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1.
《FEBS letters》1987,220(1):67-73
A photosystem II reaction centre has been isolated from peas and found to consist of D1, D2 polypeptides and the apoproteins of cytochrome b-559, being similar to that reported for spinach by Nanba and Satoh [(1987) Proc. Natl. Acad. Sci. USA 84, 109–112]. The complex binds chlorophyll a, pheophytin and the haem of cytochrome b-559 in an approximate ratio of 4:2:1 and also contains about one molecule of β-carotene. It binds no plastoquinone-9 or manganese but does contain at least one non-haem iron. In addition to a light-induced signal due to Pheo− seen under reducing conditions, a light-induced P680+ signal is seen when the reaction centre is incubated with silicomolybdate. In the presence of diphenylcarbazide, the P680+ signal is partially inhibited and net electron flow to silicomolybdate occurs. This net electron flow is insensitive to o-phenanthroline, 3-(3,4-dichlorophenyl)-1,1-dimethyl urea and 2-(3-chloro-4-trifluoromethyl)anilino-3,5-dinitrothiophene but is inhibited by proteolysis with trypsin and by other treatments. Fluorescence, from the complex, peaks at 682 nm at room temperature and at 685 nm at 77 K. This emission is significantly quenched when either the P680+Pheo or P680Pheo− states are established indicating that the fluorescence emanates from the back reaction between P680+ and Pheo−. 相似文献
2.
Extraction of PS II particles with 50 mM cholate and 1 M NaCl releases several proteins (33-, 23-, 17- and 13 kDa) and lipids from the thylakoid membrane which are essential for O2 evolution, dichlorophenolindophenol (DCIP) reduction and for stable charge separation between P680+ and QA
-. This work correlates the results on the loss of steady-state rates for O2 evolution and PS II mediated DCIP photo-reduction with flash absorption changes directly monitoring the reaction center charge separation at 830 nm due to P680+, the chlorophyll a donor. Reconstitution of the extracted lipids to the depleted membrane restores the ability to photo-oxidize P680 reversibly and to reduce DCIP, while stimulating O2 evolution minimally. Addition of the extracted proteins of masses 33-, 23- and 17- kDa produces no further stimulation of DCIP reduction in the presence of an exogenous donor like DPC, but does enhance this rate in the absence of exogenous donors while also stimulating O2 evolution. The proteins alone in the absence of lipids have little influence on charge separation in the reaction center. Thus lipids are essential for stable charge separation within the reaction center, involving formation of P680+ and QA
-.Abbreviations A830
Absorption change at 830 nm
- Chl
Chlorophyll
- D1
primary electron donor to P680
- DCIP
2,6-dichlorophenolindophenol
- DPC
1,5-diphenylcarbazide
- MOPS
3-(N-morpholino)propanesulfonic acid
- P680
reaction center chlorophyll a molecule of photosystem II
- PPBQ
Phenyl-p-benzoquinone
- PS II
Photosystem II
- QA, QB
first and second quinone acceptors in PS II
- V-DCIP
rate of DCIP reduction
- V-O2
rate of oxygen evolution
- Y
water-oxidizing enzyme system
- CHAPS
3-Cyclohexylamino-propanesulfonic acid 相似文献
3.
The rise time, of Signal IIf and the decay time of P-680+ have been measured kinetically as a function of pH by using EPR. The Photosystem II-enriched preparations which were used as samples were derived from spinach chloroplasts, and they evolved oxygen before Tris washing. The onset kinetics of Signal IIf are in agreement, within experimental error, with the fast component of the decay of an EPR signal attributable to P-680+. The signal IIf rise kinetics also show good agreement with published values of the pH dependence of the decay of P-680+ measured optically (Conjeaud, H. and Mathis, P. (1980) Biochim. Biophys. Acta 590, 353–359). These results are consistent with a model where the species Z (or D1) responsible for Signal IIf is the immediate electron donor to P-680+ in tris-washed Photosystem II fragments. 相似文献
4.
Measurements are reported on μs delayed light emission, following a single 10 ns excitation flash, in Alaska pea thylakoids treated with hydroxylamine (NH2OH) or with silicomolybdate.
