O2 evolution and recovery of the water-oxidizing enzyme

Synthetic Mn3Ce2O5-Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis

The photosynthetic oxygen-evolving center (OEC) is a unique Mn4CaO5-cluster that catalyses water splitting into electrons, protons, and dioxygen. Precisely structural and functional mimicking of the OEC is a long-standing challenge and pressingly needed for understanding the structure-function relationship and catalytic mechanism of O-O bond formation. Herein we report two simple and robust artificial Mn3Ce2O5-complexes that display a remarkable structural similarity to the OEC in regarding of the ten-atom core (five metal ions and five oxygen bridges) and the alkyl carboxylate peripheral ligands. This Mn3Ce2O5-cluster can catalyse the water-splitting reaction on the surface of ITO electrode. These results clearly show that cerium can structurally and functionally replace both calcium and manganese in the cluster. Mass spectroscopic measurements demonstrate that the oxide bridges in the cluster are exchangeable and can be rapidly replaced by the isotopic oxygen of H2 18O in acetonitrile solution, which supports that the oxide bridge(s) may serve as the active site for the formation of O-O bond during the water-splitting reaction. These results would contribute to our understanding of the structure-reactivity relationship of both natural and artificial clusters and shed new light on the development of efficient water-splitting catalysts in artificial photosynthesis.

Hey ho, where'd the proton go? Final deprotonation of O6 within the S3 state of photosystem II

The deprotonation of O6 within the S3 state marks the final deprotonation event before the formation of oxygen‑oxygen bond interactions and eventual production and release of dioxygen. Gaining a thorough understanding of this event, from the proton acceptors involved, to the exfiltration pathways available, is key in determining the nature of the resulting oxygen species, influencing the mechanism through which the first oxygen‑oxygen bond forms. Computational analysis, using BS-DFT methodologies, showed that proton abstraction by the local Glu189 residue provides consistent evidence against this being a viable mechanistic pathway due to the lack of a stable product structure. In contrast, abstraction via W3 shows an increasingly stable oxo-oxo product state between r[O5O6] = 2.1 Å & 1.9 Å. The resulting oxo-oxo state is stabilised through donation of β electron character from O6 to Mn1 and α electron character from O6 to O5. This donation from the O6 lone pair is shown to be a key factor in stabilising the oxo-oxo state, in addition to showing the initiation of first O5-O6 bond.

Closing Kok's cycle of nature's water oxidation catalysis

The Mn4CaO5(6) cluster in photosystem II catalyzes water splitting through the Si state cycle (i = 0-4). Molecular O2 is formed and the natural catalyst is reset during the final S3 → (S4) → S0 transition. Only recently experimental breakthroughs have emerged for this transition but without explicit information on the S0-state reconstitution, thus the progression after O2 release remains elusive. In this report, our molecular dynamics simulations combined with density functional calculations suggest a likely missing link for closing the cycle, i.e., restoring the first catalytic state. Specifically, the formation of closed-cubane intermediates with all hexa-coordinate Mn is observed, which would undergo proton release, water dissociation, and ligand transfer to produce the open-cubane structure of the S0 state. Thereby, we theoretically identify the previously unknown structural isomerism in the S0 state that acts as the origin of the proposed structural flexibility prevailing in the cycle, which may be functionally important for nature's water oxidation catalysis.

The Effect of Removal of External Proteins PsbO, PsbP and PsbQ on Flash-Induced Molecular Oxygen Evolution and Its Biphasicity in Tobacco PSII

The oxygen evolution within photosystem II (PSII) is one of the most enigmatic processes occurring in nature. It is suggested that external proteins surrounding the oxygen-evolving complex (OEC) not only stabilize it and provide an appropriate ionic environment but also create water channels, which could be involved in triggering the ingress of water and the removal of O2 and protons outside the system. To investigate the influence of these proteins on the rate of oxygen release and the efficiency of OEC function, we developed a measurement protocol for the direct measurement of the kinetics of oxygen release from PSII using a Joliot-type electrode. PSII-enriched tobacco thylakoids were used in the experiments. The results revealed the existence of slow and fast modes of oxygen evolution. This observation is model-independent and requires no specific assumptions about the initial distribution of the OEC states. The gradual removal of exogenous proteins resulted in a slowdown of the rapid phase (~ms) of O2 release and its gradual disappearance while the slow phase (~tens of ms) accelerated. The role of external proteins in regulating the biphasicity and efficiency of oxygen release is discussed based on observed phenomena and current knowledge.

Energetics of the H-Bond Network in Exiguobacterium sibiricum Rhodopsin

Exiguobacterium sibiricum rhodopsin (ESR) functions as a light-driven proton pump utilizing Lys96 for proton uptake and maintaining its activity over a wide pH range. Using a combination of methodologies including the linear Poisson-Boltzmann equation and a quantum mechanical/molecular mechanical approach with a polarizable continuum model, we explore the microscopic mechanisms underlying its pumping activity. Lys96, the primary proton uptake site, remains deprotonated owing to the loss of solvation in the ESR protein environment. Asp85, serving as a proton acceptor group for Lys96, does not form a low-barrier H-bond with His57. Instead, deprotonated Asp85 forms a salt-bridge with protonated His57, and the proton is predominantly located at the His57 moiety. Glu214, the only acidic residue at the end of the H-bond network exhibits a pKa value of ∼6, slightly elevated due to solvation loss. It seems likely that the H-bond network [Asp85···His57···H2O···Glu214] serves as a proton-conducting pathway toward the protein bulk surface.

