Activating anions that replace Cl- in the O2-evolving complex of photosystem II slow the kinetics of the terminal step in water oxidation and destabilize the S2 and S3 states

Role of D1-Glu65 in Proton Transfer during Photosynthetic Water Oxidation in Photosystem II

Photosynthetic water oxidation takes place at the Mn₄CaO₅ cluster in photosystem II (PSII) through a light-driven cycle of five intermediates called S states (S₀-S₄). Although the PSII structures have shown the presence of several channels around the Mn₄CaO₅ cluster leading to the lumen, the pathways for proton release in the individual S-state transitions remain unidentified. Here, we studied the involvement of the so-called Cl channel in proton transfer during water oxidation by examining the effect of the mutation of D1-Glu65, a key residue in this channel, to Ala using Fourier transform infrared difference and time-resolved infrared spectroscopies together with thermoluminescence and delayed luminescence measurements. It was shown that the structure and the redox property of the catalytic site were little affected by the D1-Glu65Ala mutation. In the S₂ → S₃ transition, the efficiency was still high and the transition rate was only moderately retarded in the D1-Glu65Ala mutant. In contrast, the S₃ → S₀ transition was significantly inhibited by this mutation. These results suggest that proton transfer in the S₂ → S₃ transition occurs through multiple pathways including the Cl channel, whereas this channel likely serves as a single pathway for proton exit in the S₃ → S₀ transition.

Photosystem 2 and the oxygen evolving complex: a brief overview

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

Proton and Water Transfer Pathways in the S2 → S3 Transition of the Water-Oxidizing Complex in Photosystem II: Time-Resolved Infrared Analysis of the Effects of D1-N298A Mutation and NO3- Substitution

Photosynthetic water oxidation is performed through a light-driven cycle of five intermediates (S₀-S₄ states) in photosystem II (PSII). The S₂ → S₃ transition, which involves concerted water and proton transfer, is a key process for understanding the water oxidation mechanism. Here, to identify the water and proton transfer pathways during the S₂ → S₃ transition, we examined the effects of D1-N298A mutation and NO₃^- substitution for Cl^-, which perturbed the O1 and Cl channels, respectively, on the S₂ → S₃ kinetics using time-resolved infrared spectroscopy. The S₂ → S₃ transition was retarded both upon NO₃^- substitution and upon D1-N298A mutation, whereas it was unaffected by further NO₃^- substitution in N298A PSII. The H/D kinetic isotope effect in N298A PSII was relatively small, revealing that water transfer is a rate-limiting step in this mutant. From these results, it was suggested that during the S₂ → S₃ transition, water delivery and proton release occur through the O1 and Cl channels, respectively.

Nitrogen Dioxide at Ambient Concentrations Induces Nitration and Degradation of PYR/PYL/RCAR Receptors to Stimulate Plant Growth: A Hypothetical Model

Exposing Arabidopsis thaliana (Arabidopsis) seedlings fed with soil nitrogen to 10-50 ppb nitrogen dioxide (NO₂) for several weeks stimulated the uptake of major elements, photosynthesis, and cellular metabolisms to more than double the biomass of shoot, total leaf area and contents of N, C P, K, S, Ca and Mg per shoot relative to non-exposed control seedlings. The ¹⁵N/¹⁴N ratio analysis by mass spectrometry revealed that N derived from NO₂ (NO₂-N) comprised < 5% of the total plant N, showing that the contribution of NO₂-N as N source was minor. Moreover, histological analysis showed that leaf size and biomass were increased upon NO₂ treatment, and that these increases were attributable to leaf age-dependent enhancement of cell proliferation and enlargement. Thus, NO₂ may act as a plant growth signal rather than an N source. Exposure of Arabidopsis leaves to 40 ppm NO₂ induced virtually exclusive nitration of PsbO and PsbP proteins (a high concentration of NO₂ was used). The PMF analysis identified the ninth tyrosine residue of PsbO1 (⁹Tyr) as a nitration site. ⁹Tyr of PsbO1 was exclusively nitrated after incubation of the thylakoid membranes with a buffer containing NO₂ and NO₂^- or a buffer containing NO₂^- alone. Nitration was catalyzed by illumination and repressed by photosystem II (PSII) electron transport inhibitors, and decreased oxygen evolution. Thus, protein tyrosine nitration altered (downregulated) the physiological function of cellular proteins of Arabidopsis leaves. This indicates that NO₂-induced protein tyrosine nitration may stimulate plant growth. We hypothesized that atmospheric NO₂ at ambient concentrations may induce tyrosine nitration of PYR/PYL/RCAR receptors in Arabidopsis leaves, followed by degradation of PYR/PYL/RCAR, upregulation of target of rapamycin (TOR) regulatory complexes, and stimulation of plant growth.

