Biosynthetic Ca2+/Sr2+ Exchange in the Photosystem II Oxygen-evolving Enzyme of Thermosynechococcus elongatus

Insights from Ca2+→Sr2+ substitution on the mechanism of O-O bond formation in photosystem II

In recent years, there has been a steady interest in unraveling the intricate mechanistic details of water oxidation mechanism in photosynthesis. Despite the substantial progress made over several decades, a comprehensive understanding of the precise kinetics underlying O-O bond formation and subsequent evolution remains elusive. However, it is well-established that the oxygen evolving complex (OEC), specifically the CaMn4O5 cluster, plays a crucial role in O-O bond formation, undergoing a series of four oxidative events as it progresses through the S-states of the Kok cycle. To gain further insights into the OEC, researchers have explored the substitution of the Ca2+ cofactor with strontium (Sr), the sole atomic replacement capable of retaining oxygen-evolving activity. Empirical investigations utilizing spectroscopic techniques such as XAS, XRD, EPR, FTIR, and XANES have been conducted to probe the structural consequences of Ca2+→Sr2+ substitution. In parallel, the development of DFT and QM/MM computational models has explored different oxidation and protonation states, as well as variations in ligand coordination at the catalytic center involving amino acid residues. In this review, we critically evaluate and integrate these computational and spectroscopic approaches, focusing on the structural and mechanistic implications of Ca2+→Sr2+ substitution in PS II. We contribute DFT modelling and simulate EXAFS Fourier transforms of Sr-substituted OEC, analyzing promising structures of the S3 state. Through the combination of computational modeling and spectroscopic investigations, valuable insights have been gained, developing a deeper understanding of the photosynthetic process.

Thermophilic cyanobacteria-exciting, yet challenging biotechnological chassis

Thermophilic cyanobacteria are prokaryotic photoautotrophic microorganisms capable of growth between 45 and 73 °C. They are typically found in hot springs where they serve as essential primary producers. Several key features make these robust photosynthetic microbes biotechnologically relevant. These are highly stable proteins and their complexes, the ability to actively transport and concentrate inorganic carbon and other nutrients, to serve as gene donors, microbial cell factories, and sources of bioactive metabolites. A thorough investigation of the recent progress in thermophilic cyanobacteria reveals a significant increase in the number of newly isolated and delineated organisms and wide application of thermophilic light-harvesting components in biohybrid devices. Yet despite these achievements, there are still deficiencies at the high-end of the biotechnological learning curve, notably in genetic engineering and gene editing. Thermostable proteins could be more widely employed, and an extensive pool of newly available genetic data could be better utilised. In this manuscript, we attempt to showcase the most important recent advances in thermophilic cyanobacterial biotechnology and provide an overview of the future direction of the field and challenges that need to be overcome before thermophilic cyanobacterial biotechnology can bridge the gap with highly advanced biotechnology of their mesophilic counterparts. KEY POINTS: • Increased interest in all aspects of thermophilic cyanobacteria in recent years • Light harvesting components remain the most biotechnologically relevant • Lack of reliable molecular biology tools hinders further development of the chassis.

Assignment of the slowly exchanging substrate water of nature's water-splitting cofactor

Identifying the two substrate water sites of nature's water-splitting cofactor (Mn4CaO5 cluster) provides important information toward resolving the mechanism of O-O bond formation in Photosystem II (PSII). To this end, we have performed parallel substrate water exchange experiments in the S1 state of native Ca-PSII and biosynthetically substituted Sr-PSII employing Time-Resolved Membrane Inlet Mass Spectrometry (TR-MIMS) and a Time-Resolved 17O-Electron-electron Double resonance detected NMR (TR-17O-EDNMR) approach. TR-MIMS resolves the kinetics for incorporation of the oxygen-isotope label into the substrate sites after addition of H218O to the medium, while the magnetic resonance technique allows, in principle, the characterization of all exchangeable oxygen ligands of the Mn4CaO5 cofactor after mixing with H217O. This unique combination shows i) that the central oxygen bridge (O5) of Ca-PSII core complexes isolated from Thermosynechococcus vestitus has, within experimental conditions, the same rate of exchange as the slowly exchanging substrate water (WS) in the TR-MIMS experiments and ii) that the exchange rates of O5 and WS are both enhanced by Ca2+→Sr2+ substitution in a similar manner. In the context of previous TR-MIMS results, this shows that only O5 fulfills all criteria for being WS. This strongly restricts options for the mechanism of water oxidation.

Interplay of two low-barrier hydrogen bonds in long-distance proton-coupled electron transfer for water oxidation

D1-Tyr161 (TyrZ) forms a low-barrier H-bond with D1-His190 and functions as a redox-active group in photosystem II. When oxidized to the radical form (TyrZ-O•), it accepts an electron from the oxygen-evolving Mn4CaO5 cluster, facilitating an increase in the oxidation state (Sn; n = 0-3). In this study, we investigated the mechanism of how TyrZ-O• drives proton-coupled electron transfer during the S2 to S3 transition using a quantum mechanical/molecular mechanical approach. In response to TyrZ-O• formation and subsequent loss of the low-barrier H-bond, the ligand water molecule at the Ca2+ site (W4) reorients away from TyrZ and donates an H-bond to D1-Glu189 at Mn4 of Mn4CaO5 together with an adjacent water molecule. The H-bond donation to the Mn4CaO5 cluster triggers the release of the proton from the lowest pKa site (W1 at Mn4) along the W1…D1-Asp61 low-barrier H-bond, leading to protonation of D1-Asp61. The interplay of the two low-barrier H-bonds, involving the Ca2+ interface and forming the extended Grotthuss-like network [TyrZ…D1-His190]-[Mn4CaO5]-[W1…D1-Asp61], rather than the direct electrostatic interaction, is likely a basis of the apparent long-distance interaction (11.4 Å) between TyrZ-O• formation and D1-Asp61 protonation.

