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Extended x ray absorption fine structure studies of bi nuclear copper site of oxy hemo cyanin deoxy hemo cyanin metaquo hemo cyanin met fluoro hemo cyanin and met azido hemo cyanin from arthropods and mollusks


, : Extended x ray absorption fine structure studies of bi nuclear copper site of oxy hemo cyanin deoxy hemo cyanin metaquo hemo cyanin met fluoro hemo cyanin and met azido hemo cyanin from arthropods and mollusks. Journal of the American Chemical Society 106(1): 86-92

The extended X-ray absorption fine structure (EXAFS) beyond the Cu K edge was analyzed for oxy-, deoxy- and metaquohemocyanin (Hc) derivatives from a mollusk, Busycon canaliculatum, and arthropods representative of the 2 subphyla Limulus polyphemus and Cancer irroratus and also for the fluoride and azide metHc adducts for Busycon and Limulus. The oxyHc EXAFS Fourier transforms (FT) show a prominent outer-shell peak due to scattering from the partner Cu, at a distance of 3.58-3.66 .ANG. The FT 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 .ANG., 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 .ANG. (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 1st-shell EXAFS fitting show the Cu coordination number to be 4 in oxy- and metHc and 2 in deoxyHc. 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; binding N3- increases the Cu-Cu separation to 3.66 .ANG. The 3 spp. studied show only minor differences in the EXAFS-derived structural parameters for any of the Hc forms. The strong catalase activity present only in mollusk hemocyanins must arise from variations in the active-site residues which do not significantly affect the Cu coordination number, ligand type or bond lengths.

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