488
13 The Structure and Metabolism of Carbohydrates
have also been reported in mammalian brain [49].
In certain active tissues, such as the eye muscle of
some mammals or the heart muscle of mammals
and lungfish (Dipnoi), the cisternae of the endoplasmic reticulum (ER) contain glycogenmembrane complexes that are about 35 nm in
diameter, the so-called glycogen bodies. In such
glycogen granulae, the polysaccharide is associated with proteins, especially the enzymes of glycogen synthesis and catabolism, and the corresponding regulatory enzyme cascade. Glycogen
in the retina has glycosidic bonds to the phenolic
hydroxyl group of tyrosine residues [6].
The primer for glycogen biosynthesis in muscle
and liver is glycogenin, a protein of 332 amino
acids which is released proteolytically from a high
molecular weight precursor. In the absence of glycogen, glycogenin is bound to glycogen synthase.
The first step in glycogen synthesis is the binding
of a glucose to the 194-Tyr of glycogenin. Up to
six further glucoses are added to the first, with
the glycogenin itself functioning as a UDPGspecific glucosyltransferase. Further extension of
the carbohydrate chain is brought about by the
glycogen synthase, which for this purpose dissociates from glycogenin. Glycogen synthase transfers glucose residues from UDP-Glc to the primer
polysaccharide with a(1-4) bonding. When the
a(1-4)-linked chain reaches a length of 12-18
glucose residues, the "branching enzyme" creates
an a(1-6)-bound sugar and, thus, a branch point.
The primer protein remains bound and consequently each glycogen particle includes one molecule of glycogenin [42, 249]. The glycogen synthases from human muscle (737 amino acids) and
rabbit muscle (734 amino acids) show 97 %
sequence agreement. The enzymes from mammalian muscle and liver have a total of up to ten sites
in their N- and C-terminal regions, and these may
be phosphorylated by one of seven different protein kinases. The enzyme of rat liver (703 amino
acids) is about 80 % similar to the mammalian
muscle enzyme in its central region, but shows
large terminal differences; the markedly shortened C-terminal region lacks two of the phosphorylation sites [9, 38,298]. Mammalian muscle
contains an almost phosphate-free I-form of the
enzyme (I for independent) together with various
D-forms (D for dependent) which arise by the
phosphorylation of one or more serines and
which are active only in the presence of glucose
phosphate. The enzyme from rabbit muscle normally has 2.4-3.0 phosphates per subunit; the
phosphate content is increased by adrenalin and
reduced by insulin. The liver enzyme includes up
to six alkali-labile phosphates. In mammalian
kidney, the dephosphorylated I form is also activated by glucose-6-phosphate, and in muscle of
the frog Rana ridibunda the activity of both forms
is increased by glucose-6-phosphate. The lID conversion of the vertebrate enzyme can occur
in vitro through the action of various protein
kinases, some of which are cAMP-dependent and
some cAMP-independent [264]. In the invertebrates so far examined, 1- and D-forms of glycogen
synthase can be distinguished; however, these differ from each other and from the vertebrate enzymes in their allosteric properties [83, 106, 109].
Some unusual reserve polysaccharides have
been found in the protozoans. Tetrahymena has a
typical glycogen with an average chain length of
13 units. In contrast, the a(1-4), a(1-6) glucans
from the rumen ciliates of the genera Entodinium, Isotricha and Dasytricha have chain lengths
of 22-25 units; this is characteristic of the plant
amylopectins. Entodinium caudatum, the gregarines and the sporozoon Eimeria ten ella have intermediate chain lengths of about 19 sugar residues
[284]. Homo- and heteroglycans with a different
composition have been identified in other Protozoa; these are probably components of glycoproteins and fulfil functions other than nutrient storage. They include, for example, D-mannan and Darabino-D-galactan in Crithidia jasciculata, and
the ~(1-2)-mannans and galactomannans found
in several trypanosomes. Trypanosoma mega has
no ~(1-2)-mannan but, instead, a mixture of
branched galactans with (1-2) and (1-3) bonds
and several fucose, glucose and mannose residues
[44, 261]. The main polysaccharide in the eggs of
the migratory locust Locusta migratoria is a mannan, with about 12 % glucosamine, which is
bound covalently to protein; it makes up about
10 % of the dry weight of freshly laid eggs but is
rapidly metabolized during embryo development
[293].
Galactans have been found in all pulmonate
snails examined and in the prosobranch Ampullarius sp. This polysaccharide was discovered by
Hammersten in 1885 and was initially known as
"sinistrin"; in view of its structural and functional
relationship to glycogen, it was later given the
name "galactogen". The galactans are produced
in the albumen glands and accumulate in the perivitelline fluid of the egg, where they constitute a
nutrient reserve for the embryo. The galactan
content of, for example, freshly laid eggs of Helix
or Lymnaea is 30-36 % of the dry weight, and the
galactan is rapidly consumed during later embryo
development (Lymnaea) or after hatching
13 The Structure and Metabolism of Carbohydrates
have also been reported in mammalian brain [49].
