Stages in iron storage in the ferritin of Escherichia coli (EcFtnA): Analysis of Mossbauer spectra reveals a new intermediate

Iron uptake into the nonheme ferritin of Escherichia coli (EcFtnA) and its site-directed variants have been investigated by Mossbauer spectroscopy. EcFtnA, like recombinant human H chain ferritin (HuHF), oxidized Fe(II) at a dinuclear ferroxidase center situated at a central position within each subunit. As with HuHF, Mossbauer subspectra observed between 1 min and 24 h after Fe(II) addition were assigned to Fe(III) monomers, 'c', μ-oxo-bridged dimers, 'b', and clusters, 'a', the latter showing magnetically split spectra, 'd', at 4.1 K. Like those of HuHF, the μ-oxo-bridged dimers were formed at the ferroxidase centers. However, the analysis also revealed the presence of a new type of dimer, 'e' (QS₁ = 0.38 mm/s, IS₁ = 0.51 mm/s and QS₂ = 0.72 mm/s, IS₂ = 0.50 mm/s), and this was also assigned to the ferroxidase center. Dimers 'b' appeared to be converted to dimers 'e' over time. Subspectra 'e' became markedly asymmetric at temperatures above 90 K, suggesting that the two Fe(III) atoms of dimers 'e' were more weakly coupled than in the μ-oxo-bridged dimers 'b', possibly due to OH^- bridging. Monomeric Fe(III), giving relaxation spectra 'c', was assigned to a unique site C that is near the dinuclear center. In EcFtnA all three iron atoms seemed to be oxidized together. In contrast to HuHF, no Fe(III) clusters were observed 24 h after the aerobic addition of 48 Fe(II) atoms/molecule in wild- type EcFtnA. This implies that iron is more evenly distributed between molecules in the bacterial ferritins, which may account for its greater accessibility.