THE METAL SITE OF PSEUDOMONAS-AERUGINOSA AZURIN, REVEALED BY A CRYSTAL-STRUCTURE DETERMINATION OF THE CO(II) DERIVATIVE AND CO-EPR SPECTROSCOPY

The crystal structure of cobalt-substituted azurin from Pseudomonas ae ruginosa has been determined to final crystallographic R value of 0.17 5 at 1.9 Angstrom resolution, There are four molecules in the asymmetr ic unit in the structure, and these four molecules are packed as a dim er of dimers, The dimer packing is very similar to that of the wild-ty pe Pseudomonas aeruginosa azurin dimer. Replacement of the native copp er by the cobalt ion has only small effects on the metal binding site presumably because of the existence of an extensive network of hydroge n bonds in its immediate neighborhood, Some differences are obvious, h owever, In wild-type azurin the copper atom occupies a distorted trigo nal bipyramidal site, while cobalt similar to zinc and nickel occupy a distorted tetrahedral site, in which the distance to the Met121,S-del ta atom is increased to 3.3-3.5 Angstrom and the distance to the carbo nyl oxygen of Gly45 has decreased to 2.1-2.4 Angstrom, The X-band EPR spectrum of the high-spin Co(II) in azurin is well resolved (apparent g values g(x)' = 5.23; g(y)' = 3.83; g(z)' = 1.995, and hyperfine spli ttings A(x)' = 31; A(y)' = 20-30; A(z)' = 53 G) and indicates that the ligand field is close to axial. (C) 1997 Wiley-Liss, Inc.