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1.
Effect of bicarbonate on the water-oxidizing complex of photosystem II in the super-reduced S-states
It is shown that the hydrazine-induced transition of the water-oxidizing complex (WOC) to super-reduced S-states depends on the presence of bicarbonate in the medium so that after a 20 min treatment of isolated spinach thylakoids with 3 mM NH(2)NH(2) at 20 degrees C in the CO(2)/HCO(3)(-)-depleted buffer the S-state populations are: 42% of S(-3), 42% of S(-2), 16% of S(-1) and even formal S(-4) state is reached, while in the presence of 2 mM NaHCO(3), the same treatment produces 30% of S(-3), 38% of S(-2), and 32% of S(-1) and there is no indication of the S(-4) state. Bicarbonate requirement for the oxygen-evolving activity, very low in untreated thylakoids, considerably increases upon the transition of the WOC to the super-reduced S-states, and the requirement becomes low again when the WOC returns back to the normal S-states using pre-illumination. The results are discussed as a possible indication of ligation of bicarbonate to manganese ions within the WOC. 相似文献
2.
In this study, we probe the effects of bicarbonate (hydrogencarbonate), BC, removal from photosystem II in spinach thylakoids
by measuring flash-induced oxygen evolution patterns (FIOPs) with a Joliot-type electrode. For this we compared three commonly
employed methods: (1) washing in BC-free medium, (2) formate addition, and (3) acetate addition. Washing of the samples with
buffers depleted of BC and CO2 by bubbling with argon (Method 1) under our conditions leads to an increase in the double hit parameter of the first flash
(β1), while the miss parameter and the overall activity remain unchanged. In contrast, addition of 40–50 mM formate or acetate
results in a significant increase in the miss parameter and to an ∼50% (formate) and ∼10% (acetate) inhibition of the overall
oxygen evolution activity, but not to an increased β1 parameter. All described effects could be reversed by washing with formate/acetate free buffer and/or addition of 2–10 mM
bicarbonate. The redox potential of the water-oxidizing complex (WOC) in samples treated by Method 1 is compared to samples
containing 2 mM bicarbonate in two ways: (1) The lifetimes of the S0, S2, and S3 states were measured, and no differences were found between the two sample types. (2) The S1, S0, S−1, and S−2 states were probed by incubation with small concentrations of NH2OH. These experiments displayed a subtle, yet highly reproducible difference in the apparent Si/S−i state distribution which is shown to arise from the interaction of BC with PSII in the already reduced states of the WOC.
These data are discussed in detail by also taking into account the CO2 concentrations present in the buffers after argon bubbling and during the measurements. These values were measured by membrane-inlet
mass spectrometry (MIMS). 相似文献
3.
András Szilárd 《BBA》2007,1767(6):876-882
The water-oxidizing complex of Photosystem II is an important target of ultraviolet-B (280-320 nm) radiation, but the mechanistic background of the UV-B induced damage is not well understood. Here we studied the UV-B sensitivity of Photosystem II in different oxidation states, called S-states of the water-oxidizing complex. Photosystem II centers of isolated spinach thylakoids were synchronized to different distributions of the S0, S1, S2 and S3 states by using packages of visible light flashes and were exposed to UV-B flashes from an excimer laser (λ = 308 nm). The loss of oxygen evolving activity showed that the extent of UV-B damage is S-state-dependent. Analysis of the data obtained from different synchronizing flash protocols indicated that the UV-sensitivity of Photosystem II is significantly higher in the S3 and S2 states than in the S1 and S0 states. The data are discussed in terms of a model where UV-B-induced inhibition of water oxidation is caused either by direct absorption within the catalytic manganese cluster or by damaging intermediates of the water oxidation process. 相似文献
4.
