Photosynthetic Light-Harvesting (Antenna) Complexes-Structures and Functions.

Chlorophylls and bacteriochlorophylls, together with carotenoids, serve, noncovalently bound to specific apoproteins, as principal light-harvesting and energy-transforming pigments in photosynthetic organisms. In recent years, enormous progress has been achieved in the elucidation of structures and functions of light-harvesting (antenna) complexes, photosynthetic reaction centers and even entire photosystems. It is becoming increasingly clear that light-harvesting complexes not only serve to enlarge the absorption cross sections of the respective reaction centers but are vitally important in short- and long-term adaptation of the photosynthetic apparatus and regulation of the energy-transforming processes in response to external and internal conditions. Thus, the wide variety of structural diversity in photosynthetic antenna "designs" becomes conceivable. It is, however, common for LHCs to form trimeric (or multiples thereof) structures. We propose a simple, tentative explanation of the trimer issue, based on the 2D world created by photosynthetic membrane systems.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures.

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

Reactive oxygen species: re-evaluation of generation, monitoring and role in stress-signaling in phototrophic organisms.

This review provides an overview about recent developments and current knowledge about monitoring, generation and the functional role of reactive oxygen species (ROS) - H2O2, HO2, HO, OH(-), (1)O2 and O2(-) - in both oxidative degradation and signal transduction in photosynthetic organisms including microscopic techniques for ROS detection and controlled generation. Reaction schemes elucidating formation, decay and signaling of ROS in cyanobacteria as well as from chloroplasts to the nuclear genome in eukaryotes during exposure of oxygen-evolving photosynthetic organisms to oxidative stress are discussed that target the rapidly growing field of regulatory effects of ROS on nuclear gene expression.

Temperature-dependent vibrational and conformational dynamics of photosystem II membrane fragments from spinach investigated by elastic and inelastic neutron scattering.

Vibrational and conformational protein dynamics of photosystem II (PS II) membrane fragments from spinach were investigated by elastic and inelastic incoherent neutron scattering (EINS and IINS). As to the EINS experiments, the average atomic mean square displacement values of PS II membrane fragments hydrated at a relative humidity of 57% exhibit a dynamical transition at ~230K. In contrast, the dynamical transition was absent at a relative humidity of 44%. These findings are in agreement with previous studies which reported a "freezing" of protein mobility due to dehydration (Pieper et al. (2008) Eur. Biophys. J. 37: 657-663) and its correlation with an inhibition of electron transfer from Q(A)(-) to Q(B) (Kaminskaya et al. (2003) Biochemistry 42, 8119-8132). IINS spectra of a sample hydrated at a relative humidity of 57% show a distinct Boson peak at ~7.5meV at 20K, which shifts towards lower energy values upon temperature increase to 250K. This unexpected effect is interpreted in terms of a "softening" of the protein matrix along with the onset of conformational protein dynamics as revealed by the EINS experiments. Information on the density of vibrational states of pigment-protein complexes is important for a realistic calculation of excitation energy transfer kinetics and spectral lineshapes and is often routinely obtained by optical line-narrowing spectroscopy at liquid helium temperature. The data presented here demonstrate that IINS is a valuable experimental tool in determining the density of vibrational states not only at cryogenic, but also at nearly physiological temperatures up to 250K. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

The rate of Q(x)→Q(y) relaxation in bacteriochlorophylls of reaction centers from Rhodobacter sphaeroides determined by kinetics of the ultrafast carotenoid bandshift.

Transient absorption changes induced by excitation of isolated reaction centers (RCs) from Rhodobacter sphaeroides with 600nm laser pulses of 20fs (full width at half maximum) were monitored in the wavelength region of 420-560nm. The spectral features of the spectrum obtained are characteristic for an electrochromic band shift of the single carotenoid (Car) molecule spheroidene, which is an integral constituent of these RCs. This effect is assigned to an electrochromic bandshift of Car due to the local electric field of the dipole moment formed by electronic excitation of bacteriochlorophyll (BChl) molecule(s) in the neighborhood of Car. Based on the known distances between the pigments, the monomeric BChl (B(B)) in the inactive B-branch is inferred to dominate this effect. The excitation of B(B) at 600nm leads to a transition into the S(2) state (Q(x) band), which is followed by rapid internal conversion to the S(1) state (Q(y) band), thus leading to a change of strength and orientation of the dipole moment, i.e., of the electric field acting on the Car molecule. Therefore, the time course of the electrochromic bandshift reflects the rate of the internal conversion from S(2) to S(1) of B(B). The evaluation of the kinetics leads to a value of 30fs for this relaxation process. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

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.

