The ferritin iron entry and exit problem

The entry and exit of Fe(II) ions in ferritin are endpoints in the process which concentrates iron as a solid (hydrated ferric oxide) to be used by living cells. Ferritin is a response to the trillion fold mismatch between the solubility of iron in neutral, aqueous, aerated solutions and the requirements for protein biosynthesis. The supramolecular structure of 24 polypeptides (subunits) joined by non-covalent bonds in a highly symmetrical (4, 3, 2), large (12 nm diameter) protein with a cavity (8 nm diameter) is found in plants, animals and microorganisms. Of the five types of iron sites which can be defined, iron entry (site 1) is likely at the junction of three subunits. A ferroxidase site (site 2), present in H-type ferritins, binds Fe(II) which reacts with dioxygen to form an initial, μ-1,2 diferric peroxo complex. The peroxo complex decays into hydrogen peroxide and the multiple diferric oxo complexes that are mineral precursors (sites 3a, 3b, 3x) in transit across the protein to the cavity; the ferroxidase site is sensitive to both natural and engineered variations in Fe ligands and ‘second shell amino’ acids such as tyrosine 30 and leucine 134. Studies of ferritin with subunits which lack the ferroxidase site (L-type) show that the mineral anchor sites of clustered R-COOH (site 4) are sensitive to RCH3 and RSH replacements. Iron exit (site 5), studied by adding NADH/FMN as a trigger, is at or near the entry site. Iron exit can be enhanced by localized unfolding of the polypeptide at the junction of three subunits, suggesting a regulation-sensitive biological signal for iron exit. The entry and exit of iron to and from the mineral, unique to ferritin has steps which parallel those in channel ion transport, and ion transport to and from biomineral such as tooth and bone. Iron entry involves oxidation at non-heme iron catalytic centers, similar to ribonucleotide reductase and methane monoxygenase, but diverging in the role of iron as a substrate rather than a cofactor. Sorting out the evolutionary and mechanistic relationships of ferritin and other proteins which use metal ions, as well as fully characterizing all five types of the functional iron sites are challenges which will keep biological inorganic chemists occupied for some time to come.