Spectroscopic evidence for the presence of a high-valent Fe(IV) species in the ferroxidase reaction of an archaeal ferritin.
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ABSTRACT:
SUBMITTER: Honarmand Ebrahimi K
PROVIDER: S-EPMC5519934 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Ferritins are ubiquitous and can be found in practically all organisms that utilize Fe. They are composed of 24 subunits forming a hollow sphere with an inner cavity of ~80 Å in diameter. The main function of ferritin is to oxidize the cytotoxic Fe(2+) ions and store the oxidized Fe in the inner cavity. It has been established that the initial step of rapid oxidation of Fe(2+) (ferroxidation) by H-type ferritins, found in vertebrates, occurs at a diiron binding center, termed the ferroxidase center. In bacterial ferritins, however, X-ray crystallographic evidence and amino acid sequence analysis revealed a trinuclear Fe binding center comprising a binuclear Fe binding center (sites A and B), homologous to the ferroxidase center of H-type ferritin, and an adjacent mononuclear Fe binding site (site C). In an effort to obtain further evidence supporting the presence of a trinuclear Fe binding center in bacterial ferritins and to gain information on the states of the iron bound to the trinuclear center, bacterial ferritin from Desulfovibrio vulgaris (DvFtn) and its E130A variant was loaded with substoichiometric amounts of Fe(2+), and the products were characterized by Mössbauer and EPR spectroscopy. Four distinct Fe species were identified: a paramagnetic diferrous species, a diamagnetic diferrous species, a mixed valence Fe(2+)Fe(3+) species, and a mononuclear Fe(2+) species. The latter three species were detected in the wild-type DvFtn, while the paramagnetic diferrous species was detected in the E130A variant. These observations can be rationally explained by the presence of a trinuclear Fe binding center, and the four Fe species can be properly assigned to the three Fe binding sites. Further, our spectroscopic data suggest that (1) the fully occupied trinuclear center supports an all ferrous state, (2) sites B and C are bridged by a μ-OH group forming a diiron subcenter within the trinuclear center, and (3) this subcenter can afford both a mixed valence Fe(2+)Fe(3+) state and a diferrous state. Mechanistic insights provided by these new findings are discussed and a minimal mechanistic scheme involving O-O bond cleavage is proposed.
| S-EPMC3390943 | biostudies-literature
Project description:Maxi ferritins, 24 subunit protein nanocages, are essential in humans, plants, bacteria, and other animals for the concentration and storage of iron as hydrated ferric oxide, while minimizing free radical generation or use by pathogens. Formation of the precursors to these ferric oxides is catalyzed at a nonheme biferrous substrate site, which has some parallels with the cofactor sites in other biferrous enzymes. A combination of circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD (VTVH MCD) has been used to probe Fe(II) binding to the substrate active site in frog M ferritin. These data determined that the active site within each subunit consists of two inequivalent five-coordinate (5C) ferrous centers that are weakly antiferromagnetically coupled, consistent with a mu-1,3 carboxylate bridge. The active site ligand set is unusual and likely includes a terminal water bound to each Fe(II) center. The Fe(II) ions bind to the active sites in a concerted manner, and cooperativity among the sites in each subunit is observed, potentially providing a mechanism for the control of ferritin iron loading. Differences in geometric and electronic structure--including a weak ligand field, availability of two water ligands at the biferrous substrate site, and the single carboxylate bridge in ferritin--coincide with the divergent reaction pathways observed between this substrate site and the previously studied cofactor active sites.
| S-EPMC2531225 | biostudies-literature
Project description:It is widely accepted that iron deposition in the iron storage protein ferritin in vitro involves Fe(II) oxidation, and that ferritin facilitates this oxidation at a ferroxidase site on the protein. However, these views have recently been questioned, with the protein ferroxidase activity instead being attributed to autoxidation from the buffer alone. Ligand exchange between another protein with ferroxidase activity and ferritin has been proposed as an alternative mechanism for iron incorporation into ferritin. In the present work, a pH stat apparatus is used to eliminate the influence of buffers on iron(II) oxidation. Here we show that the recent experiments questioning the ferroxidase activity of ferritin were flawed by inadequate pH control, that buffers actually retard rather than facilitate iron(II) oxidation, and that horse spleen ferritin has ferroxidase activity when measured under proper experimental conditions. Furthermore, high pH (7.0), a high Fe(II) concentration and the presence of Fe(III) all favour Fe(II) autoxidation in the presence or absence of ferritin.