A novel role for PsbO1 in photosynthetic electron transport as suggested by its light-triggered selective nitration in Arabidopsis thaliana

Exposure of Arabidopsis leaves to nitrogen dioxide (NO2) results in the selective nitration of specific proteins, such as PsbO1. The 9th tyrosine residue (9Tyr) of PsbO1 has been identified as the nitration site. This nitration is triggered by light and inhibited by photosynthetic electron transport inhibitors. During protein nitration, tyrosyl and NO2 radicals are formed concurrently and combine rapidly to form 3-nitrotyrosine. A selective oxidation mechanism for 9Tyr of PsbO1 is required. We postulated that, similar to 161Tyr of D1, 9Tyr of PsbO1 is selectively photo-oxidized by photosynthetic electron transport in response to illumination to a tyrosyl radical. In corroboration, after reappraising our oxygen evolution analysis, the nitration of PsbO1 proved responsible for decreased oxygen evolution from the thylakoid membranes. NO2 is reportedly taken into cells as nitrous acid, which dissociates to form NO2-. NO2- may be oxidized into NO2 by the oxygen-evolving complex. Light may synchronize this reaction with tyrosyl radical formation. These findings suggest a novel role for PsbO1 in photosynthetic electron transport.

Amino acid oxidation of the D1 and D2 proteins by oxygen radicals during photoinhibition of Photosystem II

The Photosystem II reaction center is vulnerable to photoinhibition. The D1 and D2 proteins, lying at the core of the photosystem, are susceptible to oxidative modification by reactive oxygen species that are formed by the photosystem during illumination. Using spin probes and EPR spectroscopy, we have determined that both O₂^•- and HO^• are involved in the photoinhibitory process. Using tandem mass spectroscopy, we have identified a number of oxidatively modified D1 and D2 residues. Our analysis indicates that these oxidative modifications are associated with formation of HO^• at both the Mn₄O₅Ca cluster and the nonheme iron. Additionally, O₂^•- appears to be formed by the reduction of O₂ at either Pheo_D1 or Q_A Early oxidation of D1:³³²H, which is coordinated with the Mn1 of the Mn₄O₅Ca cluster, appears to initiate a cascade of oxidative events that lead to the oxidative modification of numerous residues in the C termini of the D1 and D2 proteins on the donor side of the photosystem. Oxidation of D2:²⁴⁴Y, which is a bicarbonate ligand for the nonheme iron, induces the propagation of oxidative reactions in residues of the D-de loop of the D2 protein on the electron acceptor side of the photosystem. Finally, D1:¹³⁰E and D2:²⁴⁶M are oxidatively modified by O₂^•- formed by the reduction of O₂ either by Pheo_D1^•- or Q_A^•- The identification of specific amino acid residues oxidized by reactive oxygen species provides insights into the mechanism of damage to the D1 and D2 proteins under light stress.

An enzyme kinetics study of the pH dependence of chloride activation of oxygen evolution in photosystem II

Oxygen evolution by photosystem II (PSII) involves activation by Cl^- ion, which is regulated by extrinsic subunits PsbQ and PsbP. In this study, the kinetics of chloride activation of oxygen evolution was studied in preparations of PSII depleted of the PsbQ and PsbP subunits (NaCl-washed and Na₂SO₄/pH 7.5-treated) over a pH range from 5.3 to 8.0. At low pH, activation by chloride was followed by inhibition at chloride concentrations >100 mM, whereas at high pH activation continued as the chloride concentration increased above 100 mM. Both activation and inhibition were more pronounced at lower pH, indicating that Cl^- binding depended on protonation events in each case. The simplest kinetic model that could account for the complete data set included binding of Cl^- at two sites, one for activation and one for inhibition, and four protonation steps. The intrinsic (pH-independent) dissociation constant for Cl^- activation, K _S, was found to be 0.9 ± 0.2 mM for both preparations, and three of the four pK _as were determined, with the fourth falling below the pH range studied. The intrinsic inhibition constant, K _I, was found to be 64 ± 2 and 103 ± 7 mM for the NaCl-washed and Na₂SO₄/pH7.5-treated preparations, respectively, and is considered in terms of the conditions likely to be present in the thylakoid lumen. This enzyme kinetics analysis provides a more complete characterization of chloride and pH dependence of O₂ evolution activity than has been previously presented.

