A glycoprotein isolated from the egg jelly of
the starfish Asterias amurensis has a tri-antennary
carbohydrate
chain
with
the
formula
Man9_12GlcNAez [77]. A glycoprotein from the
mucus of the coral Acropora formosa has no similarity to the mucus of higher vertebrates; it consists of a polypeptide chain with 77 % serine/threonine bearing O-linked carbohydrate chains of
varying, consistently unusual structures which
involve mainly D-arabinose, D-mannose and Nacetylglucosamine but have no sialic acid or
uronic acid [181]. Many of the haemocyanins contain N-linked carbohydrates, whereby the structures of the oligosaccharides of the arthropods
and molluscs are very different (see Fig. 7.10,
p. 278). There is probably only one glycosylated
haemoglobin and that occurs in the pulmonate
snail Biomphalaria; the oligosaccharide structure
has not yet been determined.
The variable surface glycoproteins (VSGs) of
the African Trypanosoma species were discussed
in Chapter 6 in connection with their characteristic immunological functions. The particularly well-investigated VSG variant MITat 1.4 of T. brucei
bears two asparagine-bound oligosaccharides.
The oligosaccharide on 432-Asn is always mannose rich (Man4-7GlcNAez), whilst the initial
Man3GlcNAez on 419-Asn is later processed to
the
bi-antennary
structure
(GaIGlcNAcManhManGlcNAez. In all Trypanosomatidae
examined, formation of the asparagine-bound
oligosaccharides of the VSGs involves dolichol;
however, the dolichol-PP intermediates never
contain glucose residues but have only six, seven
or nine mannose residues and two Nacetylglucosamines in species-specific proportions. After transfer to the VSG, a single glucose
residue may be added to the asparagine-bound
oligosaccharide but this is then removed again by
a specific glucosidase [15, 34]. Thus, the Trypanosomatidae differ from the mammals, insects,
higher plants and fungi, where Glc3Man9GlcNAez
is produced on dolichol and then transferred to
the acceptor protein (Fig. 13.17). The South
American T. cruzi, the agent of Chagas disease,
has no VSGs but possesses invariable glycoproteins of another type, which appear in stage-specific
forms of 25-90 kDa (p. 241). The trypomastigotes which penetrate the cells of the mammalian
host are protected by these glycoproteins from
recognition and destruction by lysosomes. The
90-kDa glycogen (GP90) of the trypomastigotes
carries asparagine-bound oligosaccharides that
are similar to the mannose-rich type of the mammals; dolichol is present in this case. Antibodies
13.6 Carbohydrases
493
against GP72 prevent the conversion of epimastigotes from the insect host to the trypomastigotes
of the mammalian host. In addition to the typical
N-linked oligosaccharides of mannose and Nacetylglucosamine, GP72 carries O-linked oligosaccharides which contain galactose, fucose,
ribose and xylose and are highly phosphorylated.
It is the latter oligosaccharides which are recognized by the transformation inhibitory antibodies.
The smaller structures GP37 , GP31 and GP25 ,
present on the epimastigotes, have not been
investigated in detail. As well as glycoproteins,
the plasma membrane of T. cruzi includes an
unusual lipopeptidophosphoglycan which will be
described in Chapter 15.
Crithidia fascicultata, which also belongs to
the Kinetoplastida, contains the oligosaccharide
Man7GlcNAez, both bound to dolichol pyrophosphate and as carbohydrate chains on glycoproteins. Thus in this case the carbohydrate
chains transferred from lipid to the protein
differ from the typical vertebrate chains by the
absence of glucose. On the other hand, a
Glc3Man9GlcNAez bound to dolichol pyrophosphate is found in the flagellate Euglena gracilis [43]. Mature glycoproteins of C. fasciculata
also contain small amounts of GallMan6GlcNAez,
in which the galactose residue surprisingly all
have the furanose configuration [182]. The dolichol of C. fasciculata consists of only 11 isoprene
units, whereas that of T. cruzi has 13 units; thus,
both are significantly smaller than the dolichols of
yeast, higher plants and vertebrates, which have
17-20 units [44]. The surface of the kinetoplastid
parasite (Leishmania tarentolae) of the lizards has
a mixture of glycoproteins which consists mainly
of linearly bound galactose residues [292].
