The effect of pH on the midpoint oxidation-reduction potentials of components associated with plant photosystem II

A possible relationship between the effect of factors on photoactivation of photosystem II depleted of functional Mn and cytochrome b559

The photoassembly of the Mn4CaO5 cluster in Mn-depleted photosystem II preparations (photoactivation) was studied under the influence of oxidants, reductants and pH. New data on the effect of these factors on the photoactivation yield are presented. The presence of the oxidant, ferricyanide, negatively affected the photoactivation yield over the entire concentration range studied (0-1 mM). In contrast to ferricyanide, the addition of the reductant, ferrocyanide, up to 1 mM resulted in an increase in the photoactivation yield. Other reductants either did not significantly affect (diphenylcarbazide) or suppressed (ascorbate) the photoactivation yield. The effect of ferrocyanide on photoactivation were found to be similar dichlorophenolindophenol. Investigation of the photoactivation yield as a function of pH revealed that the maximum yield was observed at pH 6.5 in the presence of ferrocyanide and DCPIP, and at pH 5.5 without additives. In addition, the photoactivation yield at pH 5.5 was the same without and with the addition of ferrocyanide or dichlorophenolindophenol. Although ferricyanide suppressed the photoactivation, the photoactivation yield increased in the presence of ferricyanide by shifting the pH to the acidic region. The samples contained approximately 25 % of the HP cyt b559, which was in the reduced state, as the absorbance at 559 nm was decreased upon addition of ferricyanide and subsequent addition of ferrocyanide returned the spectrum to the baseline. A possible relationship between the effect of factors on the photoactivation and the involvement of cyt b559 in the protection of PSII from oxidative damage on the donor side is discussed.

Modeling the Characteristic Residues of Chlorophyll f Synthase (ChlF) from Halomicronema hongdechloris to Determine Its Reaction Mechanism

Photosystem II (PSII) is a quinone-utilizing photosynthetic system that converts light energy into chemical energy and catalyzes water splitting. PsbA (D1) and PsbD (D2) are the core subunits of the reaction center that provide most of the ligands to redox-active cofactors and exhibit photooxidoreductase activities that convert quinone and water into quinol and dioxygen. The performed analysis explored the putative uncoupled electron transfer pathways surrounding P680+ induced by far-red light (FRL) based on photosystem II (PSII) complexes containing substituted D1 subunits in Halomicronema hongdechloris. Chlorophyll f-synthase (ChlF) is a D1 protein paralog. Modeling PSII-ChlF complexes determined several key protein motifs of ChlF. The PSII complexes included a dysfunctional Mn4CaO5 cluster where ChlF replaced the D1 protein. We propose the mechanism of chlorophyll f synthesis from chlorophyll a via free radical chemistry in an oxygenated environment created by over-excited pheophytin a and an inactive water splitting reaction owing to an uncoupled Mn4CaO5 cluster in PSII-ChlF complexes. The role of ChlF in the formation of an inactive PSII reaction center is under debate, and putative mechanisms of chlorophyll f biosynthesis are discussed.

Computer modeling of electron and proton transport in chloroplasts

Photosynthesis is one of the most important biological processes in biosphere, which provides production of organic substances from atmospheric CO2 and water at expense of solar energy. In this review, we contemplate computer models of oxygenic photosynthesis in the context of feedback regulation of photosynthetic electron transport in chloroplasts, the energy-transducing organelles of the plant cell. We start with a brief overview of electron and proton transport processes in chloroplasts coupled to ATP synthesis and consider basic regulatory mechanisms of oxygenic photosynthesis. General approaches to computer simulation of photosynthetic processes are considered, including the random walk models of plastoquinone diffusion in thylakoid membranes and deterministic approach to modeling electron transport in chloroplasts based on the mass action law. Then we focus on a kinetic model of oxygenic photosynthesis that includes key stages of the linear electron transport, alternative pathways of electron transfer around photosystem I (PSI), transmembrane proton transport and ATP synthesis in chloroplasts. This model includes different regulatory processes: pH-dependent control of the intersystem electron transport, down-regulation of photosystem II (PSII) activity (non-photochemical quenching), the light-induced activation of the Bassham-Benson-Calvin (BBC) cycle. The model correctly describes pH-dependent feedback control of electron transport in chloroplasts and adequately reproduces a variety of experimental data on induction events observed under different experimental conditions in intact chloroplasts (variations of CO2 and O2 concentrations in atmosphere), including a complex kinetics of P700 (primary electron donor in PSI) photooxidation, CO2 consumption in the BBC cycle, and photorespiration. Finally, we describe diffusion-controlled photosynthetic processes in chloroplasts within the framework of the model that takes into account complex architecture of chloroplasts and lateral heterogeneity of lamellar system of thylakoids. The lateral profiles of pH in the thylakoid lumen and in the narrow gap between grana thylakoids have been calculated under different metabolic conditions. Analyzing topological aspects of diffusion-controlled stages of electron and proton transport in chloroplasts, we conclude that along with the NPQ mechanism of attenuation of PSII activity and deceleration of PQH2 oxidation by the cytochrome b6f complex caused by the lumen acidification, the intersystem electron transport may be down-regulated due to the light-induced alkalization of the narrow partition between adjacent thylakoids of grana. The computer models of electron and proton transport described in this article may be integrated as appropriate modules into a comprehensive model of oxygenic photosynthesis.

