Detection of an intermediary, protonated water cluster in photosynthetic oxygen evolution

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.

Infrared Difference Spectroscopy of Proteins: From Bands to Bonds

Infrared difference spectroscopy probes vibrational changes of proteins upon their perturbation. Compared with other spectroscopic methods, it stands out by its sensitivity to the protonation state, H-bonding, and the conformation of different groups in proteins, including the peptide backbone, amino acid side chains, internal water molecules, or cofactors. In particular, the detection of protonation and H-bonding changes in a time-resolved manner, not easily obtained by other techniques, is one of the most successful applications of IR difference spectroscopy. The present review deals with the use of perturbations designed to specifically change the protein between two (or more) functionally relevant states, a strategy often referred to as reaction-induced IR difference spectroscopy. In the first half of this contribution, I review the technique of reaction-induced IR difference spectroscopy of proteins, with special emphasis given to the preparation of suitable samples and their characterization, strategies for the perturbation of proteins, and methodologies for time-resolved measurements (from nanoseconds to minutes). The second half of this contribution focuses on the spectral interpretation. It starts by reviewing how changes in H-bonding, medium polarity, and vibrational coupling affect vibrational frequencies, intensities, and bandwidths. It is followed by band assignments, a crucial aspect mostly performed with the help of isotopic labeling and site-directed mutagenesis, and complemented by integration and interpretation of the results in the context of the studied protein, an aspect increasingly supported by spectral calculations. Selected examples from the literature, predominately but not exclusively from retinal proteins, are used to illustrate the topics covered in this review.

A Proton Wire Mediates Proton Coupled Electron Transfer from Hydroxyurea and Other Hydroxamic Acids to Tyrosyl Radical in Class Ia Ribonucleotide Reductase

Proton-coupled electron transfer (PCET) is fundamental to many important biological reactions, including solar energy conversion and DNA synthesis. For example, class Ia ribonucleotide reductases (RNRs) contain a tyrosyl radical-diiron cofactor with one aspartate ligand, D84. The tyrosyl radical, Y122^•, in the β2 subunit acts as a radical initiator and oxidizes an active site cysteine in the α2 subunit. A transient quaternary α2/β2 complex is induced by substrate and effector binding. The hydroxamic acid, hydroxyurea (HU), reduces Y122^• in a PCET reaction involving an electron and proton. This reaction is associated with the loss of activity, a conformational change at Y122, and a change in hydrogen bonding to the Fe1 ligand, D84. Here, we use isotopic labeling, solvent isotope exchange, proton inventories, and reaction-induced Fourier transform infrared (RIFT-IR) spectroscopy to show that the PCET reactions of hydroxamic acids are associated with a characteristic spectrum, which is assignable to electrostatic changes at nonligating aspartate residues. Notably, RIFT-IR spectroscopy reveals this characteristic spectrum when the effects of HU, hydroxylamine, and N-methylhydroxylamine are compared. A large solvent isotope effect is observed for each of the hydroxamic acid reactions, and proton inventories predict that the reactions are associated with the transfer of multiple protons in the transition state. The reduction of Y122^• with 4-methoxyphenol does not lead to these characteristic carboxylate shifts and is associated with only a small solvent isotope effect. In addition to studies of the effects of hydroxamic acids on β2 alone, the reactions involving the quaternary α2β2 complex were also investigated. HU treatment of the quaternary complex, α2/β2/ATP/CDP, leads to a similar carboxylate shift spectrum, as observed with β2 alone. The use of globally labeled ¹³C chimeras (¹³C α2, ¹³C β2) confirms the assignment. Because the spectrum is sensitive to ¹³C β2 labeling, but not ¹³C α2 labeling, the quaternary complex spectrum is assigned to electrostatic changes in β2 carboxylate groups. Examination of the β2 X-ray structure reveals a hydrogen-bonded network leading from the protein surface to Y122. This predicted network includes nonligating aspartates, glutamate ligands to the iron cluster, and predicted crystallographically resolved water molecules. The network is similar when class Ia RNR structures from Escherichia coli, human, and mouse are compared. We propose that the PCET reactions of hydroxamic acids are mediated by a hydrogen-bonded proton wire in the β2 subunit.

