Chlorophyll a fluorescence as a monitor of nanosecond reduction of the photooxidized primary donor P-680 of Photosystem II

1. Changes in the fluorescence yield of aerobic Chlorella vulgaris have been measured in laser flashes of 15 ns, 30 ns and 350 ns half time. The kinetics after the first flash given after a 3 min dark period could be simulated on a computer using the hypothesis that the oxidized acceptor Q and primary donor P⁺ are fluorescence quenchers, and Q⁻ is a weak quencher, and that the reduction time for P⁺ is 20–35 ns.

2. The P⁺ reduction time for at least an appreciable part of the reaction centers was found to be longer after the second and subsequent flashes. In the first 5 flashes an oscillation was observed. Under steady state conditions, with a pulse separation of 3 s, a reduction time for P⁺ of about 400 ns for all reaction centers gave the best correspondence between computed and experimental fluorescence kinetics.     

Effect of preillumination on the P-680+ reduction kinetics in chloride-free photosystem II membranes

The re-reduction course of P-680⁺, the photooxidized PS II primary donor, was measured as a function of excitation number in Cl⁻-depleted PS II membranes. After the 1st and 2nd excitations the signal amplitude of P-680⁺ is small, indicating a submicrosecond reduction of P-680⁺ by Z, the secondary donor of PS II. After the 3rd excitation, however, a larger P-680⁺ signal with a 40–50 μs half-life is observed. The slow decay of this signal is attributed to a back-reaction with a reduced acceptor in the presence of the Z⁺S₂ state on the donor side. The state Z⁺S₂ has a lifetime longer than 300 ms and its formation was found to depend on the presence of the abnormal S₂ state created by the 1st excitation. The P-680 data and thermoluminescence measurements show that the S-state advancement beyond S₂ is blocked in the absence of Cl⁻ and that the Cl⁻-free abnormal S₂ state has a lifetime about 10-times longer than the normal S₂ state.     

A flash photolysis ESR study of photosystem II signal IIvf, the physiological donor to P-680+.

In flash-illuminated, oxygen-evolving spinach chloroplasts and green algae, a free radical transient has been observed with spectral parameters similar to those of Signal II (g approximately 2.0045, deltaHpp approximately 19G). However, in contrast with ESR Signal II, the transient radical does not readily saturate even at microwave power levels of 200 mW. This species is formed most efficiently with "red" illumination (lambda less than 680 nm) and occurs stoichiometrically in a 1:1 ratio with P-700+. The Photosystem II transient is formed in less than 100 mus and decays via first-order kinetics with a halftime of 400-900 mus. Additionally, the t1/2 for radical decay is temperature independent between 20 and 4 degrees C; however, below 4 degrees C the transient signal exhibits Arrhenius behavior with an activation energy of approx. 10 kcal-mol-1. Inhibition of electron transport through Photosystem II by o-phenanthroline, 3-(3,4-dichlorophenyl)-1,1-dimethylurea or reduced 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone suppresses the formation of the light-induced transient. At low concentrations (0.2 mM), 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone partially inhibits the free radical formation, however, the decay kinetics are unaltered. High concentrations of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (1-5 mM) restore both the transient signal and electron flow through Photosystem II. These findings suggest that this "quinoidal" type ESR transient functions as the physiological donor to the oxidized reaction center chlorophyll, P-680+.     

Relation between microsecond reduction kinetics of photooxidized chlorophyll aII (P-680) and photosynthetic water oxidation

The reduction phases of chlorophyll a⁺_II (P-680⁺) in the microsecond range have been studied in O₂-evolving Photosystem II particles from Synechococcus sp. and in spinach subchloroplasts. (1) In selected Photosystem II preparations only approx. 15% of chlorophyll a⁺_II is reduced under repetitive excitation in the microsecond time-range (approx. 85% are reduced in the nanosecond time-range). (2) The size of the microsecond fraction varies as a function of the flash number given to dark-adapted samples, suggesting a correlation to the oxidation states of the O₂-evolving complex (S-states). The oscillatory pattern closely follows the concentration of S₂ + S₃. (3) The microsecond decay can be deconvoluted into three exponential phases with half-life times of approx. 5, 35 and 200 μs. It is the amplitude of the 35 μs phase which depends on S₂ + S₃. Therefore, the 35 μs phase (approx. 10% under repetitive excitation) is connected with water oxidation. (4) Considerably higher values of the μs fraction (up to 50%) reported in former publications were probably due to Photosystem II centers which were inactive in O₂ evolution.     

