The crystal structure of poplar apoplastocyanin at 1.8-A resolution. The geometry of the copper-binding site is created by the polypeptide.

The three-dimensional structure of apoplastocyanin from poplar leaves (Populus nigra var. italica) has been determined by x-ray diffraction at 1.8-A resolution. The structure closely resembles that of the holoprotein. In particular, the positions of the copper-binding residues in the apo- and holoproteins differ by only 0.1-0.3 A. This indicates that the irregular geometry of the "type 1" copper site is imposed upon the metal atom by the polypeptide moiety. A 180 degrees rotation of one solvent-exposed histidine imidazole ring about C beta-C gamma appears to facilitate access to the copper site. The close structural similarity between apo-, Cu-(II)-, and Cu(I)-plastocyanin was initially demonstrated by means of electron density difference maps. Two series of restrained least squares refinement calculations for apoplastocyanin, originating from different sets of atomic positional parameters, were carried out in parallel. Both refinements converged to the same model which, when fully refined, had a residual R = 0.16. Forty-two water molecules were located during the refinement.