Although the chemical repertoire of amino acid functional groups is quite varied, they often cannot meet the chemical needs required for catalysis to take place. Thus, the catalytic activity of many enzymes depends on the presence of small molecules termed cofactors. The precise role varies with the cofactor and the enzyme. An enzyme without its cofactor is referred to as an apoenzyme; the complete, catalytically active enzyme is called a holoenzyme. Cofactors can be subdivided into two groups: (1) small organic molecules, derived from vitamins, called coenzymes and (2) metals (Table 6.2). Tightly bound coenzymes are called prosthetic (helper) groups. Loosely associated coenzymes are more like cosubstrates because, like substrates and products, they bind to the enzyme and are released from it. Coenzymes are distinct from normal substrates not only because they are often derived from vitamins but also because they are used by a variety of enzymes. Different enzymes that use the same coenzyme usually carry out similar chemical transformations.

100