Metal substitution in a blue-copper protein: the crystal structure of cadmium-azurin at 1.8 Å 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 Å resolution were collected by Weissenberg photography (with image plates) using synchrotron radiation. These data were combined with a 2.2 Å diffractometer data set to give 90% coverage to 1.8 Å. 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 Å (23 349 reflections). The final model of 1954 protein atoms, two Cd^II ions (occupancy 0.75), four SO ions and 239 water molecules has r.m.s. deviations of 0.015, 0.045 and 0.013 Å 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 Å for 97% of main-chain atoms after superposition of the two structures. The Cd atom is within 0.2 Å 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 Å. The Cd-S(Cys) bond is lengthened to 2.39 Å. 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 stellacyanin.