Oxygen-evolving Complex Of Photosystem II Research Articles

Bioinspired Water Preorganization in Confined Space for Efficient Water Oxidation Catalysis in Metallosupramolecular Ruthenium Architectures.

ConspectusNature has established a sustainable way to maintain aerobic life on earth by inventing one of the most sophisticated biological processes, namely, natural photosynthesis, which delivers us with organic matter and molecular oxygen derived from the two abundant resources sunlight and water. The thermodynamically demanding photosynthetic water splitting is catalyzed by the oxygen-evolving complex in photosystem II (OEC-PSII), which comprises a distorted tetramanganese-calcium cluster (CaMn4O5) as catalytic core. As an ubiquitous concept for fine-tuning and regulating the reactivity of the active site of metalloenzymes, the surrounding protein domain creates a sophisticated environment that promotes substrate preorganization through secondary, noncovalent interactions such as hydrogen bonding or electrostatic interactions. Based on the high-resolution X-ray structure of PSII, several water channels were identified near the active site, which are filled with extensive hydrogen-bonding networks of preorganized water molecules, connecting the OEC with the protein surface. As an integral part of the outer coordination sphere of natural metalloenzymes, these channels control the substrate and product delivery, carefully regulate the proton flow by promoting pivotal proton-coupled electron transfer processes, and simultaneously stabilize short-lived oxidized intermediates, thus highlighting the importance of an ordered water network for the remarkable efficiency of the natural OEC.Transferring this concept from nature to the engineering of artificial metal catalysts for fuel production has fostered the fascinating field of metallosupramolecular chemistry by generating defined cavities that conceptually mimic enzymatic pockets. However, the application of supramolecular approaches to generate artificial water oxidation catalysts remained scarce prior to our initial reports, since such molecular design strategies for efficient activation of substrate water molecules in confined nanoenvironments were lacking. In this Account, we describe our research efforts on combining the state-of-the art Ru(bda) catalytic framework with structurally programmed ditopic ligands to guide the water oxidation process in defined metallosupramolecular assemblies in spatial proximity. We will elucidate the governing factors that control the quality of hydrogen-bonding water networks in multinuclear cavities of varying sizes and geometries to obtain high-performance, state-of-the-art water oxidation catalysts. Pushing the boundaries of artificial catalyst design, embedding a single catalytic Ru center into a well-defined molecular pocket enabled sophisticated water preorganization in front of the active site through an encoded basic recognition site, resulting in high catalytic rates comparable to those of the natural counterpart OEC-PSII.To fully explore their potential for solar fuel devices, the suitability of our metallosupramolecular assemblies was demonstrated under (electro)chemical and photocatalytic water oxidation conditions. In addition, testing the limits of structural diversity allowed the fabrication of self-assembled linear coordination oligomers as novel photocatalytic materials and long-range ordered covalent organic framework (COF) materials as recyclable and long-term stable solid-state materials for future applications.

Three-state mathematical model for the assessment of DCMU-treated photosystem II heterogeneity.

Photosystem II (PSII) is one of the main pigment-protein complexes of photosynthesis which is highly sensitive to unfavorable environmental factors. The heterogeneity of PSII properties is essential for the resistance of autotrophic organisms to stress factors. Assessment of the PSII heterogeneity may be used in environmental monitoring for on-line detection of contamination of the environment. We propose an approach to assess PSII oxygen-evolving complex and light-harvesting antenna heterogeneity that is based on mathematical modeling of the shape of chlorophyll a fluorescence rise of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated samples. The hierarchy of characteristic times of the processes considered in the model makes it possible to reduce the model to a system of three ordinary differential equations. The analytic solution of the reduced three-state model is expressed as a sum of two exponential functions, and it exactly reproduces the solution of the complete system within the time range from microseconds to hundreds of milliseconds. The combination of several such models for reaction centers with different properties made it possible to use it as an instrument to study PSII heterogeneity. PSII heterogeneity was studied for Chlamydomonas at different intensities of actinic light, for Scenedesmus under short-term heating, and for Chlorella grown in nitrate-enriched and nitrate-depleted media.

Nitrate nitrogen enhances the efficiency of photoprotection in Leymus chinensis under drought stress.

