Question

4. Under each of the following conditions, please predict the effect the mutation will have on the rate at which glycolysis will proceed in liver cells:

a. Loss of the allosteric site for ATP in phosphofructokinase

b. Loss of binding site for citrate in phosphofructokinase

c. Loss of the phosphatase domain of the bifunctional enzyme that controls the level of fructose 2,6-bisphosphate

d. Loss of the binding site for fructose 1,6-bisphosphate in pyruvate kinase.

Answer 1

Ans. a. ATP is an allosteric inhibitor of phosphofructokinase. When the cell has abundance of intracellular ATP, it does not need to continue glycolysis. Under high [ATP], ATP binds the enzyme at allosteric site and inactivates the enzyme, thus shuts down glycolysis.

The mutated allosteric site may not bind to ATP. As a result, glycolysis can’t be shut down under high [ATP] by this enzyme.

Ans. b. Citrate is an intermediate substrate of the citric acid cycle. High [citrate] indicates that the cell has abundant energy source and it does not need to carry out glycolysis at the moment. Thus, citrate acts as the inhibitor of the enzyme under high intracellular [citrate].

The mutated allosteric site may not bind to citrate. As a result, glycolysis can’t be shut down under high [citrate] by this enzyme.

Ans. c. (PFK-2/ FBPase-2) is known as bifunctional enzyme because it has dual roles- in phosphorylation/ dephosphorylation of fructose-6-phospgate. Phosphofructokinase 2 (PFK-2) phosphorylates fructose-6-phosphate (F-6-P) to produce F-2,6-bisP.

Under low intracellular [glucose], the enzyme fructose bisphosphatase 2 (FBPase-2) converts F-2.6-bisP back into F-6-P through dephosphorylation and inhibits glycolysis. Simultaneously it also triggers breakdown of glycogen to produce more glucose and maintain glucose homeostasis in blood. In case of mutated FBPase-2 (has phosphatase domain), glycolysis does not stop even at low blood glucose level, thus, blood glucose homeostasis might m=not correctly be regulated.

Ans. d. Fructose-1,6-bisphoate is an allosteric activator of pyruvate kinase.

In case this allosteric site is mutated, the enzyme may not respond to F-1,6-bisP. Thus, glycolysis shall finally stop as the enzyme pyruvate kinase is NOT being activated by F-1,6-bisP.