The use of zinc(II) to probe iron binding and oxidation by the ferritin (EcFtnA) of Escherichia coli

The ferritin of Escherichia coli (EcFtnA) is similar to human H-chain ferritin (HuHF) in having 24 subunits, each containing a dinuclear site at which two iron atoms can be oxidised (the diiron centre). In EcFtnA, unlike HuHF, fluorescence quenching of Trp122, located near site A of the dinuclear centre, can be used to monitor metal binding (this tryptophan is absent from HuHF). Metal binding also perturbs the UV absorbance spectrum of Trp122 and that of Tyr24 (a conserved residue near site B of the dinuclear centre). Using UV-difference spectroscopy and fluorescence quenching it is shown that Fe(II) and Zn(II) bind at the same sites, A and B. Sequential stopped-flow studies of Fe(II) binding and oxidation also show that Zn(II) is an effective competitor of Fe(II) binding and an inhibitor of its oxidation.