Question

Describe the way(s) in which a homotropic allosteric effector could affect a substrate activity curve.

Answer 1

The activity curve for an allosteric enzyme does not follow the Michaelis-Menten model (hyperbolic curve) because it has more than one active site, this kind of enzymes present the cooperativity phenomenon, in wich the binding of a ligand to a binding site can modify the affinity of other ligand(s) to its/their correspondent binding sites. The actual curve is a sigmoidal curve, an "s shaped" curve for homotropic allosteric enzymes (in which the activator and the modulator are the same), while for heterotropic allosteric enzymes the shape may vary.

For homotropic allosteric enzymes, the curve maintains a value of [S] in which saturation exists and a value where V₀ is 0.5V_max but the begining of the curve is "s-shaped", this is due to the increase of the affinity with the binding of each molecule of substrate and can be seen with the large increase of V₀ for a small increase of [S] before saturation for allosteric activators.