Photosynthetic water oxidation to molecular oxygen: apparatus and mechanism

PS II model-based simulations of single turnover flash-induced transients of fluorescence yield monitored within the time domain of 100 ns-10 s on dark-adapted Chlorella pyrenoidosa cells

The set up described in Steffen et al. (Biochemistry 40:173-180, 2001) was used to monitor in the time domain from 100 ns to 10 s single turnover flash-induced transients of the normalized fluorescence yield (SFITFY) on dark-adapted cells of the thermophilic algae Chlorella pyrenoidosa Chick. Perfect data fit was achieved within the framework of a previously proposed model for the PS II reaction pattern (Lebedeva et al., Biophysics 47:968-980, 2002; Belyaeva et al., Biophysics 51:860-872, 2006) after its modification by taking into account nonradiative decay processes including nonphotochemical quenching due to time-dependent populations of P680(+*) and (3)Car. On the basis of data reported in the literature, a consistent set of rate constants was obtained for electron transfer at the donor and acceptor sides of PS II, pH in lumen and stroma, the initial redox state of plastoquinone pool and the rate of plastoquinone oxidation. The evaluation of the rate constant values of dissipative processes due to quenching by carotenoid triplets in antennae and P680(+*)Q(A)(-*) recombination as well as the initial state populations after excitation with a single laser flash are close to that outlined in (Steffen et al., Biochemistry 44:3123-3133, 2005a). The simulations based on the model of the PS II reaction pattern provide information on the time courses of population probabilities of different PS II states. We analyzed the maximum (F(m)(STF)) and minimum (F(0)) of the normalized FL yield dependence on the rate of the recombination processes (radiative and dissipative nonradiative) and of P680(+*) reduction. The developed PS II model provides a basis for theoretical comparative analyses of time-dependent fluorescence signals, observed at different photosynthetic samples under various conditions (e.g. presence of herbicides, other stress conditions, excitation with actinic pulses of different intensity, and duration).

Photosystem II: The machinery of photosynthetic water splitting

This review summarizes our current state of knowledge on the structural organization and functional pattern of photosynthetic water splitting in the multimeric Photosystem II (PS II) complex, which acts as a light-driven water: plastoquinone-oxidoreductase. The overall process comprises three types of reaction sequences: (1) photon absorption and excited singlet state trapping by charge separation leading to the ion radical pair [Formula: see text] formation, (2) oxidative water splitting into four protons and molecular dioxygen at the water oxidizing complex (WOC) with P680+* as driving force and tyrosine Y(Z) as intermediary redox carrier, and (3) reduction of plastoquinone to plastoquinol at the special Q(B) binding site with Q(A)-* acting as reductant. Based on recent progress in structure analysis and using new theoretical approaches the mechanism of reaction sequence (1) is discussed with special emphasis on the excited energy transfer pathways and the sequence of charge transfer steps: [Formula: see text] where (1)(RC-PC)* denotes the excited singlet state (1)P680* of the reaction centre pigment complex. The structure of the catalytic Mn(4)O(X)Ca cluster of the WOC and the four step reaction sequence leading to oxidative water splitting are described and problems arising for the electronic configuration, in particular for the nature of redox state S(3), are discussed. The unravelling of the mode of O-O bond formation is of key relevance for understanding the mechanism of the process. This problem is not yet solved. A multistate model is proposed for S(3) and the functional role of proton shifts and hydrogen bond network(s) is emphasized. Analogously, the structure of the Q(B) site for PQ reduction to PQH(2) and the energetic and kinetics of the two step redox reaction sequence are described. Furthermore, the relevance of the protein dynamics and the role of water molecules for its flexibility are briefly outlined. We end this review by presenting future perspectives on the water oxidation process.

Computational insights into the O2-evolving complex of photosystem II

Mechanistic investigations of the water-splitting reaction of the oxygen-evolving complex (OEC) of photosystem II (PSII) are fundamentally informed by structural studies. Many physical techniques have provided important insights into the OEC structure and function, including X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy as well as mass spectrometry (MS), electron paramagnetic resonance (EPR) spectroscopy, and Fourier transform infrared spectroscopy applied in conjunction with mutagenesis studies. However, experimental studies have yet to yield consensus as to the exact configuration of the catalytic metal cluster and its ligation scheme. Computational modeling studies, including density functional (DFT) theory combined with quantum mechanics/molecular mechanics (QM/MM) hybrid methods for explicitly including the influence of the surrounding protein, have proposed chemically satisfactory models of the fully ligated OEC within PSII that are maximally consistent with experimental results. The inorganic core of these models is similar to the crystallographic model upon which they were based, but comprises important modifications due to structural refinement, hydration, and proteinaceous ligation which improve agreement with a wide range of experimental data. The computational models are useful for rationalizing spectroscopic and crystallographic results and for building a complete structure-based mechanism of water-splitting in PSII as described by the intermediate oxidation states of the OEC. This review summarizes these recent advances in QM/MM modeling of PSII within the context of recent experimental studies.

The effect of hydration on protein flexibility in photosystem II of green plants studied by quasielastic neutron scattering

The effect of hydration on protein dynamics in photosystem II (PS II) membrane fragments from spinach has been investigated by using the method of quasielastic neutron scattering (QENS) at room temperature. The QENS data obtained indicate that the protein dynamics is strongly dependent on the extent of hydration. In particular, the hydration-induced activation of localized diffusive protein motions and QA- reoxidation by QB in PS II appear to be correlated in their onset at a hydration value of about 45% relative humidity (r.h.). These findings underline the crucial functional relevance of localized diffusive protein motions on the picosecond-timescale for the reactions of light-induced photosynthetic water splitting under formation of plastoquinol and molecular oxygen in PS II of green plants.

