Mn4CaO5 Cluster Research Articles

Exploring reaction pathways for the structural rearrangements of the Mn cluster induced by water binding in the S3 state of the oxygen evolving complex of photosystem II

Photosynthetic oxidation of water to dioxygen is catalyzed by the Mn4CaO5 cluster in the protein-cofactor complex photosystem II. The light-driven catalytic cycle consists of four observable intermediates (S0, S1, S2, and S3) and one transient S4 state. Recently, using X-ray free-electron laser crystallography, two experimental groups independently observed incorporation of one additional oxygen into the cluster during the S2 to S3 transition, which is likely to represent a substrate. The present study implicates two competing reaction routes encountered during the structural rearrangement of the catalyst induced by the water binding and immediately preceding the formation of final stable forms in the S3 state. This mutually exclusive competition involves concerted versus stepwise conformational changes between two isomers, called open and closed cubane structures, which have different consequences on the immediate product in the S3 state. The concerted pathway involves a one-step conversion between two isomeric hydroxo forms without changes to the metal oxidation and total spin (Stotal = 3) states. Alternatively, in the stepwise process, the bound waters are oxidized and transformed into an oxyl–oxo form in a higher spin (Stotal = 6) state. Here, density functional calculations are used to characterize all relevant intermediates and transition structures and demonstrate that the stepwise pathway to the substrate activation is substantially favored over the concerted one, as evidenced by comparison of the activation barriers (11.1 and 20.9 kcal mol−1, respectively). Only after formation of the oxyl–oxo precursor can the hydroxo species be generated; this occurs with a slow kinetics and an activation barrier of 17.8 kcal mol−1. The overall thermodynamic driving force is likely to be controlled by the movements of two glutamate ligands, D1-Glu189 and CP43-Glu354, in the active site and ranges from very weak (+0.4 kcal mol−1) to very strong (–23.5 kcal mol−1).

PKa of the ligand water molecules in the oxygen-evolving Mn4CaO5 cluster in photosystem II

Release of the protons from the substrate water molecules is prerequisite for O2 evolution in photosystem II (PSII). Proton-releasing water molecules with low pKa values at the catalytic moiety can be the substrate water molecules. In some studies, one of the ligand water molecules, W2, is regarded as OH−. However, the PSII crystal structure shows neither proton acceptor nor proton-transfer pathway for W2, which is not consistent with the assumption of W2 = OH−. Here we report the pKa values of the four ligand water molecules, W1 and W2 at Mn4 and W3 and W4 at Ca2+, of the Mn4CaO5 cluster. pKa(W1) ≈ pKa(W2) << pKa(W3) ≈ pKa(W4) in the Mn4CaO5 cluster in water. However, pKa(W1) ≈ pKa(D1-Asp61) << pKa(W2) in the PSII protein environment. These results suggest that in PSII, deprotonation of W2 is energetically disfavored as far as W1 exists.

HYSCORE and DFT Studies of Proton-Coupled Electron Transfer in a Bioinspired Artificial Photosynthetic Reaction Center.

SummaryThe photosynthetic water-oxidation reaction is catalyzed by the oxygen-evolving complex in photosystem II (PSII) that comprises the Mn4CaO5 cluster, with participation of the redox-active tyrosine residue (YZ) and a hydrogen-bonded network of amino acids and water molecules. It has been proposed that the strong hydrogen bond between YZ and D1-His190 likely renders YZ kinetically and thermodynamically competent leading to highly efficient water oxidation. However, a detailed understanding of the proton-coupled electron transfer (PCET) at YZ remains elusive owing to the transient nature of its intermediate states involving YZ⋅. Herein, we employ a combination of high-resolution two-dimensional 14N hyperfine sublevel correlation spectroscopy and density functional theory methods to investigate a bioinspired artificial photosynthetic reaction center that mimics the PCET process involving the YZ residue of PSII. Our results underscore the importance of proximal water molecules and charge delocalization on the electronic structure of the artificial reaction center.