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

Exploring the Deprotonation Process during Incorporation of a Ligand Water Molecule at the Dangling Mn Site in Photosystem II

The Mn4CaO5 cluster, featuring four ligand water molecules (W1 to W4), serves as the water-splitting site in photosystem II (PSII). X-ray free electron laser (XFEL) structures exhibit an additional oxygen site (O6) adjacent to the O5 site in the fourth lowest oxidation state, S3, forming Mn4CaO6. Here, we investigate the mechanism of the second water ligand molecule at the dangling Mn (W2) as a potential incorporating species, using a quantum mechanical/molecular mechanical (QM/MM) approach. Previous QM/MM calculations demonstrated that W1 releases two protons through a low-barrier H-bond toward D1-Asp61 and subsequently releases an electron during the S2 to S3 transition, resulting in O•- at W1 and protonated D1-Asp61. During the process of Mn4CaO6 formation, O•-, rather than H2O or OH-, best reproduced the O5···O6 distance. Although the catalytic cluster with O•- at O6 is more stable than that with O•- at W1 in S3, it does not occur spontaneously due to the significantly uphill deprotonation process. Assuming O•- at W2 incorporates into the O6 site, an exergonic conversion from Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV) (equivalent to the open-cubane S2 valence state) to Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III) (equivalent to the closed-cubane S2 valence state) occurs. These findings provide energetic insights into the deprotonation and structural conversion events required for the Mn4CaO6 formation.

Enhanced photosynthetic efficiency by nitrogen-doped carbon dots via plastoquinone-involved electron transfer in apple

Artificially enhancing photosynthesis is critical for improving crop yields and fruit qualities. Nanomaterials have demonstrated great potential to enhance photosynthetic efficiency; however, the mechanisms underlying their effects are poorly understood. This study revealed that the electron transfer pathway participated in nitrogen-doped carbon dots (N-CDs)-induced photosynthetic efficiency enhancement (24.29%), resulting in the improvements of apple fruit qualities (soluble sugar content: 11.43%) in the orchard. We also found that N-CDs alleviated mterf5 mutant-modulated photosystem II (PSII) defects, but not psa3 mutant-modulated photosystem I (PSI) defects, suggesting that the N-CDs-targeting sites were located between PSII and PSI. Measurements of chlorophyll fluorescence parameters suggested that plastoquinone (PQ), the mobile electron carrier in the photosynthesis electron transfer chain (PETC), was the photosynthesis component that N-CDs targeted. In vitro experiments demonstrated that plastoquinone-9 (PQ-9) could accept electrons from light-excited N-CDs to produce the reduced plastoquinone 9 (PQH2-9). These findings suggested that N-CDs, as electron donors, offer a PQ-9-involved complement of PETC to improve photosynthesis and thereby fruit quality. Our study uncovered a mechanism by which nanomaterials enhanced plant photosynthesis and provided some insights that will be useful in the design of efficient nanomaterials for agricultural/horticultural applications.

Mapping the Oxygens in the Oxygen-Evolving Complex of Photosystem II by Their Nucleophilicity Using Quantum Descriptors

The oxygen-evolving complex (OEC) of Photosystem II catalyzes the water-splitting reaction using solar energy. Thus, understanding the reaction mechanism will inspire the design of biomimetic artificial catalysts that convert solar energy to chemical energy. Conceptual Density Functional Theory (CDFT) focuses on understanding the reactivity of molecules and the atomic contribution to the overall nucleophilicity and electrophilicity of the molecule using quantum descriptors. However, this method has not been applied to the OEC before. Here, we use Fukui functions and the dual descriptor to provide quantitative measures of the nucleophilicity and electrophilicity of oxygens in the OEC for different models in different S states. Our results show that the μ-oxo bridges connected to terminal Mn4 are nucleophilic, and those in the cube formed by Mn1, Mn2, and Mn3 are mostly electrophilic. The dual descriptors of the bridging oxygens in the OEC showed a similar reactivity to that of bridging oxygens in Mn model compounds. However, the terminal water W1, which is bound to Mn4, showed very strong reactivity in some of the S3 models. Thus, our calculations support the model that proposes the formation of the O2 molecule through nucleophilic attack by a terminal water.

Soluble Gd6Cu24 clusters: effective molecular electrocatalysts for water oxidation

The water oxidation half reaction in water splitting for hydrogen production is extremely rate-limiting. This study reports the synthesis of two heterometallic clusters (Gd6Cu24-IM and Gd6Cu24-AC) for application as efficient water oxidation catalysts. Interestingly, the maximum turnover frequency of Gd6Cu24-IM in an NaAc solution of a weak acid (pH 6) was 319 s-1. The trimetallic catalytic site, H2O-GdIIICuII2-H2O, underwent two consecutive two-electron two-proton coupled transfer processes to form high-valent GdIII-O-O-CuIII2 intermediates. Furthermore, the O-O bond was formed via intramolecular interactions between the CuIII and GdIII centers. The results of this study revealed that synergistic catalytic water oxidation between polymetallic sites can be an effective strategy for regulating O-O bond formation.