Where water is oxidized to dioxygen: structure of the photosynthetic Mn4Ca cluster from X-ray spectroscopy

Light-driven oxidation of water to dioxygen in plants, algae, and cyanobacteria is catalyzed within photosystem II (PS II) by a Mn 4Ca cluster. Although the cluster has been studied by many different methods, its structure and mechanism have remained elusive. X-ray absorption and emission spectroscopy and extended X-ray absorption fine structure studies have been particularly useful in probing the electronic and geometric structures and the mechanism of the water oxidation reaction. Recent progress, reviewed here, includes polarized X-ray absorption spectroscopy measurements of PS II single crystals. Analysis of those results has constrained the Mn 4Ca cluster geometry to a set of three similar high-resolution structures. The structure of the cluster from the present study is unlike either the 3.0- or 3.5-A-resolution X-ray structures or other previously proposed models. The differences between the models derived from X-ray spectroscopy and crystallography are predominantly because of damage to the Mn 4Ca cluster by X-rays under conditions used for the structure determination by X-ray crystallography. X-ray spectroscopy studies are also used for studying the changes in the structure of the Mn 4Ca catalytic center as it cycles through the five intermediate states known as the S i states ( i = 0-4). The electronic structure of the Mn 4Ca cluster has been studied more recently using resonant inelastic X-ray scattering spectroscopy (RIXS), in addition to the earlier X-ray absorption and emission spectroscopy methods. These studies are revealing that the assignment of formal oxidation states is overly simplistic. A more accurate description should consider the charge density on the Mn atoms, which includes the covalency of the bonds and delocalization of the charge over the cluster. The geometric and electronic structures of the Mn 4Ca cluster in the S states derived from X-ray spectroscopy are leading to a detailed understanding of the mechanism of O-O bond formation during the photosynthetic water-splitting process.

Computational studies of the O(2)-evolving complex of photosystem II and biomimetic oxomanganese complexes

In recent years, there has been considerable interest in studies of catalytic metal clusters in metalloproteins based on Density Functional Theory (DFT) quantum mechanics/molecular mechanics (QM/MM) hybrid methods. These methods explicitly include the perturbational influence of the surrounding protein environment on the structural/functional properties of the catalytic centers. In conjunction with recent breakthroughs in X-ray crystallography and advances in spectroscopic and biophysical studies, computational chemists are trying to understand the structural and mechanistic properties of the oxygen-evolving complex (OEC) embedded in photosystem II (PSII). Recent studies include the development of DFT-QM/MM computational models of the Mn(4)Ca cluster, responsible for photosynthetic water oxidation, and comparative quantum mechanical studies of biomimetic oxomanganese complexes. A number of computational models, varying in oxidation and protonation states and ligation of the catalytic center by amino acid residues, water, hydroxide and chloride have been characterized along the PSII catalytic cycle of water splitting. The resulting QM/MM models are consistent with available mechanistic data, Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction data and extended X-ray absorption fine structure (EXAFS) measurements. Here, we review these computational efforts focused towards understanding the catalytic mechanism of water oxidation at the detailed molecular level.

Photoassembly of the Water-Oxidizing Complex in Photosystem II

The light-driven steps in the biogenesis and repair of the inorganic core comprising the O(2)-evolving center of oxygenic photosynthesis (photosystem II water-oxidation complex, PSII-WOC) are reviewed. These steps, known collectively as photoactivation, involve the photoassembly of the free inorganic cofactors to the cofactor-depleted PSII-(apo-WOC) driven by light and produce the active O(2)-evolving core comprised of Mn(4)CaO(x)Cl(y). We focus on the functional role of the inorganic components as seen through the competition with non-native cofactors ("inorganic mutants") on water oxidation activity, the rate of the photoassembly reaction, and on structural insights gained from EPR spectroscopy of trapped intermediates formed in the initial steps of the assembly reaction. A chemical mechanism for the initial steps in photoactivation is given that is based on these data. Photoactivation experiments offer the powerful insights gained from replacement of the native cofactors, which together with the recent X-ray structural data for the resting holoenzyme provide a deeper understanding of the chemistry of water oxidation. We also review some new directions in research that photoactivation studies have inspired that look at the evolutionary history of this remarkable catalyst.

Hydroxyl radicals are not the protagonists of UV-B-induced damage in isolated thylakoid membranes

The production of reactive oxygen species (ROS) was studied in isolated thylakoid membranes exposed to 312 nm UV-B irradiation. Hydroxyl radicals (^•OH) and hydrogen peroxide were measured directly, using a newly developed method based on hydroxylation of terephthalic acid and the homovanillic acid/peroxidase assay, respectively. At the early stage of UV-B stress (doses lower than 2.0 J cm^-2), ^•OH were derived from superoxide radicals via hydrogen peroxide. Production of these ROS was dependent on photosynthetic electron transport and was not exclusive to UV-B. Both ROS were found in samples exposed to the same doses of PAR, suggesting that the observed ROS are by-products of the UV-B-driven electron transport rather than specific initiators of the UV-B-induced damage. After longer exposure of thylakoids to UV-B, leading to the inactivation of PSII centres, a small amount of ^•OH was still observed in thylakoids, even though no free hydrogen peroxide was detected. At this late stage of UV-B stress, ^•OH may also be formed by the direct cleavage of organic peroxides by UV-B. Immunodetection showed that the presence of the observed ROS alone was not sufficient to achieve the degradation of the D1 protein of PSII centres.

