SIMPLE EXPLANATION OF THE MISSES IN THE COOPERATION OF CHARGES IN PHOTOSYNTHETIC O2EVOLUTION

The Effect of Removal of External Proteins PsbO, PsbP and PsbQ on Flash-Induced Molecular Oxygen Evolution and Its Biphasicity in Tobacco PSII

The oxygen evolution within photosystem II (PSII) is one of the most enigmatic processes occurring in nature. It is suggested that external proteins surrounding the oxygen-evolving complex (OEC) not only stabilize it and provide an appropriate ionic environment but also create water channels, which could be involved in triggering the ingress of water and the removal of O2 and protons outside the system. To investigate the influence of these proteins on the rate of oxygen release and the efficiency of OEC function, we developed a measurement protocol for the direct measurement of the kinetics of oxygen release from PSII using a Joliot-type electrode. PSII-enriched tobacco thylakoids were used in the experiments. The results revealed the existence of slow and fast modes of oxygen evolution. This observation is model-independent and requires no specific assumptions about the initial distribution of the OEC states. The gradual removal of exogenous proteins resulted in a slowdown of the rapid phase (~ms) of O2 release and its gradual disappearance while the slow phase (~tens of ms) accelerated. The role of external proteins in regulating the biphasicity and efficiency of oxygen release is discussed based on observed phenomena and current knowledge.

Unequal misses during the flash-induced advancement of photosystem II: effects of the S state and acceptor side cycles

Photosynthetic water oxidation is catalyzed by the oxygen-evolving complex (OEC) in photosystem II (PSII). This process is energetically driven by light-induced charge separation in the reaction center of PSII, which leads to a stepwise accumulation of oxidizing equivalents in the OEC (S_i states, i = 0-4) resulting in O₂ evolution after each fourth flash, and to the reduction of plastoquinone to plastoquinol on the acceptor side of PSII. However, the S_i-state advancement is not perfect, which according to the Kok model is described by miss-hits (misses). These may be caused by redox equilibria or kinetic limitations on the donor (OEC) or the acceptor side. In this study, we investigate the effects of individual S state transitions and of the quinone acceptor side on the miss parameter by analyzing the flash-induced oxygen evolution patterns and the S₂, S₃ and S₀ state lifetimes in thylakoid samples of the extremophilic red alga Cyanidioschyzon merolae. The data are analyzed employing a global fit analysis and the results are compared to the data obtained previously for spinach thylakoids. These two organisms were selected, because the redox potential of Q_A/Q_A^- in PSII is significantly less negative in C. merolae (E_m = - 104 mV) than in spinach (E_m = - 163 mV). This significant difference in redox potential was expected to allow the disentanglement of acceptor and donor side effects on the miss parameter. Our data indicate that, at slightly acidic and neutral pH values, the E_m of Q_A^-/Q_A plays only a minor role for the miss parameter. By contrast, the increased energy gap for the backward electron transfer from Q_A^- to Pheo slows down the charge recombination reaction with the S₃ and S₂ states considerably. In addition, our data support the concept that the S₂ → S₃ transition is the least efficient step during the oxidation of water to molecular oxygen in the Kok cycle of PSII.

Environmental pH and a Glu364 to Gln mutation in the chlorophyll-binding CP47 protein affect redox-active TyrD and charge recombination in Photosystem II

In Photosystem II, loop E of the chlorophyll-binding CP47 protein is located near a redox-active tyrosine, Y_D , forming a symmetrical analog to loop E in CP43, which provides a ligand to the oxygen-evolving complex (OEC). A Glu364 to Gln substitution in CP47, near Y_D , does not affect growth in the cyanobacterium Synechocystis sp. PCC 6803; however, deletion of the extrinsic protein PsbV in this mutant leads to a strain displaying a pH-sensitive phenotype. Using thermoluminescence, chlorophyll fluorescence, and flash-induced oxygen evolution analyses, we demonstrate that Glu364 influences the stability of Y_D and the redox state of the OEC, and highlight the effects of external pH on photosynthetic electron transfer in intact cyanobacterial cells.

