known as glucokinase) is not subject to this type of inhibition and has a higherKmfor
glucose than have the other three forms. It is confined mainly to the liver where it is
able to metabolise high concentrations of glucose, the resulting glucose-6-phosphate
being diverted to glycogen biosynthesis via glucose-1-phosphate. In some tissues the
limiting activity of hexokinase is bypassed by the provision of glucose-6-phosphate
from glycogen via glucose-1-phosphate.
The activity of pyruvate kinase is regulated allosterically, being inhibited by ATP
and activated by AMP and fructose-1,6-bisphosphate. In muscle, pyruvate kinase is
present in large amounts, hence minimising its rate-limiting constraint. The fact that
pyruvate kinase is located at the end of the pathway makes it unlikely that it will have
a major role in the regulatory control of glycolysis.
6-Phosphofructokinase (PFK) is subject to allosteric control by a number of allo-
steric effectors that are related to the energy status of the cell. The principal activators
are AMP and fructose-2,6-bisphosphate whilst ATP is an activator at low concen-
trations but an inhibitor at higher concentrations (1 mM). AMP activates the enzyme
by releasing it from the inhibitory control of ATP disturbing the equilibrium away
from the state that contains the ATP inhibitor site. The balance of control exerted by
ATP and AMP is thus determined by their relative concentrations. This in turn is
influenced by the enzyme adenylate kinase which catalyses the reaction:2 ADP^ !ATPþAMP
ATP is normally present in a cell at much higher concentrations that the other two
nucleotides and as a consequence a small decrease in the concentration of ATP that is
too small to relieve the inhibitory effect of ATP on PFK, results in a proportionally
much larger change in AMP concentration that is normally only about 2% of that of
ATP. This large percentage increase in the concentration of AMP allows it to exert a
powerful activator effect on PFK hence facilitating increased glycolytic flux.
Additional control of glycolytic flux by PFK is exerted by its involvement in a
substrate cyclewith the enzyme fructose bisphosphatase (FBP) which is part of the
gluconeogenesis pathway from pyruvate to glucose (Fig. 15.14). Both reactions are
strongly exergonic and essentially irreversible. Whereas AMP acts as a powerful
activator of PFK, it acts as a potent inhibitor of FBP and hence plays a reciprocal
role in the control of these two opposing pathways. PFK convertsD-fructose-6-
phosphate toD-fructose-1,6-bisphosphate and simultaneously converts ATP to ADP,
while FBP converts D-fructose-1,6-bisphosphate to D-fructose-6-phosphate and
inorganic phosphate. The net result is apparently only the hydrolysis of ATP but
in fact it results in a proportionally large increase in AMP concentration via
adenylate kinase. As discussed above, this produces a large increase in flux through
the glycolytic pathway by the activation of PFK and inhibition of FBP. A two-fold
increase in AMP concentration can increase the glycolytic flux by 200-fold. How-
ever, the regulatory importance of changes in AMP concentration is not confined to
its stimulation of glycolytic flux. Equally important is the fact that decreases in AMP
concentration result in the ATP-inhibition of PFK activity becoming dominant,
resulting in the virtual switching off of the glycolytic pathway and a concomitant
increase in glycogen biosynthesis.618 Enzymes