Electron-transport properties of the isolated D1-D2-cytochrome b-559 Photosystem II reaction centre

Bicarbonate stimulates the electron donation from Mn²⁺ to P₆₈₀⁺ in isolated D1/D2/cytochrome b559 complex

Influence of bicarbonate on the efficiency of the electron donation from Mn(2+) to P₆₈₀(+) in isolated D1/D2/cytochrome b559 complex was investigated. All the experiments were carried out in a medium depleted of HCO₃(-)/CO₂. Kinetics of photoinduced absorbance changes (ΔA) at different wavelengths and decrease of chlorophyll fluorescence yield (-ΔF) related to photoaccumulation of reduced pheophytin, the intermediary electron acceptor of photosystem II (PSII), in the presence of Mn(2+) under anaerobic conditions were measured. Addition of bicarbonate (1 mM) increased the amplitude of these ΔA and -ΔF at least by a factor of 3. Measurements of the photoinduced ΔA, related to photooxidation of the primary electron donor of PSII, chlorophyll P₆₈₀, were done in the presence of silicomolybdate as electron acceptor. These results show that the addition of 0.05 mM Mn(2+) alone or jointly with 1 mM bicarbonate induces a 20% and 70%-decrease of the magnitude of the ΔA at 680 nm. The effect of Mn(2+) (in the presence and absence of bicarbonate) was completely eliminated by the addition of 12 mM EDTA. All these bicarbonate effects were not observed if MgCl₂ or formate were used instead of MnCl₂ and bicarbonate, respectively. In the absence of Mn(2+), bicarbonate induced none of the mentioned above effects (increase of photoaccumulation of reduced pheophytin and decrease of photooxidation of P680). The presented data suggest that bicarbonate stimulates the electron donation from Mn(2+) to D1/D2/cyt b559 reaction center evidently due to formation of easily oxidizable Mn-bicarbonate complexes.

Biphasic reduction of cytochrome b559 by plastoquinol in photosystem II membrane fragments: evidence for two types of cytochrome b559/plastoquinone redox equilibria

In photosystem II membrane fragments with oxidized cytochrome (Cyt) b559 reduction of Cyt b559 by plastoquinol formed in the membrane pool under illumination and by exogenous decylplastoquinol added in the dark was studied. Reduction of oxidized Cyt b559 by plastoquinols proceeds biphasically comprising a fast component with a rate constant higher than (10s)(-1), named phase I, followed by a slower dark reaction with a rate constant of (2.7min)(-1) at pH6.5, termed phase II. The extents of both components of Cyt b559 reduction increased with increasing concentrations of the quinols, with that, maximally a half of oxidized Cyt b559 can be photoreduced or chemically reduced in phase I at pH6.5. The photosystem II herbicide dinoseb but not 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) competed with the quinol reductant in phase I. The results reveal that the two components of the Cyt b559 redox reaction reflect two redox equilibria attaining in different time domains. One-electron redox equilibrium between oxidized Cyt b559 and the photosystem II-bound plastoquinol is established in phase I of Cyt b559 reduction. Phase II is attributed to equilibration of Cyt b559 redox forms with the quinone pool. The quinone site involved in phase I of Cyt b559 reduction is considered to be the site regulating the redox potential of Cyt b559 which can accommodate quinone, semiquinone and quinol forms. The properties of this site designated here as QD clearly suggest that it is distinct from the site QC found in the photosystem II crystal structure.

Trolox, a water-soluble analogue of α-tocopherol, photoprotects the surface-exposed regions of the photosystem II reaction center in vitro. Is this physiologically relevant?

