Nonheme iron proteins as an alternative system of antioxidant defense in the cells of strictly anaerobic microorganisms: A review

Enzymatic systems accounting for the relative oxygen resistance of multiple strict anaerobes are reviewed, with emphasis on molecular-biological properties and action mechanisms of nonheme iron proteins (neelaredoxins, desulfoferrodoxins, and rubrerythrins). These unique proteins, which are widespread in anaerobes, comprise a system of antioxidant defense against toxic effects of oxygen and products of its incomplete reduction (an alternative to the classic antioxidant system involving superoxide dismutase and catalase). The role of the superoxide reductase-mediated elimination of endogenous superoxide radicals is discussed. This extremely efficient means of rapid superoxide radical detoxification underlies the preferred mechanism for maintaining the optimum balance between oxidized and reduced forms of some proteins in the cells of strict anaerobes.     

The iron content of iron superoxide dismutase: determination by anomalous scattering

The number of iron atoms in the dimeric iron-containing superoxide dismutase from Pseudomonas ovalis and their atomic positions have been determined directly from anomalous scattering measurements on crystals of the native enzyme. To resolve the long-standing question of the total amount of iron per molecule for this class of dismutase, the occupancy of each site was refined against the measured Bijvoet differences. The enzyme is a symmetrical dimer with one iron site in each subunit. The iron position is 9 A from the intersubunit interface. The total iron content of the dimer is 1.2 +/- 0.2 moles per mole of protein. This is divided between the subunits in the ratio 0.65:0.55; the difference between them is probably not significant. Since each subunit contains, on average, slightly more than half an iron atom we conclude that the normal state of this enzyme is two iron atoms per dimer but that some of the metal is lost during purification of the protein. Although the crystals are obviously a mixture of holo- and apo-enzymes, the 2.9 A electron density map is uniformly clean, even at the iron site. We conclude that the three-dimensional structures of the iron-bound enzyme and the apo-enzyme are identical.     

Isolation and Role of Nonheme Iron Protein in Clostridium botulinum

A type of iron-bound protein was isolated from Clostridium botulinum by a modification of the method used for isolating ferredoxin from C. pasteurianum. This method involved acetone and diethylaminoethyl cellulose treatments followed by ammonium sulfate fractionation. The protein exhibited maximal absorption in the ultraviolet region near 260 mμ. Portions of the isolated iron protein were separated by disc electrophoresis and, following specific iron-bound protein staining, showed a positive reaction in the same position on the gel column as was first demonstrated by use of cell-free extract. Evidence accumulated by use of a cell-free extract of C. botulinum suggests that pyruvate is metabolized through a phosphoroclastic system as demonstrated in other clostridia. It is probable that ferredoxin is an electron mediator between pyruvic oxidase and hydrogenase for hydrogen evolution and acetyl phosphate formation.     

Nonheme iron(II) complexes of macrocyclic ligands in the generation of oxoiron(IV) complexes and the catalytic epoxidation of olefins

Mononuclear nonheme oxoiron(IV) complexes bearing 15-membered macrocyclic ligands were generated from the reactions of their corresponding iron(II) complexes and iodosylbenzene (PhIO) in CH(3)CN. The oxoiron(IV) species were characterized with various spectroscopic techniques such as UV-vis spectrophotometer, electron paramagnetic resonance, electrospray ionization mass spectrometer, and resonance Raman spectroscopy. The oxoiron(IV) complexes were inactive in olefin epoxidation. In contrast, when iron(II) or oxoiron(IV) complexes were combined with PhIO in the presence of olefins, high yields of epoxide products were obtained. These results indicate that in addition to the oxoiron(IV) species, there must be at least one more active oxidant (e.g., Fe(IV)-OIPh adduct or oxoiron(V) species) that effects the olefin epoxidation. We have also demonstrated that the ligand environment of iron catalysts is an important factor in controlling the catalytic activity as well as the product selectivity in the epoxidation of olefins by PhIO.     