《BBA》2023,1864(4):148998
Chloride (Cl−) is essential for O2 evolution during photosynthetic water oxidation. Two chlorides near the water-oxidizing complex (WOC) in Photosystem II (PSII) structures from Thermosynechococcus elongatus (and T. vulcanus) have been postulated to transfer protons generated from water oxidation. We monitored four criteria: primary charge separation flash yield (P* → P+QA−), rates of water oxidation steps (S-states), rate of proton evolution, and flash O2 yield oscillations by measuring chlorophyll variable fluorescence (P* quenching), pH-sensitive dye changes, and oximetry. Br-substitution slows and destabilizes cellular growth, resulting from lower light-saturated O2 evolution rate (−20 %) and proton release (−36 % ΔpH gradient). The latter implies less ATP production. In Br- cultures, protonogenic S-state transitions (S2 → S3 → S0’) slow with increasing light intensity and during O2/water exchange (S0’ → S0 → S1), while the non-protonogenic S1 → S2 transition is kinetically unaffected. As flash rate increases in Cl− cultures, both rate and extent of acidification of the lumen increase, while charge recombination is suppressed relative to Br−. The Cl− advantage in rapid proton escape from the WOC to lumen is attributed to correlated ion-pair movement of H3O+Cl− in dry water channels vs. separated Br− and H+ ion movement through different regions (>200-fold difference in Bronsted acidities). By contrast, at low flash rates a previously unreported reversal occurs that favors Br− cultures for both proton evolution and less PSII charge recombination. In Br− cultures, slower proton transfer rate is attributed to stronger ion-pairing of Br− with AA residues lining the water channels. Both anions charge-neutralize protons and shepherd them to the lumen using dry aqueous channels. 相似文献
5.
The effect of high temperature treatment (40 °C, 3 h, illumination at 100 μmol m− 2 s− 1) on the photosynthetic electron flow in barley seedlings of different age was investigated. Thermoinduced inhibition of the liner electron flow due to partial impairment of the water oxidizing complex (WOC) and the increase in the extent of QA− reoxidation by Tyrzox in thylakoids isolated from 4-day-old leaves was shown by measurements of oxygen evolution using benzoquinone or potassium ferricyanide as electron acceptors, as well as by following QA− reoxidation kinetics in the absence and presence of exogenous electron acceptors, DCBQ and DMBQ. Using HPLC analysis, an increase in the oxidation of the photoactive plastoquinone pool in young leaves under heating was shown. In older, 11-day-old leaves, heat treatment limited both photosynthetic electron flow and oxygen evolution. The same effects of heat shock on oxygen evolution caused an inhibition of electron flow on the donor side of PSII only. However, a rise in the proportion of PSII with QA− reoxidized through recombination with the S2/S3 state of the WOC was observed. The addition of exogenous electron acceptors (DCBQ and DMBQ) and a donor (DPC) showed that the thermoinduced decrease in the electron transport rate was caused by an impediment of electron flow from QA− to acceptor pool. The decrease in size of the photoactive PQ-pool and a change in the proportions of oxidized and reduced PQ in older leaves under heat treatment were shown. It was suggested that a thermoinduced change of the redox state of the PQ-pool and a redistribution of plastoquinone molecules between photoactive and non-photoactive pools are the mechanisms which reflect and regulate the response of the photosynthetic apparatus under heat stress conditions. 相似文献
6.
Raif Musa-Aziz Lihong Jiang Li-Ming Chen Kevin L. Behar Walter F. Boron 《The Journal of membrane biology》2009,228(1):15-31
Others have shown that exposing oocytes to high levels of (10–20 mM) causes a paradoxical fall in intracellular pH (pHi), whereas low levels (e.g., 0.5 mM) cause little pHi change. Here we monitored pHi and extracellular surface pH (pHS) while exposing oocytes to 5 or 0.5 mM NH3/NH4
+. We confirm that 5 mM causes a paradoxical pHi fall (−ΔpHi ≅ 0.2), but also observe an abrupt pHS fall (−ΔpHS ≅ 0.2)—indicative of NH3 influx—followed by a slow decay. Reducing [NH3/NH4
+] to 0.5 mM minimizes pHi changes but maintains pHS changes at a reduced magnitude. Expressing AmtB (bacterial Rh homologue) exaggerates −ΔpHS at both levels. During removal of 0.5 or 5 mM NH3/NH4
+, failure of pHS to markedly overshoot bulk extracellular pH implies little NH3 efflux and, thus, little free cytosolic NH3/NH4
+. A new analysis of the effects of NH3 vs. NH4
+ fluxes on pHS and pHi indicates that (a) NH3 rather than NH4
+ fluxes dominate pHi and pHS changes and (b) oocytes dispose of most incoming NH3. NMR studies of oocytes exposed to 15N-labeled show no significant formation of glutamine but substantial accumulation in what is likely an acid intracellular compartment. In conclusion, parallel measurements of pHi and pHS demonstrate that NH3 flows across the plasma membrane and provide new insights into how a protein molecule in the plasma membrane—AmtB—enhances
the flux of a gas across a biological membrane.
