There are a number of cases in which non-protein constituents called co-factors are bound to the enzyme to make the enzyme catalytically active.
In these instances, the protein portion of the enzyme is called the apoenzyme.
Three kinds of co-factors are identified: prosthetic groups, co-enzymes and metal ions.
Prosthetic groups are organic compounds and they are tightly bound with the apoenzyme.
Co-enzymes are also organic compounds but their association with the apoenzyme is only transient.
Hence, the actual protein part of a conjugated enzyme is the apoenzyme, while prosthetic group, cofactor and coenzyme refer to the non-protein partners.
So the correct answer is Apoenzyme.
