coenzymes

Coenzymes are small organic molecules that bind to enzymes and are required for catalytic activity. They participate in a wide range of metabolic reactions by carrying electrons, atoms, or specific functional groups. Many coenzymes are derived from vitamins and must be present for enzymes to function properly.

Coenzymes differ from prosthetic groups in their mode of association with the enzyme. Some act as cosubstrates that bind and release during the reaction, while others form tightly bound, often covalent, associations with the enzyme and are called prosthetic groups. When the coenzyme is present, the enzyme exists as a holoenzyme; in its absence, it is an apoenzyme.

Well-known coenzymes include NAD+ and NADP+ (nicotinamide adenine dinucleotide) and FAD and FMN (flavin adenine dinucleotide and flavin mononucleotide), which shuttle electrons in oxidation–reduction reactions. Coenzyme A carries acetyl and other acyl groups. Thiamine pyrophosphate (derived from vitamin B1) assists decarboxylation and transfer of two-carbon units. Biotin carries carbon dioxide in carboxylation reactions. Pyridoxal phosphate (vitamin B6 derivative) enables amino group transfers. Tetrahydrofolate (derived from folate) transfers one-carbon units. Lipoamide is a swinging coenzyme in certain dehydrogenase complexes. S-adenosylmethionine acts as a methyl group donor in many methylation reactions.

Most coenzymes are derived from B vitamins; niacin provides NAD+/NADP+, riboflavin provides FAD/FMN, pantothenic acid gives CoA, thiamine yields TPP, biotin, pyridoxine for PLP, and folate for THF. Some coenzymes also require metal ions for full activity.

Deficiencies in the corresponding vitamins can impair numerous enzymes, disrupting energy metabolism and biosynthetic pathways. Coenzymes are continually regenerated and recycled by cellular systems after turnover in reactions.