相似文献
Walter F. Boron (Corresponding author)Email: |
7.
G. Renger G. Christen M. Karge H.-J. Eckert K.-D. Irrgang 《Journal of biological inorganic chemistry》1998,3(4):360-366
The temperature dependence of donor side reactions was analysed within the framework of the Marcus theory of nonadiabatic electron transfer. The following results were obtained for PS II membrane fragments from spinach: (1) the reorganisation energy of P680+? reduction by YZ is of the order of 0.5?eV in samples with a functionally fully competent water oxidising complex (WOC); (2) destruction of the WOC by Tris-washing gives rise to a drastic increase of λ to values of the order of 1.6?eV; (3) the reorganisation energies of the oxidation steps in the WOC are dependent, on the redox states S i with values of about 0.6?eV for the reactions YZ OX S 0→YZ S 1 and YZ OX S 1→YZ S 2, 1.6?eV for the reaction YZ OX S 2→YZ S 3 and 1.1?eV (above a characteristic temperature uc of about 6??°C) for the reaction YZ OX S 3→→YZ S 0+O2. Using an empirical rate constant-distance relationship, the van der Waals distance between YZ and P680 was found to be about 10?Å, independent of the presence or absence of the WOC, whereas the distance between YZ and the manganese cluster in the WOC was ≥15?Å. Based on the calculated activation energies the environment of YZ is inferred to be almost "dry" and hydrophobic when the WOC is intact but becomes enriched with water molecules after WOC destruction. Furthermore, it is concluded that the transition S 2→S 3 is an electron transfer reaction gated by a conformational change, i.e. it comprises significant structural changes of functional relevance. Measurements of kinetic H/D isotope exchange effects support the idea that none of these reactions is gated by the break of a covalent O-H bond. The implications of these findings for the mechanism of water oxidation are discussed. 相似文献
8.
In Photosystem II (PS II), water is oxidized to molecular oxygen and plastoquinone is reduced to plastoquinol. The oxidation of water requires the accumulation of four oxidizing equivalents, through the so-called S-states of the oxygen evolving complex; the production of plastoquinol requires the accumulation of two reducing equivalents on a bound plastoquinone, QB. It has been generally believed that during the flash-induced transition of each of the S-states (Sn Sn+1, where n=0, 1, 2 and 3), a certain small but equal fraction of the PS II reaction centers are unable to function and, thus, miss being turned over. We used thoroughly dark-adapted thylakoids from peas (Pisum sativum) and Chenopodium album (susceptible and resistant to atrazine) starting with 100% of the oxygen evolving complex in the S1 state. Thylakoids were illuminated with saturating flashes, providing a double hit parameter of about 0.07. Our experimental data on flashnumber dependent oscillations in the amount of oxygen per flash fit very well with a binary pattern of misses: 0, 0.2, 0, 0.4 during S0 S1, S1 S2, S2 S3 and S3 S0 transitions. Addition of 2 mM ferricyanide appears to shift this pattern by one flash. These results are consistent with the bicycle model recently proposed by V. P. Shinkarev and C. A. Wraight (Oxygen evolution in photosynthesis: From unicycle to bicycle, 1993, Proc Natl Acad Sci USA 90: 1834–1838), where misses are due to the presence of P+ or QA
- among the various equilibrium states of PS II centers.Abbreviations
miss parameter
-
double hit parameter
- PS II
Photosystem II
- QA
primary one-electron acceptor of PS II, a plastoquinone molecule
- QB
secondary plastoquinone two-electron acceptor of PS II
- S-states (Sn, where n=0, 1, 2, 3 or 4)
redox states of the oxygen evolving complex 相似文献
9.
