The role of pH and membrane potential in the reactions of Photosystem II as measured by effects on delayed fluorescence

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.

A molecular mechanism for qE-quenching

We discuss energy-dependent fluorescence lowering (qE-quenching), and suggest a model to explain the experimental data currently available. The main elements of the model are: (a) the qE-quenching reflects a mechanism associated with a component of the light-harvesting antenna rather than the reaction center of photosystem (PS) II--we suggest that it occurs through formation of an efficient quencher in one of the minor chlorophyll protein (CP) complexes; (b) the minor CPs have glutamate residues instead of glutamines at positions shown in light-harvesting complex II (LHCII) to be ligands to chlorophylls near the lumenal interface. We suggest that the quenching reflects a change in ligation of chlorophyll on protonation of these glutamate residues leading to formation of an exciton coupled dimer with a neighboring pigment, in which additional energy levels allow vibrational relaxation of the excited singlet. The model accounts for the dependence on low lumenal pH, the ligand residue changes between LHCII and the minor CPs, the preferential distribution of components of the xanthophyll cycle in the minor CPs, the inhibition of qE-quenching by DCCD, and the specific binding of DCCD to the minor CPs.

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.

Photoinhibition of hydroxylamine-extracted photosystem II membranes: identification of the sites of photodamage

Electron paramagnetic resonance (EPR) analyses (g = 2 region) and optical spectrophotometric analyses of P680+ were made of NH2OH-extracted photosystem II (PSII) membranes after various durations of weak-light photoinhibition, in order to identify the sites of damage responsible for the observed kinetic components of the loss of electron transport [Blubaugh, D.J., & Cheniae, G.M. (1990) Biochemistry 29, 5109-5118]. The EPR spectra, recorded in the presence of K3Fe(CN)6, gave evidence for rapid (t1/2 = 2-3 min) and slow (t1/2 = 3-4) losses of formation of the tyrosyl radicals YZ+ and YD+, respectively, and the rapid appearance (t1/2 = 0.8 min) of a 12-G-wide signal, centered at g = 2.004, which persisted at 4 degrees C in subsequent darkness in rather constant abundance (approximately 1/2 spin per PSII). This latter EPR signal is correlated with quenching of the variable chlorophyll a fluorescence yield and is tentatively attributed to a carotenoid (Car) cation. Exogenous reductants (NH2OH greater than or equal to NH2NH2 greater than DPC much greater than Mn2+) were observed to reduce the quencher, but did not reverse other photoinhibition effects. An additional 10-G-wide signal, tentatively attributed to a chlorophyll (Chl) cation, is observed during illumination of photoinhibited membranes and rapidly decays following illumination. The amplitude of formation of the oxidized primary electron donor, P680+, was unaffected throughout 120 min of photoinhibition, indicating no impairment of charge separation from P680, via pheophytin (Pheo), to the first stable electron acceptor, QA. However, a 4-microsecond decay of P680+, reflecting YZ----P680+, was rapidly (t1/2 = 0.8 min) replaced by an 80-140 microsecond decay, presumably reflecting QA-/P680+ back-reaction. Photoinhibition caused no discernible decoupling of the antenna chlorophyll from the reaction center complex. We conclude that the order of susceptibility of PSII components to photodamage when O2 evolution is impaired is Chl/Car greater than YZ greater than YD much greater than P680, Pheo, QA.

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.

pH dependent stabilization of S2Q A (-) and S 2Q B (-) charge pairs studied by thermoluminescence

The pH dependence of emission peak temperature and decay time of thermoluminescence arising from S2QB (-) and S2QA (-) recombinations demonstrates that a stabilization of S2QB (-) occurs at low pH whereas stabilization of S2QA (-) occurs at high pH. Based on comparative analysis of thermoluminescence parameters of the two types of recombination, we suggest that in the pH range between 5.3 and 7.5, Em(S2/S1) and Em(QA/QA (-)) are constant, but Em(QB/QB (-)) gradually increases with decreasing pH, while in the pH range between 7.5 and 8.5, an unusual change occurs on S2QA (-) charge pair, which is interpreted as either a decrease in Em(S2/S1) or an increase in Em(QA/QA (-)).

pH dependence of the multiline, manganese EPR signal for the 'S2' state in PS II particles. Absence of proton release during the S1----S2 electron transfer step of the oxygen evolving system

The pH dependence of oxygen evolution rates, 2,6-dichlorophenolindophenol (DCIP) reduction rates and the intensity of the multiline manganese EPR signal associated with the S2K ok state has been studied using oxygen-evolving spinach (PS) II particles. The oxygen evolution and DCIP reduction rates are found to be very sensitive to pH, with the maximal rates occurring at pH 6.5-7.0. Both the rate and yield of the S2 multiline manganese EPR signal intensity, produced by single flash excitation at room temperature or by continuous illumination at 200 K, are found to be independent of pH, indicating that no proton is released from this manganese site during the S1----S2 electron transfer. These results agree with those from other laboratories showing no proton release on this transition, but using techniques monitoring other species.