首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
Solar energy exploitation by photosynthetic water cleavage is of central relevance for the development and sustenance of all higher forms of living matter in the biosphere. The key steps of this process take place within an integral protein complex referred to as Photosystem II (PS II) which is anisotropically incorporated into the thylakoid membrane. This minireview concentrates on mechanistic questions related to i) the generation of strongly oxidizing equivalents (holes) at a special chlorophyll a complex (designated as P680) and ii) the cooperative reaction of four holes with two water molecules at a manganese containing unit WOC (water oxidizing complex) resulting in the release of molecular oxygen and four protons. The classical work of Pierre Joliot and Bessel Kok and their coworkers revealed that water oxidation occurs via a sequence of univalent oxidation steps including intermediary redox states Si (i = number of accumulated holes within the WOC). Based on our current stage of knowledge, an attempt is made a) to identify the nature of the redox states Si, b) to describe the structural arrangement of the (four) manganese centers and their presumed coordination and ligation within the protein matrix, and c) to propose a mechanism of photosynthetic water oxidation with special emphasis on the key step, i.e. oxygen-oxygen bond formation. It is assumed that there exists a dynamic equilibrium in S3 with one state attaining the nuclear geometry and electronic configuration of a complexed peroxide. This state is postulated to undergo direct oxidation to complexed dioxygen by univalent electron abstraction with YZ ox and simultaneous internal ligand to metal charge transfer.Key questions on the mechanism will be raised. The still fragmentary answers to these questions not only reflect our limited knowledge but also illustrate the challenges for future research.Abbreviations b559 cytochrome b559 - BChl bacteriochlorophyll - Chl chlorophyll - CP47 Chl a containing a 47 kDa polypeptide - D1/D2 polypeptides of the PS II reaction center - ENDOR electron nuclear double resonance - EPR electron paramagnetic resonance - ESEEM electron spin echo envelope modulation - EXAFS extended X-ray absorption fine structure - FTIR Fourier transform infrared - NMR nuclear magnetic resonance - P680, P700 photoactive Chl a of PS II and PS I, respectively - PS II Photosystem II - QA special plastoquinone of PS II - Si redox states of WOC - WOC water oxidizing complex - WOS water oxidizing site - UV/VIS ultraviolet/visible - YD, YZ redox active tyrosines of polypeptides D2 and D1, respectively  相似文献   

2.
Formation of thermoluminescence signals is characteristics of energy- and charge storage in Photosystem II. In isolated D1/D2/cytochrome b-559 Photosystem II reaction centre preparation four thermoluminescence components were found. These appear at -180 (Z band), between -80 and -50 (Zv band), at -30 and at +35°C. The Z band arises from pigment molecules but not correlated with photosynthetic activity. The Zv and -30°C bands arise from the recombination of charge pairs stabilized in the Photosystem II reaction centre complex. The +35°C band probably corresponds to the artefact glow peak resulting from a pigment-protein-detergent interaction in subchloroplast preparations (Rózsa Zs, Droppa M and Horváth G (1989) Biochim Biophys Acta 973, 350–353).Abbreviations Chl chlorophyll - Cyt cytochrome - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - D1 psbA gene product - D2 psbD gene product - P680 primary electron donor of PS II - Pheo pheophytin - PS II Photosystem II - QA primary quinone acceptor of PS II - QB secondary quinone acceptor of PS II - RC reaction centre of PS II - TL thermoluminescence  相似文献   

