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PubMed2002

Evidence for rapid microscale bacterial redox cycling of iron in circumneutral environments.

The potential for microscale bacterial Fe redox cycling was investigated in microcosms containing ferrihydrite-coated sand and a coculture of a lithotrophic Fe(II)-oxidizing bacterium (strain TW2) and a dissimilatory Fe(III)-reducing bacterium (Shewanella alga strain BrY). The Fe(II)-oxidizing organism was isolated from freshwater wetland surface sediments which are characterized by steep gradients of dissolved 02 and high concentrations of dissolved and solid-phase Fe(II) within mm of the sediment-water interface, and which support comparable numbers (10(5)-10(6) mL(-1)) of culturable Fe(II)-oxidizing and Fe(III)-reducing reducing. The coculture systems showed minimal Fe(III) oxide accumulation at the sand-water interface, despite intensive O2 input from the atmosphere and measurable dissolved O2 to a depth of 2 mm below the sand-water interface. In contrast, a distinct layer of oxide precipitates formed in systems containing Fe(IllI)-reducing bacteria alone. Examination of materials from the cocultures by fluorescence in situ hybridization indicated close physical juxtapositioning of Fe(II)-oxidizing and Fe(III)-reducing bacteria in the upper few mm of sand. Our results indicate that Fe(II)-oxidizing bacteria have the potential to enhance the coupling of Fe(II) oxidation and Fe(III) reduction at redox interfaces, thereby promoting rapid microscale cycling of Fe.

Aerobiosis↗
PubMed1984

Suboxic diagenesis in banded iron formations.

Anomalous isotopic composition has been reported for the carbon in carbonate minerals of banded iron formations. Well studied examples show and enrichment in the light isotope of carbon, 12C. This enrichment presumably reflects unusual circumstances in the deposition of these sedimentary rocks. It is suggested here that the isotopically-light carbonate results from early diagenetic oxidation by bacteria of substantial amounts of isotopically light organic carbon. The electron acceptor that permits oxidation in the absence of free oxygen is presumed to the iron(III) which may have been significantly more abundant in the initial chemical precipitate than in the post-diagenetic sedimentary rock.

Anaerobiosis↗
PubMed1990

Ferryl and hydroxy intermediates in the reaction of oxygen with reduced cytochrome c oxidase.

Cytochrome c oxidase catalyses the 4-electron reduction of dioxygen to water and translocates protons vectorially across the inner mitochondrial membrane. Proposed reaction pathways for the catalytic cycle of the O2 reduction are difficult to verify without knowing the structures of the intermediates, but we now have such information for the catalytic intermediates in the first steps of the reaction of O2 with cytochrome c oxidase from resonance Raman spectroscopy, a technique that enables iron-ligand stretching modes to be identified. Here we report on two more key intermediates: a ferryl-oxo (Fe4 = O2-) and a ferric-hydroxy (Fe3+--OH-) intermediate at the level of 3- and 4-electron reduction, respectively. We identified these intermediates by their characteristic iron-oxygen stretching frequencies (786 cm-1 for Fe4+ = O2-, and 450 cm-1 for Fe3+ -- OH-) and oxygen and deuterium isotope shifts. The oxo atom in the ferryl intermediate is hydrogen-bonded and the iron-oxygen bond in the hydroxy intermediate is anomalously weak. With the identification of the primary, ferryl and hydroxy intermediates, the predominant structures at almost all stages of O2 reduction are now known and the catalytic pathway can be described with more certainty.

Deuterium↗
PubMed2002

Geobacter metallireducens accesses insoluble Fe(III) oxide by chemotaxis.

Microorganisms that use insoluble Fe(III) oxide as an electron acceptor can have an important function in the carbon and nutrient cycles of aquatic sediments and in the bioremediation of organic and metal contaminants in groundwater. Although Fe(III) oxides are often abundant, Fe(III)-reducing microbes are faced with the problem of how to access effectively an electron acceptor that can not diffuse to the cell. Fe(III)-reducing microorganisms in the genus Shewanella have resolved this problem by releasing soluble quinones that can carry electrons from the cell surface to Fe(III) oxide that is at a distance from the cell. Here we report that another Fe(III)-reducer, Geobacter metallireducens, has an alternative strategy for accessing Fe(III) oxides. Geobacter metallireducens specifically expresses flagella and pili only when grown on insoluble Fe(III) or Mn(IV) oxide, and is chemotactic towards Fe(II) and Mn(II) under these conditions. These results suggest that G. metallireducens senses when soluble electron acceptors are depleted and then synthesizes the appropriate appendages to permit it to search for, and establish contact with, insoluble Fe(III) or Mn(IV) oxide. This approach to the use of an insoluble electron acceptor may explain why Geobacter species predominate over other Fe(III) oxide-reducing microorganisms in a wide variety of sedimentary environments.

