of the galactans,
L-galactose, L-fucose, L-glucose and phosphate
residues may also be present. Most preparations
contain amino acids but it is not clear whether in
vivo the snail galactans really exist as protein
complexes. Isolation with the help of lectins produces almost N-free polysaccharides. The galactans of different gastropods vary greatly in their
reactivity with lectins and antibodies [152].
The D-galactose residues are predominantly
~(1-3) or ~(1-6) linked, although relatively few
are found in linear regions and most are terminal
or occur at branch points; some species also have
~(1-2) and ~(1-4) bonds. L-Galactoses probably
always occur terminally [153]. Little is known
about the galactan-synthesizing enzymes; ~(1-3)and ~(1-6)-galactosyl-transferases have been isolated from the albumen glands of Lymnaea and
Helix [98, 137]. The galactans of the aquatic
Basommatophora (e.g. Lymnaea, Biomphalaria)
are highly branched with only 5-8 % of the sugar
residues in linear sections. L-galactose and ~(1-2)
bonds are lacking, and ~(1-3) and ~(1-6) bonds
alternate more-or-Iess regularly. In the terrestrial
Stylommatophora (e.g. Helix, Arianta, Cepaea,
Achatina) up to 20 % of the sugar residues are linear ~(1-3) bound. L-galactose and linear ~(1-2)
bonds are found only in the galactans of the family
Helicidae. The galactans of Ampullarius sp., the
only prosobranch examined so far, have an unusually large proportion of linearly arranged sugars,
with 5 % ~(1-3), 26 % ~(1-6) and 10 % ~(1-2). In
contrast to those of the pulmonates, some of the
D-galactose residues in Ampullarius sp. are
replaced by fucose. Phosphate-substituted galactose residues are found in the galactans of individual species from various snail genera, e.g. in
Biomphalaria, Helix and Cepaea, but are absent
from, e.g., Lymnaea and Arianta [117]. In addition to heparin, the mammalian lung also contains
a ~(1-3), ~(1-6)-D-galactan which, however,
includes 2-3 % glucuronic acid residues [123].
13.5 Glycoproteins
Almost all proteins of the cell surface and the
extracellular space bear carbohydrate chains; gly·
cosylation is one of the most important mechanisms for the post-translational modification of
13.5 Glycoproteins
489
proteins. This is achieved by the transfer of
mono- or oligo saccharides to the amide-N of certain asparagine residues or to the -OH groups
of serine, threonine, hydroxylysine or other
hydroxy- amino acids. The biological importance
of glycosylation lies on the one hand in the carbohydrates as recognition signals, and on the other
hand in the resulting effect on the folding of the
polypeptide chain, which influences its chemical
and biological properties. The information content even of small oligo saccharides is very high;
for example, the various possibilities for the
anomeric configuration and position of the glycosidic bonding of three hexoses allows the formation of almost 1000 different trisaccharides,
whereas in the case of three amino acids or nucleotides only 6 different combinations are possible.
Several thousand carbohydrate sequences of oligosaccharides, polysaccharides and glycoconjugates have been determined and published to date
[57]. The importance of carbohydrate structure
for the recognition of proteins is clearly illustrated by the endocytosis of plasma glycoproteins
by mammalian liver cells. Enzymatic cleavage of
the terminal sialic acid of the carbohydrate chains
of plasma proteins exposes galactose residues
which are recognized by receptors on the cell surface. The asialoglycoprotein-receptor complex is
taken up into the cell, the plasma protein is
degraded in the lysosomes, and the receptor is
recycled to the cell surface. Human hepatocytes
have two receptor proteins (H1 and H2) of about
45 kDa whose cDNA-determined amino acid
sequences are about 58 % identical. However, H1
shows high agreement with the corresponding
sequence in rat, i.e. the gene duplication which
produced the two different receptor sequences
occurred before the separation of the human and
rat evolutionary lines. The biological significance
of the existence of two receptor proteins in hepatocytes is not clear; it is possible that they form
dimers [252].
The oligosaccharide chains of the glycolipids
and glycoproteins form a layer (the glycocalyx)
on the surface of many cells and this is important
in cell-cell interaction between cells of the same
organism and with foreign cells [93]. Proteolytic
cleavage of membrane proteins from tissue culture cells of all vertebrate classes releases about
60 glycopeptide fractions which are evolutionarily
very conservative within the vertebrates in their
size, lectin specificity, charge and chromatographic behaviour, but which differ greatly from,
for example, those in the eggs of the sea urchin
Arbacia punctulata [31].
reserve for gastropod embryos is not glycogen, as
it is in all other metazoans. The galactans of the
gastropods are all polysaccharides of D-galactose
residues with species-specific degrees of branching. Depending upon the origin of the galactans,
L-galactose, L-fucose, L-glucose and phosphate
residues may also be present. Most preparations
contain amino acids but it is not clear whether in
vivo the snail galactans really exist as protein
complexes. Isolation with the help of lectins produces almost N-free polysaccharides. The galactans of different gastropods vary greatly in their
reactivity with lectins and antibodies [152].
