A prosthetic group is a cofactor that is attached permanently and tightly or even covalently to an enzyme and that is regenerated in each enzymatic turnover. Thus a prostethic group is distinguished from a coenzyme or cosubstrate that is attached loosely and transiently. Like a coenzyme, the prosthetic group is required by an enzyme for its activity. A prosthetic group is 'a tightly bound, specific nonpolypeptide unit in a protein determining and involved in its biological activity' (IUPAC definition).
FMN/FMNH2 and FAD/FADH2 are prosthetic groups of Complex I and Complex II, respectively.
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MitoPedia topics: Enzyme, Substrate and metabolite