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.

Characterization of tryptophan oxidation affecting D1 degradation by FtsH in the photosystem II quality control of chloroplasts

Photosynthesis is one of the most important reactions for sustaining our environment. Photosystem II (PSII) is the initial site of photosynthetic electron transfer by water oxidation. Light in excess, however, causes the simultaneous production of reactive oxygen species (ROS), leading to photo-oxidative damage in PSII. To maintain photosynthetic activity, the PSII reaction center protein D1, which is the primary target of unavoidable photo-oxidative damage, is efficiently degraded by FtsH protease. In PSII subunits, photo-oxidative modifications of several amino acids such as Trp have been indeed documented, whereas the linkage between such modifications and D1 degradation remains elusive. Here, we show that an oxidative post-translational modification of Trp residue at the N-terminal tail of D1 is correlated with D1 degradation by FtsH during high-light stress. We revealed that Arabidopsis mutant lacking FtsH2 had increased levels of oxidative Trp residues in D1, among which an N-terminal Trp-14 was distinctively localized in the stromal side. Further characterization of Trp-14 using chloroplast transformation in Chlamydomonas indicated that substitution of D1 Trp-14 to Phe, mimicking Trp oxidation enhanced FtsH-mediated D1 degradation under high light, although the substitution did not affect protein stability and PSII activity. Molecular dynamics simulation of PSII implies that both Trp-14 oxidation and Phe substitution cause fluctuation of D1 N-terminal tail. Furthermore, Trp-14 to Phe modification appeared to have an additive effect in the interaction between FtsH and PSII core in vivo. Together, our results suggest that the Trp oxidation at its N-terminus of D1 may be one of the key oxidations in the PSII repair, leading to processive degradation by FtsH.

New insights into the proton pumping mechanism of ba3 cytochrome c oxidase: the functions of key residues and water

As the terminal oxidase of cell respiration in mitochondria and aerobic bacteria, the proton pumping mechanism of ba3-type cytochrome c oxidase (CcO) of Thermus thermophiles is still not fully understood. Especially, the functions of key residues which were considered as the possible proton loading sites (PLSs) above the catalytic center, as well as water located above and within the catalytic center, remain unclear. In this work, molecular dynamic simulations were performed on a set of designed mutants of key residues (Asp287, Asp372, His376, and Glu126II). The results showed that Asp287 may not be a PLS, but it could modulate the ability of the proton transfer pathway to transfer protons through its salt bridge with Arg225. Maintaining the closed state of the water pool above the catalytic center is necessary for the participation of inside water molecules in proton transfer. Water molecules inside the water pool can form hydrogen bond chains with PLS to facilitate proton transfer. Additional quantum cluster models of the Fe-Cu metal catalytic center are established, indicating that when the proton is transferred from Tyr237, it is more likely to reach the OCu atom directly through only one water molecule. This work provides a more profound understanding of the functions of important residues and specific water molecules in the proton pumping mechanism of CcO.

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.

Insights into the protonation state and spin structure for the g = 2 multiline electron paramagnetic resonance signal of the oxygen-evolving complex

In photosystem II (PSII), one-electron oxidation of the most stable oxidation state of the Mn4CaO5 cluster (S1) leads to formation of two distinct states, the open-cubane S2 conformation [Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV)] with low spin and the closed-cubane S2 conformation [Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III)] with high spin. In electron paramagnetic resonance (EPR) spectroscopy, the open-cubane S2 conformation exhibits a g = 2 multiline signal. However, its protonation state remains unclear. Here, we investigated the protonation state of the open-cubane S2 conformation by calculating exchange couplings in the presence of the PSII protein environment and simulating the pulsed electron-electron double resonance (PELDOR). When a ligand water molecule, which forms an H-bond with D1-Asp61 (W1), is deprotonated at dangling Mn4(IV), the first-exited energy (34 cm-1) in manifold spin excited states aligns with the observed value in temperature-dependent pulsed EPR analyses, and the PELDOR signal is best reproduced. Consequently, the g = 2 multiline signal observed in EPR corresponds to the open-cubane S2 conformation with the deprotonated W1 (OH-).

Mechanism of Asparagine-Mediated Proton Transfer in Photosynthetic Reaction Centers

In photosynthetic reaction centers from purple bacteria (PbRCs), light-induced charge separation leads to the reduction of the terminal electron acceptor quinone, QB. The reduction of QB to QB•- is followed by protonation via Asp-L213 and Ser-L223 in PbRC from Rhodobacter sphaeroides. However, Asp-L213 is replaced with nontitratable Asn-L222 and Asn-L213 in PbRCs from Thermochromatium tepidum and Blastochloris viridis, respectively. Here, we investigated the energetics of proton transfer along the asparagine-involved H-bond network using a quantum mechanical/molecular mechanical approach. The potential energy profile for the H-bond between H3O+ and the carbonyl O site of Asn-L222 shows that the proton is predominantly localized at the Asn-L222 moiety in the T. tepidum PbRC protein environment, easily forming the enol species. The release of the proton from the amide -NH2 site toward Ser-L232 via tautomerization suffers from the energy barrier. Upon reorientation of Asn-L222, the enol -OH site forms a short low-barrier H-bond with Ser-L232, facilitating protonation of QB•- in a Grotthuss-like mechanism. This is a basis of how asparagine or glutamine side chains function as acceptors/donors in proton transfer pathways.

