Temperature dependence of the high-spin S2 to S3 transition in Photosystem II: Mechanistic consequences

New insights into the involvement of residue D1/V185 in photosystem II function in Synechocystis 6803 and Thermosynechococcus vestitus

The effects of D1-V185T and D1-V185N mutations in Photosystem II (PSII) from Thermosynechococcus vestitus (formerly T. elongatus) and Synechocystis 6803, respectively, were studied using both EPR and optical kinetics. EPR spectroscopy reveals the presence of a mixture of a S2 state in a high spin configuration (S2HS) and in a low spin configuration (S2LS) in both mutants. In contrast to the S2HS in the wild type, the S2HS state in the D1-V185T mutant does not progress to the S3 state at 198 K. This inability is likely due to alterations in the protonation state and hydrogen-bonding network around the Mn4CaO5 cluster. Optical studies show that these mutations significantly affect proton release during the S3-to-S0 transition. While the initial fast proton release associated with TyrZ● formation remains unaffected within the resolution of our measurements, the second, and slower, proton release is delayed, suggesting that the mutations disrupt the hydrogen-bonding interactions necessary for efficient deprotonation of substrate water (O6). This disruption in proton transfer also correlates with slower water exchange in the S3 state, likely due to non-native hydrogen bonds introduced by the threonine or asparagine side chains at position 185. These findings point to a critical role of D1-V185 in regulating both proton transfer dynamics and water binding, underscoring a complex interplay between structural and functional changes in PSII.

On the nature of high-spin forms in the S2 state of the oxygen-evolving complex

The Mn4CaO x cluster of the oxygen-evolving complex (OEC) in photosystem II, the site of biological water oxidation, adopts different forms as it progresses through the catalytic cycle of S i states (i = 0-4) and within each S i state itself. This has been amply documented by spectroscopy, but the structural basis of spectroscopic polymorphism remains debated. The S2 state is extensively studied by magnetic resonance spectroscopies. In addition to the common type of g ≈ 2 multiline EPR signal attributed to a low-spin (S = 1/2) form of the manganese cluster, other signals at lower fields (g ≥ 4) associated with the S2 state arise from higher-spin forms. Resolving the structural identity of the high-spin species is paramount for a microscopic understanding of the catalytic mechanism. Hypotheses explored by theoretical studies implicate valence isomerism, proton tautomerism, or coordination change with respect to the low-spin form. Here we analyze structure-property correlations for multiple formulations employing a common high-level protocol based on multiscale models that combine a converged quantum mechanics region embedded within a large protein region treated semiempirically with an extended tight-binding method (DFT/xTB), surpassing conventional quantum mechanics/molecular mechanics (QM/MM) approaches. Our results provide a comprehensive comparison of magnetic topologies, spin states and energetics in relation to experimental observations. Crucial predictions are made about 14N hyperfine coupling constants and X-ray absorption Mn K-pre-edge features as criteria for discriminating between different models. This study updates our view on a persistent mystery of biological water oxidation, while providing a refined and transferable computational platform for future theoretical studies of the OEC.

Conformational Flexibility of D1-Glu189: A Crucial Determinant in Substrate Water Selection, Positioning, and Stabilization within the Oxygen-Evolving Complex of Photosystem II

Photosynthetic water oxidation is a vital process responsible for producing dioxygen and supplying the energy necessary to sustain life on Earth. This fundamental reaction is catalyzed by the oxygen-evolving complex (OEC) of photosystem II, which houses the Mn4CaO5 cluster as its catalytic core. In this study, we specifically focus on the D1-Glu189 amino acid residue, which serves as a direct ligand to the Mn4CaO5 cluster. Our primary goal is to explore, using density functional theory (DFT), how the conformational flexibility of the D1-Glu189 side chain influences crucial catalytic processes, particularly the selection, positioning, and stabilization of a substrate water molecule within the OEC. Our investigation is based on a hypothesis put forth by Li et al. (Nature, 2024, 626, 670), which suggests that during the transition from the S2 to S3 state, a specific water molecule temporarily coordinating with the Ca ion, referred to as O6*, may exist as a hydroxide ion (OH-). Our results demonstrate a key mechanism by which the detachment of the D1-Glu189 carboxylate group from its coordination with the Ca ion allows the creation of a specialized microenvironment within the OEC that enables the selective attraction of O6* in its deprotonated form (OH-) and stabilizes it at the catalytic metal (MnD) site. Our findings indicate that D1-Glu189 is not only a structural ligand for the Ca ion but may also play an active and dynamic role in the catalytic process, positioning O6* optimally for its subsequent participation in the oxidation sequence during the water-splitting cycle.

