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1.
Bruce A. Diner  René Delosme 《BBA》1983,722(3):452-459
Redox titrations of the flash-induced formation of C550 (a linear indicator of Q?) were performed between pH 5.9 and 8.3 in Chlamydomonas Photosystem II particles lacking the secondary electron acceptor, B. One-third of the reaction centers show a pH-dependent midpoint potential (Em,7.5) = ? 30 mV) for redox couple QQ?, which varies by ?60 mV/pH unit. Two-thirds of the centers show a pH-independent midpoint potential (Emm = + 10 mV) for this couple. The elevated pH-independent Em suggests that in the latter centers the environment of Q has been modified such as to stabilize the semiquinone anion, Q?. The midpoint potentials of the centers having a pH-dependent Em are within 20 mV of those observed in chloroplasts having a secondary electron acceptor. It appears therefore that the secondary electron acceptor exerts little influence on the Em of QQ?. An EPR signal at g 1.82 has recently been attributed to a semiquinone-iron complex which comprises Q?. The similar redox behavior reported here for C550 and reported by others (Evans, M.C.W., Nugent, J.H.A., Tilling, L.A. and Atkinson, Y.E. (1982) FEBS Lett. 145, 176–178) for the g 1.82 signal in similar Photosystem II particles confirm the assignment of this EPR signal to Q?. At below ?200 mV, illumination of the Photosystem II particles produces an accumulation of reduced pheophytin (Ph?). At ?420 mV Ph? appears with a quantum yield of 0.006–0.01 which in this material implies a lifetime of 30–100 ns for the radical pair P-680+Ph?.  相似文献   

2.
John H. Golbeck  Bessel Kok 《BBA》1979,547(2):347-360
The primary photochemical quencher Q and the secondary electron acceptor pool in Photosystem II have been titrated. We used particles of Scenedesmus mutant No. 8 that lack System I and allowed the system to equilibrate with external redox mediators in darkness prior to measurement of the fluorescence rise curve.The titration of Q, as indicated by the dark level of Fi, occurs in two discrete steps. The high-potential component (Qh) has a midpoint potential of +68 mV (pH 7.2) and accounts for ~67% of Q. The pH sensitivity of the midpoint potential is ?60 mV, indicating the involvement of 1 H+e. The low-potential component (Q1) accounts for the remaining 33% of Q and shows a midpoint potential near?300 mV (pH 7.2).The plastoquinone pool, assayed as the half-time of the fluorescence rise curve, titrates as a single component with a midpoint potential 30–40 mV more oxidizing than that of Qh, i.e., at 106 mV (pH 7.2). The Em shows a pH sensitivity of ?60 mV/pH unit, indicating the involvement of 1 H+e. The observation that all 12–14 electron equivalents in the pool titrate as a single component indicates that the heterogeneity otherwise observed in the secondary acceptor system is a kinetic rather than a thermodynamic property.Illumination causes peculiar, and as yet unclarified, changes of both Q and the secondary pool under anaerobic conditions that are reversed by oxygen.  相似文献   

3.
Peter Horton 《BBA》1981,635(1):105-110
The effect of alteration of redox potential on the kinetics of fluorescence induction in pea chloroplasts has been investigated. Potentiometric titration of the initial (Fi) level of fluorescence recorded upon shutter opening gave a two component curve, with Em(7) at ?20 mV and ?275 mV, almost, identical to results obtained using continuous low intensity illumination (Horton, P. and Croze, E. (1979) Biochim. Biophys. Acta 545, 188–201). The slow or tail phase of induction observed in the presence of DCMU can be eliminated by poising the redox potential at approx. 0 to +50 mV. At this potential Fi was increased by less than 10% and the higher potential quencher described above was only marginally reduced. The disappearance of the slow phase titrated as an n = 1 component with an Em(7) of +120 mV. Therefore it seems unlikely that the slow phase of fluorescence induction is due to photoreduction of the ?20 mV quencher. These results are discussed with reference to current ideas concerning heterogeneity on the acceptor side of Photosystem II.  相似文献   