Acquirement of water-splitting ability and alteration of the charge-separation mechanism in photosynthetic reaction centers

In photosynthetic reaction centers from purple bacteria (PbRC) and the water-oxidizing enzyme, photosystem II (PSII), charge separation occurs along one of the two symmetrical electron-transfer branches. Here we report the microscopic origin of the unidirectional charge separation, fully considering electron-hole interaction, electronic coupling of the pigments, and electrostatic interaction with the polarizable entire protein environments. The electronic coupling between the pair of bacteriochlorophylls is large in PbRC, forming a delocalized excited state with the lowest excitation energy (i.e., the special pair). The charge-separated state in the active branch is stabilized by uncharged polar residues in the transmembrane region and charged residues on the cytochrome c2 binding surface. In contrast, the accessory chlorophyll in the D1 protein (ChlD1) has the lowest excitation energy in PSII. The charge-separated state involves ChlD1•+ and is stabilized predominantly by charged residues near the Mn4CaO5 cluster and the proceeding proton-transfer pathway. It seems likely that the acquirement of water-splitting ability makes ChlD1 the initial electron donor in PSII.

PH dependence of photosystem II photoinhibition: relationship with structural transition of oxygen-evolving complex at the pH of thylakoid lumen.

Ca-depleted photosystem II membranes (PSII[-Ca]) do not contain PsbP and PsbQ proteins protecting the Mn4CaO5 cluster of the PSII oxygen-evolving complex (OEC). Therefore, the Mn ions in the PSII(-Ca) membranes can be reduced by exogenous bulky reductants or the charged reductant Fe(II). We have recently found that the resistance of Mn ions in the OEC to the Fe(II) action is pH dependent and that this reductant is less effective at pH 5.7 than at pH 6.5 (Semin et al. J Photochem Photobiol B 178:192, 2018). Taking these data into account, we investigated the photoinhibition in different PSII membranes at pH 5.7 and 6.5 and found that the resistance to photoinhibition of PSII and PSII(-Ca) membranes with a Mn cluster is higher at pH 5.7 than at pH 6.5, whereas the resistance of the Mn-depleted PSII membranes is pH independent. In thylakoids, light generates the transmembrane ΔpH, leading to the acidulation of lumen that results in pH 5.7. The uncouplers (NH4Cl or nigericin) that significantly prevent acidulation increase the rate of PSII photoinhibition in thylakoids. We suggest that the structural transition in the OEC at pH 5.7 plays a role of a built-in mechanism increasing the resistance of OEC to photoinhibition under illumination, since it is accompanied by a pH decrease in lumen to 5.7. The coincidence of these pH values, i.e. lumen pH under illumination and pH of the maximal resistance of the Mn cluster to the reduction by reductants, can point at the pH-dependent mechanism of PSII self-protection from photoinactivation.

Structural Dynamics of a Protein Domain Relevant to the Water-Oxidizing Complex in Photosystem II as Visualized by High-Speed Atomic Force Microscopy.

Photosystem II (PSII) is a multiprotein complex that has a function of light-driven water oxidation. The catalytic site of water oxidation is the Mn4CaO5 cluster, which is bound to the lumenal side of PSII through amino acid residues from the D1 and CP43 proteins and is further surrounded by the extrinsic proteins. In this study, we have for the first time visualized the structural dynamics of the lumenal region of a PSII core complex using high-speed atomic force microscopy (HS-AFM). The HS-AFM images of a PSII membrane fragment showed stepwise dissociation of the PsbP and PsbO extrinsic proteins. Upon subsequent destruction of the Mn4CaO5 cluster, the lumenal domain of CP43 was found to undergo a conformational fluctuation. The observed structural flexibility and conformational fluctuation of the CP43 lumenal domain are suggested to play important roles in the biogenesis of PSII and the photoassembly of the Mn4CaO5 cluster.

Evaluation of new low-valent computational models for the oxygen-evolving complex of photosystem II

The oxidation states of the Mn4CaO5 cluster of the photosynthetic oxygen-evolving complex remain controversial. New quantum chemical models for the dark-stable S1 state suggest that a low-valent Mn(III-IV-III-II) form, opposed to the commonly accepted high-valent Mn(III-IV-IV-III) assignment, agrees with available structural data. We examine the magnetic properties of the models and the consequences for electronic structure and catalytic progression of assuming a neutral second-sphere residue D1-His337. The low-valent model reproduces the experimental diamagnetic ground spin-state. However, the protonation assignment introduces a critical flaw because neutral His337 becomes itself the locus of subsequent oxidation, in contradiction to physiological catalytic progression.

