Effects of pH on reactions on the donor side of photosystem II

The modified Q-cycle: A look back at its development and forward to a functional model

On looking back at a lifetime of research, it is interesting to see, in the light of current progress, how things came to be, and to speculate on how things might be. I am delighted in the context of the Mitchell prize to have that excuse to present this necessarily personal view of developments in areas of my interests. I have focused on the Q-cycle and a few examples showing wider ramifications, since that had been the main interest of the lab in the 20 years since structures became available, - a watershed event in determining our molecular perspective. I have reviewed the evidence for our model for the mechanism of the first electron transfer of the bifurcated reaction at the Q_o-site, which I think is compelling. In reviewing progress in understanding the second electron transfer, I have revisited some controversies to justify important conclusions which appear, from the literature, not to have been taken seriously. I hope this does not come over as nitpicking. The conclusions are important to the final section in which I develop an internally consistent mechanism for turnovers of the complex leading to a state similar to that observed in recent rapid-mix/freeze-quench experiments, reported three years ago. The final model is necessarily speculative but is open to test.

Efficiency and role of loss processes in light-driven water oxidation by PSII

Its superior quantum efficiency renders PSII a model for biomimetic systems. However, also in biological water oxidation by PSII, the efficiency is restricted by recombination losses. By laser-flash illumination, the secondary radical pair, P680(+)Q(-) (A) (where P680 is the primary Chl donor in PSII and Q(A), primary quinone acceptor of PSII), was formed in close to 100% of the PSII. Investigation of the quantum efficiency (or yield) of the subsequent steps by time-resolved delayed (10 micros to 60 ms) and prompt (70 micros to 700 ms) Chl fluorescence measurements on PSII membrane particles suggests that (1) the effective rate for P680(+) Q(-) (A) recombination is approximately 5 ms(-1) with an activation energy of approximately 0.34 eV, circumstantially confirming dominating losses by reformation of the primary radical pair followed by ground-state recombination. (2) Because of compensatory influences on recombination and forward reactions, the efficiency is only weakly temperature dependent. (3) Recombination losses are several-fold enhanced at lower pH. (4) Calculation based on delayed-fluorescence data suggests that the losses depend on the state of the water-oxidizing manganese complex, being low in the S(0)-->S(1) and S(1)-->S(2) transition, clearly higher in S(2)-->S(3) and S(3)-->S(4)-->S(0). (5) For the used artificial electron acceptor, the efficiency is limited by acceptor-side processes/S-state decay at high/low photon-absorption rates resulting in optimal efficiency at surprisingly low rates of approximately 0.15-15 photons s(-1) (per PSII). The pH and S-state dependence can be rationalized by the basic model of alternate electron-proton removal proposed elsewhere. A physiological function of the recombination losses could be limitation of the lifetime of the reactive donor-side tyrosine radical (Y(.) (Z)) in the case of low-pH blockage of water oxidation.

Photosynthetic dioxygen formation studied by time-resolved delayed fluorescence measurements--method, rationale, and results on the activation energy of dioxygen formation

The analysis of the time-resolved delayed fluorescence (DF) measurements represents an important tool to study quantitatively light-induced electron transfer as well as associated processes, e.g. proton movements, at the donor side of photosystem II (PSII). This method can provide, inter alia, insights in the functionally important inner-protein proton movements, which are hardly detectable by conventional spectroscopic approaches. The underlying rationale and experimental details of the method are described. The delayed emission of chlorophyll fluorescence of highly active PSII membrane particles was measured in the time domain from 10 mus to 60 ms after each flash of a train of nanosecond laser pulses. Focusing on the oxygen-formation step induced by the third flash, we find that the recently reported formation of an S4-intermediate prior to the onset of O-O bond formation [M. Haumann, P. Liebisch, C. Müller, M. Barra, M. Grabolle, H. Dau, Science 310, 1019-1021, 2006] is a multiphasic process, as anticipated for proton movements from the manganese complex of PSII to the aqueous bulk phase. The S4-formation involves three or more likely sequential steps; a tri-exponential fit yields time constants of 14, 65, and 200 mus (at 20 degrees C, pH 6.4). We determine that S4-formation is characterized by a sizable difference in Gibbs free energy of more than 90 meV (20 degrees C, pH 6.4). In the second part of the study, the temperature dependence (-2.7 to 27.5 degrees C) of the rate constant of dioxygen formation (600/s at 20 degrees C) was investigated by analysis of DF transients. If the activation energy is assumed to be temperature-independent, a value of 230 meV is determined. There are weak indications for a biphasicity in the Arrhenius plot, but clear-cut evidence for a temperature-dependent switch between two activation energies, which would point to the existence of two distinct rate-limiting steps, is not obtained.