The temperature dependence of P680(+) reduction in oxygen-evolving photosystem II

The temperature dependence for the reduction of the oxidized primary electron donor P680(+) by the redox active tyrosine Y(Z) has been studied in oxygen-evolving photosystem II preparations from spinach. The observed temperature dependence is found to vary markedly with the S-state of the manganese cluster. In the higher oxidation states, S(2) and S(3), sub-microsecond P680(+) reduction exhibits activation energies of about 260 meV. In contrast, there is only a small temperature dependence for the sub-microsecond reaction in the S(0) and S(1) states (an activation energy of approximately 50 meV). Slower microsecond components of P680(+) reduction show an activation energy of about 250 meV which, within experimental error, is independent of the oxidation state of the Mn cluster. By combining these values with measurements of DeltaG for electron transfer, the reorganization energies for each component of P680(+) reduction have been calculated. High activation and reorganization energies are found for sub-microsecond P680(+) reduction in S(2) and S(3), demonstrating that these electron transfers are coupled to significant reorganization events which do not occur in the presence of the lower S-states. One interpretation of these results is that there is an increase in the net charge on the manganese cluster on the S(1) to S(2) transition which acts as a barrier to electron transfer in the higher S-states. This argues against the electroneutrality requirement for some models of the function of the manganese cluster and hence against a role for Y(Z) as a hydrogen abstractor on all S-state transitions. An alternative or additional possibility is that there are proton (or other ion) motions in the sub-microsecond phases in S(2) and S(3) which contribute to the large reorganization energies observed, these motions being absent in the S(0) and S(1) states. Indeed charge accumulation may directly cause the increased reorganization energy.     

Effect of photosystem II reaction center closure on nanosecond fluorescence relaxation kinetics

The fluorescence decay of chlorophyll in spinach thylakoids was measured as a function of the degree of closure of Photosystem II reaction centers, which was set for the flowed sample by varying either the preillumination by actinic light or the exposure of the sample to the exciting pulsed laser light. Three exponential kinetic components originating in Photosystem II were fitted to the decays; a fourth component arising from Photosystem I was determined to be negligible at the emission wavelength of 685 nm at which the fluorescence decays were measured. Both the lifetimes and the amplitudes of the components vary with reaction center closure. A fast (170–330 ps) component reflects the trapping kinetics of open Photosystem II reaction centers capable of reducing the plastoquinone pool; its amplitude decreases gradually with trap closure, which is incompatible with the concept of photosynthetic unit connectivity where excitation energy which encounters a closed trap can find a different, possibly open one. For a connected system, the amplitude of the fast fluorescence component is expected to remain constant. The slow component (1.7–3.0 ns) is virtually absent when the reaction centers are open, and its growth is attributable to the appearance of closed centers. The middle component (0.4–1.7 ns) with approximately constant amplitude may originate from centers that are not functionally linked to the plastoquinone pool. To explain the continuous increase in the lifetimes of all three components upon reaction center closure, we propose that the transmembrane electric field generated by photosynthetic turnover modulates the trapping kinetics in Photosystem II and thereby affects the excited state lifetime in the antenna in the trap-limited case.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - HEPES 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid - PQ plastoquinone - PSI and PSII Photosystem I and II - Q_A and Q_B primary and secondary quinone acceptor of PSII     

Kinetics of reduction of the primary donor of photosystem II. Influence of pH in various preparations

The kinetics of reduction of P-680, oxidized by a flash, have been measured with chloroplasts treated with Tris, hydroxylamine or cholate, with PS-II particles from Phormidium (Tris-treated) and with subchloroplast particles prepared with digitonin or Triton. In all cases the electron transfer from D1 to P-680+ has similar rates, influenced similarly by pH and D1 has a one-electron capacity.     

