Prosthetic groups differ from co-enzymes in that :-
They require metal ions for their activity
They (prosthetic groups) are tightly bound to apoenzymes.
Their association with apoenzymes is transient.
They can serve as co-factors in a number of enzyme-catalyzed reactions.
Prosthetic groups are organic compounds and are distinguished from other cofactors in that they are tightly bound to the apoenzyme.