492
13 The Structure and Metabolism of Carbohydrates
transferases sequenced so far via the cDNA have
a similar domain structure but absolutely no
sequence similarity [207].
Such a biosynthetic process allows for an extraordinarily large variety of carbohydrate structu·
res amongst the glycoproteins of the asparaginebound type. Individual polypeptide chains can
carry several oligosaccharides, e.g. two in the
case of transferrin and immunoglobulin IgG,
three in pro-thrombin, four in fibrinogen and
antithrombin III, five in the acidic alglycoprotein, and six in fibronectin. The carbohydrate chains may be variously branched (bi-,
trio, tetra-antennary) but in fact bi-antennary
chains occur most frequently (Fig. 13.17). Different molecules of a protein can have different carbohydrate components; for example, 60 % of
human serum transferrin molecules have two biantennary chains and the rest have a tri- or even
tetra-antennary chain in addition to a biantennary one [294].
The lower vertebrates already show deviations
from this scheme. The eggs of the rainbow trout
Salmo gairdneri contain polysialoglycoproteins
(PSGPs) with a very unusual structure. The main
component in the unfertilized egg is a PSGP of
200 kDa which contains about 85 % carbohydrate, of which more than 50 % is sialic acid. The
sequences of the core proteins (apoPSGPs) consist predominantly of about 25 tridecapeptides
with the sequence DDATSEAAIGPSG; these
are glycosylated on the underlined amino acids.
These core proteins are encoded by a multi-gene
family with more than 100 members which vary in
the number of 39-bp repeats. After fertilization,
the PSGPs enter the perivitelline space by exocyto sis ; here, they are proteolytically cleaved into 9kDa fragments with only one repeat sequence.
The oligosaccharides bound to a threonine or
serine of the core protein via a GalNAc residue
contain galactose, N-acetylgalactosamine and
fucose as well as stretches of a(2-8)-linked Nglycolylneuraminic acids, which may be capped
by a deaminoneuraminic acid (KDN) , or chains
of a(2-8)-linked KDN residues; they are resistant
to sialidases [142, 250]. The name "hyosophorins" has been proposed for these and similar proteins in the eggs of the carp Cyprinus carpio and
other teleosts. A completely new type of hyosophorin has been isolated from the flounder Paralichthys olivaceus. Its core protein is made up of
repetitive decapeptides, each of which carries a
sialic acid-free, penta-antennary oligosaccharide
bound to an asparagine [243]. The glycoproteins
from the eggs of the char Salve linus leucomaenis
have been found to contain 6-deoxyaltrose; this is
the first report of this unusual deoxyhexose in
animals [128].
The glycoproteins of the invertebrates show
similarities, but also extensive differences, to
those of the vertebrates both in biosynthesis and
in the resulting structures. Sialic acid is not present in most phyla. It has been mentioned already
that the difference between proteoglycans and
glycoproteins is much less marked in the invertebrates than in the vertebrates, and that glycosaminoglycans or glycan sulphates are also found in
invertebrates as significant components of mucous secretions and gels. However, in addition to
proteoglycans, the representatives of various
phyla have been shown to possess typical glycoproteins; such phyla include flagellates, cnidarians, cestodes, nematodes, annelids, insects, molluscs, echinoderms and tunicates [77, 123,
181, 200]. Detailed investigations of the biosynthesis, structure and function of glycoproteins
have been carried out only for insects [41, 121]
and Kinetoplastida [44, 182, 292].
The biosynthesis of asparagine-bound glycoproteins in tissue culture cells of the mosquito
Aedes albopictus is very similar to the process in
the higher vertebrates. The cells synthesize
Glc3Man9GlcNACz bound to dolichol pyrophosphate and transfer this to a protein, after cleavage
of the glucose residues. Six further mannose residues are subsequently removed, leaving a chain of
only three mannose residues, as is found in the
mammals; complex chains are not produced.
None of the N-bound carbohydrates contains
sialic acid, galactose or fucose [121]. An interesting glycoprotein has been isolated from the basement membranes of Drosophila Kc tissue culture
cells, and has been given the name papilin. This is
a glycoprotein of 900 kDa with 64 % carbohydrate, including hexosamine, uronic acid and neutral sugars. The protein component has a mass of
about 400 kDa and contains 25 % aspartate/glutamate, 22 % serine/threonine and carries about
80 O-linked sulphated oligosaccharide chains. The
molecule is a thread with a length of 225 nm and a
diameter of 15 nm and is held in the form of a
loop by disulphide bridges. Several such protomers may be bound together by further disulphide
bridges to give a cloverleaf-shaped oligomer. This
proteoglycan-like glycoprotein is very different to
the heparan sulphate proteoglycan in the basement membranes of vertebrate cells. However,
the other components of the Drosophila basement
membranes, laminin and collagen IV, are very
similar to those of the vertebrates [41].