Links between peptides and Mn oxide: nano-sized manganese oxide embedded in a peptide matrix

We report on a poly-peptide/Mn oxide nanocomposite as a model for the water-oxidizing catalyst in Photosystem II.

Probing S-state advancements and recombination pathways in photosystem II with a global fit program for flash-induced oxygen evolution pattern

The oxygen-evolving complex (OEC) in photosystem II catalyzes the oxidation of water to molecular oxygen. Four decades ago, measurements of flash-induced oxygen evolution have shown that the OEC steps through oxidation states S(0), S(1), S(2), S(3) and S(4) before O(2) is released and the S(0) state is reformed. The light-induced transitions between these states involve misses and double hits. While it is widely accepted that the miss parameter is S state dependent and may be further modulated by the oxidation state of the acceptor side, the traditional way of analyzing each flash-induced oxygen evolution pattern (FIOP) individually did not allow using enough free parameters to thoroughly test this proposal. Furthermore, this approach does not allow assessing whether the presently known recombination processes in photosystem II fully explain all measured oxygen yields during Si state lifetime measurements. Here we present a global fit program that simultaneously fits all flash-induced oxygen yields of a standard FIOP (2 Hz flash frequency) and of 11-18 FIOPs each obtained while probing the S(0), S(2) and S(3) state lifetimes in spinach thylakoids at neutral pH. This comprehensive data treatment demonstrates the presence of a very slow phase of S(2) decay, in addition to the commonly discussed fast and slow reduction of S(2) by YD and QB(-), respectively. Our data support previous suggestions that the S(0)→S(1) and S(1)→S(2) transitions involve low or no misses, while high misses occur in the S(2)→S(3) or S(3)→S(0) transitions.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

Two-electron reactions S2QB -->S0QB and S3QB -->S1QB are involved in deactivation of higher S states of the oxygen-evolving complex of Photosystem II

The oxygen-evolving complex of Photosystem II cycles through five oxidation states (S(0)-S(4)), and dark incubation leads to 25% S(0) and 75% S(1). This distribution cannot be reached with charge recombination reactions between the higher S states and the electron acceptor Q(B)(-). We measured flash-induced oxygen evolution to understand how S(3) and S(2) are converted to lower S states when the electron required to reduce the manganese cluster does not come from Q(B)(-). Thylakoid samples preconditioned to make the concentration of the S(1) state 100% and to oxidize tyrosine Y(D) were illuminated by one or two laser preflashes, and flash-induced oxygen evolution sequences were recorded at various time intervals after the preflashes. The distribution of the S states was calculated from the flash-induced oxygen evolution pattern using an extended Kok model. The results suggest that S(2) and S(3) are converted to lower S states via recombination from S(2)Q(B)(-) and S(3)Q(B)(-) and by a slow change of the state of oxygen-evolving complex from S(3) and S(2) to S(1) and S(0) in reactions with unspecified electron donors. The slow pathway appears to contain two-electron routes, S(2)Q(B) -->S(0)Q(B), and S(3)Q(B) -->S(1)Q(B). The two-electron reactions dominate in intact thylakoid preparations in the absence of chemical additives. The two-electron reaction was replaced by a one-electron-per-step pathway, S(3)Q(B) -->S(2)Q(B) -->S(1)Q(B) in PS II-enriched membrane fragments and in thylakoids measured in the presence of artificial electron acceptors. A catalase effect suggested that H(2)O(2) acts as an electron donor for the reaction S(2)Q(B) -->S(0)Q(B) but added H(2)O(2) did not enhance this reaction.