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.

Alternative Mechanism for O2 Formation in Natural Photosynthesis via Nucleophilic Oxo-Oxo Coupling

O2 formation in photosystem II (PSII) is a vital event on Earth, but the exact mechanism remains unclear. The presently prevailing theoretical model is "radical coupling" (RC) involving a Mn(IV)-oxyl unit in an "open-cubane" Mn4CaO6 cluster, which is supported experimentally by the S3 state of cyanobacterial PSII featuring an additional Mn-bound oxygenic ligand. However, it was recently proposed that the major structural form of the S3 state of higher plants lacks this extra ligand, and that the resulting S4 state would feature instead a penta-coordinate dangler Mn(V)=oxo, covalently linked to a "closed-cubane" Mn3CaO4 cluster. For this proposal, we explore here a large number of possible pathways of O-O bond formation and demonstrate that the "nucleophilic oxo-oxo coupling" (NOOC) between Mn(V)=oxo and μ3-oxo is the only eligible mechanism in such a system. The reaction is facilitated by a specific conformation of the cluster and concomitant water binding, which is delayed compared to the RC mechanism. An energetically feasible process is described starting from the valid S4 state through the sequential formation of peroxide and superoxide, followed by O2 release and a second water insertion. The newly found mechanism is consistent with available experimental thermodynamic and kinetic data and thus a viable alternative pathway for O2 formation in natural photosynthesis, in particular for higher plants.

Protonation structure of the closed-cubane conformation of the O2-evolving complex in photosystem II

In photosystem II (PSII), one-electron oxidation of the most stable state of the oxygen-evolving Mn4CaO5 cluster (S1) leads to the S2 state formation, Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV) (open-cubane S2) or Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III) (closed-cubane S2). In electron paramagnetic resonance (EPR) spectroscopy, the g = 4.1 signal is not observed in cyanobacterial PSII but in plant PSII, whereas the g = 4.8 signal is observed in cyanobacterial PSII and extrinsic-subunit-depleted plant PSII. Here, we investigated the closed-cubane S2 conformation, a candidate for a higher spin configuration that accounts for g > 4.1 EPR signal, considering all pairwise exchange couplings in the PSII protein environment (i.e. instead of considering only a single exchange coupling between the [Mn3(CaO4)] cubane region and the dangling Mn4 site). Only when a ligand water molecule that forms an H-bond with D1-Asp61 (W1) is deprotonated at dangling Mn4(IV), the g = 4.1 EPR spectra can be reproduced using the cyanobacterial PSII crystal structure. The closed-cubane S2 is less stable than the open-cubane S2 in cyanobacterial PSII, which may explain why the g = 4.1 EPR signal is absent in cyanobacterial PSII.

Binding and functions of the two chloride ions in the oxygen-evolving center of photosystem II

Light-driven water oxidation in photosynthesis occurs at the oxygen-evolving center (OEC) of photosystem II (PSII). Chloride ions (Cl^-) are essential for oxygen evolution by PSII, and two Cl^- ions have been found to specifically bind near the Mn₄CaO₅ cluster in the OEC. The retention of these Cl^- ions within the OEC is critically supported by some of the membrane-extrinsic subunits of PSII. The functions of these two Cl^- ions and the mechanisms of their retention both remain to be fully elucidated. However, intensive studies performed recently have advanced our understanding of the functions of these Cl^- ions, and PSII structures from various species have been reported, aiding the interpretation of previous findings regarding Cl^- retention by extrinsic subunits. In this review, we summarize the findings to date on the roles of the two Cl^- ions bound within the OEC. Additionally, together with a short summary of the functions of PSII membrane-extrinsic subunits, we discuss the mechanisms of Cl^- retention by these extrinsic subunits.

PALE-GREEN LEAF 1, a rice cpSRP54 protein, is essential for the assembly of the PSI-LHCI supercomplex

Although photosynthetic multiprotein complexes have received major attention, our knowledge about the assembly of these proteins into functional complexes in plants is still limited. In the present study, we have identified a chlorophyll-deficient mutant, pale-green leaf 1 (pgl1), in rice that displays abnormally developed chloroplasts. Map-based cloning of this gene revealed that OsPGL1 encodes a chloroplast targeted protein homologous to the 54-kDa subunit of the signal recognition particle (cpSRP54). Immunoblot analysis revealed that the accumulation of the PSI core proteins PsaA and PsaB, subunits from the ATP synthase, cytochrome, and light-harvesting complex (LHC) is dramatically reduced in pgl1. Blue native gel analysis of thylakoid membrane proteins showed the existence of an extra band in the pgl1 mutant, which located between the dimeric PSII/PSI-LHCI and the monomeric PSII. Immunodetection after 2D separation indicated that the extra band consists of the proteins from the PSI core complex. Measurements of chlorophyll fluorescence at 77 K further confirmed that PSI, rather than PSII, was primarily impaired in the pgl1 mutant. These results suggest that OsPGL1 might act as a molecular chaperone that is required for the efficient assembly and specific integration of the peripheral LHCI proteins into the PSI core complex in rice.