Probing the role of arginine 323 of the D1 protein in photosystem II function.

The Mn4 CaO5 cluster of photosystem II (PSII) advances sequentially through five oxidation states (S0 to S4 ). Under the enzyme cycle, two water molecules are oxidized, O2 is generated and four protons are released into the lumen. Umena et al. (2011) have proposed that, with other charged amino acids, the R323 residue of the D1 protein could contribute to regulate a proton egress pathway from the Mn4 CaO5 cluster and TyrZ via a proton channel identified from the 3D structure. To test this suggestion, a PsbA3/R323E site-directed mutant has been constructed and the properties of its PSII have been compared to those of the PsbA3-PSII by using EPR spectroscopy, polarography, thermoluminescence and time-resolved UV-visible absorption spectroscopy. Neither the oscillations with a period four nor the kinetics and S-state-dependent stoichiometry of the proton release were affected. However, several differences have been found: (1) the P680 + decay in the hundreds of ns time domain was much slower in the mutant, (2) the S2 QA - /DCMU and S3 QA - /DCMU radiative charge recombination occurred at higher temperatures and (3) the S0 TyrZ • , S1 TyrZ • , S2 TyrZ • split EPR signals induced at 4.2 K by visible light from the S0 TyrZ , S1 TyrZ , S2 TyrZ , respectively, and the (S2 TyrZ • )' induced by NIR illumination at 4.2 K of the S3 TyrZ state differed. It is proposed that the R323 residue of the D1 protein interacts with TyrZ likely via the H-bond network previously proposed to be a proton channel. Therefore, rather than participating in the egress of protons to the lumen, this channel could be involved in the relaxations of the H-bonds around TyrZ by interacting with the bulk, thus tuning the driving force required for TyrZ oxidation.

Role of PsbV-Tyr137 in photosystem II studied by site-directed mutagenesis in the thermophilic cyanobacterium Thermosynechococcus vulcanus.

PsbV (cytochrome c550) is one of the three extrinsic proteins of photosystem II (PSII) and functions to maintain the stability and activity of the Mn4CaO5 cluster, the catalytic center for water oxidation. PsbV-Y137 is the C-terminal residue of PsbV and is located at the exit of a hydrogen-bond network mediated by the D1-Y161-H190 residue pair. In order to examine the function of PsbV-Y137, four mutants, PsbV-Y137A, PsbV-Y137F, PsbV-Y137G, and PsbV-Y137W, were generated with Thermosynechococcus vulcanus (T. vulcanus). These mutants showed growth rates similar to that of the wild-type strain (WT); however, their oxygen-evolving activities were different. At pH 6.5, the oxygen evolution rates of Y137F and Y137W were almost identical to that of WT, whereas the oxygen evolution rates of the Y137A, Y137G mutants were 64% and 61% of WT, respectively. However, the oxygen evolution in the latter two mutants decreased less at higher pHs, suggesting that higher pHs facilitated oxygen evolution probably by facilitating proton egress in these two mutants. Furthermore, thylakoid membranes isolated from the PsbV-Y137A, PsbV-Y137G mutants exhibited much lower levels of oxygen-evolving activity than that of WT, which was found to be caused by the release of PsbV. In addition, PSII complexes purified from the PsbV-Y137A and PsbV-Y137G mutants lost all of the three extrinsic proteins but instead bind Psb27, an assembly cofactor of PSII. These results demonstrate that the PsbV-Tyr137 residue is required for the stable binding of PsbV to PSII, and the hydrogen-bond network mediated by D1-Y161-H190 is likely to function in proton egress during water oxidation.

