Allosteric enzymes change their conformation in response to the binding of a regulating molecule. The regulator, called a modulator or effector, can have a positive or negative effect on enzyme activity. Allosteric enzymes have two conformations, one that is active, catalytically, and the other that is inactive. Positive modulators (activators) bind to an allosteric site on the enzyme and stabilise the active conformation. Negative modulators (inhibitors) bind to an allosteric site that stabilises the inactive conformation of the molecule. Enzymes with allosteric sites usually have two or more subunits. The principle of allosteric control of enzyme activity is illustrated in Figure 13.

Cooperativity is an important mechanism in the overall control of enzyme activity. In this mechanism, where there are multiple subunits, the response of enzymes to substrate molecules is enhanced. One substrate molecule changes the conformation of the active site on one subunit of the enzyme and, by doing so, it causes the other subunits to change to their active conformation.

Figure 13 - Allosteric control of enzyme activity.