holoenzym

A holoenzyme is the catalytically active form of an enzyme, comprising an apoprotein (the protein component, or apoenzyme) bound to a nonprotein component essential for activity. The apoenzyme by itself is typically inactive or only weakly active, and the binding of the nonprotein component converts it into the functional enzyme.

The nonprotein component can be an inorganic ion, such as a metal ion (for example Zn2+, Mg2+, or Fe2+), or an organic molecule known as a cofactor. When the cofactor is tightly bound to the protein, the enzyme is described as having a prosthetic group; when the cofactor is loosely bound and can dissociate, it is more accurately called a coenzyme (a type of cofactor).

Holoenzymes carry out catalysis by enabling proper substrate binding, electron transfer, or chemical transformation that the apoenzyme cannot perform alone. Examples include lactate dehydrogenase requiring NAD+, carbonic anhydrase containing Zn2+, and several enzymes that rely on pyridoxal phosphate as a prosthetic or tightly bound cofactor. Cytochromes in the electron transport chain contain heme groups, which are essential cofactors for activity.

Formation and regulation: The holoenzyme can often be formed by the association of the apoenzyme with its cofactor, either biosynthetically or after protein synthesis. Cellular levels of cofactors and allosteric interactions can regulate holoenzyme activity, linking enzyme function to metabolic state.