In certain active tissues, such as the eye muscle of
some mammals or the heart muscle of mammals
and lungfish (Dipnoi), the cisternae of the endoplasmic reticulum (ER) contain glycogenmembrane complexes that are about 35 nm in
diameter, the so-called glycogen bodies. In such
glycogen granulae, the polysaccharide is associated with proteins, especially the enzymes of glycogen synthesis and catabolism, and the corresponding regulatory enzyme cascade. Glycogen
in the retina has glycosidic bonds to the phenolic
hydroxyl group of tyrosine residues [6].
The primer for glycogen biosynthesis in muscle
and liver is glycogenin, a protein of 332 amino
acids which is released proteolytically from a high
molecular weight precursor. In the absence of glycogen, glycogenin is bound to glycogen synthase.
The first step in glycogen synthesis is the binding
of a glucose to the 194-Tyr of glycogenin. Up to
six further glucoses are added to the first, with
the glycogenin itself functioning as a UDPGspecific glucosyltransferase. Further extension of
the carbohydrate chain is brought about by the
glycogen synthase, which for this purpose dissociates from glycogenin. Glycogen synthase transfers glucose residues from UDP-Glc to the primer
polysaccharide with a(1-4) bonding. When the
a(1-4)-linked chain reaches a length of 12-18
glucose residues, the "branching enzyme" creates
an a(1-6)-bound sugar and, thus, a branch point.
The primer protein remains bound and consequently each glycogen particle includes one molecule of glycogenin [42, 249]. The glycogen synthases from human muscle (737 amino acids) and
rabbit muscle (734 amino acids) show 97 %
sequence agreement. The enzymes from mammalian muscle and liver have a total of up to ten sites
in their N- and C-terminal regions, and these may
be phosphorylated by one of seven different protein kinases. The enzyme of rat liver (703 amino
acids) is about 80 % similar to the mammalian
muscle enzyme in its central region, but shows
large terminal differences; the markedly shortened C-terminal region lacks two of the phosphorylation sites [9, 38,298]. Mammalian muscle
contains an almost phosphate-free I-form of the
enzyme (I for independent) together with various
D-forms (D for dependent) which arise by the
phosphorylation of one or more serines and
which are active only in the presence of glucose
phosphate. The enzyme from rabbit muscle normally has 2.4-3.0 phosphates per subunit; the
phosphate content is increased by adrenalin and
reduced by insulin. The liver enzyme includes up
to six alkali-labile phosphates. In mammalian
kidney, the dephosphorylated I form is also activated by glucose-6-phosphate, and in muscle of
the frog Rana ridibunda the activity of both forms
is increased by glucose-6-phosphate. The lID conversion of the vertebrate enzyme can occur
in vitro through the action of various protein
kinases, some of which are cAMP-dependent and
some cAMP-independent [264]. In the invertebrates so far examined, 1- and D-forms of glycogen
synthase can be distinguished; however, these differ from each other and from the vertebrate enzymes in their allosteric properties [83, 106, 109].
Some unusual reserve polysaccharides have
been found in the protozoans. Tetrahymena has a
typical glycogen with an average chain length of
13 units. In contrast, the a(1-4), a(1-6) glucans
from the rumen ciliates of the genera Entodinium, Isotricha and Dasytricha have chain lengths
of 22-25 units; this is characteristic of the plant
amylopectins. Entodinium caudatum, the gregarines and the sporozoon Eimeria ten ella have intermediate chain lengths of about 19 sugar residues
[284]. Homo- and heteroglycans with a different
composition have been identified in other Protozoa; these are probably components of glycoproteins and fulfil functions other than nutrient storage. They include, for example, D-mannan and Darabino-D-galactan in Crithidia jasciculata, and
the ~(1-2)-mannans and galactomannans found
in several trypanosomes. Trypanosoma mega has
no ~(1-2)-mannan but, instead, a mixture of
branched galactans with (1-2) and (1-3) bonds
and several fucose, glucose and mannose residues
[44, 261]. The main polysaccharide in the eggs of
the migratory locust Locusta migratoria is a mannan, with about 12 % glucosamine, which is
bound covalently to protein; it makes up about
10 % of the dry weight of freshly laid eggs but is
rapidly metabolized during embryo development
[293].
Galactans have been found in all pulmonate
snails examined and in the prosobranch Ampullarius sp. This polysaccharide was discovered by
Hammersten in 1885 and was initially known as
"sinistrin"; in view of its structural and functional
relationship to glycogen, it was later given the
name "galactogen". The galactans are produced
in the albumen glands and accumulate in the perivitelline fluid of the egg, where they constitute a
nutrient reserve for the embryo. The galactan
content of, for example, freshly laid eggs of Helix
or Lymnaea is 30-36 % of the dry weight, and the
galactan is rapidly consumed during later embryo
development (Lymnaea) or after hatching