| S-EPMC1220094 | biostudies-other
Project description:Ferritin is the main protein of Fe storage in eukaryote and prokaryote cells. It is a large multifunctional, multi-subunit protein consisting of heavy H and light L subunits. In the field of nuclear toxicology, it has been suggested that some actinide elements, such as thorium and plutonium at oxidation state +IV, have a comparable `biochemistry' to iron at oxidation state +III owing to their very high tendency for hydrolysis and somewhat comparable ionic radii. Therefore, the possible mechanisms of interaction of such actinide elements with the Fe storage protein is a fundamental question of bio-actinidic chemistry. We recently described the complexation of Pu(IV) and Th(IV) with horse spleen ferritin (composed mainly of L subunits). In this article, we bring another viewpoint to this question by further combining modeling with our previous EXAFS data for Pu(IV) and Th(IV). As a result, the interaction between the L subunits and both actinides appears to be non-specific but driven only by the density of the presence of Asp and Glu residues on the protein shell. The formation of an oxyhydroxide Th or Pu core has not been observed under the experimental conditions here, nor the interaction of Th or Pu with the ferric oxyhydroxide core.
| S-EPMC8733997 | biostudies-literature
Project description:The apparent Sc(3+) adduct of [Fe(IV)(O)(TMC)](2+) (1, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) has been synthesized in amounts sufficient to allow its characterization by various spectroscopic techniques. Contrary to the earlier assignment of a +4 oxidation state for the iron center of 1, we establish that 1 has a high-spin iron(III) center based on its Mössbauer and EPR spectra and its quantitative reduction by 1 equiv of ferrocene to [Fe(II)(TMC)](2+). Thus, 1 is best described as a Sc(III)-O-Fe(III) complex, in agreement with previous DFT calculations (Swart, M. Chem. Commun. 2013, 49, 6650.). These results shed light on the interaction of Lewis acids with high-valent metal-oxo species.
| S-EPMC4939224 | biostudies-literature
Project description:Treatment of cis-Me2Fe(PMe3)4 with di-1,2-(E-2-(pyridin-2-yl)vinyl)benzene ((bdvp)H2), a tetradentate ligand precursor, afforded (bdvp)Fe(PMe3)2 (1-PMe3) and 2 equiv. CH4, via C-H bond activation. Similar treatments with tridentate ligand precursors PhCH 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 NCH2(E-CHCHPh) ((pipp)H2) and PhCHN(2-CCMe-Ph) ((pipa)H) under dinitrogen provided trans-(pipp)Fe(PMe3)2N2 (2) and trans-(pipvd)Fe(PMe3)2N2 (3), respectively; the latter via one C-H bond activation, and a subsequent insertion of the alkyne into the remaining Fe-Me bond. All three Fe(ii) vinyl species were protonated with H[BArF4] to form the corresponding Fe(iv) alkylidene cations, [(bavp)Fe(PMe3)2][BArF4] (4-PMe3), [(piap)Fe(PMe3)3][BArF4] (5), and [(pipad)Fe(PMe3)3][BArF4] (6). Mössbauer spectroscopic measurements on the formally Fe(ii) and Fe(iv) derivatives revealed isomer shifts within 0.1 mm s-1, reflecting the similarity in their bond distances.
| S-EPMC5667503 | biostudies-literature
Project description:Previous efforts to model the diiron(IV) intermediate Q of soluble methane monooxygenase have led to the synthesis of a diiron(IV) TPA complex, 2, with an O=Fe(IV)-O-Fe(IV)-OH core that has two ferromagnetically coupled Sloc = 1 sites. Addition of base to 2 at -85 °C elicits its conjugate base 6 with a novel O═Fe(IV)-O-Fe(IV)═O core. In frozen solution, 6 exists in two forms, 6a and 6b, that we have characterized extensively using Mössbauer and parallel mode EPR spectroscopy. The conversion between 2 and 6 is quantitative, but the relative proportions of 6a and 6b are solvent dependent. 6a has two equivalent high-spin (Sloc = 2) sites, which are antiferromagnetically coupled; its quadrupole splitting (0.52 mm/s) and isomer shift (0.14 mm/s) match those of intermediate Q. DFT calculations suggest that 6a assumes an anti conformation with a dihedral O═Fe-Fe═O angle of 180°. Mössbauer and EPR analyses show that 6b is a diiron(IV) complex with ferromagnetically coupled Sloc = 1 and Sloc = 2 sites to give total spin St = 3. Analysis of the zero-field splittings and magnetic hyperfine tensors suggests that the dihedral O═Fe-Fe═O angle of 6b is ∼90°. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes 6b, facilitating protonation of one oxo group to regenerate 2, a protonation step difficult to achieve for mononuclear Fe(IV)═O complexes. Complex 6 represents an intriguing addition to the handful of diiron(IV) complexes that have been characterized.