Functional domain size in aggregates of light-harvesting complex II and thylakoid membranes.

The functional domain size for efficient excited singlet state quenching was studied in artificial aggregates of the main light-harvesting complex II (LHCIIb) from spinach and in native thylakoid membranes by picosecond time-resolved fluorescence spectroscopy and quantum yield measurements. The domain size was estimated from the efficiency of added exogenous singlet excitation quenchers-phenyl-p-benzoquinone (PPQ) and dinitrobenzene (DNB). The mean fluorescence lifetimes τ(av) were quantified for a range of quencher concentrations. Applying the Stern-Volmer formalism, apparent quenching rate constants k(q) were determined from the dependencies on quencher concentration of the ratio τ(0)(av)/τ(av), where τ(0)(av) is the average fluorescence lifetime of the sample without addition of an exogenous quencher. The functional domain size was gathered from the ratio k(q)'/k(q), i.e., the apparent quenching rate constants determined in aggregates (or membranes), k(q)', and in detergent-solubilised LHCII trimers, k(q), respectively. In LHCII macroaggregates, the resulting values for the domain size were 15-30 LHCII trimers. In native thylakoid membranes the domain size was equivalent to 12-24 LHCII trimers, corresponding to 500-1000 chlorophylls. Virtually the same results were obtained when membranes were suspended in buffers promoting either membrane stacking or destacking. These domain sizes are orders of magnitude smaller than the number of physically connected pigment-protein complexes. Therefore our results imply that the physical size of an antenna system beyond the numbers of a functional domain size has little or no effect on improving the light-harvesting efficiency.

Light induced oxidative water splitting in photosynthesis: energetics, kinetics and mechanism.

The essential steps of photosynthetic water splitting take place in Photosystem II (PSII) and comprise three different reaction sequences: (i) light induced formation of the radical pair P680(+)Q(A)(-), (ii) P680(+) driven oxidative water splitting into O(2) and four protons, and (iii) two step plastoquinone reduction to plastoquinol by Q(A)(-). This mini-review briefly summarizes our state of knowledge on energetics, kinetics and mechanism of oxidative water splitting. Essential features of the two types of reactions involved are described: (a) P680(+) reduction by the redox active tyrosine Y(z) and (b) sequence of oxidation steps induced by Y(z)(ox) in the water-oxidizing complex (WOC). The rate of the former reaction is limited by the non-adiabatic electron transfer (NET) step and the multi-phase kinetics shown to originate from a sequence of relaxation processes. In marked contrast, the rate of the stepwise oxidation by Y(z)(ox) of the WOC up to the redox level S(3) is not limited by NET but by trigger reactions which probably comprise proton shifts and/or conformational changes. The overall rate of the final reaction sequence leading to formation and release of O(2) is assumed to be limited by the electron transfer step from the S(3) state of WOC to Y(z)(ox) due to involvement of an endergonic redox equilibrium. Currently discussed controversial ideas on possible pathways are briefly outlined. Several crucial points of the mechanism of oxidative water splitting, like O-O bond formation, role of local proton shift(s), details of hydrogen bonding, are still not clarified and remain a challenging topic of future research.

Protein dynamics investigated by neutron scattering.

This contribution describes incoherent quasielastic neutron scattering (QENS) as a suitable tool for investigations of protein dynamics with special emphasis on applications in photosynthesis research. QENS characterizes protein dynamics via the measurement of energy and momentum exchange between sample system and incident low-energy neutrons (1 meV

Oxygen detection in biological systems.

