Iron oxidation and hydrolysis reactions of a novel ferritin from Listeria innocua

Iron deposition in the unusual 12-subunit ferritin from the bacterium Listeria innocua proceeds in three phases: a rapid first phase in which Fe2+ binds to the apoprotein, P-Z of charge Z, according to the postulated reaction 2Fe(2+) + P-Z --> [Fe-2-P](Z+2) + 2H(+), where [Fe-2-P](Z+2) represents a dinuclear iron(II) complex formed at each of the 12 ferroxidase centres of the protein; a second phase corresponding to oxidation of this putative complex, i.e. [Fe-2-P](Z+2) + O-1/2(2) --> [Fe2O-P](Z) + 2H(+); and a third phase of iron(II) oxidation/mineralization, i.e. 4Fe(2+) + O-2 + 8H(2)O --> 8FeOOH((s)) + 8H(+) [where FeOOH(s) represents the hydrous ferric oxide mineral that precipitates from the solution], which occurs when iron is added in excess of 24Fe(2+)/protein. In contrast with other ferritins, the ferroxidation reaction in L. innocua ferritin proceeds more slowly than the oxidation/mineralization reaction. Water is the final product of dioxygen reduction in the 12-subunit L. innocua ferritin (the present work) and in the 24-subunit Escherichia coli bacterioferritin, whereas H2O2 is produced in 24-subunit mammalian ferritins. Possible reasons for this difference are discussed.