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1.
《FEBS letters》1986,205(2):275-281
EPR signals in the high-spin region were studied at 10 K in photosystem II (PS II) particles and in a purified oxygen-evolving PS II reaction center complex under oxidizing conditions. PS II particles showed EPR peaks at g = 8.0 and 5.6, confirming the recent report by Petrouleas and Diner [(1986) Biochim. Biophys. Acta 849, 264-275]. Addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) or o-phenanthroline shifted the peaks to be closer to g = 6.0 depending on the medium pH. On the other hand, the PS II reaction center complex showed peaks at g = 6.1 and 7.8, and at g = 6.1 and 6.4, in the absence and presence of o-phenanthroline, respectively. All these peaks were found to be decreased by the illumination at 10 K. These results suggest that the high-spin signals are due to Q400, Fe(III) atom interacting with the PS II primary electron acceptor quinone QA as reported and that the Fe atom also interacts with the secondary acceptor quinone QB. This interaction seems to induce the highly asymmetric ligand coordination of the Fe atom and to be affected by DCMU and o-phenanthroline in a somewhat different manner. 相似文献
2.
《BBA》1986,851(3):416-423
The ferrous ion associated with the electron acceptors in Photosystem II can be oxidized by the unstable semiquinone form of certain high-potential quinones (phenyl-p-benzoquinone, dimethylbenzoquinone and benzoquinone) which are used as electron acceptors. In a flash sequence, alternating oxidation of the iron by the photoreduced semiquinone on odd-numbered flashes is followed by photoreduction of the iron on even-numbered flashes. These reactions are detected by monitoring EPR signals arising from Fe3+. The oxidation of the iron can also occur in the frozen state (−30°C) indicating that the high-potential quinone can occupy the QB site. The reaction also takes place when the exogenous quinone is added in the dark to samples in which QB is already in the semiquinone form. The inhibitors of electron transfer between QA− and QB, DCMU and sodium formate, block the photoreductant-induced iron oxidation. It is suggested that the iron oxidation takes place through the QB site. This unexpected photochemistry occurs under experimental conditions routinely used in studies of Photosystem II. Some previously reported phenomena can be reinterpreted on the basis of these new data. 相似文献
3.
《BBA》1985,809(3):320-330
Preincubation of isolated chloroplasts with ferricyanide, prior to addition of DCMU, unmasks a high-potential electron acceptor (Q400) in Photosystem II that acts as an additional quencher and prolongs the fluorescence induction curve in the presence of DCMU (Ikegami, I. and Katoh, S. (1973) Plant Cell Physiol. 14, 829–836). This study confirms that Q400 is endogenous to Photosystem II and is not a bound ferricyanide, and several new characteristics of this high potential acceptor are established. (a) It is accessible to ferricyanide even in the presence of DCMU. The rate of oxidation, however, is very slow, consistent with access only via QA. Accessibility may be enhanced by magnesium, reminiscent of the oxidation of Q−A by ferricyanide. (b) Oxidation of Q400 drastically suppresses the binding of DCMU at neutral and alkaline pH. Below pH 6, however, DCMU binding is essentially normal. The pH dependence of DCMU binding is consistent with the known pH dependence of the redox midpoint potential of Q400. (c) Binding of many other inhibitors of QA-to-QB electron transfer is much less affected or even completely unaffected. These results have implications for current notions of herbicide binding and may also bear on the origin of slow phases of fluorescence induction in the presence of DCMU. 相似文献
4.
Effect of Oxidation Rate and Fe(II) State on Microbial Nitrate-Dependent Fe(III) Mineral Formation
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A nitrate-dependent Fe(II)-oxidizing bacterium was isolated and used to evaluate whether Fe(II) chemical form or oxidation rate had an effect on the mineralogy of biogenic Fe(III) (hydr)oxides resulting from nitrate-dependent Fe(II) oxidation. The isolate (designated FW33AN) had 99% 16S rRNA sequence similarity to Klebsiella oxytoca. FW33AN produced Fe(III) (hydr)oxides by oxidation of soluble Fe(II) [Fe(II)sol] or FeS under nitrate-reducing conditions. Based on X-ray diffraction (XRD) analysis, Fe(III) (hydr)oxide produced by oxidation of FeS was shown to be amorphous, while oxidation of Fe(II)sol yielded goethite. The rate of Fe(II) oxidation was then manipulated by incubating various cell concentrations of FW33AN with Fe(II)sol and nitrate. Characterization of products revealed that as Fe(II) oxidation rates slowed, a stronger goethite signal was observed by XRD and a larger proportion of Fe(III) was in the crystalline fraction. Since the mineralogy of Fe(III) (hydr)oxides may control the extent of subsequent Fe(III) reduction, the variables we identify here may have an effect on the biogeochemical cycling of Fe in anoxic ecosystems. 相似文献
5.
