Iron Oxidation in Escherichia coli Bacterioferritin Ferroxidase Centre, a Site Designed to React Rapidly with H(2)O(2) but Slowly with O(2)

Both O(2) and H(2)O(2) can oxidize iron at the ferroxidase center (FC) of Escherichia coli bacterioferritin (EcBfr) but mechanistic details of the two reactions need clarification. UV/Vis, EPR, and Mössbauer spectroscopies have been used to follow the reactions when apo‐EcBfr, pre‐loaded anaerobically with Fe(2+), was exposed to O(2) or H(2)O(2). We show that O(2) binds di‐Fe(2+) FC reversibly, two Fe(2+) ions are oxidized in concert and a H(2)O(2) molecule is formed and released to the solution. This peroxide molecule further oxidizes another di‐Fe(2+) FC, at a rate circa 1000 faster than O(2), ensuring an overall 1:4 stoichiometry of iron oxidation by O(2). Initially formed Fe(3+) can further react with H(2)O(2) (producing protein bound radicals) but relaxes within seconds to an H(2)O(2)‐unreactive di‐Fe(3+) form. The data obtained suggest that the primary role of EcBfr in vivo may be to detoxify H(2)O(2) rather than sequester iron.