3.
Regulation of electron transport rate through Photosystem I (PS I) was investigated in intact sunflower leaves. The rate constant of electron donation via the cytochrome b 6 f complex (kq, s–1) was obtained from the postillumination P700+ reduction rate, measured as the exponential decay of the light-dark difference (D830) of the 830 nm transmission signal. D830 corresponding to maximum oxidisable P700 (D830m) was obtained by applying white light flashes of different intensity and extrapolating the plot of the quantum yield Y vs. D830 to the axis of abscissae (Y->0). Maximum quantum yield of PS I at completely reduced P700 (Ym) was obtained by extrapolating the same plot to the axis of ordinates (D830->0). Regulation of kq, D830m and Ym under rate-limiting CO2 and O2 concentrations applied after air (21% O2, 310 ppm CO2) was investigated. The amplitude of the downregulation of kq (photosynthetic control) was maximal when electron transport rate (ETR) was limited to about 3 nmol cm–2 s–1 and decreased when ETR was higher or lower. Downregulation did not occur in the absence of CO2 and O2. These gases acted only as substrates of ribulosebisphosphate carboxylase-oxygenase, no high-affinity reaction of O2 leading to enhanced photosynthetic control (e.g. Mehler reaction) was detected. After the transition, D830m at first decreased and then increased again, showing that the reduction of the PS I acceptor side disappeared as a result of the downregulation of kq. The variation of Ym had two reasons, PS I acceptor side reduction and variable excitation capture efficiency by P700. It is concluded that electron transport through PS I is coregulated by the rate of plastoquinol oxidation at Cyt b 6 f, excitation capture efficiency by P700, and by acceptor side reduction.Abbreviations Cyt b 6 f cytochrome b 6 f complex - D830 difference of the 830 nm signal from the dark level - ETR electron transport rate - PAD photon absorption density nmol cm–2 s–1 - PFD incident photon flux density, nmol cm–2 s–1 - PS I Photosystem I - PS II Photosystem II - PQH2 plastoquinol - P700 Photosystem I donor pigment - Y quantum yield of PS I electron transport, rel. un.  相似文献   

4.
Old and very recent experiments on the extent and the rate of proton release during the four reaction steps of photosynthetic water oxidation are reviewed. Proton release is discussed in terms of three main sources, namely the chemical production upon electron abstraction from water, protolytic reactions of Mn-ligands (e.g. oxo-bridges), and electrostatic response of neighboring amino acids. The extent of proton release differs between the four oxidation steps and greatly varies as a function of pH both, but differently, in thylakoids and PS II-membranes. Contrastingly, it is about constant in PS II-core particles. In any preparation, and on most if not all reaction steps, a large portion of proton transfer can occur very rapidly (<20 s) and before the oxidation of the Mn-cluster by Yz + is completed. By these electrostatically driven reactions the catalytic center accumulates bases. An additional slow phase is observed during the oxygen evolving step, S3S4S0. Depending on pH, this phase consists of a release or an uptake of protons which accounts for the balance between the number of preformed bases and the four chemically produced protons. These data are compatible with the hypothesis of concerted electron/proton-transfer to overcome the kinetic and energetic constraints of water oxidation.Abbreviations BBY-membranes Photosystem II-enriched membrane fragments prepared after Berthold, Babcock and Yocum (1981) - BSA bovine serum albumin - Chl chlorophyll - CAB-protein chlorophyll a/b-binding protein - core particles oxygen evolving reaction center core particles of Photosystem II - Cyt cytochrome - DCBQ 2,5-dichloro-p-benzoquinone - IML intermittent light - P-680 primary electron donor of Photosystem II - PS II Photosystem II - Yz tyrosine residue on the D1 polypeptide, electron carrier between manganese and P-680 - photochemical reaction   相似文献   

5.
The light-induced oxidation of the accessory donor tyrosine-D (YD) has been studied by measurements of the EPR Signal IIslow at room temperature in the autotrophically and photoheterotrophically cultivated alga Chlamydobotrys stellata. After illumination and dark adaptation, YD Signal IIslow was observed only in autotrophic algae, i.e. under conditions of a linear photosynthetic electron transfer from water to NADP+. The addition of artificial electron acceptors phenyl-p-benzoquinone (PPQ) or dichloro-p-benzoquinone (DCQ) to the autotrophic cells caused an almost negligible increase of this signal. When photosynthetic electron flow and oxygen evolution were diminished by removal of the carbon source CO2 and addition of acetate (photoheterotrophy), a pronounced YD Signal IIslow was seen only in presence of DCQ or PPQ. Several possibilities are discussed to explain the absence of YD Signal IIslow in photoheterotrophic Chl. stellata such as the existence of a cyclic PS II electron flow very effectively reducing P680 and thereby preventing the possibility of YD oxidation. Artificial electron acceptors withdraw electrons from this cycle thus keeping the primary quinone acceptor, QA, oxidized and thereby diminishing the reduction of P680 + by cyclic PSII. This leads to the appearance of the YD Signal IIslow also in the photoheterotrophically grown algae.Abbreviations A-band- thermoluminescence band associated with S2QA - charge recombination - DCQ- 2,5-dichlorobenzoquinone - D2- structure protein of Photosystem II - EPR- electron paramagnetic resonance - OEC- oxygen evolving complex - PPQ- phenyl-p-benzoquinone - PS II- Photosystem II - P680- reaction center of Photosystem II - Q-band- thermoluminescence band associated with S2QA - charge recombination - Si- oxidation levels of the OEC - YD- tyrosine-D accessory donor to P680 - YZ- tyrosine-Z electron donor to P680 Dedicated to Prof. Dr E. Schnepf/Heidelberg.  相似文献   

