Oxygen Evolution In Photosystem Research Articles

Exploring the interdependence of calcium and chloride activation of O2 evolution in photosystem II.

Calcium and chloride are activators of oxygen evolution in photosystem II (PSII), the light-absorbing water oxidase of higher plants, algae, and cyanobacteria. Calcium is an essential part of the catalytic Mn4CaO5 cluster that carries out water oxidation and chloride has two nearby binding sites, one of which is associated with a major water channel. The co-activation of oxygen evolution by the two ions is examined in higher plant PSII lacking the extrinsic PsbP and PsbQ subunits using a bisubstrate enzyme kinetics approach. Analysis of three different preparations at pH 6.3 indicates that the Michaelis constant, KM, for each ion is less than the dissociation constant, KS, and that the affinity of PSII for Ca2+ is about ten-fold greater than for Cl-, in agreement with previous studies. Results are consistent with a sequential binding model in which either ion can bind first and each promotes the activation by the second ion. At pH 5.5, similar results are found, except with a higher affinity for Cl- and lower affinity for Ca2+. Observation of the slow-decaying Tyr Z radical, YZ•, at 77 K and the coupled S2YZ• radical at 10 K, which are both associated with Ca2+ depletion, shows that Cl- is necessary for their observation. Given the order of electron and proton transfer events, this indicates that chloride is required to reach the S3 state preceding Ca2+ loss and possibly for stabilization of YZ• after it forms. Interdependence through hydrogen bonding is considered in the context of the water environment that intervenes between Cl- at the Cl-1 site and the Ca2+/Tyr Z region.

Functions of MnOx in NaCl Aqueous Solution for Artificial Photosynthesis.

SummaryPhotoelectrochemical water splitting has been intensively investigated as artificial photosynthesis technology to convert solar energy into chemical energy. The use of seawater and salted water has advantages for minimum environmental burden; however, the oxidation of Cl− ion to hypochlorous acid (HClO), which has toxicity and heavy corrosiveness, should occur at the anode, along with the oxygen evolution. Here, O2 and HClO production in aqueous solution containing Cl− on photoanodes modified with various metal oxides was investigated. The modification of MnOx resulted in the promotion of the O2 evolution reaction (OER) specifically without HClO production over a wide range of conditions. The results will contribute not only to the practical application of artificial photosynthesis using salted water but also to the elucidation of substantial function of manganese as the element for OER center in natural photosynthesis.

Hydroxyectoine protects Mn-depleted photosystem II against photoinhibition acting as a source of electrons.

In the present study, we have investigated the effect of hydroxyectoine (Ect-OH), a heterocyclic amino acid, on oxygen evolution in photosystem II (PS II) membrane fragments and on photoinhibition of Mn-depleted PS II (apo-WOC-PS II) preparations. The degree of photoinhibition of apo-WOC-PS II preparations was estimated by the loss of the capability of exogenous electron donor (sodium ascorbate) to restore the amplitude of light-induced changes of chlorophyll fluorescence yield (∆F). It was found that Ect-OH (i) stimulates the oxygen-evolving activity of PS II, (ii) accelerates the electron transfer from exogenous electron donors (K4[Fe(CN)6], DPC, TMPD, Fe2+, and Mn2+) to the reaction center of apo-WOC-PS II, (iii) enhances the protective effect of exogenous electron donors against donor-side photoinhibition of apo-WOC-PS II preparations. It is assumed that Ect-OH can serve as an artificial electron donor for apo-WOC-PS II, which does not directly interact with either the donor or acceptor side of the reaction center. We suggest that the protein conformation in the presence of Ect-OH, which affects the extent of hydration, becomes favorable for accepting electrons from exogenous donors. To our knowledge, this is the first study dealing with redox activity of Ect-OH towards photosynthetic pigment-protein complexes.

An enzyme kinetics study of the pH dependence of chloride activation of oxygen evolution in photosystem II.