- In thylakoids treated with 2 mM NH2OH in the light, or in the dark, the quantum yield of delayed light emission is considerably enhanced. A 10 μs lifetime component of delayed light emission is not significantly changed, whereas a 50–70 μs lifetime component is increased. MnCl2 and diphenylcarbazide are unable to reverse the above effects of NH2OH treatment. Thus Mn2+ and diphenylcarbazide must not donate electrons directly to reaction center II but on the oxygen-evolution side of the NH2OH block.
- When the closed form of photosystem II reaction centers (P680Q-), where P680 is the reaction center chlorophyll and Q is a ‘stable’ electron acceptor, is generated by preillumination of NH2OH-treated thylakoids with diuron present, the μs delayed light emission is inhibited, but a low level residual delayed light emission remains. Possible origins of this emission are discussed. It is believed that the best explanation for residual DLE is the existence of another acceptor besides Q that partakes in charge separation and rapid dissipative recombination when the reaction center is in the P680Q- state.
- The quantum yield of delayed light emission from ‘closed’ reaction centers (P680 +Q-) that have all charge stabilization reactions (i.e., flow of electrons to P680 + and out of Q-) blocked by NH2OH treatment and addition of diuron is 1.1×10-3 for components measured in a range from 6 to 400 μs and extrapolated to zero time.
- The addition of silicomolybdate, which accepts electron from Q-, causes delayed light emission in the μs range to be greatly inhibited.
5.
The reversible inhibition of Photosystem II by salicylaldoxime was studied in spinach D-10 particles by fluorescence, optical absorption, and electron spin resonance spectroscopy. In the presence of 15 mM salicylaldoxime, the initial fluorescence yield was raised to the level of the maximum fluorescence, indicating efficient charge recombination between reduced pheophytin (Ph) and P680+. In agreement with the rapid (ns) backreaction expected between Ph– and P680+, the optical absorption transient at 820 mm was not observed. When the particles were washed free of salicylaldoxime, the optical absorption transient resulting from the rereduction of P680+ was restored to the µs timescale. These results, along with the previously observed inhibition of electron transport reactions and diminution of the 515-nm absorption change in chloroplasts [Golbeck, J.H. (1980) Arch Biochem Biophys 202, 458–466], are consistent with a site of inhibition between Ph and QA in Photosystem II. ESR Signal IIf and Signal Its were abolished in the presence of 25 mM salicylaldoxime, but both signals could be recovered by washing the D-10 particles free of the inhibitor. The loss of Signal Ilf is most likely a consequence of the inhibition between Ph and QA; the rapid charge recombination between Ph– and P680+ would preclude electron transfer from an electron donor on the oxidizing side of Photosystem II. The loss of Signal Its may be due to a change in the environment of the donor complex such that the semiquinone radical giving rise to Signal Its interacts with a nearby reductant.Abbreviations D1
electron donor to P680+ in oxygen-inhibited chloroplasts
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- F0
prompt chlorophyll a fluorescence yield
- Fi
initial chlorophyll a fluorescence yield
- Fmax
maximum chlorophyll a fluorescence yield
- Fvar
variable chlorophyll a fluorescence yield
- FWHM
full width at half maximum
- Mes
2-(N-morpholino) ethanesulfonic acid
- P680
reaction center chlorophyll a of photosystem II
- Ph
pheophytin intermediate electron acceptor
- QA
primary quinone electron acceptor
- QB
secondary quinone electron acceptor
- Tris
tris(hydroxymethyl)aminomethane
- Z
electron donor to P680+ 相似文献
6.
Wim Vermaas Cathy Madsen Jiujiang Yu Janine Visser James Metz Peter J. Nixon Bruce Diner 《Photosynthesis research》1995,45(2):99-104
The Photosystem II reaction center is rapidly inactivated by light, particularly at higher light intensity. One of the possible factors causing this phenomenon is the oxidized primary donor, P680+, which may be harmful to Photosystem II because of its highly oxidizing nature. However, no direct evidence specificially implicating P680+ in photoinhibition has been obtained yet. To investigate whether P680+ is harmful to Photosystem II, turnover of the D1 protein and of the Photosystem II reaction center complex were measured in vivo in a mutant of the cyanobacterium Synechocystis sp. PCC 6803, in which the physiological donor to P680+, Tyrz, was genetically deleted. In this mutant, D1 degradation in the light is an order of magnitude faster than in wild type. The most straightforward explanation of this phenomenon is that accumulation of P680+ leads to an increased rate of turnover of the Photosystem II reaction center complex, which is compatible with the hypothesis of destructive oxidation by P680+ that is damaging to the Photosystem II complex. 相似文献
7.