13 The Structure and Metabolism of Carbohydrates
transferases sequenced so far via the cDNA have
a similar domain structure but absolutely no
sequence similarity [207].
Such a biosynthetic process allows for an extraordinarily large variety of carbohydrate structu·
res amongst the glycoproteins of the asparaginebound type. Individual polypeptide chains can
carry several oligosaccharides, e.g. two in the
case of transferrin and immunoglobulin IgG,
three in pro-thrombin, four in fibrinogen and
antithrombin III, five in the acidic alglycoprotein, and six in fibronectin. The carbohydrate chains may be variously branched (bi-,
trio, tetra-antennary) but in fact bi-antennary
chains occur most frequently (Fig. 13.17). Different molecules of a protein can have different carbohydrate components; for example, 60 % of
human serum transferrin molecules have two biantennary chains and the rest have a tri- or even
tetra-antennary chain in addition to a biantennary one [294].
The lower vertebrates already show deviations
from this scheme. The eggs of the rainbow trout
Salmo gairdneri contain polysialoglycoproteins
(PSGPs) with a very unusual structure. The main
component in the unfertilized egg is a PSGP of
200 kDa which contains about 85 % carbohydrate, of which more than 50 % is sialic acid. The
sequences of the core proteins (apoPSGPs) consist predominantly of about 25 tridecapeptides
with the sequence DDATSEAAIGPSG; these
are glycosylated on the underlined amino acids.
These core proteins are encoded by a multi-gene
family with more than 100 members which vary in
the number of 39-bp repeats. After fertilization,
the PSGPs enter the perivitelline space by exocyto sis ; here, they are proteolytically cleaved into 9kDa fragments with only one repeat sequence.
The oligosaccharides bound to a threonine or
serine of the core protein via a GalNAc residue
contain galactose, N-acetylgalactosamine and
fucose as well as stretches of a(2-8)-linked Nglycolylneuraminic acids, which may be capped
by a deaminoneuraminic acid (KDN) , or chains
of a(2-8)-linked KDN residues; they are resistant
to sialidases [142, 250]. The name "hyosophorins" has been proposed for these and similar proteins in the eggs of the carp Cyprinus carpio and
other teleosts. A completely new type of hyosophorin has been isolated from the flounder Paralichthys olivaceus. Its core protein is made up of
repetitive decapeptides, each of which carries a
sialic acid-free, penta-antennary oligosaccharide
bound to an asparagine [243]. The glycoproteins
from the eggs of the char Salve linus leucomaenis
have been found to contain 6-deoxyaltrose; this is
the first report of this unusual deoxyhexose in
animals [128].
The glycoproteins of the invertebrates show
similarities, but also extensive differences, to
those of the vertebrates both in biosynthesis and
in the resulting structures. Sialic acid is not present in most phyla. It has been mentioned already
that the difference between proteoglycans and
glycoproteins is much less marked in the invertebrates than in the vertebrates, and that glycosaminoglycans or glycan sulphates are also found in
invertebrates as significant components of mucous secretions and gels. However, in addition to
proteoglycans, the representatives of various
phyla have been shown to possess typical glycoproteins; such phyla include flagellates, cnidarians, cestodes, nematodes, annelids, insects, molluscs, echinoderms and tunicates [77, 123,
181, 200]. Detailed investigations of the biosynthesis, structure and function of glycoproteins
have been carried out only for insects [41, 121]
and Kinetoplastida [44, 182, 292].
The biosynthesis of asparagine-bound glycoproteins in tissue culture cells of the mosquito
Aedes albopictus is very similar to the process in
the higher vertebrates. The cells synthesize
Glc3Man9GlcNACz bound to dolichol pyrophosphate and transfer this to a protein, after cleavage
of the glucose residues. Six further mannose residues are subsequently removed, leaving a chain of
only three mannose residues, as is found in the
mammals; complex chains are not produced.
None of the N-bound carbohydrates contains
sialic acid, galactose or fucose [121]. An interesting glycoprotein has been isolated from the basement membranes of Drosophila Kc tissue culture
cells, and has been given the name papilin. This is
a glycoprotein of 900 kDa with 64 % carbohydrate, including hexosamine, uronic acid and neutral sugars. The protein component has a mass of
about 400 kDa and contains 25 % aspartate/glutamate, 22 % serine/threonine and carries about
80 O-linked sulphated oligosaccharide chains. The
molecule is a thread with a length of 225 nm and a
diameter of 15 nm and is held in the form of a
loop by disulphide bridges. Several such protomers may be bound together by further disulphide
bridges to give a cloverleaf-shaped oligomer. This
proteoglycan-like glycoprotein is very different to
the heparan sulphate proteoglycan in the basement membranes of vertebrate cells. However,
the other components of the Drosophila basement
membranes, laminin and collagen IV, are very
similar to those of the vertebrates [41].