The prosthetic group is the non-protein component of an enzyme that is tightly (often covalently) bound to the apoenzyme and essential for catalytic activity. Examples: heme in catalase and peroxidase; FAD in succinate dehydrogenase; biotin in pyruvate carboxylase. The holoenzyme = apoenzyme + cofactor (complete, active enzyme); the apoenzyme = protein part alone (inactive without cofactor). Some sources use 'prosthetic group' for tightly bound cofactors and 'coenzyme' for loosely bound ones.