Oxygen evolution by photosystem II (PSII) involves activation by Cl- ion, which is regulated by extrinsic subunits PsbQ and PsbP. In this study, the kinetics of chloride activation of oxygen evolution was studied in preparations of PSII depleted of the PsbQ and PsbP subunits (NaCl-washed and Na2SO4/pH 7.5-treated) over a pH range from 5.3 to 8.0. At low pH, activation by chloride was followed by inhibition at chloride concentrations >100mM, whereas at high pH activation continued as the chloride concentration increased above 100mM. Both activation and inhibition were more pronounced at lower pH, indicating that Cl- binding depended on protonation events in each case. The simplest kinetic model that could account for the complete data set included binding of Cl- at two sites, one for activation and one for inhibition, and four protonation steps. The intrinsic (pH-independent) dissociation constant for Cl- activation, K S, was found to be 0.9±0.2mM for both preparations, and three of the four pK as were determined, with the fourth falling below the pH range studied. The intrinsic inhibition constant, K I, was found to be 64±2 and 103±7mM for the NaCl-washed and Na2SO4/pH7.5-treated preparations, respectively, and is considered in terms of the conditions likely to be present in the thylakoid lumen. This enzyme kinetics analysis provides a more complete characterization of chloride and pH dependence of O2 evolution activity than has been previously presented.

Mechanism of Water Delivery to the Active Site of Photosystem II along the S(2) to S(3) Transition.

The two water molecules serving as substrate for the oxygen evolution in Photosystem II are already bound in the S2 state of the Kok-Joliot's cycle. Nevertheless, an additional water molecule is supposed to bind the cluster during the transition between the S2 and S3 states, which has been recently revealed to have the Mn4CaO5 catalytic cluster arranged in an open cubane fashion. In this Letter, by means of ab initio calculations, we investigated the possible pathways for the binding of the upcoming water molecule. Upon the four different possibilities checked in our calculations, the binding of the crystallographic water molecule, originally located nearby the Cl(-) binding site, showed the lowest activation energy barrier. Our findings therefore support the view in which the W2 hydroxyl group and the O5 oxygen act as substrates for the oxygen evolution. Within this framework the role of the open and closed Mn4CaO5 conformers is clarified as well as the exact mechanistic events occurring along the S2 to S3 transition.

Net light-induced oxygen evolution in photosystem I deletion mutants of the cyanobacterium Synechocystis sp. PCC 6803

Oxygenic photosynthesis in cyanobacteria, algae, and plants requires photosystem II (PSII) to extract electrons from H2O and depends on photosystem I (PSI) to reduce NADP+. Here we demonstrate that mixotrophically-grown mutants of the cyanobacterium Synechocystis sp. PCC 6803 that lack PSI (ΔPSI) are capable of net light-induced O2 evolution in vivo. The net light-induced O2 evolution requires glucose and can be sustained for more than 30min. Utilizing electron transport inhibitors and chlorophyll a fluorescence measurements, we show that in these mutants PSII is the source of the light-induced O2 evolution, and that the plastoquinone pool is reduced by PSII and subsequently oxidized by an unidentified electron acceptor that does not involve the plastoquinol oxidase site of the cytochrome b6f complex. Moreover, both O2 evolution and chlorophyll a fluorescence kinetics of the ΔPSI mutants are highly sensitive to KCN, indicating the involvement of a KCN-sensitive enzyme(s). Experiments using 14C-labeled bicarbonate show that the ΔPSI mutants assimilate more CO2 in the light compared to the dark. However, the rate of the light-minus-dark CO2 assimilation accounts for just over half of the net light-induced O2 evolution rate, indicating the involvement of unidentified terminal electron acceptors. Based on these results we suggest that O2 evolution in ΔPSI cells can be sustained by an alternative electron transport pathway that results in CO2 assimilation and that includes PSII, the platoquinone pool, and a KCN-sensitive enzyme.

Phylogenomic and structural modeling analyses of the PsbP superfamily reveal multiple small segment additions in the evolution of photosystem II-associated PsbP protein in green plants

PsbP is a thylakoid lumen protein involved in oxygen evolution in photosystem II (PSII) in green plants. Genomic analysis identified a number of PsbP homologs in plants, algae, and cyanobacteria. To analyze the transition of cyanobacterial PsbO/U/V complex to PsbO/P/Q complex in green plants, the evolutionary history of the PsbP superfamily was reconstructed. Phylogenetic analyses suggested that PsbP homologs be classified into eight major families (A-H), which were also characterized by specific insertion/deletion of short segments, as found by sequence alignment and homology modeling. Family A represented authentic PsbP proteins involved in oxygen evolution. The cyanobacterial PsbP and plant/algal PPL (Family H), having the simplest structure, should be considered as the root of all other families of PsbP, which subsequently gained various short, family-specific structural motifs during diversification of PsbP families. Interestingly, segments specific to Family A proteins were found arranged as a ring surrounding the modeled Arabidopsis PsbP protein. These results suggest that Family A-specific additions of short segments played a decisive role in the transition of PsbO/U/V to PsbO/P/Q complex in green plants.