A nano-sized manganese oxide in a protein matrix as a natural water-oxidizing site

The purpose of this review is to present recent advances in the structural and functional studies of water-oxidizing center of Photosystem II and its surrounding protein matrix in order to synthesize artificial catalysts for production of clean and efficient hydrogen fuel.

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.

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.

Functional roles of D2-Lys317 and the interacting chloride ion in the water oxidation reaction of photosystem II as revealed by fourier transform infrared analysis

Photosynthetic water oxidation in plants and cyanobacteria is catalyzed by a Mn4CaO5 cluster within the photosystem II (PSII) protein complex. Two Cl(-) ions bound near the Mn4CaO5 cluster act as indispensable cofactors, but their functional roles remain to be clarified. We have investigated the role of the Cl(-) ion interacting with D2-K317 (designated Cl-1) by Fourier transform infrared spectroscopy (FTIR) analysis of the D2-K317R mutant of Synechocystis sp. PCC 6803 in combination with Cl(-)/NO3(-) replacement. The D2-K317R mutation perturbed the bands in the regions of the COO(-) stretching and backbone amide vibrations in the FTIR difference spectrum upon the S1 → S2 transition. In addition, this mutation altered the (15)N isotope-edited NO3(-) bands in the spectrum of NO3(-)-treated PSII. These results provide the first experimental evidence that the Cl-1 site is coupled with the Mn4CaO5 cluster and its interaction is affected by the S1 → S2 transition. It was also shown that a negative band at 1748 cm(-1) arising from COOH group(s) was altered to a positive intensity by the D2-K317R mutation as well as by NO3(-) treatment, suggesting that the Cl-1 site affects the pKa of COOH/COO(-) group(s) near the Mn4CaO5 cluster in a common hydrogen bond network. Together with the observation that the efficiency of the S3 → S0 transition significantly decreased in the core complexes of D2-K317R upon moderate dehydration, it is suggested that D2-K317 and Cl-1 are involved in a proton transfer pathway from the Mn4CaO5 cluster to the lumen, which functions in the S3 → S0 transition.

Investigation of the inhibitory effect of nitrite on Photosystem II

The role of chloride in photosystem II (PSII) is unclear. Several monovalent anions compete for the Cl(-) site(s) in PSII, and some even support activity. NO2(-) has been reported to be an activator in Cl(-)-depleted PSII membranes. In this paper, we report a detailed investigation of the chemistry of NO2(-) with PSII. NO2(-) is shown to inhibit PSII activity, and the effects on the donor side as well as the acceptor side are characterized using steady-state O2-evolution assays, electron paramagnetic resonance (EPR) spectroscopy, electron-transfer assays, and flash-induced polarographic O2 yield measurements. Enzyme kinetics analysis shows multiple sites of NO2(-) inhibition in PSII with significant inhibition of oxygen evolution at <5 mM NO2(-). By EPR spectroscopy, the yield of the S2 state remains unchanged up to 15 mM NO2(-). However, the S2-state g = 4.1 signal is favored over the g = 2 multiline signal with increasing NO2(-) concentrations. This could indicate competition of NO2(-) for the Cl(-) site at higher NO2(-) concentrations. In addition to the donor-side chemistry, there is clear evidence of an acceptor-side effect of NO2(-). The g = 1.9 Fe(II)-QA(-•) signal is replaced by a broad g = 1.6 signal in the presence of NO2(-). Additionally, a g = 1.8 Fe(II)-Q(-•) signal is present in the dark, indicating the formation of a NO2(-)-bound Fe(II)-QB(-•) species in the dark. Electron-transfer assays suggest that the inhibitory effect of NO2(-) on the activity of PSII is largely due to the donor-side chemistry of NO2(-). UV-visible spectroscopy and flash-induced polarographic O2 yield measurements indicate that NO2(-) is oxidized by the oxygen-evolving complex in the higher S states, contributing to the donor-side inhibition by NO2(-).