This article presents a brief description of analytical tools for monitoring evolution and consumption of molecular dioxygen in biological organisms. Based on its nature as a gas and its physical and chemical properties of the ground state ³Σ(g)O2; different approaches have been developed for quantitative determinations: (i) manometry, (ii) formation of titratable sediments, (iii) solid state electrodes, (iv) EPR oximetry, (v) luminescence quenching, (vi) biological sensoring, (vii) mass spectrometry and (viii) amperometry. Among these methods mass spectrometry and amperometry are of special relevance for studies on the mechanisms of photosynthetic dioxygen evolution. Mass spectrometry is described in the article of Beckman et al. in this special issue. Therefore, the major part of this contribution focuses on amperometric methods that are currently widely used. Two different types of electrodes are described: (i) Clark-type electrode and (ii) Joliot-type electrode. The complementary advantages of both systems are outlined. A more detailed description comprises the potential of the Joliot-type electrode for mechanistic studies on the reactivity of the different redox states of the water oxidizing complex (WOC).

Flash-induced structural dynamics in photosystem II membrane fragments of green plants.

Time-resolved quasielastic neutron scattering with laser excitation is a promising novel pump-probe approach, which opens up new perspectives for the study of protein-membrane dynamics in specific functional states of even complex systems. This is demonstrated here for the case of photosystem II membrane fragments with inhibited electron transfer. In contrast to the case of the model system bacteriorhodopsin, a transient reduction of the dynamics is observed approximately 160 micros after the actinic laser flash. This effect is the first observation of a modulated structural dynamics in photosystem II membrane fragments.

Photosystem II: The machinery of photosynthetic water splitting.

This review summarizes our current state of knowledge on the structural organization and functional pattern of photosynthetic water splitting in the multimeric Photosystem II (PS II) complex, which acts as a light-driven water: plastoquinone-oxidoreductase. The overall process comprises three types of reaction sequences: (1) photon absorption and excited singlet state trapping by charge separation leading to the ion radical pair [Formula: see text] formation, (2) oxidative water splitting into four protons and molecular dioxygen at the water oxidizing complex (WOC) with P680+* as driving force and tyrosine Y(Z) as intermediary redox carrier, and (3) reduction of plastoquinone to plastoquinol at the special Q(B) binding site with Q(A)-* acting as reductant. Based on recent progress in structure analysis and using new theoretical approaches the mechanism of reaction sequence (1) is discussed with special emphasis on the excited energy transfer pathways and the sequence of charge transfer steps: [Formula: see text] where (1)(RC-PC)* denotes the excited singlet state (1)P680* of the reaction centre pigment complex. The structure of the catalytic Mn(4)O(X)Ca cluster of the WOC and the four step reaction sequence leading to oxidative water splitting are described and problems arising for the electronic configuration, in particular for the nature of redox state S(3), are discussed. The unravelling of the mode of O-O bond formation is of key relevance for understanding the mechanism of the process. This problem is not yet solved. A multistate model is proposed for S(3) and the functional role of proton shifts and hydrogen bond network(s) is emphasized. Analogously, the structure of the Q(B) site for PQ reduction to PQH(2) and the energetic and kinetics of the two step redox reaction sequence are described. Furthermore, the relevance of the protein dynamics and the role of water molecules for its flexibility are briefly outlined. We end this review by presenting future perspectives on the water oxidation process.

Effects of methanol on the Si-state transitions in photosynthetic water-splitting.

From a chemical point of view methanol is one of the closest analogues of water. Consistent with this idea EPR spectroscopy studies have shown that methanol binds at-or at least very close to-the Mn(4)O(x)Ca cluster of photosystem II (PSII). In contrast, Clark-type oxygen rate measurements demonstrate that the O(2) evolving activity of PSII is surprisingly unaffected by methanol concentrations of up to 10%. Here we study for the first time in detail the effect of methanol on photosynthetic water-splitting by employing a Joliot-type bare platinum electrode. We demonstrate a linear dependence of the miss parameter for S( i ) state advancement on the methanol concentrations in the range of 0-10% (v/v). This finding is consistent with the idea that methanol binds in PSII with similar affinity as water to one or both substrate binding sites at the Mn(4)O(x)Ca cluster. The possibility is discussed that the two substrate water molecules bind at different stages of the cycle, one during the S(4) --> S(0) and the other during the S(2) --> S(3) transition.

Temperature- and hydration-dependent protein dynamics in photosystem II of green plants studied by quasielastic neutron scattering.