Absorption changes at 820 or 515 nm after a short laser flash were studied comparatively in untreated chloroplasts and in chloroplasts in which oxygen evolution is inhibited.In chloroplasts pre-treated with Tris, the primary donor of Photosystem II (P-680) is oxidized by the flash, as observed by an absorption increase at 820 nm. After the first flash it is re-reduced in a biphasic manner with half-times of 6 μs (major phase) and 22 μs. After the second flash, the 6 μs phase is nearly absent and P-680+ decays with half-times of 130 μs (major phase) and 22 μs. Exogenous electron donors (MnCl2 or reduced phenylenediamine) have no direct influence on the kinetics of P-680+.In untreated chloroplasts the 6 and 22 μs phases are of very small amplitude, either at the 1st, 2nd or 3rd flash given after dark-adaptation. They are observed, however, after incubation with 10 mM hydroxylamine.These results are interpreted in terms of multiple pathways for the reduction of P-680+: a rapid reduction (<1 μs) by the physiological donor D1; a slower reduction (6 and 22 μs) by donor D′1, operative when O2 evolution is inhibited; a back-reaction (130 μs) when D′1 is oxidized by the pre-illumination in inhibited chloroplasts. In Tris-treated chloroplasts the donor system to P-680+ has the capacity to deliver only one electron.The absorption change at 515 nm (electrochromic absorption shift) has been measured in parallel. It is shown that the change linked to Photosystem II activity has nearly the same magnitude in untreated chloroplasts or in chloroplasts treated with hydroxylamine or with Tris (first and subsequent flashes). Thus we conclude that all the donors (P-680, D1, D′1) are located at the internal side of the thylakoid membrane. 相似文献
8.
《Plant Science Letters》1980,17(2):215-220
The light-induced absorbance change of the centerII P-680 reaction was investigated in autotrophically and photoheterotrophically cultivated Chlamydobotrys stellata. In addition the light-dark A559 change was followed in order to look for a correlation between the kinetics of the cytochrome b-559 oxidation and the P-680+ reduction.In the dark period after a short laser flash, the P-680+ of the autotrophically cultivated organisms is reduced in a two-phase process with different half times, the first of approx. 30 μs and the second longer than 300 μs.Adaptation of the algae from autotrophic to photoheterotrophic growth conditions changes the 30 μs phase into one with a half time of decay of approx. 60 μs and in addition decreases the amplitude. The > 300-gms phase is changed only in its amplitude.In autotrophic organisms a significant A559 change can be detected only in the presence of CCCP or FCCP and can be eliminated by the addition of DCMU.In photoheterotrophic C. stellata, the 559 nm signal can be measured without any treatment of the algae. High light intensity causes a reduction and low light intensity an oxidation of cytochrome b-559.These results support the previously reported hypothesis that cytochrome b-559 in photoheterotrophically cultivated C. stellata, but not in autotrophic algae, is reduced by a light driven photosystem-II reaction and oxidised by the donor site of photosystem II with a half time of approx. 60 μs. Thus photosystem II is short circuited by a cyclic electron transport via cytochrome b-559. 相似文献
9.
Christer Jansson Örjan Hansson Hans-Erik Åkerlund Lars-Erik Andreasson 《Biochemical and biophysical research communications》1984,124(1):269-276
EPR measurements on inside-out thylakoids revealed that salt-washing, known to inhibit oxygen evolution and release a 23 and a 16 kDa protein, induced a Signal IIf and decreased the EPR signal from state S2. Readdition of the released 23 kDa protein restored the oxygen evolution and decreased the Signal IIf, but did not relieve the decrease in the state S2 signal. It is suggested that salt-washing inhibits the electron transfer from the oxygen-evolving site to Z, the physiological donor to P680. In inhibited photosystem II units lacking Signal IIf, Z+ is rapidly reduced, possibly by a modified S-cycle unable to evolve oxygen. 相似文献
10.