The Role of Redox-Inactive Metals in Modulating the Redox Potential of the Mn4CaO4 Model Complex

Photosynthetic oxygen-evolving center (OEC), the "engine of life", is a unique Mn₄CaO₅ cluster catalyzing the water oxidation. The role of redox-inactive component Ca²⁺, which can only be functionally replaced by Sr²⁺ in a biological environment, has been under debate for a long time. Recently, its modulating effect on the redox potential of native OEC and artificial structural OEC model complex has received great attention, and linear relationship between the potential and the Lewis acidity of the redox-inactive metal has been proposed for the MMn₃O₄ model complex. In this work, the modulating effect has been studied in detail using the Mn₄CaO₄ model complex, which is the closest structural model to OEC to date and has a similar redox potential at the S₁-S₂ transition. We found the redox-inactive metal only has a weak modulating effect on the potential, which is comparable in strength to that of the ligand environments. Meanwhile, the net charge of the complex, which could be changed along with the redox-inactive metal, has a high impact on the potential and can be unified by protonation, deprotonation, or ligand modification. Although the modulating effect of the redox-inactive metal is not very strong, the linear relationship between the potential and the Lewis acidity is still valid for Mn₄MO₄ complexes. Our results of strong modulating effects for net charge and weak modulating effects for redox-inactive metal fit with the previous experimental observations on Mn₄MO₄ (M = Ca²⁺, Y³⁺, and Gd³⁺) model complexes, and suggest that Ca²⁺ can be structurally and electrochemically replaced with other metal cations, together with proper ligand modifications.

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.

Reversible Structural Isomerization of Nature's Water Oxidation Catalyst Prior to O-O Bond Formation

Photosynthetic water oxidation is catalyzed by a manganese–calcium oxide cluster, which experiences five “S-states” during a light-driven reaction cycle. The unique “distorted chair”-like geometry of the Mn₄CaO₅₍₆₎ cluster shows structural flexibility that has been frequently proposed to involve “open” and “closed”-cubane forms from the S₁ to S₃ states. The isomers are interconvertible in the S₁ and S₂ states, while in the S₃ state, the open-cubane structure is observed to dominate inThermosynechococcus elongatus (cyanobacteria) samples. In this work, using density functional theory calculations, we go beyond the S₃⁺Y_z state to the S₃ⁿY_z^• → S₄⁺Y_z step, and report for the first time that the reversible isomerism, which is suppressed in the S₃⁺Y_z state, is fully recovered in the ensuing S₃ⁿY_z^• state due to the proton release from a manganese-bound water ligand. The altered coordination strength of the manganese–ligand facilitates formation of the closed-cubane form, in a dynamic equilibrium with the open-cubane form. This tautomerism immediately preceding dioxygen formation may constitute the rate limiting step for O₂ formation, and exert a significant influence on the water oxidation mechanism in photosystem II.

Release of a Proton and Formation of a Low-Barrier Hydrogen Bond between Tyrosine D and D2-His189 in Photosystem II

In photosystem II (PSII), the second-lowest oxidation state (S₁) of the oxygen-evolving Mn₄CaO₅ cluster is the most stable, as the radical form of the redox-active D2-Tyr160 is considered to be a candidate that accepts an electron from the lowest oxidation state (S₀) in the dark. Using quantum mechanical/molecular mechanical calculations, we investigated the redox potential (E _m) of TyrD and its H-bond partner, D2-His189. The potential energy profile indicates that the release of a proton from the TyrD...D2-His189 pair leads to the formation of a low-barrier H-bond. The E _m depends on the H⁺ position along the low-barrier H-bond, e.g., 680 mV when the H⁺ is at the D2-His189 moiety and 800 mV when the H⁺ is at the TyrD moiety, which can explain why TyrD mediates both the S₀ to S₁ oxidation and the S₂ to S₁ reduction.

Synthesizing Mechanism of the Mn4 Ca Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis

The photosynthetic oxygen-evolving center (OEC) is a unique Mn₄ CaO₅ cluster that serves as a blueprint to develop superior water-splitting catalysts for the generation of solar fuels in artificial photosynthesis. It is a great challenge and long-standing issue to reveal the synthesizing mechanism of this Mn₄ CaO₅ cluster in both natural and artificial photosynthesis. Herein, efforts were made to reveal the synthesizing mechanism of an artificial Mn₄ CaO₄ cluster, a close mimic of the OEC. Four key intermediates were successfully isolated and structurally characterized for the first time. It was demonstrated that the Mn₄ CaO₄ cluster could be formed through a reaction between a thermodynamically stable Mn₃ CaO₄ cluster and an unusual four-coordinated Mn^III ion, followed by stabilization process through binding an organic base (e.g., pyridine) on the "dangling" Mn ion. These findings shed new light on the synthesizing mechanism of the OEC and rational design of new artificial water-splitting catalysts.