Enthalpy changes during photosynthetic water oxidation tracked by time-resolved calorimetry using a photothermal beam deflection technique

The energetics of the individual reaction steps in the catalytic cycle of photosynthetic water oxidation at the Mn(4)Ca complex of photosystem II (PSII) are of prime interest. We studied the electron transfer reactions in oxygen-evolving PSII membrane particles from spinach by a photothermal beam deflection technique, allowing for time-resolved calorimetry in the micro- to millisecond domain. For an ideal quantum yield of 100%, the enthalpy change, DeltaH, coupled to the formation of the radical pair Y(Z)(.+)Q(A)(-) (where Y(Z) is Tyr-161 of the D1 subunit of PSII) is estimated as -820 +/- 250 meV. For a lower quantum yield of 70%, the enthalpy change is estimated to be -400 +/- 250 meV. The observed nonthermal signal possibly is due to a contraction of the PSII protein volume (apparent DeltaV of about -13 A(3)). For the first time, the enthalpy change of the O(2)-evolving transition of the S-state cycle was monitored directly. Surprisingly, the reaction is only slightly exergonic. A value of DeltaH(S(3)-->S(0)) of -210 meV is estimated, but also an enthalpy change of zero is within the error range. A prominent nonthermal photothermal beam deflection signal (apparent DeltaV of about +42 A(3)) may reflect O(2) and proton release from the manganese complex, but also reorganization of the protein matrix.

Photosystem II: structure and mechanism of the water:plastoquinone oxidoreductase

This mini-review briefly summarizes our current knowledge on the reaction pattern of light-driven water splitting and the structure of Photosystem II that acts as a water:plastoquinone oxidoreductase. The overall process comprises three types of reaction sequences: (a) light-induced charge separation leading to formation of the radical ion pair P680+*QA(-*) ; (b) reduction of plastoquinone to plastoquinol at the QB site via a two-step reaction sequence with QA(-*) as reductant and (c) oxidative water splitting into O2 and four protons at a manganese-containing catalytic site via a four-step sequence driven by P680+* as oxidant and a redox active tyrosine YZ acting as mediator. Based on recent progress in X-ray diffraction crystallographic structure analysis the array of the cofactors within the protein matrix is discussed in relation to the functional pattern. Special emphasis is paid on the structure of the catalytic sites of PQH2 formation (QB-site) and oxidative water splitting (Mn4OxCa cluster). The energetics and kinetics of the reactions taking place at these sites are presented only in a very concise manner with reference to recent up-to-date reviews. It is illustrated that several questions on the mechanism of oxidative water splitting and the structure of the catalytic sites are far from being satisfactorily answered.

The Possible Role of Proton-Coupled Electron Transfer (PCET) in Water Oxidation by Photosystem II

All higher life forms use oxygen and respiration as their primary energy source. The oxygen comes from water by solar-energy conversion in photosynthetic membranes. In green plants, light absorption in photosystem II (PSII) drives electron-transfer activation of the oxygen-evolving complex (OEC). The mechanism of water oxidation by the OEC has long been a subject of great interest to biologists and chemists. With the availability of new molecular-level protein structures from X-ray crystallography and EXAFS, as well as the accumulated results from numerous experiments and theoretical studies, it is possible to suggest how water may be oxidized at the OEC. An integrated sequence of light-driven reactions that exploit coupled electron-proton transfer (EPT) could be the key to water oxidation. When these reactions are combined with long-range proton transfer (by sequential local proton transfers), it may be possible to view the OEC as an intricate structure that is "wired for protons".

Time-resolved X-ray spectroscopy leads to an extension of the classical S-state cycle model of photosynthetic oxygen evolution

In oxygenic photosynthesis, a complete water oxidation cycle requires absorption of four photons by the chlorophylls of photosystem II (PSII). The photons can be provided successively by applying short flashes of light. Already in 1970, Kok and coworkers [Photochem Photobiol 11:457-475, 1970] developed a basic model to explain the flash-number dependence of O2 formation. The third flash applied to dark-adapted PSII induces the S3-->S4-->S0 transition, which is coupled to dioxygen formation at a protein-bound Mn4Ca complex. The sequence of events leading to dioxygen formation and the role of Kok's enigmatic S4-state are only incompletely understood. Recently we have shown by time-resolved X-ray spectroscopy that in the S3-->S0 transition an interesting intermediate is formed, prior to the onset of O-O bond formation [Haumann et al. Science 310:1019-1021, 2005]. The experimental results of the time-resolved X-ray experiments are discussed. The identity of the reaction intermediate is considered and the question is addressed how the novel intermediate is related to the S4-state proposed in 1970 by Bessel Kok. This leads us to an extension of the classical S-state cycle towards a basic model which describes sequence and interplay of electron and proton abstraction events at the donor side of PSII [Dau and Haumann, Science 312:1471-1472, 2006].

