Inhibition or activation of an enzyme by a small regulatory molecule that interacts with the enzyme at a site (allosteric site) other than the active site (at which catalytic activity occurs).
The interaction changes the shape of the enzyme, thus affecting the active site of the usual complex between the enzyme and its substrate (the substance on which the enzyme acts). As a result, the enzyme's ability to catalyze a reaction (see catalysis ) is either inhibited or enhanced. If the regulatory molecule inhibits an enzyme in the pathway of its own synthesis, the control is said to be feedback inhibition . Allosteric control enables the cell to regulate needed substances rapidly.