Mechanism of light induced water splitting in Photosystem II of oxygen evolving photosynthetic organisms

The reactions of light induced oxidative water splitting were analyzed within the framework of the empirical rate constant-distance relationship of non-adiabatic electron transfer in biological systems (C. C. Page, C. C. Moser, X. Chen , P. L. Dutton, Nature 402 (1999) 47-52) on the basis of structure information on Photosystem II (PS II) (A. Guskov, A. Gabdulkhakov, M. Broser, C. Glöckner, J. Hellmich, J. Kern, J. Frank, W. Saenger, A. Zouni, Chem. Phys. Chem. 11 (2010) 1160-1171, Y. Umena, K. Kawakami, J-R Shen, N. Kamiya, Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9Å. Nature 47 (2011) 55-60). Comparison of these results with experimental data leads to the following conclusions: 1) The oxidation of tyrosine Y(z) by the cation radical P680(+·) in systems with an intact water oxidizing complex (WOC) is kinetically limited by the non-adiabatic electron transfer step and the extent of this reaction is thermodynamically determined by relaxation processes in the environment including rearrangements of hydrogen bond network(s). In marked contrast, all Y(z)(ox) induced oxidation steps in the WOC up to redox state S(3) are kinetically limited by trigger reactions which are slower by orders of magnitude than the rates calculated for non-adiabatic electron transfer. 3) The overall rate of the triggered reaction sequence of Y(z)(ox) reduction by the WOC in redox state S(3) eventually leading to formation and release of O(2) is kinetically limited by an uphill electron transfer step. Alternative models are discussed for this reaction. The protein matrix of the WOC and bound water molecules provide an optimized dynamic landscape of hydrogen bonded protons for catalyzing oxidative water splitting energetically driven by light induced formation of the cation radical P680(+·). In this way the PS II core acts as a molecular machine formed during a long evolutionary process. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

The extrinsic proteins of Photosystem II

In this review we examine the structure and function of the extrinsic proteins of Photosystem II. These proteins include PsbO, present in all oxygenic organisms, the PsbP and PsbQ proteins, which are found in higher plants and eukaryotic algae, and the PsbU, PsbV, CyanoQ, and CyanoP proteins, which are found in the cyanobacteria. These proteins serve to optimize oxygen evolution at physiological calcium and chloride concentrations. They also shield the Mn(4)CaO(5) cluster from exogenous reductants. Numerous biochemical, genetic and structural studies have been used to probe the structure and function of these proteins within the photosystem. We will discuss the most recent proposed functional roles for these components, their structures (as deduced from biochemical and X-ray crystallographic studies) and the locations of their proposed binding domains within the Photosystem II complex. This article is part of a Special Issue entitled: Photosystem II.

Chloride regulation of enzyme turnover: application to the role of chloride in photosystem II

Chloride-dependent α-amylases, angiotensin-converting enzyme (ACE), and photosystem II (PSII) are activated by bound chloride. Chloride-binding sites in these enzymes contain a positively charged Arg or Lys residue crucial for chloride binding. In α-amylases and ACE, removal of chloride from the binding site triggers formation of a salt bridge between the positively charged Arg or Lys residue involved in chloride binding and a nearby carboxylate residue. The mechanism for chloride activation in ACE and chloride-dependent α-amylases is 2-fold: (i) correctly positioning catalytic residues or other residues involved in stabilizing the enzyme-substrate complex and (ii) fine-tuning of the pKa of a catalytic residue. By using examples of how chloride activates α-amylases and ACE, we can gain insight into the potential mechanisms by which chloride functions in PSII. Recent structural evidence from cyanobacterial PSII indicates that there is at least one chloride-binding site in the vicinity of the oxygen-evolving complex (OEC). Here we propose that, in the absence of chloride, a salt bridge between D2:K317 and D1:D61 (and/or D1:E333) is formed. This can cause a conformational shift of D1:D61 and lower the pKa of this residue, making it an inefficient proton acceptor during the S-state cycle. Movement of the D1:E333 ligand and the adjacent D1:H332 ligand due to chloride removal could also explain the observed change in the magnetic properties of the manganese cluster in the OEC upon chloride depletion.

Substitution of chloride by bromide modifies the low-temperature tyrosine Z oxidation in active photosystem II

Chloride is an essential cofactor for photosynthetic water oxidation. However, its location and functional roles in active photosystem II are still a matter of debate. We have investigated this issue by studying the effects of Cl- replacement by Br- in active PSII. In Br- substituted samples, Cl- is effectively replaced by Br- in the presence of 1.2 M NaBr under room light with protection of anaerobic atmosphere followed by dialysis. The following results have been obtained. i) The oxygen-evolving activities of the Br--PSII samples are significantly lower than that of the Cl--PSII samples; ii) The same S2 multiline EPR signals are observed in both Br- and Cl--PSII samples; iii) The amplitudes of the visible light induced S1TyrZ* and S2TyrZ* EPR signals are significantly decreased after Br- substitution; the S1TyrZ* EPR signal is up-shifted about 8G, whereas the S2TyrZ* signal is down-shifted about 12 G after Br- substitution. These results imply that the redox properties of TyrZ and spin interactions between TyrZ* and Mn-cluster could be significantly modified due to Br- substitution. It is suggested that Cl-/Br- probably coordinates to the Ca2+ ion of the Mn-cluster in active photosystem II.

