Ferroxidase center

Tatur, J. and Hagen, W.R. 2005. The dinuclear iron-oxo ferroxidase center of Pyrococcus furiosus ferritin is a stable prosthetic group with unexpectedly high reduction potentials. FEBS Letters 579 4729 1732. 

Kauko, A., Pullianen, A.T., Haataja, S., Mayer-Klauke, W., Finne, J. and Papageorghiou, A.C. (2006) Iron incorporation in Streptococcus suis Dps-like peroxide resistance protein Dpr requires mobility in the ferroxidase center and leads to the formation of a ferrihydrite-like core, J. Mol. Biol., 364, 97-109. 

Antitumor drugs cisplatin as, history, 37 175-179 platinum compounds future studies, 37 206-208 resistance to, 37 192-193 second-generation, 37 178 Antiviral agents, 36 37-38 AOR, see Aldehyde oxidoreductase Aphanothece sacrum, ferredoxins, amino acid sequence, 38 225-227 Apo-calcylin, 46 455 Apo-caldodulin, 46 449-450 Apoenzyme, 22 424 Apoferritin biosynthesis, 36 457 cystalline iron core, 36 423 Fe(III)distribution, 36 458-459 Fe(II) sequestration, 36 463-464 ferroxidase centers, 36 457-458 iron core reconstruction in shell, 36 457 mineralization, 36 25 Mdssbauer spectra, 36 459-460 optical absorbance spectra, 36 418-419 subunit conformation and quaternary structure, 36 470-471... 

Figure 16-4 The dinuclear iron center or ferroxidase center of human ferritin based on the structure of a terbium(III) derivative.73 Courtesy of Pauline Harrison.

Each ferritin H-chain subunit possesses a conserved dinuclear iron site known as the ferroxidase center, which catalyzes iron oxidation. Residues E27, E61, E62, H65, and E107 act as metal hgands while residues Y34 and Q144 form hydrogen bonds (the numbering refers to the human H-chain sequence) (see Figure 6). 

Figure 6 Schematic model of the hinnan H-chain ferroxidase center based on the X-ray structure of Tb(III)-containing ferritin ...

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) ... 

The oxidized iron travels from the ferroxidase center to the core, thus regenerating the ferroxidase center for finther reactions. When large amounts of iron are added (>800 Fe per protein), following oxidation of the first two ferrous ions per subunit an autooxidation reaction catalyzed by the core surface takes over and becomes the predominant mechanism (equation 5) ... 

The two iron-binding sites at the dinuclear ferroxidase center are not identical. Mutagenesis of residues E27 (in site A) and El 07 (site B) shows that site A can bind iron independently but requires site B for iron oxidation. In contrast, iron binding in site B is greatly decreased in the absence of site A, whereas oxidation is unaffected. ... 

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. ... 

Figure 7 Schematic model of EcFtnA ferroxidase center including site C based on crystal structure ...

Figure 8 The strucmre of the BFR subunit dimer showing heme boimd at monomer interface by M52 residues. The iron atoms in the ferroxidase centers are shown as black spheres. Figure created using pdb coordinates Ibcf ...

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. ... 

Figure 9 Schematic model of BFR ferroxidase center based on the crystal structure of iron-hoimd BFR from D. desulfuricans (EcBFR numhermg). The bridging ligand, shown here as a /r-oxo group, cannot he determined in the structure hut there is sufficient electron density for a /r-oxo, /r-hydroxo, H2O and even peroxo group ...

Note that reaction (9) indicates a pairwise reaction within subunit dimers rather than individual ferroxidase centers reacting separately. Although reaction (7) indicates the formation of hydrogen peroxide, no diferric peroxo intermediate has been observed for BFR. This may be because such a species is not formed, or, more likely, because such an intermediate is too short lived or has too low an extinction coefficient to be detected. ... 

Following reaction (9), the ferric iron remains at the ferroxidase center as a /x-oxo bridged species instead of moving into the core and thus regenerating the ferroxidase center. The addition of more than 48 ferrous ions per protein saturates the ferroxidase reaction and any further iron is oxidized within the core (phase 3). This process is significantly slower than phase 2. Core formation occurs according to reaction (10) below. 