Structural and energetic insights into Mn-to-Fe substitution in the oxygen-evolving complex

Manganese (Mn) serves as the catalytic center for water splitting in photosystem II (PSII), despite the abundance of iron (Fe) on earth. As a first step toward why Mn and not Fe is employed by Nature in the water oxidation catalyst, we investigated the Fe₄CaO₅ cluster in the PSII protein environment using a quantum mechanical/molecular mechanical (QM/MM) approach, assuming an equivalence between Mn(III/IV) and Fe(II/III). Substituting Mn with Fe resulted in the protonation of μ-oxo bridges at sites O2 and O3 by Arg357 and D1-His337, respectively. While the Mn₄CaO₅ cluster exhibits distinct open- and closed-cubane S₂ conformations, the Fe₄CaO₅ cluster lacks this variability due to an equal spin distribution over sites Fe1 and Fe4. The absence of a low-barrier H-bond between a ligand water molecule (W1) and D1-Asp61 in the Fe₄CaO₅ cluster may underlie its incapability for ligand water deprotonation, highlighting the relevance of Mn in natural water splitting.

Predicting the oxidation states of Mn ions in the oxygen-evolving complex of photosystem II using supervised and unsupervised machine learning

Serial Femtosecond Crystallography at the X-ray Free Electron Laser (XFEL) sources enabled the imaging of the catalytic intermediates of the oxygen evolution reaction of Photosystem II (PSII). However, due to the incoherent transition of the S-states, the resolved structures are a convolution from different catalytic states. Here, we train Decision Tree Classifier and K-means clustering models on Mn compounds obtained from the Cambridge Crystallographic Database to predict the S-state of the X-ray, XFEL, and CryoEM structures by predicting the Mn's oxidation states in the oxygen-evolving complex. The model agrees mostly with the XFEL structures in the dark S₁ state. However, significant discrepancies are observed for the excited XFEL states (S₂, S_3, and S₀) and the dark states of the X-ray and CryoEM structures. Furthermore, there is a mismatch between the predicted S-states within the two monomers of the same dimer, mainly in the excited states. We validated our model against other metalloenzymes, the valence bond model and the Mn spin densities calculated using density functional theory for two of the mismatched predictions of PSII. The model suggests designing a more optimized sample delivery and illumiation systems are crucial to precisely resolve the geometry of the advanced S-states to overcome the noncoherent S-state transition. In addition, significant radiation damage is observed in X-ray and CryoEM structures, particularly at the dangler Mn center (Mn4). Our model represents a valuable tool for investigating the electronic structure of the catalytic metal cluster of PSII to understand the water splitting mechanism.

Substitution of Ca2+ and changes in the H-bond network near the oxygen-evolving complex of photosystem II

Ca²⁺, which provides binding sites for ligand water molecules W3 and W4 in the Mn₄CaO₅ cluster, is a prerequisite for O₂ evolution in photosystem II (PSII). We report structural changes in the H-bond network and the catalytic cluster itself upon the replacement of Ca²⁺ with other alkaline earth metals, using a quantum mechanical/molecular mechanical approach. The small radius of Mg²⁺ makes W3 donate an H-bond to D1-Glu189 in Mg²⁺-PSII. If an additional water molecule binds at the large surface of Ba²⁺, it donates H-bonds to D1-Glu189 and the ligand water molecule at the dangling Mn, altering the H-bond network. The potential energy profiles of the H-bond between D1-Tyr161 (TyrZ) and D1-His190 and the interconversion between the open- and closed-cubane S₂ conformations remain substantially unaltered upon the replacement of Ca²⁺. Remarkably, the O5⋯Ca²⁺ distance is shortest among all O5⋯metal distances irrespective of the radius being larger than that of Mg²⁺. Furthermore, Ca²⁺ is the only alkaline earth metal that equalizes the O5⋯metal and O2⋯metal distances and facilitates the formation of the symmetric cubane structure.

Induction of cellulase production by Sr2+ in Trichoderma reesei via calcium signaling transduction

Trichoderma reesei RUT-C30 is a well-known high-yielding cellulase-producing fungal strain that converts lignocellulose into cellulosic sugar for resource regeneration. Calcium is a ubiquitous secondary messenger that regulates growth and cellulase production in T. reesei. We serendipitously found that adding Sr2+ to the medium significantly increased cellulase activity in the T. reesei RUT-C30 strain and upregulated the expression of cellulase-related genes. Further studies showed that Sr2+ supplementation increased the cytosolic calcium concentration and activated the calcium-responsive signal transduction pathway of Ca2+-calcineurin-responsive zinc finger transcription factor 1 (CRZ1). Using the plasma membrane Ca2+ channel blocker, LaCl3, we demonstrated that Sr2+ induces cellulase production via the calcium signaling pathway. Supplementation with the corresponding concentrations of Sr2+ also inhibited colony growth. Sr2+ supplementation led to an increase in intracellular reactive oxygen species (ROS) and upregulated the transcriptional levels of intracellular superoxide dismutase (sod1) and catalase (cat1). We further demonstrated that ROS content was detrimental to cellulase production, which was alleviated by the ROS scavenger N-acetyl cysteine (NAC). This study demonstrated for the first time that Sr2+ supplementation stimulates cellulase production and upregulates cellulase genes via the calcium signaling transduction pathway. Sr2+ leads to an increase in intracellular ROS, which is detrimental to cellulase production and can be alleviated by the ROS scavenger NAC. Our results provide insights into the mechanistic study of cellulase synthesis and the discovery of novel inducers of cellulase.

