Cofactors and Coenzymes

Some enzymes associate with non-protein molecules or cofactors to enhance their catalytic activity.

The chemically active enzyme-cofactor complex is called a holoenzyme and the enzyme alone is called an apoenzyme.

Cofactors can be present as inorganic metal ions such as the zinc ion in carbonic anhydrase or as organic molecules called coenzymes such as NAD⁺, coenzyme A and others.

Some cofactors, called prosthetic groups, are covalently bonded to apoenzymes. For example, in hemoglobin, the heme group, a porphyrin with a central iron ion, is associated with  protein chains through covalent bonding, hydrogen bonding, and extensive hydrophobic interactions.

In contrast, cofactors that are transiently bound to enzymes act as cosubstrates.

For example, NAD⁺ , a coenzyme for alcohol dehydrogenase, helps oxidize alcohol to aldehyde and gets reduced to NADH. NADH dissociates once the reaction is complete and is reoxidized to  NAD⁺ for the next reaction cycle.