Theoretical elucidation of the structure, bonding, and reactivity of the CaMn4Ox clusters in the whole Kok cycle for water oxidation embedded in the oxygen evolving center of photosystem II. New molecular and quantum insights into the mechanism of the O-O bond formation

This paper reviews our historical developments of broken-symmetry (BS) and beyond BS methods that are applicable for theoretical investigations of metalloenzymes such as OEC in PSII. The BS hybrid DFT (HDFT) calculations starting from high-resolution (HR) XRD structure in the most stable S1 state have been performed to elucidate structure and bonding of whole possible intermediates of the CaMn4Ox cluster (1) in the Si (i = 0 ~ 4) states of the Kok cycle. The large-scale HDFT/MM computations starting from HR XRD have been performed to elucidate biomolecular system structures which are crucial for examination of possible water inlet and proton release pathways for water oxidation in OEC of PSII. DLPNO CCSD(T0) computations have been performed for elucidation of scope and reliability of relative energies among the intermediates by HDFT. These computations combined with EXAFS, XRD, XFEL, and EPR experimental results have elucidated the structure, bonding, and reactivity of the key intermediates, which are indispensable for understanding and explanation of the mechanism of water oxidation in OEC of PSII. Interplay between theory and experiments have elucidated important roles of four degrees of freedom, spin, charge, orbital, and nuclear motion for understanding and explanation of the chemical reactivity of 1 embedded in protein matrix, indicating the participations of the Ca(H2O)n ion and tyrosine(Yz)-O radical as a one-electron acceptor for the O-O bond formation. The Ca-assisted Yz-coupled O-O bond formation mechanisms for water oxidation are consistent with recent XES and very recent time-resolved SFX XFEL and FTIR results.

The S1 to S2 and S2 to S3 state transitions in plant photosystem II: relevance to the functional and structural heterogeneity of the water oxidizing complex

In Photosystem II, light-induced water splitting occurs via the S state cycle of the CaMn4O5-cluster. To understand the role of various possible conformations of the CaMn4O5-cluster in this process, the temperature dependence of the S1 → S2 and S2 → S3 state transitions, induced by saturating laser flashes, was studied in spinach photosystem II membrane preparations under different conditions. The S1 → S2 transition temperature dependence was shown to be much dependent on the type of the cryoprotectant and presence of 3.5% methanol, resulting in the variation of transition half-inhibition temperature by 50 K. No similar effect was observed for the S2 → S3 state transition, for which we also show that both the low spin g = 2.0 multiline and high spin g = 4.1 EPR configurations of the S2 state advance with similar efficiency to the S3 state, both showing a transition half-inhibition temperature of 240 K. This was further confirmed by following the appearance of the Split S3 EPR signal. The results are discussed in relevance to the functional and structural heterogeneity of the water oxidizing complex intermediates in photosystem II.

Independent Mutation of Two Bridging Carboxylate Ligands Stabilizes Alternate Conformers of the Photosynthetic O2-Evolving Mn4CaO5 Cluster in Photosystem II

The O2-evolving Mn4CaO5 cluster in photosystem II is ligated by six carboxylate residues. One of these is D170 of the D1 subunit. This carboxylate bridges between one Mn ion (Mn4) and the Ca ion. A second carboxylate ligand is D342 of the D1 subunit. This carboxylate bridges between two Mn ions (Mn1 and Mn2). D170 and D342 are located on opposite sides of the Mn4CaO5 cluster. Recently, it was shown that the D170E mutation perturbs both the intricate networks of H-bonds that surround the Mn4CaO5 cluster and the equilibrium between different conformers of the cluster in two of its lower oxidation states, S1 and S2, while still supporting O2 evolution at approximately 50% the rate of the wild type. In this study, we show that the D342E mutation produces much the same alterations to the cluster's FTIR and EPR spectra as D170E, while still supporting O2 evolution at approximately 20% the rate of the wild type. Furthermore, the double mutation, D170E + D342E, behaves similarly to the two single mutations. We conclude that D342E alters the equilibrium between different conformers of the cluster in its S1 and S2 states in the same manner as D170E and perturbs the H-bond networks in a similar fashion. This is the second identification of a Mn4CaO5 metal ligand whose mutation influences the equilibrium between the different conformers of the S1 and S2 states without eliminating O2 evolution. This finding has implications for our understanding of the mechanism of O2 formation in terms of catalytically active/inactive conformations of the Mn4CaO5 cluster in its lower oxidation states.