Mimicking the Oxygen-Evolving Center in Photosynthesis

The oxygen-evolving center (OEC) in photosystem II (PSII) of oxygenic photosynthetic organisms is a unique heterometallic-oxide Mn4CaO5-cluster that catalyzes water splitting into electrons, protons, and molecular oxygen through a five-state cycle (Sn, n = 0 ~ 4). It serves as the blueprint for the developing of the man-made water-splitting catalysts to generate solar fuel in artificial photosynthesis. Understanding the structure-function relationship of this natural catalyst is a great challenge and a long-standing issue, which is severely restricted by the lack of a precise chemical model for this heterometallic-oxide cluster. However, it is a great challenge for chemists to precisely mimic the OEC in a laboratory. Recently, significant advances have been achieved and a series of artificial Mn4XO4-clusters (X = Ca/Y/Gd) have been reported, which closely mimic both the geometric structure and the electronic structure, as well as the redox property of the OEC. These new advances provide a structurally well-defined molecular platform to study the structure-function relationship of the OEC and shed new light on the design of efficient catalysts for the water-splitting reaction in artificial photosynthesis.

From manganese oxidation to water oxidation: assembly and evolution of the water-splitting complex in photosystem II

The manganese cluster of photosystem II has been the focus of intense research aiming to understand the mechanism of H₂O-oxidation. Great effort has also been applied to investigating its oxidative photoassembly process, termed photoactivation that involves the light-driven incorporation of metal ions into the active Mn₄CaO₅ cluster. The knowledge gained on these topics has fundamental scientific significance, but may also provide the blueprints for the development of biomimetic devices capable of splitting water for solar energy applications. Accordingly, synthetic chemical approaches inspired by the native Mn cluster are actively being explored, for which the native catalyst is a useful benchmark. For both the natural and artificial catalysts, the assembly process of incorporating Mn ions into catalytically active Mn oxide complexes is an oxidative process. In both cases this process appears to share certain chemical features, such as producing an optimal fraction of open coordination sites on the metals to facilitate the binding of substrate water, as well as the involvement of alkali metals (e.g., Ca²⁺) to facilitate assembly and activate water-splitting catalysis. This review discusses the structure and formation of the metal cluster of the PSII H₂O-oxidizing complex in the context of what is known about the formation and chemical properties of different Mn oxides. Additionally, the evolutionary origin of the Mn₄CaO₅ is considered in light of hypotheses that soluble Mn²⁺ was an ancient source of reductant for some early photosynthetic reaction centers ('photomanganotrophy'), and recent evidence that PSII can form Mn oxides with structural resemblance to the geologically abundant birnessite class of minerals. A new functional role for Ca²⁺ to facilitate sustained Mn²⁺ oxidation during photomanganotrophy is proposed, which may explain proposed physiological intermediates during the likely evolutionary transition from anoxygenic to oxygenic photosynthesis.

Photosystem 2 and the oxygen evolving complex: a brief overview

These special issues of photosynthesis research present papers documenting progress in revealing the many aspects of photosystem 2, a unique, one-of-a-kind complex system that can reduce a plastoquinone to a plastoquinol on every second flash of light and oxidize 2 H₂O to an O₂ on every fourth flash. This overview is a brief personal assessment of the progress observed by the author over a four-decade research career, including a discussion of some remaining unsolved issues. It will come as no surprise to readers that there are remaining questions given the complexity of PS2, and the efforts that have been needed so far to uncover its secrets. In fact, most readers will have their own lists of outstanding questions.

Why Did Nature Choose Manganese over Cobalt to Make Oxygen Photosynthetically on the Earth?

All contemporary oxygenic phototrophs─from primitive cyanobacteria to complex multicellular plants─split water using a single invariant cluster comprising Mn₄CaO₅ (the water oxidation catalyst) as the catalyst within photosystem II, the universal oxygenic reaction center of natural photosynthesis. This cluster is unstable outside of PSII and can be reconstituted, both in vivo and in vitro, using elemental aqueous ions and light, via photoassembly. Here, we demonstrate the first functional substitution of manganese in any oxygenic reaction center by in vitro photoassembly. Following complete removal of inorganic cofactors from cyanobacterial photosystem II microcrystal (PSIIX), photoassembly with free cobalt (Co²⁺), calcium (Ca²⁺), and water (OH^-) restores O₂ evolution activity. Photoassembly occurs at least threefold faster using Co²⁺ versus Mn²⁺ due to a higher quantum yield for PSIIX-mediated charge separation (P*): Co²⁺ → P* → Co³⁺Q_A^-. However, this kinetic preference for Co²⁺ over native Mn²⁺ during photoassembly is offset by significantly poorer catalytic activity (∼25% of the activity with Mn²⁺) and ∼3- to 30-fold faster photoinactivation rate. The resulting reconstituted Co-PSIIX oxidizes water by the standard four-flash photocycle, although they produce 4-fold less O₂ per PSII, suggested to arise from faster charge recombination (Co³⁺Q_A ← Co⁴⁺Q_A^-) in the catalytic cycle. The faster photoinactivation of reconstituted Co-PSIIX occurs under anaerobic conditions during the catalytic cycle, suggesting direct photodamage without the involvement of O₂. Manganese offers two advantages for oxygenic phototrophs, which may explain its exclusive retention throughout Darwinian evolution: significantly slower charge recombination (Mn³⁺Q_A ← Mn⁴⁺Q_A^-) permits more water oxidation at low and fluctuating solar irradiation (greater net energy conversion) and much greater tolerance to photodamage at high light intensities (Mn⁴⁺ is less oxidizing than Co⁴⁺). Future work to identify the chemical nature of the intermediates will be needed for further interpretation.