4.
Bruce A. Diner  René Delosme 《BBA》1983,722(3):443-451
Redox titration of the electrochromic carotenoid band shift, detected at 50 μs after a saturating actinic flash, in spinach chloroplasts, shows that only one electron acceptor in Photosystem II participates in a transmembrane primary electron transfer. This species, the primary quinone acceptor, Q, shows only one midpoint potential (Em,7.5) of approx. 0 V and is undoubtedly equivalent to the fluorescence quencher, QH. A second titration wave is observed at low potential (Em,7.5 ? ? 240 mV) and at greater than 3 ms after a saturating actinic flash. This wave has an action spectrum different from that of Photosystem II centers containing Q and could arise from a secondary but not primary electron transfer. A low-potential fluorescence quencher is observed in chloroplasts which largely disappears in a single saturating flash at ? 185 mV and which does not participate in a transmembrane electron transfer. This low-potential quencher (probably equivalent to fluorescence quencher, QL) and Q are altogether different species. Redox titration of C550 shows that if electron acceptor Qβ is indeed characterized by an Em,7 of + 120 mV, then this acceptor does not give rise to a C550 signal upon reduction and does not participate in a transmembrane electron transfer. This titration also shows that C550 is not associated with QL.  相似文献   

5.
Jane M. Bowes  Peter Horton 《BBA》1982,680(2):127-133
Fluorescence induction curves in 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-inhibited Photosystem (PS) II particles isolated from the blue-green alga Phormidium laminosum have been analysed as a function of redox potential. Redox titration of the initial fluorescence indicated a single component with Em,7.5 = +30 mV (n = 1) (Bowes, J., Horton, P. and Bendall, D.S. (1981) FEBS Lett. 135, 261–264). Despite this simplified electron acceptor system and the small number of chlorophylls per reaction centre, a sigmoidal induction curve was nevertheless seen. Sigmoidicity decreased as Q was reduced potentiometrically prior to induction such that the induction was exponential when the ratio FiFm = 0.64. These particles also showed a slow (β) phase of induction which titrated with an Em value slightly more positive than that of the major quencher. It is concluded that the sigmoidal shape of the fluorescence induction curve observed in Phormidium PS II particles is not a consequence of a requirement for two photons to close the PS II reaction centre, but is generated as a result of energy transfer between photosynthetic units comprising one reaction centre per approx. 50 chlorophylls. Also, the existence of PS II heterogeneity (PS IIα, PS IIβ centres) does not require a structurally differentiated chloroplast, but may only indicate the extent of aggregation of PS II centres.  相似文献   

6.
This study describes an analysis of different treatments that influence the relative content and the midpoint potential of HP Cyt b559 in PS II membrane fragments from higher plants. Two basically different types of irreversible modification effects are distinguished: the HP form of Cyt b559 is either predominantly affected when the heme group is oxidized (“O-type” effects) or when it is reduced (“R-type” effects). Transformation of HP Cyt b559 to lower potential redox forms (IP and LP forms) by the “O-type” mechanism is induced by high pH and detergent treatments. In this case the effects consist of a gradual decrease in the relative content of HP Cyt b559 while its midpoint potential remains unaffected. Transformation of HP Cyt b559 via an “R-type” mechanism is caused by a number of exogenous compounds denoted L: herbicides, ADRY reagents and tetraphenylboron. These compounds are postulated to bind to the PS II complex at a quinone binding site designated as QC which interacts with Cyt b559 and is clearly not the QB site. Binding of compounds L to the QC site when HP Cyt b559 is oxidized gives rise to a gradual decrease in the Em of HP Cyt b559 with increasing concentration of L (up to 10 Kox(L) values) while the relative content of HP Cyt b559 is unaffected. Higher concentrations of compounds L required for their binding to QC site when HP Cyt b559 is reduced (described by Kred(L)) induce a conversion of HP Cyt b559 to lower potential redox forms (“R-type” transformation). Two reaction pathways for transitions of Cyt b559 between the different protein conformations that are responsible for the HP and IP/LP redox forms are proposed and new insights into the functional regulation of Cyt b559 via the QC site are discussed.  相似文献   