Molecular Structure Related to an S = 5/2 High-Spin S2 State Manganese Cluster of Photosystem II Investigated by Q-Band Pulse EPR Spectroscopy

The high-spin S₂ state of the photosynthetic oxygen-evolving cluster Mn₄CaO₅, corresponding to the g = 4.1 signal for X-band electron paramagnetic resonance (EPR), was investigated using Q-band pulsed EPR, which detected a main peak at g = 3.10 and satellite peaks at 5.25, 4.55, and 2.80. We evaluated the spin state as the zero-field splitting of D = 0.465 cm^-1 and E/D = 0.245 with S = 5/2. The temperature dependence of the T₁ relaxation time revealed that the excited-state energy was 28.7 cm^-1 higher than that of the S = 5/2 ground state. By comparing present quantum mechanical (QM) calculation models, a closed-cubane structure with the protonation state of two oxygens, W1 (= OH^-) and W2 (= H₂O), was the most probable structure for the S = 5/2 state. The three-pulse electron spin-echo envelope modulation (ESEEM) detected the nuclear signal, which was assigned to nitrogen as His332 ligated to the Mn1 ion. The obtained hyperfine constant for the nitrogen signal was significantly reduced from that in the S = 1/2 low-spin state. These results indicate that the S = 5/2 spin state arises from the closed-cubane structure.

Mutation of a metal ligand stabilizes the high-spin form of the S2 state in the O2-producing Mn4CaO5 cluster of photosystem II

The residue D1-D170 bridges Mn4 with the Ca ion in the O₂-evolving Mn₄CaO₅ cluster of Photosystem II. Recently, the D1-D170E mutation was shown to substantially alter the S_n+1-minus-S_n FTIR difference spectra [Debus RJ (2021) Biochemistry 60:3841-3855]. The mutation was proposed to alter the equilibrium between different Jahn-Teller conformers of the S₁ state such that (i) a different S₁ state conformer is stabilized in D1-D170E than in wild-type and (ii) the S₁ to S₂ transition in D1-D170E produces a high-spin form of the S₂ state rather than the low-spin form that is produced in wild-type. In this study, we employed EPR spectroscopy to test if a high-spin form of the S₂ state is formed preferentially in D1-D170E PSII. Our data show that illumination of dark-adapted D1-D170E PSII core complexes does indeed produce a high-spin form of the S₂ state rather than the low-spin multiline form that is produced in wild-type. This observation provides further experimental support for a change in the equilibrium between S state conformers in both the S₁ and S₂ states in a site-directed mutant that retains substantial O₂ evolving activity.

Does Serial Femtosecond Crystallography Depict State-Specific Catalytic Intermediates of the Oxygen-Evolving Complex?

Recent advances in serial femtosecond crystallography (SFX) of photosystem II (PSII), enabled by X-ray free electron lasers (XFEL), provided the first geometric models of distinct intermediates in the catalytic S-state cycle of the oxygen-evolving complex (OEC). These models are obtained by flash-advancing the OEC from the dark-stable state (S₁) to more oxidized intermediates (S₂ and S₃), eventually cycling back to the most reduced S₀. However, the interpretation of these models is controversial because geometric parameters within the Mn₄CaO₅ cluster of the OEC do not exactly match those expected from coordination chemistry for the spectroscopically verified manganese oxidation states of the distinct S-state intermediates. Here we focus on the first catalytic transition, S₁ → S₂, which represents a one-electron oxidation of the OEC. Combining geometric and electronic structure criteria, including a novel effective oxidation state approach, we analyze existing 1-flash (1F) SFX-XFEL crystallographic models that should depict the S₂ state of the OEC. We show that the 1F/S₂ equivalence is not obvious, because the Mn oxidation states and total unpaired electron counts encoded in these models are not fully consistent with those of a pure S₂ state and with the nature of the S₁ → S₂ transition. Furthermore, the oxidation state definition in two-flashed (2F) structural models is practically impossible to elucidate. Our results advise caution in the extraction of electronic structure information solely from the literal interpretation of crystallographic models and call for re-evaluation of structural and mechanistic interpretations that presume exact correspondence of such models to specific catalytic intermediates of the OEC.

