Cofactor (Coenzyme and Prosthetic Group) In most cases,a coenzyme is firmly associated with its enzyme,perhaps even by covalent bonds, and it is difficult to separate the two.Such tightly bound coenzymes are referred to as prosthetic groups of the enzyme.The catalytically active complex of protein and prosthetic group is called the holoenzyme.The protein without the prosthetic group is called the apoenzyme;it is catalytically inactive.Cofactor (Coenzyme and Prosthetic Group) In most cases, a coenzyme is firmly associated with its enzyme, perhaps even by covalent bonds, and it is difficult to separate the two. Such tightly bound coenzymes are referred to as prosthetic groups of the enzyme. The catalytically active complex of protein and prosthetic group is called the holoenzyme. The protein without the prosthetic group is called the apoenzyme; it is catalytically inactive