13.4.4 Glycogen, Galactan and Other Reserve Polysaccharides
487
cies, organ and chitin type. Even though several
such proteins have received specific names, e.g.
arthropodin or sclerotin in the arthropods, and
conchiolin or conchagen in the molluscs, it is usually the case that little is known about their structural properties, except for their amino acid compositions [4,23, 123, 192]. The arthropod cuticula is made up of chitin fibres embedded in a
protein matrix. X-ray pictures show that both the
chitin chains and the proteins in the ovipositor of
the ichneumon Megarhyssa have a crystalline
structure. The fibres are bundles of microfibrils,
each made up of a central a-chitin fibril with a
protein coat (Fig. 13.15b), packed in a regular
hexagonal pattern. This is therefore a unique
example of a fibre-matrix structure in which both
components are strictly organized [30].
Chitin biosynthesis in the arthropods differs
from that in the fungi by the involvement of dolichol phosphate (dol-PP). It has been shown in the
brine shrimp Artemia salina that a specific transferase transfers a sugar phosphate residue from
UDP-N-acetyl-D-glucosamine to dolichol phosphate to give dolichol pyrophosphate-N-acetyl-Dglucosamine (dol-PP-GlcNAc). As will be shown,
this compound is the starting point for the production of asparagine-bound oligo saccharides in
glycoproteins. In chitin biosynthesis, the carbohydrate components bound to the dol-PP are
extended to form approximately octasaccharides
by further N-acetylglucosamines, and are then
transferred to a peptide acceptor. The resulting
chitoprotein is the primer for chitin synthase
[119]. Animal chitin synthase is a microsomal,
integral membrane enzyme which is extremely
labile and has so far proved impossible to isolate.
However, cell-free systems have been prepared
from Artemia and various insects and these catalyze transfer of N-acetylglucosamine residues
from UDP to chitin oligosaccharide or an endogenous primer. The insecticide diflubenzuron,
which in vivo inhibits the incorporation of glucosamine into chitin, is variable in its effects upon
these in vitro systems [54, 158].
Cellulose was discovered as a component of
animal tissues in 1858 in the mantle (tunica) of
the ascidians, and was named tunicin. The cellulose of the tunicates corresponds in all its properties to plant cellulose. It is made up exclusively of
linear ~(1-4) linked glucose residues; the chains
show the typical conformation where 250 chains
are associated into fibrils that are 10 nm thick,
and the fibrils are bundled into fibres of about
70 nm. The cellulose of the tunicates is bound
covalently to protein and the proteoglycan is noncovalently linked to hyaluronic acid and further
proteins. Fibres of protein-bound cellulose have
also been found in the skin and aorta wall of
humans and various mammals. The concept has
been put forward that cellulose from the chordates corresponds to the chitin of other animal
phyla, perhaps even with homologous enzymes
for their synthesis [123, 162].
13.4.4 Glycogen, Galactan
and Other Reserve Polysaccharides
Amongst the multicellular animals glycogens are
universal as storage polysaccharides, their concentrations varying by orders of magnitude with
the species, tissue and physiological state. Thus,
the glycogen content of the liver of a starving
mammal is 1-5 mglg fresh weight, but on feeding
increases by up to 100-fold. The highest values of
about 50 % of the dry weight are found in the
endoparasitic nematodes and cestodes, which
have a particularly high requirement for carbohydrate as a result of their anaerobic metabolism.
All the glycogens are branched glucans with
a(1-4) and a(1-6) bonds and an average chain
length of 10-13 sugar residues. Rabbit and porcine liver glycogen normally includes glucosamine in addition to glucose in the ratio 1: 50 000
[149]. The degree of branching and the chain
length of the glycogens vary with the species and
the tissue, and may even be heterogenous in the
same cell. Thus, for example, the muscle glycogens of the teleost species Catla catla have consistently longer chains than those of Clarias batrachus, and the glycogens of the white muscle of
both species have longer chains than those of the
red skeletal muscle and heart muscle. In the mussel Mytilus galloprovincialis, the average chain
length in the hepatopancreas is shorter than in
the mantle tissue [123, 161].