Protein film voltammetry and co-factor electron transfer dynamics in spinach photosystem II core complex

Direct protein film voltammetry (PFV) was used to investigate the redox properties of the photosystem II (PSII) core complex from spinach. The complex was isolated using an improved protocol not used previously for PFV. The PSII core complex had high oxygen-evolving capacity and was incorporated into thin lipid and polyion films. Three well-defined reversible pairs of reduction and oxidation voltammetry peaks were observed at 4 °C in the dark. Results were similar in both types of films, indicating that the environment of the PSII-bound cofactors was not influenced by film type. Based on comparison with various control samples including Mn-depleted PSII, peaks were assigned to chlorophyll a (Chl a) (Em = -0.47 V, all vs. NHE, at pH 6), quinones (-0.12 V), and the manganese (Mn) cluster (Em = 0.18 V). PFV of purified iron heme protein cytochrome b-559 (Cyt b-559), a component of PSII, gave a partly reversible peak pair at 0.004 V that did not have a potential similar to any peaks observed from the intact PSII core complex. The closest peak in PSII to 0.004 V is the 0.18 V peak that was found to be associated with a two-electron process, and thus is inconsistent with iron heme protein voltammetry. The -0.47 V peak had a peak potential and peak potential-pH dependence similar to that found for purified Chl a incorporated into DMPC films. The midpoint potentials reported here may differ to various extents from previously reported redox titration data due to the influence of electrode double-layer effects. Heterogeneous electron transfer (hET) rate constants were estimated by theoretical fitting and digital simulations for the -0.47 and 0.18 V peaks. Data for the Chl a peaks were best fit to a one-electron model, while the peak assigned to the Mn cluster was best fit by a two-electron/one-proton model.

Factors determining the special redox properties of photosynthetic cytochrome b559

Factors controlling the redox properties of the two conventional forms of cytochrome b559, i.e. the unstable high-potential form and the stable low-potential form, have been further investigated using PSII-enriched membranes from pea and spinach chloroplasts. The redox potential of the stable form of cytochrome b559 is pH independent both above pH 7.5 (E'm approximately +110 mV) and below pH 6.0 (E'm approximately +203 mV), but it changes with a slope of 58 mV per pH unit between these two pH values. Thus, cytochrome b559 seems to have a single ionizing group influencing its redox potential, with a higher affinity for protons in the reduced form (pK(red) = 7.5) and a lower affinity in the oxidized form (pK(ox) = 6.0); consequently, one unprotonated low-potential form (LP) and one protonated intermediate-potential form (IP). The redox potential of the high-potential form (HP) is pH-independent between pH 5.0 and 8.0, but its relative content (compared to the total amount of protein) decreases progressively above pH 7.0. This conversion to the stable LP form is interpreted as corresponding to the loss of a proton by one ionizing group, the protonation of which is essential for maintaining the unstable HP state. According to chemical modification experiments with diethylpyrocarbonate, one of the two histidine ligands of the heme seems to be the ionizing group responsible for the existence of both the protonated IP and HP forms. It is proposed that the difference between the IP and HP forms is due to the formation of an additional hydrogen bond between the protonated histidine and the protein in the HP state that stabilizes a special hydrophobic heme environment responsible for its high redox potential.

