Iron detoxification properties of Escherichia coli bacterioferritin -: Attenuation of oxyradical chemistry

Bacterioferritin (EcBFR) of Escherichia coli is an iron-mineralizing hemoprotein composed of 24 identical subunits, each containing a dinuclear metal-binding site known as the "ferroxidase center." The chemistry of Fe(II) binding and oxidation and Fe(III) hydrolysis using H2O2 as oxidant was studied by electrode oximetry, pH-stat, UV-visible spectrophotometry, and electron paramagnetic resonance spin trapping experiments. Absorption spectroscopy data demonstrate the oxidation of two Fe(II) per H2O2 at the ferroxidase center, thus avoiding hydroxyl radical production via Fenton chemistry. The oxidation reaction with H2O2 corresponds to [Fe(II)(2)-P](Z) + H2O2 --> [Fe(III)(2)O-P](Z) + H2O, where [Fe(II)(2)-P](Z) represents a diferrous ferroxidase center complex of the protein P with net charge Z and [Fe(III)(2)O-P](Z) a mu-oxo-bridged diferric ferroxidase complex. The mineralization reaction is given by 2Fe(2+) + H2O2 + 2H(2)O --> 2FeOOH((core)) + 4H(+), where two Fe(II) are again oxidized by one H2O2. Hydrogen peroxide is shown to be an intermediate product of dioxygen reduction when O-2 is used as the oxidant in both the ferroxidation and mineralization reactions. Most of the H2O2 produced from O-2 is rapidly consumed in a subsequent ferroxidase reaction with Fe(II) to produce H2O. EPR spin trapping experiments show that the presence of EcBFR greatly attenuates the production of hydroxyl radical during Fe(II) oxidation by H2O2, consistent with the ability of the bacterioferritin to facilitate the pairwise oxidation of Fe(II) by H2O2, thus avoiding odd electron reduction products of oxygen and therefore oxidative damage to the protein and cellular components through oxygen radical chemistry.