Allosteric enzymes are enzymes that change their shape, or conformation, upon bining of a modulator. The word allosteric comes from the Greek allos,'other' and 'stereos', 'shape'. An allosteric enzyme is an oligomer whose biological activity is affected by altering the conformation(s) of its tertiary structure. Allosteric enzymes tend to have several subunits. In some cases the regulatory site(s) and the active site are on separate subunits.