A new method of measuring the rate of the back reaction from the state Z+ P680 QA? in Tris-washed chloroplasts is described. By using ratios of back reaction rates we demonstrate a Tris-induced change in the equilibrium between Z and P680 and attribute this change to an alteration of the midpoint potential of Z by Tris treatment. We also demonstrate that the previously observed inhibition of the back reaction by ADRY reagents can be localized at Z and understood in terms of electron donation to Z+ by ADRY reagents. 相似文献
11.
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
12.
Inhibition of photosystem 2 by the peptide-modification reagent, tetranitromethane, has been investigated with spinach digitonin particles. In the presence of tetranitromethane, (1) the initial fluoresence yield is suppressed with a concomitant elimination of the variable component of fluorescence; (2) the optical absorption transient at 820 nm, attributed to P680+, is greatly attenuated; (3) diphenylcarbazide-supported photoreduction of dichlorophenol indophenol is abolished; and (4) electron spin resonance Signal 2f and Signal 2s are eliminated. These results are consistent with multiple sites of modification in photosystem 2 by tetranitromethane, and suggest further that this reagent can inhibit charge stabilization in the reaction center.Abbreviations D1
electron donor to P680+ in oxygen-inhibited photosystem 2 preparations
- DPIP
2,6-dichlorophenol indophenol
- esr
electron spin resonance
- Fi
initial chlorophyll a fluorescence yield
- Fmax
maximum chlorophyll a fluorescence yield
- Fv
variable chlorophyll a fluorescence yield
- FWHM
full width at half maximum
- Mes
2-(N-morpholino)ethanesulfonic acid
- P680
primary electron donor chlorophyll of photosystem 2
- Ph
pheophytin
- PS
2-photosystem 2
- Qa
primary quinone electron acceptor
- Qb
secondary quinone acceptor
- Tricine
N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine
- TNM
tetranitromethane 相似文献
13.
Effect of preillumination on the P-680+ reduction kinetics in chloride-free photosystem II membranes
《FEBS letters》1986,203(2):215-219
The re-reduction course of P-680+, the photooxidized PS II primary donor, was measured as a function of excitation number in Cl−-depleted PS II membranes. After the 1st and 2nd excitations the signal amplitude of P-680+ is small, indicating a submicrosecond reduction of P-680+ by Z, the secondary donor of PS II. After the 3rd excitation, however, a larger P-680+ signal with a 40–50 μs half-life is observed. The slow decay of this signal is attributed to a back-reaction with a reduced acceptor in the presence of the Z+S2 state on the donor side. The state Z+S2 has a lifetime longer than 300 ms and its formation was found to depend on the presence of the abnormal S2 state created by the 1st excitation. The P-680 data and thermoluminescence measurements show that the S-state advancement beyond S2 is blocked in the absence of Cl− and that the Cl−-free abnormal S2 state has a lifetime about 10-times longer than the normal S2 state. 相似文献
14.
Photosynthetie water oxidation is unique to plants and cyanobacteria, it occurs in thylakoid membranes. The components associated with this process include: a reaction center polypeptide, having a molecular weight (Mr) of 47–50 kilodaltons (kDa), containing a reaction center chlorophyll a labeled as P680, a plastoquinol(?)-electron donor Z, a primary electron acceptor pheophytin, and a quinone electron acceptor QA; three ‘extrinsic’ polypeptides having Mr of approximately 17 kDa, 23 kDa, and 33 kDa; and, in all likelihood, an approximately 34 kDa ‘intrinsic’ polypeptide associated with manganese (Mn) atoms. In addition, chloride and calcium ions appear to be essential components for water oxidation. Photons, absorbed by the so-called photosystem II, provide the necessary energy for the chemical oxidation-reduction at P680; the oxidized P680 (P680+), then, oxidizes Z, which then oxidizes the water-manganese system contained, perhaps, in a protein matrix. The oxidation of water, leading to O2 evolution and H+ release, requires four such independent acts, i.e., there is a charge accumulating device (the so-called S-states). In this minireview, we have presented our current understanding of the reaction center P680, the chemical nature of Z, a possible working model for water oxidation, and the possible roles of manganese atoms, chloride ions, and the various polypeptides, mentioned above. A comparison with cytochrome c oxidase, which is involved in the opposite process of the reduction of O2 to H2O, is stressed. This minireview is a prelude to the several minireviews, scheduled to be published in the forthcoming issues of Photosynthesis Research, including those on photosystem II (by H.J. van Gorkom); polypeptides of the O2-evolving system (by D.F. Ghanotakis and C.F. Yocum); and the role of chloride in O2 evolution (by S. Izawa). 相似文献
15.
