Some kinds of biological processes require catalytic functions beyond those built into protein molecules alone. In such cases, a protein may require the help of some other small molecule or ion to carry out the reaction. Molecules bound to enzymes for this purpose are called coenzymes. The water soluble vitamin B complexes are metabolic precursors of a number of coenzymes. Table 11.5 lists several important coenzymes together with their related vitamins.

NAD⁺ - Nicotinamide adenine dinucleotide (NAD⁺) is derived from the vitamin niacin. The nicotinamide portion of the molecule is capable of being reduced and can thus serve as an oxidizing agent (see here), where 'R' stands for the remainder of the molecule. NAD⁺/NADH behave both like a second substrate in a reaction (because each is converted to the other by the enzyme) and like a coenzyme (because they are recycled over and over). They are generally classified as coenzymes.

Metal Ions in Coenzymes - Many enzymes contain metal ions, usually held by coordinate covalent bonds from amino acid side chains, but sometimes bound by a prosthetic group like heme. Such enzymes are called metalloenzymes. Figure 11.27 shows the active site of the protease carboxypeptidase A, which contains a zinc ion.