Efficiency and role of loss processes in light-driven water oxidation by PSII

Its superior quantum efficiency renders PSII a model for biomimetic systems. However, also in biological water oxidation by PSII, the efficiency is restricted by recombination losses. By laser-flash illumination, the secondary radical pair, P680(+)Q(-) (A) (where P680 is the primary Chl donor in PSII and Q(A), primary quinone acceptor of PSII), was formed in close to 100% of the PSII. Investigation of the quantum efficiency (or yield) of the subsequent steps by time-resolved delayed (10 micros to 60 ms) and prompt (70 micros to 700 ms) Chl fluorescence measurements on PSII membrane particles suggests that (1) the effective rate for P680(+) Q(-) (A) recombination is approximately 5 ms(-1) with an activation energy of approximately 0.34 eV, circumstantially confirming dominating losses by reformation of the primary radical pair followed by ground-state recombination. (2) Because of compensatory influences on recombination and forward reactions, the efficiency is only weakly temperature dependent. (3) Recombination losses are several-fold enhanced at lower pH. (4) Calculation based on delayed-fluorescence data suggests that the losses depend on the state of the water-oxidizing manganese complex, being low in the S(0)-->S(1) and S(1)-->S(2) transition, clearly higher in S(2)-->S(3) and S(3)-->S(4)-->S(0). (5) For the used artificial electron acceptor, the efficiency is limited by acceptor-side processes/S-state decay at high/low photon-absorption rates resulting in optimal efficiency at surprisingly low rates of approximately 0.15-15 photons s(-1) (per PSII). The pH and S-state dependence can be rationalized by the basic model of alternate electron-proton removal proposed elsewhere. A physiological function of the recombination losses could be limitation of the lifetime of the reactive donor-side tyrosine radical (Y(.) (Z)) in the case of low-pH blockage of water oxidation.

Interaction of N,N,N',N'-tetramethyl-p-phenylenediamine with photosystem II as revealed by thermoluminescence: reduction of the higher oxidation states of the Mn cluster and displacement of plastoquinone from the Q(B) niche

The flash-induced thermoluminescence (TL) technique was used to investigate the action of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) on charge recombination in photosystem II (PSII). Addition of low concentrations (muM range) of TMPD to thylakoid samples strongly decreased the yield of TL emanating from S(2)Q(B)(-) and S(3)Q(B)(-) (B-band), S(2)Q(A)(-) (Q-band), and Y(D)(+)Q(A)(-) (C-band) charge pairs. Further, the temperature-dependent decline in the amplitude of chlorophyll fluorescence after a flash of white light was strongly retarded by TMPD when measured in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Though the period-four oscillation of the B-band emission was conserved in samples treated with TMPD, the flash-dependent yields (Y(n)) were strongly declined. This coincided with an upshift in the maximum yield of the B-band in the period-four oscillation to the next flash. The above characteristics were similar to the action of the ADRY agent, carbonylcyanide m-chlorophenylhydrazone (CCCP). Simulation of the B-band oscillation pattern using the integrated Joliot-Kok model of the S-state transitions and binary oscillations of Q(B) confirmed that TMPD decreased the initial population of PSII centers with an oxidized plastoquinone molecule in the Q(B) niche. It was deduced that the action of TMPD was similar to CCCP, TMPD being able to compete with plastoquinone for binding at the Q(B)-site and to reduce the higher S-states of the Mn cluster.

Flash-induced oxygen evolution in photosynthesis: simple solution for the extended S-state model that includes misses, double-hits, inactivation, and backward-transitions

Flash-induced oxygen evolution in higher plants, algae, and cyanobacteria exhibits damped period-four oscillations. To explain such oscillations, Kok suggested a simple phenomenological S-state model, in which damping is due to empirical misses and double-hits. Here we developed an analytical solution for the extended Kok model that includes misses, double-hits, inactivation, and backward-transitions. The solution of the classic Kok model (with misses and double-hits only) can be obtained as a particular case of this solution. Simple equations describing the flash-number dependence of individual S-states and oxygen evolution in both cases are almost identical and, therefore, the classic Kok model does not have a significant advantage in its simplicity over the extended version considered in this article. Developed equations significantly simplify the fitting of experimental data via standard nonlinear regression analysis and make unnecessary the use of many previously developed methods for finding parameters of the model. The extended Kok model considered here can provide additional insight into the effect of dark relaxation between flashes and inactivation.