7.
Succinate:quinone oxidoreductase (SQR) from Bacillus subtilis consists of two hydrophilic protein subunits comprising succinate dehydrogenase, and a di-heme membrane anchor protein harboring two putative quinone binding sites, Qp and Qd. In this work we have used spectroelectrochemistry to study the electronic communication between purified SQR and a surface modified gold capillary electrode. In the presence of two soluble quinone mediators the midpoint potentials of both hemes were revealed essentially as previously determined by conventional redox titration (heme bH, Em = + 65 mV, heme bL, Em = − 95 mV). In the absence of mediators the enzyme still communicated with the electrode, albeit with a reproducible hysteresis, resulting in the reduction of both hemes occurring approximately at the midpoint potential of heme bL, and with a pronounced delay of reoxidation. When the specific inhibitor 2-n-heptyl-4 hydroxyquinoline N-oxide (HQNO), which binds to Qd in B. subtilis SQR, was added together with the two quinone mediators, rapid reductive titration was still possible which can be envisioned as an electron transfer occurring via the HQNO insensitive Qp site. In contrast, the subsequent oxidative titration was severely hampered in the presence of HQNO, in fact it completely resembled the unmediated reaction. If mediators communicate with Qp or Qd, either event is followed by very rapid electron redistribution within the enzyme. Taken together, this strongly suggests that the accessibility of Qp depended on the redox state of the hemes. When both hemes were reduced, and Qd was blocked by HQNO, quinone-mediated communication via the Qp site was no longer possible, revealing a redox-dependent conformational change in the membrane anchor domain.  相似文献   

8.
Chromatophore membranes from Rhodopseudomonas sphaeroides activated by light display a carotenoid band shift (phase III) that occurs in response to the electrogenic event (charge separation) in the ubiquinol-cytochrome c oxidoreductase. The rate of formation of this electrogenic event has previously been shown to be strongly dependent on the initial redox state of a bound ubiquinone species (designated Qz) associated with the oxidoreductase. When Qz is reduced (quinol form; QzH2) the electrogenic event takes place in less than 5 ms. When Qz is oxidized (quinone form; Qz) it is much slower; under these conditions the fact that it occurs has been ignored. In this report, we address this issue and describe events that lead to the generation of carotenoid band shift phase III when the total population of Qz of the chromatophore is oxidized before flash activation. The following characteristics are apparent: (1) When oxidized Qz is present before activation, the half-time of formation of carotenoid band-shift phase III is 10–20-times slower than when QzH2 is present before activation. (2) When oxidized Qz is present, the measured full extent of phase III generated by a single-turnover flash is diminished by about one-half of that observed when QzH2 is present before activation. (3) The rate of formation of the carotenoid band shift phase III when Qz is initially oxidized corresponds closely to the rate of completion of the flash-activated electron-transfer cycle. This can be seen under two different conditions: (a) as the partial reduction of cytochrome c1 + c2 (at redox potentials of 200–300 mV) or (b) as the partial reduction of flash-oxidized bacteriochlorophyll dimer, (BChl)2+ (at redox potentials above 300 mV). (4) At the higher redox potentials (above 300 mV), antimycin-sensitive proton binding shares a common, rate-limiting step with the carotenoid band shift phase III and (BChl)2+ reduction. (5) However, proton binding at redox potentials above 300 mV is not observed at all unless valinomycin (K+) is present. Thus, proton binding occurs only when the carotenoid band shift is collapsed in milliseconds, whereas, conversely, the carotenoid band shift is stably generated when proton binding is not observed. These and other observations are the basis of a reevaluation of our current views on the coupling of electron transfer and proton translocation in photosynthetic bacteria.  相似文献   

