Which of the following contains Zinc?

CARBOXYPEPTIDASE is a metalloenzyme which contains zinc. it can be de-activated by removal of zinc and that activity can be restored by the addition of zinc, cobaltous, ferrous, nickel and manganous ions but not by addition of cadmium, magnesium or calcium ions. This metalloenzyme therefore falls in the group of enzymes which have no specific metal requirement for activity. In biological systems, however, a paicular metal, zinc, is associated with the enzyme presumably because complex ion equilibria or this paicular association over all others. The situation is not very different from that found in the case of the enzyme enolase, where magnesium and manganous are the paicular ions associated with activity in the biological system, but where other cations such as zinc can activate the enzyme in vitro 3. The apparent stability constants for the association of zinc and cobaltous ions with carboxypeptidase are 2 x 108 and 1 x 106 respectively1. The pH range of activation by the different cations indicates that association with the active center is in the order zinc (II) > nickel(II) and cobalt (II). The study of complexes of a wide variety of simple organic ligands has shown2 that this stability sequence is observed only when one of the coordinating centers is sulfide, R--S -. I conclude that zinc is bound by a sulfide group in carboxypeptidase and give the following additional evidence in suppo of this binding.