H-chain ferritin

This is done a contre coeur, since the L-chain sequence was the first to be established. However, it makes little sense to refer to the amino terminus of H-chain ferritins as —1, —2, etc. 

The detailed high-resolution structure of the non-haem-containing ferritin of E. coli EcFTNA has been recently published (Stillman et al, 2000) and shows considerable structural similarity to human H-chain ferritin (r.m.s. deviation of main chain... 

Several binding sites for Tb3+ or Cd2+ ions have been identified in the interior of the apoferritin protein shell, some of which may be iron-binding sites (Harrison et ai, 1989 Granier et ah, 1998). In HoSF and HoLF, two sites were identified on the inner surface of the B helix at the subunit dimer interface (Figure 6.15, Plate 11) which bind two Cd2+ ions. One involves Glu-57 and Glu-60 as ligands and the other Glu-61 and Glu-64 (Granier et al., 1998). In H-chain ferritins the first pair of Glu-57 and Glu-60 are both replaced by His and only a single Tb3+ is found bound to Glu-61 and Glu-64 (Lawson et al, 1991). 

Figure 19.1 (a) View of the 24-subunit structure of human H-chain ferritin (rHuHF) viewed down the four-fold symmetry axis, (b) The subunit of rHuHF, with the short E-helix at the top of the four-helix bundle, (c) rHuHF viewed down the three-fold symmetry axis. (From Lewin et al., 2005. Copyright with permission from The Royal Society of Chemistry, 2005.)... 

Scheme 2.12 Representations of the structure of human H chain ferritin viewed down the four- and three-fold symmetry axes (left and right respectively). The interactions about the fourfold axis are at 90° and are based on four subunit tetramers. Protein trimers arranged at 60 degrees to one another form the three-fold axis. (Reproduced with permission from Reference 20).

Ferrous iron binds to H-chain ferritin with the release of 0.25H+ per Fe(II) ion. Iron oxidation then takes place via a combination of three pathways, with the proportions of each dependent on the amount of iron added. At low iron loading (<50 Fe per H-chain homopolymer, that is, less than required to saturate the ferroxidase centers) the dominant reaction is at the ferroxidase center (equation 4) ... 

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. ... 

In L-chain subunits, the ferroxidase center present in the H-chain is absent the space occupied by it in H-chain ferritin has a salt bridge between K62 and El07 that stabilizes the protein. Nevertheless, L-chain homopolymers can accumulate... 

Fe(III) particles through oxidation of Fe(n) at sites different from the intra-subunit ferroxidase centers of H-chain ferritins. These sites involve glutamic acid residues 53, 56, 57 and 60, and face the inner cavity of the molecule. Because ferritins rich in 1-chains oxidize iron more slowly than H-chain ferritins, they form iron particles of greater average size, crystallinity, and magnetic ordering. ... 

Oxidation of iron by EcFtnA produces a blue-colored intermediate with an absorption maximum at 650 nm and a shoulder at 370mn. This intermediate in human H-chain ferritin corresponds to a diferric peroxo species and is likely to be the same for EcFtnA, as mutation of Y24 has indicated that it caimot be an iron-tyrosinate. ... 

A significant difference between BFR and H-chain ferritins is the mechanism of iron uptake and core formation. In H-chain ferritins, initial iron uptake takes place by ferrous iron being oxidized at the ferroxidase center and moving into the core, and once a sizeable core has been established, autooxidation at the core surface takes over from the ferroxidase center reaction (see Section 7.2). In BFR, iron uptake takes place in three distinct phases. Phase I is the binding of two ferrous ions per ferroxidase center (48 per protein). Phase 2 corresponds to rapid oxidation at the ferroxidase center according to reactions (7) and (8), where z represents the charge on the protein. ... 

The next example shows how to choose a kinetic model. The unusual kinetic curves observed by stopped-flow spectrophotometry have been successfully simulated for all levels of iron loading of human H-chain ferritin. The presence of an additional intermediate, postulated to be a hydroperoxo diFe(III) complex, was suggested by a detailed analysis of the kinetic data of 48Fe/protein samples. A mathematical model for catalysis was developed that explains the observed kinetics. The model consists of two sequential mechanisms. ... 

Toussaint, L., Cuypers, M. G., Bertrand, L., Hue, L., Romao, C. V., Saraiva, L. M., et al. (2009). Comparative Fe and Zn K-edge X-ray absorption spectroscopic study of the ferroxidase centres of human H-chain ferritin and bacterioferritin from desulfovibrio desulfuricans. Journal of Biological Inorganic Chemistry, 14, 35—49. 

Fe(II) must then move from the 12 A long channel, and traverse a further distance of about 8 A along a hydrophilic pathway from the inner side of the three-fold channel to the ferroxidase site, and a putative pathway for Fe(II) is shown in Figure 19.6. The diiron ferroxidase centre is located in the central region of the four-helix subunit bundle and the coordination geometry of the ferroxidase centre of human H-chain ferritin is shown in Figure 19.7. Detailed analysis of the ferroxidase reaction in H-chain ferritin has allowed the identification of a number of intermediates, which are illustrated in Figure 19.8. 

FIGURE 19.7 Schematic view of the di-iron ferroxidase centre on the four-heUx of the H-chain ferritin (left panel). Dinuclear ferroxidase centre diagram (right panel) showing the amino acid residues. (From Bou Abdallah, 2010. Copyright 2010 with permission from Elsevier.)... 

As with H-chain ferritins, the three-dimensional structure of BFR results in channels through the protein at the symmetry axes. However, while the hydrophilic threefold channels... 

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