Oxidative photosynthetic water splitting: energetics, kinetics and mechanism

This minireview is an attempt to summarize our current knowledge on oxidative water splitting in photosynthesis. Based on the extended Kok model (Kok, Forbush, McGloin (1970) Photochem Photobiol 11:457-476) as a framework, the energetics and kinetics of two different types of reactions comprising the overall process are discussed: (i) P680+* reduction by the redox active tyrosine YZ of polypeptide D1 and (ii) Yz (ox) induced oxidation of the four step sequence in the water oxidizing complex (WOC) leading to the formation of molecular oxygen. The mode of coupling between electron transport (ET) and proton transfer (PT) is of key mechanistic relevance for the redox turnover of YZ and the reactions within the WOC. The peculiar energetics of the oxidation steps in the WOC assure that redox state S1 is thermodynamically most stable. This is a general feature in all oxygen evolving photosynthetic organisms and assumed to be of physiological relevance. The reaction coordinate of oxidative water splitting is discussed on the basis of the available information about the Gibbs energy differences between the individual redox states Si+1 and Si and the data reported for the activation energies of the individual oxidation steps in the WOC. Finally, an attempt is made to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and on the active role of the protein in tuning the local proton activity that depends on time and redox state Si. The O-O linkage is assumed to take place at the level of a complexed peroxide.

The sensitivity of Photosystem II to damage by UV-B radiation depends on the oxidation state of the water-splitting complex

The water-oxidizing complex of Photosystem II is an important target of ultraviolet-B (280-320 nm) radiation, but the mechanistic background of the UV-B induced damage is not well understood. Here we studied the UV-B sensitivity of Photosystem II in different oxidation states, called S-states of the water-oxidizing complex. Photosystem II centers of isolated spinach thylakoids were synchronized to different distributions of the S(0), S(1), S(2) and S(3) states by using packages of visible light flashes and were exposed to UV-B flashes from an excimer laser (lambda=308 nm). The loss of oxygen evolving activity showed that the extent of UV-B damage is S-state-dependent. Analysis of the data obtained from different synchronizing flash protocols indicated that the UV-sensitivity of Photosystem II is significantly higher in the S(3) and S(2) states than in the S(1) and S(0) states. The data are discussed in terms of a model where UV-B-induced inhibition of water oxidation is caused either by direct absorption within the catalytic manganese cluster or by damaging intermediates of the water oxidation process.

Molecular mechanisms of light stress of photosynthesis

Photosynthesis is the basic energy conversion process on Earth, which makes possible the utilization of the energy of sunlight for living organisms. However, light is not only the basic driving force of photosynthesis, but also an important stress factor at the same time. Light-induced decline of photosynthetic activity, generally denoted as photoinhibition, is a general phenomenon in all oxygenic photosynthetic organism under conditions when the metabolic processes cannot keep up with the electron flow produced by the primary photoreactions. Although light-induced damage occurs in all pigmented photosynthetic complexes the primary site of photoinhibition is the photosystem II (PSII) complex, which performs light-driven oxidation of water to protons and oxygen. The main factors, which are responsible for the light sensitivity of photosystem II, are excited pigment molecules, oxygen, manganese, as well as electron donors with high-oxidizing potential. Photosystem II can be efficiently protected from photodamage by the combination of harmless dissipation of absorbed light energy, nonradiative charge recombination, and repair of damaged reaction center complexes, making possible the safe utilization of light, the highly energetic substrate of photosynthesis.

Quantum mechanics/molecular mechanics structural models of the oxygen-evolving complex of photosystem II

The annual production of 260 Gtonnes of oxygen, during the process of photosynthesis, sustains life on earth. Oxygen is produced in the thylakoid membranes of green-plant chloroplasts and the internal membranes of cyanobacteria by photocatalytic water oxidation at the oxygen-evolving complex (OEC) of photosystem II (PSII). Recent breakthroughs in X-ray crystallography and advances in quantum mechanics/molecular mechanics (QM/MM) hybrid methods have enabled the construction of chemically sensible models of the OEC of PSII. The resulting computational structural models suggest the complete ligation of the catalytic center by amino acid residues, water, hydroxide and chloride, as determined from the intrinsic electronic properties of the oxomanganese core and the perturbational influence of the surrounding protein environment. These structures are found to be consistent with available mechanistic data, and are also compatible with X-ray diffraction models and extended X-ray absorption fine structure measurements. It is therefore conjectured that these OEC models are particularly relevant for the elucidation of the catalytic mechanism of water oxidation.

Radiative and non-radiative charge recombination pathways in Photosystem II studied by thermoluminescence and chlorophyll fluorescence in the cyanobacterium Synechocystis 6803

