three isoenzymes by its smaller size (50 kDa), its
much lower affinity for glucose and other substrates, and its reduced sensitivity to the inhibitory
effect of Glc-6-P. At substrate concentrations of
about 100 mmol/l, hexokinase D metabolizes
fructose with much lower efficiency than it metabolizes glucose; however, reference to this
enzyme as a glucokinase is misleading, as may be
seen from the values of V max (Table 14.2a). The
liver contains a 60-kDa inhibitory protein that
forms an inactive complex with glucokinase in the
presence of Fru-6-P; this is inhibited by Fru-1-P
[264]. The hexokinase D of the pancreatic ~ cells
differs from the liver enzyme only at the 15 Nterminal positions, and is encoded by the same
gene. The glucokinase gene has two promoters
and is regulated by insulin in the liver and by glucose in the ~ cells [146].
The hexokinase spectrum found in the rat may
not be taken as a model even for the mammals,
and certainly not for the vertebrates as a whole.
The D type with low substrate affinity is absent
from the horse, llama, cow, goat, sheep and chinchilla, and the glucose-inhibited C type is not
found in the domestic dog and many rodents;
both are lacking in the domestic cat. In investigations of 97 vertebrate species from all classes, the
D type was detected only in the turtles arid
amphibians in addition to the mammals; and the
C type was detected only in several amphibians
that lacked hexokinase A, and perhaps also in
several teleosts. Neither the D nor the C type was
found in any of 13 avian or 13 reptilian species
(excluding the turtles); the hexokinases of the A
and B types found in these latter species differed
significantly from those of the other vertebrates
in having a lower glucose affinity and a higher
V max with fructose [262]. The C-type hexokinase
found to have high activity in the liver of the frog
Calyptocephalella caudiverba shows sigmoidal
ATP kinetics and other features that are missing
in the corresponding enzyme from Rana catesbeiana. The hexokinases of the vertebrates are, in
general, ATP specific, although the enzyme from
mammalian brain can also use ITP. A C-like hexokinase isolated from the liver of the rainbow trout
shows an unusually wide nucleotide specificity; in
addition to the different nucleotide triphosphates, ADP can be used as the phosphate donor,
with the production of AMP [174]. Although
multiple hexokinases are found in all other vertebrates, the lamprey Entosphenus japonicus has
only one hexokinase of 90 kDa that is similar to
the A type of mammals [165].
The hexokinase spectrum is dependent upon
14.3 Hexokinases
523
the developmental stage. In developing rat liver,
A appears first, followed by Band C, and finally
D. Each hepatocyte of the adult liver contains all
four isoenzymes, whereby type D accounts for
about 95 % of the glucose-phosphorylating activity. On the 10th day of brooding in the chicken
there are four hexokinases present, of which one
disappears on day 17 and a further one during
hatching. In the frog Calyptocephalella, type D
predominates in the liver of the tadpoles, but during metamorphosis C gradually completely replaces D. Type D reappears later and constitutes
about 30 % of the total activity in the adult liver.
B appears after metamorphosis but remains at a
relatively low level, and A is absent from the liver
at all stages [203, 262].
The 100-kDa hexokinases A-C are polypeptides of 917 amino acids, the C- and N-terminal
halves of which agree in more than 50 % of their
sequences. The binding sites for glucose and ATP
in the C-terminal catalytic domain are present in
the N-terminal regulatory domain as allosteric
binding sites [256]. The 50-kDa hexokinase D
from the rat liver agrees in 53 % of its 465 amino
acids with the C-terminal catalytic domain of hexokinase A from the rat brain [4]. The 100-kDa
hexokinases apparently arose by gene duplication
from 50-kDa enzymes such as those found in
yeast and higher plants. The 50-kDa enzymes in
turn may have arisen from 25-kDa enzymes such
as those found in the bacteria. The rat hexokinase
D shows immunological cross-reactivity not only
with type-D enzymes of other mammals but also
with those of the turtles, although not with those
of the amphibians. In contrast, antisera raised
against rat type-A react only to a minor extent
with the corresponding enzymes from other
rodents, and not at all with those of the cow,
sheep or pig. The rate of evolution of type A
appears to far exceed that of type D [132,
262].