9.
《BBA》1986,849(1):150-161
Flash-induced absorption changes at 450 nm were investigated in isolated chromatophores of Rhodopseudomonas sphaeroides and Rhodospirillum rubrum non-sulfur purple bacteria to follow the redox changes of the semiquinone species of the secondary quinone acceptor of the photosynthetic reaction center. Excitation of a dark-adapted chromatophore suspension by a series of successive flashes in the presence of electron donors capable of rapidly reducing the photooxidized reaction-center pigment causes the formation of a stable semiquinone species (QB) with a lifetime which is shown to be proportional to the amount of the oxidized redox mediator in the incubation medium. It is shown that the disappearance of the flash-induced absorption changes at 450 nm on lowering the ambient redox potential (Eh) to 200–300 mV is the result of increasing the lifetime of QB, as the amount of the oxidized mediator diminishes; consequently, in these circumstances, the 2–5 min dark interval between the flash cycles appears insufficient for QB recovery. After the addition of redox mediators with a low midpoint potential, acting as an oxidant for QB, the flash-induced redox changes of QB were observed at low Eh values unless Eh reached a value at which QB underwent reduction at equilibrium to form QBH2. The data provide evidence that reaction centers with a fully oxidized secondary acceptor can donate electrons to the cyclic electron-transport chain only after two turnovers, leading to the formation of the doubly reduced ubiquinone species (QBH2) of the secondary acceptor.  相似文献   

10.
(1) Two populations of reaction centers in the chromatophore membrane can be distinguished under some conditions of initial redox poise (300 mV < Eh < 400 mV): those which transfer a reducing equivalent after the first flash from the secondary quinone (QII) of the reaction center to cytochrome b of the ubiquinone-cytochrome c2 oxidoreductase; and those which retain the reducing equivalent on Q?II until a second flash is given. These two populations do not exchange on a time scale of tens of seconds. (2) At redox potentials higher than 400 mV, Q?II generated after the first flash is no longer able to reduce cytochrome b-560 even in those reaction centers associated with an oxidoreductase. Under these conditions, doubly reduced QII generated by a second flash is required for cytochrome b reduction, so that the QII effectively functions as a two-electron gate into the oxidoreductase at these high potentials. (3) At redox potentials below 300 mV, although the two populations of QII are no longer distinguishable, cytochrome b reduction is still dependent on only part of the reaction center population. (4) Proton binding does not oscillate under any condition tested.  相似文献   

11.
Reaction centers (RC) from the species Erythrobacter (Eb.) litoralis, Erythromonas (Em.) ursincola and Sandaracinobacter (S.) sibiricus have been purified by LDAO treatment of light-harvesting-reaction center complexes and DEAE chromatography. The content and overall organisation of the RCs' chromophores, determined by linear dichroism (LD) and absorption spectroscopy, are similar to those isolated from anaerobic photosynthetic bacteria. The redox properties of the primary electron donor are pH-independent and very similar to those determined for anaerobic photosynthetic bacteria with midpoint potential values equal to 445 (± 10), 475 and 510 mV for Eb. litoralis, S. sibiricus and Em. ursincola, respectively. The RC purified from Eb. litoralis does not contain bound cytochrome (cyt), whereas RCs isolated from S. sibiricus and Em. ursincola possess a tetraheme cyt c. Each of these tetraheme cyts contains two high potential hemes and two low potential hemes. Their redox properties are very similar, with midpoint potentials equal to 385 (± 10), 305, 40, -40 mV for Em. ursincola and 355, 285, 30, -48 mV for S. sibiricus. At physiological pH, the midpoint potential of the primary electron acceptor (QA) varies with a slope of -60 mV/pH unit. The reduced form of QA presents pK values of 9, 9.8, 10.5 for S. sibiricus, Em. ursincola and Eb. litoralis, respectively. The main difference observed between RCs isolated from anaerobic photosynthetic and from obligate aerobic bacteria is the Emvalues of QA which are 65 to 120 mV higher in the last case. This difference is proposed to be a major reason for the inability of these species to grow under anaerobic photosynthetic conditions.  相似文献   