Hemicubane topological analogs of the oxygen-evolving complex of photosystem II mediating water-assisted propylene carbonate oxidation

A series of Ca-Mn clusters with the ligand 2-pyridinemethoxide (Py-CH₂O) have been prepared with varying degrees of topological similarity to the biological oxygen-evolving complex. These clusters activate water as a substrate in the oxidative degradation of propylene carbonate, with activity correlated with topological similarity to the OEC, lowering the onset potential of the oxidation by as much as 700 mV.

Suppression of Charge Recombination by Auxiliary Atoms in Photoinduced Charge Separation Dynamics with Mn Oxides: A Theoretical Study

Charge separation is one of the most crucial processes in photochemical dynamics of energy conversion, widely observed ranging from water splitting in photosystem II (PSII) of plants to photoinduced oxidation reduction processes. Several basic principles, with respect to charge separation, are known, each of which suffers inherent charge recombination channels that suppress the separation efficiency. We found a charge separation mechanism in the photoinduced excited-state proton transfer dynamics from Mn oxides to organic acceptors. This mechanism is referred to as coupled proton and electron wave-packet transfer (CPEWT), which is essentially a synchronous transfer of electron wave-packets and protons through mutually different spatial channels to separated destinations passing through nonadiabatic regions, such as conical intersections, and avoided crossings. CPEWT also applies to collision-induced ground-state water splitting dynamics catalyzed by Mn₄CaO₅ cluster. For the present photoinduced charge separation dynamics by Mn oxides, we identified a dynamical mechanism of charge recombination. It takes place by passing across nonadiabatic regions, which are different from those for charge separations and lead to the excited states of the initial state before photoabsorption. This article is an overview of our work on photoinduced charge separation and associated charge recombination with an additional study. After reviewing the basic mechanisms of charge separation and recombination, we herein studied substituent effects on the suppression of such charge recombination by doping auxiliary atoms. Our illustrative systems are X–Mn(OH)₂ tied to N-methylformamidine, with X=OH, Be(OH)₃, Mg(OH)₃, Ca(OH)₃, Sr(OH)₃ along with Al(OH)₄ and Zn(OH)₃. We found that the competence of suppression of charge recombination depends significantly on the substituents. The present study should serve as a useful guiding principle in designing the relevant photocatalysts.

Impact of energy limitations on function and resilience in long-wavelength Photosystem II

Photosystem II (PSII) uses the energy from red light to split water and reduce quinone, an energy-demanding process based on chlorophyll a (Chl-a) photochemistry. Two types of cyanobacterial PSII can use chlorophyll d (Chl-d) and chlorophyll f (Chl-f) to perform the same reactions using lower energy, far-red light. PSII from Acaryochloris marina has Chl-d replacing all but one of its 35 Chl-a, while PSII from Chroococcidiopsis thermalis, a facultative far-red species, has just 4 Chl-f and 1 Chl-d and 30 Chl-a. From bioenergetic considerations, the far-red PSII were predicted to lose photochemical efficiency and/or resilience to photodamage. Here, we compare enzyme turnover efficiency, forward electron transfer, back-reactions and photodamage in Chl-f-PSII, Chl-d-PSII, and Chl-a-PSII. We show that: (i) all types of PSII have a comparable efficiency in enzyme turnover; (ii) the modified energy gaps on the acceptor side of Chl-d-PSII favour recombination via P_D1⁺Phe^- repopulation, leading to increased singlet oxygen production and greater sensitivity to high-light damage compared to Chl-a-PSII and Chl-f-PSII; (iii) the acceptor-side energy gaps in Chl-f-PSII are tuned to avoid harmful back reactions, favouring resilience to photodamage over efficiency of light usage. The results are explained by the differences in the redox tuning of the electron transfer cofactors Phe and Q_A and in the number and layout of the chlorophylls that share the excitation energy with the primary electron donor. PSII has adapted to lower energy in two distinct ways, each appropriate for its specific environment but with different functional penalties.

Self-Assembled Heterometallic Complexes by Incorporation of Calcium or Strontium Ion into a Manganese(II) 12-Metallacrown-3 Framework Supported by a Tripodal Ligand with Pyridine-Carboxylate Motifs: Stability in Their Manganese(III) Oxidized Form