Oxidation of Indole and Indole Derivatives Catalyzed by Nonheme Chloroperoxidases

Indole, indolylacetic acid, and tryptophan were oxidized by chloroperoxidases isolated from strains of Streptomyces lividansand Pseudomonas pyrrocinia. Indigo (indoxyl), isatin, and anthranilic acid (intermediate products of oxidative degradation of indole and indole derivatives) were isolated from the reaction medium.     

Single-cell determination of iron content in magnetotactic bacteria: implications for the iron biogeochemical cycle

Magnetotactic bacteria (MTB) are ubiquitous aquatic microorganisms that mineralize dissolved iron into intracellular magnetic crystals. After cell death, these crystals are trapped into sediments that remove iron from the soluble pool. MTB may significantly impact the iron biogeochemical cycle, especially in the ocean where dissolved iron limits nitrogen fixation and primary productivity. A thorough assessment of their impact has been hampered by a lack of methodology to measure the amount of, and variability in, their intracellular iron content. We quantified the iron mass contained in single MTB cells of Magnetospirillum magneticum strain AMB-1 using a time-resolved inductively coupled plasma-mass spectrometry methodology. Bacterial iron content depends on the external iron concentration, and reaches a maximum value of ~10⁻⁶ ng of iron per cell. From these results, we calculated the flux of dissolved iron incorporation into environmental MTB populations and conclude that MTB may mineralize a significant fraction of dissolved iron into crystals.     

The determination of ferric iron in plants by HPLC using the microbial iron chelator desferrioxamine E

Common methods for plant iron determination are based on atomic absorption spectroscopy, radioactive measurements or extraction with subsequent spectrophotometry. However, accuracy is often a problem due to background, contamination and interfering compounds. We here describe a novel method for the easy determination of ferric iron in plants by chelation with a highly effective microbial siderophore and separation by high performance liquid chromatography (HPLC). After addition of colourless desferrioxamine E (DFE) to plant fluids, the soluble iron is trapped as a brown-red ferrioxamine E (FoxE) complex which is subsequently separated by HPLC on a reversed phase column. The formed FoxE complex can be identified due to its ligand-to-metal charge transfer band at 435 nm. Alternatively, elution of both, DFE and FoxE can be followed as separate peaks at 220 nm wavelength with characteristic retention times. The extraordinarily high stability constant of DFE with ferric iron of K=10³² enables extraction of iron from a variety of ferrous and ferric iron compounds and allows quantitation after separation by HPLC without interference by coloured by-products. Thus, iron bound to protein, amino acids, citrate and other organic acid ligands and even insoluble ferric hydroxides and phosphates can be solubilized in the presence desferrioxamine E. The “Ferrioxamine E method” can be applied to all kinds of plant fluids (apoplasmic, xylem, phloem, intracellular) either at physiological pH or even at acid pH values. The FoxE complex is stable down to pH 1 allowing protein removal by perchloric acid treatment and HPLC separation in the presence of trifluoroacetic acid containing eluents. Published online December 2004     

Voltammetric determination of dissolved iron and its speciation in freshwater

Analytical methods were developed to determine the concentration of total dissolved iron and its chemical speciation in freshwater using cathodic stripping voltammetry (CSV) with 1-nitroso-2-naphthol (NN) at pH 8.1. The concentrations of total dissolved iron in river water that iron concentration was certified and in natural water samples from Lake Kasumigaura were determined successfully. The natural iron ligand concentration and the conditional stability constant were determined by ligand competition between NN and the natural ligands present in the sample. In the water samples from Lake Kasumigaura, the concentrations of total dissolved iron and natural ligand were 47.8 ± 4.4nM and 80.0 ± 19.6nM and the conditional stability constant (K_FeL) was 10^25.9±0.4M^–1 (n = 3). The value of K_FeL was greater than any reported K_FeL for seawater. More than 99.9% of the dissolved iron existed as organic species due to the very high value of the conditional stability constant. The inorganic iron concentration calculated from these results was 10^–13.4M, indicating that the inorganic iron level in Lake Kasumigaura was similar to that in the open ocean and therefore that iron can be a limiting factor for algal growth in Lake Kasumigaura. This is the first report of the complexation of iron(III) and inorganic iron levels in lake water determined by CSV.     

