Question

4. If you were studying an enzyme that catalyzed the reaction of ATP and fructose 1-
phosphate to form fructose 1,6-bisphosphate and ADP and discovered that a plot of the
initial velocity of formation of fructose 1,6-bisphosphate versus ATP concentration was
not hyperbolic, but rather sigmoid, what would you suspect?

Answer 1

Sigmoidal shape of the enzyme activity plot demonstrates the presence of cooperativity. It means that the enzyme consists of several subunits that act as regulators of each other. Moreover, if a sigmoid curve has a decline, it can be suggested that ATP acts as a negative allosteric regulator (inhibitor) of the enzyme. As a result, at low ATP concentrations, enzyme activity increases gradually to a certain maximal level whereas high ATP concentrations inhibit the enzyme.