Studies on the proton release pattern of the donor side of system II: Correlation between oxidation and deprotonization of donor D 1 in Tris-washed inside-out thylakoids

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 differences in microenvironments and functions of tyrosine radicals YZ and YD in photosystem II studied by EPR

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were performed to investigate the difference in microenvironments and functions between tyrosine Z (Y(Z)) and tyrosine D (Y(D)). Mn-depletion or Ca(2+)-depletion causes extension of the lifetime of tyrosine radical Y(Z)(*), which can be trapped by rapid freezing after illumination at about 250 K. Above pH 6.5, Y(Z)(*) radical in Mn-depleted PS II shows similar EPR and ENDOR spectra similar to that of Y(D)(*) radical, which are ascribed to a typical neutral tyrosine radical. Below pH 6.5, Y(Z)(*) radical shows quite different EPR and ENDOR spectra. ENDOR spectra show the spin density distribution of the low-pH form of Y(Z)(*) that has been quite different from the high-pH form of Y(Z)(*). The spin density distribution of the low-pH Y(Z)(*) can be explained by a cation radical or the neutral radical induced by strong electrostatic interaction. The pH dependence of the activation energy of the recombination rate between Y(Z)(*) and Q(A)(-) shows a gap of 4.4 kJ/mol at pH 6.0-6.5. In the Ca(2+)-depleted PS II, Y(Z)(*) signal was the mixture of the cation-like and normal neutral radicals, and the pH dependence of Y(Z)(*) spectrum in Ca(2+)-depleted PS II is considerably different from the neutral radical found in Mn-depleted PS II. Based on the recent structure data of cyanobacterial PS II, the pH dependence of Y(Z)(*) could be ascribed to the modification of the local structure and hydrogen-bonding network induced by the dissociation of ASP170 near Y(Z).

Oxidative photosynthetic water splitting: energetics, kinetics and mechanism

This minireview is an attempt to summarize our current knowledge on oxidative water splitting in photosynthesis. Based on the extended Kok model (Kok, Forbush, McGloin (1970) Photochem Photobiol 11:457-476) as a framework, the energetics and kinetics of two different types of reactions comprising the overall process are discussed: (i) P680+* reduction by the redox active tyrosine YZ of polypeptide D1 and (ii) Yz (ox) induced oxidation of the four step sequence in the water oxidizing complex (WOC) leading to the formation of molecular oxygen. The mode of coupling between electron transport (ET) and proton transfer (PT) is of key mechanistic relevance for the redox turnover of YZ and the reactions within the WOC. The peculiar energetics of the oxidation steps in the WOC assure that redox state S1 is thermodynamically most stable. This is a general feature in all oxygen evolving photosynthetic organisms and assumed to be of physiological relevance. The reaction coordinate of oxidative water splitting is discussed on the basis of the available information about the Gibbs energy differences between the individual redox states Si+1 and Si and the data reported for the activation energies of the individual oxidation steps in the WOC. Finally, an attempt is made to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and on the active role of the protein in tuning the local proton activity that depends on time and redox state Si. The O-O linkage is assumed to take place at the level of a complexed peroxide.

Functional Characterization of Monomeric Photosystem II Core Preparations fromThermosynechococcus elongatuswith or without the Psb27 Protein

Two monomeric fractions of photosystem II (PS II) core pacticles from the thermophilic cyanobacterium Thermosynechococcus elongatus have been investigated using flash-induced variable fluorescence kinetics and EPR spectroscopy. One fraction was highly active in oxygen evolution and contained the extrinsic protein subunits PsbO, PsbU, and PsbV. The other monomeric fraction lacked oxygen evolving activity as well as the three extrinsic subunits, but the luminally located, extrinsic Psb27 lipoprotein was present. In the monomeric fraction with bound Psb27, flash-induced variable fluorescence showed an absence of oxidizable Mn on the donor side of PS II and impaired forward electron transfer from the primary quinone acceptor, QA. These results were confirmed with EPR spectroscopy by the absence of the "split S1" interaction signal from YZ* and the CaMn4 cluster and by the absence of the S2-state multiline signal. A different protein composition on the donor side of PS II monomers with Psb27 was also supported by the lack of an EPR signal from cytochrome c550 (in the PsbV subunit). In addition, we did not observe any oxidation of cytochrome b559 at low temperature in this fraction. The presence of Psb27 and the absence of the CaMn4 cluster did not affect the protein matrix around YD or the acceptor side quinones as can be judged from the appearance of the corresponding EPR signals. The diminished electron transport capabilities on both the donor and the acceptor side of PS II when Psb27 is present give further indications that this PS II complex is involved in the earlier steps of the PS II repair cycle.