12.
The fluorescence yield of chloroplasts reflects the redox state of the electron acceptor of the Photosystem II reaction center, with increasing yield as the acceptor is reduced. Chemical reductive titrations of fluorescence yield in chloroplasts at room temperature indicate two distinct midpoint potentials, suggesting the possibility of Photosystem II electron acceptor heterogeneity. We have carried out a potentiometric titration of the fluorescence decay kinetics in spinach chloroplasts using a continuous mode-locked dye laser with low-intensity excitation pulses and a picosecond-resolution single-photon timing system. At all potentials the fluorescence decay is best described by three exponential components. As the potential is lowered, the slow phase changes 30-fold in yield with two distinct midpoint potentials, accompanied by a modest (3-fold) increase in the lifetime. The titration curve for the slow component of the fluorescence decay of spinach chloroplasts is best characterized by two single-electron redox reactions with midpoint potentials at pH 8.0 of +119 and ?350 mV, with corresponding relative contributions to the fluorescence yield of 49 and 51%, respectively. There is little change in the fast and middle components of the fluorescence decay. We found that the oxidized form of the redox mediator 2-hydroxy-1,4-naphthoquinone preferentially quenches the fluorescence, causing an anomalous decrease in the apparent midpoint of the high-potential transition. This effect accounts for a significant difference between the midpoint potentials that we observe and some of those previously reported. The selective effect of reduction potentials on particular fluorescence decay components provides useful information about the organization and distribution of the Photosystem II electron acceptor.  相似文献   

13.
《BBA》1985,809(3):320-330
Preincubation of isolated chloroplasts with ferricyanide, prior to addition of DCMU, unmasks a high-potential electron acceptor (Q400) in Photosystem II that acts as an additional quencher and prolongs the fluorescence induction curve in the presence of DCMU (Ikegami, I. and Katoh, S. (1973) Plant Cell Physiol. 14, 829–836). This study confirms that Q400 is endogenous to Photosystem II and is not a bound ferricyanide, and several new characteristics of this high potential acceptor are established. (a) It is accessible to ferricyanide even in the presence of DCMU. The rate of oxidation, however, is very slow, consistent with access only via QA. Accessibility may be enhanced by magnesium, reminiscent of the oxidation of QA by ferricyanide. (b) Oxidation of Q400 drastically suppresses the binding of DCMU at neutral and alkaline pH. Below pH 6, however, DCMU binding is essentially normal. The pH dependence of DCMU binding is consistent with the known pH dependence of the redox midpoint potential of Q400. (c) Binding of many other inhibitors of QA-to-QB electron transfer is much less affected or even completely unaffected. These results have implications for current notions of herbicide binding and may also bear on the origin of slow phases of fluorescence induction in the presence of DCMU.  相似文献   

14.
Hiroshi Ishikita 《BBA》2007,1767(11):1300-1309
In bacterial photosynthetic reaction centers (bRC), the electron is transferred from the special pair (P) via accessory bacteriochlorophyll (BA), bacteriopheopytin (HA), the primary quinone (QA) to the secondary quinone (QB). Although the non-heme iron complex (Fe complex) is located between QA and QB, it was generally supposed not to be redox-active. Involvement of the Fe complex in electron transfer (ET) was proposed in recent FTIR studies [A. Remy and K. Gerwert, Coupling of light-induced electron transfer to proton uptake in photosynthesis, Nat. Struct. Biol. 10 (2003) 637-644]. However, other FTIR studies resulted in opposite results [J. Breton, Steady-state FTIR spectra of the photoreduction of QA and QB in Rhodobacter sphaeroides reaction centers provide evidence against the presence of a proposed transient electron acceptor X between the two quinones, Biochemistry 46 (2007) 4459-4465]. In this study, we calculated redox potentials of QA/B (Em(QA/B)) and the Fe complex (Em(Fe)) based on crystal structure of the wild-type bRC (WT-bRC), and we investigated the energetics of the system where the Fe complex is assumed to be involved in the ET. Em(Fe) in WT-bRC is much less pH-dependent than that in PSII. In WT-bRC, we observed significant coupling of ET with Glu-L212 protonation upon oxidation of the Fe complex and a dramatic Em(Fe) downshift by 230 mV upon formation of QA (but not QB) due to the absence of proton uptake of Glu-L212. Changes in net charges of the His ligands of the Fe complex appear to be the nature of the redox event if we assume the involvement of the Fe complex in the ET.  相似文献   