Effects of CeCl 3 on Energy Transfer and Oxygen Evolution in Spinach Photosystem II

Due to 4f electron characteristics and alternation valence, cerium involved in an oxidation-reduction reaction in plant, closely relating to photosynthesis. Our studies proved that cerium could promote photosynthesis and greatly improve spinach growth. However, the mechanism of promoting energy transfer and conversion by cerium remains unclear. Here we reported that the effects of Ce 3+ on energy transfer and oxygen evolution in photosystem II (PS II) isolated from spinach, which was related to 4f electron characteristics and alternation valence in Ce molecule. The methods of absorption spectrum, fluorescence spectrum were used in the research. Results showed that Ce 3+ treatment at low concentration could suitably change PS II microenvironment, increase the absorbance of visible light, improve the energy transfer among amino acids within PS II protein-pigment complex, and accelerate energy transport from tyrosine residue to chlorophyll a. In summary, the photochemical activity of PS II (fluorescence quantum yield) and its oxygen evolving rate were enhanced by Ce 3+.

Probing the Functional Role of Ca2+ in the Oxygen-Evolving Complex of Photosystem II by Metal Ion Inhibition

Photosynthetic oxygen evolution in photosystem II (PSII) takes place in the oxygen-evolving complex (OEC) that is comprised of a tetranuclear manganese cluster (Mn4), a redox-active tyrosine residue (YZ), and Ca2+ and Cl- cofactors. The OEC is successively oxidized by the absorption of 4 quanta of light that results in the oxidation of water and the release of O2. Ca2+ is an essential cofactor in the water-oxidation reaction, as its depletion causes the loss of the oxygen-evolution activity in PSII. In recent X-ray crystal structures, Ca2+ has been revealed to be associated with the Mn4 cluster of PSII. Although several mechanisms have been proposed for the water-oxidation reaction of PSII, the role of Ca2+ in oxygen evolution remains unclear. In this study, we probe the role of Ca2+ in oxygen evolution by monitoring the S1 to S2 state transition in PSII membranes and PSII core complexes upon inhibition of oxygen evolution by Dy3+, Cu2+, and Cd2+ ions. By using a cation-exchange procedure in which Ca2+ is not removed prior to addition of the studied cations, we achieve a high degree of reversible inhibition of PSII membranes and PSII core complexes by Dy3+, Cu2+, and Cd2+ ions. EPR spectroscopy is used to quantitate the number of bound Dy3+ and Cu2+ ions per PSII center and to determine the proximity of Dy3+ to other paramagnetic centers in PSII. We observe, for the first time, the S2 state multiline electron paramagnetic resonance (EPR) signal in Dy3+- and Cd2+-inhibited PSII and conclude that the Ca2+ cofactor is not specifically required for the S1 to S2 state transition of PSII. This observation provides direct support for the proposal that Ca2+ plays a structural role in the early S-state transitions, which can be fulfilled by other cations of similar ionic radius, and that the functional role of Ca2+ to activate water in the O-O bond-forming reaction that occurs in the final step of the S state cycle can only be fulfilled by Ca2+ and Sr2+, which have similar Lewis acidities.

Effect of α- and β-cyclodextrins on the photochemical activity of thylakoid membranes and photosystem II particles from barley (Hordeum vulgare): oxygen evolution and whole chain electron transport