Bacterial Proteins↗
PubMed1995

The role of iron chelators and oxygen in the reduced nicotinamide adenine dinucleotide phosphate-cytochrome P450 oxidoreductase-dependent chromium(VI) reduction.

Chromium(VI) reduction was studied in a system composed of reduced nicotinamide adenine dinucleotide phosphate-cytochrome P450 oxidoreductase (NADPH-P450 reductase) and different iron chelators and iron sources. In an aerobic phosphate buffer containing iron(II), chromium(VI) was not reduced by the Fe2+ probably because of spontaneous autoxidation of Fe2+, but freshly made Fe2+, added directly to a CrVI-containing buffer, reduced CrVI. Under anaerobic conditions, iron(II) reduced chromium(VI) stoichiometrically. A systemic containing ethylenediaminetetraacetic acid (EDTA)-Fe3+, NADPH-P450 reductase and NADPH effectively reduced chromium(VI) anaerobically. Under aerobic conditions this reaction was inhibited by about 45%. Adenosine diphosphate (ADP)-Fe3+, which is a poor acceptor of electrons from NADPH-P450 reductase, reduced chromium(VI) only marginally, Mannitol slightly increased the aerobic CrVI reduction. Addition of superoxide dismutase and catalase, which both regenerate some O2, led to inhibition of CrVI reduction. Ferritin, NADPH-P450 reductase and the iron chelators, EDTA and citrate, reduced CrVI, indicating mobilization of Fe2+ from ferritin. Low levels of EDTA (55 mumol l-1) and citrate (100 mumol l-1) in contrast to high levels (5 mmol l-1) did not increase CrVI reduction in microsomes. Using 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid buffer instead of phosphate buffer, the CrVI-reducing activity was increased.

Catalase↗
PubMed2004

A diiron complex mediates an intramolecular aliphatic hydroxylation by various oxygen donors.

In the presence of hydrogen peroxide, m-chloroperbenzoic acid or an iodosyl arene, the tert-butyl group of the ligand H(L-t-Bu) in the complex [Fe2(L-t-Bu)(mpdp)]2+ is quantitatively hydroxylated to a butanolate terminally bound to one iron in [Fe2(L-t-Bu - H + O)(mpdp)]2+, and mass spectrometry experiments indicate that the reaction proceeds according to different mechanisms.

Ferric Compounds↗
PubMed2005

Unusual reactions of [{micro-(phthalazine-N2:N3)}Fe2(micro-CO)(CO)6] with organolithium reagents. A novel coordination mode of 1,2-diazane diiron carbonyl compounds.

[{Micro-(phthalazine-N2:N3)}Fe2(micro-CO)(CO)6](1) reacts with organolithium reagents, RLi (R = CH3, C6H5, p-CH3C6H4, p-CH3OC6H4, p-CF3C6H4, p-C6H5C6H4), followed by treatment with Me3SiCl to give the novel diiron carbonyl complexes with a saturated N-N six-membered diazane ring ligand, [{C6H4CH(R)NNCH2}Fe2(C=O)(CO)6](2, R = CH3; 3, R = C6H5; 4, R =p-CH3C6H4; 5, R =p-CH3OC6H4; 6, R =p-CF3C6H4; 7, R =p-C6H5C6H4). Compounds 4 and 5 were treated with [(NH4)2Ce(NO3)6] to afford the aryl-substituted phthalazine-coordinated diiron carbonyl compounds [(micro-{1-(p-CH3C6H4)-phthalazine-N2:N3})Fe2(micro-CO)(CO)6](8) and [(micro-{1-(p-CH3OC6H4)-phthalazine-N2:N3})Fe2(micro-CO)(CO)6](9), respectively. The structures of complexes 4 and 9 have been established by X-ray diffraction studies.

Diazonium Compounds↗
PubMed2005

Versatility in the binding of 2-pyrazinecarboxylate with iron. Synthesis, structure and magnetic properties of iron(II) and iron(III) complexes.