Determining the Activation Energy of Photoassembly of the Oxygen‐evolving Complex Using an Alternative Redox Mediator

The oxygen‐evolving complex (OEC) is a well‐researched Mn4CaO5 cluster with a cubane‐like structure that is responsible for oxidizing water to fuel photosynthetic electron transfer. It is a cofactor of photosystem II (PSII), which catalyzes light‐induced charge separation and oxidation of the OEC. PSII is comprised of pigment molecules and cofactors that transport electrons to a mobile plastoquinone that is located at the QB site. These reducing equivalents can eventually be used to reduce CO2 into simple sugars. In nature, the OEC is constantly degraded as a result of PSII photodamage and must be reassembled through a light‐dependent process after the affected PSII is repaired. OEC assembly occurs in the presence of light in vivo, but if conducted in situ is significantly enhanced by including a dark incubation after the first light dependent step. This dark‐dependent step is the rate‐limiting step of OEC assembly. Determining the activation energy will provide insight into the mechanism of OEC formation and provide clues on how artificial systems could be engineered.To conduct these studies, subcellular thylakoid membranes were isolated and enriched in PSII and the Mn4CaO5 cluster was removed via an alkaline incubation to generate apo‐PSII. The apo‐PSII enriched membranes were then used to measure the efficiency of OEC assembly in situ at temperatures ranging from 5 – 45 °C using Clark type oximetry. However, when OEC assembly occurs in apo‐PSII‐enriched membranes the rate‐determining step is electron shuttling from the QB site to the terminal electron acceptor {K3[Fe(CN)6]} via the native plastoquinones, thus preventing an accurate determination of the activation energy. This issue was remedied by supplementing with an alternative redox mediator know as phenyl‐p‐benzoquinone (PPBQ), which has been shown to increase the rate of electron shuttling from the QB site to a terminal electron acceptor. OEC photoassembly was successful in the presence of PPBQ and the oxygen production at varying temperatures was used to produce an Arrhenius plot to calculate the activation energy of OEC assembly.Currently, the activation energies have been determined for photoassembled PSII‐enriched membranes in the presence of PPBQ (18 kJ/mol) and native plastoquinones (49 kJ/mol). From these results, we speculate that OEC photoassembly in apo‐PSII membranes with PPBQ allows us to measure the true rate‐limiting step.Support or Funding InformationE.N. was supported by a fellowship from the SURE Program funded through the WV Research Challenge Fund, and administered by the WV HEPC, Division of Science and Research, Award Number DSR.17.09. E.N. had additional support from the WV NASA Space Grant Consortium’s NASA WVSG Undergraduate Fellowship Program and from the DOW Undergraduate Research Fellowship.D.K. and A.S. were supported by the National Science Foundation under Cooperative Agreement No. OIA‐1458952. Amanda Smythers was also funded by the NASA West Virginia Space Grant Consortium, Grant #NNX15AK74A and Training Grant #NNX15AI01H.J.B. had additional support from the WV NASA Space Grant Consortium’s NASA WVSG Undergraduate Fellowship Program and from the DOW Undergraduate Research Fellowship.

PH-Dependent Protonation of Surface Carboxylate Groups in PsbO Enables Local Buffering and Triggers Structural Changes.

Photosystem II (PSII) catalyzes the splitting of water, releasing protons and dioxygen. Its highly conserved subunit PsbO extends from the oxygen‐evolving center (OEC) into the thylakoid lumen and stabilizes the catalytic Mn4CaO5 cluster. The high degree of conservation of accessible negatively charged surface residues in PsbO suggests additional functions, as local pH buffer or by affecting the flow of protons. For this discussion, we provide an experimental basis, through the determination of pK a values of water‐accessible aspartate and glutamate side‐chain carboxylate groups by means of NMR. Their distribution is strikingly uneven, with high pK a values around 4.9 clustered on the luminal PsbO side and values below 3.5 on the side facing PSII. pH‐dependent changes in backbone chemical shifts in the area of the lumen‐exposed loops are observed, indicating conformational changes. In conclusion, we present a site‐specific analysis of carboxylate group proton affinities in PsbO, providing a basis for further understanding of proton transport in photosynthesis.

