Direct observation of the iron binding sites in a ferritin

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Abstract
X‐Ray analysis of the ferritin of Escherichia coli (Ec‐FTN) and of Ec‐FTN crystals soaked in (NH4)2Fe(SO4)2 has revealed the presence of three iron‐binding sites per subunit. Two of these form a di‐iron site in the centre of the subunit as has been proposed for the ‘ferroxidase centres’ of human ferritin H chains. This di‐iron site, lying within the 4‐alpha‐helix bundle, resemble those of ribonucleotide reductase, methane monoxygenase and haemerythrin. The third iron is bound by ligands unique to Ec‐FTN on the inner surface of the protein shell. It is speculated that this state may represent the nucleation centre of a novel type of Fe(III) cluster, recently observed in Ec‐FTN.