coenzimi

Coenzimi are organic non-protein molecules that bind to enzymes and are essential for catalytic activity. They are a subset of cofactors, distinguished from inorganic ions by their organic nature and, in many cases, their vitamin origin. Most coenzymes act as carriers, shuttling electrons, atoms, or functional groups between reactions and are recycled after each catalytic cycle.

Enzymes that require a coenzyme are called holoenzymes when the coenzyme is present; apoproteins lack it. Prosthetic groups are coenzymes that stay tightly bound to the enzyme, whereas many coenzymes bind reversibly and can dissociate after the reaction.

Common examples include NAD+ and NADH, FAD and FMN, which transfer electrons in oxidation–reduction reactions; coenzyme A (CoA), which carries acyl groups; thiamine pyrophosphate (TPP) for decarboxylation; pyridoxal phosphate (PLP) for amino group transfer; biotin for carboxylation; folate derivatives for one-carbon transfers; and cobalamin (vitamin B12) for certain rearrangements and methyl transfers. Coenzymes are typically derived from vitamins and are essential across central metabolism.

In metabolism, coenzymes enable pathways such as glycolysis, the citric acid cycle, fatty acid synthesis, and amino acid metabolism. Deficiency of the vitamin precursors can impair coenzyme synthesis and enzyme activity, with downstream physiological effects. Coenzymes are regenerated after reactions, allowing them to participate in multiple catalytic cycles, and they may be soluble in the cytosol or associated with organelles like mitochondria.