Theory of chemical bonds in metalloenzymes XXIII fundamental principles for the photo-induced water oxidation in oxygen evolving complex of photosystem II

Molecular quantum mechanics (MQM) investigations have been performed for elucidation of fundamental principles of the photo-induced water oxidation in oxygen evolving complex (OEC) of photosystem II (PSII). To this end, as a first theoretical step, broken symmetry (BS) quantum mechanics (QM) and QM(BS)/molecular mechanics (MM) calculations have been conducted for elucidation of geometrical, electronic and spin structures of the CaMn4Ox (X = 5, 6) cluster in the five steps Si (i = 0∼4) of the Kok cycle for water oxidation. The QM and QM/MM calculations have provided full optimised geometries of short-lived key intermediates and transition state structure for the O-O bond formation in the native solar-energy conversion. The interplay between theory and experiment have clearly indicated that the CaMn4O5 cluster in OEC of PSII exhibits typical physicochemical properties of strong correlation electron system (SCES) confined with effective protein field. Our QM and QM/MM computational results for key intermediates and transition structure for the O-O bond formation in the Kok cycle are now plentiful for derivation of fundamental principles (FP) for understanding of photo-induced water oxidation in OEC of PSII. We summarize twenty nine fundamental principles (FPs) in systematic manner by QM and QM/MM calculations for understanding of water oxidation in Oxygen Evolving Complex(OEC) of Photosystem II(PSII). One of our final aims is theoretical design of the next-generation artificial photosystem materials composed of abundant metal ions.

Role of the O4 Channel in Photosynthetic Water Oxidation as Revealed by Fourier Transform Infrared Difference and Time-Resolved Infrared Analysis of the D1-S169A Mutant.

Photosynthetic water oxidation takes place at the Mn4CaO5 cluster in photosystem II. Although the atomic structures of its intermediates called S states have recently been reported, the catalytic mechanism of water oxidation has not been well understood. Here, to investigate the involvement of the O4 site of the Mn4CaO5 cluster and a water channel from O4 in the water oxidation reaction, we examined the effects of D1-S169A mutation, which perturbs the interaction of a water molecule hydrogen-bonded with O4, by thermoluminescence (TL), Fourier transform infrared (FTIR) difference, and time-resolved infrared (TRIR) measurements. The observed upshifts of TL peaks and some changes in FTIR spectra upon S169A mutation revealed the perturbations of the redox potential of the Mn4CaO5 cluster and the interactions of the surrounding hydrogen bond network. In contrast, FTIR oscillation patterns and TRIR traces showed only minor effects of the mutation on the efficiencies and kinetics of individual S-state transitions. It was thus concluded that the O4 site plays a role in retaining the redox potential and the structure of the hydrogen bond network, whereas it is unlikely to be directly involved in the catalytic reaction of substrate water except for proton transfer through the O4 water chain.

Recent Advances in First‐Row Transition Metal Clusters for Photocatalytic Water Splitting

AbstractWater splitting based on first‐row transition metal (3d) photocatalysts is a cost‐effective technology for the conversion of abundant solar energy into useful energy on a large scale. The catalytic core of photosynthetic enzymes is a cubic CaMn4O5 cluster. Illuminated by natural photosynthesis, artificial solar water splitting photocatalysts composed of metal clusters are now being designed and tested. Ideally, such photocatalysts composed of atomically precise metal clusters with regular crystal structures are promising model catalysts that can be used to study structure–performance relationships. Recent advances based on metal cluster designs have improved our understanding of photoinduced charge separation and catalytic water redox reactions. This Minireview summarizes the recent advances in 3d metal cluster photocatalysts with well‐defined structures for photocatalytic water splitting and focuses on different clusters with tunable structures that are capable of achieving better photocatalytic properties.