13.6 Carbohydrases
Enzymes that hydrolyse complex carbohydrates
were traditionally termed carbohydrases and
assigned to either the polysaccharidases or the
glycosidases, according to the molecular weights
of their substrates. However, the International
Union of Biochemistry (IUB) enzyme commission uses the term glycosidase in a broad sense
for all enzymes that hydrolyse glycosidic bonds,
e.g. in glycosides, oligo- or polysaccharides, glycopeptides, glycolipids or nucleosides. Glycosidases are specific for the sugar released and the
anomeric configuration of the glycosidic bond
the starfish Asterias amurensis has a tri-antennary
carbohydrate
chain
with
the
formula
Man9_12GlcNAez [77]. A glycoprotein from the
mucus of the coral Acropora formosa has no similarity to the mucus of higher vertebrates; it consists of a polypeptide chain with 77 % serine/threonine bearing O-linked carbohydrate chains of
varying, consistently unusual structures which
involve mainly D-arabinose, D-mannose and Nacetylglucosamine but have no sialic acid or
uronic acid [181]. Many of the haemocyanins contain N-linked carbohydrates, whereby the structures of the oligosaccharides of the arthropods
and molluscs are very different (see Fig. 7.10,
p. 278). There is probably only one glycosylated
haemoglobin and that occurs in the pulmonate
snail Biomphalaria; the oligosaccharide structure
has not yet been determined.
The variable surface glycoproteins (VSGs) of
the African Trypanosoma species were discussed
in Chapter 6 in connection with their characteristic immunological functions. The particularly well-investigated VSG variant MITat 1.4 of T. brucei
bears two asparagine-bound oligosaccharides.
The oligosaccharide on 432-Asn is always mannose rich (Man4-7GlcNAez), whilst the initial
Man3GlcNAez on 419-Asn is later processed to
the
bi-antennary
structure
(GaIGlcNAcManhManGlcNAez. In all Trypanosomatidae
examined, formation of the asparagine-bound
oligosaccharides of the VSGs involves dolichol;
however, the dolichol-PP intermediates never
contain glucose residues but have only six, seven
or nine mannose residues and two Nacetylglucosamines in species-specific proportions. After transfer to the VSG, a single glucose
residue may be added to the asparagine-bound
oligosaccharide but this is then removed again by
a specific glucosidase [15, 34]. Thus, the Trypanosomatidae differ from the mammals, insects,
higher plants and fungi, where Glc3Man9GlcNAez
is produced on dolichol and then transferred to
the acceptor protein (Fig. 13.17). The South
American T. cruzi, the agent of Chagas disease,
has no VSGs but possesses invariable glycoproteins of another type, which appear in stage-specific
forms of 25-90 kDa (p. 241). The trypomastigotes which penetrate the cells of the mammalian
host are protected by these glycoproteins from
recognition and destruction by lysosomes. The
90-kDa glycogen (GP90) of the trypomastigotes
carries asparagine-bound oligosaccharides that
are similar to the mannose-rich type of the mammals; dolichol is present in this case. Antibodies
13.6 Carbohydrases
493
against GP72 prevent the conversion of epimastigotes from the insect host to the trypomastigotes
of the mammalian host. In addition to the typical
N-linked oligosaccharides of mannose and Nacetylglucosamine, GP72 carries O-linked oligosaccharides which contain galactose, fucose,
ribose and xylose and are highly phosphorylated.
It is the latter oligosaccharides which are recognized by the transformation inhibitory antibodies.
The smaller structures GP37 , GP31 and GP25 ,
present on the epimastigotes, have not been
investigated in detail. As well as glycoproteins,
the plasma membrane of T. cruzi includes an
unusual lipopeptidophosphoglycan which will be
described in Chapter 15.
Crithidia fascicultata, which also belongs to
the Kinetoplastida, contains the oligosaccharide
Man7GlcNAez, both bound to dolichol pyrophosphate and as carbohydrate chains on glycoproteins. Thus in this case the carbohydrate
chains transferred from lipid to the protein
differ from the typical vertebrate chains by the
absence of glucose. On the other hand, a
Glc3Man9GlcNAez bound to dolichol pyrophosphate is found in the flagellate Euglena gracilis [43]. Mature glycoproteins of C. fasciculata
also contain small amounts of GallMan6GlcNAez,
in which the galactose residue surprisingly all
have the furanose configuration [182]. The dolichol of C. fasciculata consists of only 11 isoprene
units, whereas that of T. cruzi has 13 units; thus,
both are significantly smaller than the dolichols of
yeast, higher plants and vertebrates, which have
17-20 units [44]. The surface of the kinetoplastid
parasite (Leishmania tarentolae) of the lizards has
a mixture of glycoproteins which consists mainly
of linearly bound galactose residues [292].
13.6 Carbohydrases
Enzymes that hydrolyse complex carbohydrates
were traditionally termed carbohydrases and
assigned to either the polysaccharidases or the
glycosidases, according to the molecular weights
of their substrates. However, the International
Union of Biochemistry (IUB) enzyme commission uses the term glycosidase in a broad sense
for all enzymes that hydrolyse glycosidic bonds,
e.g. in glycosides, oligo- or polysaccharides, glycopeptides, glycolipids or nucleosides. Glycosidases are specific for the sugar released and the
anomeric configuration of the glycosidic bond