Oxygen evolution in photosynthesis: simple analytical solution for the Kok model

The light-induced oxidation of water by Photosystem II (PS II) of higher plants, algae, and cyanobacteria, is the main source of atmospheric oxygen. The discovery of the flash-induced period four oscillations in the oxygen evolution made by Pierre Joliot in 1969 has a lasting impact on current photosynthesis research. Bessel Kok explained such oscillations by introducing the cycle of flash-induced transitions of states (S-states) of an oxygen-evolving complex governed by the values of miss and double hit. Although this Kok model has been successfully used over 30 years for interpretation of experimental data in photosynthesis, until now there has been no simple analytical solution for it. Such an analytical solution for individual S-states and for oxygen evolution is presented here. When only the S(1) state is present before flash series, and when both the miss and double hit are zero, the oxygen evolved by the PSII after the n(th) flash, Y(n), is given by the following equation: 4Y(n)=1 + (-1)(n-1)-2 cos((n-1)pi/2). It is found here that binary oscillations of the secondary acceptor semiquinone at the acceptor side of the reaction center of PS II and release of reducing equivalents from reaction center to b(6)f complex can also be determined in the framework of the Kok model. The simple solutions found here for individual S-states, semiquinone, and oxygen evolution provide a basis for quantitative description of the charge accumulation processes at the donor and acceptor sides of PSII. It also provides a rare example of a significant problem in biology, which can be solved analytically.

Regulation of thermal dissipation of absorbed excitation energy and violaxanthin deepoxidation in the thylakoids of lactuca sativa. Photoprotective mechanism of a population of photosystem II centers

Non-photochemical quenching of chlorophyll a fluorescence is thought to be mainly associated with thermal dissipation of excitation energy taking place within the antenna and reaction center of PS II. In this report, non-photochemical fluorescence quenching was investigated in the fluorescence yields induced by a series of short and high-energy flashes after dark adaptation. The observation of period four fluorescence oscillations with increasing flash number indicates functioning O2 evolving centers. It was found that these PS II centers could not be identical to all the O2 evolving centers. Appreciable differences in antenna size and the number of centers were observed between the PS II centers contributing to fluorescence oscillations and the PS II centers that evolve the flash-induced steady-state O2 yield. Direct evidence for non-photochemical fluorescence quenching was provided by the numerical fitting of the fluorescence oscillations. This procedure revealed that a proportion of the centers exhibiting oscillating fluorescence yields, converted into quenching centers after each flash of a series (7% in February; 17% in June). The observed quenching could not be related to a dissipative process inside the reaction center. Instead, it was attributed to a change in the organization of some PS II centers in the membrane, possibly a conversion of PS II dimers into PS II monomers, resulting in a decreased absorption cross-section for these centers. Quenching resulting from energy de-excitation in the antenna was also observed. This was a light-initiated process, but the modification of the antenna occurred in the dark on a time scale of a few minutes. After this dark period and only on the first flash of a series, antenna quenching was revealed by a smaller absorption cross-section of the PS II centers involved in fluorescence oscillations. This process was reversed on the following flashes. The same period of darkness after illumination was necessary to allow maximum zeaxanthin formation to occur in the dark at a higher pH than the pH for optimum violaxanthin deepoxidation in the absence of preillumination. To explain this effect, comparable to that referred to as light activation for non-photochemical quenching (Ruban and Horton, Aust. J. Plant Physiol. 22 (1995) 221-230), we propose that upon preillumination (before darkness), the protons released in response to a net positive charge in these PS II centers, have access to proton binding groups acting in a cooperative way in LHC II. This accounts for the proton cooperativity as can be deduced from the pH dependence of the rate constant of violaxanthin deepoxidation (Hill coefficient n from 2 to 6). Copyright 1998 Elsevier Science B.V.

