noun, plural: apoenzymes
(biochemistry) The protein component in which together with a cofactor forms a complete enzyme
Enzymes can speed up biochemical processes. Some enzymes need cofactors (non-protein molecules) to carry out catalysis while others do not. Those that do not require cofactors are referred to as simple enzymes. Examples are pepsin, trypsin, and urease. Those that require a particular cofactor are referred to as conjugate enzymes.
Conjugate enzymes are comprised of two main components: (1) cofactor, which is the non-protein part and (2) apoenzyme, the protein part. The cofactor may be an organic compound (e.g. flavin) or an inorganic compound (e.g. metal ion). The organic cofactor may either be a coenzyme or a prosthetic group. A coenzyme is a cofactor that is loosely bound to the enzyme and therefore may be released readily from the active site of the enzyme. Examples of coenzymes are those made of water-soluble vitamins (i.e. B vitamins and vitamin C), and elements (e.g. Cu, Ca, Zn, Mg, K, Ni, Co, Fe, etc.).1 A prosthetic group is rather firmly attached to the enzyme.
The term holoenzyme is used to refer to the apoenzyme bound to a cofactor.
1 Jain, V.K. and Sharma, J.P. (2005). Comprehensive Objective Biology. New Delhi, India: Golden Bells. p.99