Biological systems are dependent on the proper functioning of enzymes. Specific enzymes act to aid specific reactions. Enzymes achieve this specificity through differences in the shape and composition that reflect genetic differences (i.e., differences in the genetic code that carry instructions to construct the enzyme). A region of the enzyme molecule termed the active site achieves an important component of specificity. The active site is a specific region of the enzyme molecule that bonds only with a limited number of molecules (substrates) to form the enzyme-substrate complex.

The binding of a specific substrate (or a limited number of substrates) to a specific region of the enzyme molecule is also termed the "lock and key" mechanism of proteins. Only specific keys (substrates) can fit into and properly interact with an active site on an enzyme molecule. The broadness with which an enzyme can act within a...