Characterization of chlorophyll triplet promoting states in photosystem II sequentially induced during photoinhibition

It has recently been demonstrated that strong illumination under anaerobic conditions leads to the double reduction of the primary quinone acceptor, QA, which in turn promotes the light-induced formation of triplet reaction center chlorophyll, 3P680, a potentially dangerous species to its protein surroundings in the presence of oxygen [Vass, I., Styring, S., Hundal, T., Koivuniemi, A., Aro, E.-M., & Anderson, B. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1408-1412]. Here we have studied in further detail the formation of 3P680 producing centers in anaerobically photoinhibited photosystem II membranes by using low-temperature EPR spectroscopy. The results show that 3P680 formation occurs in three different populations of modified photosystem II centers. After a short period of photoinhibitory illumination, a very stable form of singly reduced QA is observed, with a decay halftime of several minutes at room temperature, and our results indicate that already this population of centers promotes the light-induced formation of the spin-polarized EPR signal from 3P680. The formation of these centers is enhanced below pH 6.0, indicating the involvement of a protonation event in neutralizing the negative charge on QA-, a prerequisite for efficient primary charge separation and subsequent triplet formation via the radical pair mechanism. If these centers are incubated in the dark, the stable singly reduced QA species is slowly reoxidized concomitant with the loss of its triplet forming ability. Extended photoinhibitory illumination converts the stable form of singly reduced QA to an EPR-silent species indicating the second reduction of QA-. The second negative charge on the double-reduced QA is neutralized most likely by a second protonation.(ABSTRACT TRUNCATED AT 250 WORDS)

Redox and acid-base characterization of cytochrome b-559 in photosystem II particles

The redox and acid/base states and midpoint potentials of cytochrome b-559 have been determined in oxygen-evolving photosystem II (PS II) particles at room temperature in the pH range from 6.5 to 8.5. At pH 7.5 the fresh PS II particles present about 2/3 of their cytochrome b-559 in its reduced and protonated (non-auto-oxidizable) high-potential form and about 1/3 in its oxidized and non-protonated low-potential form. Potentiometric reductive titration shows that the protonated high-potential couple is pH-independent (E'0, + 380 mV), whereas the low-potential couple is non-protonated and pH-independent above pH 7.6 (E'0, pH greater than 7.6, + 140 mV), but becomes pH-dependent below this pH, with a slope of -72 mV/pH unit. Moreover, evidence is presented that in PS II particles cytochrome b-559 can cycle, according to its established redox and acid/base properties, as an energy transducer at two alternate midpoint potentials and at two alternate pKa values. Red light absorbed by PS II induces reduction of cytochrome b-559 in these particles at room temperature, the reaction being completely blocked by dichlorophenyldimethylurea.

Heterogeneity in chloroplast photosystem II

Photosystem-two (PSII) in the chloroplasts of higher plants and green algae is not homogeneous. A review of PSII heterogeneity is presented and a model is proposed which is consistent with much of the data presented in the literature. It is proposed that the non-quinone electron acceptor of PSII is preferentially associated with the sub-population of PSII known as PSIIß.

pH dependent stabilization of S2Q A (-) and S 2Q B (-) charge pairs studied by thermoluminescence

The pH dependence of emission peak temperature and decay time of thermoluminescence arising from S2QB (-) and S2QA (-) recombinations demonstrates that a stabilization of S2QB (-) occurs at low pH whereas stabilization of S2QA (-) occurs at high pH. Based on comparative analysis of thermoluminescence parameters of the two types of recombination, we suggest that in the pH range between 5.3 and 7.5, Em(S2/S1) and Em(QA/QA (-)) are constant, but Em(QB/QB (-)) gradually increases with decreasing pH, while in the pH range between 7.5 and 8.5, an unusual change occurs on S2QA (-) charge pair, which is interpreted as either a decrease in Em(S2/S1) or an increase in Em(QA/QA (-)).

Characterization of photosystem ii electron acceptors in Phormidium laminosum

Chlorophyll a fluorescence has been used to monitor the redox state of the primary electron acceptor of photosystem II (PS II) in the blue-green alga Phormidium laminosum during equilibrium titrations. The shape of induction curves measured in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) have been analyzed. The induction curves were very similar in unfractionated thylakoid membranes and PS II particles. In both, the fast (alpha) phase was sigmoidal, and was followed by a slow (beta) exponential tail. Thus, the structural organization and complexity of the particles (J. M. Bowes and P. Horton, Biochim, Biophys. Acta 680, 127-133 (1982), as indicated by the occurrence of energy transfer between alpha centers and presence of beta centers, must preexist in the membranes. Redox titration of the initial level of fluorescence indicated the presence of a single quencher QH in the unfractionated thylakoids, midpoint potential: Em7.0 approximately -35 mV (n = 1). Thus, the occurrence of a single acceptor is characteristic for P. laminosum and the absence of a low potential acceptor in PS II particles (J.M. Bowes, P. Horton, and D.S. Bendall, FEBS Lett. 135, 261-264 (1981] was not the result of its removal during their preparation. The midpoint potential of Q varied by -60 mV/pH unit in PS II particles and membrane fragments, with a pK at pH greater than 8.5 (particles) and at pH 7.5 (fragments). In PS II particles, DCMU raised the pK by approximately 0.5 pH units. It is argued that the pH dependence of Q is conferred by protonation of a protein which accompanies its reduction rather than protonation of the semiquinone Q X itself.

