Enzymes are protein catalysts found in all living cells. For more than a century chemists and biochemists have been studying the structure and function of these remarkable molecules, It is now known that the majority of enzymes are globular in structure with molecular weights in the range of 10, 000 to 600, 000 (Fig. 1). The active center, which generally represents a relatively small portion of the surface area of the molecule, is composed of a binding site and a catalytic site. In addition to amino acid side chains such as imidazole and thiol groups, the catalytic site often contains metal ions and non-protein “coenzymes”.