The D-galactose residues are predominantly
~(1-3) or ~(1-6) linked, although relatively few
are found in linear regions and most are terminal
or occur at branch points; some species also have
~(1-2) and ~(1-4) bonds. L-Galactoses probably
always occur terminally [153]. Little is known
about the galactan-synthesizing enzymes; ~(1-3)and ~(1-6)-galactosyl-transferases have been isolated from the albumen glands of Lymnaea and
Helix [98, 137]. The galactans of the aquatic
Basommatophora (e.g. Lymnaea, Biomphalaria)
are highly branched with only 5-8 % of the sugar
residues in linear sections. L-galactose and ~(1-2)
bonds are lacking, and ~(1-3) and ~(1-6) bonds
alternate more-or-Iess regularly. In the terrestrial
Stylommatophora (e.g. Helix, Arianta, Cepaea,
Achatina) up to 20 % of the sugar residues are linear ~(1-3) bound. L-galactose and linear ~(1-2)
bonds are found only in the galactans of the family
Helicidae. The galactans of Ampullarius sp., the
only prosobranch examined so far, have an unusually large proportion of linearly arranged sugars,
with 5 % ~(1-3), 26 % ~(1-6) and 10 % ~(1-2). In
contrast to those of the pulmonates, some of the
D-galactose residues in Ampullarius sp. are
replaced by fucose. Phosphate-substituted galactose residues are found in the galactans of individual species from various snail genera, e.g. in
Biomphalaria, Helix and Cepaea, but are absent
from, e.g., Lymnaea and Arianta [117]. In addition to heparin, the mammalian lung also contains
a ~(1-3), ~(1-6)-D-galactan which, however,
includes 2-3 % glucuronic acid residues [123].
13.5 Glycoproteins
Almost all proteins of the cell surface and the
extracellular space bear carbohydrate chains; gly·
cosylation is one of the most important mechanisms for the post-translational modification of
13.5 Glycoproteins
489
proteins. This is achieved by the transfer of
mono- or oligo saccharides to the amide-N of certain asparagine residues or to the -OH groups
of serine, threonine, hydroxylysine or other
hydroxy- amino acids. The biological importance
of glycosylation lies on the one hand in the carbohydrates as recognition signals, and on the other
hand in the resulting effect on the folding of the
polypeptide chain, which influences its chemical
and biological properties. The information content even of small oligo saccharides is very high;
for example, the various possibilities for the
anomeric configuration and position of the glycosidic bonding of three hexoses allows the formation of almost 1000 different trisaccharides,
whereas in the case of three amino acids or nucleotides only 6 different combinations are possible.
Several thousand carbohydrate sequences of oligosaccharides, polysaccharides and glycoconjugates have been determined and published to date
[57]. The importance of carbohydrate structure
for the recognition of proteins is clearly illustrated by the endocytosis of plasma glycoproteins
by mammalian liver cells. Enzymatic cleavage of
the terminal sialic acid of the carbohydrate chains
of plasma proteins exposes galactose residues
which are recognized by receptors on the cell surface. The asialoglycoprotein-receptor complex is
taken up into the cell, the plasma protein is
degraded in the lysosomes, and the receptor is
recycled to the cell surface. Human hepatocytes
have two receptor proteins (H1 and H2) of about
45 kDa whose cDNA-determined amino acid
sequences are about 58 % identical. However, H1
shows high agreement with the corresponding
sequence in rat, i.e. the gene duplication which
produced the two different receptor sequences
occurred before the separation of the human and
rat evolutionary lines. The biological significance
of the existence of two receptor proteins in hepatocytes is not clear; it is possible that they form
dimers [252].
The oligosaccharide chains of the glycolipids
and glycoproteins form a layer (the glycocalyx)
on the surface of many cells and this is important
in cell-cell interaction between cells of the same
organism and with foreign cells [93]. Proteolytic
cleavage of membrane proteins from tissue culture cells of all vertebrate classes releases about
60 glycopeptide fractions which are evolutionarily
very conservative within the vertebrates in their
size, lectin specificity, charge and chromatographic behaviour, but which differ greatly from,
for example, those in the eggs of the sea urchin
Arbacia punctulata [31].