Oscillatory Control of Glycolysis as Model for Timing Processes
213
modifying control site further down which can be localized at the GAPDH
reaction, because there is no further phase shift between the metabolites
further down.
This indirect evidence of the alternately changing flow rates in the
upper and in the lower part of the glycolytic pathway is confined by direct
measurements [18]. In the glycolysing cell-free extract, inorganic phosphate
is predominantly incorporated into A TP and FDP. Pulse labelling experiments show that FDP synthesis is almost completely restricted to the
phase of rising NADH, whereas ATP synthesis is never completely
suppressed but rather is twice as efficient during the NADH oxidation
phase.
It might be of interest to mention that only 50 % of all glucose taken
up by yeast cells is worked up into glycolysis. However, the uptake of all
glucose is controlled in a pulsed manner and in phase with its conversion to
FOP. The other 50 % of glucose which is not worked up into glycolysis is
transformed into an insoluble carbohydrate, presumably glycogen. UDPG,
the direct precursor of glycogen synthesis also oscillates, being in phase
with G-6-P. Glucose uptake and ATP breakdown are also very closely
balanced in resynthesis of A TP [19].
Oscillatory control in glycolysis was interpreted by HIGGINS [20]. He
postulated that a single enzymatic step can produce oscillations when it is
susceptible to substrate inhibition and product activation. PFK, the enzyme
at the most obvious control site as seen by phase analysis as well as in
studies of the incorporation of3 2 P, actually satisfies both conditions [21, 22].
This enzyme is excessively inhibited by its substrate ATP and activated by
F-6-P. It is further activated by AMP, a compound closely correlated with
ADP, one of the products of the PFK reaction. Some activation is also
possible by FOP, the second product. We found this compound effective
only at low concentrations of F-6-P [23]; it is unlikely to be an important
factor in oscillatory control. The activation by the substrate F-6-P is more
important, since it does not only work by mass action but is also most efficient
in neutralizing the inhibitory action of ATP. Plotting F-6-P concentration
against velocity gives a sigmoid curve indicating that a cooperative process is involved which is typical of allosteric enzymes. In the Hill plot
F-6-P gives a coordination number of 4, which means that 4 steps (or 4
subunits) are involved. Coordination numbers of 2 and higher are to be
observed with AMP and FDP too.
Any interpretation of PFK activity is extremely difficult, because any
substance acting on this enzyme has only a limited range of activating concentration, and this range depends on the actual concentration of all other
substrates and effectors.
At present allosteric conditions of PFK are theoretically sufficient to
explain that this enzyme makes the pathway in which it works to become an
213
modifying control site further down which can be localized at the GAPDH
reaction, because there is no further phase shift between the metabolites
further down.
This indirect evidence of the alternately changing flow rates in the
upper and in the lower part of the glycolytic pathway is confined by direct
measurements [18]. In the glycolysing cell-free extract, inorganic phosphate
is predominantly incorporated into A TP and FDP. Pulse labelling experiments show that FDP synthesis is almost completely restricted to the
phase of rising NADH, whereas ATP synthesis is never completely
suppressed but rather is twice as efficient during the NADH oxidation
phase.
It might be of interest to mention that only 50 % of all glucose taken
up by yeast cells is worked up into glycolysis. However, the uptake of all
glucose is controlled in a pulsed manner and in phase with its conversion to
FOP. The other 50 % of glucose which is not worked up into glycolysis is
transformed into an insoluble carbohydrate, presumably glycogen. UDPG,
the direct precursor of glycogen synthesis also oscillates, being in phase
with G-6-P. Glucose uptake and ATP breakdown are also very closely
balanced in resynthesis of A TP [19].
Oscillatory control in glycolysis was interpreted by HIGGINS [20]. He
postulated that a single enzymatic step can produce oscillations when it is
susceptible to substrate inhibition and product activation. PFK, the enzyme
at the most obvious control site as seen by phase analysis as well as in
studies of the incorporation of3 2 P, actually satisfies both conditions [21, 22].
This enzyme is excessively inhibited by its substrate ATP and activated by
F-6-P. It is further activated by AMP, a compound closely correlated with
ADP, one of the products of the PFK reaction. Some activation is also
possible by FOP, the second product. We found this compound effective
only at low concentrations of F-6-P [23]; it is unlikely to be an important
factor in oscillatory control. The activation by the substrate F-6-P is more
important, since it does not only work by mass action but is also most efficient
in neutralizing the inhibitory action of ATP. Plotting F-6-P concentration
against velocity gives a sigmoid curve indicating that a cooperative process is involved which is typical of allosteric enzymes. In the Hill plot
F-6-P gives a coordination number of 4, which means that 4 steps (or 4
subunits) are involved. Coordination numbers of 2 and higher are to be
observed with AMP and FDP too.
Any interpretation of PFK activity is extremely difficult, because any
substance acting on this enzyme has only a limited range of activating concentration, and this range depends on the actual concentration of all other
substrates and effectors.
At present allosteric conditions of PFK are theoretically sufficient to
explain that this enzyme makes the pathway in which it works to become an