Variable features of ferroxidation and translocation

The initial complexes of Fe(III) and (apo)-ferritin differ in spectral properties and rate of formation. H-type ferritins form transient blue or piiik Fe(III)-species (absorbance maximum over the range 650-550 nm) with different kinetics [40, 42, 43] and Fe(III)-oxy species which absorb over the range 350-450 nm [42, 43], L-type ferritins form only the Fe(III)-oxo species, which are indistinguishable from the mineral, preventing the measurement of decay rates by UV-visible spectroscopy. 

Rates of Fe binding/ oxidation by recombinant H and L ferritins differ over a 1000-fold. The L type of ferritin protein forms polynuclear complexes, as soon as the iron is oxidized, that are indistinguishable from the mineral (B. H. Huynh and E. C. Theil, unpublished results), whereas the H type ferritin proteins form a series of ferric intermediates that include a diferric-peroxo as the first product. The di-ferric peroxo species is similar to complexes that form in methane monoxygenase and ribonucleotide reductase (see Chapter 16). Thus, the Fe - - O2 inorganic chemistry 

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same conditions, Fe(III) in L ferritins reaches the cavity immediately (polynuclear Fe) (B. H. Huynh and E. C. Theil, unpublished results).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. 

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