Evolution of photosynthetic reaction centers

The common ancestor of all photosynthetic prokaryotes and organelles contained chlorophyll (Chl) a. All green and purple photosynthetic bacteria descended from a common bacteriochlorophyll (Bchl) a-containing ancestor which diverged from the Chl a line. Separate PS-I and PS-II reaction centers may have evolved before the appearance of Bchl a. When the transition to Bchl a occurred, the resultant organism contained two types of reaction center, "PS-I" and "PS-II." One line of development eliminated "PS-II" and evolved into the green bacteria. The other line eliminated "PS-I" and became the purple bacteria. In the Chl a-containing organisms the evolution of PS-II continued until oxygen evolution was achieved.

Observation of two quenchers of chlorophyll fluorescence in chloroplasts at -196 degrees C

Fluorescence induction at -196 degrees C has been monitored in chloroplasts rapidly frozen after poising at different redox potentials at room temperature. It was found that, as at room temperature, the initial level of fluorescence observed upon shutter opening (Fo), relative to the final level observed after 10 seconds of illumination (Fm) increased as the redox potential of the chloroplasts was lowered. Redox titration and revealed the presence of two quenching components with Em, 7.8 at -70 mV and -275 mV accounting for approx. 75% and 25% of the variable fluorescence (Fv). Parallel observation of fluorescence yield at room temperature similarly gave two components, with Em, 7.8 at -95 mV and -290 mV, also accounting for appox. 75% and 25%. Simultaneous measurement of fluorescence emission at -196 degrees C at 695 nm and 735 nm indicated that both emissions are quenched by the same redox components.

Binary oscillations in the rate of reoxidation of the primary acceptor of photosystem II

The decay of fluorescence yield following each of a series of saturating laser flashes has been used to monitor the kinetics of reoxidation of the primary acceptor of Photosystem II under conditions of varied redox potential. 1. In dark-adapted chloroplasts, a damped binary oscillation as a function of flash number was observed in the kinetics of the decay of the fluorescence yield. The decay was faster on odd than on even-numbered flashes. 2. In the presence of low concentrations of 1,4-benzoquinone, the oscillation was more marked, and over the range approx 200--350 mV, independent of redox potential. The decay following flash 1 under these conditions had a half-time of approx. 200--400 microseconds. The decay following flash 2 was decelerated; the initial rate was up to 10-fold slower than after flash 1. 3. We suggest that the kinetics following a single flash reflect the rate of the reaction Q-B leads to QB-, and following the second flash, Q-B- leads to QB2-. Benzoquinone at low concentrations oxidises a residual fraction of B- which is usually reduced in the dark before the flash sequence. 4. A faster component in the decay (t u/2 approximately 140 microseconds) following the first flash titrated in over the range Eh greater than 350 mV. The binary oscillation was still apparent but delayed by one flash. 5. We discuss the relative redox potentials of the couples B/B- and B-/B2-, and the role of the component which titrates in at Eh greater than 350 mV.

Redox titration of electron acceptor Q and the plastoquinone pool in photosystem II

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 approximately 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.

Studies on the proton transport at system II in trypsin-treated spinach chloroplasts

The proton transport coupled with the DCMU-insensitive oxygen evolution mediated by K3[Fe(CN)6] in trypsin-treated chloroplasts (Renger, G. (1976) FEBS Lett. 69, 225--230) has been investigated with the aid of the pH indicator bromcresol purple. It was found that (1) the proton uptake from the outer aqueous phase observed in normal chloroplasts is completely suppressed by mild trypsin treatment; (2) a rather slow proton release into the external phase is detected which is insensitive to DCMU; (3) in the presence of DCMU, the extent of the proton release depends on the incubation time with trypsin in a similar manner as the average oxygen yield per flash. The results are interpreted by the assumption, that: (i) the reduced primary electron acceptor of System II, X 320-, does not become protonated, and (ii) the external acidification is caused by a passive efflux of protons, which are released by the watersplitting enzyme system Y into the inner phase of the thylakoids. The pK value of X 320- in trypsinated chloroplasts is estimated to be below 4.5. A possible pK shift caused by a modification of the proteinaceous barrier, which earlier (Renger, G. (1976) Biochim. Biophys. Acta 440, 287--300) was postulated to cover up the primary electron acceptor X 320, is discussed. Furthermore, the watersplitting enzyme system Y is inferred to be sensitive to deletereous attack from the outer aqueous phase mainly by secondary structural effects. Trypsination does not change the direction of the proton release in system Y.