The mechanism of charge recombination was studied in Photosystem II by using flash induced chlorophyll fluorescence and thermoluminescence measurements. The experiments were performed in intact cells of the cyanobacterium Synechocystis 6803 in which the redox properties of the primary pheophytin electron acceptor, Phe, the primary electron donor, P(680), and the first quinone electron acceptor, Q(A), were modified. In the D1Gln130Glu or D1His198Ala mutants, which shift the free energy of the primary radical pair to more positive values, charge recombination from the S(2)Q(A)(-) and S(2)Q(B)(-) states was accelerated relative to the wild type as shown by the faster decay of chlorophyll fluorescence yield, and the downshifted peak temperature of the thermoluminescence Q and B bands. The opposite effect, i.e. strong stabilization of charge recombination from both the S(2)Q(A)(-) and S(2)Q(B)(-) states was observed in the D1Gln130Leu or D1His198Lys mutants, which shift the free energy level of the primary radical pair to more negative values, as shown by the retarded decay of flash induced chlorophyll fluorescence and upshifted thermoluminescence peak temperatures. Importantly, these mutations caused a drastic change in the intensity of thermoluminescence, manifested by 8- and 22-fold increase in the D1Gln130Leu and D1His198Lys mutants, respectively, as well as by a 4- and 2.5-fold decrease in the D1Gln130Glu and D1His198Ala mutants, relative to the wild type, respectively. In the presence of the electron transport inhibitor bromoxynil, which decreases the redox potential of Q(A)/Q(A)(-) relative to that observed in the presence of DCMU, charge recombination from the S(2)Q(A)(-) state was accelerated in the wild type and all mutant strains. Our data confirm that in PSII the dominant pathway of charge recombination goes through the P(680)(+)Phe(-) radical pair. This indirect recombination is branched into radiative and non-radiative pathways, which proceed via repopulation of P(680)(*) from (1)[P(680)(+)Ph(-)] and direct recombination of the (3)[P(680)(+)Ph(-)] and (1)[P(680)(+)Ph(-)] radical states, respectively. An additional non-radiative pathway involves direct recombination of P(680)(+)Q(A)(-). The yield of these charge recombination pathways is affected by the free energy gaps between the Photosystem II electron transfer components in a complex way: Increase of DeltaG(P(680)(*)<-->P(680)(+)Phe(-)) decreases the yield of the indirect radiative pathway (in the 22-0.2% range). On the other hand, increase of DeltaG(P(680)(+)Phe(-)<-->P(680)(+)Q(A)(-)) increases the yield of the direct pathway (in the 2-50% range) and decreases the yield of the indirect non-radiative pathway (in the 97-37% range).

Comparative study of the water oxidizing reactions and the millisecond delayed chlorophyll fluorescence in photosystem II at different pH

Water splitting activity, the multiline EPR signal associated with S(2)-state of the CaMn(4)-cluster and the fast and slow phases of the induction curve of the millisecond delayed chlorophyll fluorescence from photosystem II (PSII) in the pH range of 4.5-8.5 were studied in the thylakoid membranes and purified PSII particles. It has been found that O(2) evolution and the multiline EPR signal were inhibited at acidic (pK approximately 5.3) and alkaline (pK approximately 8.1) pH values, and were maximal at pH 6.0-7.0. Our results indicate that the loss of O(2) evolution and the S(2)-state multiline EPR signal associated with the decrease of the millisecond delayed chlorophyll fluorescence only in alkaline region (pH 7.0-8.5). Possible correlations of the millisecond delayed chlorophyll fluorescence components with the donor side reactions in PSII are discussed.

Photosystem II: an enzyme of global significance

Photosystem II (PSII) is a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Powered by light, this enzyme catalyses the chemically and thermodynamically demanding reaction of water splitting. In so doing, it releases dioxygen into the atmosphere and provides the reducing equivalents required for the conversion of CO2 into the organic molecules of life. Recently, a fully refined structure of a 700 kDa cyanobacterial dimeric PSII complex was elucidated by X-ray crystallography which gave organizational and structural details of the 19 subunits (16 intrinsic and three extrinsic) which make up each monomer and provided information about the position and protein environments of 57 different cofactors. The water-splitting site was revealed as a cluster of four Mn ions and a Ca2+ ion surrounded by amino acid side chains, of which six or seven form direct ligands to the metals. The metal cluster was modelled as a cubane-like structure composed of three Mn ions and the Ca2+ linked by oxo-bonds with the fourth Mn attached to the cubane via one of its oxygens. The overall structure of the catalytic site is providing a framework to develop a mechanistic scheme for the water-splitting process, knowledge which could have significant implications for mimicking the reaction in an artificial chemical system.

Low-barrier hydrogen bond plays key role in active photosystem II--a new model for photosynthetic water oxidation

The function and mechanism of Tyr(Z) in active photosystem II (PSII) is one of the long-standing issues in the study of photosynthetic water oxidation. Based on recent investigations on active PSII and theoretical studies, a new model is proposed, in which D1-His190 acts as a bridge, to form a low-barrier hydrogen bond (LBHB) with Tyr(Z), and a coordination bond to Mn or Ca ion of the Mn-cluster. Accordingly, this new model differs from previous proposals concerning the mechanism of Tyr(Z) function in two aspects. First, the LBHB plays a key role to decrease the activation energy for Tyr(Z) oxidation and Tyr(Z)(.) reduction during photosynthetic water oxidation. Upon the oxidation of Tyr(Z), the hydrogen bond between Tyr(Z) and His190 changes from a LBHB to a weak hydrogen bond, and vice versa upon Tyr(Z)(.) reduction. In both stages, the electron transfer and proton transfer are coupled. Second, the positive charge formed after Tyr(Z) oxidation may play an important role for water oxidation. It can be delocalized on the Mn-cluster, thus helps to accelerate the proton release from substrate water on Mn-cluster. This model is well reconciled with observations of the S-state dependence of Tyr(Z) oxidation and Tyr(Z)(.) reduction, proton release, isotopic effect and recent EPR experiments. Moreover, the difference between Tyr(Z) and Tyr(D) in active PSII can also be readily rationalized. The His190 binding to the Mn-cluster predicted in this model is contradictious to the recent structure data, however, it has been aware that the crystal structure of the Mn-cluster and its environment are significantly modified by X-ray due to radiation damage and are different from that in active PSII. It is suggested that the His190 may be protonated during the radiation damage, which leads to the loss of its binding to Mn-cluster and the strong hydrogen bond with Tyr(Z). This type of change arising from radiation damage has been confirmed in other enzyme systems.