Global climate change exerts a significant impact on the nitrogen supply and photosynthesis ability in land-based plants. The photosynthetic capacity of dominant grassland species is important if we are to understand carbon cycling under climate change. Drought stress is one of the major factors limiting plant photosynthesis, and nitrogen (N) is an essential nutrient involved in the photosynthetic activity of leaves. The regulatory mechanisms responsible for the effects of ammonium (NH4 +) and nitrate (NO3 -) on the drought-induced photoinhibition of photosystem II (PSII) in plants have yet to be fully elucidated. Therefore, there is a significant need to gain a better understanding of the role of electron transport in the photoinhibition of PSII. In the present study, we conducted experiments with normal watering (LD), severe drought (MD), and extreme drought (HD) treatments, along with no nitrogen (N0), ammonium (NH4), nitrate (NO3), and mixed nitrogen (NH4NO3) treatments. We analyzed pigment accumulation, reactive oxygen species (ROS) accumulation, photosynthetic enzyme activity, photosystem activity, electron transport, and O-J-I-P kinetics. Analysis showed that increased nitrate application significantly increased the leaf chlorophyll content per unit area (Chlarea) and nitrogen content per unit area (Narea) (p< 0.05). Under HD treatment, ROS levels were lower in NO3-treated plants than in N0 plants, and there was no significant difference in photosynthetic enzyme activity between plants treated with NO3 and NH4NO3. Under drought stress, the maximum photochemical efficiency of PSII (Fv/Fm), PSII electron transport rate (ETR), and effective quantum yield of PSII (φPSII) were significant higher in NO3-treated plants (p< 0.05). Importantly, the K-band and G-band were higher in NO3-treated plants. These results suggest that drought stress hindered the formation of NADPH and ATP in N0 and NH4-treated L. chinensis plants, thus damaging the donor side of the PSII oxygen-evolving complex (OEC). After applying nitrate, higher photosynthetic enzyme and antioxidant enzyme activity not only protected PSII from photodamage under drought stress but also reduced the rate of damage in PSII during the growth of L. chinensis growth under drought stress.

Physiological Role of pH-Dependent Structural Transition in Oxygen-Evolving Complex of PSII

Photosystem II (PSII) of the photosynthetic apparatus in oxygenic organisms contains a catalytic center that performs one of the most important reactions in bioenergetics: light-dependent water oxidation to molecular oxygen. The catalytic center is a Mn4CaO5 cluster consisting of four cations of manganese and one calcium cation linked by oxygen bridges. The authors reported earlier that a structural transition occurs at pH 5.7 in the cluster resulting in changes in manganese cation(s) redox potential and elevation of the Mn‑clus-ter resistance to reducing agents. The discovered effect was examined in a series of investigations that are reviewed in this work. It was found that, at pH 5.7, Fe(II) cations replace not two manganese cations as it happens at pH 6.5 but only one cation; as a result, a chimeric Mn3Fe1 cluster is produced. In the presence of exogenous calcium ions, membrane preparations of PSII with such a chimeric cluster are capable of evolving oxygen in the light (at a rate of approximately 25% of the rate in native PSII). It was found that photoinhibition that greatly depends on the processes of oxidation or reduction at pH 5.7 slows down as compared with pH 6.5. PSII preparations were also more resistant to thermal inactivation at pH 5.7 than at pH 6.5. However, in PSII preparations lacking manganese cations in the oxygen-evolving complex, the rates of photoinhibition at pH 6.5 and 5.7 did not differ. In thylakoid membranes, protonophores that abolish the proton gradient and increase pH in the lumen (where the manganese cluster is located) from 5.7 to 7.0 considerably elevated the rate of PSII photoinhibition. It is assumed that the structural transition in the Mn-cluster at pH 5.7 is involved in the mechanisms of PSII defense against photoinhibition.

Physiological Role of pH-Dependent Structural Transition in Oxygen-Evolving Complex of PSII