Water oxidation in oxygenic photosynthesis studied by magnetic resonance techniques

The understanding of light-induced biological water oxidation in oxygenic photosynthesis is of great importance both for biology and (bio)technological applications. The chemically difficult multistep reaction takes place at a unique protein-bound tetra-manganese/calcium cluster in photosystem II whose structure has been elucidated by X-ray crystallography (Umena et al. Nature 2011, 473, 55). The cluster moves through several intermediate states in the catalytic cycle. A detailed understanding of these intermediates requires information about the spatial and electronic structure of the Mn₄ Ca complex; the latter is only available from spectroscopic techniques. Here, the important role of Electron Paramagnetic Resonance (EPR) and related double resonance techniques (ENDOR, EDNMR), complemented by quantum chemical calculations, is described. This has led to the elucidation of the cluster's redox and protonation states, the valence and spin states of the manganese ions and the interactions between them, and contributed substantially to the understanding of the role of the protein surrounding, as well as the binding and processing of the substrate water molecules, the O-O bond formation and dioxygen release. Based on these data, models for the water oxidation cycle are developed.

Glycerol binding at the narrow channel of photosystem II stabilizes the low-spin S2 state of the oxygen-evolving complex

The oxygen-evolving complex (OEC) of photosystem II (PSII) cycles through redox intermediate states Si (i = 0-4) during the photochemical oxidation of water. The S2 state involves an equilibrium of two isomers including the low-spin S2 (LS-S2) state with its characteristic electron paramagnetic resonance (EPR) multiline signal centered at g = 2.0, and a high-spin S2 (HS-S2) state with its g = 4.1 EPR signal. The relative intensities of the two EPR signals change under experimental conditions that shift the HS-S2/LS-S2 state equilibrium. Here, we analyze the effect of glycerol on the relative stability of the LS-S2 and HS-S2 states when bound at the narrow channel of PSII, as reported in an X-ray crystal structure of cyanobacterial PSII. Our quantum mechanics/molecular mechanics (QM/MM) hybrid models of cyanobacterial PSII show that the glycerol molecule perturbs the hydrogen-bond network in the narrow channel, increasing the pKa of D1-Asp61 and stabilizing the LS-S2 state relative to the HS-S2 state. The reported results are consistent with the absence of the HS-S2 state EPR signal in native cyanobacterial PSII EPR spectra and suggest that the narrow water channel hydrogen-bond network regulates the relative stability of OEC catalytic intermediates during water oxidation.

Temperature dependence of the formation of the g ~ 5 EPR signal in the oxygen evolving complex of photosystem II

The temperature dependence of the formation of the g ~ 5 S₂ state electron paramagnetic resonance (EPR) signal in photosystem II (PSII) was investigated. The g ~ 5 signal was produced at an illumination above 200 K. The half inhibition temperature of the formation of the g ~ 5 EPR signal was approximately 215 K. The half inhibition temperature is close to that of the transition from the S₂ state-to-S₃ state in the untreated PSII, and not to that of the transition from S₁ state -to-S₂ state in the untreated PSII. The upshift of the half inhibition temperature of the transition from the S₁ state -to-S₂ state (g ~ 5) reflects the structural change upon transition from the S₁ state to the S₂ state. The activation energy of the g ~ 5 state formation was estimated as 40.7 ± 4.4 kJ/mol, which is comparable to the reported activation energy for the S₂ formation in the untreated PSII. The activation enthalpy and entropy were estimated to be 39.0 ± 4.4 kJ/mol and - 103 ± 19 J/mol K at 210 K, respectively. Based on these parameters, the formation process of the g ~ 5 state is discussed in this study.