We report on the isolation of a new family of μ-carboxylato-bridged metallocrown (MC) compounds by self-assembly of the recently isolated hexadentate tris(2-pyridylmethyl)amine ligand tpada^2- incorporating two carboxylate units with metal cations. Twelve-membered MCs of manganese of the type 12-MC-3, namely, [{Mn^II(tpada)}₃(M)(H₂O)_n]²⁺ (Mn₃M) (M = Mn²⁺ (n = 0), Ca²⁺ (n = 1), or Sr²⁺ (n = 2)), were structurally characterized. The metallamacrocycles connectivity consisting in three -[Mn-O-C-O]- repeating units is provided by one carboxylate unit of the three tpada^2- ligands, while the second carboxylate coordinated a fourth cation in the central cavity of the MC, Mn²⁺ or an alkaline earth metal, Ca²⁺ or Sr²⁺. Mn₃Ca and {Mn₃Sr}₂ join the small family of heterometallic manganese-calcium complexes and even rarer manganese-strontium complexes as models of the OEC of photosystem II (PSII). A 8-MC-4 of strontium of the molecular wheel type with four -[Sr-O]- repeating unit was also isolated by self-assembly of the tpada^2- ligand with Sr²⁺. This complex, namely, [Sr(tpada)(OH₂)]₄ (Sr₄), does not incorporate any cation in the central cavity but instead four water molecules coordinated to each Sr²⁺. Electrochemical investigations coupled to UV-visible absorption and EPR spectroscopies as well as electrospray mass spectrometry reveal the stability of the 12-MC-3 tetranuclear structures in solution, both in the initial oxidation state, Mn^II₃M, as well as in the three-electrons oxidized state, Mn^III₃M. Indeed, the cyclic voltammogram of all these complexes exhibits three-successive reversible oxidation waves between +0.5 and +0.9 V corresponding to the successive one-electron oxidation of the Mn(II) ion into Mn(III) of the three {Mn(tpada)} units constituting the ring, which are fully maintained after bulk electrolysis.

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 Tyr_Z 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 Q_A ^- /DCMU and S₃ Q_A ^- /DCMU radiative charge recombination occurred at higher temperatures and (3) the S₀ Tyr_Z ^• , S₁ Tyr_Z ^• , S₂ Tyr_Z ^• split EPR signals induced at 4.2 K by visible light from the S₀ Tyr_Z , S₁ Tyr_Z , S₂ Tyr_Z , respectively, and the (S₂ Tyr_Z ^• )' induced by NIR illumination at 4.2 K of the S₃ Tyr_Z state differed. It is proposed that the R323 residue of the D1 protein interacts with Tyr_Z 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 Tyr_Z by interacting with the bulk, thus tuning the driving force required for Tyr_Z oxidation.

The D1-V185N mutation alters substrate water exchange by stabilizing alternative structures of the Mn4Ca-cluster in photosystem II

In photosynthesis, the oxygen-evolving complex (OEC) of the pigment-protein complex photosystem II (PSII) orchestrates the oxidation of water. Introduction of the V185N mutation into the D1 protein was previously reported to drastically slow O₂-release and strongly perturb the water network surrounding the Mn₄Ca cluster. Employing time-resolved membrane inlet mass spectrometry, we measured here the H₂¹⁸O/H₂¹⁶O-exchange kinetics of the fast (W_f) and slow (W_s) exchanging substrate waters bound in the S₁, S₂ and S₃ states to the Mn₄Ca cluster of PSII core complexes isolated from wild type and D1-V185N strains of Synechocystis sp. PCC 6803. We found that the rate of exchange for W_s was increased in the S₁ and S₂ states, while both W_f and W_s exchange rates were decreased in the S₃ state. Additionally, we used EPR spectroscopy to characterize the Mn₄Ca cluster and its interaction with the redox active D1-Tyr161 (Y_Z). In the S₂ state, we observed a greatly diminished multiline signal in the V185N-PSII that could be recovered by addition of ammonia. The split signal in the S₁ state was not affected, while the split signal in the S₃ state was absent in the D1-V185N mutant. These findings are rationalized by the proposal that the N185 residue stabilizes the binding of an additional water-derived ligand at the Mn1 site of the Mn₄Ca cluster via hydrogen bonding. Implications for the sites of substrate water binding are discussed.

Structural Changes in the Acceptor Site of Photosystem II upon Ca2+/Sr2+ Exchange in the Mn4CaO5 Cluster Site and the Possible Long-Range Interactions

The Mn4CaO5 cluster site in the oxygen-evolving complex (OEC) of photosystem II (PSII) undergoes structural perturbations, such as those induced by Ca2+/Sr2+ exchanges or Ca/Mn removal. These changes have been known to induce long-range positive shifts (between +30 and +150 mV) in the redox potential of the primary quinone electron acceptor plastoquinone A (QA), which is located 40 Å from the OEC. To further investigate these effects, we reanalyzed the crystal structure of Sr-PSII resolved at 2.1 Å and compared it with the native Ca-PSII resolved at 1.9 Å. Here, we focus on the acceptor site and report the possible long-range interactions between the donor, Mn4Ca(Sr)O5 cluster, and acceptor sites.

Five-coordinate MnIV intermediate in the activation of nature's water splitting cofactor

Nature's water splitting cofactor passes through a series of catalytic intermediates (S₀-S₄) before O-O bond formation and O₂ release. In the second last transition (S₂ to S₃) cofactor oxidation is coupled to water molecule binding to Mn1. It is this activated, water-enriched all Mn^IV form of the cofactor that goes on to form the O-O bond, after the next light-induced oxidation to S₄ How cofactor activation proceeds remains an open question. Here, we report a so far not described intermediate (S₃') in which cofactor oxidation has occurred without water insertion. This intermediate can be trapped in a significant fraction of centers (>50%) in (i) chemical-modified cofactors in which Ca²⁺ is exchanged with Sr²⁺; the Mn₄O₅Sr cofactor remains active, but the S₂-S₃ and S₃-S₀ transitions are slower than for the Mn₄O₅Ca cofactor; and (ii) upon addition of 3% vol/vol methanol; methanol is thought to act as a substrate water analog. The S₃' electron paramagnetic resonance (EPR) signal is significantly broader than the untreated S₃ signal (2.5 T vs. 1.5 T), indicating the cofactor still contains a 5-coordinate Mn ion, as seen in the preceding S₂ state. Magnetic double resonance data extend these findings revealing the electronic connectivity of the S₃' cofactor is similar to the high spin form of the preceding S₂ state, which contains a cuboidal Mn₃O₄Ca unit tethered to an external, 5-coordinate Mn ion (Mn4). These results demonstrate that cofactor oxidation regulates water molecule insertion via binding to Mn4. The interaction of ammonia with the cofactor is also discussed.