Reaction pattern and mechanism of light induced oxidative water splitting in photosynthesis

This mini review is an attempt to briefly summarize our current knowledge on light driven oxidative water splitting in photosynthesis. The reaction leading to molecular oxygen and four protons via photosynthesis comprises thermodynamic and kinetic constraints that require a balanced fine tuning of the reaction coordinates. The mode of coupling between electron (ET) and proton transfer (PT) reactions is shown to be of key mechanistic relevance for the redox turnover of Y(Z) and the reactions within the WOC. The WOC is characterized by peculiar energetics of its oxidation steps in the WOC. In all oxygen evolving photosynthetic organisms the redox state S(1) is thermodynamically most stable and therefore this general feature is assumed to be of physiological relevance. Available information on the Gibbs energy differences between the individual redox states S(i+1) and S(i) and on the activation energies of their oxidative transitions are used to construct a general reaction coordinate of oxidative water splitting in photosystem II (PS II). Finally, an attempt is presented to cast our current state of knowledge into a mechanism of oxidative water splitting with special emphasis on the formation of the essential O-O bond and the active role of the protein environment in tuning the local proton activity that depends on time and redox state S(i). The O-O linkage is assumed to take place within a multistate equilibrium at the redox level of S(3), comprising both redox isomerism and proton tautomerism. It is proposed that one state, S(3)(P), attains an electronic configuration and nuclear geometry that corresponds with a hydrogen bonded peroxide which acts as the entatic state for the generation of complexed molecular oxygen through S(3)(P) oxidation by Y(Z)(ox).

Coupling of electron and proton transfer in oxidative water cleavage in photosynthesis

This minireview addresses questions on the mechanism of oxidative water cleavage with special emphasis on the coupling of electron (ET) and proton transfer (PT) of each individual redox step of the reaction sequence and on the mode of O-O bond formation. The following topics are discussed: (1) the multiphasic kinetics of Y(Z)(ox) formation by P680(+*) originate from three different types of rate limitations: (i) nonadiabatic electron transfer for the "fast" ns reaction, (ii) local "dielectric" relaxation for the "slow" ns reaction, and (iii) "large-scale" proton shift for the micros kinetics; (2) the ET/PT-coupling mode of the individual redox transitions within the water oxidizing complex (WOC) driven by Y(Z)(ox) is assumed to depend on the redox state S(i): the oxidation steps of S(0) and S(1) comprise separate ET and PT pathways while those of S(2) and S(3) take place via proton-coupled electron transfer (PCET) analogous to Jerry Babcock's hydrogen atom abstractor model [Biochim. Biophys. Acta, 1458 (2000) 199]; (3) S(3) is postulated to be a multistate redox level of the WOC with fast dynamic equilibria of both redox isomerism and proton tautomerism. The primary event in the essential O-O bond formation is the population of a state S(3)(P) characterized by an electronic configuration and nuclear geometry that corresponds with a complexed hydrogen peroxide; (4) the peroxidic type S(3)(P) is the entatic state for formation of complexed molecular oxygen through S(3) oxidation by Y(Z)(ox); and (5) the protein matrix itself is proposed to exert catalytic activity by functioning as "PCET director". The WOC is envisaged as a supermolecule that is especially tailored for oxidative water cleavage and acts as a molecular machine.