Recent evidence has indicated the presence of novel plastoquinone-binding sites, QC and QD, in photosystem II (PSII). Here, we investigated the potential involvement of loosely bound plastosemiquinones in superoxide anion radical (O2•−) formation in spinach PSII membranes using electron paramagnetic resonance (EPR) spin-trapping spectroscopy. Illumination of PSII membranes in the presence of the spin trap EMPO (5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide) resulted in the formation of O2•−, which was monitored by the appearance of EMPO-OOH adduct EPR signal. Addition of exogenous short-chain plastoquinone to PSII membranes markedly enhanced the EMPO-OOH adduct EPR signal. Both in the unsupplemented and plastoquinone-supplemented PSII membranes, the EMPO-OOH adduct EPR signal was suppressed by 50% when the urea-type herbicide DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) was bound at the QB site. However, the EMPO-OOH adduct EPR signal was enhanced by binding of the phenolic-type herbicide dinoseb (2,4-dinitro-6-sec-butylphenol) at the QD site. Both in the unsupplemented and plastoquinone-supplemented PSII membranes, DCMU and dinoseb inhibited photoreduction of the high-potential form of cytochrome b559 (cyt b559). Based on these results, we propose that O2•− is formed via the reduction of molecular oxygen by plastosemiquinones formed through one-electron reduction of plastoquinone at the QB site and one-electron oxidation of plastoquinol by cyt b559 at the QC site. On the contrary, the involvement of a plastosemiquinone formed via the one-electron oxidation of plastoquinol by cyt b559 at the QD site seems to be ambiguous. In spite of the fact that the existence of QC and QD sites is not generally accepted yet, the present study provided more spectroscopic data on the potential functional role of these new plastoquinone-binding sites. 相似文献
6.
Verena Nikeleit Markus Maisch James M. Byrne Caroline Harwood Andreas Kappler Casey Bryce 《Environmental microbiology》2024,26(3):e16608
Rhodopseudomonas palustris TIE-1 grows photoautotrophically with Fe(II) as an electron donor and photoheterotrophically with a variety of organic substrates. However, it is unclear whether R. palustris TIE-1 conducts Fe(II) oxidation in conditions where organic substrates and Fe(II) are available simultaneously. In addition, the effect of organic co-substrates on Fe(II) oxidation rates or the identity of Fe(III) minerals formed is unknown. We incubated R. palustris TIE-1 with 2 mM Fe(II), amended with 0.6 mM organic co-substrate, and in the presence/absence of CO2. We found that in the absence of CO2, only the organic co-substrates acetate, lactate and pyruvate, but not Fe(II), were consumed. When CO2 was present, Fe(II) and all organic substrates were consumed. Acetate, butyrate and pyruvate were consumed before Fe(II) oxidation commenced, whereas lactate and glucose were consumed at the same time as Fe(II) oxidation proceeded. Lactate, pyruvate and glucose increased the Fe(II) oxidation rate significantly (by up to threefold in the case of lactate). 57Fe Mössbauer spectroscopy revealed that short-range ordered Fe(III) oxyhydroxides were formed under all conditions. This study demonstrates phototrophic Fe(II) oxidation proceeds even in the presence of organic compounds, and that the simultaneous oxidation of organic substrates can stimulate Fe(II) oxidation. 相似文献
7.
