holoenzimet

Holoenzyme is the active form of an enzyme composed of the catalytic protein component, the apoenzyme, together with its bound cofactors. The apoenzyme alone is typically catalytically inactive or has reduced activity; binding of one or more cofactors converts it into a fully functional holoenzyme.

Cofactors are non-polypeptide components that are required for enzyme activity and are classified as inorganic ions (for example Mg2+, Zn2+, Fe2+) or organic molecules called coenzymes. Some cofactors are tightly bound as prosthetic groups and remain part of the enzyme even between catalytic cycles, while others bind and release reversibly and must be present for each turnover.

Coenzymes are often derived from vitamins; common examples include NAD+ and NADP+ (niacin), FAD (riboflavin), and CoA (pantothenic acid). Prosthetic groups include metal ions bound in the active site or organic molecules such as heme.

Examples include carbonic anhydrase, which requires Zn2+ for catalysis; dehydrogenases that use cofactors such as NAD+, FAD, or Fe-S clusters; and cytochromes that contain heme prosthetic groups.

The holoenzyme concept helps explain regulation and metabolism; availability of cofactors or vitamins can affect enzyme activity, and deficiencies can impair metabolic pathways.