Alteration of the O2-Producing Mn4Ca Cluster in Photosystem II by the Mutation of a Metal Ligand

The O₂-evolving Mn₄Ca cluster in photosystem II (PSII) is arranged as a distorted Mn₃Ca cube that is linked to a fourth Mn ion (denoted as Mn4) by two oxo bridges. The Mn4 and Ca ions are bridged by residue D1-D170. This is also the only residue known to participate in the high-affinity Mn(II) site that participates in the light-driven assembly of the Mn₄Ca cluster. In this study, we use Fourier transform infrared difference spectroscopy to characterize the impact of the D1-D170E mutation. On the basis of analyses of carboxylate and carbonyl stretching modes and the O-H stretching modes of hydrogen-bonded water molecules, we show that this mutation alters the extensive network of hydrogen bonds that surrounds the Mn₄Ca cluster in the same manner as that of many other mutations. It also alters the equilibrium between conformers of the Mn₄Ca cluster in the dark-stable S₁ state so that a high-spin form of the S₂ state is produced during the S₁-to-S₂ transition instead of the low-spin form that gives rise to the S₂ state multiline electron paramagnetic resonance signal. The mutation may also change the coordination mode of the carboxylate group at position 170 to unidentate ligation of Mn4. This is the first mutation of a metal ligand in PSII that substantially impacts the spectroscopic signatures of the Mn₄Ca cluster without substantially eliminating O₂ evolution. The results have significant implications for our understanding of the roles of alternate active/inactive conformers of the Mn₄Ca cluster in the mechanism of O₂ formation.

The exchange of the fast substrate water in the S2 state of photosystem II is limited by diffusion of bulk water through channels - implications for the water oxidation mechanism

The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn4CaO5 cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the 'slow exchanging substrate water (Ws)', while, in the S2 state, the terminal water ligands W2 and W3 are both discussed as the 'fast exchanging substrate water (Wf)'. A critical point for the assignment of Wf is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn4CaO5 cluster. In this study, we measured the rates of H2 16O/H2 18O substrate water exchange in the S2 and S3 states of PSII core complexes from wild-type (WT) Synechocystis sp. PCC 6803, and from two mutants, D1-D61A and D1-E189Q, that are expected to alter water access via the Cl1/O4 channels and the O1 channel, respectively. We found that the exchange rates of Wf and Ws were unaffected by the E189Q mutation (O1 channel), but strongly perturbed by the D61A mutation (Cl1/O4 channel). It is concluded that all channels have restrictions limiting the isotopic equilibration of the inner water pool near the Mn4CaO5 cluster, and that D61 participates in one such barrier. In the D61A mutant this barrier is lowered so that Wf exchange occurs more rapidly. This finding removes the main argument against Ca-bound W3 as fast substrate water in the S2 state, namely the indifference of the rate of Wf exchange towards Ca/Sr substitution.

Redox Isomerism in the S3 State of the Oxygen-Evolving Complex Resolved by Coupled Cluster Theory

The electronic and geometric structures of the water-oxidizing complex of photosystem II in the steps of the catalytic cycle that precede dioxygen evolution remain hotly debated. Recent structural and spectroscopic investigations support contradictory redox formulations for the active-site Mn4 CaOx cofactor in the final metastable S3 state. These range from the widely accepted MnIV 4 oxo-hydroxo model, which presumes that O-O bond formation occurs in the ultimate transient intermediate (S4 ) of the catalytic cycle, to a MnIII 2 MnIV 2 peroxo model representative of the contrasting "early-onset" O-O bond formation hypothesis. Density functional theory energetics of suggested S3 redox isomers are inconclusive because of extreme functional dependence. Here, we use the power of the domain-based local pair natural orbital approach to coupled cluster theory, DLPNO-CCSD(T), to present the first correlated wave function theory calculations of relative stabilities for distinct redox-isomeric forms of the S3 state. Our results enabled us to evaluate conflicting models for the S3 state of the oxygen-evolving complex (OEC) and to quantify the accuracy of lower-level theoretical approaches. Our assessment of the relevance of distinct redox-isomeric forms for the mechanism of biological water oxidation strongly disfavors the scenario of early-onset O-O formation advanced by literal interpretations of certain crystallographic models. This work serves as a case study in the application of modern coupled cluster implementations to redox isomerism problems in oligonuclear transition metal systems.