Based on the electron-transport properties on the reducing side of the reaction center, photosystem II (PS II) in green plants and algae occurs in two distinct forms. Centers with efficient electron-transport from QA to plastoquinone (QB-reducing) account for 75% of the total PS II in the thylakoid membrane. Centers that are photochemically competent but unable to transfer electrons from QA to QB (QB-nonreducing) account for the remaining 25% of total PS II and do not participate in plastoquinone reduction. In Dunaliella salina, the pool size of QB-nonreducing centers changes transiently when the light regime is perturbed during cell growth. In cells grown under moderate illumination intensity (500 E m-2s-1), dark incubation induces an increase (half-time 45 min) in the QB-nonreducing pool size from 25% to 35% of the total PS II. Subsequent illumination of these cells restores the steady-state concentration of QB-nonreducing centers to 25%. In cells grown under low illumination intensity (30 µE m–2s–1), dark incubation elicits no change in the relative concentration of QB-nonreducing centers. However, a transfer of low-light grown cells to moderate light induces a rapid (half-time 10 min) decrease in the QB-nonreducing pool size and a concomitant increase in the QB-reducing pool size. These and other results are explained in terms of a pool of QB-nonreducing centers existing in a steady-state relationship with QB-reducing centers and with a photochemically silent form of PS II in the thylakoid membrane of D. salina. It is proposed that QB-nonreducing centers are an intermediate stage in the process of damage and repair of PS II. It is further proposed that cells regulate the inflow and outflow of centers from the QB-nonreducing pool to maintain a constant pool size of QB-nonreducing centers in the thylakoid membrane.Abbreviations Chl
chlorophyll
- PS
photosystem
- QA
primary quinone electron acceptor of PS II
- QB
secondary quinone electron acceptor of PS II
- LHC
light harvesting complex
- Fo
non-variable fluorescence yield
- Fpl
intermediate fluorescence yield plateau level
- Fmax
maximum fluorescence yield
- Fi
mitial fluorescence yield increase from Fo to Fpl(Fpl-Fo)
- Fv
total variable fluorescence yield (Fmax-Fo)
- DCMU
dichlorophenyl-dimethylurea 相似文献
8.
The functional state of the PS II population localized in the stroma exposed non-appressed thylakoid region was investigated by direct analysis of the PS II content of isolated stroma thylakoid vesicles. This PS II population, possessing an antenna size typical for PS II, was found to have a fully functional oxygen evolving capacity in the presence of an added quinone electron acceptor such as phenyl-p-benzoquinone. The sensitivity to DCMU for this PS II population was the same as for PS II in control thylakoids. However, under more physiological conditions, in the absence of an added quinone acceptor, no oxygen was evolved from stroma thylakoid vesicles and their PS II centers were found to be incapable to pass electrons to PS I and to yield NADPH. By comparison of the effect of a variety of added quinone acceptors with different midpoint potentials, it is concluded that the inability of PS II in the stroma thylakoid membranes to contribute to NADPH formation probably is due to that QA of this population is not able to reduce PQ, although it can reduce some artificial acceptors like phenyl-p-benzoquinone. These data give further support to the notion of a discrete PS II population in the non-appressed stroma thylakoid region, PS II, having a higher midpoint potential of QA than the PS II population in the appressed thylakoid region, PS II. The physiological significance of a PS II population that does not produce any NADPH is discussed.Abbreviations pBQ
p-benzoquinone
- Chl
chlorophyll
- DCBQ
2,6-dichloro-p-benzoquinone
- DCIP
2,6-dichloroindophenol
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- DMBQ
2,5-dimethyl-p-benzoquinone
- DQ
duroquinone(tetramethyl-p-benzoquinone)
- FeCN
ferricyanide (potassium hexacyanoferrat)
- MV
methylviologen
- NADPH,NADP+
reduced or oxidized form of nicotinamide adenine dinucleotide phosphate respectively
- PpBQ
phenyl-p-benzoquinone
- PQ
plastoquinone
- PS II
photosystem II
- PS I
photosystem I
- QA
primary quinone acceptor of PS II
- QB
secondary quinone acceptor of PS II
- E
microEinstein 相似文献
9.
