Structural and spectroscopic studies of the copper site of stellacyanin

The structure of the copper site in oxidized and reduced Rhus vernicifera stellacyanin has been studied by X-ray absorption (XAFS) spectroscopy at different pH values. Data for the oxidized protein are consistent with the fourth ligand being an O- or N-donating ligand rather than a cysteine from the disulfide bridge. The fourth ligand is not present in the inner coordination sphere, but makes a more distant interaction 2.7 A from the copper atom. Only minor changes in the details of the Cu(II) coordination occur when the pH is varied. Direct structural information on reduced stellacyanin is provided. Upon reduction, one of the histidine ligands moves away from the copper atom by at least 0.2 A. A low-Z (O or N) scatterer is present approximately 2.4 A from the Cu(I) atom in the protein at low pH, and this ligand is lost at high pH. There is no evidence for an S-donating fourth ligand in the reduced protein. The XAFS results are presented in relation to the spectroscopic and structural information available for some methionine-121 mutants of azurin. The data reveal that there are spectroscopic similarities between stellacyanin and some of the mutant proteins, but distinct structural differences exist that preclude these proteins as suitable models for the copper site of stellacyanin.