What is the apoenzyme in enzymes?

Prosthetic groups are organic compounds and are distinguished from other cofactors in that they are tightly bound to the apoenzyme. For example, in peroxidase and catalase, which catalyze the breakdown of hydrogen peroxide to water and oxygen, haem is the prosthetic group and it is a part of the active site of the enzyme. The apoenzyme is the protein portion of the enzyme in cases where non-protein constituents called co-factors are bound to the enzyme to make it catalytically active.