METAL SUBSTITUTION IN A BLUE-COPPER PROTEIN - THE CRYSTAL-STRUCTURE OF CADMIUM-AZURIN AT 1.8 ANGSTROM RESOLUTION

Crystals of cadmium-substituted azurin have been prepared by diffusing Cd(II) into crystals of apo-azurin grown previously and their structure has been determined at high resolution by X-ray crystallography. Data to 1.8 angstrom resolution were collected by Weissenberg photography (with image plates) using synchrotron radiation. These data were combined with a 2.2 angstrom diffractometer data set to give 90% coverage to 1.8 angstrom. An initial model was derived from the isomorphous Cu(II)-azurin structure, and the cadmium and ligand positions added from 'omit' maps. Refinement was by restrained least squares (program PROLSQ), to a final R value of 0.168 for all data in the range 10.0-1.8 angstrom (23 349 reflections). The final model of 1954 protein atoms, two Cd(II) ions (occupancy 0.75), four SO42- ions and 239 water molecules has r.m.s. deviations of 0.015, 0.045 and 0.013 angstrom from standard bond lengths, angle distances and planar groups. The protein structure is essentially the same as that of Cu(II)-azurin, with an r.m.s. deviation of 0.18 angstrom for 97% of main-chain atoms after superposition of the two structures. The Cd atom is within 0.2 angstrom of the equivalent copper position, displaced slightly away from the axial Met ligand towards the carbonyl O atom of Gly45. The latter has also moved slightly towards the metal, by a rotation of the peptide unit, to give a Cd-O bond of 2.76 angstrom. The Cd-S(Cys) bond is lengthened to 2.39 angstrom. The coordination geometry is slightly more tetrahedral than for Cu(II), and the cadmium-oxygen interaction is consistent with the presence of an oxygen ligand in the coordination sphere of stella-cyanin.