Photosystem II (PSII) of the photosynthetic apparatus in oxygenic organisms contains a catalytic center that performs one of the most important reactions in bioenergetics: light-dependent water oxidation to molecular oxygen. The catalytic center is a Mn4CaO5 cluster consisting of four cations of manganese and one calcium cation linked by oxygen bridges. The authors reported earlier that a structural transition occurs at pH 5.7 in the cluster resulting in changes in manganese cation(s) redox potential and elevation of the Mn‑clus-ter resistance to reducing agents. The discovered effect was examined in a series of investigations that are reviewed in this work. It was found that, at pH 5.7, Fe(II) cations replace not two manganese cations as it happens at pH 6.5 but only one cation; as a result, a chimeric Mn3Fe1 cluster is produced. In the presence of exogenous calcium ions, membrane preparations of PSII with such a chimeric cluster are capable of evolving oxygen in the light (at a rate of approximately 25% of the rate in native PSII). It was found that photoinhibition that greatly depends on the processes of oxidation or reduction at pH 5.7 slows down as compared with pH 6.5. PSII preparations were also more resistant to thermal inactivation at pH 5.7 than at pH 6.5. However, in PSII preparations lacking manganese cations in the oxygen-evolving complex, the rates of photoinhibition at pH 6.5 and 5.7 did not differ. In thylakoid membranes, protonophores that abolish the proton gradient and increase pH in the lumen (where the manganese cluster is located) from 5.7 to 7.0 considerably elevated the rate of PSII photoinhibition. It is assumed that the structural transition in the Mn-cluster at pH 5.7 is involved in the mechanisms of PSII defense against photoinhibition.

Elucidation of a multiple S3 intermediates model for water oxidation in the oxygen evolving complex of photosystem II. Calcium-assisted concerted O[sbnd]O bond formation

A multiple S3 intermediates model for water oxidation in the oxygen evolving complex (OEC) of photosystem II (PSII) is examined on the basis of key intermediates elucidated by UB3LYP/def2-TZVP and DLPNO-CCSD(T0) computations. The kinetic experimental results by this model are consistent with the main contribution of the Mn-Hydroxide configuration and partial contribution of Mn-Oxyl in the S3 state, for which the oxyl-radical character is largely reduced by the coordination of the Ca ion in OEC of PSII.

Water oxidation reaction in the presence of a dinuclear Mn(II)-semicarbohydrazone coordination compound.

Water splitting, producing of oxygen, and hydrogen molecules, is an essential reaction for clean energy resources and is one of the challenging reactions for artificial photosynthesis. The Mn4Ca cluster in photosystem II (PS-II) is responsible for water oxidation in natural photosynthesis. Due to this, water oxidation reaction by Mn coordination compounds is vital for mimicking the active core of the oxygen-evolving complex in PS-II. Here, a new dinuclear Mn(II)-semicarbohydrazone coordination compound, [Mn(HL)(µ-N3)Cl]2 (1), was synthesized and characterized by various methods. The structure of compound 1 was determined by single crystal X-ray analysis, which revealed the Mn(II) ions have distorted octahedral geometry as (MnN4OCl). This geometry is created by coordinating of oxygen and two nitrogen donor atoms from semicarbohydrazone ligand, two nitrogen atoms from azide bridges, and chloride anion. Compound 1 was used as a catalyst for electrochemical water oxidation, and the surface of the electrode after the reaction was investigated by scanning electron microscopy, energy dispersive spectrometry, and powder X-ray diffraction analyses. Linear sweep voltammetry (LSV) experiments revealed that the electrode containing 1 shows high activity for chemical water oxidation with an electrochemical overpotential as low as 377mV. Although our findings showed that the carbon paste electrode in the presence of 1 is an efficient electrode for water oxidation, it could not withstand water oxidation catalysis under bulk electrolysis and finally converted to Mn oxide nanoparticles which were active for water oxidation along with compound 1.

Responses of Linear and Cyclic Electron Flow to Nitrogen Stress in an N-Sensitive Species Panax notoginseng.

Nitrogen (N) is a primary factor limiting leaf photosynthesis. However, the mechanism of N-stress-driven photoinhibition of the photosystem I (PSI) and photosystem II (PSII) is still unclear in the N-sensitive species such as Panax notoginseng, and thus the role of electron transport in PSII and PSI photoinhibition needs to be further understood. We comparatively analyzed photosystem activity, photosynthetic rate, excitation energy distribution, electron transport, OJIP kinetic curve, P700 dark reduction, and antioxidant enzyme activities in low N (LN), moderate N (MN), and high N (HN) leaves treated with linear electron flow (LEF) inhibitor [3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU)] and cyclic electron flow (CEF) inhibitor (methyl viologen, MV). The results showed that the increased application of N fertilizer significantly enhance leaf N contents and specific leaf N (SLN). Net photosynthetic rate (Pn) was lower in HN and LN plants than in MN ones. Maximum photochemistry efficiency of PSII (Fv/Fm), maximum photo-oxidation P700+ (Pm), electron transport rate of PSI (ETRI), electron transport rate of PSII (ETRII), and plastoquinone (PQ) pool size were lower in the LN plants. More importantly, K phase and CEF were higher in the LN plants. Additionally, there was not a significant difference in the activity of antioxidant enzyme between the MV- and H2O-treated plants. The results obtained suggest that the lower LEF leads to the hindrance of the formation of ΔpH and ATP in LN plants, thereby damaging the donor side of the PSII oxygen-evolving complex (OEC). The over-reduction of PSI acceptor side is the main cause of PSI photoinhibition under LN condition. Higher CEF and antioxidant enzyme activity not only protected PSI from photodamage but also slowed down the damage rate of PSII in P. notoginseng grown under LN.

