The organic non-protein part of an enzyme tightly bond to the active site is called as 

Most of the enzymes are conjugated enzymes having both protein and non-protein parts. The protein part is called Apoenzyme and non-protein part as Cofactor. Cofactor and apoenzyme together called as Holoenzyme. If the cofactor is organic and tightly bound to the active site of enzyme, it is called as Prosthetic group. If loosely associated only at the time of catalysis it is called as coenzyme. The inorganic cofactor is tightly bound to the active site of apoenzyme and forms coordinate bonds with both substrate and apoenzyme.