Crystal structure determinations of oxidized and reduced pseudoazurins from Achromobacter cycloclastes -: Concerted movement of copper site in redox forms with the rearrangement of hydrogen bond at a remote histidine

The crystal structures of oxidized and reduced pseudoazurins from a denitrifying bacterium, Achromobacter cycloclastes IAM1013, have been determined at 1.35- and 1.6-Angstrom resolutions, respectively. The copper site in the oxidized state exhibits a distorted tetrahedral structure like those of other pseudoazurins. However, not only a small change of the copper geometry, but concerted peptide bond flips are identified. The imidazole ring of remote His(6) has a hydrogen bonding distance of 2.73 Angstrom between N-delta 1(His(6)) and O-gamma 1(Thr(36)) in the oxidized protein. When the protein is reduced at pH 6.0, the imidazole ring rotates by 30.3 degrees and moves 1.00 Angstrom away from the position of the oxidized state. A new hydrogen bond between N-epsilon 2(His(6)) and O-epsilon 1(Glu(4)) is formed with a distance of 3.03 Angstrom, while the hydrogen bond between N-delta 1(His(6))-O-gamma 1(Thr(36)) is maintained with an interatomic distance of 2.81 Angstrom. A concomitant peptide bond flip of main chain between Ile(34) and Thr(36) occurs.