6.
Manganese in the oxygen-evolving complex is a physiological electron donor to Photosystem II. PS II depleted of manganese may oxidize exogenous reductants including benzidine and Mn2+. Using flash photolysis with electron spin resonance detection, we examined the room-temperature reaction kinetics of these reductants with Yz +, the tyrosine radical formed in PS II membranes under illumination. Kinetics were measured with membranes that did or did not contain the 33 kDa extrinsic polypeptide of PS II, whose presence had no effect on the reaction kinetics with either reductant. The rate of Yz + reduction by benzidine was a linear function of benzidine concentration. The rate of Yz + reduction by Mn2+ at pH 6 increased linearly at low Mn2+ concentrations and reached a maximum at the Mn2+ concentrations equal to several times the reaction center concentration. The rate was inhibited by K+, Ca2+ and Mg2+. These data are described by a model in which negative charge on the membrane causes a local increase in the cation concentration. The rate of Yz + reduction at pH 7.5 was biphasic with a fast 400 s phase that suggests binding of Mn2+ near Yz + at a site that may be one of the native manganese binding sites.Abbreviations PS II Photosystem II - YD tyrosine residue in Photosystem II that gives rise to the stable Signal II EPR spectrum - Yz tyrosine residue in Photosystem II that mediates electron transfer between the reaction center chlorophyll and the site of water oxidation - ESR electron spin resonance - DPC diphenylcarbazide - DCIP dichlorophenolindophenol  相似文献   

7.
The kinetics of the postillumination reduction of P700+ which reflects the rate constant for plastoquinol (PQH2) oxidation was recorded in sunflower leaves at different photon absorption densities (PAD), CO2 and O2 concentrations. The P700 oxidation state was calculated from the leaf transmittance at 830 nm logged at 50 s intervals. The P700+ dark reduction kinetics were fitted with two exponents with time constants of 6.5 and about 45 ms at atmospheric CO2 and O2 concentrations. The time constant of the fast component, which is the major contributor to the linear electron transport rate (ETR), did not change over the range of PADs of 14.5 to 134 nmol cm-2 s-1 in 21% O2, but it increased up to 40 ms under severe limitation of ETR at low O2 and CO2. The acceptor side of Photosystem I (PS I) became reduced in correlation with the downregulation of the PQH2 oxidation rate constant. It is concluded that thylakoid pH-related downregulation of the PQH2 oxidation rate constant (photosynthetic control) is not present under normal atmospheric conditions but appears under severe limitation of the availability of electron acceptors. The measured range of photosynthetic control fits with the maximum variation of ETR under natural stress in C3 plants. Increasing the carboxylase/oxygenase specificity would lead to higher reduction of the PS I acceptor side under stress.Abbreviations Cyt b 6 f cytochrome b 6 f complex - Cw cell-wall CO2 concentration, M - ETR electron transport rate - Fd ferredoxin - FNR ferredoxin-NADP reductase - FRL far-red light - PC plastocyanin - PAD photon absorption density nmol cm-2 s-1 - PFD photon flux density nmol cm-2 s-1 - PS I Photosystem I complex - PQ plastoquinon - PQH2 plastoquinol - PS II Photosystem II complex - P700 Photosystem I donor pigment, reduced - S830 830 nm signal (D830, difference of S830 from the dark level) - WL white light - Yl maximum quantum yield of PS I electron transport, rel. un  相似文献   