Electron, proton and hydrogen-atom transfers in photosynthetic water oxidation

When photosynthetic organisms developed so that they could use water as an electron source to reduce carbon dioxide, the stage was set for efficient proliferation. Algae and plants spread globally and provided the foundation for our atmosphere and for O(2)-based chemistry in biological systems. Light-driven water oxidation is catalysed by photosystem II, the active site of which contains a redox-active tyrosine denoted Y(Z), a tetramanganese cluster, calcium and chloride. In 1995, Gerald Babcock and co-workers presented the hypothesis that photosynthetic water oxidation occurs as a metallo-radical catalysed process. In this model, the oxidized tyrosine radical is generated by coupled proton/electron transfer and re-reduced by abstracting hydrogen atoms from substrate water or hydroxide-ligated to the manganese cluster. The proposed function of Y(Z) requires proton transfer from the tyrosine site upon oxidation. The oxidation mechanism of Y(Z) in an inhibited and O(2)-evolving photosystem II is discussed. Domino-deprotonation from Y(Z) to the bulk solution is shown to be consistent with a variety of data obtained on metal-depleted samples. Experimental data that suggest that the oxidation of Y(Z) in O(2)-evolving samples is coupled to proton transfer in a hydrogen-bonding network are described. Finally, a dielectric-dependent model for the proton release that is associated with the catalytic cycle of photosystem II is discussed.

Water oxidation chemistry of photosystem II

The O(2)-evolving complex of photosystem II catalyses the light-driven four-electron oxidation of water to dioxygen in photosynthesis. In this article, the steps leading to photosynthetic O(2) evolution are discussed. Emphasis is given to the proton-coupled electron-transfer steps involved in oxidation of the manganese cluster by oxidized tyrosine Z (Y(*)(Z)), the function of Ca(2+) and the mechanism by which water is activated for formation of an O-O bond. Based on a consideration of the biophysical studies of photosystem II and inorganic manganese model chemistry, a mechanism for photosynthetic O(2) evolution is presented in which the O-O bond-forming step occurs via nucleophilic attack on an electron-deficient Mn(V)=O species by a calcium-bound water molecule. The proposed mechanism includes specific roles for the tetranuclear manganese cluster, calcium, chloride, Y(Z) and His190 of the D1 polypeptide. Recent studies of the ion selectivity of the calcium site in the O(2)-evolving complex and of a functional inorganic manganese model system that test key aspects of this mechanism are also discussed.

Mechanism of photosynthetic water oxidation: combining biophysical studies of photosystem II with inorganic model chemistry

A mechanism for photosynthetic water oxidation is proposed based on a structural model of the oxygen-evolving complex (OEC) and its placement into the modeled structure of the D1/D2 core of photosystem II. The structural model of the OEC satisfies many of the geometrical constraints imposed by spectroscopic and biophysical results. The model includes the tetranuclear manganese cluster, calcium, chloride, tyrosine Z, H190, D170, H332 and H337 of the D1 polypeptide and is patterned after the reversible O2-binding diferric site in oxyhemerythrin. The mechanism for water oxidation readily follows from the structural model. Concerted proton-coupled electron transfer in the S2-->S3 and S3-->S4 transitions forms a terminal Mn(V)=O moiety. Nucleophilic attack on this electron-deficient Mn(V)=O by a calcium-bound water molecule results in a Mn(III)-OOH species, similar to the ferric hydroperoxide in oxyhemerythrin. Dioxygen is released in a manner analogous to that in oxyhemerythrin, concomitant with reduction of manganese and protonation of a mu-oxo bridge.

Proton equilibria in the manganese cluster of photosystem II control the intensities of the S(0) and S(2) state g approximately 2 electron paramagnetic resonance signals

We have studied the pH effect on the S(0) and S(2) multiline electron paramagnetic resonance (EPR) signals from the water-oxidizing complex of photosystem II. Around pH 6, the maximum signal intensities were detected. On both the acidic and alkaline sides of pH 6, the intensities of the EPR signals decreased. Two pKs were determined for the S(0) multiline signal; pK(1) = 4.2 +/- 0.2 and pK(2) = 8.0 +/- 0.1, and for the S(2) multiline signal the pKs were pK(1) = 4.5 +/- 0.1 and pK(2) = 7.6 +/- 0.1. The intensity of the S(0)-state EPR signal was partly restored when the pH was changed from acidic or alkaline pH back to pH approximately 6. In the S(2) state we observed partial recovery of the multiline signal when going from alkaline pH back to pH approximately 6, whereas no significant recovery of the S(2) multiline signal was observed when the pH was changed from acidic pH back to pH approximately 6. Several possible explanations for the intensity changes as a function of pH are discussed. Some are ruled out, such as disintegration of the Mn cluster or decay of the S states and formal Cl(-) and Ca(2+) depletion. The altered EPR signal intensities probably reflect the protonation/deprotonation of ligands to the Mn cluster or the oxo bridges between the Mn ions. Also, the possibility of decreased multiline signal intensities at alkaline pH as an effect of changed redox potential of Y(Z) is put forward.