15.
A Photosystem-II reaction-center particle derived from spinach chloroplasts by Triton treatment contains only one kind of cytochrome, namely, cytochrome b559, in the amount of slightly more than 2 per 100 total chlorophyll molecules. Cytochrome b559 is present in the oxidized form, has a standard redox potential of 58 mV, and undergoes photoreduction.  相似文献   

16.
The kinetics of the photoreduction of C-550, the photooxidation of cytochrome b559 and the fluorescence yield changes during irradiation of chloroplasts at ?196 °C were measured and compared. The photoreduction of C-550 proceeded more rapidly than the photooxidation of cytochrome b559 and the fluorescence yield increase followed the cytochrome b559 oxidation. These results suggest that fluorescence yield under these conditions indicates the dark reduction of the primary electron donor to Photosystem II, P680+, by cytochrome b559 rather than the photoreduction of the primary electron acceptor.The photoreduction of C-550 showed little if any temperature dependence over the range of ?196 to ?100 °C. The amount of cytochrome b559 photooxidized was sensitive to temperature decreasing from the maximal change at temperatures between ?196 to ?160 °C to no change at ?100 °C. To the extent that the reaction occurred at temperatures between ?160 and ?100 °C the rate was largely independent of temperature. The rate of the fluorescence increase was dependent on temperature over this range being 3–4 times more rapid at ?100 than at ?160 °C. At ?100 °C the light-induced fluorescence increase and the photoreduction of C-550 show similar kinetics. The temperature dependence of the fluorescence induction curve is attributed to the temperature dependence of the dark reduction of P680+.The intensity dependence of the photoreduction of C-550 and of the photooxidation of cytochrome b559 are linear at low intensities (below 200 μW/cm2) but fall off at higher intensities. The failure of reciprocity in the photoreduction of C-550 at the higher intensities is not explained by the simple model proposed for the Photosystem II reaction centers.  相似文献   

17.
C.A. Wraight 《BBA》1979,548(2):309-327
The photoreduction of ubiquinone in the electron acceptor complex (Q1Q11) of photosynthetic reaction centers from Rhodopseudomonas sphaeroides, R26, was studied in a series of short, saturating flashes. The specific involvement of H+ in the reduction was revealed by the pH dependence of the electron transfer events and by net H+ binding during the formation of ubiquinol, which requires two turnovers of the photochemical act. On the first flash Q11 receives an electron via Q1 to form a stable ubisemiquinone anion (Q??11); the second flash generates Q??1. At low pH the two semiquinones rapidly disproportionate with the uptake of 2 H+, to produce Q11H2. This yields out-of-phase binary oscillations for the formation of anionic semiquinone and for H+ uptake. Above pH 6 there is a progressive increase in H+ binding on the first flash and an equivalent decrease in binding on the second flash until, at about pH 9.5, the extent of H+ binding is the same on all flashes. The semiquinone oscillations, however, are undiminished up to pH 9. It is suggested that a non-chromophoric, acid-base group undergoes a pK shift in response to the appearance of the anionic semiquinone and that this group is the site of protonation on the first flash. The acid-base group, which may be in the reaction center protein, appears to be subsequently involved in the protonation events leading to fully reduced ubiquinol. The other proton in the two electron reduction of ubiquinone is always taken up on the second flash and is bound directly to Q??11. At pH values above 8.0, it is rate limiting for the disproportionation and the kinetics, which are diffusion controlled, are properly responsive to the prevailing pH. Below pH 8, however, a further step in the reaction mechanism was shown to be rate limiting for both H+ binding electron transfer following the second flash.  相似文献   