Can Trolox, a water-soluble analogue of α-tocopherol and a scavenger of singlet oxygen ((1)O(2)), provide photoprotection, under high irradiance, to the isolated photosystem II (PSII) reaction center (RC)? To answer the question, we studied the endogenous production of (1)O(2) in preparations of the five-chlorophyll PSII RC (RC5) containing only one β-carotene molecule. The temporal profile of (1)O(2) emission at 1270 nm photogenerated by RC5 in D(2)O followed the expected biexponential behavior, with a rise time, unaffected by Trolox, of 13 ± 1 μs and decay times of 54 ± 2 μs (without Trolox) and 38 ± 2 μs (in the presence of 25 μM Trolox). The ratio between the total (k(t)) and chemical (k(r)) bimolecular rate constants for the scavenging of (1)O(2) by Trolox in aqueous buffer was calculated to be ~1.3, with a k(t) of (2.4 ± 0.2) × 10(8) M(-1) s(-1) and a k(r) of (1.8 ± 0.2) × 10(8) M(-1) s(-1), indicating that most of the (1)O(2) photosensitized by methylene blue chemically reacts with Trolox in the assay buffer. The photoinduced oxygen consumption in the oxygen electrode, when RC5 and Trolox were mixed, revealed that Trolox was a better (1)O(2) scavenger than histidine and furfuryl alcohol at low concentrations (i.e., <1 mM). After its incorporation into detergent micelles in unbuffered solutions, Trolox was able to photoprotect the surface-exposed regions of the D1-D2 heterodimer, but not the RC5 pigments, which were oxidized, together with the membrane region of the protein matrix of the PSII RC, by (1)O(2). These results are discussed and compared with those of studies dealing with the physiological role of tocopherol molecules as a (1)O(2) scavenger in thylakoid membranes of photosynthetic organisms.

Isolation of photosystem II reaction center complexes from plants

Methods to isolate and purify 6- and 5-Chl D1/D2/Cyt b559 photosystem II (PSII) reaction center (RC) complexes from plants are presented, and the advantages and disadvantages of each procedure are discussed. One of the simpler 6-Chl procedures and a procedure for isolating 5-Chl complexes are described in detail. Furthermore, a rapid procedure that produces relatively large amounts of less pure 6-Chl material (i.e., more nonpigmented protein) is also described. Criteria to assess the purity of PSII RC preparations are presented, and problems associated with each of the isolation procedures are discussed.

Glycinebetaine protects the D1/D2/Cytb559 complex of photosystem II against photo-induced and heat-induced inactivation

The presence of 1.0 mol/L glycinebetaine during isolation of D1/D2/Cytb559 reaction centre (RC) complexes from photosystem II (PSII) membrane fragments preserved the photochemical activity, monitored as the light-induced reduction of pheophytin and electron transport from diphenylcarbazide to 2.6-dichlorophenol-indophenol.-Glycinebetaine also protected the D1/D2/Cytb559 complexes against strong light-induced damage to the photochemical reactions and the irreversible bleaching of beta-carotene and chlorophyll. The presence of glycinebetaine also enhanced thermotolerance of the D1/D2/Cytb559 complexes isolated in the presence of 1.0 mol/L betaine with an increase in the temperature for 50% inactivation from 29 degrees C to 35 degrees C. The results indicate an increased supramolecular structural stability in the presence of glycinebetaine.

Redox changes of cytochrome b(559) in the presence of plastoquinones

We have found that short chain plastoquinones effectively stimulated photoreduction of the low potential form of cytochrome b(559) and were also active in dark oxidation of this cytochrome under anaerobic conditions in Triton X-100-solubilized photosystem II (PSII) particles. It is also shown that molecular oxygen competes considerably with the prenylquinones in cytochrome b(559) oxidation under aerobic conditions, indicating that both molecular oxygen and plastoquinones could be electron acceptors from cytochrome b(559) in PSII preparations. alpha-Tocopherol quinone was not active in the stimulation of cytochrome photoreduction but efficiently oxidized it in the dark. Both the observed photoreduction and dark oxidation of the cytochrome were not sensitive to 3-(3,4-dichlorophenyl)-1, 1-dimethylurea. It was concluded that both quinone-binding sites responsible for the redox changes of cytochrome b(559) are different from either the Q(A) or Q(B) site in PSII and represent new quinone-binding sites in PSII.