《BBA》1987,893(3):452-469
Absorption changes coupled with the individual transitions S0–S3 and redox reactions in the water-splitting enzyme system S of photosynthesis have been measured. The principal difficulties of measuring the very small absorption changes in the ultraviolet coupled with those reactions have been reduced drastically through the use of a highly purified Photosystem II complex isolated from the Cyanobacterium synechococcus. The general problem caused by the mixing of the S states during a train of flashes and the falsification through the overlap with absorption changes of QB (binary oscillations) have been treated as follows. (1) The binary oscillations were bypassed through the use of silicomolybdate and high concentrations of DCBQ, respectively, as external electron acceptor. (2) Stable absorption changes of the mixed S-state transitions have been deconvoluted through fitting procedures to get the changes of the individual transitions of S1 → S2 → S3 → S0 → S1. (3) Kinetically resolved absorption changes of the S-states in the 100-μs range gave independent information on the individual transitions. (4) Stable absorption changes of the S0 → S1 transitions in the forefront were induced after shifting the S states through low concentrations of NH2OH two units backwards. Analysis of the resulting sequence Sx → S0 → S1 → S2 → S3 → S0, beginning with an NH2OH depending pre-state, Sx, and followed by an S0 → S1 transition not mixed with the opposite S3 → S0 transition, increased the conclusiveness considerably. It results that the ultraviolet spectrum of the S0 → S1 transition is different from the spectra of the S1 → S2 and S2 → S3 transition. Possible states of manganese, water and surplus charges responsible for these spectra are presented. 相似文献
10.
Junichi Mano Kunio Kawamoto G. Charles Dismukes Kozi Asada 《Photosynthesis research》1993,38(3):433-440
A new binding site for anions which inhibit the water oxidizing complex (WOC) of Photosystem II in spinach has been identified. Anions which bind to this site inhibit the flash-induced S2/S0 catalase reaction (2H2O22H2O+O2) of the WOC by displacing hydrogen peroxide. Using a mass spectrometer and gas permeable membrane to detect the 32O2 product, the yield and lifetime of the active state of the flash-induced catalase (to be referred to simply as flash-catalase) reaction were measured after forming the S2 or S0-states by a short flash. The increase in flash-catalase activity with H2O2 concentration exhibits a Km=10–20 mM, and originates from an increase in the lifetime by 20-fold of the active state. The increased lifetime in the presence of peroxide is ascribed to formation of the long-lived S0-state at the expense of the unstable S2-state. The anion inhibition site differs from the chloride site involved in stimulating the photolytic water oxidation reaction (2H2OO2+4e-+4H+). Whereas water oxidation requires Cl- and is inhibited with increasing effectiveness by F-CN-N3
-, the flash-catalase reaction is weakly inhibited by Cl-, and with increasing effectiveness by F-CN-, N3
-. Unlike water oxidation, chloride is unable to suppress or reverse inhibition of the flash-catalase reaction caused by these anions. The inhibitor effectiveness correlates with the pKa of the conjugate acid, suggesting that the protonated species may be the active inhibitor. The reduced activity arises from a shortening of the lifetime of the flash-induced catalase active state by 3–10 fold owing to stronger anion binding in the flash-induced states, S2 and S0, than in the dark S-states, S1 and S-1. To account for the paradoxical result that higher anion concentrations are required to inhibit at lower H2O2 concentrations, where S2 forms initially after the flash, than at higher H2O2 concentrations, where S0 forms initially after the flash, stronger anion binding to the S0-state than to the S2-state is proposed. A kinetic model is given which accounts for these equilibria with anions and H2O2. The rate constant for the formation/release of O2 by reduction of S2 in the WOC is <0.4 s-1.Abbreviations ADRY
acceleration of the deactivation reactions of the water splitting enzyme system Y
- BTP
bis [tris(hydroxymethyl)methylamino]-propane
- CCCP
carbonylcyanide m-chlorophenylhyrazone
- DCBQ
2,5-dichlorobenzoquinone
- DMBQ
2,3-dimethylbenzoquinone
- WOC
water oxidizing complex 相似文献
11.