Reaction pattern and mechanism of light induced oxidative water splitting in photosynthesis

This mini review is an attempt to briefly summarize our current knowledge on light driven oxidative water splitting in photosynthesis. The reaction leading to molecular oxygen and four protons via photosynthesis comprises thermodynamic and kinetic constraints that require a balanced fine tuning of the reaction coordinates. The mode of coupling between electron (ET) and proton transfer (PT) reactions is shown to be of key mechanistic relevance for the redox turnover of Y(Z) and the reactions within the WOC. The WOC is characterized by peculiar energetics of its oxidation steps in the WOC. In all oxygen evolving photosynthetic organisms the redox state S(1) is thermodynamically most stable and therefore this general feature is assumed to be of physiological relevance. Available information on the Gibbs energy differences between the individual redox states S(i+1) and S(i) and on the activation energies of their oxidative transitions are used to construct a general reaction coordinate of oxidative water splitting in photosystem II (PS II). Finally, an attempt is presented to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and the active role of the protein environment in tuning the local proton activity that depends on time and redox state S(i). The O-O linkage is assumed to take place within a multistate equilibrium at the redox level of S(3), comprising both redox isomerism and proton tautomerism. It is proposed that one state, S(3)(P), attains an electronic configuration and nuclear geometry that corresponds with a hydrogen bonded peroxide which acts as the entatic state for the generation of complexed molecular oxygen through S(3)(P) oxidation by Y(Z)(ox).

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.

Six specific lysine residues are crucial in maintaining the structure and function of soluble manganese stabilizing protein

When manganese stabilizing protein (MSP) was treated with 0.5 mM N-succinimidyl propionate (NSP), the rebinding ability and oxygen-releasing capabilities of the modified MSP were not altered, in spite of changes of MSP surface Lys residues. Furthermore, far-ultraviolet circular dichroism and intrinsic fluorescence spectra analysis revealed that 0.5 mM NSP-modified MSP retained most of its native secondary and tertiary structure. Mapping of the sites of NSP modification by Staphylococcus V(8) protease digestion of the modified protein, as well as analysis by matrix-assisted laser desorption ionization-time of flight mass spectrometry, indicated that seven Lys residues were modified. The results suggested that these residues are not absolutely essential to the structure and function of MSP. However, when the NSP concentration was increased to 4 mM, the modified MSP was unable to bind photosystem II and completely lost its reactivating capability. Both far-ultraviolet circular dichroism and intrinsic fluorescence spectra analysis revealed a clear conformational change in MSP after 4 mM NSP treatment, suggesting that some Lys residues are involved in maintaining the structure and function of MSP. Analysis by matrix-assisted laser desorption ionization-time of flight mass spectrometry indicated that another six Lys residues, namely Lys20, Lys101, Lys196, Lys207, Lys130 (or Lys137) and Lys66 (or Lys76), were modified by 4 mM NSP. Therefore, these six Lys residues are crucial in maintaining the structure and function of soluble MSP.

Comparative proteomics of high light stress in the model algaChlamydomonas reinhardtii

High light (HL) stress adversely affects growth, productivity and viability of photosynthetic organisms. The green alga Chlamydomonas reinhardtii is a model system to study photosynthesis and light stress. Comparative proteomics of wild-type and two very high light (VHL)-resistant mutants, VHL(R)-S4 and VHL(R)-S9, revealed complex alterations in response to excess light. A two-dimensional reference map of the soluble subproteome was constructed representing about 1500 proteins. A total of 83 proteins from various metabolic pathways were identified by peptide mass fingerprinting. Quantitative comparisons of 444 proteins showed 105 significantly changed proteins between wild type and mutants under different light conditions. Commonly, more proteins were decreased than increased, but different proteins were affected in each genotype. Proteins uniquely altered in either VHL(R) mutant may be involved in VHL resistance. Such candidate proteins similarly altered without light stress, thus possibly contributing to "pre-adaptation" of mutants to VHL, included decreased levels of a DEAD box RNA helicase (VHL(R)-S4) and NAB1 and RB38 proteins (VHL(R)-S9), and increased levels of an oxygen evolving enhancer 1 (OEE1) isoform and an unknown protein (VHL(R)-S4). Changes from increased levels in HL to decreased levels in excess light, included OEE1 (VHL(R)-S9) or the reverse change for NAB1, RB38, beta-carbonic anhydrase and an ABC transporter-like protein (VHL(R)-S4).

Chemical approaches to artificial photosynthesis. 2

The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.

Interaction between tyrosineZ and substrate water in active photosystem II

In the field of photosynthetic water oxidation it has been under debate whether Tyrosine(Z) (Tyr(Z)) acts as a hydrogen or an electron acceptor from water. In the former concept, direct contact of Tyr(Z) with substrate water has been assumed. However, there is no direct evidence for the interaction between Tyr(Z) and substrate water in active Photosystem II (PSII), instead most experiments have been performed on inhibited PSII. Here, this problem is tackled in active PSII by combining low temperature EPR measurements and quantum chemistry calculations. EPR measurements observed that the maximum yield of Tyr(Z) oxidation at cryogenic temperature in the S(0) and S(1) states was around neutral pH and was essentially pH-independent. The yield of Tyr(Z) oxidation decreased at acidic and alkaline pH, with pKs at 4.7-4.9 and 7.7, respectively. The observed pH-dependent parts at low and high values of pH can be explained as due to sample inactivation, rather than active PSII. The reduction kinetics of Tyr(Z)(.) in the S(0) and S(1) states were pH independent at pH range from 4.5 to 8. Therefore, the change of the pH in bulk solution probably has no effect on the Tyr(Z) oxidation and Tyr(Z)(.) reduction at cryogenic temperature in the S(0) and S(1) states of the active PSII. Theoretical calculations indicate that Tyr(Z) becomes more difficult to oxidize when a H(2)O molecule interacts directly with it. It is suggested that Tyr(Z) is probably located in a hydrophobic environment with no direct interaction with the substrate H(2)O in active PSII. These results provide new insights on the function and mechanism of water oxidation in PSII.