Thermal sensitivity of the red absorption tail of the photosystem II reaction center complex

The red tail of the absorption spectrum of the D1-D2-cytb559 complex, defined as the absorption signal not described by the two Gaussian sub-bands associated with the intense electronic transitions at 680 and 683 nm, exhibits anomalous temperature behavior. This tail was analyzed in the temperature interval between 80 and 300 K in terms of the mean square deviation (sigma2) of the total Qy absorption band and by Gaussian sub-band decomposition. The value of the average optical reorganization energy (Snum) obtained from the temperature dependence of sigma2 for the whole absorption band was 32 cm(-1), and changed to 16-20 cm(-1) after subtraction of the sub-bands describing the red tail. This latter value is in agreement with the hole burning literature data for chlorophyll bound to proteins, and indicates that the rather high value for the apparent optical reorganization energy obtained by analysis of the total Qy band of the D1-D2-cytb559 complex is determined by the temperature sensitivity of the red tail. This suggests that the long wavelength absorption tail might be due to vibrational transitions associated with vibrational modes in the range of 80-150 cm(-1) which are thermally accessible and give rise to an absorption signal on the low-energy side of the (0,0) transition. On the basis of this assumption, the electron-phonon coupling strength (S) for these modes is estimated to be in the range 0.028-0.18. This interpretation furthermore supports the idea that the electronic transition near 683 nm is that of a monomer chlorophyll.

The origin of chlorophyll fluorescence In vivo and its quenching by the photosystem II reaction centre

Isolated chlorophyll a , in contrast to when it is dissolved in organic solvents, shows a lower and variable yield of fluorescence when bound to protein and embedded in the thylakoid membrane of photosynthetic organisms. There are two current theories that attempt to explain the origin of this variable yield of fluorescence, (i) It may be emitted directly from the photosystem II (PSII) antenna system and therefore in competition with photochemical trapping (prompt fluorescence), (ii) It may be derived from a recombination reaction between oxidized P 680 and reduced pheophytin within the PS II reaction centre (delayed fluorescence). We have isolated a PS II reaction centre complex that binds only four chlorophyll a molecules and can carry out primary charge separation. The complex contains no plastoquinone and therefore is devoid of the secondary electron acceptor Q A . It does, however, contain two pheophytin a molecules, and one of these acts as a primary electron acceptor. The electron donor is P 680 , which is either a monomeric or dimeric form of chlorophyll a . The isolated PS II reaction centre fluoresces at room temperature with a maximum at 683 nm, and the intensity of this emission is almost totally quenched when reduced pheophytin (bright light plus sodium dithionite) or oxidized P 680 (bright light plus silicomolybdate) is photoaccumulated. The photo-induced quenching of chlorophyll fluorescence when sodium dithionite is present is also observed in intact PS II preparations containing plastoquinone Q A . In the latter case Q A is chemically reduced in the dark by dithionite. Bearing in mind the above two postulates for the origin of variable chlorophyll fluorescence it has been possible to investigate the relative quantum yields for the photoproduction of the P 680 Pheo - state either in the absence (with isolated PS II reaction centres) or presence (with PSII-enriched membranes) of reduced Q A . It has been shown that in the absence of Q - A the quantum efficiency for production of the P 680 Pheo - is several orders of magnitude greater than when Q - A is present. This difference probably partly reflects the coulombic restraints on primary charge separation when Q A is reduced and would suggest that under these conditions the PS II reaction centre is a less efficient trap. Such a conclusion is therefore consistent with postulate (i) that the increase inyield of chlorophyll fluorescence as Q A becomes reduced is not due to a back reaction between P + 680 and Pheo - but rather to a decrease in competition between emission and trapping. The results do emphasize however, that the P 680 Pheo - and P + 680 Pheo states are quenchers of chlorophyll fluorescence. In addition to the above, it has been noted that at 77 K fluorescence from the isolated PS II reaction centre reaches a maximum at 685 nm and does not have a peak at 695 nm. This observation appears to invalidate the postulate that the 695 nm emission is from the pheophytin of the PS II reaction centre.