Requirement of Chloride for the Downhill Electron Transfer Pathway from the Water-Splitting Center in Natural Photosynthesis

In photosystem II (PSII), Cl^- is a prerequisite for the second flash-induced oxidation of the Mn₄CaO₅ cluster (the S₂ to S₃ transition). We report proton transfer from the substrate water molecule via D1-Asp61 and electron transfer via redox-active D1-Tyr161 (TyrZ) to the chlorophyll pair in Cl^--depleted PSII using a quantum mechanical/molecular mechanical approach. The low-barrier H-bond formation between the substrate water molecule and D1-Asp61 remained unaffected upon the depletion of Cl^-. However, the binding site, D2-Lys317, formed a salt bridge with D1-Asp61, leading to the inhibition of the subsequent proton transfer. Remarkably, the redox potential (E_m) of S₂/S₃ increased significantly, making electron transfer from S₂ to TyrZ energetically uphill, as observed in Ca²⁺-depleted PSII. The uphill electron transfer pathway was induced by the significant increase in E_m(S₂/S₃) caused by the loss of charge compensation for D2-Lys317 upon the depletion of Cl^-, whereas it was induced by the significant decrease in E_m(TyrZ) caused by the rearrangement of the water molecules at the Ca²⁺ binding moiety upon the depletion of Ca²⁺.

Structural dynamics in the water and proton channels of photosystem II during the S2 to S3 transition

Light-driven oxidation of water to molecular oxygen is catalyzed by the oxygen-evolving complex (OEC) in Photosystem II (PS II). This multi-electron, multi-proton catalysis requires the transport of two water molecules to and four protons from the OEC. A high-resolution 1.89 Å structure obtained by averaging all the S states and refining the data of various time points during the S₂ to S₃ transition has provided better visualization of the potential pathways for substrate water insertion and proton release. Our results indicate that the O1 channel is the likely water intake pathway, and the Cl1 channel is the likely proton release pathway based on the structural rearrangements of water molecules and amino acid side chains along these channels. In particular in the Cl1 channel, we suggest that residue D1-E65 serves as a gate for proton transport by minimizing the back reaction. The results show that the water oxidation reaction at the OEC is well coordinated with the amino acid side chains and the H-bonding network over the entire length of the channels, which is essential in shuttling substrate waters and protons.

Rare-Earth Elements Can Structurally and Energetically Replace the Calcium in a Synthetic Mn4CaO4-Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis

The oxygen-evolving center (OEC) in photosynthesis is a unique biological Mn₄CaO₅ cluster catalyzing the water-splitting reaction. A great current challenge is to achieve a robust and precise mimic of the OEC in the laboratory. Herein, we report synthetic Mn₄XO₄ clusters (X = calcium, yttrium, gadolinium) that closely resemble the OEC with regard to the main metal-oxide core and peripheral ligands, as well as the oxidation states of the four Mn ions and the redox potential of the cluster. We demonstrate that rare-earth elements can structurally replace the calcium in neutral Mn₄XO₄ clusters. All three Mn₄XO₄ clusters with different redox-inactive metal ions display essentially the same redox properties, challenging the conventional view that the Lewis acidity of the redox-inactive metal ions could modulate the redox potential of the heteronuclear-oxide clusters. The new synthetic rare-earth element-containing Mn₄XO₄ clusters reported here provide robust and structurally well-defined chemical models and shed new light on the design of new water-splitting catalysts in artificial photosynthesis.

Mechanism of Oxygen Evolution and Mn4CaO5 Cluster Restoration in the Natural Water-Oxidizing Catalyst

Water oxidation occurring in the first steps of natural oxygenic photosynthesis is catalyzed by the pigment/protein complex Photosystem II. This process takes place on the Mn₄Ca cluster located in the core of Photosystem II and proceeds along the five steps (S₀-S₄) of the so-called Kok-Joliot cycle until the release of molecular oxygen. The catalytic cycle can therefore be started afresh through insertion of a new water molecule. Here, combining quantum mechanics/molecular mechanics simulations and minimum energy path calculations, we characterized on different spin surfaces the events occurring in the last sector of the catalytic cycle from structural, electronic, and thermodynamic points of view. We found that the process of oxygen evolution and water insertion can be described well by a two-step mechanism, with oxygen release being the rate-limiting step of the process. Moreover, our results allow us to identify the upcoming water molecule required to regenerate the initial structure of the Mn₄Ca cluster in the S₀ state. The insertion of the water molecule was found to be coupled with the transfer of a proton to a neighboring hydroxide ion, thus resulting in the reconstitution of the most widely accepted model of the S₀ state.