Direct and Fe(II)-Mediated Reduction of Technetium by Fe(III)-Reducing Bacteria 总被引:10,自引:4,他引:6
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The dissimilatory Fe(III)-reducing bacterium Geobacter sulfurreducens reduced and precipitated Tc(VII) by two mechanisms. Washed cell suspensions coupled the oxidation of hydrogen to enzymatic reduction of Tc(VII) to Tc(IV), leading to the precipitation of TcO2 at the periphery of the cell. An indirect, Fe(II)-mediated mechanism was also identified. Acetate, although not utilized efficiently as an electron donor for direct cell-mediated reduction of technetium, supported the reduction of Fe(III), and the Fe(II) formed was able to transfer electrons abiotically to Tc(VII). Tc(VII) reduction was comparatively inefficient via this indirect mechanism when soluble Fe(III) citrate was supplied to the cultures but was enhanced in the presence of solid Fe(III) oxide. The rate of Tc(VII) reduction was optimal, however, when Fe(III) oxide reduction was stimulated by the addition of the humic analog and electron shuttle anthaquinone-2,6-disulfonate, leading to the rapid formation of the Fe(II)-bearing mineral magnetite. Under these conditions, Tc(VII) was reduced and precipitated abiotically on the nanocrystals of biogenic magnetite as TcO2 and was removed from solution to concentrations below the limit of detection by scintillation counting. Cultures of Fe(III)-reducing bacteria enriched from radionuclide-contaminated sediment using Fe(III) oxide as an electron acceptor in the presence of 25 μM Tc(VII) contained a single Geobacter sp. detected by 16S ribosomal DNA analysis and were also able to reduce and precipitate the radionuclide via biogenic magnetite. Fe(III) reduction was stimulated in aquifer material, resulting in the formation of Fe(II)-containing minerals that were able to reduce and precipitate Tc(VII). These results suggest that Fe(III)-reducing bacteria may play an important role in immobilizing technetium in sediments via direct and indirect mechanisms. 相似文献
10.
The iron-storage protein, ferritin, is widely found in all Domains of life. A conserved diiron center in ferritin catalyzes oxidation of Fe(II) and regulates storage of the resultant Fe(III) oxidation product. When this center is filled with Fe(III), in bacterial or archaeal ferritin the presence of phosphate accelerates the rate of Fe(II) oxidation. The molecular mechanism underlying this stimulatory effect of phosphate is unknown. Using site directed mutagenesis of the residues in the diiron center of the archaeal ferritin from Pyrococcus furiosus we show that phosphate facilitates displacement of Fe(III) by Fe(II) from this site. Therefore, the rate of Fe(II) oxidation increases only when the ferroxidase center is filled with Fe(III). 相似文献
11.
12.
Anirban Chakraborty Flynn Picardal 《World journal of microbiology & biotechnology》2013,29(4):617-623
A species of Dechloromonas, strain UWNR4, was isolated from a nitrate-reducing, enrichment culture obtained from Wisconsin River (USA) sediments. This strain was characterized for anaerobic oxidation of both aqueous and chelated Fe(II) coupled to nitrate reduction at circumneutral pH. Dechloromonas sp. UWNR4 was incubated in anoxic batch reactors in a defined medium containing 4.5–5 mM NO3 ?, 6 mM Fe2+ and 1–1.8 mM acetate. Strain UWNR4 efficiently oxidized Fe2+ with 90 % oxidation of Fe2+ after 3 days of incubation. However, oxidation of Fe2+ resulted in Fe(III)-hydroxide-encrusted cells and loss of metabolic activity, suggested by inability of the cells to utilize further additions of acetate. In similar experiments with chelated iron (Fe(II)-EDTA), encrusted cells were not produced and further additions of acetate and Fe(II)-EDTA could be oxidized. Although members of the genus Dechloromonas are primarily known as perchlorate and nitrate reducers, our findings suggest that some species could be members of microbial communities influencing iron redox cycling in anoxic, freshwater sediments. Our work using Fe(II)-EDTA also demonstrates that Fe(II) oxidation was microbially catalyzed rather than a result of abiotic oxidation by biogenic NO2 ?. 相似文献
13.