Characterization of two quenchers of chlorophyll fluorescence with different midpoint oxidation-reduction potentials in chloroplasts

The properties of two redox quenchers of chlorophyll fluorescence in chloroplasts at room temperature have been investigated. (1) Redox titration of the fluorescence yield reveals two n = 1 components with Em7.8 at--45 and --247 mV, accounting for approx. 70 and 30% of the total yield, respectively. (2) Neutral red, a redox mediator often used at redox potentials below --300 mV, preferentially quenches the fluorescence controlled by the --247 mV component. Titrations using neutral red artifactually create an n = 2 quenching component with Em7.8 = --375 mV. (3) Analysis of fluorescence induction curves recorded at different redox potentials indicates that both the --45 and --247 mV components can be photochemically reduced. The reduction of the --247 mV component corresponds to a fast phase of the induction curve whilst the slower reduction of the 45 mV component accounts for the tail phase. (4) The excitation spectra for the fluorescence controlled by the two quenchers show small differences in the ratio of chlorophyll a and b. (5) Whereas the --247 mV component readily shows a 60 mV per pH unit dependency on solution pH, the ability of the --45 mV component to respond to pH change is restricted. (6) Triton Photosystem II particles contain both quenchers but the --247 mV component accounts for approx. 70% of the fluorescence and the high component has an Em7.8 of +48 mV. The relative merits of sequential and parallel models in explaining the presence of the two quenchers are considered.

The role of plastoquinone and beta-carotene in the primary reaction of plant photosystem II

Extraction of Triton Photosystem II chloroplast fragments with 0.2% methanol in hexane for 3 h results in the removal of 90 to 95% of the plastoquinone in the original preparation. The extracted fragments (chlorophyll:plastoquinone ratio, 900: 1) showed no P-680 photooxidation at 15 K after a single laser flash. The extracted fragments also showed no light-induced C-550 absorbance change at 77 K. Reconstitution of the primary reaction of Photosystem II, as evidenced by restoration of low-temperature photooxidation of P-680, could be obtained by the addition of plastoquinone A but not by the addition of beta-carotene. The addition of beta-carotene plus plastoquinone A restored the C-550 absorbance change. These results indicate that plastoquinone functions as the primary electron acceptor of Photosystem II and that beta-carotene does not play a direct role in the primary photochemistry but is required for the C-550 absorbance change.

Studies on the structural and functional organization of system II of photosynthesis. The use of trypsin as a structurally selective inhibitor at the outer surface of the thylakoid membrane

The effect of trypsin on the photosynthetic electron transport has been investigated in the presence of various electron acceptors (benzyl viologen, p-benzo-quinone, K3[Fe(CN)6]) by measurements of flash-induced oxygen evolution and of the absorption changes at 334 nm, indicating the primary electron acceptor of System II, X 320, and at 515 nm, indicating via electrochromism the electrical potential gradient across the thylakoid membrane. It was found that the effect of trypsin is strongly dependent on the nature of the electron acceptor: (1) Oxygen evolution is completely inhibited in the presence of p-benzo-quinone, but remains nearly unaffected by K3[Fe(CN)]6. (2) The initial amplitude deltaAO of the 334 nm absorption change is insensitive to trypsin in the presence of K3[Fe(CN)6], but the absorption change is abolished if benzyl viologen is used as acceptor. (3) The initial amplitude deltaAO of the 515 nm absorption change decreases by trypsin down to 50% with K3[Fe(CN)6] and is completely suppressed with benzyl viologen. (4) In trypsinated chloroplasts, the above-mentioned activities appear to be rather insensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea, in contrast to normal chloroplasts. On the basis of these results it is inferred that the primary electron acceptor of System II, X 320, is covered by a proteinaceous component susceptible to tryptic digestion. In addition, it is postulated that this component acts as well as an allosteric protein responsible for the regulation of the electronic interaction between X 320 and the plastoquinone pool, as for the inhibitory effect of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Various other possible effects caused by the proteinaceous shield and its modification by trypsin are discussed. The present results are in complete agreement with asymmetric membrane models postulating a zig-zag arrangement of the electron transport chain with the reducing side located towards the outer phase and the oxidizing side near the inner phase of the thylakoids.