Metal binding at the active centre of the ferritin of Escherichia coli (EcFtnA). A Mössbauer spectroscopic study

Iron storage in the ferritin of Escherichia coli (EcFtnA) involves (1) the catalytic oxidation of 2Fe(II) at a dinuclear centre (sites A and B) and of a single Fe(II) at a nearby site C; (2) the formation of oxo-bridged Fe(III) dimers (designated ‘b’) at A and B and isolated Fe3+ ions (‘c’) at site C; (3) the conversion of the oxo-bridged dimers to a second dimer form (‘e’) which is probably hydroxo-bridged; (4) the movement of iron into the iron storage cavity where Fe(III) ferrhydrite clusters (‘a’ and ‘d’) form, and (5) the provision on the surfaces of the mineral clusters of alternative sites for further Fe(II) oxidation as well as for the addition of Fe(III) from the dinuclear centres. Each of the iron species gives rise to a characteristic Mössbauer subspectrum and Mössbauer spectroscopy has been used to study the early stages of iron storage with the aid of competing metal ions Zn2+ and Tb3+. The effects of adding these metal ions both before and after Fe(II) have been examined with both wild type EcFtnA and its site directed variants. These studies have provided evidence that both types of Fe(III) dimers are located at the dinuclear centre and that with time dimers ‘b’ are replaced by dimers ‘e’. The effects of the competing metals are different: Zn(II) has a lower affinity than Fe(III) for site C and does not displace it from C but is able to compete with both Fe(III) and Fe(II) for at least one of the dimer sites. Tb(III) competes with Fe(III) for site C displacing Fe(III) to the cavity to form clusters. The presence of these clusters may accelerate loss of Fe(III) from the dimer sites to the cavity.