Effect of Photosystem II inhibitor K-15 on photochemical reactions of the isolated D1/D2 cytochrome b559 complex

Effect of a highly efficient inhibitor of Photosystem II (PS II), K-15 (4-[methoxy-bis-(trifluoromethyl)methyl)-2,6-dinitrophenyl hydrazone methyl ketone), was investigated using the D1/D2/cytochrome b559 reaction centre (RC) complex. A novel approach for photoaccumulating reduced pheophytin (Pheo^–) in the absence of the strong reducing agent, sodium dithionite, was demonstrated which involved illumination in the presence of TMPD (from 5 to 100 M) under anaerobic conditions. The addition of K-15 at concentrations of 0.5 M and 2 M resulted in approx. 50% and near 100%, respectively, inhibition of this photoreaction, while subsequent additions of dithionite eliminated the inhibitory effect of K-15. Methyl viologen induced similar inhibition at much higher concentrations (>1 mM). Moreover, K-15 efficiently quenched the variable part of chlorophyll fluorescence (which is the recombination luminescence of the pair P₆₈₀ ⁺ Pheo^–). A 50% inhibition was induced by 5 M K-15 and the effect was maximal in the range 20 to 200 M. Photooxidation of P₆₈₀ in the presence of 0.1 mM silicomolybdate was also efficiently inhibited by K-15 (50% inhibition at 15 M). The data are consistent with the idea put forward earlier (Klimov et al. 1992) that the inhibitory effect of K-15 is based on facilitating a rapid recombination between Pheo^– and P₆₈₀ ⁺ (or Z⁺) via its redox properties. The inhibitor can be useful for suppressing PS II reactions in isolated RCs of PS II which are resistant to all traditional inhibitors, like diuron, and probably functions by substituting for Q_A missing in the preparation.At a concentration of 0.5–50 M K-15 considerably increased both the rate and extent of cytochrome b559 photoreduction in the presence, as well as in the absence, of 5 mM MnCl₂. Consequently it is suggested that K-15 also serves as a mediator for electron transfer from Pheo^– to cytochrome b559.Abbreviations K-15 4-[methoxy-bis-(trifluoromethyl)methyl]-2,6-dinitrophenyl hydrazone methyl ketone - P₆₈₀ the primary electron donor of PS II - Pheo pheophytin - PS II Photosystem II - Q_A and Q_B the primary and the secondary electron acceptor of PS II - RC reaction centre - SiMo silicomolybdate - TMPD N,N,N,,N,-tetramethyl-p-phenylenediamine - Z secondary electron donor of PS II     

Light-Induced Damage of Histidine Residues in Photosystem II Reaction Center D1/D2/cyt b559 Complex

Histidine residue content of photosystem Ⅱ reaction center D1/D2/cytochrome b559 complex decreased by about 26% after illumination. The result suggests that some histidine residues are damaged by illumination. The damage of histidine residues may be related to the changes of the spectra properties during the incubation in the dark following preillumination of the reaction center complex.     

Superoxide,Hydrogen Peroxide and Hydroxyl Radical in D1/D2/cytochrome b-559 Photosystem II Reaction Center Complex

Spin-trapping electron spin resonance (ESR) was used to monitor the formation of superoxide and hydroxyl radicals in D1/D2/cytochrome b-559 Photosystem II reaction center (PS II RC) Complex. When the PS II RC complex was strongly illuminated, superoxide was detected in the presence of ubiquinone. SOD activity was detected in the PS II RC complex. A primary product of superoxide, hydrogen peroxide, resulted in the production of the most destructive reactive oxygen species, *OH, in illuminated PS II RC complex. The contributions of ubiquinone, SOD and H(2)O(2) to the photobleaching of pigments and protein photodamage in the PS II RC complex were further studied. Ubiquinone protected the PS II RC complex from photodamage and, interestingly, extrinsic SOD promoted this damage. All these results suggest that PS II RC is an active site for the generation of superoxide and its derivatives, and this process protects organisms during strong illumination, probably by inhibiting more harmful ROS, such as singlet oxygen.     

Spectral properties of stabilized D1/D2/cytochrome b-559 photosystem II reaction center complex Effects of Triton X-100, the redox state of pheophytin, and β-carotene

Absorption, fluorescence, and CD spectral properties of the isolated D1/D2/cytochrome b-559 photosystem II reaction center complex were examined in stabilized reaction center material at 77 K. Spectral properties were dependent on the presence or absence of 0.05% Triton X-100 in the RC suspension medium, on the redox state of pheophytin, and on the state of inactivation of the complex. The specific spectral properties of the PS II RC complex in the red suggest that the primary donor is not a bacterial-type special pair and could be a monomer. Furthermore, the spectral properties in the PS II RC may be the result of excitonic interactions among all the porphyrin molecules in the complex. Interactions between β-carotene and porphyrins indicate a significant role for β-carotene in the PS II RC.     