8.
Many of the core proteins in Photosystem II (PS II) undergo reversible phosphorylation. It is known that protein phosphorylation controls the repair cycle of Photosystem II. However, it is not known how protein phosphorylation affects the partial electron transport reactions in PS II. Here we have applied variable fluorescence measurements and EPR spectroscopy to probe the status of the quinone acceptors, the Mn cluster and other electron transfer components in PS II with controlled levels of protein phosphorylation. Protein phosphorylation was induced in vivo by varying illumination regimes. The phosphorylation level of the D1 protein varied from 10 to 58% in PS II membranes isolated from pre-illuminated spinach leaves. The oxygen evolution and QA to QB(QB ) electron transfer measured by flash-induced fluorescence decay remained similar in all samples studied. Similar measurements in the presence of DCMU, which reports on the status of the donor side in PS II, also indicated that the integrity of the oxygen-evolving complex was preserved in PS II with different levels of D1 protein phosphorylation. With EPR spectroscopy we examined individual redox cofactors in PS II. Both the maximal amplitude of the charge separation reaction (measured as photo-accumulated pheophytin) and the EPR signal from the QA Fe2+ complex were unaffected by the phosphorylation of the D1 protein, indicating that the acceptor side of PS II was not modified. Also the shape of the S2 state multiline signal was similar, suggesting that the structure of the Mn-cluster in Photosystem II did not change. However, the amplitude of the S2 multiline signal was reduced by 35% in PS II, where 58% of the D1 protein was phosphorylated, as compared to the S2 multiline in PS II, where only 10% of the D1 protein was phosphorylated. In addition, the fraction of low potential Cyt b 559 was twice as high in phosphorylated PS II. Implications from these findings, were precise quantification of D1 protein phosphorylation is, for the first time, combined with high-resolution biophysical measurements, are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

9.
In this minireview we discuss effects of excitation stress on the molecular organization and function of PS II as induced by high light or low temperature in the cyanobacterium Synechococcus sp. PCC 7942. Synechococcus displays PS II plasticity by transiently replacing the constitutive D1 form (D1:1) with another form (D1:2) upon exposure to excitation stress. The cells thereby counteract photoinhibition by increasing D1 turn over and modulating PS II function. A comparison between the cyanobacterium Synechococcus and plants shows that in cyanobacteria, with their large phycobilisomes, resistance to photoinhibition is mainly through the dynamic properties (D1 turnover and quenching) of the reaction centre. In contrast, plants use antenna quenching in the light-harvesting complex as an important means to protect the reaction center from excessive excitation.Abbreviations D1 reaction center protein of Photosystem II - P680 the reaction center of Photosystem II - QA the primary quinone acceptor of Photosystem II - TyrZ tyrosine electron donor to P680  相似文献   

10.
A light-driven reaction model for the Ca2+-depleted Photosystem (PS) II is proposed to explain the split signal observed in electron paramagnetic resonance (EPR) spectra based on a comparison of EPR assignments with recent x-ray structural data. The split signal has a splitting linewidth of 160 G at around g = 2 and is seen upon illumination of the Ca2+-depleted PS II in the S2 state associated with complete or partial disappearance of the S2 state multiline signal. Another g=2 broad ESR signal with a 110 G linewidth was produced by 245 K illumination for a short period in the Ca2+-depleted PS II in S1 state. At the same time a normal YZ· radical signal was also efficiently trapped. The g=2 broad signal is attributed to an intermediate S1X· state in equilibrium with the trapped YZ· radical. Comparison with x-ray structural data suggests that one of the split signals (doublet signal) is attributable to interaction between His 190 and the YZ· radical, and other signals is attributable to interaction between His 337 and the manganese cluster, providing further clues as to the mechanism of water oxidation in photosynthetic oxygen evolution.  相似文献   

11.
Excitation of photosynthetic systems with short intense flashes is known to lead to exciton-exciton annihilation processes. Here we quantify the effect of competition between annihilation and trapping for Photosystem II, Photosystem I (thylakoids from peas and membranes from the cyanobacterium Synechocystis sp.), as well as for the purple bacterium Rhodospirillum rubrum. In none of the cases it was possible to reach complete product saturation (i.e. closure of reaction centers) even with an excitation energy exceeding 10 hits per photosynthetic unit. The parameter introduced by Deprez et al. ((1990) Biochim. Biophys. Acta 1015: 295–303) describing the competition between exciton-exciton annihilation and trapping was calculated to range between 4.5 (PS II) and 6 (Rs. rubrum). The rate constants for bimolecular exciton-exciton annihilation ranged between (42 ps)-1 and (2.5 ps)-1 for PS II and PS I-membranes of Synechocystis, respectively. The data are interpreted in terms of hopping times (i.e. mean residence time of the excited state on a chromophore) according to random walk in isoenergetic antenna.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - LHC II light harvesting complex II - P primary donor - PS I Photosystem I - PS II Photosystem II - PSU photosynthetic unit - RC reaction center  相似文献   