Two Distinct Oxygen-Radical Conformations in the X-ray Free Electron Laser Structures of Photosystem II

We report the existence of two distinct oxygen-radical-containing Mn₄CaO_5/6 conformations with short O···O bonds in the crystal structures of the oxygen-evolving enzyme photosystem II (PSII), obtained using an X-ray free electron laser (XFEL). A short O···O distance of <2.3 Å between the O4 site of the Mn₄CaO₅ complex and the adjacent water molecule (W539) in the proton-conducting O4-water chain was observed in the second flash-induced (2F) XFEL structure (2F-XFEL), which may correspond to S₃. By use of a quantum mechanical/molecular mechanical approach, the OH^• formation at W539 and the short O4···O_W539 distance (<2.3 Å) were reproduced in S₂ and S₃ with reduced Mn1(III), which lacks the additional sixth water molecule O6. As the O^•- formation at O6 and the short O5···O6 distance (1.9 Å) have been reported in another 2F-XFEL structure with reduced Mn4(III), two distinct oxygen-radical conformations exist in the 2F-XFEL crystals.

Successes, challenges, and opportunities for quantum chemistry in understanding metalloenzymes for solar fuels research

Overview of the rich and diverse contributions of quantum chemistry to understanding the structure and function of the biological archetypes for solar fuel research, photosystem II and hydrogenases.

Kinetic modeling of substrate-water exchange in Photosystem II

We derive a model that provides an exact solution to the substrate-water exchange kinetics in a double-conformation system and use this model to interpret recently published data for Ca²⁺- and Sr²⁺-containing PSII in the S₂ state, in which the g = 2.0 and g = 4.1 conformations coexist. The component concentrations derived from the kinetic model provide an analytic description of the substrate-water exchange kinetics, allowing us to more accurately interpret the results. Based on this model and the previously reported data on the S₂ state g = 2.0 conformation, we obtain the substrate-water exchange rates of the g = 4.1 conformation and the conformational change rates. Two conclusions are made from the analyses. First, contrary to previous reports, there is no significant effect of substituting Sr²⁺ for Ca²⁺ on any of the exchange rate constants. Second, the exchange rate of the slowly-exchanging water (W_s) in the S₂ state g = 4.1 conformation is much faster than that in the S₂ state g = 2.0 conformation. The second conclusion is consistent with the assignment of W_s to W1 or W2 bound as terminal ligands to Mn4; Mn4 has been proposed to undergo an oxidation state change from Mn(IV) in the g = 2.0 conformation to Mn(III) in the g = 4.1 conformation.

Formation of the High-Spin S2 State Related to the Extrinsic Proteins in the Oxygen Evolving Complex of Photosystem II

The high-spin S₂ state was investigated with photosystem II (PSII) from spinach, Thermosynechococcus vulcanus, and Cyanidioschyzon merolae. In extrinsic protein-depleted PSII, high-spin electron paramagnetic resonance (EPR) signals were not detected in either species, whereas all species showed g ∼ 5 signals in the presence of a high concentration of Ca²⁺ instead of the multiline signal. In the intact and PsbP/Q-depleted PSII from spinach, the g = 4.1 EPR signal was detected. These results show that formation of the high-spin S₂ state of the manganese cluster is regulated by the extrinsic proteins through a charge located near the Mn4 atom in the Mn₄CaO₅ cluster but is independent of the intrinsic proteins. The shift to the g ∼ 5 state is caused by tilting of the z-axis in the Mn4 coordinates through hydrogen bonds or external divalent cations. The structural modification may allow insertion of an oxygen atom during the S₂-to-S₃ transition.

A Theoretical Study of the Recently Suggested MnVII Mechanism for O-O Bond Formation in Photosystem II

The mechanism for water oxidation in photosystem II has been a major topic for several decades. The active catalyst has four manganese atoms connected by bridging oxo bonds, in a complex termed the oxygen-evolving complex (OEC), which also includes a calcium atom. The O-O bond of oxygen is formed after absorption of four photons in a state of the OEC termed S₄. There has been essential consensus that in the S₄ state, all manganese atoms are in the Mn(IV) oxidation state. However, recently there has been a suggestion that one of the atoms is in the Mn(VII) state. In the present computational study, the feasibility of that proposal has been investigated. It is here shown that the mechanism involving Mn(VII) has a much higher barrier for forming O₂ than the previous proposal with four Mn(IV) atoms.