Oxygenic Photoreactivity in Photosystem II Studied by Rotating Ring Disk Electrochemistry

Protein film photoelectrochemistry has previously been used to monitor the activity of photosystem II, the water-plastoquinone photooxidoreductase, but the mechanistic information attainable from a three-electrode setup has remained limited. Here we introduce the four-electrode rotating ring disk electrode technique for quantifying light-driven reaction kinetics and mechanistic pathways in real time at the enzyme-electrode interface. This setup allows us to study photochemical H₂O oxidation in photosystem II and to gain an in-depth understanding of pathways that generate reactive oxygen species. The results show that photosystem II reacts with O₂ through two main pathways that both involve a superoxide intermediate to produce H₂O₂. The first pathway involves the established chlorophyll triplet-mediated formation of singlet oxygen, which is followed by its reduction to superoxide at the electrode surface. The second pathway is specific for the enzyme/electrode interface: an exposed antenna chlorophyll is sufficiently close to the electrode for rapid injection of an electron to form a highly reducing chlorophyll anion, which reacts with O₂ in solution to produce O₂^•-. Incomplete H₂O oxidation does not significantly contribute to reactive oxygen formation in our conditions. The rotating ring disk electrode technique allows the chemical reactivity of photosystem II to be studied electrochemically and opens several avenues for future investigation.

Ultrafast infrared observation of exciton equilibration from oriented single crystals of photosystem II

In oxygenic photosynthesis, two photosystems work in series. Each of them contains a reaction centre that is surrounded by light-harvesting antennae, which absorb the light and transfer the excitation energy to the reaction centre where electron transfer reactions are driven. Here we report a critical test for two contrasting models of light harvesting by photosystem II cores, known as the trap-limited and the transfer-to-the trap-limited model. Oriented single crystals of photosystem II core complexes of Synechococcus elongatus are excited by polarized visible light and the transient absorption is probed with polarized light in the infrared. The dichroic amplitudes resulting from photoselection are maintained on the 60 ps timescale that corresponds to the dominant energy transfer process providing compelling evidence for the transfer-to-the-trap limitation of the overall light-harvesting process. This finding has functional implications for the quenching of excited states allowing plants to survive under high light intensities.

Photocurrents from photosystem II in a metal oxide hybrid system: Electron transfer pathways

We have investigated the nature of the photocurrent generated by Photosystem II (PSII), the water oxidizing enzyme, isolated from Thermosynechococcus elongatus, when immobilized on nanostructured titanium dioxide on an indium tin oxide electrode (TiO2/ITO). We investigated the properties of the photocurrent from PSII when immobilized as a monolayer versus multilayers, in the presence and absence of an inhibitor that binds to the site of the exchangeable quinone (QB) and in the presence and absence of exogenous mobile electron carriers (mediators). The findings indicate that electron transfer occurs from the first quinone (QA) directly to the electrode surface but that the electron transfer through the nanostructured metal oxide is the rate-limiting step. Redox mediators enhance the photocurrent by taking electrons from the nanostructured semiconductor surface to the ITO electrode surface not from PSII. This is demonstrated by photocurrent enhancement using a mediator incapable of accepting electrons from PSII. This model for electron transfer also explains anomalies reported in the literature using similar and related systems. The slow rate of the electron transfer step in the TiO2 is due to the energy level of electron injection into the semiconducting material being below the conduction band. This limits the usefulness of the present hybrid electrode. Strategies to overcome this kinetic limitation are discussed.

Photoactivation: The Light-Driven Assembly of the Water Oxidation Complex of Photosystem II

Photosynthetic water oxidation is catalyzed by the Mn4CaO5 cluster of photosystem II. The assembly of the Mn4O5Ca requires light and involves a sequential process called photoactivation. This process harnesses the charge-separation of the photochemical reaction center and the coordination environment provided by the amino acid side chains of the protein to oxidize and organize the incoming manganese ions to form the oxo-bridged metal cluster capable of H2O-oxidation. Although most aspects of this assembly process remain poorly understood, recent advances in the elucidation of the crystal structure of the fully assembled cyanobacterial PSII complex help in the interpretation of the rich history of experiments designed to understand this process. Moreover, recent insights on the structure and stability of the constituent ions of the Mn4CaO5 cluster may guide future experiments. Here we consider the literature and suggest possible models of assembly including one involving single Mn(2+) oxidation site for all Mn but requiring ion relocation.