An oxyl/oxo mechanism for oxygen-oxygen coupling in PSII revealed by an x-ray free-electron laser

Photosynthetic water oxidation is catalyzed by the Mn4CaO5 cluster of photosystem II (PSII) with linear progression through five S-state intermediates (S0 to S4). To reveal the mechanism of water oxidation, we analyzed structures of PSII in the S1, S2, and S3 states by x-ray free-electron laser serial crystallography. No insertion of water was found in S2, but flipping of D1 Glu189 upon transition to S3 leads to the opening of a water channel and provides a space for incorporation of an additional oxygen ligand, resulting in an open cubane Mn4CaO6 cluster with an oxyl/oxo bridge. Structural changes of PSII between the different S states reveal cooperative action of substrate water access, proton release, and dioxygen formation in photosynthetic water oxidation.

Orientation of non-spherical protonated water clusters revealed by infrared absorption dichroism

Infrared continuum bands that extend over a broad frequency range are a key spectral signature of protonated water clusters. They are observed for many membrane proteins that contain internal water molecules, but their microscopic mechanism has remained unclear. Here we compute infrared spectra for protonated and unprotonated water chains, discs, and droplets from ab initio molecular dynamics simulations. The continuum bands of the protonated clusters exhibit significant anisotropy for chains and discs, with increased absorption along the direction of maximal cluster extension. We show that the continuum band arises from the nuclei motion near the excess charge, with a long-ranged amplification due to the electronic polarizability. Our experimental, polarization-resolved light–dark difference spectrum of the light-driven proton pump bacteriorhodopsin exhibits a pronounced dichroic continuum band. Our results suggest that the protonated water cluster responsible for the continuum band of bacteriorhodopsin is oriented perpendicularly to the membrane normal.

Protein-bound water clusters play a key role for proton transport and storage in molecular biology. Here, the authors show by simulations and experiments that the orientation of non-spherical protonated water clusters in bacteriorhodopsin is unveiled by polarization-resolved infrared spectroscopy.

Tracking Reactive Water and Hydrogen-Bonding Networks in Photosynthetic Oxygen Evolution