Neutrophilic Fe(II) oxidizing microorganisms are found in many natural environments. It has been hypothesized that, at low oxygen concentrations, microbial iron oxidation is favored over abiotic oxidation. Here, we compare the kinetics of abiotic Fe(II) oxidation to oxidation in the presence of the bacterium Leptothrix cholodnii Appels isolated from a wetland sediment. Rates of Fe(II) oxidation were determined in batch experiments at 20°C, pH 7 and oxygen concentrations between 3 and 120 μmol/l. The reaction progress in experiments with and without cells exhibited two distinct phases. During the initial phase, the oxygen dependency of microbial Fe(II) oxidation followed a Michaelis-Menten rate expression (KM = 24.5 ± 10 μmol O2/l, vmax = 1.8 ± 0.2 μmol Fe(II)/(l min) for 108 cells/ml). In contrast, abiotic rates increased linearly with increasing oxygen concentrations. At similar oxygen concentrations, initial Fe(II) oxidation rates were faster in the experiments with bacteria. During the second phase, the accumulated iron oxides catalyzed further oxidative iron precipitation in both abiotic and microbial reaction systems. That is, abiotic oxidation also dominated the reaction progress in the presence of bacteria. In fact, in some experiments with bacteria, iron oxidation during the second phase proceeded slower than in the absence of bacteria, possibly due to an inhibitory effect of extracellular polymeric substances on the growth of Fe(III) oxides. Thus, our results suggest that the competitive advantage of microbial iron oxidation in low oxygen environments may be limited by the autocatalytic nature of abiotic Fe(III) oxide precipitation, unless the accumulation of Fe(III) oxides is prevented, for example, through a close coupling of Fe(II) oxidation and Fe(III) reduction. 相似文献
14.
Incubation of PS II membranes with herbicides results in changes in EPR signals arising from reaction centre components. Dinoseb, a phenolic herbicide which binds to the reaction centre polypeptide, changes the width and form of the EPR signal arising from photoreduced Q?AFe. o-Phenanthroline slightly broadens the Q?AFe signal. These effects are attributed to changes in the interaction between the semi-quinone and the iron. DCMU, which binds to the 32 kDa protein, has virtually no effect on the width of the Q?AFe signal but does give rise to an increase in its amplitude. This could result from a change in redox state of an interacting component. Herbicide effects can also be seen when Q?AFe is chemically reduced and these seen to be reflected by changes in splitting and amplitude of the split pheophytin? signal. Dinoseb also results in the loss of ‘Signal II dark’, the conversion of reduced high-potential cytochrome b559 to its oxidized low-potential form and the presence of transiently photooxidized carotenoid after a flash at 25°C; these effects indicate that dinoseb may also act as an ADRY reagent. 相似文献
15.
Phototrophic Fe(II) Oxidation Promotes Organic Carbon Acquisition by Rhodobacter capsulatus SB1003
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Anoxygenic phototrophic Fe(II) oxidation is usually considered to be a lithoautotrophic metabolism that contributes to primary production in Fe-based ecosystems. In this study, we employed Rhodobacter capsulatus SB1003 as a model organism to test the hypothesis that phototrophic Fe(II) oxidation can be coupled to organic carbon acquisition. R. capsulatus SB1003 oxidized Fe(II) under anoxic conditions in a light-dependent manner, but it failed to grow lithoautotrophically on soluble Fe(II). When the strain was provided with Fe(II)-citrate, however, growth was observed that was dependent upon microbially catalyzed Fe(II) oxidation, resulting in the formation of Fe(III)-citrate. Subsequent photochemical breakdown of Fe(III)-citrate yielded acetoacetic acid that supported growth in the light but not the dark. The deletion of genes (RRC00247 and RRC00248) that encode homologs of atoA and atoD, required for acetoacetic acid utilization, severely impaired the ability of R. capsulatus SB1003 to grow on Fe(II)-citrate. The growth yield achieved by R. capsulatus SB1003 in the presence of citrate cannot be explained by lithoautotrophic growth on Fe(II) enabled by indirect effects of the ligand [such as altering the thermodynamics of Fe(II) oxidation or preventing cell encrustation]. Together, these results demonstrate that R. capsulatus SB1003 grows photoheterotrophically on Fe(II)-citrate. Nitrilotriacetic acid also supported light-dependent growth on Fe(II), suggesting that Fe(II) oxidation may be a general mechanism whereby some Fe(II)-oxidizing bacteria mine otherwise inaccessible organic carbon sources. 相似文献
16.