Probing S-state advancements and recombination pathways in photosystem II with a global fit program for flash-induced oxygen evolution pattern

The oxygen-evolving complex (OEC) in photosystem II catalyzes the oxidation of water to molecular oxygen. Four decades ago, measurements of flash-induced oxygen evolution have shown that the OEC steps through oxidation states S(0), S(1), S(2), S(3) and S(4) before O(2) is released and the S(0) state is reformed. The light-induced transitions between these states involve misses and double hits. While it is widely accepted that the miss parameter is S state dependent and may be further modulated by the oxidation state of the acceptor side, the traditional way of analyzing each flash-induced oxygen evolution pattern (FIOP) individually did not allow using enough free parameters to thoroughly test this proposal. Furthermore, this approach does not allow assessing whether the presently known recombination processes in photosystem II fully explain all measured oxygen yields during Si state lifetime measurements. Here we present a global fit program that simultaneously fits all flash-induced oxygen yields of a standard FIOP (2 Hz flash frequency) and of 11-18 FIOPs each obtained while probing the S(0), S(2) and S(3) state lifetimes in spinach thylakoids at neutral pH. This comprehensive data treatment demonstrates the presence of a very slow phase of S(2) decay, in addition to the commonly discussed fast and slow reduction of S(2) by YD and QB(-), respectively. Our data support previous suggestions that the S(0)→S(1) and S(1)→S(2) transitions involve low or no misses, while high misses occur in the S(2)→S(3) or S(3)→S(0) transitions.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

Redox potential tuning by redox-inactive cations in nature's water oxidizing catalyst and synthetic analogues

Fundamental differences between synthetic manganese clusters and the biological water oxidizing catalyst are demonstrated in the modulation of their redox potential by redox-inactive cations.

Magnetic Behavior of Heterometallic Wheels Having a [Mn(IV)6M2O9](10+) Core with M = Ca(2+) and Sr(2+)

Two new heterometallic Mn(IV)-M(2+) compounds with formula [Mn6M2O9(4-(t)BuC6H4COO)10(4-(t)BuC6H4COOH)5] (M = Ca(2+) (1), Sr(2+) (2)) have been crystallized. The core of both compounds consists of a planar Mn6 ring, where the Mn(IV) ions are alternatively bridged by (μ3-O)2(μ-RCOO) and (μ4-O)(μ-RCOO)2 ligands, and the two alkaline earth ions are located to both sides of the wheel, linked to the oxo bridges, generating three fused [Mn2M2O4](4+) cuboids. These compounds show a net antiferromagnetic behavior, more important for 2 (Sr(2+)) than for 1 (Ca(2+)). The fitting of the experimental data was performed with the support of DFT calculations, considering four different exchange pathways: two between adjacent Mn(IV) ions (J1 and J2) and two between nonadjacent Mn(IV) ions (J3 and J4). The results of the analysis show that J1 and J2 are of the opposite sign, the ferromagnetic contribution corresponding to the [Mn2(μ4-O)(μ-RCOO)2](4+) unit (J2). The influence of the M(2+) ions in the magnetic behavior is analyzed for 1 and 2 and for three hypothetical models with the structural parameters of 1 containing Mg(2+), Sr(2+) or without the M(2+) ions. In spite of the diamagnetic character of the alkaline earth ions, their influence on the magnetic behavior has been evidenced and correlated with their polarizing effect. Moreover, the magnetic interactions between nonadjacent ions are non-negligible.

Structural rearrangements preceding dioxygen formation by the water oxidation complex of photosystem II

Photosynthetic water oxidation is catalyzed by the Mn4CaO5 cluster of photosystem II. Recent studies implicate an oxo bridge atom, O5, of the Mn4CaO5 cluster, as the "slowly exchanging" substrate water molecule. The D1-V185N mutant is in close vicinity of O5 and known to extend the lag phase and retard the O2 release phase (slow phase) in this critical last [Formula: see text] transition of water oxidation. The pH dependence, hydrogen/deuterium (H/D) isotope effect, and temperature dependence on the O2 release kinetics for this mutant were studied using time-resolved O2 polarography, and comparisons were made with WT and two mutants of the putative proton gate D1-D61. Both kinetic phases in V185N are independent of pH and buffer concentration and have weaker H/D kinetic isotope effects. Each phase is characterized by a parallel or even lower activation enthalpy but a less favorable activation entropy than the WT. The results indicate new rate-determining steps for both phases. It is concluded that the lag does not represent inhibition of proton release but rather, slowing of a previously unrecognized kinetic phase involving a structural rearrangement or tautomerism of the S3 (+) ground state as it approaches a configuration conducive to dioxygen formation. The parallel impacts on both the lag and O2 formation phases suggest a common origin for the defects surmised to be perturbations of the H-bond network and the water cluster adjacent to O5.

Proton Matrix ENDOR Studies on Ca2+-depleted and Sr2+-substituted Manganese Cluster in Photosystem II

Proton matrix ENDOR spectra were measured for Ca(2+)-depleted and Sr(2+)-substituted photosystem II (PSII) membrane samples from spinach and core complexes from Thermosynechococcus vulcanus in the S2 state. The ENDOR spectra obtained were similar for untreated PSII from T. vulcanus and spinach, as well as for Ca(2+)-containing and Sr(2+)-substituted PSII, indicating that the proton arrangements around the manganese cluster in cyanobacterial and higher plant PSII and Ca(2+)-containing and Sr(2+)-substituted PSII are similar in the S2 state, in agreement with the similarity of the crystal structure of both Ca(2+)-containing and Sr(2+)-substituted PSII in the S1 state. Nevertheless, slightly different hyperfine separations were found between Ca(2+)-containing and Sr(2+)-substituted PSII because of modifications of the water protons ligating to the Sr(2+) ion. Importantly, Ca(2+) depletion caused the loss of ENDOR signals with a 1.36-MHz separation because of the loss of the water proton W4 connecting Ca(2+) and YZ directly. With respect to the crystal structure and the functions of Ca(2+) in oxygen evolution, it was concluded that the roles of Ca(2+) and Sr(2+) involve the maintenance of the hydrogen bond network near the Ca(2+) site and electron transfer pathway to the manganese cluster.