In oxygenic photosynthesis, photosystem II (PSII) converts water to molecular oxygen through four photodriven oxidation events at a Mn₄CaO₅ cluster. A tyrosine, YZ (Y161 in the D1 polypeptide), transfers oxidizing equivalents from an oxidized, primary chlorophyll donor to the metal center. Calcium or its analogue, strontium, is required for activity. The Mn₄CaO₅ cluster and YZ are predicted to be hydrogen bonded in a water-containing network, which involves amide carbonyl groups, amino acid side chains, and water. This hydrogen-bonded network includes amino acid residues in intrinsic and extrinsic subunits. One of the extrinsic subunits, PsbO, is intrinsically disordered. This extensive (35 Å) network may be essential in facilitating proton release from substrate water. While it is known that some proteins employ internal water molecules to catalyze reactions, there are relatively few methods that can be used to study the role of water. In this Account, we review spectroscopic evidence from our group supporting the conclusion that the PSII hydrogen-bonding network is dynamic and that water in the network plays a direct role in catalysis. Two approaches, transient electron paramagnetic resonance (EPR) and reaction-induced FT-IR (RIFT-IR) spectroscopies, were used. The EPR experiments focused on the decay kinetics of YZ• via recombination at 190 K and the solvent isotope, pH, and calcium dependence of these kinetics. The RIFT-IR experiments focused on shifts in amide carbonyl frequencies, induced by photo-oxidation of the metal cluster, and on the isotope-based assignment of bands to internal, small protonated water clusters at 190, 263, and 283 K. To conduct these experiments, PSII was prepared in selected steps along the catalytic pathway, the S_n state cycle (n = 0-4). This cycle ultimately generates oxygen. In the EPR studies, S-state dependent changes were observed in the YZ• lifetime and in its solvent isotope effect. The YZ• lifetime depended on the presence of calcium at pH 7.5, but not at pH 6.0, suggesting a two-donor model for PCET. At pH 6.0 or 7.5, barium and ammonia both slowed the rate of YZ• recombination, consistent with disruption of the hydrogen-bonding network. In the RIFT-IR studies of the S state transitions, infrared bands associated with the transient protonation and deprotonation of internal waters were identified by D₂O and H₂¹⁸O labeling. The infrared bands of these protonated water clusters, W_n⁺ (or nH₂O(H₃O)⁺, n = 5-6), exhibited flash dependence and were produced during the S₁ to S₂ and S₃ to S₀ transitions. Calcium dependence was observed at pH 7.5, but not at pH 6.0. S-state induced shifts were observed in amide C═O frequencies during the S₁ to S₂ transition and attributed to alterations in hydrogen bonding, based on ammonia sensitivity. In addition, isotope editing of the extrinsic subunit, PsbO, established that amide vibrational bands of this lumenal subunit respond to the S state transitions and that PsbO is a structural template for the reaction center. Taken together, these spectroscopic results support the hypothesis that proton transfer networks, extending from YZ to PsbO, play a functional and dynamic role in photosynthetic oxygen evolution.

Water is an active matrix of life for cell and molecular biology

Szent-Győrgi called water the "matrix of life" and claimed that there was no life without it. This statement is true, as far as we know, on our planet, but it is not clear whether it must hold throughout the cosmos. To evaluate that question requires a close consideration of the many varied and subtle roles that water plays in living cells-a consideration that must be free of both an assumed essentialism that gives water an almost mystical life-giving agency and a traditional tendency to see it as a merely passive solvent. Water is a participant in the "life of the cell," and here I describe some of the features of that active agency. Water's value for molecular biology comes from both the structural and dynamic characteristics of its status as a complex, structured liquid as well as its nature as a polar, protic, and amphoteric reagent. Any discussion of water as life's matrix must, however, begin with an acknowledgment that our understanding of it as both a liquid and a solvent is still incomplete.

What computational chemistry and magnetic resonance reveal concerning the oxygen evolving centre in Photosystem II

Density Functional Theory (DFT) computational studies of the Mn₄/Ca Oxygen Evolving Complex (OEC) region of Photosystem II in the paramagnetic S₂ and S₃ states of the water oxdizing catalytic cycle are described. These build upon recent advances in computationally understanding the detailed S₁ state OEC geometries, revealed by the recent high resolution Photosystem II crystal structures of Shen et al., at 1.90Å and 1.95Å (Petrie et al., 2015, Angew. Chem. Int. Ed., 54, 7120). The models feature a 'Low Oxidation Paradigm' assumption for the mean Mn oxidation states in the functional enzyme, with the mean oxidation levels being 3.0, 3.25 and 3.5 in S₁, S₂ and S₃, respectively. These calculations are used to infer magnetic exchange interactions within the coupled OEC cluster, particularly in the Electron Paramagnetic Resonance (EPR)-visible S₂ and S₃ states. Detailed computational estimates of the intrinsic magnitudes and molecular orientations of the ⁵⁵Mn hyperfine tensors in the S₂ state are presented. These parameters, together with the resultant spin projected hyperfine values are compared with recent appropriate experimental EPR data (Continuous Wave (CW), Electron-Nuclear Double Resonance (ENDOR) and ELDOR (Electron-Electron Double Resonance)-Detected Nuclear Magnetic Resonance (EDNMR)) from the OEC. It is found that an effective Coupled Dimer magnetic organization of the four Mn in the OEC cluster in the S₂ and S₃ states is able to quantitatively rationalize the observed ⁵⁵Mn hyperfine data. This is consistent with structures we propose to represent the likely state of the OEC in the catalytically active form of the enzyme.