Characterization of the water oxidizing complex of photosystem II of the Chl d-containing cyanobacterium Acaryochloris marina via its reactivity towards endogenous electron donors and acceptors

Acaroychloris (A.) marina is a unique oxygen evolving organism that contains a large amount of chlorophyll d (Chl d) and only very few Chl a molecules. This feature raises questions on the nature of the photoactive pigment, which supports light-induced oxidative water splitting in Photosystem II (PS II). In this study, flash-induced oxygen evolution patterns (FIOPs) were measured to address the question whether the S(i) state transition probabilities and/or the redox-potentials of the water oxidizing complex (WOC) in its different S(i) states are altered in A. marina cells compared to that of spinach thylakoids. The analysis of the obtained data within the framework of different versions of the Kok model reveals that in light activated A. marina cells the miss probability is similar compared to spinach thylakoids. This finding indicates that the redox-potentials and kinetics within the WOC, of the reaction center (P680) and of Y(Z) are virtually the same for both organisms. This conclusion is strongly supported by lifetime measurements of the S(2) and S(3) states. Virtually identical time constants were obtained for the slow phase of deactivation. Kinetic differences in the fast phase of S(2) and S(3) decay between A. marina cells and spinach thylakoids reflect a shift of the E(m) of Y(D)/Y(D)(ox) to lower values in the former compared to the latter organisms, as revealed by the observation of an opposite change in the kinetics of S(0) oxidation to S(1) by Y(D)(ox). A slightly increased double hit probability in A. marina cells is indicative of a faster Q(A)(-) to Q(B) electron transfer in these cells compared to spinach thylakoids.

Effect of bicarbonate on the water-oxidizing complex of photosystem II in the super-reduced S-states

It is shown that the hydrazine-induced transition of the water-oxidizing complex (WOC) to super-reduced S-states depends on the presence of bicarbonate in the medium so that after a 20 min treatment of isolated spinach thylakoids with 3 mM NH(2)NH(2) at 20 degrees C in the CO(2)/HCO(3)(-)-depleted buffer the S-state populations are: 42% of S(-3), 42% of S(-2), 16% of S(-1) and even formal S(-4) state is reached, while in the presence of 2 mM NaHCO(3), the same treatment produces 30% of S(-3), 38% of S(-2), and 32% of S(-1) and there is no indication of the S(-4) state. Bicarbonate requirement for the oxygen-evolving activity, very low in untreated thylakoids, considerably increases upon the transition of the WOC to the super-reduced S-states, and the requirement becomes low again when the WOC returns back to the normal S-states using pre-illumination. The results are discussed as a possible indication of ligation of bicarbonate to manganese ions within the WOC.

Herbicide binding and thermal stability of photosystem II isolated from Thermosynechococcus elongatus

Binding of herbicides to photosystem II inhibits the electron transfer from Q(A) to Q(B) due to competition of herbicides with plastoquinone bound at the Q(B) site. We investigated herbicide binding to monomeric and dimeric photosystem II core complexes (PSIIcc) isolated from Thermosynechococcus elongatus by a combination of different methods (isothermal titration and differential scanning calorimetry, CD spectroscopy and measurements of the oxygen evolution) yielding binding constants, enthalpies and stoichiometries for various herbicides as well as information regarding stabilization/destabilization of the complex. Herbicide binding to detergent-solubilized PSIIcc can be described by a model of single independent binding sites present on this important membrane protein. Interestingly, binding stoichiometries herbicide:PSIIcc are lower than 1:1 and vary depending on the herbicide under study. Strong binding herbicides such as terbutryn stabilize PSIIcc in thermal unfolding experiments and endothermically binding herbicides like ioxynil probably cause large structural changes accompanied with the binding process as shown by differential scanning calorimetry experiments of the unfolding reaction of PSIIcc monomer in the presence of ioxynil. In addition we studied the occupancy of the Q(B) sites with plastoquinone (PQ9) by measuring flash induced fluorescence relaxation yielding a possible explanation for the deviations of herbicide binding from a 1:1 herbicide/binding site model.

Mimicking the electron donor side of Photosystem II in artificial photosynthesis

This review focuses on our recent efforts in synthetic ruthenium-tyrosine-manganese chemistry mimicking the donor side reactions of Photosystem II. Tyrosine and tryptophan residues were linked to ruthenium photosensitizers, which resulted in model complexes for proton-coupled electron transfer from amino acids. A new mechanistic model was proposed and used to design complexes in which the mechanism could be switched between concerted and step-wise proton-coupled electron transfer. Moreover, a manganese dimer linked to a ruthenium complex could be oxidized in three successive steps, from Mn (2) (II,II) to Mn (2) (III,IV) by the photo-oxidized ruthenium sensitizer. This was possible thanks to a charge compensating ligand exchange in the manganese complex. Detailed studies of the ligand exchange suggested that at high water concentrations, each oxidation step is coupled to a proton-release of water-derived ligands, analogous to the oxidation steps of the manganese cluster of Photosystem II.

