Co-factors

Co-factors

Many enzymes cannot catalyse a reaction on their own; the protein portion alone, called the apoenzyme, is catalytically inactive until it is joined by an additional, non-protein component called a co-factor. Together, the apoenzyme plus its co-factor make up the complete, catalytically active enzyme, called the holoenzyme. Co-factors are generally grouped into three kinds. Prosthetic groups are organic co-factors that are bound tightly, often covalently, to the apoenzyme on a more or less permanent basis - the haem group found in enzymes such as catalase and peroxidase is a typical example. Co-enzymes are also organic, but unlike prosthetic groups they are bound loosely and only associate with the enzyme transiently, during the catalytic step itself, before separating again; many co-enzymes are derived from vitamins, such as NAD+, NADP+ and coenzyme A. Metal-ion co-factors (also called activators) are inorganic ions - such as Zn2+, Mg2+ or Mn2+ - that form co-ordination bonds with the enzyme and/or its substrate and are essential for the enzyme to fold or function correctly. …