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.