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.

Current Understanding of the Mechanism of Water Oxidation in Photosystem II and Its Relation to XFEL Data

The investigation of water oxidation in photosynthesis has remained a central topic in biochemical research for the last few decades due to the importance of this catalytic process for technological applications. Significant progress has been made following the 2011 report of a high-resolution X-ray crystallographic structure resolving the site of catalysis, a protein-bound Mn4CaOxcomplex, which passes through ≥5 intermediate states in the water-splitting cycle. Spectroscopic techniques complemented by quantum chemical calculations aided in understanding the electronic structure of the cofactor in all (detectable) states of the enzymatic process. Together with isotope labeling, these techniques also revealed the binding of the two substrate water molecules to the cluster. These results are described in the context of recent progress using X-ray crystallography with free-electron lasers on these intermediates. The data are instrumental for developing a model for the biological water oxidation cycle.

What can we still learn from the electrochromic band-shifts in Photosystem II?

Electrochromic band-shifts have been investigated in Photosystem II (PSII) from Thermosynechoccocus elongatus. Firstly, by using Mn-depleted PsbA1-PSII and PsbA3-PSII in which the Q_X absorption of Phe_D1 differs, a band-shift in the Q_X region of Phe_D2 centered at ~ 544 nm has been identified upon the oxidation, at pH 8.6, of Tyr_D. In contrast, a band-shift due to the formation of either Q_A^•- or Tyr_Z^• is observed in PsbA3-PSII at ~ 546 nm, as expected with E130 H-bonded to Phe_D1 and at ~ 544 nm as expected with Q130 H-bonded to Phe_D1. Secondly, electrochromic band-shifts in the Chla Soret region have been measured in O₂-evolving PSII in PsbA3-PSII, in the PsbA3/H198Q mutant in which the Soret band of P_D1 is blue shifted and in the PsbA3/T179H mutant. Upon Tyr_Z^•Q_A^•- formation the Soret band of P_D1 is red shifted and the Soret band of Chl_D1 is blue shifted. In contrast, only P_D1 undergoes a detectable S-state dependent electrochromism. Thirdly, the time resolved S-state dependent electrochromism attributed to P_D1 is biphasic for all the S-state transitions except for S₁ to S₂, and shows that: i) the proton release in S₀ to S₁ occurs after the electron transfer and ii) the proton release and the electron transfer kinetics in S₂ to S₃, in T. elongatus, are significantly faster than often considered. The nature of S₂Tyr_Z^• is discussed in view of the models in the literature involving intermediate states in the S₂ to S₃ transition.

Substrate water exchange in the S2 state of photosystem II is dependent on the conformation of the Mn4Ca cluster

The structural flexibility of the Mn₄Ca cluster in photosystem II supports the exchange of the central O5 bridge.

Early Binding of Substrate Oxygen Is Responsible for a Spectroscopically Distinct S2 State in Photosystem II

The biological generation of oxygen by the oxygen-evolving complex (OEC) in photosystem II (PS II) is one of nature's most important reactions. The OEC is a Mn₄Ca cluster that has multiple Mn-O-Mn and Mn-O-Ca bridges and binds four water molecules. Previously, binding of an additional oxygen was detected in the S₂ to S₃ transition. Here we demonstrate that early binding of the substrate oxygen to the five-coordinate Mn₁ center in the S₂ state is likely responsible for the S₂ high-spin EPR signal. Substrate binding in the Mn₁-OH form explains the prevalence of the high-spin S₂ state at higher pH and its low-temperature conversion into the S₃ state. The given interpretation was confirmed by X-ray absorption spectroscopic measurements, DFT, and broken symmetry DFT calculations of structures and magnetic properties. Structural, electronic, and spectroscopic properties of the high-spin S₂ state model are provided and compared with the available S₃ state models. New interpretation of the high-spin S₂ state opens opportunity for analysis of factors controlling the oxygen substrate binding in PS II.