Tyrosine-Z in oxygen-evolving photosystem II: a hydrogen-bonded tyrosinate

In oxygen-evolving photosystem II (PSII), a tyrosine residue, D1Tyr161 (YZ), serves as the intermediate electron carrier between the catalytic Mn cluster and the photochemically active chlorophyll moiety P680. A more direct catalytic role of YZ, as a hydrogen abstractor from bound water, has been postulated. That YZox appears as a neutral (i.e. deprotonated) radical, YZ*, in EPR studies is compatible with this notion. Data based on electrochromic absorption transients, however, are conflicting because they indicate that the phenolic proton remains on or near to YZox. In Mn-depleted PSII the electron transfer between YZ and P680+ can be almost as fast as in oxygen-evolving material, however, only at alkaline pH. With an apparent pK of about 7 the fast reaction is suppressed and converted into an about 100-fold slower one which dominates at acid pH. In the present work we investigated the optical difference spectra attributable to the transition YZ --> YZox as function of the pH. We scanned the UV and VIS range and used Mn-depleted PSII core particles and also oxygen-evolving ones. Comparing these spectra with published in vitro and in vivo spectra of phenolic compounds, we arrived at the following conclusions: In oxygen-evolving PSII YZ resembles a hydrogen-bonded tyrosinate, YZ(-).H(+).B. The phenolic proton is shifted toward a base B already in the reduced state and even more so in the oxidized state. The retention of the phenolic proton in a hydrogen-bonded network gives rise to a positive net charge in the immediate vicinity of the neutral radical YZ*. It may be favorable both for the very rapid reduction by YZ of P680+ and for electron (not hydrogen) abstraction by YZ* from the Mn-water cluster.

Involvement of histidine 190 on the D1 protein in electron/proton transfer reactions on the donor side of photosystem II

Flash-induced chlorophyll fluorescence kinetics from photosystem II in thylakoids from the dark-grown wild type and two site-directed mutants of the D1 protein His190 residue (D1-H190) in Chlamydomonas reinhardtii have been characterized. Induction of the chlorophyll fluorescence on the first flash, reflecting electron transport from YZ to P680(+), exhibited a strong pH dependence with a pK of 7.6 in the dark-grown wild type which lacks the Mn cluster. The chlorophyll fluorescence decay, measured in the presence of DCMU, which reflects recombination between QA- and YZox, was also pH-dependent with a similar pK of 7.5. These results indicate participation by the same base, which is suggested to be D1-H190, in oxidation and reduction of YZ in forward electron transfer and recombination pathways, respectively. This hypothesis was tested in the D1-H190 mutants. Induction of chlorophyll fluorescence in these H190 mutants has been observed to be inefficient due to slow electron transfer from YZ to P680(+) [Roffey, R. A., et al. (1994) Biochim. Biophys. Acta 1185, 257-270]. We show that this reaction is pH-dependent, with a pK of 8. 1, and at pH >/=9, the fluorescence induction is efficient in the H190 mutants, suggesting direct titration of YZ. The efficient oxidation of YZ ( approximately 70% at pH 9.0) at high pH was confirmed by kinetic EPR measurements. In contrast to the wild type, the H190 mutants show little or no observable fluorescence decay. Our data suggest that H190 is an essential component in the electron transfer reactions in photosystem II and acts as a proton acceptor upon YZ oxidation. In the H190 mutants, this reaction is inefficient and YZ oxidation only occurs at elevated pHs when YZ itself probably is deprotonated. We also propose that H190 is able to return a proton to YZox during electron recombination from QA- in a reaction which does not take place in the D1-H190 mutants.