Reevaluation of colorimetric iron determination methods commonly used in geomicrobiology

The ferrozine and phenanthroline colorimetric assays are commonly applied for the determination of ferrous and total iron concentrations in geomicrobiological studies. However, accuracy of both methods depends on slight changes in their protocols, on the investigated iron species, and on geochemical variations in sample conditions. Therefore, we tested the performance of both methods using Fe(II)((aq)), Fe(III)((aq)), mixed valence solutions, synthetic goethite, ferrihydrite, and pyrite, as well as microbially-formed magnetite and a mixture of goethite and magnetite. The results were compared to concentrations determined with aqua regia dissolution and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Iron dissolution prior to the photometric assays included dissolution in 1M or 6M HCl, at 21 or 60°C, and oxic or anoxic conditions. Results indicated a good reproducibility of quantitative total iron determinations by the ferrozine and phenanthroline assays for easily soluble iron forms such as Fe(II)((aq)), Fe(III)((aq)), mixed valence solutions, and ferrihydrite. The ferrozine test underestimated total iron contents of some of these samples after dissolution in 1M HCl by 10 to 13%, whereas phenanthroline matched the results determined by ICP-AES with a deviation of 5%. Total iron concentrations after dissolution in 1M HCl of highly crystalline oxides such as magnetite, a mixture of goethite and magnetite, and goethite were underestimated by up to 95% with both methods. When dissolving these minerals in 6M HCl at 60°C, the ferrozine method was more reliable for total iron content with an accuracy of ±5%, related to values determined with ICP-AES. Phenanthroline was more reliable for the determination of total pyritic iron as well as ferrous iron after incubation in 1M HCl at 21°C in the Fe(II)((aq)) sample with a recovery of 98%. Low ferrous iron concentrations of less than 0.5mM were overestimated in a Fe(III) background by up to 150% by both methods. Heating of mineral samples in 6M HCl increased their solubility and susceptibility for both photometric assays which is a need for total iron determination of highly crystalline minerals. However, heating also rendered a subsequent reliable determination of ferrous iron impossible due to fast abiotic oxidation. Due to the low solubility of highly crystalline samples, the determination of total iron is solely possible after dissolution in 6M HCl at 60°C which on the other hand makes determination of ferrous iron impossible. The recommended procedure for ferrous iron determination is therefore incubation at 21°C in 6M HCl, centrifugation, and subsequent measurement of ferrous iron in the supernatant. The different procedures were tested during growth of G. sulfurreducens on synthetic ferrihydrite. Here, the phenanthroline test was more accurate compared to the ferrozine test. However, the latter provided easy handling and seemed preferable for larger amounts of samples.     

Calcein as a fluorescent iron chemosensor for the determination of low molecular weight iron in biological fluids

The fluorescence quenching of calcein (CA) is not iron specific and results in a negative calibration curve. In the present study, deferoxamine (DFO), a strong iron chelator, was used to regenerate the fluorescence quenched by iron. Therefore, the differences in fluorescence reading of the same sample with or without addition of DFO are positively and specifically proportional to the amounts of iron. We found that the same iron species but different anions (e.g. ferric sulfate or ferric citrate) differed in CA fluorescence quenching, so did the same anions but different iron (e.g. ferrous or ferric sulfates). Excessive amounts of citrate competed with CA for iron and citrate could be removed by barium precipitation. After optimizing the experimental conditions, the sensitivity of the fluorescent CA assay is 0.02 M of iron, at least 10 times more sensitive than the colorimetric assays. Sera from 6 healthy subjects were tested for low molecular weight (LMW) chelator bound iron in the filtrates of 10 kDa nominal molecular weight limit (NMWL). The LMW iron was marginally detectable in the normal sera. However, increased levels of LMW iron were obtained at higher transferrin (Tf) saturation (1.64–2.54 M range at 80% Tf saturation, 2.77–3.15 M range at 100% Tf saturation and 3.09–3.39 M range at 120% Tf saturation). The application of the assay was further demonstrated in the filtrates of human liver HepG2 and human lung epithelial A549 cells treated with iron or iron-containing dusts.