The effect of α- and β-cyclodextrin (CD) concentration (0–16 mM) on oxygen evolution in photosystem II (PSII) and whole chain electron transport (H2O to photosystem I (PSI)) was studied in isolated thylakoid membranes and PSII particles from barley (Hordeum vulgare L.). The CDs are cyclic oligosaccharides containing, for example, six (α-CD) or seven (β-CD) α-D-glucose residues linked by α-1,4 glycosidic bonds. These compounds alter the lipid composition of the thylakoids and most likely also the structure of their membrane proteins. We show for the first time that in the thylakoid membranes, but not in the isolated PSII particles, the relationship between oxygen evolution in PSII and the CD concentration is represented by a S-shaped (sigmoidal) curve displaying a sharp inflexion point or transition. We found, in addition, that the CDs inhibit the whole chain electron transport from H2O to methyl viologen, that is, PSI, measured as oxygen uptake, according to a nonlinear dependence that is also sigmoidal. Moreover, another interesting observation is that in the thylakoid membranes the electron transport from H2O to PSI is quite well inhibited at low CD concentrations (<4–6 mM), whereas the oxygen evolution in PSII is only substantially enhanced at CD concentrations greater than 8–10 mM. To explain this, we suggest that the mechanisms underlying the inhibition of electron transfer from H2O to PSI become operative before those giving origin to the enhancement of oxygen evolution in PSII.Key words: cyclodextrins, electron transfer, nonlinearity, oxygen evolution, photosystem, thylakoid membrane.

Increase in electron transfer activity in photosystem II of spinach thylakoids caused by conversion of phosphatidylglycerol to phosphatidic acid molecules

The techniques of oxygen electrode polarography, sodium dodecyl sulfate-polyacryamide gel electrophoresis (SDS-PAGE) and thin layer chromatography (TLC) were employed to investigate the effect of phospholipase D treatment on physiological function of spinach thylakoids. It was shown that the phospholipase D treatment on thylakoid resulted in the degradation of phosphatidylglycerol (PG) and occurrence of phosphatidic acid (PA). The changes of PG to PA molecules caused an increase in oxygen evolution in photosystem II (PSII), which was accompanied by an uncoupling effect on thylakoid membrane. It was revealed that the head-groups of PG molecules play an important role in the maintenance of the appropriate physiological activity of thylakoid membrane.

Inhibition of oxygen evolution in Photosystem II by Cu(II) ions is associated with oxidation of cytochrome b559.

We have found that elevated copper concentrations, apart from the inhibition of oxygen evolution, changed the initial states distribution of the oxygen-evolving complex. Already at low concentrations, copper ions oxidized the low-potential form of cytochrome b (559) and also its high-potential form at higher concentrations at which fluorescence quenching was observed. We suggest that the primary target sites in Photosystem II for copper is tyrosine(z), both cytochrome b (559) forms and chlorophyll(z), and that these sites are the source of the copper-induced fluorescence quenching and oxygen evolution inhibition in Photosystem II.

A quantum chemical study of tyrosyl reduction and O—O bond formation in photosystem II

Density functional theory has been used to study two of the main topics of oxygen evolution in photosystem II. The first concerns the reduction of the tyrosyl radical that occurs in every S-state transition. The two leading models, hydrogen atom transfer and electron transfer, have been investigated. In the present study, hydrogen atom transfer has the lower barrier. The second topic addressed is O—O bond formation in the S3 to S0 transition, where a Mn3 complex including calcium and chloride cofactors was used as a model. The reaction is suggested to proceed from a bridging oxyl group to a radical hydroxyl, which combines to form the O—O bond with a hydroxide ligand of the Mn complex.

18O isotope effect in the photosynthetic water splitting process

In mass spectroscopic experiments of oxygen evolution in Photosystem II at 50% enrichment of H 2 18O, one expects equal signals of 18O 2 and 16O 2 unless one of the isotopes is favored by the oxygen evolving complex (OEC). We have observed a deviation from this expectation, being a clear indication of an isotope effect. We have measured the effect to be 1.14–1.30, which is higher than the theoretically predicted value of 1.014–1.06. This together with the strong temperature variation of the measured effect with a discontinuity at 11 °C observed for wild-type tobacco and at 9 °C for a yellow-green tobacco mutant suggest that an additional mechanism is responsible for the observed high isotope effect. The entry of a finite size of water clusters to the cleavage site of the OEC can explain the observation.

Stimulation of Oxygen Evolution in Photosystem II by Copper(II) Ions

We have found that copper(II) ions at about equimolar Cu2+/photosystem II (PS II) reaction center proportions stimulate oxygen evolution nearly twofold. This high affinity Cu-binding site is different from the binding sites of Mn and Ca ions. The analysis of the Cu2+ content in PS II preparations isolated from wild-type tobacco and a tobacco mutant deficient in light-harvesting complex suggests that Cu2+ may be a native component of PS II and may take part in the oxygen evolution process. At higher concentrations, Cu2+ ions inhibit oxygen evolution and quench fluorescence.