Oxygen flash yield patterns of dark adapted thylakoid membranes as measured with a Joliot-type O2-electrode indicate that inhibitors that block the oxidation of the reduced primary quinone Q?A of Photosystem II vary greatly in the rate of binding to and release from the inhibitor / QB binding environment. The ‘classical’ Photosystem-II herbicides like diuron and atrazine exhibit slow binding and release kinetics, whereas, for example, phenolic inhibitors, o-phenanthroline and synthetic quinones are exchanging quite rapidly with QB (about once per second or faster at inhibitor concentrations causing about 50% inhibition of O2 evolution). No general relationship between the efficiency of the inhibitor and the exchange rate is observed; it depends mainly on the type of inhibitor. Based on the classical Kok model, equations are derived in order to calculate oxygen yields evolved by thylakoids in single-turnover flashes as a function of the rate constants of inhibitor binding to and release from the inhibitor / QB binding environment in the presence of an oxidized or semireduced QA · QB or QA · inhibitor complex. Fitting of theoretical and experimental values yields that o-phenanthroline binds much faster to an oxidized than to a semireduced QA · QB complex. This fits very well with the hypothesis that the Q?B affinity to the site is much higher than that of QB. In the case of i-dinoseb, however, inhibitor / quinone exchange seems to occur mainly in the semiquinone state. Possibilities to explain this result are discussed. 相似文献
17.
Inhibition of electron flow from H2O to methylviologen by 3-(34 dichlorophenyl)-1,1 dimethyl urea (DCMU), yields a biphasic curve — an initial high sensitivity phase and a subsequent low sensitivity phase. The two phases of electron flow have a different pH dependence and differ in the light intensity required for saturation.Preincubation of chloroplasts with ferricyanide causes an inhibition of the high sensitivity phase, but has no effect on the low sensitivity phase. The extent of inhibition increases as the redox potential during preincubation becomes more positive. Tris-treatment, contrary to preincubation with ferricyanide, affects, to a much greater extent, the low sensitivity phase.Trypsin digestion of chloroplasts is known to block electron flow between Q
A and Q
B, allowing electron flow to ferricyanide, in a DCMU insensitive reaction. We have found that in trypsinated chloroplasts, electron flow becomes progressively inhibited by DCMU with increase in pH, and that DCMU acts as a competitive inhibitor with respect to [H+]. The sensitivity to DCMU rises when a more negative redox potential is maintained during trypsin treatment. Under these conditions, only the high sensitivity, but not the low sensitivity phase is inhibited by DCMU.The above results indicate the existence of two types of electron transport chains. One type, in which electron flow is more sensitive to DCMU contains, presumably Fe in a Q
A Fe complex and is affected by its oxidation state, i.e., when Fe is reduced, it allows electron flow to Q
B in a DCMU sensitive step; and a second type, in which electron transport is less sensitive to DCMU, where Fe is either absent or, if present in its oxidized state, is inaccessible to reducing agents.Abbreviations DCMU
3-(34 dichlorophenyl)-1, 1 Dimethyl urea
- MV
methyl viologen
- PS II
Photosystem II
- Tris
tris (hydroxymethyl)aminomethane 相似文献
18.
Suboxic Deposition of Ferric Iron by Bacteria in Opposing Gradients of Fe(II) and Oxygen at Circumneutral pH 总被引:4,自引:1,他引:3
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The influence of lithotrophic Fe(II)-oxidizing bacteria on patterns of ferric oxide deposition in opposing gradients of Fe(II) and O2 was examined at submillimeter resolution by use of an O2 microelectrode and diffusion microprobes for iron. In cultures inoculated with lithotrophic Fe(II)-oxidizing bacteria, the majority of Fe(III) deposition occurred below the depth of O2 penetration. In contrast, Fe(III) deposition in abiotic control cultures occurred entirely within the aerobic zone. The diffusion microprobes revealed the formation of soluble or colloidal Fe(III) compounds during biological Fe(II) oxidation. The presence of mobile Fe(III) in diffusion probes from live cultures was verified by washing the probes in anoxic water, which removed ca. 70% of the Fe(III) content of probes from live cultures but did not alter the Fe(III) content of probes from abiotic controls. Measurements of the amount of Fe(III) oxide deposited in the medium versus the probes indicated that ca. 90% of the Fe(III) deposited in live cultures was formed biologically. Our findings show that bacterial Fe(II) oxidation is likely to generate reactive Fe(III) compounds that can be immediately available for use as electron acceptors for anaerobic respiration and that biological Fe(II) oxidation may thereby promote rapid microscale Fe redox cycling at aerobic-anaerobic interfaces. 相似文献
19.