12.
A three-dimensional model of the core proteins D1 and D2, including the cofactors, that form the Photosystem II reaction centre of pea (Pisum sativum), has been generated. This model was built with a rule-based computer modelling system using the information from the crystal structures of the photosynthetic reaction centres of Rhodopseudomonas viridis and Rhodobacter sphaeroides. An alignment of the primary sequences of twenty three D1, nine D2, eight bacterial L and eight bacterial M subunits predicts strong similarity between bacterial and higher plant reaction centres, especially in the transmembrane region where the cofactors responsible for electron transport are located. The sequence to be modelled was aligned to the bacterial structures using environment-dependent substitution tables to construct matrices, improving the alignment procedure. The ancestral relationship between the bacteria and higher plant sequences allowed both the L and M subunits to be used as structural templates as they were equally related to the higher plant polypeptides. The regions with the highest predicted structural homology were used as a framework for the construction of the structurally conserved regions. The structurally conserved region of the model shows strong similarity to the bacterial reaction centre in the transmembrane helices. The stromal and lumenal loops show greater sequence variation and are therefore predicted to be the structurally variable regions in the model. The key sidechain assignments and residues that may interact with cofactors are discussed.Abbreviations D Tyr161 in the D2 polypeptide - PS II Photosystem II - QA primary plastoquinone acceptor of Photosystem II - QB secondary plastoquinone acceptor of Photosystem II - Z Tyr161 in the D1 polypeptide  相似文献   

13.
Target theory and the photoinactivation of Photosystem II   总被引:1,自引:0,他引:1  
Application of target theory to the photoinactivation of Photosystem II in pea leaf discs (Park et al. 1995, 1996a,b) reveals that there is a critical light dosage below which there is complete photoprotection and above which there is photoinactivation (i.e a light-induced loss of oxygen flash yield). The critical dosage is about 3 mol photons m–2 for medium and high light-grown leaves and 0.36 mol photons m–2 for low light-grown leaves. Photoinactivation is a one-hit process with an effective cross-section of 0.045 m2 mol–1 photons which does not vary with growth irradiance, unlike the cross-section for oxygen evolution which increases with decreasing growth irradiance. The cross-section for oxygen evolution increased by about 20% following exposure to 6.8 mol photons m–2 which may be due to energy transfer from photoinactivated units to functional Photosystem II units. We propose that the photoinactivation of PS II begins when a small group of PS II pigment molecules whose structure is uninfluenced by growth irradiance, becomes uncoupled energetically from the rest of the photosynthetic unit and thus no longer transfers excitions to P680. De-excitation of this group of pigment molecules provides the energy which leads to the damage of Photosystem II. Treatment of pea leaves with dithiothreitol, an inhibitor of the xanthophyll cycle, decreases the critical dosage i.e. decreases photoprotection but has no effect on the PS II photoinactivation cross-section. Treatment with 1 M nigericin increased the photoinactivation cross-section of PS II as did exposure to lincomycin which inhibits D1 protein synthesis and thus the repair of PS II reaction centres.Abbreviations DTT- dithiothreitol - PS II- Photosystem II - Fm- maximum fluorescence - Fv- variable fluorescence - LHCIIb- main light harvesting pigment-protein complex of PS II - D1 protein- psbA gene product - P680- reaction centre chlorophyll of Photosystem II - Qa- first quinone electron acceptor of Photosystem II - (o2)- cross-section for oxygen evolution - (pi)- cross-section for photoinactivation  相似文献   

