Allosteric enzymes are are of a general class of compounds called cooperative enzymes. With cooperative enzymes, the binding of substrate to the molecule influences further binding and the overall kinetics of the reaction. Hemoglobin is the classic example. Allosteric enzymes have multiple subunits which interact positively or negatively in a cooperative fashion.

Allosteric behavior can usually be described by a modification of the Michaelis-Menten equation:

One feature of cooperative enzymes is that they are more sensitive to changes in substrate concentration in the vicinity of the K_0.5 than enzymes displaying Michaelis-Menten kinetics are at their K_m values (i.e. the slope of the kinetic response is more steep).  For this reason, enzymes that are regulated in biochemical systems are often cooperative enzymes.  Regulation by cellular metabolites involves a site, separate from the active site which binds the substrate(s), which binds cellular metabolites indicating whether the particular reaction should be activated or inhibited.  Enzymes which respond to the binding of ligands to other sites than the active site are termed 'allosteric' (i.e. other site).  Cooperativity is one case of allosteric response; binding of substrate at one active site responds to the binding of another substrate molecule at a different active site.

Allosteric responses may also involve changes in affinity for substrate or Vmax due to the binding of non-substrate molecules at allosteric sites on the enzyme.  They usually are regulated by cellular molecules which let them "sense" the need for their products. Such allosteric responses are usually important for cellular regulation. Investigate the biomolecules course for more information.

Tutorial: Kinetics of an allosteric enzyme at The Biology Project, developed at The University of Arizona.