The synthesis and characterization of two new iron(II) complexes, [Fe(pca)2(py)2].py (1) and {[Fe(pca)2(H2O)].H2O}n (2) and one new iron(III) complex, Na2{[Fe(pca)()]2O}.2H2O.2CH3CN (3) (pca- stands for 2-pyrazinecarboxylate), are reported. Complex 1 is obtained from the reaction of iron powder with 2-pyrazinecarboxylic acid. The reaction of Fe(ClO4)3.10H2O with Hpca in the presence of 3 equiv. of Bu4NOH yields 2, whereas the presence of NaOH yields 3. The molecular structure of 1 contains an iron(II) ion with a pseudo-octahedral environment resulting from the coordination of two pca- ligands in a bidentate chelating fashion and two pyridine molecules; pi-pi stacking interactions between pyridine and pyrazine rings lead to a one-dimensional chain. Complex 2 is an iron(II) coordination polymer with an infinite zig-zag motif and an Fe...Fe separation of 7.1 A. In 2, the pi-pi stacking interactions involving the pyrazine rings and the strong hydrogen bonds between the coordinated water molecule and the carboxylate oxygens of two pca- ligands result in a three-dimensional network structure. Complex 3 consists of an anionic micro-oxo-bridged diiron(III) core with two crystallographically distinct iron(iii) ions; the negative charge is compensated by two sodium cations. Complex 3 is assembled in a three dimensional network structure through coordination of Na(I) and hydrogen bond interactions. Temperature dependent magnetic susceptibility and Mössbauer spectroscopic studies indicate that 1 and 2 have similar magnetic properties. Both complexes are paramagnetic above 12 K, whereas antiferromagnetic ordering is observed below 12 K. The magnetic properties of reveal strong intramolecular antiferromagnetic interactions between the two iron(III) ions with a J value of -221 cm(-1); no long range intermolecular magnetic coupling is observed between 295 and 4.2 K.

Binding Sites↗
PubMed2006

Reaction route control by microperoxidase-9/CTAB micelle ratios.

Microperoxidases (MP) as water-soluble models attract interest to studying the reaction mechanism of peroxidases because these heme peptides are able to form the same enzyme intermediates during the reaction with peroxides. In this work we have demonstrated that the association of Fe(III)MP-9 and Fe(III)MP-11 with CTAB micelles (MP-9/CTAB and MP11/CTAB) provides a microenvironment with an alkaline interface and a hydrophobic core that exhibits peroxidase behavior. This microenvironment shifts positively the redox potential of microperoxidases by approximately 100 mV. tert-Butylhydroperoxide (t-BuOOH) when added to the medium, converted Fe(III)MP-9/CTAB to MP-9/CTAB Compound II, a high valence oxidized intermediate of the heme peptide. Subsequent addition of diphenylacetaldehyde (DPAA) to MP-9/CTAB Compound II regenerated the native form of the enzyme, Fe(III)MP-9/CTAB, what characterizes the occurrence of a peroxidase cycle. Fe(III)MP-9/CTAB regenerated during the peroxidase cycle reacted with residual DPAA in the medium to form Fe(II)MP-9/CTAB, which indicates that both Fe(III)MP-9/CTAB and its oxyferryl form can use aldehydes as reducing agents. According to the determined reduction potential, Fe(III)MP-9 and Fe(III)MP-9/CTAB should be able to oxidize DPAA (reduction potential -630 mV). The reaction of MP-9/CTAB with DPAA produced benzophenone as final product, detected by infrared spectroscopy and mass spectrometry. Interestingly, a significant difference was observed in the benzophenone yield according to the micelle/MP-9 molar ratio.

Aldehydes↗
PubMed2004

Engineering peroxidase activity in myoglobin: the haem cavity structure and peroxide activation in the T67R/S92D mutant and its derivative reconstituted with protohaemin-l-histidine.