Mechanism of inhibition and decoupling of oxygen evolution from electron transfer in photosystem II by fluoride, ammonia and acetate

Ca(2+) extraction from oxygen-evolving complex (OEC) of photosystem II (PSII) is accompanied by decoupling of oxygen evolution/electron transfer processes [Semin et al. Photosynth. Res. 98 (2008) 235] and appearance of a broad EPR signal at g=2 (split "S3" signal) what can imply the relationship between these effects. Split signal have been observed not only in Ca-depleted PSII but also in PSII membranes treated by fluoride anions, sodium acetate, and NH4Cl. Here we investigated the question: can such compounds induce the decoupling effect during treatment of PSII like Ca(2+) extraction does? We found that F(-), sodium acetate, and NH4Cl inhibit O2 evolution in PSII membranes more effectively than the reduction of artificial electron acceptor 2,6-dichlorophenolindophenol, i.e. the action of these compounds is accompanied by decoupling of these processes in OEC. Similarity of effects observed after Ca(2+) extraction and F(-), CH3COO(-) or NH4Cl treatments suggests that these compounds can inactivate function of Ca(2+). Such inactivation could originate from disturbance of the network of functionally active hydrogen bonds around OEC formed with participation of Ca(2+). This inhibition effect is observed in the region of low concentration of inhibitors. Increasing of inhibitor concentration is accompanied by appearance of other sites of inhibition.

Infrared spectral marker bands characterizing a transient water wire inside a hydrophobic membrane protein

Proton conduction along protein-bound "water wires" is an essential feature in membrane proteins. Here, we analyze in detail a transient water wire, which conducts protons via a hydrophobic barrier within a membrane protein to create a proton gradient. It is formed only for a millisecond out of three water molecules distributed at inactive positions in a polar environment in the ground state. The movement into a hydrophobic environment causes characteristic shifts of the water bands reflecting their different chemical properties. These band shifts are identified by time-resolved Fourier Transform Infrared difference spectroscopy and analyzed by biomolecular Quantum Mechanical/Molecular Mechanical simulations. A non-hydrogen bonded ("dangling") O-H stretching vibration band and a broad continuum absorbance caused by a combined vibration along the water wire are identified as characteristic marker bands of such water wires in a hydrophobic environment. The results provide a basic understanding of water wires in hydrophobic environments.

Reaction dynamics and proton coupled electron transfer: studies of tyrosine-based charge transfer in natural and biomimetic systems

In bioenergetic reactions, electrons are transferred long distances via a hopping mechanism. In photosynthesis and DNA synthesis, the aromatic amino acid residue, tyrosine, functions as an intermediate that is transiently oxidized and reduced during long distance electron transfer. At physiological pH values, oxidation of tyrosine is associated with a deprotonation of the phenolic oxygen, giving rise to a proton coupled electron transfer (PCET) reaction. Tyrosine-based PCET reactions are important in photosystem II, which carries out the light-induced oxidation of water, and in ribonucleotide reductase, which reduces ribonucleotides to form deoxynucleotides. Photosystem II contains two redox-active tyrosines, YD (Y160 in the D2 polypeptide) and YZ (Y161 in the D1 polypeptide). YD forms a light-induced stable radical, while YZ functions as an essential charge relay, oxidizing the catalytic Mn₄CaO₅ cluster on each of four photo-oxidation reactions. In Escherichia coli class 1a RNR, the β2 subunit contains the radical initiator, Y122O•, which is reversibly reduced and oxidized in long range electron transfer with the α2 subunit. In the isolated E. coli β2 subunit, Y122O• is a stable radical, but Y122O• is activated for rapid PCET in an α2β2 substrate/effector complex. Recent results concerning the structure and function of YD, YZ, and Y122 are reviewed here. Comparison is made to recent results derived from bioengineered proteins and biomimetic compounds, in which tyrosine-based charge transfer mechanisms have been investigated. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

