aprotein

Apoprotein, occasionally referred to as an aprotein in older or informal literature, is the protein component of a conjugated protein that lacks its nonprotein constituent, such as a prosthetic group or cofactor. The term apoprotein is used to distinguish the protein part from the complete, functioning holoprotein. The association of an apoprotein with its prosthetic group or cofactor converts it into a holoprotein, often enabling catalytic activity or binding capability. In some cases the apoprotein can fold and stabilize in its own right, whereas in others the apoprotein is unstable without the bound cofactor and depends on it for proper structure.

The concept is central to understanding how many proteins achieve activity through the binding of nonprotein components. The terms apo-, holo-, and prosthetic group are used to describe these states: apo- refers to the protein lacking its nonprotein component, holo- to the complete, active complex, and a prosthetic group to a tightly bound nonprotein moiety. Cofactors can include metal ions, organic molecules, or tightly bound organic cofactors that remain associated with the protein as part of its functional form.

Examples include the apoproteins of heme-containing proteins such as the globin chains of hemoglobin and myoglobin when the heme group is removed. Enzymes that require metal ions or organic cofactors can also exist as apoproteins before the cofactor is incorporated. In laboratory or physiological contexts, reconstitution experiments often add the prosthetic group to the apoprotein to restore activity, illustrating the interdependence of protein structure and its nonprotein component.