Function of tyrosine Z in water oxidation by photosystem II: electrostatical promotor instead of hydrogen abstractor

Photosynthetic water oxidation by photosystem II is mediated by a Mn4 cluster, a cofactor X still chemically ill-defined, and a tyrosine, YZ (D1-Tyr161). Before the final reaction with water proceeds to yield O2 (transition S4-->S0), two oxidizing equivalents are stored on Mn4 (S0-->S1-->S2), a third on X (S2-->S3), and a forth on YZ(S3-->S4). It has been proposed that YZ functions as a pure electron transmitter between Mn4X and P680, or, more recently, that it acts as an abstractor of hydrogen from bound water. We scrutinized the coupling of electron and proton transfer during the oxidation of YZ in PSII core particles with intact or impaired oxygen-evolving capacity. The rates of electron transfer to P680+, of electrochromism, and of pH transients were determined as a function of the pH, the temperature, and the H/D ratio. In oxygen-evolving material, we found only evidence for electrostatically induced proton release from peripheral amino acid residues but not from YZox itself. The positive charge stayed near YZox, and the rate of electron transfer was nearly independent of the pH. In core particles with an impaired Mn4 cluster, on the other hand, the rate of the electron transfer became strictly dependent on the protonation state of a single base (pK approximately 7). At pH < 7, the rate of electron transfer revealed the same slow rate (t1/2 approximately 35 microseconds) as that of proton release into the bulk. The deposition of a positive charge around YZox was no longer detected. A large H/D isotope effect (approximately 2.5) on these rates was also indicative of a steering of electron abstraction by proton transfer. That YZox was deprotonated into the bulk in inactive but not in oxygen-evolving material argues against the proposed role of YZox as an acceptor of hydrogen from water. Instead, the positive charge in its vicinity may shift the equilibrium from bound water to bound peroxide upon S3-->S4 as a prerequisite for the formation of oxygen upon S4-->S0.

Charge recombination and proton transfer in manganese-depleted photosystem II

The proton transfer reactions induced by the oxidation and reduction of the secondary donor, tyrosine YZ, have been studied in photosystem II after inactivation (Mn-depletion) of the oxygen-evolving complex. The rate of the recombination reaction of YZox with the reduced primary acceptor QA- appears modulated by a protonatable group with pK approximately 6 in the presence of YZox. The finding of monophasic recombination kinetics requires that the proton equilibration of this group is faster than the recombination rate. The same group modulates the extent of proton release, from 0 below pH 5 to 1 per center above pH 7. The kinetics of proton appearance and disappearance in the bulk medium are markedly dependent on the material used. In PSII core particles, the release is observed in the 100 micros range and the uptake accompanies the recombination reaction. In PSII membranes, both of these reactions are markedly delayed, so that the uptake considerably lags behind the completion of the recombination reaction. An electrochromic shift of a chlorophyll is present during the whole lifetime of YZox, suggesting a charged character of this species. A fast decreasing phase of this signal was observed in particles in the same time range as proton release. These results are discussed in the framework of a model where the proton originating from the formation of the neutral oxidized tyrosine radical (YZ.) remains locally trapped. In turn, this proton shifts the pK of a nearby group from a value >/=9 to a value of 6.

Fourier transform infrared difference study of tyrosineD oxidation and plastoquinone QA reduction in photosystem II

