Many enzymes show catalytic activity only in association with certain non-protein substances. Those substances are known as cofactors (cofactors may be simple metal ions, e.g. Mg++ or K+ etc.). The enzymes which works only in presence of cofactors are called apoenzymes. An organic non-protein cofactor which is easily separable from the apoenzyme is called coenzyme, and the non-protein organic cofactor that is tightly bound to and non-dissociable from the enzyme is termed as prosthetic group, e.g. biotin of carboxylases, haem of hemoglobin, etc.