18.
The polyphasic patterns of fluorescence induction rise in pea leaves in vivo and after the treatment with ionophores have been studied using a Plant Efficiency Analyzer. To analyze in detail photosystem II (PS II) electron transfer processes, an extended PS II model was applied, which included the sums of exponential functions to specify explicitly the light-driven formation of the transmembrane electric potential (ΔΨ(t)) as well as pH in the lumen (pHL(t)) and stroma (pHS(t)). PS II model parameters and numerical coefficients in ΔΨ(t), pHL(t), and pHS(t) were evaluated to fit fluorescence induction data for different experimental conditions: leaf in vivo or after ionophore treatment at low or high light intensity. The model imitated changes in the pattern of fluorescence induction rise due to the elimination of transmembrane potential in the presence of ionophores, when ΔΨ = 0 and pHL(t), pHS(t) changed to small extent relative to control values in vivo, with maximum ΔΨ(t) ∼ 90 mV and ΔΨ(t) ∼ 40 mV for the stationary state at ΔpH ≅ 1.8. As the light intensity was increased from 300 to 1200 μmol m−2 s−1, the heat dissipation rate constants increased threefold for nonradiative recombination of P680+Phe and by ∼30% for P680+QA. The parameters ΔΨ, pHS and pHL were analyzed as factors of PS II redox state populations and fluorescence yield. The kinetic mechanism of fluorescence quenching is discussed, which is related with light-induced lumen acidification, when +QA and P680+ recombination probability increases to regulate the QA reduction.  相似文献   

19.
High-temperature-induced inhibition of the acceptor side of Photosystem II (PS II) was studied in tobacco thylakoids using oxygen evolution, chlorophyll a (Chl a) fluorescence and redox potential measurements. When thylakoids were heated at 2 °C/min from 25 to 50 °C, the oxygen evolving complex became inhibited between 32 and 45 °C, whereas the acceptor side of PS II tolerated higher temperatures. Variable Chl a fluorescence decreased more slowly than oxygen evolution, suggesting that transitions between some S-states occurred even after heat-induced inhibition of the oxygen evolving activity. 77 K emission spectroscopy reveals that heating does not cause detachment of the light-harvesting complex II from PS II, and thus the heat-induced increase in the initial F0 fluorescence is due to loss of exciton trapping in the heated PS II centers. Redox titrations showed a heat-induced increase in the midpoint potential of the QA/QA -) couple from the control value of –80 mV to +40 mV at 50 °C, indicating a loss of the reducing power of QA -). When its driving force thus decreased, electron transfer from QA -) to QB in the PS II centers that still could reduce QA became gradually inhibited, as shown by measurements of the decay of Chl a fluorescence yield after a single turnover flash. Interestingly, the heat-induced loss of variable fluorescence and inhibition of electron transfer from QA -) to QB could be partially prevented by the presence of 5 mM bicarbonate during heating, suggesting that high temperatures cause release of the bicarbonate bound to PS II. We speculate that both the upshift in the redox potential of the QA/QA -) couple and the release of bicarbonate may be caused by a heat-induced structural change in the transmembrane D1 or D2 proteins. This structural change may, in turn, be caused by the inhibition of the oxygen evolving complex during heating.  相似文献   

20.
Gerald T. Babcock  Kenneth Sauer 《BBA》1975,376(2):329-344
Rapid light-induced transients in EPR Signal IIf (F?+) are observed in 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated, Tris-washed chloroplasts until the state F P680 Q? is reached. In the absence of exogenous redox mediators several flashes are required to saturate this photoinactive state. However, the Signal IIf transient is observed on only the first flash following DCMU addition if an efficient donor to Signal IIf, phenylenediamine or hydroquinone, is present. Complementary polarographic measurements show that under these conditions oxidized phenylenediamine is produced only on the first flash of a series. The DCMU inhibition of Signal IIf can be completely relieved by oxidative titration of a one-electron reductant with E08.0 = +480 mV. At high reduction potentials the decay time of Signal IIf is constant at about 300 ms, whereas in the absence of DCMU the decay time is longer and increases with increasing reduction potential.A model is proposed in which Q?, the reduced Photosystem II primary acceptor, and D, a one-electron 480 mV donor endogenous to the chloroplast suspension, compete in the reduction of Signal IIf (F?+). At high potentials D is oxidized in the dark, and the (Q? + F?+) back reaction regenerates the photoactive F P680 Q state. The electrochemical and kinetic evidence is consistent with the hypothesis that the Signal IIf species, F, is identical with Z, the physiological donor to P680.  相似文献   

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