Mutagenesis of CP43-arginine-357 to serine reveals new evidence for (bi)carbonate functioning in the water oxidizing complex of Photosystem II

The chlorophyll-binding protein CP43 is an inner subunit of the Photosystem II (PSII) reaction center core complex of all oxygenic photoautotrophs. X-Ray structural evidence places the guanidinium cation of the conserved arginine 357 residue of CP43 within a few Angstroms to the Mn(4)Ca cluster of the water-oxidizing complex (WOC) and has been implicated as a possible carbonate binding site. To test the hypothesis, the serine mutant, CP43-R357S, from Synechocystis PCC 6803 was investigated by PSII variable fluorescence (F(v)/F(m)) and simultaneous flash O(2) yield measurements in cells and thylakoid membranes. The R357S mutant assembles PSII-WOC centers, but is unable to grow photoautotrophically. Reconstitution of O(2) evolution by photoactivation and the occurrence of period-four oscillations of F(v)/F(m) establishes that the R357S mutant contains an assembled Mn(4)Ca cluster, but turnover is impaired as seen by an 11-fold larger Kok double miss parameter and faster decay of upper S states. Using pulsed light to avoid photoinactivation, wild-type cells and thylakoid membranes exhibit a 2-4-fold loss in O(2) evolution rate upon partial bicarbonate depletion under multiple turnover conditions, while the R357S mutant is unaffected by bicarbonate. Arginine R357 appears to function in binding a (bi)carbonate ion essential to normal catalytic turnover of the WOC. The quantum yield of electron donation from the WOC into PSII increases with decreasing turnover rate in R357S mutant cells and involves an aborted two-flash pathway that is distinct from the classical four-flash pattern. We speculate that an altered photochemical mechanism for O(2) production occurs via formation of hydrogen peroxide, by analogy to other treatments that retard the kinetics of proton release into the lumen.

Extinction coefficients of cytochromes b559 and c550 of Thermosynechococcus elongatus and Cyt b559/PS II stoichiometry of higher plants

"Reduced minus oxidized" difference extinction coefficients Deltavarepsilon in the alpha-bands of Cyt b559 and Cyt c550 were determined by using functionally and structurally well-characterized PS II core complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus. Values of 25.1+/-1.0 mM(-1) cm(-1) and 27.0+/-1.0 mM(-1) cm(-1) were obtained for Cyt b559 and Cyt c550, respectively. Anaerobic redox titrations covering the wide range from -250 up to +450 mV revealed that the heme groups of both Cyt b559 and Cyt c550 exhibit homogenous redox properties in the sample preparation used, with E(m) values at pH 6.5 of 244+/-11 mV and -94+/-21 mV, respectively. No HP form of Cyt b559 could be detected. Experiments performed on PS II membrane fragments of higher plants where the content of the high potential form of Cyt b559 was varied by special treatments (pH, heat) have shown that the alpha-band extinction of Cyt b559 does not depend on the redox form of the heme group. Based on the results of this study the Cyt b559/PSII stoichiometry is inferred to be 1:1 not only in thermophilic cyanobacteria as known from the crystal structure but also in PSII of plants. Possible interrelationships between the structure of the Q(B) site and the microenvironment of the heme group of Cyt b559 are discussed.

Reaction pattern of photosystem II: oxidative water cleavage and protein flexibility

This short communication addresses three topics of photosynthetic water cleavage in Photosystem II (PS II): (a) effect of protonation in the acidic range on the extent of the 'fast' ns kinetics of P680+. reduction by YZ, (b) mechanism of O-O bond formation and (c) role of protein flexibility in the functional integrity of PS II. Based on measurements of light-induced absorption changes and quasielastic neutron scattering in combination with mechanistic considerations, evidence is presented for the protein acting as a functionally active constituent of the water cleavage machinery, in particular, for directed local proton transfer. A specific flexibility emerging above a threshold of about 230 K is an indispensable prerequisite for oxygen evolution and plastoquinol formation.

Optimized Spin Crossings and Transition States for Short-range Electron Transfer in Transition Metal Dimers

Electron-transfer reactions in eight mixed-valence manganese dimers are studied using B3LYP. One of the dimers is a model of the active site of manganese catalase, while another represents a basic building block of the oxygen-evolving complex in photosystem II. The adiabatic reactions are characterized by fully optimized transition states where the single imaginary frequency represents the electron-transfer coordinate. When there is antiferromagnetic coupling between different high-spin centers, electron transfer must be accompanied by a spin transition. Spin transitions are characterized by minimum-energy crossing points between spin surfaces. Three reaction mechanisms have been investigated. First, a single-step reaction where spin flip is concerted with electron transfer. Second, an initial transition to a center with intermediate spin that can be followed by electron transfer. Third, an initial transition to a ferromagnetic state from which the electron can be transferred adiabatically. The complexes prefer the third route with rate-determining barriers ranging from 5.7 kcal/mol to 17.2 kcal/mol for different complexes. The origins of these differences are discussed in terms of oxidation states and ligand environments. Many DFT functionals overestimate charge-transfer interactions, but for the present complexes, the error should be limited because of short Mn-Mn distances.