Spectroelectrochemistry of cytochrome b559 in the D1-D2-Cyt b559 complex from spinach

The redox potential of cytochrome b559 (Cyt b559) in the D1-D2-Cyt b559 complex from spinach has been determined to be +90+/-2mV vs. SHE at pH 6.0, by thin-layer cell spectroelectrochemistry for the first time. The redox potential, corresponding uniquely to the so-called "low-potential form", exhibited a sigmoidal pH-dependence from pH 4.0 to 9.0, ranging from +115 to +50mV. An analysis of the pH-dependence based on model equations suggests that two histidine residues coordinating to the heme iron in the protein subunits may exert electrostatic influence on the redox potential of Cyt b559.     

Photodamage to Pigment in the Photosystem Ⅱ Reaction Center D1/D2/Cytochrome b559 Complex

Strong light （800μmol photons/m^2 per s）-induced bleaching of the pigment in the isolated photosystem Ⅱ reaction center （PSII RC） under aerobic conditions （in the absence of electron donors or acceptors） was studied using high-pressure liquid chromatography （HPLC）, absorption spectra, 77K fluorescence spectra and resonance Raman spectra. Changes in pigment composition of the PSII RC as determined by HPLC after light treatment were as follows： with Increasing illumination time chlorophyll （Chl） a and β-carotene （β-car） content decreased. However, decreases in pheophytin （Pheo） could not be observed because of the mixture of the Pheo formed by degraded chlorophyll possibly. On the basis of absorption spectra, it was determined that, with a short time of illuminatlon, the initial bleaching occurred maximally at 680 nm but that with Increasing Illumination time there was a blue shift to 678 nm. It was suggested that P680 was destroyed Initially, followed by the accessory chlorophyll. The activity of P680 was almost lost after 10 mln light treatment. Moreover, the bleaching of Pheo and β-car was observed at the beginning of illumination. After Illumination, the fluorescence emission Intensity changed and the fluorescence maximum blue shifted, showing that energy transfer was disturbed. Resonance Raman spectra of the PSII RC excited at 488.0 and 514.5 nm showed four main bands, peaking at 1 527 cm^-1 （υ101）, 1 159 cm^-1 （υ2）, 1 006 cm^-1 （υ3）, 966 cm^-1 （υ4） for 488.0 nm excitation and 1 525 cm^-1 （υ1）, 1 159 cm^-1 （υ2）, 1 007 cm^-1 （υ3）, 968 cm^-1 （υ4） for 514.5 nm excitation. It was confirmed that two spectroscopically different β-car molecules exist In the PSII RC. After light treatment for 20 mln, band positions and bandwidths were unchanged. This indicates that carotenoid configuration Is not the parameter that regulates photoprotectlon in the PSII RC.     

Reconstitution of plastoquinone in the D1/D2/cytochrome b-559 photosystem II reaction centre complex

Reconstitution of plastoquinone in the photosystem II D1/D2/cytochrome b-559 reaction centre complex, in the presence of the detergent Triton X-100, is reported. Illumination of the reconstituted system results in the reduction of cytochrome b-559, the process being partly herbicide-sensitive. In addition, the reconstitution of plastoquinone results in the ability of the isolated reaction centre to catalyse the photoreduction of 2,6-dichlorophenolindophenol in the presence of the exogenous electron donor diphenylcarbazide.     

Predominant Protection of D2-Protein against Photodestruction in Isolated D1/D2/Cytochrome b 559 by K15, a Phenolic-Type Inhibitor of Electron Transfer in Photosystem 2

A protective action of K15 (4-[methoxy-bis(trifluoromethyl)methyl]-2,6-dinitrophenylhydrazone methyl ketone), an inhibitor of electron transport in photosystem 2 (PS 2), against photoinactivation of the PS 2 reaction center (RC) D1/D2/cytochrome b(559) complex, isolated from pea chloroplasts, by red light (0.7 mmol photons/sec per m(2)) has been investigated under aerobic conditions. The inhibitor K15 causing cyclic electron transfer around PS 2 and thus prohibiting stabilization of separated charges has been shown to effectively protect RC both against the loss of photochemical activity (measured as reversible photoinduced absorbance changes related to photoreduction of pheophytin) and aggregation and degradation of the proteins D2 and D1 during photoinactivation. Comparison of the protective action of K15 and of another inhibitor of electron transfer in PS 2, diuron, against light-induced destruction of proteins D1 and D2 shows that diuron stabilizes protein D1 and K15 stabilizes protein D2. The preferential protection of D2 against photoinduced destruction revealed in our work is in accord with the concept of a specific binding of K15 with this protein. It is proposed that this binding site may be that of the primary quinone electron acceptor Q(A) located on the D2 protein (in contrast to diuron, which is known to replace the secondary electron acceptor Q(B) from its binding site on D1).     

