Holoenzyme, Apoenzyme

A number of enzymes rely on the presence of a prosthetic group at their active site in order for them to be catalytically active. For example, aminotransferases require pyridoxal phosphate. Prosthetic groups are small molecules that are covalently or tightly bound at the active site. They provide groups that the protein component of the enzyme does not possess, but that are essential for catalysis. The enzyme with a prosthetic group bound is referred to as a holoenzyme, whereas after removal of the prosthetic group, the protein component alone is termed an apoenzyme. Thus, 

Holoenzyme = Apoenzyme + prosthetic group

The term apoenzyme is also applied to allosteric enzymes such as aspartate transcarbamoylase from which the regulatory subunits have been removed.