Two redox active tyrosines are present in the homologous polypeptides D1 and D2 of photo-system II (PS II). TyrZ (D1-161) is involved in the electron transfer reactions resulting in oxygen evolution, while TyrD (D2-160) usually forms a dark-stable radical. In Mn-depleted PS II, TyrD. can be slowly reduced by exogenous reductants. Charge separation then results in the oxidation of TyrD and TyrZ and the reduction of the primary electron acceptor QA. The semiquinone QA- can be reoxidized by oxidants like ferricyanide. In the present work, experimental conditions leading to the generation of pure QA-/QA or TyrD./TyrD FTIR difference spectra have been optimized. Therefore, single-turnover flashes or short illuminations were performed on PS II samples in the presence of exogenous reductants or oxidants. The QA- and TyrD. radicals were generated with high yield and with a lifetime of several seconds or minutes allowing averaging of FTIR difference spectra with high signal to noise ratio. Both QA- formation and contributions at the electron donor side of PS II were monitored by EPR spectroscopy. In PS II samples at pH 6 in the presence of PMS, NH2OH, and DCMU, EPR measurements show that QA- is formed with high yield upon a 1 s illumination at 10 degrees C, while no radical from the electron donor side of PS II is detected. Therefore the QA-/QA FTIR spectrum obtained in these conditions shows only vibrational changes due to QA reduction in PS II. In contrast, a similar spectrum was recently interpreted in terms of dominant contributions from Chl+/Chl signals [MacDonald, G. M., Steenhuis, J. J., & Barry, B. A. (1995) J. Biol. Chem. 270, 8420-8428], although the contribution from the electron acceptor QA was not quantified. In particular, it is shown here that the large positive signal at 1478 cm-1 is due to the QA- state and not to a Chl+ mode. This band is not downshifted upon 15N-labeling of spinach PS II membranes within the +/- 1 cm-1 accuracy of the method and is therefore tentatively assigned to the v(C[symbol: see text]O) mode of the plastosemiquinone QA-. Also unchanged upon 15N-labeling, signals at 1644 and/or 1630 cm-1 are possible candidates for the v(C = O) mode(s) of neutral QA in PS II. The TyrD./TyrD FTIR spectrum is recorded at 4 degrees C on Tris-washed PS II membranes from spinach at pH 6 in the presence of phosphate, formate, and ferricyanide. EPR experiments performed on these samples show that almost all TyrD. is formed upon a 1 s illumination at 4 degrees C and that TyrD. is then reduced within 12 min in the dark. No contributions from TyrZ. or QA- are detected 2 s after illumination. It is thus possible to optimize experimental conditions to record the FTIR difference spectrum only due to TyrD photooxidation in PS II-enriched membranes of spinach. The TyrD./TyrD FTIR spectrum is compared to a cresol./cresol FTIR difference spectrum obtained by UV irradiation at 10 K of cresol at pH 8. The spectral analogies observed between the in vivo and in vitro spectra recorded either in H2O or in D2O suggest that IR modes of TyrD contribute at 1513 and 1252 cm-1. These frequencies are characteristic of a protonated tyrosine. A positive signal is observed at 1506 cm-1 for cresol. and at 1504 cm-1 for the TyrD. state. This suggests contribution of the TyrD. side chain at 1504 cm-1. A band at 1473 cm-1 was previously assigned to the v(CO) mode of TyrD. [MacDonald, G. M., Bixby, K. A., & Barry, B. A. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 11024-11028]. In contrast, no positive signal is observed at 1473 cm-1 in the TyrD./TyrD FTIR difference spectrum presented here. The TyrD./TyrD spectrum also shows vibrational changes from peptide groups and amino acid side chains which are modified upon TyrD. formation. Proton release at the PS II protein surface upon TyrD. formation is deduced from differential signals at the v(PO) modes of phosphate.

The effects of ultraviolet irradiation on P680(+) reduction in PS II core complexes measured for individual S-states and during repetitive cycling of the oxygen-evolving complex

Flash-induced absorbance measurements at 830 nm on both nanosecond and microsecond timescales have been used to characterise the effect of ultraviolet light on Photosystem II core particles. A combination of UV-A and UV-B, closely simulating the spectrum of sunlight below 350 nm, was found to have a primary effect on the donor side of P680. Repetitive measurements indicated reductions in the nanosecond components of the absorbance decay with a concomitant appearance and increase in the amplitude of a component with a 10 μs time constant attributed to slow reduction of P680(+) by Tyrz when the function of the oxygen evolving complex is inhibited. Single-flash measurements show that the nanosecond components have amplitudes which vary with S-state. Increasing UV irradiation inhibited the amplitude of these components without changing their S-state dependence. In addition, UV irradiation resulted in a reduction in the total amplitude, with no change in the proportion of the 10 μs contribution.

Analyses of pH-induced modifications of the period four oscillation of flash-induced oxygen evolution reveal distinct structural changes of the photosystem II donor side at characteristic pH values

This study presents a thorough analysis of the reaction pattern of flash-induced oxygen evolution in spinach thylakoids as a function of pH (4.5 < or = pH < or = 9) and the redox state of tyrosine YD in polypeptide D2. Evaluation of the experimental data within the conventional Kok model [Kok, B., Forbush, B., & McGloin, M. (1970) Photochem. Photobiol. 11, 457-475] led to the following results: (1) the probability of the miss factor is strongly pH dependent (with a pronounced minimum near neutral pH) while the double hit factor is less affected; (2) a marked increase of the apparent S0 population arises at alkaline pH in dark-adapted samples where most of the YD is reduced, but this effect is absent if the percentage of PS II containing the oxidized form YDox is high; and (3) the lifetimes of S2 and S3 exhibit a characteristic pH dependence that is indicative of conformational changes of functional relevance within the water-oxidizing complex and its environment; (4) the kinetic interaction of redox states S2 and S3 with YD is characterized by a change of its behavior at a threshold pH of 6.5-7.0; and (5) at acidic pH values the extent of S2 and S3 reduction by YD decreases concomitant with the occurrence of a very fast decay kinetics. On the basis of a detailed discussion of these results and data from the literature, the water oxidase is inferred to undergo structural changes at pH values of 5-5.5 and 6.5-7.0. These transitions are almost independent of the redox state Si and modify the reaction coordinates of the water oxidase toward endogenous reductants.