Pressure effects on absorption spectra of the isolated reaction center of Photosystem II

Absorption spectra of the D1-D2-cytochrom b₅₅₉ complex at 4°C were investigated at pressures up to 300 MPa. Pressure effects were mostly reversible and independent of the detergent used (CHAPS or dodecyl--D-maltoside). Red-shifts were observed under pressure for the chlorophyll Q_y- and the -carotene S₀ S₂ bands. The relatively small Q_y-shift of approximately 0.15 cm^-1/MPa is an indication for the absence of strongly coupled chlorophyll dimers within the reaction center and supports earlier reports from low-temperature measurements (Chang HC, Jankowiak R, Reddy NRS and Small GJ (1995) Chem Phys 197: 307–321). The carotene red-shift (seen in CHAPS) is much larger (0.5 – 0.6 cm^-1/MPa) and within the range observed for excitonically coupled chlorophylls. However, since carotenes are more sensitive to changes of refractive index, we do not consider this evidence for excitonically coupled carotenes. Varying the pH and the detergent induced only small effects. Pigment exchange using high pressure instead of elevated temperature was not possible under the conditions tested.     

Characterization of a 160 kD Photosystem II reaction center complex isolated from photoinhibited Dunaliella salina thylakoids

Photoinhibition in the green alga Dunaliella salina is accompanied by the formation of inactive Photosystem II reaction centers. In SDS-PAGE analysis, the latter appear as 160 kD complexes. These complexes are structurally stable, enough to withstand re-electrophoresis of excised gel slices from the 160 kD region. Western blot analyses with specific polyclonal antibodies raised against the D1 or D2 reaction center proteins provided evidence for the presence of both of these polypeptides in the re-electrophoresed 160 kD complex. Incubation of excised gel slices from the 160 kD region, under aerobic conditions at 4°C for a prolonged period of time, caused a break-up of the 160 kD complex into a 52 kD D1-containing and 80 and 26 kD D2-containing pieces. Western blot analysis with polyclonal antibodies raised against the apoproteins of CPI (reaction center proteins of PS I) did not show cross-reaction either with the 160 kD complex or with the 52, 80 and 26 kD pieces. The results show the presence of both D1 and D2 in the 160 kD complex and strengthen the notion of a higher molecular weight D1- and D2-containing complex that forms upon disassembly of photodamaged PS II units.Abbreviations Chl chlorophyll - PS II Photosystem II - D1 the 32 kD reaction center protein of PS II, encoded by the chloroplast psbA gene - D2 the 34 kD reaction center protein of PS II, encoded by the chloroplast psbD gene - CPI the 82 and 83 kD reaction center proteins of PS I, encoded by the chloroplast psaA and psaB genes - HL high light - LL low light This publication is dedicated to the memory of the late Professor Daniel Arnon, whom the first author will fondly remember for his many accounts of past scientific discovery and debate.     

Electron transfer between cytochrome c2 and the tetraheme cytochrome c in Rhodopseudomonas viridis

Kinetics of electron transfer from soluble cytochrome c₂ to the tetraheme cytochrome c have been measured in isolated reaction centers and in membrane fragments of the photosynthetic purple bacterium Rhodopseudomonas viridis by time-resolved flash absorption spectroscopy. Absorbance changes kinetics in the region of cytochrome -bands (540–560 nm) were measured at 21 °C under redox conditions where the two high-potential hemes (c-559 and c-556) of the tetraheme cytochrome were chemically reduced. After flash excitation, the heme c-559 donates an electron to the special pair of bacteriochlorophylls and is then re-reduced by heme c-556. The data show that oxidized heme c-556 is subsequently re-reduced by electron transfer from reduced cytochrome c₂ present in the solution. The rate of this reaction has a non-linear dependence on the concentration of cytochrome c₂, suggesting a (minimal) two-step mechanism involving the f ormation of a complex between cytochrome c₂ and the reaction center, followed by intracomplex electron transfer. To explain the monophasic character of the reaction kinetics, we propose a collisional mechanism where the lifetime of the temporary complex is short compared to electron transfer. The limit of the halftime of the bimolecular process when extrapolated to high concentrations of cytochrome c₂ is 60 ± 20 s. There is a large ionic strength effect on the kinetics of electron transfer from cytochrome c₂ to heme c-556. The pseudofirst-order rate constant decreases from 1.1 × 10⁷ M^-1 s^-1 to 1.3 × 10⁶ M^-1 s^-1 when the ionic strength is increased from 1 to 1000 mM. The maximum rate (1.1 × 10⁷ M^-1 s^-1) was obtained at about 1 mM ionic strength. This dependence of the rate on ionic strength s uggests that attractive electrostatic interactions contribute to the binding of cytochrome c₂ with the tetraheme cytochrome. On the basis of our data and of previous molecular modelling, it is proposed that cytochrome c₂ docks close to the low-potential heme c-554 and reduces heme c-556 via c-554.     