Heterogeneous Composition of Oxygen-Evolving Complexes in Crystal Structures of Dark-Adapted Photosystem II.

Photosystem II (PSII) is a homodimeric protein complex that catalyzes water oxidation at the oxygen-evolving complex (OEC), a heterocubanoid calcium-tetramanganese cluster. Here, we analyze the omit electron density peaks of the OEC's metal ions in five X-ray free-electron laser PSII structures at resolutions between 2.15 and 1.95 Å. The omit peaks can be described by the total number of electrons and approximated by the variance of electron density distribution when the distributions are spherically symmetric. We show that the number of electrons of metal centers is different in the two OECs of PSII dimers, implying that the oxidation states and/or occupancies of individual metal ions are different in the two monomers. In either case, we find that the two OECs of dark-adapted PSII dimers in crystals are not fully synchronized in the S1 state. Differences in redox states of the OEC in PSII only partially account for the observation that the electron densities integrate to a smaller number of electrons than expected. Differences between the determined and expected relative electron numbers are much larger than the estimated errors, indicating heterogeneity in the OEC composition.

Succinic acid inhibits photosynthesis of Microcystis aeruginosa via damaging PSII oxygen-evolving complex and reaction center.

To elucidate the mechanism of succinic acid (SA) inhibition of Microcystis aeruginosa, the chlorophyll fluorescence transients, photosynthesis, photosynthetic electron transport activity, and gene expression of M. aeruginosa were evaluated under various doses of SA. The results demonstrated that, after treatment with 60 mg L-1 SA for 1 h, the chlorophyll fluorescence transients and related parameters changed significantly, indicating that the function and structure of photosynthetic apparatuses of Microcystis were seriously damaged. The initial quantum efficiency α, maximum net photosynthetic rate Pnmax, dark respiration rate Rd, and gross photosynthetic rate decreased to 57%, 49%, 49%, and 46%, respectively, relative to the control. Furthermore, photosystem II (PSII) activity (H2O→p-BQ) and the electron transport activity of H2O→MV and DPC→MV significantly decreased. Real-time PCR analysis revealed that, following incubation with 60 mg L-1 SA for 24 h, the expression level of core protein genes (psbA, psaB, psbD, and psbO) of the photosynthesis centers photosystem I (PSI) and PSII decreased significantly. However, the transcription of gene nblA encoding phycobilisome degradation protein was elevated. The downregulation of the rbcL gene, which encodes the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), resulted in the suppression of CO2 fixation and assimilation. High concentration (60 mg L-1) of SA resulted in damage to oxygen-evolving complex (OEC) and reaction center of PSII, blocking photosynthetic electron transport, thereby lowering the rate photosynthesis and inhibiting the growth of Microcystis. We concluded that inhibition of photosynthesis is an important mechanism of SA inhibition in M. aeruginosa.

Psb35 Protein Stabilizes the CP47 Assembly Module and Associated High-Light Inducible Proteins during the Biogenesis of Photosystem II in the Cyanobacterium Synechocystis sp. PCC6803.