Values of over 100 MDa have been recorded
for the molecular mass, and both species- and
tissue-specific differences have been described;
however, in view of the large molecular masses,
artefacts of isolation are difficult to exclude. In
most cells, glycogen forms self-limiting spherical
particles that are about 15-30 nm in diameter and
have a mass of 4-10 MDa (~-particles). In particularly glycogen-rich tissues, such as the liver
and kidney of various vertebrates or the heart
muscle of diving mammals, the ~-particles may
aggregate to form rosette-shaped structures with
a diameter of 100 nm and a mass of 400 MDa (aparticles); molecular masses of up to 100 MDa
487
cies, organ and chitin type. Even though several
such proteins have received specific names, e.g.
arthropodin or sclerotin in the arthropods, and
conchiolin or conchagen in the molluscs, it is usually the case that little is known about their structural properties, except for their amino acid compositions [4,23, 123, 192]. The arthropod cuticula is made up of chitin fibres embedded in a
protein matrix. X-ray pictures show that both the
chitin chains and the proteins in the ovipositor of
the ichneumon Megarhyssa have a crystalline
structure. The fibres are bundles of microfibrils,
each made up of a central a-chitin fibril with a
protein coat (Fig. 13.15b), packed in a regular
hexagonal pattern. This is therefore a unique
example of a fibre-matrix structure in which both
components are strictly organized [30].
Chitin biosynthesis in the arthropods differs
from that in the fungi by the involvement of dolichol phosphate (dol-PP). It has been shown in the
brine shrimp Artemia salina that a specific transferase transfers a sugar phosphate residue from
UDP-N-acetyl-D-glucosamine to dolichol phosphate to give dolichol pyrophosphate-N-acetyl-Dglucosamine (dol-PP-GlcNAc). As will be shown,
this compound is the starting point for the production of asparagine-bound oligo saccharides in
glycoproteins. In chitin biosynthesis, the carbohydrate components bound to the dol-PP are
extended to form approximately octasaccharides
by further N-acetylglucosamines, and are then
transferred to a peptide acceptor. The resulting
chitoprotein is the primer for chitin synthase
[119]. Animal chitin synthase is a microsomal,
integral membrane enzyme which is extremely
labile and has so far proved impossible to isolate.
However, cell-free systems have been prepared
from Artemia and various insects and these catalyze transfer of N-acetylglucosamine residues
from UDP to chitin oligosaccharide or an endogenous primer. The insecticide diflubenzuron,
which in vivo inhibits the incorporation of glucosamine into chitin, is variable in its effects upon
these in vitro systems [54, 158].
Cellulose was discovered as a component of
animal tissues in 1858 in the mantle (tunica) of
the ascidians, and was named tunicin. The cellulose of the tunicates corresponds in all its properties to plant cellulose. It is made up exclusively of
linear ~(1-4) linked glucose residues; the chains
show the typical conformation where 250 chains
are associated into fibrils that are 10 nm thick,
and the fibrils are bundled into fibres of about
70 nm. The cellulose of the tunicates is bound
covalently to protein and the proteoglycan is noncovalently linked to hyaluronic acid and further
proteins. Fibres of protein-bound cellulose have
also been found in the skin and aorta wall of
humans and various mammals. The concept has
been put forward that cellulose from the chordates corresponds to the chitin of other animal
phyla, perhaps even with homologous enzymes
for their synthesis [123, 162].
13.4.4 Glycogen, Galactan
and Other Reserve Polysaccharides
Amongst the multicellular animals glycogens are
universal as storage polysaccharides, their concentrations varying by orders of magnitude with
the species, tissue and physiological state. Thus,
the glycogen content of the liver of a starving
mammal is 1-5 mglg fresh weight, but on feeding
increases by up to 100-fold. The highest values of
about 50 % of the dry weight are found in the
endoparasitic nematodes and cestodes, which
have a particularly high requirement for carbohydrate as a result of their anaerobic metabolism.
All the glycogens are branched glucans with
a(1-4) and a(1-6) bonds and an average chain
length of 10-13 sugar residues. Rabbit and porcine liver glycogen normally includes glucosamine in addition to glucose in the ratio 1: 50 000
[149]. The degree of branching and the chain
length of the glycogens vary with the species and
the tissue, and may even be heterogenous in the
same cell. Thus, for example, the muscle glycogens of the teleost species Catla catla have consistently longer chains than those of Clarias batrachus, and the glycogens of the white muscle of
both species have longer chains than those of the
red skeletal muscle and heart muscle. In the mussel Mytilus galloprovincialis, the average chain
length in the hepatopancreas is shorter than in
the mantle tissue [123, 161].
Values of over 100 MDa have been recorded
for the molecular mass, and both species- and
tissue-specific differences have been described;
however, in view of the large molecular masses,
artefacts of isolation are difficult to exclude. In
most cells, glycogen forms self-limiting spherical
particles that are about 15-30 nm in diameter and
have a mass of 4-10 MDa (~-particles). In particularly glycogen-rich tissues, such as the liver
and kidney of various vertebrates or the heart
muscle of diving mammals, the ~-particles may
aggregate to form rosette-shaped structures with
a diameter of 100 nm and a mass of 400 MDa (aparticles); molecular masses of up to 100 MDa