Structure and function in the isolated reaction center complex of Photosystem II: energy and charge transfer dynamics and mechanism

The dynamics of energy and charge transfer in the Photosystem II reaction center complex is an area of great interest today. These processes occur on a time scale ranging from femtoseconds to tens of picoseconds or longer. Steady-state and ultrafast spectroscopy techniques have provided a great deal of quantitative and qualitative data that have led to varied interpretations and phenomenological models. More recently, microscopic models that identify specific charge separated states have been introduced, and offer more insight into the charge transfer mechanism. The structure and energetics of PS II reaction centers are reviewed, emphasizing the effects on the dynamics of the initial charge transfer. This revised version was published online in June 2006 with corrections to the Cover Date.     

Light-Induced Damage of Amino Acid Residues and Degradation of Polypeptides in D1/D2/Cytochrome b559 Complex

Photosystem Ⅱ reaction center D1/Dg/Cyt b559 complex is very sensitive to light. Besides pigments, some amino acids, like histidine and methionine residues on the polypeptide chain, were damaged and D1 and D2 proteins were degraded by illumination. SDS-PAGE analysis demonstrated an increased content of the D1 and D2 protein dimers and a new band with molecular weight of 41 kD after light treatment. Meanwhile, the D1 and D2 bands were shifted to apparent positions of higher molecular weight. During the consequent incubation in the dark following illumination, although there was no change in the composition of amino acids, the degradation process of D1 and D2 proteins and the production of 41 kD fragment continued. It was proposed that degradation of D1 and D2 proteins was probably due to the photodamage of some amino acids via chemical splitting and co-valent cross-linkage in this process.     

Introduction of a [4Fe-4S (S-cys)4]+1,+2 iron-sulfur center into a four-alpha helix protein using design parameters from the domain of the Fx cluster in the Photosystem I reaction center.

We describe the insertion of an iron-sulfur center into a designed four alpha-helix model protein. The model protein was re-engineered by introducing four cysteine ligands required for the coordination of the mulinucleate cluster into positions in the main-chain directly analogous to the domain predicted to ligand the interpeptide [4Fe-4S (S-cys)4] cluster, Fx, from PsaA and PsaB of the Photosystem I reaction center. This was achieved by inserting the sequence, CDGPGRGGTC, which is conserved in PsaA and PsaB, into interhelical loops 1 and 3 of the four alpha-helix model. The holoprotein was characterized spectroscopically after insertion of the iron-sulfur center in vitro. EPR spectra confirmed the cluster is a [4Fe-4S] type, indicating that the cysteine thiolate ligands were positioned as designed. The midpoint potential of the iron-sulfur center in the model holoprotein was determined via redox titration and shown to be -422 mV (pH 8.3, n = 1). The results support proposals advanced for the structure of the domain of the [4Fe-4S] Fx cluster in Photosystem I based upon sequence predictions and molecular modeling. We suggest that the lower potential of the Fx cluster is most likely due to factors in the protein environment of Fx rather than the identity of the residues proximal to the coordinating ligands.     

Isolation and purification of reaction center from Rhodopseudomonas viridis NHTC 133 by means of LDAO

Two different procedures are described to isolate and purify the reaction center complex from Rhodopseudomonas viridis NHTC 133 by means of the non-ionic detergent dodecyldimethylamine oxide. Both reaction center particles thus obtained were active, as shown by a photobleaching centered at 975 nm.The reaction center also contained, in addition to bacteriochlorophyll, bacteriopheophytin. Other components were also found in this particle: cytochromes C₅₅₃ and C₅₅₈ and a menaquinone-like substance.The SDS gel electrophoresis of reaction centers is shown. The molecular weights of the subunits forming the reaction center in 0.5% sodium dodecyl sulfate and 1% mercaptoethanol were calculated as being: 45±1.5 and 37±1.5 kdalton, 29±1.5 and 23±1.5 kdalton.The molecular weight of the complex determined by means of gel filtration (Sepharose 6-B and Bio-Gel P-300) gives a value of approximately 240 kdalton.The minimum molecular weight of the complex calculated by disc gel electrophoresis was 231 kdalton.     

