Structure of superoxide reductase bound to ferrocyanide and active site expansion upon X-ray-induced photo-reduction

TL;DR

This study reveals the structure of superoxide reductase (SOR) bound to ferrocyanide and shows how X-ray-induced photo-reduction causes active site expansion, providing insights into enzyme-inhibitor interactions and redox-induced structural changes.

Contribution

First reported structure of a protein-ferrocyanide complex showing active site plugging and structural changes upon photo-reduction in SOR.

Findings

Ferrocyanide fully plugs SOR active site, coordinating through a bent cyano bridge.

Photo-reduction from Fe(III) to Fe(II) causes active site expansion.

Structural differences between mixed-valence and reduced SOR-ferrocyanide states were characterized.

Abstract

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.