14.
The functional connection between redox component Y z identified as Tyr-161 of polypeptide D-1 (Debus et al. 1988) and P680+ was analyzed by measurements of laser flash induced absorption changes at 830 nm in PS II membrane fragments from spinach. It was found that neither DCMU nor the ADRY agent 2-(3-chloro-4-trifluoromethyl) anilino-3,5-dinitrothiophene (ANT 2p) affects the rate of P680+ reduction by Y z under conditions where the catalytic site of water oxidation stays in the redox state S1. In contrast to that, a drastic retardation is observed after mild trypsin treatment at pH=6.0. This effect which is stimualted by flash illumination can be largely reversed by Ca2+. The above mentioned data lead to the following conclusions: (a) the segment of polypeptide D-1 containing Tyr-161 and coordination sites of P680 is not allosterically affected by structural changes due to DCMU binding at the QB-site which is also located in D-1. (b) ANT 2p as a strong protonophoric uncoupler and ADRY agent does not modify the reaction coordinate of P680+ reduction by Y z , and (c) Ca2+ could play a functional role for the electronic and vibrational coupling between the redox groups Y z and P680. The electron transport from Y z to P680+ is discussed within the framework of a nonadiabatic process. Based on thermodynamic considerations the reorganization energy is estimated to be in the order of 0.5 V.Abbreviations ADRY acceleration of the deactivation reactions of the water splitting enzyme system Y - ANT 2p 2-(3-chloro-4-trifluoromethyl)anilino-3,5 dinitrothiophene - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - MES 2[N-Morpholino]ethanesulfonic acid - PS II photosystem II - QA, QB primary and secondary plastoquinone acceptor of photosystem II - S i redox states of the catalytic site of water oxidation - Y z redox active Tyr-161 of polypeptide D-1  相似文献   

15.
Transthylakoid proton transport based on Photosystem I-dependent cyclic electron transport has been demonstrated in isolated intact spinach chloroplasts already at very low photon flux densities when the acceptor side of Photosystem I (PS I) was largely closed. It was under strict redox control. In spinach leaves, high intensity flashes given every 50 s on top of far-red, but not on top of red background light decreased the activity of Photosystem II (PS II) in the absence of appreciable linear electron transport even when excitation of PS II by the background light was extremely weak. Downregulation of PS II was a consequence of cyclic electron transport as shown by differences in the redox state of P700 in the absence and the presence of CO2 which drained electrons from the cyclic pathway eliminating control of PS II. In the presence of CO2, cyclic electron transport comes into play only at higher photon flux densities. At H+/e=3 in linear electron transport, it does not appear to contribute much ATP for carbon reduction in C3 plants. Rather, its function is to control the activity of PS II. Control is necessary to prevent excessive reduction of the electron transport chain. This helps to protect the photosynthetic apparatus of leaves against photoinactivation under light stress.  相似文献   

16.
Synechococcus sp. PCC 7942 (Anacystis nidulans R2) contains two forms of the Photosystem II reaction centre protein D1, which differ in 25 of 360 amino acids. D1: 1 predominates under low light but is transiently replaced by D1:2 upon shifts to higher light. Mutant cells containing only D1:1 have lower photochemical energy capture efficiency and decreased resistance to photoinhibition, compared to cells containing D1:2. We show that when dark-adapted or under low to moderate light, cells with D1:1 have higher non-photochemical quenching of PS II fluorescence (higher qN) than do cells with D1:2. This is reflected in the 77 K chlorophyll emission spectra, with lower Photosystem II fluorescence at 697–698 nm in cells containing D1:1 than in cells with D1:2. This difference in quenching of Photosystem II fluorescence occurs upon excitation of both chlorophyll at 435 nm and phycobilisomes at 570 nm. Measurement of time-resolved room temperature fluorescence shows that Photosystem II fluorescence related to charge stabilization is quenched more rapidly in cells containing D1:1 than in those with D1:2. Cells containing D1:1 appear generally shifted towards State II, with PS II down-regulated, while cells with D1:2 tend towards State I. In these cyanobacteria electron transport away from PS II remains non-saturated even under photoinhibitory levels of light. Therefore, the higher activity of D1:2 Photosystem II centres may allow more rapid photochemical dissipation of excess energy into the electron transport chain. D1:1 confers capacity for extreme State II which may be of benefit under low and variable light.Abbreviations D1 the atrazine-binding 32 kDa protein of the PS II reaction centre core - D1:1 the D1 protein constitutively expressed during acclimated growth in Synechococcus sp. PCC 7942 - D1:2 an alternate form of the D1 protein induced under excess excitation in Synechococcus sp. PCC 7942 - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - Fo minimal fluorescence in the dark-adapted state - Fo minimal fluorescence in a light-adapted state - FM maximum fluorescence with all quenching mechanisms at a minimum, measured in presence of DCMU - FM maximal fluorescence in a light-adapted state, measured with a saturating flash - FMdark maximal fluorescence in the dark-adapted state - FV variable fluorescence in a light-adapted state (FM-Fo) - PAM pulse amplitude modulated fluorometer - qN non-photochemical quenching of PS II fluorescence - qN (dark) qN in the dark adapted state - qP photochemical quenching of fluorescence  相似文献   

