Spectral and Magnetic Studies of Metallocarboxypeptidase A's

Spectral and magnetic studies on the cobalt(II), nickel(II), and copper(II) derivatives of the zinc containing metalloenzyme carboxypeptidase A (CPA) are reported and discussed. The absorption spectrum and magnetic moment of Co(II)-CPA are shown to be consistent with a five coordinate metal structure by comparison to the spectra and magnetic moments of a large number of pseudo-tetrahedral and five coordinate cobalt(II) complexes. The usefulness of magnetic susceptibility data in assigning coordination number for high-spin cobalt(II) metalloenzyme systems is demonstrated. The absorption spectrum and magnetic moment of Ni(II)-CPA are consistent only with an octahedral ground state geometry. The effect of substrate and inhibitor on the spectrum of Ni(II)-CPA was studied. The absorption and EPR spectra of Cu(II)-CPA were also investigated. A significant pseudo-tetrahedral distortion from planar coordination geometry is indicated for this derivative. The interaction of Cu(II)-CPA and the inhibitor sodium B-phenylpropionate was also studied. The results of these studies and especially those of Ni (II)-C PA do not agree with the predictions of the entatic state hypothesis.