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

Abstract

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, PZ of charge Z, according to the postulated reaction 2Fe2++PZ → [Fe2–P]Z+2+2H+, where [Fe2–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. [Fe2–P]Z+2+½O2 → [Fe2O–P]Z+2H+; and a third phase of iron(II) oxidation/mineralization, i.e. 4Fe2++O2+8H2O → 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 24Fe2+/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.