Role of redox-inactive metals in controlling the redox potential of heterometallic manganese-oxido clusters

Photosystem II (PSII) contains Ca2+, which is essential to the oxygen-evolving activity of the catalytic Mn4CaO5 complex. Replacement of Ca2+ with other redox-inactive metals results in a loss/decrease of oxygen-evolving activity. To investigate the role of Ca2+ in this catalytic reaction, we investigate artificial Mn3[M]O2 clusters redox-inactive metals  [M] ([M]  = Mg2+, Ca2+, Zn2+, Sr2+, and Y3+), which were synthesized by Tsui et al. (Nat Chem 5:293, 2013). The experimentally measured redox potentials (Em) of these clusters are best described by the energy of their highest occupied molecular orbitals. Quantum chemical calculations showed that the valence of metals predominantly affects Em(MnIII/IV), whereas the ionic radius of metals affects Em(MnIII/IV) only slightly.

Capturing structural changes of the S1 to S2 transition of photosystem II using time-resolved serial femtosecond crystallography

Photosystem II (PSII) catalyzes light-induced water oxidation through an S i -state cycle, leading to the generation of di-oxygen, protons and electrons. Pump-probe time-resolved serial femtosecond crystallography (TR-SFX) has been used to capture structural dynamics of light-sensitive proteins. In this approach, it is crucial to avoid light contamination in the samples when analyzing a particular reaction intermediate. Here, a method for determining a condition that avoids light contamination of the PSII microcrystals while minimizing sample consumption in TR-SFX is described. By swapping the pump and probe pulses with a very short delay between them, the structural changes that occur during the S1-to-S2 transition were examined and a boundary of the excitation region was accurately determined. With the sample flow rate and concomitant illumination conditions determined, the S2-state structure of PSII could be analyzed at room temperature, revealing the structural changes that occur during the S1-to-S2 transition at ambient temperature. Though the structure of the manganese cluster was similar to previous studies, the behaviors of the water molecules in the two channels (O1 and O4 channels) were found to be different. By comparing with the previous studies performed at low temperature or with a different delay time, the possible channels for water inlet and structural changes important for the water-splitting reaction were revealed.

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.

Activation energies for two steps in the S2→ S3 transition of photosynthetic water oxidation from time-resolved single-frequency infrared spectroscopy

The mechanism of water oxidation by the Photosystem II (PSII) protein-cofactor complex is of high interest, but specifically, the crucial coupling of protonation dynamics to electron transfer (ET) and dioxygen chemistry remains insufficiently understood. We drove spinach-PSII membranes by nanosecond-laser flashes synchronously through the water-oxidation cycle and traced the PSII processes by time-resolved single-frequency infrared (IR) spectroscopy in the spectral range of symmetric carboxylate vibrations of protein side chains. After the collection of IR-transients from 100 ns to 1 s, we analyzed the proton-removal step in the S₂ ⇒ S₃ transition, which precedes the ET that oxidizes the Mn₄CaO_x-cluster. Around 1400 cm^-1, pronounced changes in the IR-transients reflect this pre-ET process (∼40 µs at 20 °C) and the ET step (∼300 µs at 20 °C). For transients collected at various temperatures, unconstrained multi-exponential simulations did not provide a coherent set of time constants, but constraining the ET time constants to previously determined values solved the parameter correlation problem and resulted in an exceptionally high activation energy of 540 ± 30 meV for the pre-ET step. We assign the pre-ET step to deprotonation of a group that is re-protonated by accepting a proton from the substrate-water, which binds concurrently with the ET step. The analyzed IR-transients disfavor carboxylic-acid deprotonation in the pre-ET step. Temperature-dependent amplitudes suggest thermal equilibria that determine how strongly the proton-removal step is reflected in the IR-transients. Unexpectedly, the proton-removal step is only weakly reflected in the 1400 cm^-1 transients of PSII core complexes of a thermophilic cyanobacterium (T. elongatus).

The Nature of the Short Oxygen-Oxygen Distance in the Mn4CaO6 Complex of Photosystem II Crystals

The O···O distance for a typical H-bond is ∼2.8 Å, whereas the radiation-damage-free structures of photosystem II (PSII), obtained using the X-ray free electron laser (XFEL), shows remarkably short O···O distances of ∼2 Å in the oxygen-evolving Mn₄CaO_5/6 complex. Herein, we report the protonation/oxidation states of the short O···O atoms in the XFEL structures using a quantum mechanical/molecular mechanical approach. The O5···O6 distance of 1.9 Å is reproduced only when O6 is an unprotonated O radical (O^•-) with Mn(IV)₃Mn(III), i.e., the S₃ state. The potential energy profile shows a barrier-less energy minimum region when O5···O6 = 1.90-2.05 Å (O^•- ↓) or 2.05-2.20 Å (O^•- ↑). Formation of such a short O5···O6 distance is not possible when O6 is OH^- with Mn(IV)₄. In the case in which the O5···O6 distance is 1.9 Å, it seems likely that the O radical species exists in the oxygen-evolving complex of the XFEL-S₃ crystals.

