The bacterioferritin (BFR) of Escherichia coliconsists of 24 identical subunits, each containing a dinuclear metal-binding site consisting of two histidines and four carboxylic acid residues. Earlier studies showed that the characterization of iron binding to BFR could be aided by EPR analysis of iron–nitrosyl species resulting from the addition of NO to the protein [Le Brun, Cheesman, Andrews, Harrison, Guest, Moore and Thomson (1993) FEBS Lett. 323, 261–266]. We now report data from gas chromatographic head space analysis combined with EPR spectroscopy to show that NO is not an inert probe: iron(II)–BFR catalyses the reduction of NO to N2O, resulting in oxidation of iron(II) at the dinuclear centre and the subsequent detection of mononuclear iron(III). In the presence of excess reductant (sodium ascorbate), iron(II)–BFR also catalyses the reduction of NO to N2O, giving rise to three mononuclear iron–nitrosyl species which are detectable by EPR. One of these, a dinitrosyl–iron complex of S = ½, present at a maximum of one per subunit, is shown by EPR studies of site-directed variants of BFR not to be located at the dinuclear centre. This is consistent with a proposal that the diferric form of the centre is unstable and breaks down to form mononuclear iron species.

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