Relationship between chlorophyll a fluorescence induction and oxygen evolution in barley (Hordeum vulgare) thylakoids treated with alpha-, beta-, and gamma-cyclodextrins

Cyclodextrins, a class of cyclic oligomeric compounds consisting of 6–8 units of D-glucose, alter the oxygen evolution of photosystem II (PSII) in barley thylakoids as well as chlorophyll a fluorescence induction, i.e., Fv/Fo and Fv/Fm where Fm is the maximal level of fluorescence when all PSII centres are closed, Fo is the minimal level of chlorophyll fluorescence when all PSII centres are open, and Fv is the variable fluorescence (= Fm – Fo). The highest Fv/Fm and Fv/Fo values are observed in samples treated with alpha- and beta-cyclodextrins, i.e., 0.618 and 0.629 (Fv/Fm) and 1.617 and 1.667 (Fv/Fo), respectively, whereas in untreated and gamma-cyclodextrin-treated thylakoids, one observes 0.608 and 0.594 (Fv/Fm) and 1.568 and 1.460 (Fv/Fo). This trend is also seen in the oxygen evolution of control and alpha-, beta-, and gamma-cyclodextrin-treated thylakoids, i.e., 183.6, 214.9, 301.7, and 174.0 µmol O2·mg chlorophyll–1·h–1, respectively. First, the fluorescence induction data indicate that in intact thylakoid membranes, the enhancement of oxygen evolution induced by alpha- and beta-cyclodextrins originates in the opening of blocked photochemical centres in PSII. Second, the results show that the correlation between the oxygen evolution of PSII and chlorophyll a fluorescence induction is a nonlinear phenomenon represented by a Boltzman expression.Key words: chlorophyll, cyclodextrins, fluorescence induction, oxygen evolution, photosystem II, thylakoid membrane.

Thermal stability of oxygen evolution in photosystem ? core complex in the presence of digalactosyl diacylglycerol

The influence of digalactosyldiacylglycerol (DGDG), one of the photosynthetic membrane lipids, on heat inactivation of the process of oxygen evolution has been studied in vitro in photosystem II (PS II) core complex. It was found that the temperature of semi-inactivation of oxygen evolution in the complex increased from 40.0 to about 43.0°C in the presence of DGDG with 5-min heat treatment in the dark. Furthermore, when PS II core complex was incubated for 5 min at 45.0°C, the oxygen evolution in the complex was completely lost, whilst the DGDG-complexed PS II core complex still retained a 16% of activity (100% for 25.0°C). In addition, a 1-h incubation at 38.0°C inactivated absolutely the oxygen evolution for the PS II core complex. By contrast, there remained about 20% of activity (zero time for 100%) for the complex in the presence of DGDG under the same condition. These results indicate a new role of DGDG in the protection of PS II core complex against the deleterious effects of temperature. It was most likely that DGDG-mediated stability toward thermal denaturation of oxygen evolution in PS II core complex is due to the protective effect of DGDG on the release of the 33 kD protein from PS II core complex.

Digalactosyl diacylglycerol-mediated Stability toward Thermal Denaturation of Oxygen Evolution in Photosystem II Core Complex

The influence of digalactosyldiacylglycerol (DGDG), one of the photosynthetic membrane lipids, on heat inactivation of the process of oxygen evolution has been studied in vitro in photosystem II (PSII) core complex. It was found that the temperature of semi-inactivation of oxygen evolution in the complex increased from 40 to about 43 degrees C in the presence of DGDG. Furthermore, when incubated for 5 min at 45° C, the oxygen evolution in PSII core complex was completely lost, whilst DGDG-complexed PSII core complex still retained a 16% of activity (100% for 25° C). In addition, a one-hour incubation at 38° C inactivated absolutely the oxygen evolution for the PSII core complex. By contrast, there remained about 20% of activity (zero time for 100%) for the complex in the presence of DGDG on the same condition. These results might indicate the role of DGDG in the protection of PSII core complex against the deleterious effects of temperature. It was suggested that the DGDG-mediated stability toward thermal denaturation of oxygen evolution in PSII core complex is due to the redistribution of secondary bonds in PSII core complex proteins through interactions with the polar and/or fatty acid parts of DGDG, which results in DGDG-induced restraint on the inhibitory effect of heat.

