Coenzyme

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Coenzymes are small organic non-protein molecules that carry chemical groups between enzymes. Many coenzymes are phosphorylated water-soluble vitamins. Although coenzymes are often vitamins, they remain proteins, unlike vitamins which can have many chemical makeups other than protein folding. However, nonvitamins may also be coenzymes, such as ATP, the biochemical carrier of phosphate groups.

Coenzymes are used up in the reactions in which they assist, (for example: NADH coenzyme is converted to NAD⁺ by oxidoreductses). Coenzymes are however regenerated and their concentration maintained at a steady level in the cell.

A special subset of coenzymes is prosthetic groups. These have more in common with cofactors since they are tightly bound to enzymes and are not released as part of the reaction. Prosthetic groups include molybdopterin, lipoamide and biotin.

A second subset of coenzymes is cosubstrates. Cosubstrates, in contrast to prosthetic groups, are loosely bound to their enzymes and behave as true substrates, being modified during catalysis. Unlike traditional substrates, which usually undergo further modification in their biochemical pathways, cosubstrates are reverted to their original forms following catalysis and are once again able to react with the active sites of their apoenzymes to reform catalytically active holoenzymes.