Binary oscillations in the Kok model of oxygen evolution in oxygenic photosynthesis

The flash-induced kinetics of various characteristics of Photosystem II (PS II) in the thylakoids of oxygenic plants are modulated by a period of two, due to the function of a two-electron gate in the electron acceptor side, and by a period of four, due to the changes in the state of the oxygen-evolving complex. In the absence of inhibitors of PS II, the assignment of measured signal to the oxygen-evolving complex or to quinone acceptor side has frequently been done on the basis of the periodicity of its flash-induced oscillations, i.e. four or two. However, in some circumstances, the period four oscillatory processes of the donor side of PS II can generate period two oscillations. It is shown here that in the Kok model of oxygen evolution (equal misses and equal double hits), the sum of the concentrations of the S 0 and S 2 states (as well as the sum of concentrations of S 1 and S 3 states) oscillates with period of two: S 0+S 2→S 1+S 3→S 0+S 2→S 1+S 3. Moreover, in the generalized Kok model (with specific miss factors and double hits for each S-state) there always exist such ε0, ε1, ε2, ε3 that the sum ε0[S0] + ε1[S1] + ε2[S2] + ε3[S3] oscillates with period of two as a function of flash number. Any other coefficients which are linearly connected with these coefficients, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaiqbew7aLzaaja% aaaa!3917!\[\hat \varepsilon \]i = c1εi + c2, also generate binary oscillations of this sum. Therefore, the decomposition of the flash-induced oscillations of some measured parameters into binary oscillations, depending only on the acceptor side of PS II, and quaternary oscillations, depending only on the donor side of PS II, becomes practically impossible when measured with techniques (such as fluorescence of chlorophyll a, delayed fluorescence, electrochromic shift, transmembrane electrical potential, changes of pH and others) that could not spectrally distinguish the donor and acceptor sides. This property of the Kok cycle puts limits on the simultaneous analysis of the donor and acceptor sides of the RC of PS II in vivo and suggests that binary oscillations are no longer a certain indicator of the origin of a signal in the acceptor side of PS II.

Improved 5-step modeling of the Photosystem II S-state mechanism in cyanobacteria

We present a model of the S-state mechanism, as well as an improved eigenvalue analysis, that integrate into a coherent ensemble several features found since the S-state model was initially developed. These features include the presence of S-1, deactivations in the dark interval between flashes, and the change in the number of active PS II centers by photoinhibition or photoactivation. A new feature is the capacity to predict the steady-state distribution of S-states under conditions of steady photoinhibition or photoactivation. The improved eigenvalue analysis allowed the calculation of the initial S-state distribution. In addition, the model resolved 'true' photochemical misses from apparent misses due to deactivations in the dark interval between flashes. The model suggested that most of the misses that are commonly reported are due to deactivations, and not to an intrinsic inefficiency of the photochemical mechanism of PS II. Because models that allow double-hits encompassing the S2 to S3 transition often predict negative initial quantities of S2 in cyanobacteria, our proposed model specifically prohibited them. The model accounts for inhomogeneous misses and a steady-state distribution of the type (S2)≈(S1)>(S3)≈(S0). This 5-step model uses only 4 probabilities, and is therefore easy to handle. The use of this model is critical for the analysis of several cyanobacterial strains, as well as for any species that show non-negligible deactivations in the dark interval between flashes.

Oxygen evolution by Photosystem II: The contribution of backward transitions to the anomalous behaviour of double-hits revealed by a new analysis method

Backward transitions in the analysis of oxygen production under flashing light were introduced by Packham et al., 1988, Photosynth. Res. 15: 221-232. In order to take backward transitions into account, a new method of analysis is presented: the 'eigenvalue method'. This method is based on the recurrence relation of oxygen production with four coefficients (also known as the four 'sigma' coefficients). It shows less susceptibility to round-off errors than other methods and permits the computation of double-hits directly from the coefficients, which was not possible before. With it we discovered that the inconsistent behaviour of double-hits observed previously under low flash intensities or low flash frequencies was mainly due to the inclusion of the backward transitions into the double-hit probability. In these conditions backward transitions seemed to be due either to the combination of an S-state deactivation and a miss, or to two S-state deactivations and a single-hit.In the presence of 3-(3, 4-Dichlorophenyl)-1, 1-dimethylurea (DCMU), the previous methods of 'sigma' analysis failed. In contrast, the new method resolved all four S-state probabilities; thus it has the further advantage of being more 'robust' (robustness being defined as the ability to yield a meaningful answer under difficult conditions).