Protein dynamics in hydrated and vacuum-dried photosystem II (PS II) membrane fragments from spinach has been investigated by quasielastic neutron scattering (QENS) in the temperature range between 5 and 300 K. Three distinct temperature ranges can be clearly distinguished by active type(s) of protein dynamics: (A) At low temperatures (T < 120 K), the protein dynamics of both dry and hydrated PS II is characterized by harmonic vibrational motions. (B) In the intermediate temperature range (120 < T < 240 K), the total mean square displacement total slightly deviates from the predicted linear behavior. The QENS data indicate that this deviation, which is virtually independent of the extent of hydration, is due to a partial onset of diffusive protein motions. (C) At temperatures above 240 K, the protein flexibility drastically changes because of the onset of diffusive (large-amplitude) protein motions. This dynamical transition is clearly hydration-dependent since it is strongly suppressed in dry PS II. The thermally activated onset of protein flexibility as monitored by QENS is found to be strictly correlated with the temperature-dependent increase of the electron transport efficiency from Q(A)(-) to QB (Garbers et al. (1998) Biochemistry 37, 11399-11404). Analogously, the freezing of protein mobility by dehydration in dry PS II appears to be responsible for the blockage of Q(A)(-) reoxidation by Q(B) at hydration values lower than 45% r.h. (Kaminskaya et al. (2003) Biochemistry 42, 8119-8132). Similar effects were observed for reactions of the water-oxidizing complex as outlined in the Discussion section.

Photosystem II: structure and mechanism of the water:plastoquinone oxidoreductase.

This mini-review briefly summarizes our current knowledge on the reaction pattern of light-driven water splitting and the structure of Photosystem II that acts as a water:plastoquinone oxidoreductase. The overall process comprises three types of reaction sequences: (a) light-induced charge separation leading to formation of the radical ion pair P680+*QA(-*) ; (b) reduction of plastoquinone to plastoquinol at the QB site via a two-step reaction sequence with QA(-*) as reductant and (c) oxidative water splitting into O2 and four protons at a manganese-containing catalytic site via a four-step sequence driven by P680+* as oxidant and a redox active tyrosine YZ acting as mediator. Based on recent progress in X-ray diffraction crystallographic structure analysis the array of the cofactors within the protein matrix is discussed in relation to the functional pattern. Special emphasis is paid on the structure of the catalytic sites of PQH2 formation (QB-site) and oxidative water splitting (Mn4OxCa cluster). The energetics and kinetics of the reactions taking place at these sites are presented only in a very concise manner with reference to recent up-to-date reviews. It is illustrated that several questions on the mechanism of oxidative water splitting and the structure of the catalytic sites are far from being satisfactorily answered.

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.

Two reaction pathways for transformation of high potential cytochrome b559 of PS II into the intermediate potential form.

This study describes an analysis of different treatments that influence the relative content and the midpoint potential of HP Cyt b559 in PS II membrane fragments from higher plants. Two basically different types of irreversible modification effects are distinguished: the HP form of Cyt b559 is either predominantly affected when the heme group is oxidized ("O-type" effects) or when it is reduced ("R-type" effects). Transformation of HP Cyt b559 to lower potential redox forms (IP and LP forms) by the "O-type" mechanism is induced by high pH and detergent treatments. In this case the effects consist of a gradual decrease in the relative content of HP Cyt b559 while its midpoint potential remains unaffected. Transformation of HP Cyt b559 via an "R-type" mechanism is caused by a number of exogenous compounds denoted L: herbicides, ADRY reagents and tetraphenylboron. These compounds are postulated to bind to the PS II complex at a quinone binding site designated as Q(C) which interacts with Cyt b559 and is clearly not the Q(B) site. Binding of compounds L to the Q(C) site when HP Cyt b559 is oxidized gives rise to a gradual decrease in the E(m) of HP Cyt b559 with increasing concentration of L (up to 10 K(ox)(L) values) while the relative content of HP Cyt b559 is unaffected. Higher concentrations of compounds L required for their binding to Q(C) site when HP Cyt b559 is reduced (described by K(red)(L)) induce a conversion of HP Cyt b559 to lower potential redox forms ("R-type" transformation). Two reaction pathways for transitions of Cyt b559 between the different protein conformations that are responsible for the HP and IP/LP redox forms are proposed and new insights into the functional regulation of Cyt b559 via the Q(C) site are discussed.