Proton release during the redox cycle of the water oxidase

Old and very recent experiments on the extent and the rate of proton release during the four reaction steps of photosynthetic water oxidation are reviewed. Proton release is discussed in terms of three main sources, namely the chemical production upon electron abstraction from water, protolytic reactions of Mn-ligands (e.g. oxo-bridges), and electrostatic response of neighboring amino acids. The extent of proton release differs between the four oxidation steps and greatly varies as a function of pH both, but differently, in thylakoids and PS II-membranes. Contrastingly, it is about constant in PS II-core particles. In any preparation, and on most if not all reaction steps, a large portion of proton transfer can occur very rapidly (<20 μs) and before the oxidation of the Mn-cluster by Yz (+) is completed. By these electrostatically driven reactions the catalytic center accumulates bases. An additional slow phase is observed during the oxygen evolving step, S3⇒S4→S0. Depending on pH, this phase consists of a release or an uptake of protons which accounts for the balance between the number of preformed bases and the four chemically produced protons. These data are compatible with the hypothesis of concerted electron/proton-transfer to overcome the kinetic and energetic constraints of water oxidation.

Studies on the electron transfer from Tyr-161 of polypeptide D-1 to P680(+) in PS II membrane fragments from spinach

The functional connection between redox component Y z identified as Tyr-161 of polypeptide D-1 (Debus et al. 1988) and P680(+) was analyzed by measurements of laser flash induced absorption changes at 830 nm in PS II membrane fragments from spinach. It was found that neither DCMU nor the ADRY agent 2-(3-chloro-4-trifluoromethyl) anilino-3,5-dinitrothiophene (ANT 2p) affects the rate of P680(+) reduction by Y z under conditions where the catalytic site of water oxidation stays in the redox state S1. In contrast to that, a drastic retardation is observed after mild trypsin treatment at pH=6.0. This effect which is stimualted by flash illumination can be largely reversed by Ca(2+). The above mentioned data lead to the following conclusions: (a) the segment of polypeptide D-1 containing Tyr-161 and coordination sites of P680 is not allosterically affected by structural changes due to DCMU binding at the QB-site which is also located in D-1. (b) ANT 2p as a strong protonophoric uncoupler and ADRY agent does not modify the reaction coordinate of P680(+) reduction by Y z , and (c) Ca(2+) could play a functional role for the electronic and vibrational coupling between the redox groups Y z and P680. The electron transport from Y z to P680(+) is discussed within the framework of a nonadiabatic process. Based on thermodynamic considerations the reorganization energy is estimated to be in the order of 0.5 V.

Studies on the protolytic reactions coupled with water cleavage in photosystem II membrane fragments from spinach

The protolytic reactions of PSII membrane fragments were analyzed by measurements of absorption changes of the water soluble indicator dye bromocresol purple induced by a train of 10 μs flashes in dark-adapted samples. It was found that: a) in the first flash a rapid H(+)-release takes place followed by a slower H(+)-uptake. The deprotonation is insensitive to DCMU but is completely eliminated by linolenic acid treatment of the samples; b) the extent of the H(+)-uptake in the first flash depends on the redox potential of the suspension. In this time domain no H(+)-uptake is observed in the subsequent flashes; c) the extent of the H(+)-release as a function of the flash number in the sequence exhibits a characteristic oscillation pattern. Multiphasic release kinetics are observed. The oscillation pattern can be satisfactorily described by a 1, 0, 1, 2 stoichiometry for the redox transitions Si → Si+1 (i=0, 1, 2, 3) in the water oxidizing enzyme system Y. The H(+)-uptake after the first flash is assumed to be a consequence of the very fast reduction of oxidized Q400(Fe(3+)) formed due to dark incubation with K3[Fe(CN)6]. The possible participation of component Z in the deprotonation reactions at the PSII donor side is discussed.