Characterization of the Sr(2+)- and Cd(2+)-Substituted Oxygen-Evolving Complex of Photosystem II by Quantum Mechanics/Molecular Mechanics Calculations

The Mn4CaO5 cluster in the oxygen-evolving complex is the catalytic core of the Photosystem II (PSII) enzyme, responsible for the water splitting reaction in oxygenic photosynthesis. The role of the redox-inactive ion in the cluster has not yet been fully clarified, although several experimental data are available on Ca2+-depleted and Ca2+-substituted PSII complexes, indicating Sr2+-substituted PSII as the only modification that preserves oxygen evolution. In this work, we investigated the structural and electronic properties of the PSII catalytic core with Ca2+ replaced with Sr2+ and Cd2+ in the S2 state of the Kok−Joliot cycle by means of density functional theory and ab initio molecular dynamics based on a quantum mechanics/ molecular mechanics approach. Our calculations do not reveal significant differences between the substituted and wild-type systems in terms of geometries, thermodynamics, and kinetics of two previously identified intermediate states along the S2 to S3 transition, namely, the open cubane S2 A and closed cubane S2 B conformers. Conversely, our calculations show different pKa values for the water molecule bound to the three investigated heterocations. Specifically, for Cd-substituted PSII, the pKa value is 5.3 units smaller than the respective value in wild type Ca-PSII. On the basis of our results, we conclude that, assuming all the cations sharing the same binding site, the induced difference in the acidity of the binding pocket might influence the hydrogen bonding network and the redox levels to prevent the further evolution of the cycle toward the S3 state.

The biological water-oxidizing complex at the nano-bio interface

Photosynthesis is one of the most important processes on our planet, providing food and oxygen for the majority of living organisms on Earth. Over the past 30 years scientists have made great strides in understanding the central photosynthetic process of oxygenic photosynthesis, whereby water is used to provide the hydrogen and reducing equivalents vital to CO2 reduction and sugar formation. A recent crystal structure at 1.9-1.95Å has made possible an unparalleled map of the structure of photosystem II (PSII) and particularly the manganese-calcium (Mn-Ca) cluster, which is responsible for splitting water. Here we review how knowledge of the water-splitting site provides important criteria for the design of artificial Mn-based water-oxidizing catalysts, allowing the development of clean and sustainable solar energy technologies.

Lessons from Nature: A Bio-Inspired Approach to Molecular Design

Metalloproteins contain actives sites with intricate structures that perform specific functions with high selectivity and efficiency. The complexity of these systems complicates the study of their function and the understanding of the properties that give rise to their reactivity. One approach that has contributed to the current level of understanding of their biological function is the study of synthetic constructs that mimic one or more aspects of the native metalloproteins. These systems allow individual contributions to the structure and function to be analyzed and also permit spectroscopic characterization of the metal cofactors without complications from the protein environment. This Current Topic is a review of synthetic constructs as probes for understanding the biological activation of small molecules. These topics are developed from the perspective of seminal molecular design breakthroughs from the past that provide the foundation for the systems used today.

Electron transfer pathways from the S2-states to the S3-states either after a Ca2+/Sr2+ or a Cl-/I- exchange in Photosystem II from Thermosynechococcus elongatus

The site for water oxidation in Photosystem II (PSII) goes through five sequential oxidation states (S0 to S4) before O2 is evolved. It consists of a Mn4CaO5-cluster close to a redox-active tyrosine residue (YZ). Cl- is also required for enzyme activity. By using EPR spectroscopy it has been shown that both Ca2+/Sr2+ exchange and Cl-/I- exchange perturb the proportions of centers showing high (S=5/2) and low spin (S=1/2) forms of the S2-state. The S3-state was also found to be heterogeneous with: i) a S=3 form that is detectable by EPR and not sensitive to near-infrared light; and ii) a form that is not EPR visible but in which Mn photochemistry occurs resulting in the formation of a (S2YZ)' split EPR signal upon near-infrared illumination. In Sr/Cl-PSII, the high spin (S=5/2) form of S2 shows a marked heterogeneity with a g=4.3 form generated at low temperature that converts to a relaxed form at g=4.9 at higher temperatures. The high spin g=4.9 form can then progress to the EPR detectable form of S3 at temperatures as low as 180K whereas the low spin (S=1/2) S2-state can only advance to the S3 state at temperatures≥235 K. Both of the two S2 configurations and the two S3 configurations are each shown to be in equilibrium at ≥235 K but not at 198 K. Since both S2 configurations are formed at 198 K, they likely arise from two specific populations of S1. The existence of heterogeneous populations in S1, S2 and S3 states may be related to the structural flexibility associated with the positioning of the oxygen O5 within the cluster highlighted in computational approaches and which has been linked to substrate exchange. These data are discussed in the context of recent in silico studies of the electron transfer pathways between the S2-state(s) and the S3-state(s).

Principles of Natural Photosynthesis

Nature relies on a unique and intricate biochemical setup to achieve sunlight-driven water splitting. Combined experimental and computational efforts have produced significant insights into the structural and functional principles governing the operation of the water-oxidizing enzyme Photosystem II in general, and of the oxygen-evolving manganese-calcium cluster at its active site in particular. Here we review the most important aspects of biological water oxidation, emphasizing current knowledge on the organization of the enzyme, the geometric and electronic structure of the catalyst, and the role of calcium and chloride cofactors. The combination of recent experimental work on the identification of possible substrate sites with computational modeling have considerably limited the possible mechanistic pathways for the critical O-O bond formation step. Taken together, the key features and principles of natural photosynthesis may serve as inspiration for the design, development, and implementation of artificial systems.