The crystal structures of reduced pseudoazurin from Alcaligenes faecalis S-6 at two pH values.

The structures of the reduced (Cu1+) blue-copper protein pseudoazurin from Alcaligenes faecalis strain S-6 are refined at pH 7.8 and 4.4 using X-ray diffraction data to 1.8 A resolution. The final R-factors for the high and low pH structures are 0.178 and 0.177, respectively. Comparing the reduced pseudoazurin at pH 7.8 with the oxidised (Cu2+) molecule, small changes are observed in the vicinity of the copper site and on the protein surface. At pH 4.4 the copper substituent imidazole of His81 rotates away from the metal with a concurrent movement of the latter towards the plane of the remaining three ligands (S gamma-Cys78, N delta 1-His40 and S delta-Met86) thus the geometry of the copper site becomes planar trigonal.