Proton transfer pathway from the oxygen-evolving complex in photosystem II substantiated by extensive mutagenesis

We report a structure-based biological approach to identify the proton-transfer pathway in photosystem II. First, molecular dynamics (MD) simulations were conducted to analyze the H-bond network that may serve as a Grotthuss-like proton conduit. MD simulations show that D1-Asp61, the H-bond acceptor of H₂O at the Mn₄CaO₅ cluster (W1), forms an H-bond via one water molecule with D1-Glu65 but not with D2-Glu312. Then, D1-Asp61, D1-Glu65, D2-Glu312, and the adjacent residues, D1-Arg334, D2-Glu302, and D2-Glu323, were thoroughly mutated to the other 19 residues, i.e., 114 Chlamydomonas chloroplast mutant cells were generated. Mutation of D1-Asp61 was most crucial. Only the D61E and D61C cells grew photoautotrophically and exhibit O₂-evolving activity. Mutations of D2-Glu312 were less crucial to photosynthetic growth than mutations of D1-Glu65. Quantum mechanical/molecular mechanical calculations indicated that in the PSII crystal structure, the proton is predominantly localized at D1-Glu65 along the H-bond with D2-Glu312, i.e., pK_a(D1-Glu65) > pK_a(D2-Glu312). The potential-energy profile shows that the release of the proton from D1-Glu65 leads to the formation of the two short H-bonds between D1-Asp61 and D1-Glu65, which facilitates downhill proton transfer along the Grotthuss-like proton conduit in the S₂ to S₃ transition. It seems possible that D1-Glu65 is involved in the dominant pathway that proceeds from W1 via D1-Asp61 toward the thylakoid lumen, whereas D2-Glu312 and D1-Arg334 may be involved in alternative pathways in some mutants.

High light intensity aggravates latent manganese deficiency in maize

Manganese (Mn) plays an important role in the oxygen-evolving complex, where energy from light absorption is used for water splitting. Although changes in light intensity and Mn status can interfere with the functionality of the photosynthetic apparatus, the interaction between these two factors and the underlying mechanisms remain largely unknown. Here, maize seedlings were grown hydroponically and exposed to two different light intensities under Mn-sufficient or -deficient conditions. No visual Mn deficiency symptoms appeared even though the foliar Mn concentration in the Mn-deficient treatments was reduced to 2 µg g-1. However, the maximum quantum yield efficiency of PSII and the net photosynthetic rate declined significantly, indicating latent Mn deficiency. The reduction in photosynthetic performance by Mn depletion was further aggravated when plants were exposed to high light intensity. Integrated transcriptomic and proteomic analyses showed that a considerable number of genes encoding proteins in the photosynthetic apparatus were only suppressed by a combination of Mn deficiency and high light, thus indicating interactions between changes in Mn nutritional status and light intensity. We conclude that high light intensity aggravates latent Mn deficiency in maize by interfering with the abundance of PSII proteins.

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 pK_a 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 pK_a values of the four ligand water molecules, W1 and W2 at Mn4 and W3 and W4 at Ca²⁺, of the Mn₄CaO₅ cluster. pK_a(W1) ≈ pK_a(W2) << pK_a(W3) ≈ pK_a(W4) in the Mn₄CaO₅ cluster in water. However, pK_a(W1) ≈ pK_a(D1-Asp61) << pK_a(W2) in the PSII protein environment. These results suggest that in PSII, deprotonation of W2 is energetically disfavored as far as W1 exists.

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 c ₂ binding surface. In contrast, the accessory chlorophyll in the D1 protein (Chl_D1) has the lowest excitation energy in PSII. The charge-separated state involves Chl_D1 ^•+ 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 Chl_D1 the initial electron donor in PSII.

Mimicking the Catalytic Center for the Water-Splitting Reaction in Photosystem II

The oxygen-evolving center (OEC) in photosystem II (PSII) of plants, algae and cyanobacteria is a unique natural catalyst that splits water into electrons, protons and dioxygen. The crystallographic studies of PSII have revealed that the OEC is an asymmetric Mn4CaO5-cluster. The understanding of the structure-function relationship of this natural Mn4CaO5-cluster is impeded mainly due to the complexity of the protein environment and lack of a rational chemical model as a reference. Although it has been a great challenge for chemists to synthesize the OEC in the laboratory, significant advances have been achieved recently. Different artificial complexes have been reported, especially a series of artificial Mn4CaO4-clusters that closely mimic both the geometric and electronic structures of the OEC in PSII, which provides a structurally well-defined chemical model to investigate the structure-function relationship of the natural Mn4CaO5-cluster. The deep investigations on this artificial Mn4CaO4-cluster could provide new insights into the mechanism of the water-splitting reaction in natural photosynthesis and may help the development of efficient catalysts for the water-splitting reaction in artificial photosynthesis.

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 (P_D1/P_D2). Using a quantum mechanical/molecular mechanical approach, we report the redox potentials (E_m) of these cofactors in the PSII protein environment. The E_m values suggest that the Mn₄CaO₅ cluster, TyrZ, and P_D1/P_D2 form a downhill electron transfer pathway. E_m for the first oxidation step, E_m(S₀/S₁), is uniquely low (730 mV) and is ∼100 mV lower than that for the second oxidation step, E_m(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.

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.

Redox potentials along the redox-active low-barrier H-bonds in electron transfer pathways

Local proton transfer along redox-active low-barrier H-bonds can alter the driving force or electronic coupling for electron transfer, as the redox potential values depend on the H⁺ position in low-barrier H-bonds.