4.6: Allosteric Regulation
Allosteric regulation of enzymes occurs when the binding of an effector molecule to a site that is different from the active site causes a change in the enzymatic activity. This alternate site is called an allosteric site, and an enzyme can contain more than one of these sites. Allosteric regulation can either be positive or negative, resulting in an increase or decrease in enzyme activity. Most enzymes that display allosteric regulation are metabolic enzymes involved in the degradation or synthesis of specific cellular molecules.
In allosteric inhibition, the binding of an effector molecule to the allosteric site causes a conformational 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 type of feedback inhibition prevents overproduction of products. As a classic example, isoleucine is an allosteric inhibitor of an enzyme important in its own synthesis.
In contrast, an allosteric activator causes a conformational change that increases the affinity of the enzyme for its substrate. Allosteric activation dramatically increases the rate of reaction, as represented by the hyperbolic curve on a reaction rate versus substrate concentration graph. As an example, extracellular ligand binding to the transmembrane EGF receptor causes a conformational change that results in an increase in the intracellular kinase activity of the receptor. 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.