Inorganic cofactor stabilization and retention: the unique functions of the two PsbO subunits of eukaryotic photosystem II

Eukaryotic PsbO, the photosystem II (PSII) manganese-stabilizing protein, has two N-terminal sequences that are required for binding of two copies of the protein to PSII [Popelkova, H., et al. (2002) Biochemistry 41, 10038-10045; Popelkova, H., et al. (2003) Biochemistry 42, 6193-6200]. In the work reported here, a set of selected N-terminal truncation mutants of PsbO that affect subunit binding to PSII were used to determine the effects of PsbO stoichiometry on the Mn, Ca(2+), and Cl(-) cofactors and to characterize the roles of each of the PsbO subunits in PSII function. Results of the experiments with the PsbO-depleted PSII membranes reconstituted with the PsbO deletion mutants showed that the presence of PsbO does not affect Ca(2+) retention by PSII in steady-state assays of activity, nor is it required for Ca(2+) to protect the Mn cluster against reductive inhibition in darkness. In contrast to the results with Ca(2+), PsbO increases the affinity of Cl(-) for the active site of the O(2)-evolving complex (OEC) as expected. These results together with other data on activity retention suggest that PsbO can stabilize the Mn cluster by facilitating retention of Cl(-) in the OEC. The data presented here indicate that each of two copies of PsbO has a distinctive function in PSII. Binding of the first PsbO subunit fully stabilizes the Mn cluster and enhances Cl(-) retention, while binding of the second subunit optimizes Cl(-) retention, which in turn maximizes O(2) evolution activity. Nonspecific binding of some PsbO truncation mutants to PSII has no functional significance.

Azide as a probe of proton transfer reactions in photosynthetic oxygen evolution

In oxygenic photosynthesis, photosystem II (PSII) is the multisubunit membrane protein responsible for the oxidation of water to O2 and the reduction of plastoquinone to plastoquinol. One electron charge separation in the PSII reaction center is coupled to sequential oxidation reactions at the oxygen-evolving complex (OEC), which is composed of four manganese ions and one calcium ion. The sequentially oxidized forms of the OEC are referred to as the S(n) states. S(1) is the dark-adapted state of the OEC. Flash-induced oxygen production oscillates with period four and occurs during the S(3) to S(0) transition. Chloride plays an important, but poorly understood role in photosynthetic water oxidation. Chloride removal is known to block manganese oxidation during the S(2) to S(3) transition. In this work, we have used azide as a probe of proton transfer reactions in PSII. PSII was sulfate-treated to deplete chloride and then treated with azide. Steady state oxygen evolution measurements demonstrate that azide inhibits oxygen evolution in a chloride-dependent manner and that azide is a mixed or noncompetitive inhibitor. This result is consistent with two azide binding sites, one at which azide competes with chloride and one at which azide and chloride do not compete. At pH 7.5, the K(i) for the competing site was estimated as 1 mM, and the K(i)' for the uncompetitive site was estimated as 8 mM. Vibrational spectroscopy was then used to monitor perturbations in the frequency and amplitude of the azide antisymmetric stretching band. These changes were induced by laser-induced charge separation in the PSII reaction center. The results suggest that azide is involved in proton transfer reactions, which occur before manganese oxidation, on the donor side of chloride-depleted PSII.

Elucidating the site of action of oxalate in photosynthetic electron transport chain in spinach thylakoid membranes

The effects of oxalate on PS II and PS I photochemistry were studied. The results suggested that in chloride-deficient thylakoid membranes, oxalate inhibited activity of PS II as well as PS I. To our knowledge, this is the only anion so far known which inhibits both the photosystems. Measurements of fluorescence induction kinetics, YZ* decay, and S2 state multiline EPR signal suggested that oxalate inhibited PS II at the donor side most likely on the oxygen evolving complex. Measurements of re-reduction of P700+ signal in isolated PS I particles in oxalate-treated samples suggested a binding site of oxalate on the donor, as well as the acceptor side of PS I.

