title: Zinc environment and cis peptide bonds in carboxypeptidase A at 1.75-A resolution.
abstract: The structure of the metalloenzyme carboxypeptidase A (peptidyl-L-amino-acid hydrolase, EC 3.4.17.1) has been refined at 1.75 A by a restrained least-squares procedure to a conventional crystallographic R factor of 0.162. Significant results of the refined structure relative to the catalytic mechanism are described. In the native enzyme, the zinc coordination number is five (two imidazole N delta 1 nitrogens, the two carboxylate oxygens of glutamate-72, and a water molecule). In the complex (at 2.0-A resolution) of carboxypeptidase A with the dipeptide glycyl-L-tyrosine, however, the water ligand is replaced by both the carbonyl oxygen and the amino nitrogen of the dipeptide. The amino nitrogen also statistically occupies a second position near glutamate-270. Consequently, the coordination number of zinc may vary from five to six in carboxypeptidase A-substrate complexes. Implications of these results for the catalytic mechanism of carboxypeptidase A are discussed. In addition, three cis peptide bonds, none of which involves proline as the amino nitrogen donor, have been located fairly near the active site.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002268:Carboxypeptidases / Q000037:antagonists & inhibitors; D043422:Carboxypeptidases A; D002384:Catalysis; D002417:Cattle; D004151:Dipeptides; D008024:Ligands; D010455:Peptides; D011487:Protein Conformation; D014867:Water; D014961:X-Ray Diffraction; D015032:Zinc
chemical_list: D004151:Dipeptides; D008024:Ligands; D010455:Peptides; D014867:Water; D002268:Carboxypeptidases; D043422:Carboxypeptidases A; D015032:Zinc