High misses after odd flashes in oxygen evolution in thoroughly dark-adapted thylakoids from pea and Chenopodium album

In Photosystem II (PS II), water is oxidized to molecular oxygen and plastoquinone is reduced to plastoquinol. The oxidation of water requires the accumulation of four oxidizing equivalents, through the so-called S-states of the oxygen evolving complex; the production of plastoquinol requires the accumulation of two reducing equivalents on a bound plastoquinone, QB. It has been generally believed that during the flash-induced transition of each of the S-states (Sn → Sn+1, where n=0, 1, 2 and 3), a certain small but equal fraction of the PS II reaction centers are unable to function and, thus, 'miss' being turned over. We used thoroughly dark-adapted thylakoids from peas (Pisum sativum) and Chenopodium album (susceptible and resistant to atrazine) starting with 100% of the oxygen evolving complex in the S1 state. Thylakoids were illuminated with saturating flashes, providing a double hit parameter of about 0.07. Our experimental data on flashnumber dependent oscillations in the amount of oxygen per flash fit very well with a binary pattern of misses: 0, 0.2, 0, 0.4 during S0 → S1, S1 → S2, S2 → S3 and S3 → S0 transitions. Addition of 2 mM ferricyanide appears to shift this pattern by one flash. These results are consistent with the 'bicycle' model recently proposed by V. P. Shinkarev and C. A. Wraight (Oxygen evolution in photosynthesis: From unicycle to bicycle, 1993, Proc Natl Acad Sci USA 90: 1834-1838), where misses are due to the presence of P(+) or QA (-) among the various equilibrium states of PS II centers.

The primary structure of D1 near the QB pocket influences oxygen evolution

Photosystem II (PS II) is the site of oxygen evolution. Activation of dark adapted samples by a train of saturating flashes produces oxygen with a yield per flash which oscillates with a periodicity of four. Damping of the oxygen oscillations is accounted for by misses and double hits. The mechanisms hidden behind these parameters are not yet fully understood. The components which participate in charge transfer and storage in PS II are believed to be anchored to the heterodimer formed by the D1 and D2 proteins. The secondary plastoquinone acceptor QB binds on D1 in a loop connecting the fourth and fifth helices (the QB pocket). Several D1 mutants, mutated in the QB binding region, have been studied over the past ten years.In the present report, our results on nine D1 mutants of Synechocystis PCC 6714 and 6803 are analyzed. When oxygen evolution is modified, it can be due to a change in the electron transfer kinetics at the level of the acceptor side of PS II and also in some specific mutants to a long ranging effect on the donor side of PS II. The different properties of the mutants enable us to propose a classification in three categories. Our results can fit in a model in which misses are substantially determined by the fraction of centers which have QA (-) before each flash due to the reversibility of the electron transfer reactions. This idea is not new but was more thoroughly studied in a recent paper by Shinkarev and Wraight (1993). However, we will show in the discussion that some doubts remain as to the true origin of misses and double hits.

Flash oxygen yield patterns of autotrophically and photoheterotrophically grown Chlamydobotrys stellata in the presence and absence of lipophilic acceptors

The obligate phototrophic green alga Chlamydobotrys stellata does not evolve oxygen when grown in CO2-free atmosphere on acetate. With the application of the lipophilic acceptor 2,6-dichloro-p-benzoquinone it was investigated whether this phenomenon is caused by the inactivation of the water-splitting system or by an inhibition of the electron transport chain. It was found that in the presence of DCQ, the photoheterotrophic alga exhibited a normal period-4 flash oxygen pattern, but the steady state yield was only 25% of that measured in the autotrophic cells. After DCQ addition, the initial distribution of S-states and the values of the transition probabilities proved to be the same in the autotrophic and photoheterotrophic algae. These results indicate that photoheterotrophic growth conditions inhibit the electron transport of Chl. stellata behind the acceptor site of DCQ, but the water-splitting system remains active with a reduced oxygen evolving capacity.