Using small molecule complexes to elucidate features of photosynthetic water oxidation

The molecular oxygen produced in photosynthesis is generated via water oxidation at a manganese-calcium cluster called the oxygen-evolving complex (OEC). While studies in biophysics, biochemistry, and structural and molecular biology are well known to provide deeper insight into the structure and workings of this system, it is often less appreciated that biomimetic modelling provides the foundation for interpreting photosynthetic reactions. The synthesis and characterization of small model complexes, which either mimic structural features of the OEC or are capable of providing insight into the mechanism of O2 evolution, have become a vital contributor to this scientific field. Our group has contributed to these findings in recent years through synthesis of model complexes, spectroscopic characterization of these systems and probing the reactivity in the context of water oxidation. In this article we describe how models have made significant contributions ranging from understanding the structure of the water-oxidation centre (e.g. contributions to defining a tetrameric Mn3Ca-cluster with a dangler Mn) to the ability to discriminate between different mechanistic proposals (e.g. showing that the Babcock scheme for water oxidation is unlikely).

Interactions of photosystem II with bicarbonate, formate and acetate

In this study, we probe the effects of bicarbonate (hydrogencarbonate), BC, removal from photosystem II in spinach thylakoids by measuring flash-induced oxygen evolution patterns (FIOPs) with a Joliot-type electrode. For this we compared three commonly employed methods: (1) washing in BC-free medium, (2) formate addition, and (3) acetate addition. Washing of the samples with buffers depleted of BC and CO2 by bubbling with argon (Method 1) under our conditions leads to an increase in the double hit parameter of the first flash (beta 1), while the miss parameter and the overall activity remain unchanged. In contrast, addition of 40-50 mM formate or acetate results in a significant increase in the miss parameter and to an approximately 50% (formate) and approximately 10% (acetate) inhibition of the overall oxygen evolution activity, but not to an increased beta 1 parameter. All described effects could be reversed by washing with formate/acetate free buffer and/or addition of 2-10 mM bicarbonate. The redox potential of the water-oxidizing complex (WOC) in samples treated by Method 1 is compared to samples containing 2 mM bicarbonate in two ways: (1) The lifetimes of the S0, S2, and S3 states were measured, and no differences were found between the two sample types. (2) The S1, S0, S(-1), and S(-2) states were probed by incubation with small concentrations of NH2OH. These experiments displayed a subtle, yet highly reproducible difference in the apparent Si/S(-i) state distribution which is shown to arise from the interaction of BC with PSII in the already reduced states of the WOC. These data are discussed in detail by also taking into account the CO2 concentrations present in the buffers after argon bubbling and during the measurements. These values were measured by membrane-inlet mass spectrometry (MIMS).

An Activated Glutamate Residue Identified in Photosystem II at the Interface between the Manganese-stabilizing Subunit and the D2 Polypeptide

Photosystem II (PSII) catalyzes the oxidation of water during oxygenic photosynthesis. PSII is composed both of intrinsic subunits, such as D1, D2, and CP47, and extrinsic subunits, such as the manganese-stabilizing subunit (MSP). Previous work has shown that amines covalently bind to amino acid residues in the CP47, D1, and D2 subunits of plant and cyanobacterial PSII, and that these covalent reactions are prevented by the addition of chloride in plant preparations depleted of the 18- and 24-kDa extrinsic subunits. It has been proposed that these reactive groups are carbonyl-containing, post-translationally modified amino acid side chains (Ouellette, A. J. A., Anderson, L. B., and Barry, B. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 2204-2209 and Anderson, L. B., Ouellette, A. J. A., and Barry, B. A. (2000) J. Biol. Chem. 275, 4920-4927). To identify the amino acid binding site in the spinach D2 subunit, we have employed a biotin-amine labeling reagent, which can be used in conjunction with avidin affinity chromatography to purify biotinylated peptides from the PSII complex. Multidimensional chromato-graphic separation and multistage mass spectrometry localizes a novel post-translational modification in the D2 subunit to glutamate 303. We propose that this glutamate is activated for amine reaction by post-translational modification. Because the modified glutamate is located at a contact site between the D2 and manganese-stabilizing subunits, we suggest that the modification is important in vivo in stabilizing the interaction between these two PSII subunits. Consistent with this conclusion, mutations at the modified glutamate alter the steady-state rate of photosynthetic oxygen evolution.