Functional and Structural Cooperativity in Photosystem II

Objectives and Methods: To investigate the dependence of oxygen evolution in photosystem II (PSII) on the lipid content of the thylakoid membrane, experiments were performed since 1990 to determine the effect of the lipid/PSII molar ratio on the function of the complexes prepared with PSII particles incorporated into phosphatidylglycerol (PG) and digalactosyldiacylglycerol (DGDG) vesicles, i.e., PSII-PG and PSII-DGDG. Results: The major finding is the observation of the oxygen evolution (OE) enhancement in the PSII-lipid vesicles with increasing lipid/PSII molar ratios up to OE maxima which are seen in both PSII-PG and PSII-DGDG; then, at higher lipid/PSII molar ratios the OE decreases to values as low as 20 % of the activity in control (delipidated) PSII particles. Conclusions: To explain the results, a mechanism of lipid-mediated cooperative function of PSII units (CF model) was postulated by M Fragata, K Strzalka and EK Nenonene [J. Photochem. Photobiol. B: Biol. 11 (1991) 329-342]. According to the CF model, the OE enhancement detected in the PSII-lipid vesicles has its origin in the formation of PSII aggregates of a yet undetermined size at the optimum lipid/PSII molar ratio. Recently, it became clear that the CF model has its structural counterpart in the PSII units arranged as dimers which were identified in the last few years (1996-2001). The data indicate that the `functional and structural cooperativity in PSII? is a mechanism which is instrumental not only in stimulating OE in the thylakoids but as well in the regulation of PSII function.

Interaction of photosystem II proteins with non-aggregated membranes constituted of phosphatidylglycerol and the electrically neutral phosphatidylcholine enhances the oxygen-evolving activity

This work investigates the role of the relative content of phosphatidylglycerol (PG) and the electrically neutral phosphatidylcholine (PC) on the oxygen evolution (OE) in the photosystem II (PSII) complex in the pH range 4.5–9.5. For this purpose, PSII particles were incorporated into non-aggregated membranes constituted of PC (PSII–PC) or PG (PSII–PG), and mixtures of PC and PG (PSII–PCPG). The major results are: (i) In solutions at pH 6.5, the PSII–PCPG membranes having a low PC content, e.g. 0.3 mol PC/mol PG, display only a slight OE augmentation in relation to the oxygen evolution observed in PSII particles in the absence of lipids. This contrasts with the OE activity observed in PSII–PCPG vesicles containing a high PC concentration, e.g. 1 or 3 mol PC/mol PG, where the oxygen evolution increases to 132 or 143% of the OE rate observed in PSII in the absence of lipids. Moreover, these OE levels are of the order of those observed previously in membranes prepared with the nonionic digalactosyldiacylglycerol (PSII–DGDG) [Fragata, M., Menikh, A., Nénonéné, E.K., 1994. Functional and structural aspects of the thylakoid lipids in oxygen evolution in photosystem II. Trends Photochem. Photobiol. 3, 201–210.]; (ii) In solutions at pH<6.5 or pH>6.5, the PSII–PCPG membranes containing a low PC content, e.g. 0.3 mol PC/mol PG, display systematically a significant decrease of oxygen evolution to levels below those observed in PSII–PG. The cooperative effect of PC and PG on the enhancement of oxygen evolution is only apparent in PSII–PCPG vesicles prepared with a high PC concentration, e.g. 1 or 3 mol PC/mol PG. In addition, the loss of oxygen-evolving activity observed in PSII particles in the absence of lipids is reversed in PSII–PCPG membranes having a high PC content. That is, the OE activity increases from 18% at pH 4.5 or 13% at pH 9.5, to about 60% at either pH 4.5 or 9.5. An important outcome in this study is that in mixed-lipid membranes constituted of phosphatidylglycerol and the electrically neutral phosphatidylcholine, the hydrophobic effect or the formation of hydrogen bonds have a more preponderant role than the electrostatic interactions in the enhancement of the oxygen evolution in the PSII complex.