  Some sulfate-reducing and microaerophilic bacteria rely on the enzyme
superoxide reductase (SOR) to eliminate the toxic superoxide anion radical
(O2*-). SOR catalyses the one-electron reduction of O2*- to hydrogen peroxide
at a nonheme ferrous iron center. The structures of Desulfoarculus baarsii SOR
(mutant E47A) alone and in complex with ferrocyanide were solved to 1.15 and
1.7 A resolution, respectively. The latter structure, the first ever reported
of a complex between ferrocyanide and a protein, reveals that this
organo-metallic compound entirely plugs the SOR active site, coordinating the
active iron through a bent cyano bridge. The subtle structural differences
between the mixed-valence and the fully reduced SOR-ferrocyanide adducts were
investigated by taking advantage of the photoelectrons induced by X-rays. The
results reveal that photo-reduction from Fe(III) to Fe(II) of the iron center,
a very rapid process under a powerful synchrotron beam, induces an expansion of
the SOR active site.
