REGULATING ENZYMES: ALLOSTERIC ENZYMES
A chemical molecule can inhibit an enzyme in two ways: 1) by combining to an allosteric site or 2) by competitive inhibition.

An allosteric site is a location on an enzyme where a regulating molecule can attach. The regulating molecule can be a final product of a metabolic pathway. Such a molecule is called an allosteric regulator and does not directly block the active site. This is noncompetitive inhibition (that is, the inhibitor is not competing with the substrate for binding to the active site).

Allosteric regulators are specific to the enzyme they regulate. When an allosteric regulator attaches to its enzyme, the enzyme's active site changes shape. This shape change prevents substrate molecules from binding to the enzyme's active site, and thus the enzyme can no longer catalyze its reaction. This inhibits the metabolic pathway from producing the final product, resulting in a lower concentration of the allosteric regulator.