We have analyzed flash-induced period-four damped oscillation of oxygen evolution and chlorophyll fluorescence with the aid of a kinetic model of photosystem II. We have shown that, for simulation of the period-four oscillatory behavior of oxygen evolution, it is essential to consider the so-called intermediate S-state as an initial phase of each of the Sn-Sn+1, (n = 0, 1, 2, 3) transitions. The intermediate S-states are defined as []-states (n = 0, 1, 2, 3) and are formed with rate constant kiSn ∼1.5 × 106 s−1, which was determined from comparison of theoretical predictions with experimental data. The assumed intermediate S-states shift the equilibrium in reaction more to the right and we suggest that kinetics of the intermediate S-states reflects a relaxation process associated with changes of the redox equilibrium in the above reaction. The oxygen oscillation is simulated without the miss and double-hit parameters, if the intermediate S-states, which are not the source of the misses or the double-hits, are included in the simulation. Furthermore, we have shown that the intermediate S-states, together with charge recombination, are prerequisites for the simulation of the period-four oscillatory behavior of the chlorophyll fluorescence. 相似文献
12.
Mechanistic and structural aspects of photosynthetic water oxidation 总被引:10,自引:0,他引:10
G. Renger 《Physiologia plantarum》1997,100(4):828-841
Conclusions on the functional and structural organisation of photosynthetic water oxidation are gathered from a critical survey of the wealth of data reported in the literature and author's own experimental research: (1) the water oxidising complex (WOC) contains a tetranuclear manganese cluster of dimer of dimers' structure and functional heterogeneity of the metal centers, (2) the four step univalent oxidative pathway leading to water oxidation into molecular oxygen and four protons comprises only manganese, tyrosine YZ of polypeptide Dl and the substrate as redox active species, (3) the redox transitions S0→ S1 and S1→ S2 are manganese centered whereas S2→ S3 is most likely a ligand-centered reaction, (4) there exist several lines of evidence for a marked structural change that accompanies the redox transition S2→ S3, (5) one Ca2+ is an indispensible constituent of a functionally competent WOC while the role of Cl is much less clear and a direct participation disputable, (6) substrate water is most likely bound in all redox states S0,…,S3 and exchangeable with the bulk phase. The protonation state is determined by the redox state S1 and the protein microenvironment. A mechanism is proposed for water oxidation in the WOC that is based on three key postulates: (1) water oxidation takes place in the first coordination sphere of one manganese dimer [MnaMnb]; (2) the essential O-O bond is preformed in S3 as part of a rapid redox isomerism S3(I)→S3(II) where in S3(II) a nuclear geometry and electronic configuration is attained that corresponds to a peroxidic-type species; and (3) S3(II) is an ‘entatic state’ for the formation of complexed dioxygen triggered by YZOX induced electron abstraction from the WOC and electronic redistribution to S0(O2). 相似文献
13.
Yashar Feyziyev Zsuzsanna Deák Stenbjörn Styring Gábor Bernát 《Journal of bioenergetics and biomembranes》2013,45(1-2):111-120
The Mn4CaO5 cluster of photosystem II (PSII) catalyzes the oxidation of water to molecular oxygen through the light-driven redox S-cycle. The water oxidizing complex (WOC) forms a triad with TyrosineZ and P680, which mediates electrons from water towards the acceptor side of PSII. Under certain conditions two other redox-active components, TyrosineD (YD) and Cytochrome b 559 (Cyt b 559) can also interact with the S-states. In the present work we investigate the electron transfer from Cyt b 559 and YD to the S2 and S3 states at 195 K. First, YD ? and Cyt b 559 were chemically reduced. The S2 and S3 states were then achieved by application of one or two laser flashes, respectively, on samples stabilized in the S1 state. EPR signals of the WOC (the S2-state multiline signal, ML-S2), YD ? and oxidized Cyt b 559 were simultaneously detected during a prolonged dark incubation at 195 K. During 163 days of incubation a large fraction of the S2 population decayed to S1 in the S2 samples by following a single exponential decay. Differently, S3 samples showed an initial increase in the ML-S2 intensity (due to S3 to S2 conversion) and a subsequent slow decay due to S2 to S1 conversion. In both cases, only a minor oxidation of YD was observed. In contrast, the signal intensity of the oxidized Cyt b 559 showed a two-fold increase in both the S2 and S3 samples. The electron donation from Cyt b 559 was much more efficient to the S2 state than to the S3 state. 相似文献
14.