FTIR studies of metal ligands, networks of hydrogen bonds, and water molecules near the active site Mn₄CaO₅ cluster in Photosystem II

The photosynthetic conversion of water to molecular oxygen is catalyzed by the Mn₄CaO₅ cluster in Photosystem II and provides nearly our entire supply of atmospheric oxygen. The Mn₄CaO₅ cluster accumulates oxidizing equivalents in response to light-driven photochemical events within Photosystem II and then oxidizes two molecules of water to oxygen. The Mn₄CaO₅ cluster converts water to oxygen much more efficiently than any synthetic catalyst because its protein environment carefully controls the cluster's reactivity at each step in its catalytic cycle. This control is achieved by precise choreography of the proton and electron transfer reactions associated with water oxidation and by careful management of substrate (water) access and proton egress. This review describes the FTIR studies undertaken over the past two decades to identify the amino acid residues that are responsible for this control and to determine the role of each. In particular, this review describes the FTIR studies undertaken to characterize the influence of the cluster's metal ligands on its activity, to delineate the proton egress pathways that link the Mn₄CaO₅ cluster with the thylakoid lumen, and to characterize the influence of specific residues on the water molecules that serve as substrate or as participants in the networks of hydrogen bonds that make up the water access and proton egress pathways. This information will improve our understanding of water oxidation by the Mn₄CaO₅ catalyst in Photosystem II and will provide insight into the design of new generations of synthetic catalysts that convert sunlight into useful forms of storable energy. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Fourier transform infrared difference and time-resolved infrared detection of the electron and proton transfer dynamics in photosynthetic water oxidation

Photosynthetic water oxidation, which provides the electrons necessary for CO₂ reduction and releases O₂ and protons, is performed at the Mn₄CaO₅ cluster in photosystem II (PSII). In this review, studies that assessed the mechanism of water oxidation using infrared spectroscopy are summarized focusing on electron and proton transfer dynamics. Structural changes in proteins and water molecules between intermediates known as Si states (i=0-3) were detected using flash-induced Fourier transform infrared (FTIR) difference spectroscopy. Electron flow in PSII and proton release from substrate water were monitored using the infrared changes in ferricyanide as an exogenous electron acceptor and Mes buffer as a proton acceptor. Time-resolved infrared (TRIR) spectroscopy provided information on the dynamics of proton-coupled electron transfer during the S-state transitions. In particular, a drastic proton movement during the lag phase (~200μs) before electron transfer in the S3→S0 transition was detected directly by monitoring the infrared absorption of a polarizable proton in a hydrogen bond network. Furthermore, the proton release pathways in the PSII proteins were analyzed by FTIR difference measurements in combination with site-directed mutagenesis, isotopic substitutions, and quantum chemical calculations. Therefore, infrared spectroscopy is a powerful tool for understanding the molecular mechanism of photosynthetic water oxidation. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Fourier transform infrared detection of a polarizable proton trapped between photooxidized tyrosine YZ and a coupled histidine in photosystem II: relevance to the proton transfer mechanism of water oxidation

