An enzyme that has two structurally distinct forms, one of which is active and the other inactive. In the active form, the quaternary structure (see protein) of the enzyme is such that a substrate can interact with the enzyme at the active site (see enzyme-substrate complex). In the inactive form the conformation of the substrate-binding site is altered so that interaction with the substrate is not possible. The changes in the binding site are the result of regulatory molecules binding to a second site—the allosteric site—elsewhere on the enzyme. 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 (see inhibition) and the enzyme is converted to the inactive form, thereby controlling the amount of product synthesized.