The functional state of the Photosystem (PS) II complex in Arabidopsis psbR T-DNA insertion mutant was studied. The ΔPsbR thylakoids showed about 34% less oxygen evolution than WT, which correlates with the amounts of PSII estimated from YDox radical EPR signal. The increased time constant of the slow phase of flash fluorescence (FF)-relaxation and upshift in the peak position of the main TL-bands, both in the presence and in the absence of DCMU, confirmed that the S2QA− and S2QB− charge recombinations were stabilized in ΔPsbR thylakoids. Furthermore, the higher amount of dark oxidized Cyt-b559 and the increased proportion of fluorescence, which did not decay during the 100s time span of the measurement thus indicating higher amount of YD+QA− recombination, pointed to the donor side modifications in ΔPsbR. EPR measurements revealed that S1-to-S2-transition and S2-state multiline signal were not affected by mutation. The fast phase of the FF-relaxation in the absence of DCMU was significantly slowed down with concomitant decrease in the relative amplitude of this phase, indicating a modification in QA to QB electron transfer in ΔPsbR thylakoids. It is concluded that the lack of the PsbR protein modifies both the donor and the acceptor side of the PSII complex. 相似文献
15.
Horst Kunkely 《Inorganica chimica acta》2004,357(8):2401-2403
The ion pair [PtIV(NH3)5Cl]3+S2O82− shows a S2O82− → [Pt(NH3)5Cl]3+ outer-sphere charge transfer (OSCT) absorption at λmax=267 nm. OSCT excitation leads to the reduction of Pt(IV) by S2O82− to Pt(II) with φ=3×10−3 at λirr=280 nm. 相似文献
16.
A detailed electronic structure of the Mn4Ca cluster is required before two key questions for understanding the mechanism of photosynthetic water oxidation can be addressed.
They are whether all four oxidizing equivalents necessary to oxidize water to O2 accumulate on the four Mn ions of the oxygen-evolving complex, or do some ligand-centered oxidations take place before the
formation and release of O2 during the S3 → [S4] → S0 transition, and what are the oxidation state assignments for the Mn during S-state advancement. X-ray absorption and emission
spectroscopy of Mn, including the newly introduced resonant inelastic X-ray scattering spectroscopy have been used to address
these questions. The present state of understanding of the electronic structure and oxidation state changes of the Mn4Ca cluster in all the S-states, particularly in the S2 to S3 transition, derived from these techniques is described in this review. 相似文献
17.
Oil sands tailings ponds contain large volumes (∼108 m3) of fine tailings, originating from bitumen production by surface mining. These sediment rapidly in dilute suspension but then form a network, which consolidates much more slowly. The overall process increases solid content to up to 85% (w/w) and is referred to as tailings densification. Addition of gypsum (CaSO4·2H2O) to a mixture of sand and fines gives a non-segregating, consolidated tailings slurry in which calcium ions serve as a cross-linking agent. Tailings ponds also harbor active anaerobic microbial consortia, which are thought to contribute to densification through microbial activity, including gas production, creating dewatering channels. To determine the roles of calcium ions and anaerobic microbial activity in tailings sedimentation, we placed 70% (v/v) tailings, containing 77% (w/w) solids, and 30% (v/v) defined medium with various amendments in anaerobic test tubes with an N2–CO2 headspace. Following mixing the initial sedimentation rate R of the water-tailings boundary and the final percentage (v/v) of sedimentation SF were measured. Amendment with 0–20 mM CaCl2 increased R from 0.006 to up to 0.012 day−1, but decreased SF from 14–15% to 8–10% (v/v), whereas subsequent amendment with lactate increased both R and SF. To determine the effect of the type of anaerobic microbial activity, tubes were amended with (i) 20 mM NaCl or 10 mM CaCl2, (ii) 10 mM Na2SO4 or 10 mM CaSO4, (iii) 20 mM NaNO3 or 10 mM Ca(NO3)2, or (iv) no additions. Following mixing, duplicate tubes were monitored continuously to determine SF, whereas another set of duplicate tubes was re-mixed once per week to determine R, as well as headspace methane, and the concentrations of sulfate, sulfide, nitrate and nitrite in the supernatant fluid. Microbial activity was boosted after 63 days by adding 20 mM lactate to all tubes. The data for this experiment also indicated that R increased, whereas SF decreased by addition of calcium ions. Lactate significantly boosted microbial activity with increased methanogenesis, sulfate reduction or nitrate reduction being observed in tubes amended with no electron acceptor, sulfate or nitrate, respectively. Addition of lactate increased SF by 2–4% (v/v) in most tubes, except in tubes with Ca(NO3)2 in which SF increased by 15% (v/v). The solids content increased from 69 to 78% under these conditions, representing a significant progression to the maximum values observed in tailings ponds over a short period of time. 相似文献
18.
