Structural Studies of the Hemocyanin Active Site. 1. Extended X-ray Absorption Fine Structure (EXAFS) Analysis1

X-ray absorption spectra near the Cu K edge have been obtained for oxy- and deoxyhemocyanin (He) from Busycon canaliculatum using synchrotron radiation. Comparison with copper complexes shows the edge structure to be consistent with imidazole coordination of Cu(ll) and Cu(l), respectively. The Fourier transform of the extended fine structure (EXAFS) shows two well-defined peaks. The positions and shape of the first peaks require coordination by low-Z atoms only, at average distances of 1.96 and 1.95 Å for the copper atoms of oxy- and deoxyHc. Their coordination numbers are calculated to be four and two, respectively, but the uncertainties are large; five and three are considered to be likelier values on the basis of chemical and spectroscopic evidence. The outer Fourier peak places a backscattering atom at 3.7 and 3.4 Å from Cu in oxy- and deoxyHc. For oxyHc this atom is shown to be the other Cu by the shape of the retransformed amplitude function. For deoxyHc the outer shell peak can be fit either with Cu or with a first-row scatterer. In the latter case, however, neither the distance nor the required Debye-Waller factor is sensible chemically, and a Cu-Cu interaction is the preferred interpretation. These results, along with those from resonance Raman and electronic absorption spectroscopy, lead to a model of the He binding site. It has two Cu atoms bound to the protein via three histidine ligands each; in oxyHc the Cu(II) ions are bridged by the bound O22- and by an atom from a protein ligand, possibly tyrosine.