μ-1,2-Peroxobridged di-iron(III) dimer formation in human H-chain ferritin

Abstract

Biomineralization of the ferritin iron core involves a complex series of events in which H2O2 is produced during iron oxidation by O2 at a dinuclear centre, the ‘ferroxidase site’, located on the H-subunit of mammalian proteins. Rapid-freeze quench Mössbauer spectroscopy was used to probe the early events of iron oxidation and mineralization in recombinant human ferritin containing 24 H-subunits. The spectra reveal that a μ-1,2-peroxodiFe(III) intermediate (species P) with Mössbauer parameters δ (isomer shift) = 0.58mm/s and ΔEQ (quadrupole splitting) = 1.07mm/s at 4.2K is formed within 50ms of mixing Fe(II) with the apoprotein. This intermediate accounts for almost all of the iron in the sample at 160ms. It subsequently decays within 10s to form a μ-oxodiFe(III)—protein complex (species D), which partially vacates the ferroxidase sites of the protein to generate Fe(III) clusters (species C) at a reaction time of 10min. The intermediate peroxodiFe(III) complex does not decay under O2-limiting conditions, an observation suggesting inhibition of decay by unreacted Fe(II), or a possible role for O2 in ferritin biomineralization in addition to that of direct oxidation of iron(II).