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)₂O-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)₂O-P]^Z a μ-oxo-bridged diferric ferroxidase complex. The mineralization reaction is given by 2Fe²⁺ + H₂O₂ + 2H₂O → 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 O₂ is used as the oxidant in both the ferroxidation and mineralization reactions. Most of the H₂O₂ produced from O₂ 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 pair-wise 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.