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

Biomineralization of the ferritin iron core involves a complex series of events in which 11.0, is produced during iron oxidation by 0, at a dinuclear centre, the 'ferroxidase site', located on the H-subunit of mammalian proteins. Rapid-freeze quench Mossbauer 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 mu-1,2peroxodiFe(III) intermediate (species P) with Mossbauer parameters 8 (isomer shift) = 0.58 mm/s and DeltaE(Q) (quadrupole splitting) = 1.07 mm/s at 4.2 K is formed within 50 ms of mixing Fe(II) with the apoprotein, This intermediate accounts for almost all of the iron in the sample at 160 ms. It subsequently decays within 10 s to form a mu-oxodiFe(III) -protein complex (species D), which partially vacates the ferroxidase sites of the protein to generate Fe(III) clusters (species Q at a reaction time of 10 min. The intermediate peroxodiFe(III) complex does not decay under O-2-limiting conditions, an observation suggesting inhibition of decay by unreacted Fe(II), or a possible role for 0, in ferritin biomineralization in addition to that of direct oxidation of iron(II).