EXAFS Studies of Binuclear Copper Site of Oxy-, Deoxy-, Metaquo-, Metfluoro-, and Metazidohemocyanin from Arthropods and Molluscs

The extended X-ray absorption fine structure (EXAFS) beyond the Cu K edge has been analyzed for oxy-, deoxy-, and metaquohemocyanin (He) derivatives from a mollusc, Busycon canaliculatum, and arthropods representative of the two subphyla Limulus Polyphemus and Cancer Irroratus and also for the fluoride and azide metHc adducts for Busycon and Limulus. The oxyHc EXAFS Fourier transforms (FTs) show a prominent outer-shell peak due to scattering from the partner Cu, at a distance of 3.58-3.66 Å. The FTs for deoxy-and metHc show a pair of outer-shell peaks attributable to the distant atoms of bound imidazole, but imidazole alone does not account for the backtransformed outer-shell EXAFS. An additional scattering atom, at 3.39-3.48 Å, is required in all cases to reproduce the data in the beat-node region. Due to a phase ambiguity, the extra atom could be Cu, N, or O. As the antiferromagnetic behavior of the met derivative is associated with an endogenous protein bridge which requires a Cu-Cu distance of <4 Å (a group 1 ligand effect), the additional scattering atom is most logically assigned as the partner Cu; only a small decrease is observed in Cu-Cu distance upon deoxygenation. The results of the first-shell EXAFS fitting show the Cu coordination number to be four in oxy-and metHc and two in deoxyHc. Thus a significant change in ligation correlates with cooperativity in hemocyanin oxygenation, consistent with the strong involvement of the endogenous protein bridge indicated by recent spectral probe studies. Binding F^- to metHc produces very little change in the EXAFS pattern, while binding N₃^- increases the Cu-Cu separation to 3.66 Å. The three species studied show only minor differences in the EXAFS-derived structural parameters for any of the He forms. Hence, the strong catalase activity present only in mollusc hemocyanins must arise from variations in the active-site residues which do not significantly affect the Cu coordination number, ligand type, or bond lengths.