H. -J. Eckert B. Geiken J. Bernarding A. Napiwotzki H. -J. Eichler G. Renger 《Photosynthesis research》1991,27(2):97-108
Photoinhibition was analyzed in O2-evolving and in Tris-treated PS II membrane fragments by measuring flash-induced absorption changes at 830 nm reflecting the transient P680+ formation and oxygen evolution. Irradiation by visible light affects the PS II electron transfer at two different sites: a) photoinhibition of site I eliminates the capability to perform a stable charge separation between P680+ and QA
- within the reaction center (RC) and b) photoinhibition of site II blocks the electron transfer from YZ to P680+. The quantum yield of site I photoinhibition (2–3×10-7 inhibited RC/quantum) is independent of the functional integrity of the water oxidizing system. In contrast, the quantum yield of photoinhibition at site II depends strongly on the oxygen evolution capacity. In O2-evolving samples, the quantum yield of site II photoinhibition is about 10-7 inhibited RC/quantum. After selective elimination of the O2-evolving capacity by Tris-treatment, the quantum yield of photoinhibition at site II depends on the light intensity. At low intensity (<3 W/m2), the quantum yield is 10-4 inhibited RC/quantum (about 1000 times higher than in oxygen evolving samples). Based on these results it is inferred that the dominating deleterious effect of photoinhibition cannot be ascribed to an unique target site or a single mechanism because it depends on different experimental conditions (e.g., light intensity) and the functional status of the PS II complex.Abbreviations A830
absorption change at 830 nm
- P680
primary electron donor of PS II
- PS II
photosystem II
- Mes
2(N-morpholino)ethansulfonic acid
- QA, QB
primary and secondary acceptors of PS II
- DCIP
2,6-dichlorophenolindophenol
- DPC
1,5-diphenylcarbohydrazide
- FWHM
fullwidth at half maximum
- Ph-p-BQ
phenyl-p-benzoquinone
- PFR
photon fluence rate
- Pheo
pheophytin
- RC
reaction center 相似文献
16.
Alexander G. Ivanov Vaughan Hurry Prafullachandra V. Sane Gunnar Öquist Norman P. A. Huner 《Journal of Plant Biology》2008,51(2):85-96
In addition to the energy dissipation of excess light occurring in PSII antenna via the xanthophyll cycle, there is mounting
evidence of a zeaxanthin-independent pathway for non-photochemical quenching based within the PSII reaction centre (reaction
centre quenching) that may also play a significant role in photoprotection. It has been demonstrated that acclimation of higher
plants, green algae and cyanobacteria to low temperature or high light conditions which potentially induce an imbalance between
energy supply and energy utilization is accompanied by the development of higher reduction state of QA and higher resistance to photoinhibition (Huner et al., 1998). Although this is a fundamental feature of all photoautotrophs,
and the acquisition of increased tolerance to photoinhibition has been ascribed to growth and development under high PSII
excitation pressure, the precise mechanism controlling the redox state of QA and its physiological significance in developing higher resistance to photoinhibition has not been fully elucidated. In this
review we summarize recent data indicating that the increased resistance to high light in a broad spectrum of photosynthetic
organisms acclimated to high excitation pressure conditions is associated with an increase probability for alternative non-radiative
P680+QA
- radical pair recombination pathway for energy dissipation within the reaction centre of PSII. The various molecular mechanisms
that could account for non-photochemical quenching through PSII reaction centre are also discussed. 相似文献
17.
18.
