Biomolecules

Many enzymes consist of a protein part and a non-protein part.
The protein part is called the apoenzyme, and the non-protein part that is essential for the activity of the enzyme is called the cofactor.
The complete, catalytically active enzyme (apoenzyme + cofactor) is called the holoenzyme.
Cofactors are of three kinds: prosthetic groups (organic, tightly bound), coenzymes (organic, bound only transiently during catalysis) and metal ions.
Coenzymes are organic compounds, many of which contain vitamins; for example NAD and NADP contain the vitamin niacin.
Isoenzymes are different molecular forms of the same enzyme.
So an organic substance bound to an enzyme and essential for its activity is called a coenzyme.