Control of misses in oxygen evolution by the oxido-reduction state of plastoquinone in Dunaliella tertiolecta

The way misses happen in oxygen evolution is subject to debate (Govindjee et al. 1985). We recently observed a linear lowering of the miss probability with the flash number (Meunier and Popovic 1989). Therefore, we investigated in Dunaliella tertiolecta the link between the average miss probability and the redox state of plastoquinone after n flashes. The effect of flashes was to oxidize the plastoquinone pool; we found that the oxidation of plastoquinone highly correlated (linear regression: R (2)=0.996) with the lowering of the miss probability. The flash frequency was found to affect both the miss probability and the redox state of plastoquinone. When pre-flashes were given using a high flash frequency (10 Hz), the plastoquinone pool was oxidized and misses were low; however, if long dark intervals between flashes were used, the oxidizing effect of flashes was lost and the misses were high. We could not explain our results by assuming equal misses over all S-states; but unequal misses, caused by deactivations, were coherent with our results. We deduced that chlororespiration was responsible for the reduction of plastoquinone in the dark interval between flashes. We compared oxygen evolution with and without benzoquinone, using a low flash frequency (0.5 Hz) for maximum misses. Benzoquinone lowered the misses from 34% to 3%, and raised the amplitude of oxygen evolution by more than a factor of two (2). From this we deduced that the charge carrier "C" postulated to explain misses (Lavorel and Maison-Peteri 1983) did not account for more than 3% of miss probability in Dunaliella tertiolecta. These results indicate that the misses in oxygen evolution are controlled by the redox state of plastoquinone, through deactivations.

Investigation of double turnovers in photosystem II charge separation and oxygen evolution with excitation flashes of different duration

The characteristics of double hitting in Photosystem II charge separation and oxygen evolution in algae and chloroplasts were investigated with saturating excitation flashes of 3 microseconds, 300 ns and 5 ns duration. Two types of double hitting or advancement in S-states were found to occur in oxygen evolution: a non-photochemical type found even with 5 ns flashes and a photochemical type seen only with microsecond-long flashes, which have extensive tails. The non-photochemical type, occurring with a probability of about 3%, is sensitive to the physiological condition of the sample, and is only present in algae or chloroplast samples that have been freshly prepared. In chloroplasts incubated with ferricyanide, a 3-fold increase in double advancement of S-states is observed with xenon-flash illumination but not with 300 ns or 5 ns laser illumination. However, double turnovers in Photosystem II reaction center charge separation are large with xenon flash or 300 ns laser illumination but not with 5 ns laser illumination. This indicates that quite different kinetic processes are involved in double advancement in S-states for oxygen evolution and double turnovers in charge separation. Various models of the Photosystem II reaction center are discussed. Also, based on experiments with chloroplasts incubated with ferricyanide, an unique solution to the oxygen S-state distribution in the dark suggested by Thibault (Thibault, P. (1978) C.R. Acad. Sci. Paris 287, 725-728) can be rejected.

Light intensity saturation properties of O2 yields in a sequence of flashes in Chlorella

As a function of the light intensity of flash n in a sequence, the O2 yields Yn, Yn+1 and Yn+2 have been measured: n = 1, 2, 3 and 6 in the examples given. It is shown that: (1) No double hit exists in the first saturating flash in Chlorella. (2) the flash saturation curve of the O2 yield Yn+1 as a function of the intensity of flash n exhibits a small sigmoidal shape at weak light. (3) If Yn+1 is detected at different times after the flash n of variable intensity, a well developed lag distinguishes the saturation curve of the O2 yield measured a long time after flash n (200 ms) with respect to that measured at shorter time (300 microseconds). Nevertheless, a large amount of double hits with the transitions S1 leads to S3 cannot occur in each flash, because it would lead to a periodicity of three rather than four in the O2 yield pattern. The saturation curve of the transition S2* leads to S3 is different from the other S-state saturation curves which are close to an exponential function; even with a short flash (0.3 microseconds), this curve shows a small lag at low light intensity, and its saturation intensity is higher than that of the other transitions. The low quantum yield of the transition S2* leads to S3 at low flash light intensity is explained by a product, T, partially inhibiting the formation of S3; at higher intensity, the quantity of formed S2* being larger than that of available T, only a part of S2* is inhibited and the quantum yield is higher than at low intensity.