Differential response of chloride binding sites to elevated temperature: a comparative study in spinach thylakoids and PSII-enriched membranes

A study of heat effects was performed in thylakoids and photosystem II (PSII)-enriched membranes isolated from spinach in relation to Cl(-)-induced activation of PSII catalyzed oxygen evolution and the retention of Cl(-) in the PSII complex. For this, Cl(-)-sufficient membranes and low-Cl(-) membranes were used. The presence of Cl(-) in the reaction medium did accelerate oxygen evolution, which remained unaffected by heat treatment up to 40 degrees C in PSII membranes and up to 42.5 degrees C in thylakoids. Heat resistance of Cl(-)-induced activation of oxygen evolution was found to be independent of the presence of 'bound Cl(-)' in the preparations. However, the functional stability of the PSII complex during heat treatment showed a marked dependence on the presence of bound Cl(-) in PSII. Electron paramagnetic resonance study of manganese (Mn) release per reaction center/Y (D) (+) showed that there was little loss of Mn(2+) up to 42 degrees C in our preparations, although the PSII activity was significantly lowered. These observations together with data from steady state chlorophyll a fluorescence imply that the site of action of Cl(-) causing direct activation of oxygen evolution was different from the site of primary heat damage. A differential response of chloride binding sites to heat stress was observed. The high-affinity (tightly bound, slow exchanging) site of chloride is affected earlier ( approximately 37 degrees C) while low-affinity (loosely bound, fast exchanging) site gets affected at higher temperatures (42.5 degrees C in thylakoids and 40 degrees C in the case of PSII-enriched membranes).

Current status of the role of Cl(-) ion in the oxygen-evolving complex

This minireview summarizes the current state of knowledge concerning the role of Cl(-) in the oxygen-evolving complex (OEC) of photosystem II (PSII). The model that proposes that Cl(-) is a Mn ligand is discussed in light of more recent work. Studies of Cl(-) specificity, stoichiometry, kinetics, and retention by extrinsic polypeptides are discussed, as are the results that fail to detect Cl(-) ligation to Mn and results that show a lack of a requirement for Cl(-) in PSII-catalyzed H(2)O oxidation. Mutagenesis experiments in cyanobacteria and higher plants that produce evidence for a correlation between Cl(-) retention and stable interactions among intrinsic and extrinsic polypeptides are summarized, and spectroscopic data on the interaction between PSII and Cl(-) are discussed. Lastly, the question of the site of Cl(-) action in PSII is discussed in connection with the current crystal structures of the enzyme.

Perturbations at the chloride site during the photosynthetic oxygen-evolving cycle

Photosystem II (PSII) catalyzes the oxidation of water to O2 at the manganese-containing, oxygen-evolving complex (OEC). Photoexcitation of PSII results in the oxidation of the OEC; four sequential oxidation reactions are required for the generation and release of molecular oxygen. Therefore, with flash illumination, the OEC cycles among five Sn states. Chloride depletion inhibits O2 evolution. However, the binding site of chloride in the OEC is not known, and the role of chloride in oxygen evolution has not as yet been elucidated. We have employed reaction-induced FT-IR spectroscopy and selective flash excitation, which cycles PSII samples through the S state transitions. On the time scale employed, these FT-IR difference spectra reflect long-lived structural changes in the OEC. Bromide substitution supports oxygen evolution and was used to identify vibrational bands arising from structural changes at the chloride-binding site. Contributions to the vibrational spectrum from bromide-sensitive bands were observed on each flash. Sulfate treatment led to an elimination of oxygen evolution activity and of the FT-IR spectra assigned to the S3 to S0 (third flash) and S0 to S1 transitions (fourth flash). However, sulfate treatment changed, but did not eliminate, the FT-IR spectra obtained with the first and second flashes. Solvent isotope exchange in chloride-exchanged samples suggests flash-dependent structural changes, which alter protein dynamics during the S state cycle.

Influence of the Redox Potential of the Primary Quinone Electron Acceptor on Photoinhibition in Photosystem II

We report the characterization of the effects of the A249S mutation located within the binding pocket of the primary quinone electron acceptor, Q(A), in the D2 subunit of photosystem II in Thermosynechococcus elongatus. This mutation shifts the redox potential of Q(A) by approximately -60 mV. This mutant provides an opportunity to test the hypothesis, proposed earlier from herbicide-induced redox effects, that photoinhibition (light-induced damage of the photosynthetic apparatus) is modulated by the potential of Q(A). Thus the influence of the redox potential of Q(A) on photoinhibition was investigated in vivo and in vitro. Compared with the wild-type, the A249S mutant showed an accelerated photoinhibition and an increase in singlet oxygen production. Measurements of thermoluminescence and of the fluorescence yield decay kinetics indicated that the charge-separated state involving Q(A) was destabilized in the A249S mutant. These findings support the hypothesis that a decrease in the redox potential of Q(A) causes an increase in singlet oxygen-mediated photoinhibition by favoring the back-reaction route that involves formation of the reaction center chlorophyll triplet. The kinetics of charge recombination are interpreted in terms of a dynamic structural heterogeneity in photosystem II that results in high and low potential forms of Q(A). The effect of the A249S mutation seems to reflect a shift in the structural equilibrium favoring the low potential form.