Under physiological conditions (278 K) femtosecond pump-probe laser spectroscopy with 20-fs time resolution was applied to study primary charge separation in spinach photosystem II (PSII) core complexes excited at 710 nm. It was shown that initial formation of anion radical band of pheophytin molecule (Pheo−) at 460 nm is observed with rise time of ~ 11 ps. The kinetics of the observed rise was ascribed to charge separation between Chl (chlorophyll a) dimer, primary electron donor in PSII (P680*) and Pheo located in D1 protein subunit (PheoD1) absorbing at 420 nm, 545 nm and 680 nm with formation of the ion-radical pair P680+PheoDI−. The subsequent electron transfer from PheoD1− to primary plastoquinone electron acceptor (QA) was accompanied by relaxation of the 460-nm band and occurred within ~ 250 ps in good agreement with previous measurements in Photosystem II-enriched particles and bacterial reaction centers. The subtraction of the P680+ spectrum measured at 455 ps delay from the spectra at 23 ps or 44 ps delay reveals the spectrum of PheoDI−, which is very similar to that measured earlier by accumulation method. The spectrum of PheoDI− formation includes a bleaching (or red shift) of the 670 nm band indicating that Chl-670 is close to PheoD1. According to previous measurements in the femtosecond–picosecond time range this Chl-670 was ascribed to ChlD1 [Shelaev, Gostev, Vishnev, Shkuropatov, Ptushenko, Mamedov, Sarkisov, Nadtochenko, Semenov and Shuvalov, J. Photochemistry and Photobiology, B: Biology 104 (2011) 45–50]. Stimulated emission at 685 nm was found to have two decaying components with time constants of ~ 1 ps and ~ 14 ps. These components appear to reflect formation of P680+ChlD1− and P680+PheoD1−, respectively, as found earlier. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy. 相似文献
19.
The room-temperature EPR characteristics of Photosystem II reaction center preparations from spinach, pokeweed and Chlamydomonas reinhardii have been investigated. In all preparations a light-induced increase in EPR Signal II, which arises from the oxidized form of a donor to P-680+, is observed. Spin quantitation, with potassium nitrosodisulfonate as a spin standard, demonstrates that the Signal II species, Z?, is present in approx. 60% of the reaction centers. In response to a flash, the increase in Signal II spin concentration is complete within the 98 μs response time of our instrument. The decay of Z? is dependent on the composition of the particle suspension medium and is accelerated by addition of either reducing agents or lipophilic anions in a process which is first order in these reagents. Comparison of these results with optical data reported previously (Diner, B.A. and Bowes, J.M. (1981) in Proceedings of the 5th International Congress on Photosynthesis (Akoyunoglou, G., ed.), Vol. 3, pp. 875–883, Balaban, Philadelphia), supports the identification of Z with the P-680+ donor, D1. From the polypeptide composition of the particles used in this study, we conclude that Z is an integral component of the reaction center and use this conclusion to construct a model for the organization of Photosystem II. 相似文献
20.
Wim F.J. Vermaas 《BBA》1982,680(2):202-209
We investigated the effect of HCO?3 addition to CO2-depleted thylakoids by means of fluorescence techniques. (1) In the presence of diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea), the net reduction of the primary quinone-type electron acceptor (Q) of Photosystem (PS) II is about 2-times faster in the absence of HCO?3 than in its presence, whether normal, heat-treated or NH2OH-treated samples are used. This effect of HCO?3 is, therefore, not on the O2-evolving apparatus. It is, however, interpreted to be due to an influence of HCO?3 on the kinetics of the reduction of Q, perhaps combined with an effect on the back reaction of Q? with P-680+, the oxidized form of the PS II reaction center chlorophyll a. (2) Fluorescence experiments in the absence of diuron indicate that the absence of HCO?3 results in a complete block at the quinone level; the area over the fluorescence induction curve in the absence of HCO?3 was found to be 2.2-times higher in the absence than in the presence of diuron, pointing to a complete block of BH2 oxidation in the absence of HCO?3. (3) No change in the midpoint potential of Q is observed when HCO?3 is added to CO2-depleted membranes. HCO?3 not only has a large (on/off) effect on the reoxidation of BH2, but also a smaller effect between P-680 and Q. We propose that HCO?3 binding to its specific site in the thylakoid membrane results in a conformational change, allowing normal electron transport between the two photosystems. 相似文献