Apoprotein

An apoprotein is the protein component of a conjugated protein that lacks its nonprotein prosthetic group or cofactor. The term highlights that the protein by itself is not fully functional until the prosthetic group binds. In enzymology, the protein portion of an enzyme is often referred to as the apoenzyme; when a cofactor such as a metal ion, a heme group, or another organic molecule binds, the complete active enzyme is called a holoenzyme.

In many enzymes, removing the prosthetic group converts the functional enzyme into an apoprotein that cannot perform its catalytic activity. Conversely, reintroduction of the prosthetic group can restore activity, demonstrating the interdependence of the protein and its nonprotein component. The apoprotein concept also extends to other conjugated proteins where the protein part determines structure and interactions while the prosthetic group provides chemical reactivity or binding properties.

In the context of lipoproteins, apoproteins (often termed apolipoproteins) are the protein constituents that stabilize lipid particles, influence lipid binding, and mediate receptor recognition. Examples include apolipoprotein B-100 in low-density lipoproteins and apolipoprotein A-I in high-density lipoproteins. Apoproteins can be modified by glycosylation or lipidation and are typically synthesized in tissues such as the liver and intestine.

Overall, apoproteins are essential for understanding how protein structure interacts with nonprotein cofactors to yield functional biological molecules, and they play key roles across enzymatic and lipid transport systems.