A spectroscopic study of metal ion and ligand binding to beta-lactamase II

beta-Lactamase II has two metal-binding sites. The electronic spectra of Cd(II)- and Co(II)-substituted beta-lactamase II have been investigated. It is suggested that a thiol ligand is involved in metal binding at the first site. The stoichiometric dissociation constants for Co(II) binding to beta-lactamase II were estimated to be 0.13 and 2.66 mM (pH 6.0, 4 degrees C, 1 M NaCl) by equilibrium dialysis. Competition between Zn(II) and Co(II) for the first metal binding site suggests a value of 0.7 microM (pH 6.0, 30 degrees C, 1 M NaCl) for the dissociation constant of Zn(II). The electronic spectra of the Co(II) enzyme lead to the suggestion that the coordination geometries around the metal ions in the first and second sites are related to those of a distorted tetrahedron and octahedron, respectively.