Photosystem II (PSII) is a large membrane protein complex performing primary charge separation in oxygenic photosynthesis. The biogenesis of PSII is a complicated process that involves a coordinated linking of assembly modules in a precise order. Each such module consists of one large chlorophyll (Chl)-binding protein, number of small membrane polypeptides, pigments and other cofactors. We isolated the CP47 antenna module from the cyanobacterium Synechocystis sp. PCC 6803 and found that it contains a 11-kDa protein encoded by the ssl2148 gene. This protein was named Psb35 and its presence in the CP47 module was confirmed by the isolation of FLAG-tagged version of Psb35. Using this pulldown assay, we showed that the Psb35 remains attached to CP47 after the integration of CP47 into PSII complexes. However, the isolated Psb35-PSIIs were enriched with auxiliary PSII assembly factors like Psb27, Psb28-1, Psb28-2 and RubA while they lacked the lumenal proteins stabilizing the PSII oxygen-evolving complex. In addition, the Psb35 co-purified with a large unique complex of CP47 and photosystem I trimer. The absence of Psb35 led to a lower accumulation and decreased stability of the CP47 antenna module and associated high-light-inducible proteins but did not change the growth rate of the cyanobacterium under the variety of light regimes. Nevertheless, in comparison with WT, the Psb35-less mutant showed an accelerated pigment bleaching during prolonged dark incubation. The results suggest an involvement of Psb35 in the life cycle of cyanobacterial Chl-binding proteins, especially CP47.

Development of broken-symmetry (BS) methods in chemical reactions. A theoretical view of water oxidation in photosystem II and related systems

Fundamental concepts and basic theories for the broken-symmetry (BS) methods have been reviewed in relation to theoretical elucidation and understanding of the mechanism for water oxidation in the oxygen evolving complex (OEC) of photosystem II (PSII). The HOMO-LUMO mixings by the BS method have provided the BS orbitals which are mainly localized on the metal and oxygen sites of the high-valent transition-metal oxo (MO) bonds, respectively. The oxyl-radical character (M–O) is responsible for radical reactivity such as the radical coupling in accord with various experimental results. The Lewis acids play important roles for reduction of the oxyl-radical character, indicating the participation of the water-coordinated Ca(II) ion of the CaMn3O4 cubane to stabilization of the Mn(V)=O…Ca(II) bond for essentially non-radical reactions. Several chemical indices have been calculated to elucidate the radical character for quantitative purpose. Implications of the computational results are discussed in relation to possible mechanisms of water oxidation in OEC of PSII.

Photochemistry and proteomics of mulberry (Morus alba L.) seedlings under NaCl and NaHCO3 stress

In order to explore the response and adaptation mechanisms of photosynthesis of the leaves of mulberry (Morus alba L.) seedlings to saline-alkali stress. Photosynthetic activity, and the response of related proteomics of M. alba seedling leaves under NaCl and NaHCO3 stress were studied by using chlorophyll fluorescence and gas exchange technique combined with TMT proteomics. The results showed that NaCl stress had no significant effect on photosystem II (PSII) activity in M. alba seedling leaves. In addition, the expressions of proteins of the PSII oxygen-evolving complex (OEE3-1 and PPD4) and the LHCII antenna (CP24 10A, CP26, and CP29) were increased, and the photosystem I (PSI) activity in the leaves of M. alba seedlings was increased, as well as expressions of proteins, such as PsaF, PsaG, PsaH, PsaL, PsaN, and Ycf4. Under NaHCO3 stress, the activity of PSII and PSI and the expression of their protein complexes and the electron transfer-related proteins significantly decreased. NaCl stress had little effect on RuBP regeneration during dark reaction in the leaves and the expressions of glucose synthesis related proteins and net photosynthetic rate (Pn) did not decrease significantly. The leaves could adapt to NaCl stress by reducing stomatal conductance (Gs) and increasing water use efficiency (WUE). Under NaHCO3 stress, the expression of dark reaction-related proteins was mostly down-regulated, while Gs was reduced, which indicated that non-stomatal factors can be responsible for inhibition of carbon assimilation.

Cationic penetrating antioxidants switch off Mn cluster of photosystem II in situ.

Mitochondria-targeted antioxidants (also known as 'Skulachev Ions' electrophoretically accumulated by mitochondria) exert anti-ageing and ROS-protecting effects well documented in animal and human cells. However, their effects on chloroplast in photosynthetic cells and corresponding mechanisms are scarcely known. For the first time, we describe a dramatic quenching effect of (10-(6-plastoquinonyl)decyl triphenylphosphonium (SkQ1) on chlorophyll fluorescence, apparently mediated by redox interaction of SkQ1 with Mn cluster in Photosystem II (PSII) of chlorophyte microalga Chlorella vulgaris and disabling the oxygen-evolving complex (OEC). Microalgal cells displayed a vigorous uptake of SkQ1 which internal concentration built up to a very high level. Using optical and EPR spectroscopy, as well as electron donors and in silico molecular simulation techniques, we found that SkQ1 molecule can interact with Mn atoms of the OEC in PSII. This stops water splitting giving rise to potent quencher(s), e.g. oxidized reaction centre of PSII. Other components of the photosynthetic apparatus proved to be mostly intact. This effect of the Skulachev ions might help to develop in vivo models of photosynthetic cells with impaired OEC function but essentially intact otherwise. The observed phenomenon suggests that SkQ1 can be applied to study stress-induced damages to OEC in photosynthetic organisms.