Influence of state-2 transition on the proton motive force across the thylakoid membrane in spinach chloroplasts

The proton motive force (pmf) across the thylakoid membrane is composed of the proton gradient and the membrane potential, which promotes millisecond-delayed light emission (ms-DLE). In this study, the time courses of LHC II phosphorylation and ms-DLE were investigated in spinach chloroplast during State-2 transition. Red light illumination resulted in an exponential rise in LHC II phosphorylation and a biphasic time course of ms-DLE. The phospho-LHC II appeared upon approximately 1 min illumination. The phosphorylation level increased exponentially when illumination was elongated to 20 min. The t((1/2) )of saturated LHC II phosphorylation was estimated 4-5 min under present illumination. During this process, the amplitudes of ms-DLE increased transiently to a maximal amplitude within 0.5 min illumination, and the reached maximum of the fast phase of ms-DLE was approximately 140% of the dark control. Then, ms-DLE decreased from the maximum. After > or =3 min illumination, ms-DLE decreased to a lower level than the dark control. In the presence of uncouplers and inhibitors, the transient increase in the biphasic time course of ms-DLE was removed by nigericin and DCMU, and the sequential decrease was delayed by DCCD. The time course was not affected significantly by valinomycin and DBMIB. Moreover, the level of LHC II phosphorylation was enhanced by nigericin, valinomycin and DCCD, and was inhibited completely by DCMU and partially by DBMIB. Taken together, we proposed that the PS II photochemical activity remained unaffected even with a higher level of LHC II phosphorylation, which was reflected by the effect of DCCD on the time course of ms-DLE. Probably, the evidence of LHC II phosphorylation is the rearrangement of LHC II-PS II complex and the thylakoid, a feedback to light-exposure, rather than the redistribution of excitation energy from PS II to PS I.

Delayed fluorescence emitted from light harvesting complex II and photosystem II of higher plants in the 100 ns-5 micros time domain

This study presents the first report on delayed fluorescence (DF) emitted from spinach thylakoids, D1/D2/Cytb-559 preparations and solubilized light harvesting complex II (LHCII) in the ns time domain after excitation with saturating laser flashes. The use of a new commercially available multichannel plate with rapid gating permitted a sufficient suppression of detector distortions due to the strong prompt fluorescence. The following results were obtained: (a) in dark-adapted thylakoids, the DF amplitudes at 100 ns and 5 micros after each flash of a train of saturating actinic pulses exhibit characteristic period four oscillations of opposite sign: the DF amplitudes at 100 ns oscillate in the same manner as the quantum yield of prompt fluorescence, whereas those at 5 micros resemble the oscillation of the micros kinetics of P680(.) reduction in samples with an intact water oxidizing complex, (b) the quantum yield of total DF emission in the range up to a few micros is estimated to be <10(-4) for thylakoids, (c) the DF of D1/D2/Cytb-559 exhibits a monophasic decay with tau approximately 50 ns, (d) DF emission is also observed in isolated LHCII with biphasic decay kinetics characterized by tau values of 65 ns and about 800 ns, (e) in contrast to thylakoids, the amplitudes of DF in D1/D2/Cytb-559 preparations and solubilized LHCII do not exhibit any oscillation pattern and (f) all spectra of DF from the different sample types are characteristic for emission from the lowest excited singlet state of chlorophyll a. The implications of these findings and problems to be addressed in future research are briefly discussed.

On the origin of the '35-mus kinetics' of P680(+.) reduction in photosystem II with an intact water oxidising complex

