Ferroxidation

Figure 18 Fe(II) ferroxidation and Fe(lll) holding sites at the T1 Cu site inhCp. (Reprinted from Ref. 59. 1997 Society of Biological Inorganic Chemistry)...

During the ferroxidation reaction, a blue color with an absorption maximum of 650 run appears. This persists in oxygen-limited conditions and decays as iron oxidation proceeds. " In frog H-chain ferritin, resonance Raman studies indicate a similar absorption is associated with an Fe(III)-tyrosinate. Harrison and Treffty have considered these and other studies and attribute the transient color to formation of a /x-l,2-peroxodiferric intermediate, which decays to a more stable /x-oxodiferric species as occurs in methane monooxygenase, ribonucleotide reductase, and model compounds. Protein radicals distinct from reactive oxygen species have been observed that have been attributed to damage caused by Fenton chemistry. ... 

The mechanism of iron oxidation and core formation in Dps differs from that of ferritin and bacterioferritin in that the rate of minerahzation is faster than oxidation at the ferroxidase center in L. innocua ferritin and E. coli Dps core formation is 60% faster than the ferroxidation reaction. ... 

Post-oxidation ferritin species in H-type ferritins (Fe(III)-oxo dimers/trimers) appear to be translocation intermediates trapped in the protein coat by using rapid mixing freeze quenching (milliseconds) and small amounts of iron (average 1.5/ subunit). (Ferroxidation sites in H-type ferritins are in the center of the four-helix bundle of the subunits, based on mutagenesis studies [2]). Under the same conditions, Fe(III) in L ferritins reaches the cavity immediately (polynuclear Fe) (B. H. Huynh and E. C. Theil, unpublished results). Thus, H-type ferritins have rapid oxidation, followed by slow (multi-site ) translocation to the cavity. In contrast, ferroxidation in L-type ferritins is slow, but Fe(IIl) is rapidly (simultaneously ) translocated to the cavity. 

Studies on the H-type animal ferritins to elucidate the mechanism of Fe oxidation show that the ferroxidase center catalytically converts two Fe and one dioxygen to a diferric oxo/hydroxo mineral precursor and hydrogen peroxide via a diferric peroxo intermediate.Overall these reactions can be summarized in Equations (1) and (2), where represents the apoprotein of net charge z, [Fe2-Pf " a diferrous ferroxidase complex, and [Fe20(0H)2-P] a hydrolyzed /u-oxo-bridged iron(III) complex at the same site. Thus in the ferroxidation reaction hydrogen peroxide is produced from the two-electron reduction of dioxygen at the diiron site (Equation... 

---