17.
A series of experiments have been conducted with isolated reaction centers of photosystem two (PS II) with the aim to elucidate the functional role of cytochrome (Cyt b 559). At pH 6.5 it was found that Cyt b 559 was reversibly photoreduced by red actinic light when Mn2+ was present as an electron donor while at pH 8.5 a photo-oxidation was observed under the same lighting conditions, which was dark reversible in the presence of hydroquinone. These pH dependent light induced changes were measured under anaerobic conditions and correlated with changes in the relative levels of high (HP) and low (LP) potential forms of the cytochrome. At pH 6.5 the cytochrome was mainly in its LP form while at pH 8.5 a significant proportion was converted to the HP form as detected by dark titrations with hydroquinone. This pH dependent difference in the levels of HP and LP Cyt b 559 was also detected when bright white light was used to monitor the level of the LP form using a novel reaction involving direct electron donation from the flavin of glucose oxidase (present in the medium and used together with glucose and catalase as an oxygen trap). The results suggest that PS II directly oxidises and reduces the HP and LP forms, respectively and that the extent of these photo-reactions is dependent on the relative levels of the two forms, which are in turn governed by the pH. This conclusion is interpreted in terms of the model presented previously (Barber J and De Las Rivas J (1993) Proc Natl Acad Sci USA 90: 10942–10946) whereby the pH induced effect is considered as a possible mechanism by which interconversion of LP and HP forms of Cyt b 559 is achieved. In agreement with this was the finding that as the extent of photo-oxidisable HPCyt b 559 increases, with increasing pH, the rate of irreversible photo-oxidation of -carotene decreases, a result expected if the HP form protects against donor side photoinhibition.Abbreviations -car -carotene - CCCP carbonylcyanide m-chloro-phenylhydrazone - Chl chlorophyll - Cyt b 559 cytochrome b 559 - HPCyt b 559 high potential form of cytochrome b 559 which is reducible by hydroquinone - LPCyt b 559 low potential form of cytochrome b 559 which is non-reducible by hydroquinone - D1 and D2 products of the psbA and psbD genes, respectively - LHC II light-harvesting chlorophyll protein complex associated with PS II - Mes 2-(N-morpholino) ethanesulphonic acid - P680 primary electron donor of PS II - Pheo pheophytin - PQ plastoquinone - PS II Photosystem II - QA first stable quinone electron acceptor of PS II - QB second stable quinone electron acceptor of PS II - RC reaction center - SDS sodium dodecyl sulphate - SiMo silicomolybdate - Tris tris(hydroxymethyl) amino methane - YZ and YD tyrosine residues 161 in D1 and D2 proteins of the PS II RC which act as secondary electron donors to P680  相似文献   

18.
Photoinactivation of Photosystem (PS) II in vivo was investigated by cumulative exposure of pea, rice and spinach leaves to light pulses of variable duration from 2 to 100 s, separated by dark intervals of 30 min. During each light pulse, photosynthetic induction occurred to an extent depending on the time of illumination, but steady-state photosynthesis had not been achieved. During photosynthetic induction, it is clearly demonstrated that reciprocity of irradiance and duration of illumination did not hold: hence the same cumulative photon exposure (mol m–2) does not necessarily give the same extent of photoinactivation of PS II. This contrasts with the situation of steady-state photosynthesis where the photoinactivation of PS II exhibited reciprocity of irradiance and duration of illumination (Park et al. (1995) Planta 196: 401–411). We suggest that, for reciprocity to hold between irradiance and duration of illumination, there must be a balance between photochemical (qP) and non-photochemical (NPQ) quenching at all irradiances. The index of susceptibility to light stress, which represents an intrinsic ability of PS II to balance photochemical and non-photochemical quenching, is defined by the quotient (1-qP)/NPQ. Although constant in steady-state photosynthesis under a wide range of irradiance (Park et al. (1995). Plant Cell Physiol 36: 1163–1169), this index of susceptibility for spinach leaves declined extremely rapidly during photosynthetic induction at a given irradiance, and, at a given cumulative photon exposure, was dependent on irradiance. During photosynthetic induction, only limited photoprotective strategies are developed: while the transthylakoid pH gradient conferred some degree of photoprotection, neither D1 protein turnover nor the xanthophyll cycle was operative. Thus, PS II is more easily photoinactivated during photosynthetic induction, a phenomenon that may have relevance for understorey leaves experiencing infrequent, short sunflecks.Abbreviations D1 protein psbA gene product - DTT dithiothreitol - Fv, Fm, Fo variable, maximum, and initial (corresponding to open traps) chlorophyll fluorescence yield, respectively - NPQ non-photochemical quenching - PS Photosystem - QA primary quinone acceptor of PS II - qP photochemical quenching coefficient  相似文献   