Certain phenolic compounds represent a distinct class of Photosystem (PS) II QB site inhibitors. In this paper, we report a detailed study of the effects of 2,4,6-trinitrophenol (TNP) and other phenolic inhibitors, bromoxynil and dinoseb, on PS II energetics. In intact PS II, phenolic inhibitors bound to only 90-95% of QB sites even at saturating concentrations. The remaining PS II reaction centers (5-10%) showed modified QA to QB electron transfer but were sensitive to urea/triazine inhibitors. The binding of phenolic inhibitors was 30- to 300-fold slower than the urea/triazine class of QB site inhibitors, DCMU and atrazine. In the sensitive centers, the S2QA− state was 10-fold less stable in the presence of phenolic inhibitors than the urea/triazine herbicides. In addition, the binding affinity of phenolic herbicides was decreased 10-fold in the S2QA− state than the S1QA state. However, removal of the oxygen-evolving complex (OEC) and associated extrinsic polypeptides by hydroxylamine (HA) washing abolished the slow binding kinetics as well as the destabilizing effects on the charge-separated state. The S2-multiline electron paramagnetic resonance (EPR) signal and the ‘split’ EPR signal, originating from the S2YZ state showed no significant changes upon binding of phenolic inhibitors at the QB site. We thus propose a working model where QA redox potential is lowered by short-range conformational changes induced by phenolic inhibitor binding at the QB niche. Long-range effects of HA-washing eliminate this interaction, possibly by allowing more flexibility in the QB site. 相似文献
20.
Nitrate‐reducing, Fe(II)‐oxidizing bacteria were suggested to couple with enzymatic Fe(II) oxidation to nitrate reduction. Denitrification proceeds via intermediates (, NO) that can oxidize Fe(II) abiotically at neutral and particularly at acidic pH. Here, we present a revised Fe(II) quantification protocol preventing artifacts during acidic Fe extraction and evaluate the contribution of abiotic vs. enzymatic Fe(II) oxidation in cultures of the nitrate‐reducing, Fe(II) oxidizer Acidovorax sp. BoFeN1. Sulfamic acid used instead of HCl reacts with nitrite and prevents abiotic Fe(II) oxidation during Fe extraction. Abiotic experiments without sulfamic acid showed that acidification of oxic Fe(II) nitrite samples leads to 5.6‐fold more Fe(II) oxidation than in anoxic samples because the formed NO becomes rapidly reoxidized by O2, therefore leading to abiotic oxidation and underestimation of Fe(II). With our revised protocol using sulfamic acid, we quantified oxidation of approximately 7 mm of Fe(II) by BoFeN1 within 4 days. Without addition of sulfamic acid, the same oxidation was detected within only 2 days. Additionally, abiotic incubation of Fe(II) with nitrite in the presence of goethite as surface catalyst led to similar abiotic Fe(II) oxidation rates as observed in growing BoFeN1 cultures. BoFeN1 growth was observed on acetate with N2O as electron acceptor. When adding Fe(II), no Fe(II) oxidation was observed, suggesting that the absence of reactive N intermediates (, NO) precludes Fe(II) oxidation. The addition of ferrihydrite [Fe(OH)3] to acetate/nitrate BoFeN1 cultures led to growth stimulation equivalent to previously described effects on growth by adding Fe(II). This suggests that elevated iron concentrations might provide a nutritional effect rather than energy‐yielding Fe(II) oxidation. Our findings therefore suggest that although enzymatic Fe(II) oxidation by denitrifiers cannot be fully ruled out, its contribution to the observed Fe(II) oxidation in microbial cultures is probably lower than previously suggested and has to be questioned in general until the enzymatic machinery‐mediating Fe(II) oxidation is identified. 相似文献