524
14 Glycolysis
various invertebrates, e.g. in nematodes and cestodes [9, 179].
Most of the characterized hexokinases of the
invertebrates show the typical wide substrate
specificity, although with large differences in
substrate affinities and relative turnover rates for
different sugars. As shown in Table 14.2b, several arthropod enzymes have been found to have
low glucose affinities, similar to that of vertebrate hexokinase D. On the other hand, many
invertebrate enzymes have much greater affinities for fructose than do any of the four rat liver
hexokinases. The predominating isoenzyme II of
Limulus hepatopancreas may be classified as a
fructomannokinase, whereas the muscle enzyme
of this species shows the typical wide specificity
of a hexokinase. The trematode Schistosoma
mansoni possesses specific kinases for glucose,
fructose, mannose and glucosamine, all of which
phosphorylate in the 6-position. Kinases for specific hexoses have also been described for Echinococcus granulosus, Ascaridia galli and Entamoeba histolytica, whereas other parasitic
worms have hexokinases with a wide specificity
[9, 179]. As a rule, the invertebrate enzymes are
also composed of 50- or 100-kDa subunits, corresponding to the scheme for hexokinase evolution
described above. For example, the enzymes of
the echinoderms Asterias amurensis, Ophioplocus japonicus and Cucumaria frondosa have subunits of about 50 kDa, although the latter species has dimers of 2 X 50 kDa [165]. The isoenzymes of the lobster Homarus, the silkworm
Bombyx mori and the mosquito Culex pipiens
are also about 50 kDa [235, 280]. The enzymes
from the nematode Angiostrongylus cantonensis
and from the fat body of the silkworm Hyalophora cecropia were found to be 60 and 64 kDa,
respectively [63, 179]. The monomeric hexokinases from the roundworm Ascaris suum and from
the flight muscles of the migratory locust Schistocerca gregaria, and isoenzyme I of the edible
mussel Mytilus galloprovincialis have a mass of
about 100 kDa. The isoenzyme II of the edible
mussel species is in fact reported to be 165 kDa
[214].
The majority of the invertebrate hexokinases
examined are inhibited, like the vertebrate enzymes A-C, by G1c-6-P' Only a few of the invertebrate hexokinases show the absence of product
inhibition that is typical for the rat enzyme D;
examples include the hexokinases of the flagellates Trypanosoma cruzi and Leishmania donovani, the enzyme of the nematode AngiostrongyIus cantonensis, and isoenzyme I from the hepatopancreas of the lobster Homarus american us
[25, 179, 235]. The isoenzyme III of Bombyx
mori is the only known hexokinase to show substrate inhibition by ATP [280]. The hexokinase of
Apis mellifera is inhibited by G1c-6-P but not by
Man-6-P; thus, mannose is toxic for the honey
bee because the absence of product inhibition
results, after mannose feeding, in the excessive
consumption of ATP, reduction in intracellular
ATP and accumulation of Man-6-P. The physiological regulation of hexokinase has been examined in some detail in the flight muscles of the
migratory locust Schistocera gregaria. The rate of
glycolysis increases by about 50-fold at the beginning of flight. The muscle glycogen is used up in
the first 10 s, and in the following minutes the
trehalose and glucose of the haemolymph serve
as a fuel; triacylglycerol gradually assumes this
role after 10-20 min. The hexokinase must be at
its maximal activity in order for the blood sugar
to be introduced into glycolysis at the required
rate. Although the G1c-6-P concentration of
200 mmol/l is far above the Ki value for the hexokinase, the substrate inhibition is completely
counteracted by Pi from arginine-P, the alanine
derived from pyruvate and the glycerol-3-P
released [238].
A substrate cycle between glucose and G1c-6-P
is active in mammalian liver, with a cycle rate of
about 1 !-tmol/min per g fresh weight. At normal
blood-sugar levels, the turnover rates of the hexokinase and glucose-6-phosphatase are about
equal and the net flux is zero. Even small changes
in the glucose or G1c-6-P concentration leads to a
directed flux, and thus to the excretion or uptake
of glucose by the liver cells. This regulation
mechanism makes up for the lack of product
inhibition of the predominant hexokinase D of
the liver [100]. Comparative studies of the flight
muscles of various insects have shown a significant correlation between the activities of hexokinase and glucose-6-phosphatase; this indicates
that a substrate cycle between glucose and G1c-6P is widespread in this tissue. In the moth Acherontia atropos, the cycle rate in the flight muscles
at the beginning of flight increases from 0.06 to
3.9 !-tmol/min per g fresh weight; this allows for
better regulation of the hexokinase reaction and,
for example, more efficient adaptation to sudden
changes in flight velocity [248].