Highly efficient photoactivation of Mn-depleted photosystem II by imidazole-liganded manganese complexes

The oxygen-evolving complex (OEC) of Mn-depleted photosystem II (PSII) can be reconstituted in the presence of exogenous Mn or a Mn complex under weak illumination, a process called photoactivation. Synthetic Mn complexes could provide a powerful system to analyze the assembly of the OEC. In this work, four mononuclear Mn complexes, [(terpy)2Mn(II)(OOCH3)] x 2 H2O (where terpy is 2,2':6',2''-terpyridine), Mn(II)(bzimpy)2, Mn(II)(bp)2(CH3CH2OH)2 [where bzimpy is 2,6-bis(2-benzimidazol-2-yl)pyridine] and [Mn(III)(HL)(L)(py)(CH3OH)]CH3OH (where py is pyridine) were used in photoactivation experiments. Measurements of the photoreduction of 2,6-dichorophenolindophenol and oxygen evolution demonstrate that photoactivation is more efficient when Mn complexes are used instead of MnCl2 in reconstructed PSII preparations. The most efficient recoveries of oxygen evolution and electron transport activities are obtained from a complex, [Mn(III)(HL)(L)(py)(CH3OH)]CH3OH, that contains both imidazole and phenol groups. Its recovery of the rate of oxygen evolution is as high as 79% even in the absence of the 33-kDa peptide. The imidazole ligands of the Mn complex probably accelerate P680*+ reduction and consequently facilitate the process of photoactivation. Also, the strong intermolecular hydrogen bond probably facilitates interaction with the Mn-depleted PSII via reorganization of the hydrogen-bonding network, and therefore promotes the recovery of oxygen evolution and electron transport activities.

EPR characteristics of chloride-depleted photosystem II membranes in the presence of other anions

Chloride is an essential cofactor for the oxidation of water to oxygen. Anion substitution (Br(-), I(-), NO(2)(-), F(-)) in Cl(-)-depleted PS II membranes brings out significant changes in the EPR signals arising from the S(2) state and from the iron-quinone complex of PS II. On the basis of the changes observed in the S(2) state multiline signal and the Q(A)Fe(3+) EPR signal in Cl(-)-depleted PS II membranes after substituting with various anions, we report a possible binding site of anions such as chloride and bromide at the PS II donor side as well as at the acceptor side.

Oxidation of the Mn cluster induces structural changes of NO3- functionally bound to the Cl- site in the oxygen-evolving complex of photosystem II

Cl(-) is an indispensable cofactor for photosynthetic O(2) evolution and is functionally replaced by NO(3)(-). Structural changes of an isotopically labeled NO(3)(-) ion, induced by the oxidation of the Mn cluster (S(1)-to-S(2)), were detected by FTIR spectroscopy. NO(3)(-)-substituted photosystem II core particles showed (14)N(16)O(3)(-)/(15)N(16)O(3)(-) and (14)N(16)O(3)(-)/(14)N(18)O(3)(-) isotopic bands in the S(2)/S(1) spectra with markedly high signal/noise ratio. These bands appeared only in the region from 1415 to 1284 cm(-1), indicating that the bands do not arise from a metal-bound NO(3)(-) but from an ionic NO(3)(-). The intensity of the bands exhibited a quantitatively proportional relationship with the O(2) activity. These results demonstrate that the NO(3)(-) functionally bound to the Cl(-) site couples to the Mn cluster structurally, but is not associated with the cluster as a direct ligand. Comparison of the bands for two isotopes ((15)N and (18)O) and their simulations enable us to assign each band to the S(1) and S(2) states. The results indicate that the NO(3)(-) ion bound to the Cl(-) site is highly asymmetric in S(1) but rather symmetric in S(2). Since NO(3)(-) functionally replaces Cl(-), most of the conclusions drawn from this study will be also applicable to Cl(-).