Evidence for breaking of the active site dimetal cluster in Cu,Co superoxide dismutase upon copper reduction: a polarized absorption spectra study

The polarized absorption spectra of the cobalt chromophore in orthorhombic crystals of bovine Cu,Co superoxide dismutase (SOD), bearing the copper ion in both the oxidized and the reduced states, are reported together with the calculated isotropic spectrum. All the absorption bands are polarized and a spectral shift, from 598 nm to 588 nm, is observed in only one of them upon copper reduction. This shift, previously described in solution studies, is unequivocally assigned to the detachment of the copper side from the imidazolate bridging copper and cobalt ions in the oxidized catalytic center. At variance with a recent X-ray diffraction investigation on Cu(I),Zn SOD, this result indicates that reduction of copper in Cu,Co SOD is associated to the breaking of the dimetal cluster also in the crystalline state.