The redox-active tyrosine YZ (D1-Tyr161) in photosystem II (PSII) functions as an immediate electron acceptor of the Mn4Ca cluster, which is the catalytic center of photosynthetic water oxidation. YZ is also located in the hydrogen bond network that connects the Mn4Ca cluster to the lumen and hence is possibly related to the proton transfer process during water oxidation. To understand the role of YZ in the water oxidation mechanism, we have studied the hydrogen bonding interactions of YZ and its photooxidized neutral radical YZ(•) together with the interaction of the coupled His residue, D1-His190, using light-induced Fourier transform infrared (FTIR) difference spectroscopy. The YZ(•)-minus-YZ FTIR difference spectrum of Mn-depleted PSII core complexes exhibited a broad positive feature around 2800 cm(-1), which was absent in the corresponding spectrum of another redox-active tyrosine YD (D2-Tyr160). Analyses by (15)N and H/D substitutions, examination of the pH dependence, and density functional theory and quantum mechanics/molecular mechanics (QM/MM) calculations showed that this band arises from the N-H stretching vibration of the protonated cation of D1-His190 forming a charge-assisted strong hydrogen bond with YZ(•). This result provides strong evidence that the proton released from YZ upon its oxidation is trapped in D1-His190 and a positive charge remains on this His. The broad feature of the ~2800 cm(-1) band reflects a large proton polarizability in the hydrogen bond between YZ(•) and HisH(+). QM/MM calculations further showed that upon YZ oxidation the hydrogen bond network is rearranged and one water molecule moves toward D1-His190. From these data, a novel proton transfer mechanism via YZ(•)-HisH(+) is proposed, in which hopping of the polarizable proton of HisH(+) to this water triggers the transfer of the proton from substrate water to the luminal side. This proton transfer mechanism could be functional in the S2 → S3 transition, which requires proton release before electron transfer because of an excess positive charge on the Mn4Ca cluster.

First site-specific incorporation of a noncanonical amino acid into the photosynthetic oxygen-evolving complex

In photosystem II (PSII), water is oxidized at the oxygen-evolving complex. This process occurs through a light-induced cycle that produces oxygen and protons. While coupled proton and electron transfer reactions play an important role in PSII and other proteins, direct detection of internal proton transfer reactions is challenging. Here, we demonstrate that the unnatural amino acid, 7-azatryptophan (7AW), has unique, pH-sensitive vibrational frequencies, which are sensitive markers of proton transfer. The intrinsically disordered, PSII subunit, PsbO, which contains a single W residue (Trp241), was engineered to contain 7AW at position 241. Fluorescence shows that 7AW-241 is buried in a hydrophobic environment. Reconstitution of 7AW(241)PsbO to PSII had no significant impact on oxygen evolution activity or flash-dependent protein dynamics. We conclude that directed substitution of 7AW into other structural domains is likely to provide a nonperturbative spectroscopic probe, which can be used to define internal proton pathways in PsbO.

Ab Initio modeling of the effect of oxidation coupled with HnO deprotonation on carboxylate ligands in Mn/Ca clusters

Oxidation of some manganese complexes containing both carboxylate and water/hydroxo ligands does not result in changes to the carboxylate stretching frequencies. The water oxidizing complex of photosystem II is one motivating example. On the basis of electronic structure theory calculations, we here suggest that the deprotonation of water or hydroxo ligands minimizes changes in the vibrational frequencies of coligating carboxylates, rendering the carboxylate modes "invisible" in FTIR difference spectroscopy. This deprotonation of water/hydroxo ligands was also found to balance the redox potentials of the Mn(II)/Mn(III) and Mn(III)/Mn(IV) couples, allowing the possibility for successive manganese oxidations at a relatively constant redox potential.

Fluorescence kinetics of PSII crystals containing Ca(2+) or Sr(2+) in the oxygen evolving complex

Photosystem II (PSII) is the pigment-protein complex which converts sunlight energy into chemical energy by catalysing the process of light-driven oxidation of water into reducing equivalents in the form of protons and electrons. Three-dimensional structures from x-ray crystallography have been used extensively to model these processes. However, the crystal structures are not necessarily identical to those of the solubilised complexes. Here we compared picosecond fluorescence of solubilised and crystallised PSII core particles isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus. The fluorescence of the crystals is sensitive to the presence of artificial electron acceptors (K3Fe(CN)3) and electron transport inhibitors (DCMU). In PSII with reaction centres in the open state, the picosecond fluorescence of PSII crystals and solubilised PSII is indistinguishable. Additionally we compared picosecond fluorescence of native PSII with PSII in which Ca(2) in the oxygen evolving complex (OEC) is biosynthetically replaced by Sr(2+). With the Sr(2+) replaced OEC the average fluorescence decay slows down slightly (81ps to 85ps), and reaction centres are less readily closed, indicating that both energy transfer/trapping and electron transfer are affected by the replacement.

