hand by variation in enzymes activities, and on
the other hand through differences in the kineticregulatory properties of the enzymes. The activities ofthe glycolytic enzymes have been investigated in numerous tissues (Table 14.1). During
comparisons of such enzyme spectra, it may be
assumed (with some reservation) that differences
in in vitro enzyme activities under more-or-Iess
optimal conditions allow conclusions to be drawn
about the flux rates in vivo. In this way, for example, the ratio of hexokinase: phosphorylase would
indicate the proportions in which free or glycogen-bound glucose is involved in glycolysis, the
ratio lactate or opine dehydrogenase:phosphofructokinase suggests the relative importance of
anaerobic glycolysis, and the ratio of pyruvate
kinase: phosphoenolpyruvate carboxykinase indicates the relative activities of glycolysis and gluconeogenesis or the relative importance of anaerobic energy release in the succinate pathway.
Consecutive enzymes in those parts of the glycoTable 14.1. The activities (U/g fro wt.) at 25 °C of glycolysis
enzymes in different cell types
1
2
3
4
5
Phosphorylase
18
8
0.69
3.34
Hexokinase
2.4
12
0.05
0.13
Phosphoglucomutase 21
6.0
12.2
Phosphoglucose
112
20
18.9
43
isomerase
Phosphofructokinase 1.5
17
11.7 2.3
12.0
Aldolase
5.0
13
2.7
15.7
Triose phosphate
476
2200
7.9
isomerase
Glyceraldehyde
85
330 124
27
42
phosphate
dehydrogenase
Phosphoglycerate
75
340
11.9
14
kinase
Phosphoglycerate
36
425
8.8
7.4
mutase
Enolase
8.0
42
9.9
16.3
Pyruvate kinase
25
90
6.9 7.5
40
Lactate
115
1.5 28
8.1
4.0
dehydrogenase
Octopine
283
17.5 117
dehydrogenase
Strombine
2.7
18.1
dehydrogenase
Alanopine
97
4.9
21.5
dehydrogenase
Phosphoenolpyruvate
11.3 0.38
0.08
carboxykinase
1, Rat liver [237]; 2, flight muscle of the migratory locust
Locusta migratoria [237]; 3, heart of the snail Busycon contrarium [51]; 4, adductor of the edible mussel Mytilus edulis [68]; 5, foot of the mussel Cardium tuberculatum [68]
14.1 The Pathway from Hexose to Pyruvate
517
lytic reaction chain that have no branch points
always occur in the same relative proportions,
irrespective of the absolute activities; for this,
Pette in 1965 coined the term "constant proportions group".
Comparative biochemical analysis of the regulatory mechanisms is of critical importance for
understanding biochemical adaptation. Although
there are more relevant observations available for
glycolysis than for any other reaction pathway,
the presentation of a complete picture of all regulatory variants in the animal kingdom is not yet
possible. Phosphofructokinase occupies the most
important regulatory position, followed by pyruvate kinase, the membrane transport of glucose
when this is the substrate, and hexokinase; where
glycogen is involved, the phosphorylase is regulatory (Fig. 14.1). All animal cells have a membrane glucose transporter which allows facilitated
diffusion, and the cells of the mammalian small
intestine also have a Na+/glucose cotransporter
(symporter). In humans there are at least five different tissue-specific facilitative glucose transporters (GLUTI to GLUTS), the genes for which lie
on different chromosomes; their sequences of
492-509 amino acids differ by as much as 60 %
[81, 236, 281]. Enzymes whose activities are influenced by factors other than substrate concentration can be described as regulatory. Regulatory
alterations of enzyme properties may occur via
two different molecular mechanisms: the binding
of allosteric effectors and the post-translational
modification of the enzyme protein, e.g. phosphorylation/dephosphorylation.
For a reaction which lies far outside thermodynamic equilibrium, the catalysis in opposing
directions by two enzymes existing side-by-side in
the same compartment leads to a substrate cycle
(futile cycle). Two possible regulatory effects on
the enzymes of such a cycle can be distinguished.
