CARBOXYPEPTIDASE-A - NATIVE, ZINC-REMOVED AND MERCURY-REPLACED FORMS

The crystal structure of the zinc-containing exopeptidase bovine carbo xypeptidase A (CPA) has been refined to high resolution, based on a da ta set collected from a single crystal, incorporating new sequence inf ormation based on cloning of the bovine gene. In addition, new refined structures are available for the zinc-removed form of the enzyme, ape -CPA, as well as the mercury-replaced form, Hg-CPA. The native structu re reveals that the zinc-bound water molecule does not appear to be mo re loosely bound than the rest of the zinc ligands, at least when B-fa ctor values are considered. Nor is there any evidence for a secondary location of this water molecule. The ape-enzyme structure does not sho w any density in the place of the removed zinc ion. The only significa nt change appears to be a chi(2), rotation of one zinc histidine ligan d to form an ion-pair interaction with a glutamic acid side chain. The structure of Hg-CPA reveals a solvent Tris molecule bound to the merc ury cation, as well as an unidentified cation bound to Glu270. The loc ation of this cation agrees with previous proposals for the binding si te of inhibitory zinc. These observations may explain some of the diff erences in kinetics observed in metal-replaced CPA.