Biosynthesis of chlorophyll P-680 of photosystem II in greening leaves of plants adapted to heat shock

In etiolated pea and maize leaves illuminated after incubation at 38 degreesC, a new dark reaction was shown manifested in the bathochromic shift of spectral bands and accompanied by esterification of the product of protochlorophyllide photochemical reduction--Chld 684/676: Chld 684/676 --> Chl 688/680. After completion of the reaction a rapid (20-30 sec) quenching of the fluorescence of the reaction product (Chl 688/680) was observed. The reaction Chld 684/676 --> Chl 688/680 is inhibited under anaerobic conditions and in the presence of cyanide; the reaction accompanied by Chl 688/680 fluorescence quenching is not observed in pea mutants with impaired function of photosystem II reaction centers. The spectral properties of the formed Chl form with the absorption maximum at 680 nm, fluorescence quenching, and simultaneous synthesis of pheophytin suggest that the reaction is connected with the chlorophyll of photosystem II reaction center--P-680.     

Kinetics of reduction of the oxidized primary electron donor of photosystem II in spinach chloroplasts and in Chlorella cells in the microsecond and nanosecond time ranges following flash excitation

Absorption changes (deltaA) at 820 nm, following laser flash excitation of spinach chloroplasts and Chlorella cells, were studied in order to obtain information on the reduction time of the photooxidized primary donor of Photosystem II at physiological temperatures. In the microsecond time range the difference spectrum of deltaA between 750 and 900 nm represents a peak at 820 nm, attributable to a radical-cation of chlorophyll a. In untreated dark-adapted material the signal can be attributed solely to P+-700; it decays in a polyphasic manner with half-times of 17 microseconds, 210 microseconds and over 1 ms. The oxidized primary donor of Photosystem II (P+II) is not detected with a time resolution of 3 microseconds. After treatment with 3--10 mM hydroxylamine, which inhibits the donor side of Photosystem II, P+II is observed and decays biphasically (a major phase with t1/2=20--40 microseconds, and a minor phase with t1/2 congruent to 200 microseconds), probably by reduction by an accessory electron donor. In the nanosecond range, which was made accessible by a new fast-response flash photometer operating at 820 nm, it was found the P+II is reduced with a half-time of 25--45 ns in untreated dark-adapted chloroplasts. It is assumed that the normal secondary electron donor is responsible for this fast reduction.     

Total recovery of O2 evolution and nanosecond reduction kinetics of chlorophyll-a II + (P-680+) after inhibition of water cleavage with acetate

Oxygen evolution and reduction kinetics of the photooxidized Chl-a_II ⁺ have been measured in oxygen-evolving complexes from the thermophilic cyanobacterium Synechococcus sp.

1. Incubation of PS II particles with acetate resulted in an inhibition of oxygen evolution and a retardation of the Chl-a_II ⁺=reduction kinetics from the nanosecond range to the microsecond range, indicating a modification of the donor side of photosystem II (PS II).
2. After the first two flashes given to a dark-adapted, acetate treated sample, Chl-a_II ⁺ was re-reduced with a half-life time of 160 μs by a component of the donor side of PS II. Under repetitive excitation Chl-a_II ⁺ was re-reduced in 500 μs by electron back reaction from the primary acceptor Q_A ^- (X-320^-). Obviously, in the presence of acetate only two electrons are available from the donor side.
3. Both oxygen evolution and nanosecond reduction kinetics of Chl-a_II ⁺ were restored to the control level when acetate was removed.
4. The results indicate a tight coupling between O₂ evolution and nanosecond reduction kinetics of Chl-a_II ⁺.
5. The reversible inhibition is probably due to a replacement of Cl^- by acetate within the water splitting enzyme.
6. Due to its strongly retarded kinetics, the reversibly modified system may facilitate investigations of the mechanism of the donor side.

     

Characterization of the photosystem II centers inactive in plastoquinone reduction by fluorescence induction

In order to characterize the photosystem II (PS II) centers which are inactive in plastoquinone reduction, the initial variable fluorescence rise from the non-variable fluorescence level F_o to an intermediate plateau level F_i has been studied. We find that the initial fluorescence rise is a monophasic exponential function of time. Its rate constant is similar to the initial rate of the fastest phase (-phase) of the fluorescence induction curve from DCMU-poisoned chloroplasts. In addition, the initial fluorescence rise and the -phase have the following common properties: their rate constants vary linearly with excitation light intensity and their fluorescence yields are lowered by removal of Mg⁺⁺ from the suspension medium. We suggest that the inactive PS II centers, which give rise to the fluorescence rise from F_o to F_i, belong to the -type PS II centers. However, since these inactive centers do not display sigmoidicity in fluorescence, they thus do not allow energy transfer between PS II units like PS II.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - DMQ 2,5-dimethyl-p-benzoquinone - F_o initial non-variable fluorescence yield - F_m maximum fluorescence yield - F_i intermediate fluorescence yield - PS II photosystem II - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II     