Effects that influence both enzymes to the same
degree (ec effects) may change the cycle rate but
not the net flux. Effects that influence only one of
the two enzymes (er effects) change the net flux
and, correspondingly, the ratio cycle rate: net flux
rate. The regulatory effect in this case is amplified. Whilst allosteric regulation of a single
enzyme can only lead to relatively minor changes
in the flux rate, there is in theory no limit to the
amplification by allosteric effects in a substrate
cycle [166]. However, each passage of the cycle
results in energy losses, and in special cases such
a substrate cycle serves for the release of the
chemical energy of ATP as heat energy. Comparative biochemical data on the substrate cycle
the other hand through differences in the kineticregulatory properties of the enzymes. The activities ofthe glycolytic enzymes have been investigated in numerous tissues (Table 14.1). During
comparisons of such enzyme spectra, it may be
assumed (with some reservation) that differences
in in vitro enzyme activities under more-or-Iess
optimal conditions allow conclusions to be drawn
about the flux rates in vivo. In this way, for example, the ratio of hexokinase: phosphorylase would
indicate the proportions in which free or glycogen-bound glucose is involved in glycolysis, the
ratio lactate or opine dehydrogenase:phosphofructokinase suggests the relative importance of
anaerobic glycolysis, and the ratio of pyruvate
kinase: phosphoenolpyruvate carboxykinase indicates the relative activities of glycolysis and gluconeogenesis or the relative importance of anaerobic energy release in the succinate pathway.
Consecutive enzymes in those parts of the glycoTable 14.1. The activities (U/g fro wt.) at 25 °C of glycolysis
enzymes in different cell types
1
2
3
4
5
Phosphorylase
18
8
0.69
3.34
Hexokinase
2.4
12
0.05
0.13
Phosphoglucomutase 21
6.0
12.2
Phosphoglucose
112
20
18.9
43
isomerase
Phosphofructokinase 1.5
17
11.7 2.3
12.0
Aldolase
5.0
13
2.7
15.7
Triose phosphate
476
2200
7.9
isomerase
Glyceraldehyde
85
330 124
27
42
phosphate
dehydrogenase
Phosphoglycerate
75
340
11.9
14
kinase
Phosphoglycerate
36
425
8.8
7.4
mutase
Enolase
8.0
42
9.9
16.3
Pyruvate kinase
25
90
6.9 7.5
40
Lactate
115
1.5 28
8.1
4.0
dehydrogenase
Octopine
283
17.5 117
dehydrogenase
Strombine
2.7
18.1
dehydrogenase
Alanopine
97
4.9
21.5
dehydrogenase
Phosphoenolpyruvate
11.3 0.38
0.08
carboxykinase
1, Rat liver [237]; 2, flight muscle of the migratory locust
Locusta migratoria [237]; 3, heart of the snail Busycon contrarium [51]; 4, adductor of the edible mussel Mytilus edulis [68]; 5, foot of the mussel Cardium tuberculatum [68]
14.1 The Pathway from Hexose to Pyruvate
517
lytic reaction chain that have no branch points
always occur in the same relative proportions,
irrespective of the absolute activities; for this,
Pette in 1965 coined the term "constant proportions group".
Comparative biochemical analysis of the regulatory mechanisms is of critical importance for
understanding biochemical adaptation. Although
there are more relevant observations available for
glycolysis than for any other reaction pathway,
the presentation of a complete picture of all regulatory variants in the animal kingdom is not yet
possible. Phosphofructokinase occupies the most
important regulatory position, followed by pyruvate kinase, the membrane transport of glucose
when this is the substrate, and hexokinase; where
glycogen is involved, the phosphorylase is regulatory (Fig. 14.1). All animal cells have a membrane glucose transporter which allows facilitated
diffusion, and the cells of the mammalian small
intestine also have a Na+/glucose cotransporter
(symporter). In humans there are at least five different tissue-specific facilitative glucose transporters (GLUTI to GLUTS), the genes for which lie
on different chromosomes; their sequences of
492-509 amino acids differ by as much as 60 %
[81, 236, 281]. Enzymes whose activities are influenced by factors other than substrate concentration can be described as regulatory. Regulatory
alterations of enzyme properties may occur via
two different molecular mechanisms: the binding
of allosteric effectors and the post-translational
modification of the enzyme protein, e.g. phosphorylation/dephosphorylation.
For a reaction which lies far outside thermodynamic equilibrium, the catalysis in opposing
directions by two enzymes existing side-by-side in
the same compartment leads to a substrate cycle
(futile cycle). Two possible regulatory effects on
the enzymes of such a cycle can be distinguished.
Effects that influence both enzymes to the same
degree (ec effects) may change the cycle rate but
not the net flux. Effects that influence only one of
the two enzymes (er effects) change the net flux
and, correspondingly, the ratio cycle rate: net flux
rate. The regulatory effect in this case is amplified. Whilst allosteric regulation of a single
enzyme can only lead to relatively minor changes
in the flux rate, there is in theory no limit to the
amplification by allosteric effects in a substrate
cycle [166]. However, each passage of the cycle
results in energy losses, and in special cases such
a substrate cycle serves for the release of the
chemical energy of ATP as heat energy. Comparative biochemical data on the substrate cycle