Iron and Hydrogen Peroxide Detoxification Properties of Dps: A Ferritin-like DNA Binding Protein of Escherichia coli

The DNA-binding proteins from starved cells (Dps) are a family of proteins induced in microorganisms by oxidative or nutritional stress. Escherichia coli Dps, a structural analog of the 12-subunit Listeria innocua ferritin, binds and protects DNA against oxidative damage mediated by H2O2. Dps is shown to be a Fe-binding and storage protein where Fe(II) oxidation is most effectively accomplished by H2O2 rather than by O2 as in ferritins. Two Fe2+ ions bind at each of the 12 putative dinuclear ferroxidase sites (PZ) in the protein according to the equation, 2Fe2+ + PZ→ [(Fe(II)2-P]Formula + 2H+. The ferroxidase site (FS) bound iron is then oxidized according to the equation, [(Fe(II)2-P]Formula + H2O2 + H2O → [Fe(III)2O2(OH)-P]Formula + 3H+, where two Fe(II) are oxidized per H2O2 reduced, thus avoiding hydroxyl radical production through Fenton chemistry. Dps acquires a ferric core of ∼500 Fe(III) according to the mineralization equation, 2Fe2+ + H2O2 + 2H2O → 2Fe(III)OOH(core) + 4H+, again with a 2 Fe(II)/H2O2 stoichiometry. The protein forms a similar ferric core with O2 as the oxidant, albeit at a slower rate. In the absence of H2O2 and O2, Dps forms a ferrous core of ∼400 Fe(II) by the reaction Fe2+ + H2O + Cl− → Fe(II)OHCl(core) + H+. The ferrous core also undergoes oxidation with a stoichiometry of 2 Fe(II)/H2O2. Spin trapping experiments demonstrate that Dps greatly attenuates hydroxyl radical production during Fe(II) oxidation by H2O2. These results and in vitro DNA damage assays indicate that the protective effect of Dps on DNA most likely is exerted through a dual action, the physical association with DNA and the ability to nullify the toxic combination of Fe(II) and H2O2. In the latter process a hydrous ferric oxide mineral core is produced within the protein, thus avoiding oxidative damage mediated by Fenton chemistry