Proton transport facilitating water-oxidation: the role of second sphere ligands surrounding the catalytic metal cluster

The ability of PSII to extract electrons from water, with molecular oxygen as a by-product, is a remarkable biochemical and evolutionary innovation. From an evolutionary perspective, the invention of PSII approximately 2.7 Ga led to the accelerated accumulation of biomass in the biosphere and the accumulation of oxygen in the atmosphere, a combination that allowed for the evolution of a much more complex and extensive biosphere than would otherwise have been possible. From the biochemical and enzymatic perspective, PSII is remarkable because of the thermodynamic and kinetic obstacles that needed to have been overcome to oxidize water as the ultimate photosynthetic electron donor. This article focuses on how proton release is an integral part of how these kinetic and thermodynamic obstacles have been overcome: the sequential removal of protons from the active site of H2O-oxidation facilitates the multistep oxidation of the substrate water at the Mn4CaOx, the catalytic heart of the H2O-oxidation reaction. As noted previously, the facilitated deprotonation of the Mn4CaOx cluster exerts a redox-leveling function preventing the accumulation of excess positive charge on the cluster, which might otherwise hinder the already energetically difficult oxidation of water. Using recent results, including the characteristics of site-directed mutants, the role of the second sphere of amino acid ligands and the associated network of water molecules surrounding the Mn4CaOx is discussed in relation to proton transport in other systems. In addition to the redox-leveling function, a trapping function is assigned to the proton release step occurring immediately prior to the dioxygen chemistry. This trapping appears to involve a yet-to-be clarified gating mechanism that facilitates to coordinated release of a proton from the neighborhood of the active site thereby insuring that the backward charge-recombination reaction does not out-compete the forward reaction of dioxygen chemistry during this final step of H2O-oxidation.

An intrinsically disordered photosystem II subunit, PsbO, provides a structural template and a sensor of the hydrogen-bonding network in photosynthetic water oxidation

Photosystem II (PSII) is a membrane-bound enzyme that utilizes solar energy to catalyze the photooxidation of water. Molecular oxygen is evolved after four sequential light-driven oxidation reactions at the Mn4CaO5 oxygen-evolving complex, producing five sequentially oxidized states, Sn. PSII is composed of 17 membrane-spanning subunits and three extrinsic subunits, PsbP, PsbQ, and PsbO. PsbO is intrinsically disordered and plays a role in facilitation of the water oxidizing cycle. Native PsbO can be removed and substituted with recombinant PsbO, thereby restoring steady-state activity. In this report, we used reaction-induced Fourier transform infrared spectroscopy to obtain information concerning the role of PsbP, PsbQ, and PsbO during the S state cycle. Light-minus-dark difference spectra were acquired, monitoring structural changes associated with each accessible flash-induced S state transition in a highly purified plant PSII preparation (Triton X-100, octylthioglucoside). A comparison of S2 minus S1 spectra revealed that removal of PsbP and PsbQ had no significant effect on the data, whereas amide frequency and intensity changes were associated with PsbO removal. These data suggest that PsbO acts as an organizational template for the PSII reaction center. To identify any coupled conformational changes arising directly from PsbO, global (13)C-PsbO isotope editing was employed. The reaction-induced Fourier transform infrared spectra of accessible S states provide evidence that PsbO spectral contributions are temperature (263 and 277 K) and S state dependent. These experiments show that PsbO undergoes catalytically relevant structural dynamics, which are coupled over long distance to hydrogen-bonding changes at the Mn4CaO5 cluster.