Modification of oxygen evolving center by Tris-washing

Tris-washing inhibits the O2-evolving center of chloroplasts and their particles specifically and reversibly, and it was applied to many investigations on O2-evolving center and PS II reaction center. In this review are introduced the various photosynthetic investigations in which Tris-washing was applied and are also discussed briefly on the site and the mechanism of Tris-inactivation, properties of P680 and Z, characteristic change in fluorescence and delayed light emission, and reactivation of O2-evolving center by DCPIP.H2-treatment and photo-reactivation of Tris-washed chloroplasts and their particles.

Endor characterization and D2O exchange in the [Formula: see text] radical in photosystem II

The early suggestion by Lozier and Butler (Photochem. Photobiol. 17, 133-137 (1973)) that EPR Signal II arises from radicals associated with the water-splitting process in PSII has been confirmed and extended over the intervening years. Recent work has identified the Signal II radicals, [Formula: see text] and [Formula: see text], with plastosemiquinone cation species. In the experiments presented here we have used ENDOR spectroscopy and D2O/H2O exchange to characterize these paramagnets in more detail. The ENDOR matrix region, which arises from protons which interact weakly with the unpaired electron spin, is well-resolved at 4 K and at least seven resonances are apparent. A number of hyperfine couplings in the 3-8 MHz range are observed and are suggested to arise from methyl or hydroxyl protons which occur as substituents on the plastosemiquinone cation ring or from amino acid protons hydrogen-bonded to the 1,4-hydroxyl groups. Orientation selection experiments are consistent with these possibilities. D2O/H2O exchange shows that the D(+)/Z(+) site is accessible to solvent. However, the exchange occurs slowly and is not complete even after 72 hours which suggests that the free radicals are functionally isolated from solvent water.

The mechanism of photosynthetic water oxidation

Photosynthetie water oxidation is unique to plants and cyanobacteria, it occurs in thylakoid membranes. The components associated with this process include: a reaction center polypeptide, having a molecular weight (Mr) of 47-50 kilodaltons (kDa), containing a reaction center chlorophyll a labeled as P680, a plastoquinol(?)-electron donor Z, a primary electron acceptor pheophytin, and a quinone electron acceptor QA; three 'extrinsic' polypeptides having Mr of approximately 17 kDa, 23 kDa, and 33 kDa; and, in all likelihood, an approximately 34 kDa 'intrinsic' polypeptide associated with manganese (Mn) atoms. In addition, chloride and calcium ions appear to be essential components for water oxidation. Photons, absorbed by the so-called photosystem II, provide the necessary energy for the chemical oxidation-reduction at P680; the oxidized P680 (P680(+)), then, oxidizes Z, which then oxidizes the water-manganese system contained, perhaps, in a protein matrix. The oxidation of water, leading to O2 evolution and H(+) release, requires four such independent acts, i.e., there is a charge accumulating device (the so-called S-states). In this minireview, we have presented our current understanding of the reaction center P680, the chemical nature of Z, a possible working model for water oxidation, and the possible roles of manganese atoms, chloride ions, and the various polypeptides, mentioned above. A comparison with cytochrome c oxidase, which is involved in the opposite process of the reduction of O2 to H2O, is stressed.This minireview is a prelude to the several minireviews, scheduled to be published in the forthcoming issues of Photosynthesis Research, including those on photosystem II (by H.J. van Gorkom); polypeptides of the O2-evolving system (by D.F. Ghanotakis and C.F. Yocum); and the role of chloride in O2 evolution (by S. Izawa).

EPR studies on the photosystem II donor side in salt-washed and reconstituted inside-out thylakoids

EPR measurements on inside-out thylakoids revealed that salt-washing, known to inhibit oxygen evolution and release a 23 and a 16 kDa protein, induced a Signal IIf and decreased the EPR signal from state S2. Readdition of the released 23 kDa protein restored the oxygen evolution and decreased the Signal IIf, but did not relieve the decrease in the state S2 signal. It is suggested that salt-washing inhibits the electron transfer from the oxygen-evolving site to Z, the physiological donor to P680. In inhibited photosystem II units lacking Signal IIf, Z+ is rapidly reduced, possibly by a modified S-cycle unable to evolve oxygen.