Dps has ferroxidase activity and has 12 ferroxidase centers, two situated at each subunit dimer interface. The iron ligands are donated from both subunits H31, H43 and E47 from one subunit and D58 and E62 from the other. The iron-bound ferroxidase center of the Dps from Bacillus brevis is shown in Figure 12. 

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. ... 

Mineralization in apoferritin involves a prior oxidation of Fe(II) as it enters channels in the protein shell. The ferroxidase center seems to be composed of Glu (Gin) and His residues situated between four helices (P. M. Harrison, personal communication). There is scope for exploring the design of agents that could block the entry of iron into the core of the protein or hasten its passage out. It is possible that non-redox-active metal ions such as Ga(III), In(III), and Al(III) can act in this way. The nature of the Fe(II) complex in the cytoplasm, which acts as a donor to ferritin, is not clear yet, but perhaps it could be Fe(II) glutathione. 

Stefanini et al. 128) have also attempted to identify which carboxyl-ates form the ferroxidase center, using horse spleen ferritin. Their study, which involved the application of various spectroscopic techniques to chemically modified ferritin, identified Asp 127 and Glu 130, groups present in the threefold channel, as the ferroxidase center. However, studies by Treffry et al. 141) of site-directed mutants of human heavy-chain ferritins indicate that the ferroxidase center is not in the threefold channels. 

The presence on apoferritin of ferroxidase centers is suggested by four observations (1) that tbe initial oxidation step when Fe(II) is added to apoferritin requires a specific oxidant, dioxygen (48Y, (2) that Fe(III) can be produced from Fe(II) in the presence of apoferritin—and intercepted by transferrin under conditions in wbicb hydrolysis is kept... 

There may be cellular conditions under which Fe(II) binding becomes significant, but the highly conserved nature of ligands associated with the ferroxidase center seems to emphasize the importance of oxidative iron storage mechanisms. 

Fig 11. Stereo ribbon diagram of part of the outer surface of a molecule of human rHF. The positions of ferroxidase centers are shown as spheres. Note that they occupy roughly central positions within the subunit. 

Glu 27, Glu 62, and His 65. In human and horse L ferritin these residues are replaced, respectively, by Tyr, Lys and Gly and in rat L ferritin, by His, Lys, and Gly. Examination of the sequences of ferritins given in Table VI shows that residues Glu 27, Glu 62, His 65, and the nearby Glu 107 and Gin 141 that make hydrogen bonds to a metal-coordinated water are conserved in H chains of human, rat, and chicken. They are also found in tadpole H and M subunits and the two sequences of Schistosoma ferritin (76) (which were designated H on the basis of their greater similarity to H than to L chains). Thus the ferroxidase center seems to be a property of primitive H chains as well as of mammalian H ferritins. The tadpole L chain (18) does not have these residues conserved, although it has 61% identity with human H chain and only 49% with human L and is also more similar to rat H chains (63%) than L chain (49%). It has residues Lys 27, Gin 107, and Ser 141 in place of Glu 27, Glu 107, and Gin 141 and hence would not be expected to show activity. 

Fig 12. Schematic diagram of the ferroxidase center of human rHF (a), and the equivalent region of horse light-chain ferritin (b) showing the ferroxidase site replaced by a salt bridge. 

Can any of the spectroscopically characterized Fe(III) atoms be identified with iron at the ferroxidase center EPR (56, 60), Mbssbauer (57), and UV-difference (51) spectroscopy suggest that the first iron oxidized is at isolated sites, but that the Fe(III) then migrates to clus-... 

An alternative route through the shell has been found in rHF. There appears to be a narrow passage (or one-fold channel) in the subunit leading directly to the ferroxidase center 94, 115). This would give a pathway of about 12 A to this site compared with over 50 A via the three-fold channels. However, the route or routes by which Fe(II) reaches the ferroxidase center remains uncertain. In L ferritins, in which the one-fold channel is blocked, entry into the cavity may be via the three-fold channels. 

A means of shepherding Fe ions formed at the ferroxidase center into the cavity is suggested by crystallographic analysis of the Tb +... 

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