Question

1. What is the rate-limiting, regulated enzyme in glycolysis?

2. Draw the complete reaction catalyzed by this enzyme with the structures of the glycolytic intermediates (substrates or products coming directly from glucose)

3. Draw an idealized Vo vs [S] plot for this enzyme. On the graph, identify and show the effect of the major allosteric inhibitor of this enzyme in muscle. Also identify and show the effect of the major allosteric activator of this enzyme in muscle.

Answer 1

1) The rate-limiting, regulated enzyme in glycolysis is phosphofructokinase.

Fructose 6-phosphate is phosphorylated by ATP to fructose 1,6-bisphosphate. This irreversible reaction is catalysed by an allosteric enzyme phosphofructokinase.

A high AMP concentration activates phosphofructokinase. In contrast a high ATP concentration inhibits it. Also citrate, which indicate that alternative energy sources are available, inhibit phosphofructokinase. Other allosteric effectors like fructose 6-phosphate and fructose-2,6-bisphosphate stimulates glycolysis by activating phosphofructokinase.

So the rate at which the glycolytic pathway operates is controlled primarily by allosteric regulation of phosphofructokinase enzyme.

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