An enzyme that has two structurally distinct forms, one of which is active and the other inactive. In the active form, the quaternary structure of the enzyme is such that a substrate can interact with the enzyme at the active site. The conformation of the substrate-binding site becomes altered in the inactive form and interaction with the substrate is not possible. Allosteric enzymes tend to catalyse the initial step in a pathway leading to the synthesis of molecules. The end product of this synthesis can act as a feedback inhibitor and the enzyme is converted to the inactive form, thereby controlling the amount of product synthesized.