Chlorophyll a fluorescence rise induced by high light illumination of dark-adapted plant tissue studied by means of a model of photosystem II and considering photosystem II heterogeneity

Chlorophyll a fluorescence rise (FLR) measured in vivo in dark-adapted plant tissue immediately after the onset of high light continuous illumination shows complex O-K-J-I-P transient. The steps typically appear at about 400 micros (K), 2 ms (J), 30 ms (I), and 200 - 500 ms (P) and a transient decrease of fluorescence to local minima (dips D) can be observed after the K, J, and I steps. As the FLR reflects a function of photosystem II (PSII) and to more understand the FLR, a PSII reactions model was formulated comprising equilibrium of excited states among all light harvesting and reaction centre pigments and P680, reversible radical pair formation and the donor and acceptor side functions. Such a formulated model is the most detailed and complex model of PSII reactions used so far for simulations of the FLR. By varying of selected model parameters (rate constants and initial conditions) several conclusions can be made as for the origin of and changes in shape of the theoretical FLR and compare them with in-literature-reported results. For homogeneous population of PSII and using standard in-literature-reported values of the model parameters, the simulated FLR is characterized by reaching the minimal fluorescence F(0) at about 3 ns after the illumination is switched on lasting to about 1 micros, followed by fluorescence rise to a plateau located at about 2 ms and subsequent fluorescence rise to a global maximum that is reached at about 60 ms. Varying of the values of rate constants of fast processes that can compete for utilization of the excited states with fluorescence emission does not change qualitatively the shape of the FLR. However, primary photochemistry of PSII (the charge separation, recombination and stabilization), non-radiative loss of excited states in light harvesting antennae and excited states quenching by oxidized plastoquisnone (PQ) molecules from the PQ pool seem to be the main factors controlling the maximum quantum yield of PSII photochemistry as expressed by the F(V)/F(M) ratio. The appearance of the plateau at about 2 ms in the FLR is affected by several factors: the height of the plateau in the FLR increases when the fluorescence quenching by oxidized P680(+) is not considered in the simulations or when the electron transfer from Q(A)(-) to Q(B)((-)) is slowed down whereas the height of the plateau decreases and its position is shifted to shorter times when OEC is initially in higher S state. The plateau at about 2 ms is changed into the local fluorescence maximum followed by a dip when the fluorescence quenching by oxidized PQ molecules or the charge recombination between P680(+) and Q(A)(-) is not considered in the simulations or when all OEC is initially in the S(0) state or when the S -state transitions of OEC are slowed down. Slowing down of the S -state transitions of OEC as well as of the electron transfer from Q(A)(-) to Q(B)((-)) also causes a decrease of maximal fluorescence level. In the case of full inhibition of the S -state transitions of OEC as well as in the case of full inhibition of the electron donation to P680(+) by Y(Z), the local fluorescence maximum becomes the global fluorescence maximum. Assuming homogeneous PSII population, theoretical FLR curve that only far resembles experimentally measured O-J-I-P transient at room temperature can be simulated when slowly reducing PQ pool is considered. Assuming heterogeneous PSII population (i.e. the alpha/beta and the Q(B) -reducing/Q(B)-non-reducing heterogeneity and heterogeneity in size of the PQ pool and rate of its reduction) enables to simulate the FLR with two steps between minimal and maximal fluorescence whose relative heights are in agreement with the experiments but not their time positions. A cause of this discrepancy is discussed as well as different approaches to the definition of fluorescence signal during the FLR.     

