Iron oxidation in Escherichia coli bacterioferritin ferroxidase centre, a site designed to react rapidly with H2O2 but slowly with O2

Both O 2 and H 2 O 2 can oxidise iron at the ferroxidase centre (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 oxidised in concert and a H 2 O 2 molecule is formed and released to solution. This peroxide molecule further oxidises another di‐Fe 2+ FC, at a rate ~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.