In this communication, evidence is presented from the kinetics of QA ? decay (where QA is the first plastoquinone electron acceptor of photosystem II) and oxygen evolution for the requirement of bicarbonate in the electron transport in a cyanobacteriumSynechocystis (Pasteur Culture Collection 6803). A large slowing down of QA ? oxidation, measured from the variable chlorophylla fluorescence after saturating actinic flashes, was observed in the thylakoids ofSynechocystis 6803 depleted of bicarbonate in the presence of 25 mM formate. Qualitatively similar results were obtained with DCMU-treated thylakoids. This shows that bicarbonate depletion inhibits electron transport on the acceptor side of photosystem II between QA and the plastoquinone (PQ) pool in cyanobacteria. Addition of 2.5 mM HCO3 ? fully reversed the inhibition of electron flow caused by bicarbonate depletion. Two exponential phases of QA ? decay, a fast one and a slow one, were observed with halftimes of approx. 400 μs (fast) and 26 ms (slow) at pH 6.5. At pH 7.5, these phases were approx. 330 μs (fast) and 21 ms (slow), respectively. The amplitude, but not the halftime, of the fast component decreased by about 70% (pH 6.5) or 50% (pH 7.5); this was accompanied by a concomittant increase in the slow phase. Twenty mM bicarbonate stimulated, by a factor of 4, the Hill reaction in bicarbonate-depletedSynechocystis cells. This effect is independent of CO2 fixation as it was observed even in the presence of an inhibitor DBMIB. 相似文献
19.
The rapid development of human activities has caused serious eutrophication of coastal waters in China in the recent decades. The study of the biofiltration capacity of Laminaria japonica under laboratory conditions showed a significant nutrient uptake. After 36 h of incubation, around 42%, 46%, 44% of N and 45%, 42%, 35% of P were removed from three gradients of medium concentrations, respectively. In the conditions of different ratios of N/P and NO3–N/NH4–N, the optimum N/P ratio for nutrient uptake was 7.4 and L. japonica prefered NO3–N rather than NH4–N as nitrogen source. Temperature and irradiance affected uptake rates significantly. The maximal N uptake rate appeared at 10 °C and 18 μmol photons m−2 s−1 and the maximal P uptake rate was found at 15 °C and 144 μmol photons m−2 s−1. Moreover, further studies were needed to investigate the bioremediation potential of L. japonica in the open sea. 相似文献
20.
The kinetics of Mn release during NH2OH inactivation of the water oxidizing reaction is largely insensitive to the S-state present during addition of NH2OH. This appears to reflect reduction by NH2OH of higher S-states to a common more reduced state (S0 or S?1) which alone is susceptible to NH2OH inactivation. Sequences of saturating flashes with dark intervals in the range 0.2–5 s?1 effectively prevent NH2OH inactivation and the associated liberation of manganese. This light-induced protection disappears rapidly when the dark interval is longer than about 5 s. Under continuous illumination, protection against NH2OH inactivation is maximally effective at intensities in the range 103–104 erg · cm?2 · s?1. This behavior differs from that of NH2OH-induced Mn release, which is strongly inhibited at all intensities greater than 103 erg · cm?2 · s?1. This indicates that two distinct processes are responsible for inactivation of water oxidation at high and low intensities. Higher S-states appear to be immune to the reaction by which NH2OH liberates manganese, although the overall process of water oxidation is inactivated by NH2OH in the presence of intense light. The light-induced protection phenomenon is abolished by 50 μM DCMU, but not by high concentrations of carbonyl cyanide m-chlorophenylhydrazone, which accelerates inactivation reactions of the water-splitting enzyme, Y (an ADRY reagent). The latter compound accelerates both inactivation of water oxidation and manganese extraction in the dark. 相似文献