| S-EPMC3941033 | biostudies-literature
Project description:At least three ferritins are found in the bacterium Escherichia coli : the heme-containing bacterioferritin (EcBFR) and two nonheme bacterial ferritins (EcFtnA and EcFtnB). In addition to the conserved A and B sites of the diiron ferroxidase center, EcFtnA has a third iron-binding site (the C site) of unknown function that is nearby the diiron site. In the present work, the complex chemistry of iron oxidation and deposition in EcFtnA was further defined through a combination of oximetry, pH stat, stopped-flow and conventional kinetics, UV-vis, fluorescence, and EPR spectroscopic measurements on both the wild-type protein and site-directed variants of the A, B, and C sites. The data reveal that although H2O2 is a product of dioxygen reduction in EcFtnA and oxidation occurs with a stoichiometry of Fe(2+)/O2 ? 3:1 most of the H2O2 produced is consumed in subsequent reactions with a 2:1 Fe(2+)/H2O2 stoichiometry, thus suppressing hydroxyl-radical formation. Although the A and B sites are essential for rapid iron oxidation, the C site slows oxidation and suppresses iron turnover at the ferroxidase center. A tyrosyl radical, assigned to Tyr24 near the ferroxidase center, is formed during iron oxidation, and its possible significance to the function of the protein is discussed. Taken as a whole, the data indicate that there are multiple iron-oxidation pathways in EcFtnA with O2 and H2O2 as oxidants. Furthermore, our data do not support a universal mechanism for iron oxidation in all ferritins whereby the C site acts as transit site, as has been recently proposed.
| S-EPMC3951517 | biostudies-literature
Project description:Many advanced oxidation processes (AOPs) use Fenton-like reactions to degrade organic pollutants by activating peroxymonosulfate (HSO5-, PMS) or peroxydisulfate (S2O82-, PDS) with Fe(H2O)62+ (FeaqII). This paper presents results on the kinetics and mechanisms of reactions between FeaqII and PMS or PDS in the absence and presence of bicarbonate (HCO3-) at different pH. In the absence of HCO3-, FeaqIV, rather than the commonly assumed SO4•-, is the dominant oxidizing species. Multianalytical methods verified the selective conversion of dimethyl sulfoxide (DMSO) and phenyl methyl sulfoxide (PMSO) to dimethyl sulfone (DMSO2) and phenyl methyl sulfone (PMSO2), respectively, confirming the generation of FeaqIV by the FeaqII-PMS/PDS systems without HCO3-. Significantly, in the presence of environmentally relevant concentrations of HCO3-, a carbonate radical anion (CO3•-) becomes the dominant reactive species as confirmed by the electron paramagnetic resonance (EPR) analysis. The new findings suggest that the mechanisms of the persulfate-based Fenton-like reactions in natural environments might differ remarkably from those obtained in ideal conditions. Using sulfonamide antibiotics (sulfamethoxazole (SMX) and sulfadimethoxine (SDM)) as model contaminants, our study further demonstrated the different reactivities of FeaqIV and CO3•- in the FeaqII-PMS/PDS systems. The results shed significant light on advancing the persulfate-based AOPs to oxidize pollutants in natural water.
| S-EPMC10134498 | biostudies-literature
Project description:The gene encoding the cyanobacterial ferritin SynFtn is up-regulated in response to copper stress. Here, we show that, while SynFtn does not interact directly with copper, it is highly unusual in several ways. First, its catalytic diiron ferroxidase center is unlike those of all other characterized prokaryotic ferritins and instead resembles an animal H-chain ferritin center. Second, as demonstrated by kinetic, spectroscopic, and high-resolution X-ray crystallographic data, reaction of O2 with the di-Fe2+ center results in a direct, one-electron oxidation to a mixed-valent Fe2+/Fe3+ form. Iron-O2 chemistry of this type is currently unknown among the growing family of proteins that bind a diiron site within a four α-helical bundle in general and ferritins in particular. The mixed-valent form, which slowly oxidized to the more usual di-Fe3+ form, is an intermediate that is continually generated during mineralization. Peroxide, rather than superoxide, is shown to be the product of O2 reduction, implying that ferroxidase centers function in pairs via long-range electron transfer through the protein resulting in reduction of O2 bound at only one of the centers. We show that electron transfer is mediated by the transient formation of a radical on Tyr40, which lies ∼4 Å from the diiron center. As well as demonstrating an expansion of the iron-O2 chemistry known to occur in nature, these data are also highly relevant to the question of whether all ferritins mineralize iron via a common mechanism, providing unequivocal proof that they do not.
| S-EPMC6369749 | biostudies-literature