organic compounds of nonprotein origin that take part in enzyme reactions as acceptors of atoms or groups of atoms split off by the enzyme from a molecule of substrate.

Coenzymes are connected to the protein part of an enzyme molecule, or apoenzyme, by an unstable bond that easily ruptures in the presence of acids and alkalis or during dialysis. The rupture results in the formation of catalytically inactive components.

In general, the formation of a complex involving a coenzyme, a substrate, and an apoenzyme in what is called the active center of the enzyme precedes enzymatic activity. The coenzyme does not undergo irreversible chemical transformation during the catalytic process; therefore, it can participate repeatedly in enzyme reactions. A huge number of biochemical reactions proceed with the participation of a limited set of coenzymes.

The majority of coenzymes are vitamin derivatives or contain vitamins. The chemical nature of the coenzyme largely determines the mechanism and type of enzyme reaction. The coenzymes most widely occurring in animal and plant tissues are nicotinamide adenine dinucleotide, nicotinamide adenine dinucleotide phosphate, thiamine pyrophosphate, lipoic acid, coenzyme A, adenosine diphosphate, and other nucleoside diphosphates.