4.6: Allosteric Regulation
Allosteric regulation of enzymes occurs when the binding of a molecule to a different location from the active site causes a change in enzymatic activity. This type of regulation can be either positive or negative, increasing or decreasing the activity of the enzyme. Most enzymes that display allostery are metabolic enzymes involved in degradation or synthesis of specific cellular molecules.
In allosteric inhibition, the binding of a molecule to the allosteric site causes a shape change that reduces the affinity of the enzyme for the substrate. Frequently, the allosteric inhibitor is a product of the enzyme or the enzyme pathway, allowing enzymatic products to limit their own production. This is a type of feedback inhibition, preventing overproduction of products. As a classic example, isoleucine is an allosteric inhibitor of an enzyme important in its synthesis.
In contrast, an allosteric activator causes a conformational change that increases the affinity of an enzyme for the substrate. Allosteric activation dramatically increases the rate of reaction, as represented by an S-shaped rate-substrate reaction. As an example, extracellular ligand binding to the transmembrane EGF receptor causes a conformational change that results in the activation of intracellular kinase activity. If an enzyme is composed of multiple subunits, binding of an allosteric activator to a single subunit can cause an increase in affinity, and shape change, for all of the affiliated subunits.