apoenzymy

An apoenzyme is the protein component of an enzyme that is catalytically inactive by itself. Most enzymes require a non-protein factor to become active; when such a factor binds, the complete catalytically competent complex is called a holoenzyme. The non-protein component can be an inorganic cofactor (often a metal ion), an organic molecule called a coenzyme, or a prosthetic group that remains tightly bound to the enzyme.

Apoenzymes provide the protein scaffold, whereas cofactors participate in catalysis by stabilizing charged intermediates, orienting substrates, or participating in redox reactions. The binding of the cofactor can induce conformational changes that enable substrate binding or chemical transformation.

Cofactors are classified as inorganic (e.g., Zn2+, Fe2+/Fe3+, Mg2+) or organic. Organic cofactors are often vitamins and are called coenzymes (e.g., NAD+, FAD, coenzyme A). Prosthetic groups are cofactor types that are tightly or covalently bound, such as the heme group in cytochromes.

Not all enzymes require cofactors; many ribozymes and some protein-only enzymes function without them, while others are active only in holoenzyme form. The terms apoenzyme and holoenzyme highlight the apo form’s dependence on the cofactor for activity.

In metabolism and research, distinguishing apoenzymes from holoenzymes helps in understanding enzyme regulation, activation mechanisms, and drug targeting.