Thermodynamics of the charge recombination in photosystem II

Kinetics and pathways of charge recombination in photosystem II

The mechanism of charge recombination of the S(2)Q(A)(-) state in photosystem II was investigated by modifying the free energy gap between the quinone acceptor Q(A) and the primary pheophytin acceptor Ph. This was done either by changing the midpoint potential of Ph (using mutants of the cyanobacterium Synechocystis with a modified hydrogen bond to this cofactor), or that of Q(A) (using different inhibitors of the Q(B) pocket). The results show that the recombination rate is dependent on the free energy gap between Ph and Q(A), which confirms that the indirect recombination pathway involving formation of Ph(-) has a significant contribution. In the mutant with the largest free energy gap, direct electron transfer from Q(A)(-) to P(+) predominates. The temperature dependence of the recombination rate was investigated, showing a lower activation enthalpy in this mutant compared with the WT. The data allow the determination of the rate of the direct route and of its relative weight in the various strains. The set of currently accepted values for the midpoint potentials of the Q(A)/Q(A)(-), Ph/Ph(-), and P(+)/P* couples is not consistent with the relatively rapid rate of the indirect recombination pathway found here, nor with the 3% yield of delayed fluorescence as previously estimated by de Grooth and van Gorkom (1981, Biochim. Biophys. Acta 635, 445-456). It is argued that a likely explanation is that the midpoint potentials of the two latter couples are more positive than believed due to electrostatic interactions. If such is the case, the estimation of the midpoint potential of the P(+)/P and S(2)/S(1) couples must also be revised upward, with values of 1260 and 1020 mV, respectively.

Electroluminescence

An overview is presented of research based on the observation by Arnold and Azzi (1971) (Photochem Photobiol 14: 233-240), that an electric field induces charge-recombination luminescence in a suspension of photosynthetic membrane vesicles. The 'electroluminescence' signals from Photosystems I and II are discussed in relation to the shape of the vesicles and the membrane potentials generated by the externally applied electric field. The use of the electroluminescence amplitude as a probe to study the kinetics and energetics of charge separation, and of its kinetics to monitor the electric-field induced charge recombination process are reviewed. Currently unresolved issues regarding the emission yield of electroluminescence are briefly discussed and the properties are summarized of the unexplained Photosystem II luminescence which is not sensitive to the membrane potential.

Oscillations of reaction center II-D1 protein degradation in vivo induced by repetitive light flashes. Correlation between the level of RCII-QB- and protein degradation in low light

The D1 protein subunit of the photochemical reaction center II (RCII) turns over rapidly in oxygenic photosynthetic organisms exposed to the light. At high photon flux densities (PFD), photoinactivation of RCII precedes the degradation of the D1 protein. We found that the apparent quantum yield for the D1 protein degradation in Chlamydomonas cells is severalfold higher at low PFDs (10-100 mumol m-2 s-1) as compared to that observed at PFDs which induce photoinactivation of RCII (1.5-3 x 10(3) mumol m-2 s-1). Relative high levels of reduced RCII secondary plastoquinone acceptor, QB-, are induced in cells exposed to low PFDs as determined by thermoluminescence measurements. The probability of generating elevated levels of QB- which may recombine with the S2,3 oxidized states of the oxygen evolving complex decreases with increase in the light intensities at which consecutive double reduction of QB and exchange with the plastoquinone pool prevail. We have used light flashes to test if a correlation exists between the degradation of D1 protein and the relative level of QB-. D1 protein degradation could be induced in dark-incubated cells exposed to a series of 1.4 x 10(3) single light flashes given at intervals compatible with generation of elevated levels of QB- and its decay by charge recombination. Oscillations of the QB- level in cells exposed to 960-1440 series of 1 to several flashes correlated with oscillations of the D1 protein degradation in Chlamydomonas cells and in the Scenedesmus wild type but not in the LF-1 mutant lacking photosystem II donor side activity. In this mutant the "S state cycle" and QB- oscillations are abolished. We propose that the process of recombination of long lived RCII-QB- with the S2,3 states may involve damaging events related to the D1 protein degradation induced by light flashes or continuous low light in vivo.

O2-dependent electron flow, membrane energization and the mechanism of non-photochemical quenching of chlorophyll fluorescence

Recent progress in chlorophyll fluorescence research is reviewed, with emphasis on separation of photochemical and non-photochemical quenching coefficients (qP and qN) by the 'saturation pulse method'. This is part of an introductory talk at the Wageningen Meeting on 'The use of chlorophyll fluorescence and other non-invasive techniques in plant stress physiology'. The sequence of events is investigated which leads to down-regulation of PS II quantum yield in vivo, expressed in formation of qN. The role of O2-dependent electron flow for ΔpH- and qN-formation is emphasized. Previous conclusions on the rate of 'pseudocyclic' transport are re-evaluated in view of high ascorbate peroxidase activity observed in intact chloroplasts. It is proposed that the combined Mehler-Peroxidase reaction is responsible for most of the qN developed when CO2-assimilation is limited. Dithiothreitol is shown to inhibit part of qN-formation as well as peroxidase-induced electron flow. As to the actual mechanism of non-photochemical quenching, it is demonstrated that quenching is favored by treatments which slow down reactions at the PS II donor side. The same treatments are shown to stimulate charge recombination, as measured via 50 μs luminescence. It is suggested that also in vivo internal thylakoid acidification leads to stimulation of charge recombination, although on a more rapid time scale. A unifying model is proposed, incorporating reaction center and antenna quenching, with primary control of ΔpH at the PS II reaction center, involving radical pair spin transition and charge recombination to the triplet state in a first quenching step. In a second step, triplet excitation is trapped by zeaxanthin (if present) which in its triplet excited state causes additional quenching of singlet excited chlorophyll.

Photoelectric study on the kinetics of trapping and charge stabilization in oriented PS II membranes

Excitation energy trapping and charge separation in Photosystem II were studied by kinetic analysis of the fast photovoltage detected in membrane fragments from peas with picosecond excitation. With the primary quinone acceptor oxidized the photovoltage displayed a biphasic rise with apparent time constants of 100-300 ps and 550±50 ps. The first phase was dependent on the excitation energy whereas the second phase was not. We attribute these two phases to trapping (formation of P-680(+) Phe(-)) and charge stabilization (formation of P-680(+) QA (-)), respectively. A reversibility of the trapping process was demonstrated by the effect of the fluorescence quencher DNB and of artificial quinone acceptors on the apparent rate constants and amplitudes. With the primary quinone acceptor reduced a transient photoelectric signal was observed and attributed to the formation and decay of the primary radical pair. The maximum concentration of the radical pair formed with reduced QA was about 30% of that measured with oxidized QA. The recombination time was 0.8-1.2 ns.The competition between trapping and annihilation was estimated by comparison of the photovoltage induced by short (30 ps) and long (12 ns) flashes. These data and the energy dependence of the kinetics were analyzed by a reversible reaction scheme which takes into account singlet-singlet annihilation and progressive closure of reaction centers by bimolecular interaction between excitons and the trap. To put on firmer grounds the evaluation of the molecular rate constants and the relative electrogenicity of the primary reactions in PS II, fluorescence decay data of our preparation were also included in the analysis. Evidence is given that the rates of radical pair formation and charge stabilization are influenced by the membrane potential. The implications of the results for the quantum yield are discussed.