Theoretical and computational investigations of geometrical, electronic and spin structures of the CaMn4 OX (X = 5, 6) cluster in the Kok cycle Si (i = 0-3) of oxygen evolving complex of photosystem II.

The optimized geometries of the CaMn4 OX (X = 5, 6) cluster in the oxygen evolving complex (OEC) of photosystem II (PSII) by large-scale quantum mechanics (QM) and molecular mechanics (MM) calculations are compared with recent serial femtosecond crystallography (SFX) results for the Si (i = 0-3) states. The valence states of four Mn ions by the QM/MM calculations are also examined in relation to the experimental results by the X-ray emission spectroscopy (XES) for the Si intermediates. Geometrical and valence structures of right-opened Mn-hydroxide, Mn-oxo and Mn-peroxide intermediates in the S3 state are investigated in detail in relation to recent SFX and XES experiments for the S3 state. Interplay between theory and experiment indicates that the Mn-oxo intermediate is a new possible candidate for the S3 state. Implications of the computational results are discussed in relation to possible mechanisms of the oxygenoxygen bond formation for water oxidation in OEC of PSII.

Seasonal Changes in Primary Photosynthetic Events during Low Temperature Adaptation of Pinus sylvestris in Central Yakutia

The methods of chlorophyll fluorescence induction and HPLC were used to study the influence of autumnal temperature decrease on photochemical electron-transport activity of photosystem II (PSII), nonphotochemical quenching of excessive excitation energy, and the composition of pigments in the firstyear needles of Pinus sуlvestris L. trees grown naturally in Central Yakutia. In the period from the beginning of September to October 10, the chlorophyll content was reduced by half, while the Chl a/b ratio increased from 2.9 to 4.3–4.5, indicating the degradation of peripheral antenna complexes. The decrease in average daily temperature to 4.9–6.4°C led to a transient increase in the quantum yield of nonphotochemical quenching (ΔpH-dependent parameter φNPQ). These changes were accompanied by a slow accumulation of unregulated zeaxanthin fraction insensitive to illumination conditions. The further decrease in average daily temperature to near-zero levels was paralleled by a sharp increase in zeaxanthin content, while the pH-dependent quenching was replaced with the constitutive quenching (parameter φf,D) because of the supposed structural reorganization of PSII. These processes were accompanied by a fast decrease in PSII functional activity, which was mostly due to the impairment of plastoquinone photochemical reduction. Freezing temperatures (from–3.6 to–12.1) destructed the oxygen-evolving complex in PSII and completely inactivated the PSII reaction centers. It is concluded that the largest changes in the condition of photosynthetic apparatus occur at a near-zero temperature range and proceed until complete inactivation of PSII under the action of freezing temperatures.

Understanding Two Different Structures in the Dark Stable State of the Oxygen-Evolving Complex of Photosystem II: Applicability of the Jahn-Teller Deformation Formula.

Tanaka et al. (J. Am. Chem. Soc., 2017, 139, 1718) recently reported the three‐dimensional (3D) structure of the oxygen evolving complex (OEC) of photosystem II (PSII) by X‐ray diffraction (XRD) using extremely low X‐ray doses of 0.03 and 0.12 MGy. They observed two different 3D structures of the CaMn4O5 cluster with different hydrogen‐bonding interactions in the S1 state of OEC keeping the surrounding polypeptide frameworks of PSII the same. Our Jahn–Teller (JT) deformation formula based on large‐scale quantum mechanics/molecular mechanics (QM/MM) was applied for these low‐dose XRD structures, elucidating important roles of JT effects of the MnIII ion for subtle geometric distortions of the CaMn4O5 cluster in OEC of PSII. The JT deformation formula revealed the similarity between the low‐dose XRD and damage‐free serial femtosecond X‐ray diffraction (SFX) structures of the CaMn4O5 cluster in the dark stable state. The extremely low‐dose XRD structures were not damaged by X‐ray irradiation. Implications of the present results are discussed in relation to recent SFX results and a blue print for the design of artificial photocatalysts for water oxidation.