Atomic co-ordinates and structure factors for the T67R/S92D metMbCN mutant have been deposited with the Protein Data Bank, under accession codes 1h1x and r1h1xsf, respectively. Protein engineering and cofactor replacement have been employed as tools to introduce/modulate peroxidase activity in sperm whale Mb (myoglobin). Based on the rationale that haem peroxidase active sites are characterized by specific charged residues, the Mb haem crevice has been modified to host a haem-distalpropionate Arg residue and a proximal Asp, yielding the T67R/S92D Mb mutant. To code extra conformational mobility around the haem, and to increase the peroxidase catalytic efficiency, the T67R/S92D Mb mutant has been subsequently reconstituted with protohaem-L-histidine methyl ester, yielding a stable derivative, T67R/S92D Mb-H. The crystal structure of T67R/S92D cyano-metMb (1.4 A resolution; R factor, 0.12) highlights a regular haem-cyanide binding mode, and the role for the mutated residues in affecting the haem propionates as well as the neighbouring water structure. The conformational disorder of the haem propionate-7 is evidenced by the NMR spectrum of the mutant. Ligand-binding studies show that the iron(III) centres of T67R/S92D Mb, and especially of T67R/S92D Mb-H, exhibit higher affinity for azide and imidazole than wild-type Mb. In addition, both protein derivatives react faster than wild-type Mb with hydrogen peroxide, showing higher peroxidase-like activity towards phenolic substrates. The catalytic efficiency of T67R/S92D Mb-H in these reactions is the highest so far reported for Mb derivatives. A model for the protein-substrate interaction is deduced based on the crystal structure and on the NMR spectra of protein-phenol complexes.

Animals↗
PubMed1982

Iron-induced DNA damage and synthesis in isolated rat liver nuclei.

Incubation of iron with isolated rat liver nuclei stimulated fragmentation of single-stranded DNA, incorporation of [3H]thymidine into DNA and the binding of 59Fe to DNA. FeCl2 was about twice as active as FeCl3. Lipid peroxidation took place in nuclei incubated with FeCl2, but not with FeCl3. Generation of reactive forms of oxygen was required for iron-mediated DNA damage, but evidence for direct interaction of reactive oxygen with DNA was not found. Apparent adducts of iron bound to DNA seemed to be formed by an enzymic mechanism.

Animals↗
PubMed1989

Stimulation of 5-lipoxygenase activity under conditions which promote lipid peroxidation.

The characteristics of hydroperoxide activation of 5-lipoxygenase were examined in the high speed supernatant fraction prepared from rat polymorphonuclear leukocytes. Stimulation of 5-lipoxygenase activity by the 5-hydroperoxyeicosatetraenoic acid (5-HPETE) reaction product was strongly dependent on the presence of thiol compounds. Various reducing agents such as mercaptoethanol and glutathione (0.5-2 mM) inhibited the reaction and increased the concentrations of 5-HPETE (1-10 microM) necessary to achieve maximal arachidonic acid oxidation. The requirement for 5-HPETE was not specific and could be replaced by H2O2 (10 microM) but not by the 5-hydroxyeicosatetraenoic acid (5-HETE) analogue. Furthermore, gel filtration chromatography of the soluble extract from leukocytes resolved different fractions which can increase the hydroperoxide dependence or fully replace the stimulation by 5-HPETE. Maximal activity of the 5-HPETE-stimulated reaction required Ca2+ ions (0.2-1 mM) and ATP with the elimination of the HPETE requirement at high ATP concentrations (2-4 mM). In addition, NADPH (1-2 mM), FAD (1 mM), Fe2+ ions (20-100 microM) and chelated Fe3+ (0.1 mM-EDTA/0.1 mM-FeCl3) all markedly increased product formation by 5-lipoxygenase whereas NADH (1 mM) was inhibitory and Fe3+ (20-100 microM) alone had no effect on the reaction. The stimulation by Fe2+ ions and NADPH was also observed under various conditions which increase the hydroperoxide dependence such as pretreatment of the enzyme preparation with glutathione peroxidase or chemical reduction with 0.015% NaBH4. These results provide evidence for an hydroperoxide activation of 5-lipoxygenase which is not product-specific and is modulated by thiol levels and several soluble components of the leukocytes. They also indicate that stimulation of 5-lipoxygenase activity can contribute to increase lipid peroxidation in iron and nucleotide-promoted reactions.

Adenosine Triphosphate↗
PubMed1993

Formation of peroxides in amino acids and proteins exposed to oxygen free radicals.

