apoenzim

An apoenzyme is the protein component of an enzyme that is catalytically inactive on its own. It requires a non-protein molecule to become active. When the apoenzyme binds its required cofactor, the resulting complete enzyme is called a holoenzyme, or simply an active enzyme.

Cofactors that activate apoenzymes can be inorganic ions, such as zinc, iron, or magnesium, or organic molecules known as coenzymes, such as NAD+, FAD, or heme. The binding of the cofactor to the apoenzyme often induces the proper conformation of the active site and enables catalysis. Some cofactors are tightly bound as prosthetic groups, while others associate more loosely and reversibly.

Many enzymes are synthesized as apoenzymes and require one or more cofactors to become functional. The apoenzyme provides the protein scaffold, while the cofactor contributes chemical functionality essential for the reaction. The distinction between apoenzyme and holoenzyme is a useful way to describe the enzyme’s dependence on non-protein components for activity.

Examples illustrate the concept: carbonic anhydrase is an apoenzyme that gains activity upon binding a zinc ion; dehydrogenases often require NAD+ or NADP+ as cofactors; apoproteins such as apocytochrome c lack the heme prosthetic group and are inactive until the heme is incorporated. Understanding apoenzymes helps explain how enzyme activity can be regulated by availability of cofactors and by cellular conditions.