Assessment of the manganese cluster’s oxidation state via photoactivation of photosystem II microcrystals

Knowledge of the manganese oxidation states of the oxygen-evolving Mn4CaO5 cluster in photosystem II (PSII) is crucial toward understanding the mechanism of biological water oxidation. There is a 4 decade long debate on this topic that historically originates from the observation of a multiline electron paramagnetic resonance (EPR) signal with effective total spin of S = 1/2 in the singly oxidized S2 state of this cluster. This signal implies an overall oxidation state of either Mn(III)3Mn(IV) or Mn(III)Mn(IV)3 for the S2 state. These 2 competing assignments are commonly known as "low oxidation (LO)" and "high oxidation (HO)" models of the Mn4CaO5 cluster. Recent advanced EPR and Mn K-edge X-ray spectroscopy studies converge upon the HO model. However, doubts about these assignments have been voiced, fueled especially by studies counting the number of flash-driven electron removals required for the assembly of an active Mn4CaO5 cluster starting from Mn(II) and Mn-free PSII. This process, known as photoactivation, appeared to support the LO model since the first oxygen is reported to evolve already after 7 flashes. In this study, we improved the quantum yield and sensitivity of the photoactivation experiment by employing PSII microcrystals that retained all protein subunits after complete manganese removal and by oxygen detection via a custom built thin-layer cell connected to a membrane inlet mass spectrometer. We demonstrate that 9 flashes by a nanosecond laser are required for the production of the first oxygen, which proves that the HO model provides the correct description of the Mn4CaO5 cluster's oxidation states.

Redox Potential of the Oxygen-Evolving Complex in the Electron Transfer Cascade of Photosystem II.

In photosystem II (PSII), water oxidation occurs in the Mn4CaO5 cluster with the release of electrons via the redox-active tyrosine (TyrZ) to the reaction-center chlorophylls (PD1/PD2). Using a quantum mechanical/molecular mechanical approach, we report the redox potentials (Em) of these cofactors in the PSII protein environment. The Em values suggest that the Mn4CaO5 cluster, TyrZ, and PD1/PD2 form a downhill electron transfer pathway. Em for the first oxidation step, Em(S0/S1), is uniquely low (730 mV) and is ∼100 mV lower than that for the second oxidation step, Em(S1/S2) (830 mV) only when the O4 site of the Mn4CaO5 cluster is protonated in S0. The O4-water chain, which directly forms a low-barrier H-bond with the Mn4CaO5 cluster and mediates proton-coupled electron transfer in the S0 to S1 transition, explains why the second lowest oxidation state, S1, is the most stable and S0 is converted to S1 even in the dark.

Fourier transform infrared and mass spectrometry analyses of a site-directed mutant of D1-Asp170 as a ligand to the water-oxidizing Mn4CaO5 cluster in photosystem II

The Mn4CaO5 cluster, the catalytic center of water oxidation in photosystem II (PSII), is coordinated by six carboxylate and one imidazole ligands. The roles of these ligands in the water oxidation mechanism remain largely unknown. In this study, we constructed a D1-D170H mutant, in which the Asp ligand bridging Mn and Ca ions was replaced with His, in the cyanobacterium Synechocystis sp. PCC 6803, and analyzed isolated PSII core complexes using Fourier transform infrared (FTIR) difference spectroscopy and mass spectrometry (MS). The S2-minus-S1 FTIR difference spectrum of the PSII complexes of the D1-D170H mutant showed features virtually identical to those of the wild-type PSII. MS analysis further showed that ~70% of D1 proteins from the PSII complexes of D1-D170H possessed the wild-type amino acid sequence, although only the mutated sequence was detected in genomic DNA in the same batch of cells for PSII preparations. In contrast, a D1-S169A mutant as a control showed a modified FTIR spectrum and only a mutated D1 protein. It is thus concluded that the FTIR spectrum of the D1-D170H mutant actually reflects that of wild-type PSII, whereas the Mn4CaO5 cluster is not formed in PSII with D1-D170H mutation. Although the mechanism of production of the wild-type D1 protein in the D1-D170H mutant is unknown at present, a caution is necessary in the analysis of site-directed mutants of crucial residues in the D1 protein, and mutation has to be confirmed not only at the DNA level but also at the amino acid level.