Dilute aqueous solutions of BSA or lysozyme gave positive tests for peroxides after exposure to reactive oxygen species. The reactive species were generated by gamma-irradiation, reduction of H2O2 with Fe2+ ions or thermal decomposition of an azo compound. Peroxides were assayed by an iodometric method. Identification of the new groups as hydroperoxides was confirmed by their ability to oxidize a range of compounds and by the kinetics of their reaction with iodide. The hydroperoxide groups were bound to the proteins and their yields (G values) corresponded to 1.2 -OOH groups per 100 eV of radiation energy absorbed for BSA, and 0.8 for lysozyme. The oxygen free radicals effective in protein peroxidation were the hydroxyl and organic peroxyl, but not superoxide or its protonated form. The efficiency of BSA peroxidation initiated by the hydroxyl radicals was 40%. Protein peroxides decayed spontaneously with a half-life of about 1.5 days at 20 degrees C. Exposure of the common amino acids to hydroxyl free radicals showed that six of them (glutamate, isoleucine, leucine, lysine, proline and valine) were peroxidized with similar efficiency to the proteins, whereas the rest were inert or much less susceptible. These results suggest that some proteins may be peroxidized by a variety of agents in vivo and that their subsequent reactions with protective agents, such as ascorbate or glutathione, may decrease the antioxidant potential of cells and tissues.

Amino Acids↗
PubMed1995

Oxidative modulation and inactivation of rabbit cardiac adenylate deaminase.

Oxidative stress and adenine nucleotide catabolism occur concomitantly in several disease states, such as cardiac ischaemia-reperfusion, and may act as synergistic determinants of tissue injury. However, the mechanisms underlying this potential interaction remain ill-defined. We examined the influence of oxidative stress on the molecular, kinetic and regulatory properties of a ubiquitous AMP-catabolizing enzyme, adenylate deaminase (AMPD) (EC 3.5.4.6). To this intent, rabbit heart AMPD and an H2O2/ascorbate/iron oxidation system were employed. Enzyme exposure to the complete oxidation system acutely impaired its catalytic activity, lowered the Vmax. by 7-fold within 5 min, and rendered the enzyme unresponsive to nucleotide effectors. Irreversible AMPD inactivation resulted within about 15 min of oxidative insult and was not prevented by free-radical scavengers. Oxidative stress did not affect the molecular mass, tetrameric nature, Km, immunoreactivity or trypsinolytic pattern of the enzyme; nor did it induce carbonyl formation, Zn2+ release from the holoenzyme or net AMPD S-thiolation. This injury pattern is inconsistent with a radical-fragmentation mechanism as the basis for the oxidative AMPD inactivation observed. Rather, the sensitivity of the enzyme to both S-thiolation and thiol alkylation and the significant (3 of 9/mol of denatured enzyme) net loss of DTNB-reactive thiols on exposure to oxidant strongly implicate the conversion of essential thiol moieties into stable higher-oxidation states in the oxidative inactivation of cardiac AMPD. The altered thiol status of the enzyme on oxidative insult may prohibit a catalytically permissible conformation and, in so doing, increase AMP availability to 5'-nucleotidase in vivo.

5'-Nucleotidase↗
PubMed1995

Biological fate of amino acid, peptide and protein hydroperoxides.

In the course of searching for a suitable marker for studying protein oxidation, we have successfully elucidated the structures of three valine hydroperoxides, i.e. beta-hydroperoxyvaline, (2S,3S)-gamma-hydroperoxyvaline and (2S,3R)-gamma-hydroperoxyvaline, which are novel products of protein oxidation. The corresponding valine hydroxides were obtained by sodium borohydride reduction [Fu, Hick, Sheil and Dean (1995) Free Rad. Biol. Med. 19, 281-292]. We hypothesized that valine hydroxides might be the major biological degradation products of valine hydroperoxides and, as such, could be useful markers for the study of protein oxidation in vivo. The aim of this study was to investigate the fate of valine hydroperoxide in selected biological systems by the use of chemiluminescence detection of hydroperoxides and HPLC analysis of O-phthaldialdehyde derivatives of amino acid residues. The degradation of hydroperoxides present on gamma-radiolysed solutions of valine, Pro-Val-Gly, or BSA occurred in the presence of: (1) transition metals (Fe2+, Fe3+, or Cu2+), (2) the detoxifying enzyme GSH peroxidase, (3) human plasma, and (4) J774 mouse monocyte macrophage cells. The major degradation product of valine hydroperoxide recovered in each case was found to be a valine hydroxide. These results suggest that valine hydroxide (derived from the hydroperoxide) may well be a useful in vivo marker for studying protein damage under oxidative stress.

Amino Acid Sequence↗