Effect of different methods of Ca2+ extraction from PSII oxygen-evolving complex on the QA- oxidation kinetics.

Lumenal extrinsic proteins PsbO, PsbP, and PsbQ of photosystem II (PSII) protect the catalytic cluster Mn4CaO5 of oxygen-evolving complex (OEC) from the bulk solution and from soluble compounds in the surrounding medium. Extraction of PsbP and PsbQ proteins by NaCl-washing together with chelator EGTA is followed also by the depletion of Ca2+ cation from OEC. In this study, the effects of PsbP and PsbQ proteins, as well as Ca2+ extraction from OEC on the kinetics of the reduced primary electron acceptor (QA-) oxidation, have been studied by fluorescence decay kinetics measurements in PSII membrane fragments. We found that in addition to the impairment of OEC, removal of PsbP and PsbQ significantly slows the rate of electron transfer from QA- to the secondary quinone acceptor QB. Electron transfer from QA- to QB in photosystem II membranes with an occupied QB site was slowed down by a factor of 8. However, addition of EGTA or CaCl2 to NaCl-washed PSII did not change the kinetics of fluorescence decay. Moreover, the kinetics of QA- oxidation by QB in Ca-depleted PSII membranes obtained by treatment with citrate buffer at pH 3.0 (such treatment keeps all extrinsic proteins in PSII but extracts Ca2+ from OEC) was not changed. The results obtained indicate that the effect of NaCl-washing on the QA- to QB electron transport is due to PsbP and PsbQ extrinsic proteins extraction, but not due to Ca2+ depletion.

On the guiding principles for understanding of geometrical structures of the CaMn4O5 cluster in oxygen-evolving complex of photosystem II. Proposal of estimation formula of structural deformations via the Jahn–Teller effects

ABSTRACTAtmospheric oxygenation and evolution of aerobic life on our earth are a result of water oxidation by oxygenic photosynthesis in photosystem II (PSII) of plants, algae and cyanobacteria. The water oxidation in the oxygen-evolving complex (OEC) in PSII is expected to proceed through five oxidation states, known as the Si (i = 0, 1, 2, 3 and 4) states in the Kok cycle, with the S1 being the most stable state in the dark. The OEC in PSII involves the active catalytic site made of four Mn ions and one Ca ion, namely the CaMn4O5 cluster. Past decades, molecular structures of the CaMn4O5 cluster in OEC in PSII have been investigated by the extended X-ray absorption fine structure (EXAFS). The magneto-structural correlations were extensively investigated by electron paramagnetic resonance (EPR) spectroscopy. Recently, Kamiya and Shen groups made great breakthrough for determination of the S1 structure of OEC of PSII by the X-ray diffraction (XRD) and X-ray free-electron laser (XFEL) experiments, providing structural foundations that are crucial for theoretical investigations of the CaMn4O5 cluster. Large-scale quantum mechanics/molecular mechanics calculations starting from the XRD structures elucidated geometrical, electronic and spin structures of the CaMn4O5 cluster, indicating an important role of the Jahn–Teller (JT) effect of Mn(III) ions. This paper presents theoretical formulas for estimation of the JT deformations of the CaMn4O5 cluster in OEC of PSII. Scope and applicability of the formulas are examined in relation to several different structures of the CaMn4O5 cluster proposed by XRD, XFEL, EXAFS and other experiments. Implications of the computational results are discussed for further refinements of geometrical parameters of the CaMn4O5 cluster.

Large-scale QM/MM calculations of the CaMn4O5 cluster in the S3 state of the oxygen evolving complex of photosystem II. Comparison between water-inserted and no water-inserted structures

Large-scale QM/MM calculations were performed to elucidate an optimized geometrical structure of a CaMn4O5 cluster with and without water insertion in the S3 state of the oxygen evolving complex (OEC) of photosystem II (PSII). The left (L)-opened structure was found to be stable under the assumption of no hydroxide anion insertion in the S3 state, whereas the right (R)-opened structure became more stable if one water molecule is inserted to the Mn4Ca cluster. The optimized Mna(4)-Mnd(1) distance determined by QM/MM was about 5.0 Å for the S3 structure without an inserted hydroxide anion, but this is elongated by 0.2-0.3 Å after insertion. These computational results are discussed in relation to the possible mechanisms of O-O bond formation in water oxidation by the OEC of PSII.