Does the water-oxidizing Mn4CaO5 cluster regulate the redox potential of the primary quinone electron acceptor QA in photosystem II? A study by Fourier transform infrared spectroelectrochemistry

Redox titration using fluorescence measurements of photosystem II (PSII) has long shown that impairment of the water-oxidizing Mn4CaO5 cluster upshifts the redox potential (Em) of the primary quinone electron acceptor QA by more than 100 mV, which has been proposed as a photoprotection mechanism of PSII. However, the molecular mechanism of this long-distance interaction between the Mn4CaO5 cluster and QA in PSII remains unresolved. In this study, we reinvestigated the effect of depletion of the Mn4CaO5 cluster on Em(QA−/QA) using Fourier transform infrared (FTIR) spectroelectrochemistry, which can directly monitor the redox state of QA at an intended potential. Light-induced FTIR difference measurements at a series of electrode potentials for intact and Mn-depleted PSII preparations from spinach and Thermosynechococcus elongatus showed that depletion of the Mn4CaO5 cluster hardly affected the Em(QA−/QA) values. In contrast, fluorescence spectroelectrochemical measurement using the same PSII sample, electrochemical cell, and redox mediators reproduced a large upshift of apparent Em upon Mn depletion, whereas a smaller shift was observed when weaker visible light was used for fluorescence excitation. Thus, the possibility was suggested that the measuring light for fluorescence disturbed the titration curve in Mn-depleted PSII, in contrast to no interference of infrared light with the PSII reactions in FTIR measurements. From these results, it was concluded that the Mn4CaO5 cluster does not directly regulate Em(QA−/QA) to control the redox reactions on the electron acceptor side of PSII.

Bound manganese oxides capable of reducing the bacteriochlorophyll dimer of modified reaction centers from Rhodobacter sphaeroides

A biohybrid model system is described that interfaces synthetic Mn-oxides with bacterial reaction centers to gain knowledge concerning redox reactions by metal clusters in proteins, in particular the Mn4CaO5 cluster of photosystem II. The ability of Mn-oxides to bind to modified bacterial reaction centers and transfer an electron to the light-induced oxidized bacteriochlorophyll dimer, P+, was characterized using optical spectroscopy. The environment of P was altered to obtain a high P/P+ midpoint potential. In addition, different metal-binding sites were introduced by substitution of amino acid residues as well as extension of the C-terminus of the M subunit with the C-terminal region of the D1 subunit of photosystem II. The Mn-compounds MnO2, αMn2O3, Mn3O4, CaMn2O4, and Mn3(PO4)2 were tested and compared to MnCl2. In general, addition of the Mn-compounds resulted in a decrease in the amount of P+ while the reduced quinone was still present, demonstrating that the Mn-compounds can serve as secondary electron donors. The extent of P+ reduction for the Mn-oxides was largest for αMn2O3 and CaMn2O4 and smallest for Mn3O4 and MnO2. The addition of Mn3(PO4)2 resulted in nearly complete P+ reduction, similar to MnCl2. Overall, the activity was correlated with the initial oxidation state of the Mn-compound. Transient optical measurements showed a fast kinetic component, assigned to reduction of P+ by the Mn-oxide, in addition to a slow component due to charge recombination. The results support the conjecture that the incorporation of Mn-oxides by ancient anoxygenic phototrophs was a step in the evolution of oxygenic photosynthesis.

Inspecting S states in photosynthesis

Water Oxidation Oxygenic photosynthesis uses a Mn4CaO5 cluster in the oxygen-evolving complex to extract electrons from water and produce dioxygen. Visualizing each of the chemical states in this process, S to S4, and assigning chemical identities and mechanisms on the basis of structures has been a challenge addressed recently by work at x-ray free-electron lasers. Suga et al. used serial crystallography at cryogenic temperatures to trap and determine the structures of several stable states during photosystem II water oxidation (see the Perspective by Britt and Marchiori). Changes around the water cluster already happen in the S2 state and set the stage for water insertion that occurs during transition to the S3 state. A short 1.9-angstrom distance between the two oxygen atoms in the S3 state is consistent with theoretical studies supporting an oxyl/oxo mechanism for oxygen-oxygen coupling. Science , this issue p. [334][1]; see also p. [305][2] [1]: /lookup/doi/10.1126/science.aax6998 [2]: /lookup/doi/10.1126/science.aaz4522