The origin of the '35-micros kinetics' of P680(+.) reduction in photosystem II (PS II) with an intact water oxidising complex has been analysed by comparative measurements of laser flash induced changes of the 830-nm absorption and the relative quantum yield of chlorophyll (Chl) fluorescence. The latter parameter was monitored at a time resolution of 500 ns by using newly developed home built equipment [Reifarth, F., Christen, G. and Renger, G. (1997) Photosynth. Res. 51, 231-2421. It was found that: (i) the amplitudes of the unresolved ns-kinetics of both 830-nm absorption changes and the rise of fluorescence yield exhibit virtually the same period four oscillation pattern when dark adapted samples are excited with a train of saturating laser flashes; (ii) the corresponding oscillation patterns of the normalised extent of the 35-micros kinetics under identical excitation conditions are strikingly different with maxima after the 3rd and 5th flash for the 830-nm absorption changes vs. pronounced maxima after the 4th and 8th flash for the rise of the fluorescence yield. The period four oscillations unambiguously show that the '35-micros kinetics' of P680(+.) reduction are characteristic for reactions in PS II entities with an intact water oxidising complex. However, the disparity of the oscillation patterns of (ii) indicates that in contrast to the ns components of P680(+.) reduction the 35-micros kinetics do not reflect exclusively an electron transfer from Y(Z) to P680(+.). It is inferred that a more complex reaction takes place which comprises at least two processes: (a) P680(+.) reduction by Y(Z) and (b) coupled and/or competing reaction(s) which give rise to additional changes of the chlorophyll fluorescence yield.

Interaction of photosystem I-derived protons with the water-splitting enzyme complex. Evidence for localized domains

The induction of millisecond delayed fluorescence mediated by PS I-dependent proton pumping has been used as an indicator of the time course with which those protons equilibrate with sites on the oxygen-evolving enzyme complex (Bowes, J. M., and Crofts, A. R. (1978). Z. Naturforsch. 33C, 271-275). We found that the induction curves were retarded by a reversible exposure of non-energized thylakoids to low concentrations of the uncoupler, desaspidin, at alkaline, but not at neutral, pH. The induction curves were not retarded by increasing the buffering capacity of the thylakoid lumen with Tricine, and were inhibited by the energy transfer inhibitors, dicyclohexylcarbodiimide (DCCD) and triphenyltin chloride (TPT). These data suggest that the catalytic site of the water-splitting complex is located in proton-sequestering membrane domains, rather than at the lumen-exposed inner membrane surface, protons released during PS I-mediated electron transport might equilibrate with protonatable sites on the oxygen-evolving complex without passing through the lumen, and those protons may travel over specific conducting pathways which can be blocked by DCCD and TPT.

The effects of pH on the reductions kinetics of P-680 in Tris-treated chloroplasts

The primary donor of Photosystem II (PS II), P-680, was photo-oxidized by a short flash and its rate of reduction was measured at different pH values by following the recovery of the absorption change at 820 nm in chloroplasts pretreated with a high concentration of Tris. The re-duction is biphasic with a fast phase (dominant after the first flash) attributed to the donation by a donor, D1, and a slow phase (usually dominant after the second flash) attributed to a back-reaction with the primary acceptor. It is found that pH has a strong influence on the donation from D1 (PI = 2 MICROSECONDS AT PH 9, 44 microseconds at pH 4), but no influence on the back reaction (pi approximately 200 microseconds). pH also influences the stability of the charge separation since the contribution of donation from D1 at the second flash increases at lower pH, getting close to 100% at pH 4.

Photosystem II oxidation of charged electron donors. Surface charge effects

Reactions occurring on the oxidizing side of Photosystem II have been studied in Tris-washed chloroplasts by monitoring the decay kinetics of EPR signal IIf, arising from the photoinduced oxidation of Z, an intermediate in the electron transport chain between P-680 and the water-splitting enzyme. Upon addition of electron donors, signal IIf follows pseudo-first order decay kinetics with rates dependent on the chemical nature of the donor. Negatively charged donors (I-, Fe(CN)6(4-), W(CN)8(4-) are poor reducing agents for Z.+ whereas neutral donors (benzidine, hydroquinone, diphenylcarbazide) are more efficient, their effectiveness paralleling their lipophilicity. The slow signal IIf reduction observed with the charged donors is consistent with the non-polar nature of the thylakoid membrane and a location for Z toward the inner membrane surface. It most probably exists in a hydrophobic site as indicated by the positive correlation between rate constant and lipophilicity for the neutral donors. A detailed study of the mechanism of Photosystem II reduction by ascorbic acid has been carried out. The pH dependence of the decay kinetics of signal IIf in the presence of this donor is consistent with a model in which both the neutral acid and the ascorbate mono-anion serve as reducing agents to Z.+. The second-order rate constant for reduction by the mono-anion is less than that of the neutral acid and is found to vary with the suspension pH. This observation is interpreted to indicate the occurrence of negative charge on the inner membrane surface in the vicinity of Z. Additional experiments, which assessed the effect of mono- and divalent cations and of cationic detergents on the signal IIf reaction rate constants, support both the presence of negative surface charge and its location on the membrane inner surface.