Koenzymes

Koenzymes (coenzymes) are small organic molecules that associate with enzymes to enable catalysis. They are distinct from inorganic cofactors and prosthetic groups in that they are organic and often serve as carriers for electrons, atoms, or functional groups during a reaction. Koenzymes typically bind reversibly to apoproteins and are regenerated after each catalytic cycle. Many koenzymes are derived from vitamins and must be present in adequate amounts for enzyme activity.

Notable examples include NAD+ and NADP+, which shuttle electrons in redox reactions; FAD and FMN, also electron carriers; coenzyme A (CoA), which transfers acyl groups; tetrahydrofolate, which carries one-carbon units; pyridoxal phosphate (vitamin B6 derivative), involved in amino-group transfer; biotin, for carboxylation reactions; thiamine pyrophosphate, a thiamine derivative for decarboxylation; lipoic acid; and vitamin B12 derived coenzymes, such as methylcobalamin and adenosylcobalamin.

Koenzymes are essential players in metabolism, often derived from dietary vitamins. Their deficiency can impair multiple pathways and lead to metabolic disorders. Because they cycle between oxidized and reduced forms or carry group units, they are repeatedly utilized and regenerated by enzymes, enabling continuous catalytic turnover. They differ from enzyme prosthetic groups, which are tightly bound, whereas koenzymes typically bind more loosely and may dissociate after the reaction.