Influence of histidine-198 of the D1 subunit on the properties of the primary electron donor, P680, of photosystem II in Thermosynechococcus elongatus

The influence of the histidine axial ligand to the PD1 chlorophyll of photosystem II on the redox potential and spectroscopic properties of the primary electron donor, P680, was investigated in mutant oxygen-evolving photosystem II (PSII) complexes purified from the thermophilic cyanobacterium Thermosynechococcus elongatus. To achieve this aim, a mutagenesis system was developed in which the psbA1 and psbA2 genes encoding D1 were deleted from a His-tagged CP43 strain (to generate strain WT*) and mutations D1-H198A and D1-H198Q were introduced into the remaining psbA3 gene. The O2-evolving activity of His-tagged PSII isolated from WT* was found to be significantly higher than that measured from His-tagged PSII isolated from WT in which psbA1 is expected to be the dominantly expressed form. PSII purified from both the D1-H198A and D1-H198Q mutants exhibited oxygen-evolving activity as high as that from WT*. Surprisingly, a variety of kinetic and spectroscopic measurements revealed that the D1-H198A and D1-H198Q mutations had little effect on the redox and spectroscopic properties of P680, in contrast to the earlier results from the analysis of the equivalent mutants constructed in Synechocystis sp. PCC 6803 [B.A. Diner, E. Schlodder, P.J. Nixon, W.J. Coleman, F. Rappaport, J. Lavergne, W.F. Vermaas, D.A. Chisholm, Site-directed mutations at D1-His198 and D2-His197 of photosystem II in Synechocystis PCC 6803: sites of primary charge separation and cation and triplet stabilization, Biochemistry 40 (2001) 9265-9281]. We conclude that the nature of the axial ligand to PD1 is not an important determinant of the redox and spectroscopic properties of P680 in T. elongatus.     

Time sequence of the damage to the acceptor and donor sides of photosystem II by UV-B radiation as evaluated by chlorophyll a fluorescence

The effects of ultraviolet-B (UV-B) radiation on photosystem II (PS II) were studied in leaves of Chenopodium album. After the treatment with UV-B the damage was estimated using chlorophyll a fluorescence techniques. Measurements of modulated fluorescence using a pulse amplitude modulated fluorometer revealed that the efficiency of photosystem II decreased both with increasing time of UV-B radiation and with increasing intensity of the UV-B. Fluorescence induction rise curves were analyzed using a mechanistic model of energy trapping. It appears that the damage by UV-B radiation occurs first at the acceptor side of photosystem II, and only later at the donor side.     

Electrogenic reduction of the primary electron donor P700 by plastocyanin in photosystem I complexes

An electrometric technique was used to investigate electron transfer between spinach plastocyanin (Pc) and photooxidized primary electron donor P700 in photosystem I (PS I) complexes from the cyanobacterium Synechocystis sp. PCC 6803. In the presence of Pc, the fast unresolvable kinetic phase of membrane potential generation related to electron transfer between P700 and the terminal iron–sulfur acceptor F_B was followed by additional electrogenic phases in the microsecond and millisecond time scales, which contribute approximately 20% to the overall electrogenicity. These phases are attributed to the vectorial electron transfer from Pc to the protein-embedded chlorophyll dimer P700⁺ within the PsaA/PsaB heterodimer. The observed rate constant of the millisecond kinetic phase exhibited a saturation profile at increasing Pc concentration, suggesting the formation of a transient complex between Pc and PS I with the dissociation constant K_d of about 80 μM. A small but detectable fast electrogenic phase was observed at high Pc concentration. The rate constant of this phase was independent of Pc concentration, indicating that it is related to a first-order process.     

Directed mutagenesis indicates that the donor to P+680 in photosystem II is tyrosine-161 of the D1 polypeptide

Photosystem II contains two redox-active tyrosines. One of these, YZ, reduces the reaction center chlorophyll, P680, and transfers the oxidizing equivalent to the oxygen-evolving complex. The second, YD, has a long-lived free radical state of unknown function. We recently established that YD is Tyr-160 of the D2 polypeptide by site-directed mutagenesis of a psbD gene in the unicellular cyanobacterium Synechocystis 6803 [Debus, R. J., Barry, B. A., Babcock, G. T., & McIntosh, L. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 427-430]. YZ is most likely the symmetry-related Tyr-161 of the D1 polypeptide. To test this hypothesis, we have changed Tyr-161 to phenylalanine by site-directed mutagenesis of a psbA gene in Synechocystis. The resulting mutant assembles PSII, as judged by its ability to produce the stable Y+D radical, but is unable to grow photosynthetically and exhibits altered fluorescence properties. The nature of the fluorescence change indicates that forward electron transfer to P+680 is disrupted in the mutant. These results provide strong support for our identification of Tyr-161 in the D1 polypeptide with YZ.