title: Spectral properties of cobalt carboxypeptidase A. Interaction of the metal atom with anions.
abstract: At pH greater than 7 the absorption and magnetic circular dichroic spectra of cobalt carboxypeptidase A are insensitive to anions [Latt, S. A., & Vallee, B. L. (1971) Biochemistry 10, 4263-4270], but at pH less than 6 chloride and other anions perturb them in a manner specific for each anion. Lowering of the pH apparently facilitates the entry of an anion into the metal coordination sphere, suggesting that an acidic group normally stabilizes a metal-coordinated water molecule against displacement. The lack of sensitivity to anions at pHs between 7 and 9--when the enzyme is maximally active--and its evident abolition upon protonation of an active-site group are consistent with this interpretation. Selective modification of cobalt carboxypeptidase at Glu-270 using a carbodiimide affinity reagent generates sensitivity to anions at pH 7 very similar to that of the unmodified enzyme at pH approximately 5. This suggests that the group stabilizing the metal-coordinated water is the catalytically essential carboxylate of Glu-270. These and related results provide evidence for a mechanistically important interaction of Glu-270 with a metal-bound water molecule.
mesh_terms: D001665:Binding Sites; D002268:Carboxypeptidases / Q000378:metabolism*; D043422:Carboxypeptidases A; D002942:Circular Dichroism; D003035:Cobalt; D006863:Hydrogen-Ion Concentration; D011485:Protein Binding; D011487:Protein Conformation; D013053:Spectrophotometry
chemical_list: D003035:Cobalt; D002268:Carboxypeptidases; D043422:Carboxypeptidases A