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).

Evidence for a novel quinone-binding site in the photosystem II (PS II) complex that regulates the redox potential of cytochrome b559.

The present study provides a thorough analysis of effects on the redox properties of cytochrome (Cyt) b559 induced by two photosystem II (PS II) herbicides [3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,4-dinitro-6-sec-butylphenol (dinoseb)], an acceleration of the deactivation reactions of system Y (ADRY) agent carbonylcyanide-m-chlorophenylhydrazone (CCCP), and the lipophilic PS II electron-donor tetraphenylboron (TPB) in PS II membrane fragments from higher plants. The obtained results revealed that (1) all four compounds selectively affected the midpoint potential (E(m)) of the high potential (HP) form of Cyt b559 without any measurable changes of the E(m) values of the intermediate potential (IP) and low potential (LP) forms; (2) the control values from +390 to +400 mV for HP Cyt b559 gradually decreased with increasing concentrations of DCMU, dinoseb, CCCP, and TPB; (3) in the presence of high TPB concentrations, a saturation of the E(m) decrease was obtained at a level of about +240 mV, whereas no saturation was observed for the other compounds at the highest concentrations used in this study; (4) the effect of the phenolic herbicide dinoseb on the E(m) is independent of the occupancy of the Q(B)-binding site by DCMU; (5) at high concentrations of TPB or dinoseb, an additional slow and irreversible transformation of HP Cyt b559 into IP Cyt b559 or a mixture of the IP and LP Cyt b559 is observed; and (6) the compounds stimulate autoxidation of HP Cyt b559 under aerobic conditions. These findings lead to the conclusion that a binding site Q(C) exists for the studied substances that is close to Cyt b559 and different from the Q(B) site. On the basis of the results of the present study and former experiments on the effect of PQ extraction and reconstitution on HP Cyt b559 [Cox, R. P., and Bendall, D. S. (1974) The functions of plastoquinone and beta-carotene in photosystem II of chloroplasts, Biochim. Biophys. Acta 347, 49-59], it is postulated that the binding of a plastoquinone (PQ) molecule to Q(C) is crucial for establishing the HP form of Cyt b559. On the other hand, the binding of plastoquinol (PQH2) to Q(C) is assumed to cause a marked decrease of E(m), thus, giving rise to a PQH2 oxidase function of Cyt b559. The possible physiological role of the Q(C) site as a regulator of the reactivity of Cyt b559 is discussed.

Characterization of the water oxidizing complex of photosystem II of the Chl d-containing cyanobacterium Acaryochloris marina via its reactivity towards endogenous electron donors and acceptors.

Acaroychloris (A.) marina is a unique oxygen evolving organism that contains a large amount of chlorophyll d (Chl d) and only very few Chl a molecules. This feature raises questions on the nature of the photoactive pigment, which supports light-induced oxidative water splitting in Photosystem II (PS II). In this study, flash-induced oxygen evolution patterns (FIOPs) were measured to address the question whether the S(i) state transition probabilities and/or the redox-potentials of the water oxidizing complex (WOC) in its different S(i) states are altered in A. marina cells compared to that of spinach thylakoids. The analysis of the obtained data within the framework of different versions of the Kok model reveals that in light activated A. marina cells the miss probability is similar compared to spinach thylakoids. This finding indicates that the redox-potentials and kinetics within the WOC, of the reaction center (P680) and of Y(Z) are virtually the same for both organisms. This conclusion is strongly supported by lifetime measurements of the S(2) and S(3) states. Virtually identical time constants were obtained for the slow phase of deactivation. Kinetic differences in the fast phase of S(2) and S(3) decay between A. marina cells and spinach thylakoids reflect a shift of the E(m) of Y(D)/Y(D)(ox) to lower values in the former compared to the latter organisms, as revealed by the observation of an opposite change in the kinetics of S(0) oxidation to S(1) by Y(D)(ox). A slightly increased double hit probability in A. marina cells is indicative of a faster Q(A)(-) to Q(B) electron transfer in these cells compared to spinach thylakoids.