14 Glycolysis
various invertebrates, e.g. in nematodes and cestodes [9, 179].
Most of the characterized hexokinases of the
invertebrates show the typical wide substrate
specificity, although with large differences in
substrate affinities and relative turnover rates for
different sugars. As shown in Table 14.2b, several arthropod enzymes have been found to have
low glucose affinities, similar to that of vertebrate hexokinase D. On the other hand, many
invertebrate enzymes have much greater affinities for fructose than do any of the four rat liver
hexokinases. The predominating isoenzyme II of
Limulus hepatopancreas may be classified as a
fructomannokinase, whereas the muscle enzyme
of this species shows the typical wide specificity
of a hexokinase. The trematode Schistosoma
mansoni possesses specific kinases for glucose,
fructose, mannose and glucosamine, all of which
phosphorylate in the 6-position. Kinases for specific hexoses have also been described for Echinococcus granulosus, Ascaridia galli and Entamoeba histolytica, whereas other parasitic
worms have hexokinases with a wide specificity
[9, 179]. As a rule, the invertebrate enzymes are
also composed of 50- or 100-kDa subunits, corresponding to the scheme for hexokinase evolution
described above. For example, the enzymes of
the echinoderms Asterias amurensis, Ophioplocus japonicus and Cucumaria frondosa have subunits of about 50 kDa, although the latter species has dimers of 2 X 50 kDa [165]. The isoenzymes of the lobster Homarus, the silkworm
Bombyx mori and the mosquito Culex pipiens
are also about 50 kDa [235, 280]. The enzymes
from the nematode Angiostrongylus cantonensis
and from the fat body of the silkworm Hyalophora cecropia were found to be 60 and 64 kDa,
respectively [63, 179]. The monomeric hexokinases from the roundworm Ascaris suum and from
the flight muscles of the migratory locust Schistocerca gregaria, and isoenzyme I of the edible
mussel Mytilus galloprovincialis have a mass of
about 100 kDa. The isoenzyme II of the edible
mussel species is in fact reported to be 165 kDa
[214].
The majority of the invertebrate hexokinases
examined are inhibited, like the vertebrate enzymes A-C, by G1c-6-P' Only a few of the invertebrate hexokinases show the absence of product
inhibition that is typical for the rat enzyme D;
examples include the hexokinases of the flagellates Trypanosoma cruzi and Leishmania donovani, the enzyme of the nematode AngiostrongyIus cantonensis, and isoenzyme I from the hepatopancreas of the lobster Homarus american us
[25, 179, 235]. The isoenzyme III of Bombyx
mori is the only known hexokinase to show substrate inhibition by ATP [280]. The hexokinase of
Apis mellifera is inhibited by G1c-6-P but not by
Man-6-P; thus, mannose is toxic for the honey
bee because the absence of product inhibition
results, after mannose feeding, in the excessive
consumption of ATP, reduction in intracellular
ATP and accumulation of Man-6-P. The physiological regulation of hexokinase has been examined in some detail in the flight muscles of the
migratory locust Schistocera gregaria. The rate of
glycolysis increases by about 50-fold at the beginning of flight. The muscle glycogen is used up in
the first 10 s, and in the following minutes the
trehalose and glucose of the haemolymph serve
as a fuel; triacylglycerol gradually assumes this
role after 10-20 min. The hexokinase must be at
its maximal activity in order for the blood sugar
to be introduced into glycolysis at the required
rate. Although the G1c-6-P concentration of
200 mmol/l is far above the Ki value for the hexokinase, the substrate inhibition is completely
counteracted by Pi from arginine-P, the alanine
derived from pyruvate and the glycerol-3-P
released [238].
A substrate cycle between glucose and G1c-6-P
is active in mammalian liver, with a cycle rate of
about 1 !-tmol/min per g fresh weight. At normal
blood-sugar levels, the turnover rates of the hexokinase and glucose-6-phosphatase are about
equal and the net flux is zero. Even small changes
in the glucose or G1c-6-P concentration leads to a
directed flux, and thus to the excretion or uptake
of glucose by the liver cells. This regulation
mechanism makes up for the lack of product
inhibition of the predominant hexokinase D of
the liver [100]. Comparative studies of the flight
muscles of various insects have shown a significant correlation between the activities of hexokinase and glucose-6-phosphatase; this indicates
that a substrate cycle between glucose and G1c-6P is widespread in this tissue. In the moth Acherontia atropos, the cycle rate in the flight muscles
at the beginning of flight increases from 0.06 to
3.9 !-tmol/min per g fresh weight; this allows for
better regulation of the hexokinase reaction and,
for example, more efficient adaptation to sudden
changes in flight velocity [248].