19.
Adaptive responses to excess (supraoptimal) level of cobalt supplied to the growth medium were studied in the cyanobacterium Synechocystis PCC 6803. Growth of cells in the medium containing 10 M CoCl2 led to a large stimulation (50%) in O2-evolution and an overall increase (30%) in the photosynthetic electron transport rates. Analysis of variable Chl a fluorescence yield of PS II and immuno-detection of Photosystem II (PS II) reaction-center protein D1, showed a small increase (15–20%) in the number of PS II units in cobalt-grown cells. Cobalt-grown cells, therefore, had a slightly elevated PS II/PS I ratio compared to control.We observed alteration in the extent of energy distribution between the two photosystems in the eobalt grown cells. Energy was preferentially distributed in favour of PS II accompanied by a reduction in the extent of energy transfer from PS II to PS I in cobalt-grown cells. These cells also showed a smaller PS I absorption cross-section and a smaller size of intersystem electron pool than the control cells. Thus, our results suggest that supplementation of 10 M CoCl2, to the normal growth medium causes multiple changes involving small increase in PS II to PS I ratio, enhanced funneling of energy to PS II and an increase in PS I electron transport, decrease PS I cross section and reduction in intersystem pool size. The cumulative effects of these alterations cause stimulation in electron transport and O2 evolution.Abbreviations BCIP 5-bromo-4-chloro-3-indolylphosphate - Chl a Chlorophyll a - Cyt blf Cytochrome blf - DCBQ 2,6-dichlorobenzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - DCPIP 2,6-dichlorophenol indophenol - DPC Diphenyl carbazide - Fo fluorescence when all reaction centers are open - FM fluorescence yield when all reaction centers are closed - Fv variable chlorophyll fluorescence - HEPES N-2-hydroxyethyl piperazine-N'-2-ethanesulphonic acid - MV methyl viologen - NBT nitro-blue tetrazolium - pBQ para-benzoquinone - PB somes phycobilisomes - PC Phycocyanin - PQ plastoquinone - PS I Photosystem I - PS II Photosystem II - P700 reaction center Chl a of PS 1 - ST-and MT-flash single turnover and multiple turnover flash  相似文献   

20.
We investigated the influence of CO2/HCO3 -depletion and of the presence of acetate and formate on the in vivo photosynthetic electron transport in the two green algae Chlamydobotrys stellata and Chlamydomonas reinhardtii by means of thermoluminescence technique and mathematical glow curve analysis. The main effects of the removal of CO2 from the algal cultures was: (1) A shift of the glow curve peak position to lower temperatures resulting from a decrease of the B band and an increase of the Q band. (2) Treatment of CO2-deficient Chl. stellata with DCMU yielded two thermoluminescence bands in the Q band region peaking at around +12°C and +5°C; in case of Chl. reinhardtii DCMU treatment induced only one band with an emission maximum at +5°C. The presence of acetate or formate in CO2-depleted algal cultures lowered the intensities of all of the individual TL bands but that of a HT band (TL+37). The effects of CO2-depletion and of the presence of anions were fully reversible.Abbreviations DCMU 3-(3,4)-dichlorophenyl-1,1-dimethylurea - HT band high temperature TL band - P680 reaction center chlorophyll of PS II - QA and QB primary and secondary quinone acceptors of PS II, respectively - PS II Photosystem II - S2/3 redox states of the oxygen evolving complex of PS II - TL thermoluminescence  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号