Inhibition by ammonium chloride of the oxygen yield of photosynthesis

Flash O2 yield experiments are described in spinach chloroplasts and Chlorella after addition of NH4Cl. (1) The damping of the sequence is increased by NH4Cl. (2) The turnover times are accelerated but the reaction during which O2 is released (S3bv leads to S4 leads to S0) is slowed. (3) We observed between the end of the turnover kinetic and the beginning of deactivation a latency time tl during which the S2 and S3 states are perfectly stable. In the presence of NH4Cl, this latency time for S2 is shortened, diminishing from 1 s to less than 12 ms; whereas it is lengthened for S3 (up to 4 s, 5 s). (4) After this latency time, two phases are clearly distinguished in the S2 deactivation: the beginning of the deactivation is abrupt, varying like the square root of the time, i.e. as is characteristic of a diffusive process. During the second phase, S2 is stabilized. Our experiment shows that S2 deactivation is monitored by the release of some product F after the latency time from one particular locus, so that nearby Photosystem II reaction centers are rapidly deactivated by diffusion of the product, whereas centers far from this locus are very slowly deactivated. These results are qualitatively complementary to the luminescence experiments of Velthuys (Biochim. Biophys, Acta (1975) 396, 392-401) except for the latency time which is invisible in the Velthuys results. We propose a modified model in which the binding of NH3 on states S2 and S3 occurs during deactivation.

Anomalies in the kinetics of photosynthetic oxygen emission in sequences of flashes revealed by matrix analysis. Effects of carbonyl cyanide m-chlorophenylhydrazone and variation in time parameters

The model of Kok et al. (Kok, B., Forbush, B. and McGloin, M. (1970) Photochem. Photobiol. 11, 457-475) is considered the best kinetic explanation of the damped oscillations of O2 evolution induced in higher plants by a sequence of brief saturating flashes. Matrix analysis applied to this model shows that the parameters involved (distribution of S states at zero time, probabilities of transition between states induced by a flash) cannot be completely known from the O2 yield sequence, Yn. However, four quantities, with limited content of information, are readily derived from data, without additional assumptions. They are sigma1, sigma2 and sigma3, three quasisymmetrical functions of the transition coefficients, and Y, a weighed average of four consecutive Yn values. The extent of misses and double hits and their variations can be qualitatively ascertained by inspection of the relative values of sigma1, sigma2and sigma3. In a regular sequence (stricly obeying Kok's model), all four quantities should be constant along the time axis. It is shown that actual sequences are seldom regular, in particular in the following conditions: (1) variable flashing frequency, (2) addition of carbonylcyanide m-chlorophenylhydrazone, (3) incomplete deactivation, (4) change of flashing frequency at steady state. In order to account for these anomalies, it is proposed to modify Kok's model by introducing, in parallel to the four state storage entity (S states), a side carrier C, which can reversibly exchange a positive charge with it. In the new model, the transition coefficients are essentially time varying, thus producing a nonregular behaviour of Yn sequences.

Photo-inactivation of system II centers by carbonyl cyanide m-chlorophenylhydrazone in Chlorella pyrenoidosa

In the presence of a high concentration of carbonyl cyanide m-chlorophenylhydrazone (CCCP) (4-10(-6) M), the S2 and S3 dark decays are accelerated and become biphasic with a first half-time of 0.6 s. The first fast phase of the decays does not correspond to a simple reduction of S2, S3 back to S0, S1 (i.e. to an acceleration of the deactivation reaction), but to a decrease in the number of oxygen-evolving System II centers. This photo-inactivation produced by CCCP is rapidly reversible in the dark.