Recombination reduction of photooxidized cytochrome c in reaction centers of Rhodopseudomonas viridis at low temperature

The temperature dependence of dark reduction of photooxidized cytochrome c was studied in isolated preparations of Rhodopseudomonas viridis reaction centers. Within the range from room temperature to 260 K this process was found to be mediated by thermal diffusion of exogenous donor molecules, whereas at lower temperatures photooxidized cytochrome is reduced as a result of indirect recombination with photoreduced primary quinone acceptor. Kinetic simulation allowed certain thermodynamic characteristics of this reaction to be calculated. To the first approximation, these characteristics correlate with the estimates obtained from the results of direct redox titration.     

Reaction center light harvesting B875 complexes from Rhodocyclus gelatinosus: characterization and identification of quinones

Reaction center-B875 pigment-protein complexes were purified from Rhodocyclus gelatinosus. The proteic components consist of 7–8 polypeptides among which some were identified by their apparent molecular weights: the light harvesting B875 polypeptides and of 8 and 6 kDa, reaction center L (23 kDa), M (28 kDa) and H (34 kDa), cytochrome c (43 kDa). Four c-type hemes were found per reaction center. Flash-induced absorbance changes showed the presence of both Q_A and Q_B in the complex. Charge recombination times were determined to be: 1.16±0.2 (n=30) for P⁺Q_AQ_B ^- and 7–10 ms for P⁺Q_A ^- in presence of herbicides. From quinone analysis on one hand and kinetics of charge recombination on the other hand, we proposed that in the reaction center of Rhodocyclus gelatinosus Q_A is menaquinone 8 and Q_B is ubiquinone 8.     

Gaussian band analysis of absorption,fluorescence and photobleaching difference spectra of D1/D2/cytb-559 complex

A study of the absorption and fluorescence characteristics of the D1/D2/cytb-559 reaction centre complex of Photosystem II has been carried out by gaussian decomposition of absorption spectra both at room temperature and 72 K and of the room temperature fluorescence spectrum. A five component fit was found in which the absorption and fluorescence sub-bands could be connected by the Stepanov relation. The photobleaching and light-activated degradation in the dark of long wavelength pigments permitted a further characterisation of the absorption bands. The absorption (fluorescence) maxima of the five bands at room temperature are 660 nm (670 nm), 669 nm (675 nm), 675 nm (681 nm), 680 nm (683 nm), 681 nm (689 nm). A novel feature of this analysis is the presence of two approximately isoenergetic absorption bands near 680 nm at room temperature. The narrower one (FWHM=12.5 nm) is attributed to pheophytin while the broader band (FWHM=23 nm) is thought to be P680. The P680 band width is discussed in terms of homogeneous and site inhomogeous band broadening. The P680 fluorescence has a large Stokes shift (9 nm) and most fluorescence in the 690–700 nm range is associated with this chromophore.The three accessory pigment bands are broad (FWHM=17–24 nm) and the 660 nm gaussian is largely temperature insensitive thus indicating significant site inhomogeneous broadening.The very slight narrowing of the D1/D2/cytb-559 Q_y absorption at crytogenic temperatures is discussed in terms of the coarse spectral inhomogeneity associated with the spectral forms and the apparently large site inhomogeneous broadening of short wavelength accessory pigments.     

Photosystem I reaction centers fromChlamydomonas and higher plant chloroplasts

A photosystem I reaction center has been isolated fromChlamydomonas chloroplasts and compared with the photosystem I reaction center from higher plants. While the higher plant reaction center is active in cytochrome 552 photooxidation, theChlamydomonas preparation was not active unless salts were included in the assay medium or the pH was lowered to 5. Subunit III-depleted photosystem I reaction center from higher plants is also inactive in cytochrome 552 photooxidation in the absence of salts. As with theChlamydomonas reaction center, salts induced its activity. Subunit I of the photosystem I reaction center has tentatively been identified as the binding site of cytochrome 552.     

The identification of the Photosystem II reaction center: a personal story

This minireview is about the path that led me to the identification of the Photosystem II reaction center in oxygenic photosynthesis. It is based mostly on my own experiences and viewpoints. Thus, the article is essentially a personal account, and does not include all contributions that led to the identification of this functional unit of Photosystem II. This revised version was published online in August 2006 with corrections to the Cover Date.