Multiple hexokinases are also found in many
invertebrate animals (Table 14.2b). However,
only one hexokinase was found in 11 out of 25
insect species, some echinoderms and the ascidian Halocynthia roretzi [165, 250]. Tissuespecific isoenzymes have been demonstrated in
Bombyx mori and Drosophila melanogaster. In
the silkworm, isoenzyme I is found in all organs
including muscle; II occurs in the midgut, testis
and Malpighian tubules; III is found only in the
fat body; and IV is present in the midgut and fat
body [280]. Particulate hexokinases that are
bound to cell structures and are like those found
in the vertebrates have also been described in
much lower affinity for glucose and other substrates, and its reduced sensitivity to the inhibitory
effect of Glc-6-P. At substrate concentrations of
about 100 mmol/l, hexokinase D metabolizes
fructose with much lower efficiency than it metabolizes glucose; however, reference to this
enzyme as a glucokinase is misleading, as may be
seen from the values of V max (Table 14.2a). The
liver contains a 60-kDa inhibitory protein that
forms an inactive complex with glucokinase in the
presence of Fru-6-P; this is inhibited by Fru-1-P
[264]. The hexokinase D of the pancreatic ~ cells
differs from the liver enzyme only at the 15 Nterminal positions, and is encoded by the same
gene. The glucokinase gene has two promoters
and is regulated by insulin in the liver and by glucose in the ~ cells [146].
The hexokinase spectrum found in the rat may
not be taken as a model even for the mammals,
and certainly not for the vertebrates as a whole.
The D type with low substrate affinity is absent
from the horse, llama, cow, goat, sheep and chinchilla, and the glucose-inhibited C type is not
found in the domestic dog and many rodents;
both are lacking in the domestic cat. In investigations of 97 vertebrate species from all classes, the
D type was detected only in the turtles arid
amphibians in addition to the mammals; and the
C type was detected only in several amphibians
that lacked hexokinase A, and perhaps also in
several teleosts. Neither the D nor the C type was
found in any of 13 avian or 13 reptilian species
(excluding the turtles); the hexokinases of the A
and B types found in these latter species differed
significantly from those of the other vertebrates
in having a lower glucose affinity and a higher
V max with fructose [262]. The C-type hexokinase
found to have high activity in the liver of the frog
Calyptocephalella caudiverba shows sigmoidal
ATP kinetics and other features that are missing
in the corresponding enzyme from Rana catesbeiana. The hexokinases of the vertebrates are, in
general, ATP specific, although the enzyme from
mammalian brain can also use ITP. A C-like hexokinase isolated from the liver of the rainbow trout
shows an unusually wide nucleotide specificity; in
addition to the different nucleotide triphosphates, ADP can be used as the phosphate donor,
with the production of AMP [174]. Although
multiple hexokinases are found in all other vertebrates, the lamprey Entosphenus japonicus has
only one hexokinase of 90 kDa that is similar to
the A type of mammals [165].
The hexokinase spectrum is dependent upon
14.3 Hexokinases
523
the developmental stage. In developing rat liver,
A appears first, followed by Band C, and finally
D. Each hepatocyte of the adult liver contains all
four isoenzymes, whereby type D accounts for
about 95 % of the glucose-phosphorylating activity. On the 10th day of brooding in the chicken
there are four hexokinases present, of which one
disappears on day 17 and a further one during
hatching. In the frog Calyptocephalella, type D
predominates in the liver of the tadpoles, but during metamorphosis C gradually completely replaces D. Type D reappears later and constitutes
about 30 % of the total activity in the adult liver.
B appears after metamorphosis but remains at a
relatively low level, and A is absent from the liver
at all stages [203, 262].
The 100-kDa hexokinases A-C are polypeptides of 917 amino acids, the C- and N-terminal
halves of which agree in more than 50 % of their
sequences. The binding sites for glucose and ATP
in the C-terminal catalytic domain are present in
the N-terminal regulatory domain as allosteric
binding sites [256]. The 50-kDa hexokinase D
from the rat liver agrees in 53 % of its 465 amino
acids with the C-terminal catalytic domain of hexokinase A from the rat brain [4]. The 100-kDa
hexokinases apparently arose by gene duplication
from 50-kDa enzymes such as those found in
yeast and higher plants. The 50-kDa enzymes in
turn may have arisen from 25-kDa enzymes such
as those found in the bacteria. The rat hexokinase
D shows immunological cross-reactivity not only
with type-D enzymes of other mammals but also
with those of the turtles, although not with those
of the amphibians. In contrast, antisera raised
against rat type-A react only to a minor extent
with the corresponding enzymes from other
rodents, and not at all with those of the cow,
sheep or pig. The rate of evolution of type A
appears to far exceed that of type D [132,
262].
Multiple hexokinases are also found in many
invertebrate animals (Table 14.2b). However,
only one hexokinase was found in 11 out of 25
insect species, some echinoderms and the ascidian Halocynthia roretzi [165, 250]. Tissuespecific isoenzymes have been demonstrated in
Bombyx mori and Drosophila melanogaster. In
the silkworm, isoenzyme I is found in all organs
including muscle; II occurs in the midgut, testis
and Malpighian tubules; III is found only in the
fat body; and IV is present in the midgut and fat
body [280]. Particulate hexokinases that are
bound to cell structures and are like those found
in the vertebrates have also been described in