Analysis of the absorption spectrum of photosystem II reaction centers: temperature dependence, pigment assignment, and inhomogeneous broadening

In this study a model for decomposition and pigment assignment of the low-temperature (10 K) absorption spectrum of the photosystem II reaction center (D1-D2-cytochrome b559 complex, PSII-RC) is developed. It is based on theoretical calculations of the line shapes of the inhomogeneously broadened pigment spectra, taking into account electron-phonon coupling. The analysis is performed under the hypothesis that exciton coupling is weak, except for the P680 special pair. In this way a detailed decomposition of the absorption spectrum is obtained. Within the model the temperature dependence of the spectrum can be well explained. It is mainly caused by the temperature-dependent changes of the homogeneous absorption spectra of the individual pigments in the PSII-RC. In addition, slight changes in the inhomogeneous distribution functions have to be taken into account. Two slightly different parameter sets are found. We prefer one of these parameter sets which indicates that an accessory chlorophyll (Chl) is the lowest energy pigment in the RC core and that the two antenna Chls have their spectral maxima at 667.7 and 677.9 nm, respectively. The relationship between the shape of the absorption spectrum and the pigment stoichiometry of the sample (ratio of chlorophyll a:pheophytin a), which was noticed by comparison of a variety of different independently prepared samples, can be explained by the presence of "additional" Chl molecules which are nonstoichiometrically bound to part of the PSII-RCs. These Chls can be grouped into three spectrally distinguishable pools. One of them has its absorption maximum at about 683 nm and is responsible for the prominent shoulder that is present in the 10 K absorption spectra of most PSII-RC preparations. Our results suggest that the Chl content of the samples has been underestimated in many spectroscopic studies on the PSII-RC.

Perfusion chromatography-a new procedure for very rapid isolation of integral photosynthetic membrane proteins

The biochemical isolation of pure and active proteins or chlorophyll protein complexes has been crucial for elucidating the mechanism of photosynthetic energy conversion. Most of the proteins involved in this process are embedded in the photosynthetic membrane. The isolation of such hydrophobic integral membrane proteins is not trivial, and involves the use of detergents often combined with various time-consuming isolation procedures. We have applied the new procedure of perfusion chromatography for the rapid isolation of photosynthetic membrane proteins. Perfusion chromatography combines a highly reactive surface per bed volume with extremely high elution flow rates. We present an overview of this chromatographic method and show the rapid isolation of reaction centres from plant Photosystems I and II and photosynthetic purple bacteria, as well as the fractionation of the chlorophyll a/b-binding proteins of Photosystem I (LHC I). The isolation times have been drastically reduced compared to earlier approaches. The pronounced reduction in time for separation of photosynthetic complexes is convenient and permits purification of proteins in a more native state, including the maintainance of ligands and the possibility to isolate proteins trapped in intermediate metabolic or structural states.

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 μM) under anaerobic conditions. The addition of K-15 at concentrations of 0.5 μM and 2 μM 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 μM K-15 and the effect was maximal in the range 20 to 200 μM. Photooxidation of P680 in the presence of 0.1 mM silicomolybdate was also efficiently inhibited by K-15 (50% inhibition at 15 μM). 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 QA missing in the preparation.At a concentration of 0.5-50 μM 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.

Purification of a Photosystem II reaction center from a thermophilic cyanobacterium using immobilized metal affinity chromatography

Oxygen-evolving PS II particles from the thermophilic cyanobacterium Synechococcus elongatus are partially purified by centrifugation on a sucrose gradient and are bound to a Chelating Sepharose column loaded with Cu(2+) ions. Bound particles are then transformed into PS II RC complexes by two washing steps. First, washing with a phosphate buffer (pH=6.5) containing 0.02% of SB 12 removes the rest of phycobilins and leaves pure PS II core particles on the column. Second, washing with a phosphate buffer (pH=6.2) containing 0.2 M LiClO4 and 0.05% of DM removes CP 47 and CP 43 and leaves bare PS II RC complexes on the column. These are then eluted with a phosphate buffer containing 1% of dodecylmaltoside (DM). The molar ratio of pigments in the eluate changes with the progress of elution but around the middle of the elution period a nearly stable ratio is maintained of Chl a: Pheo a: Car: Cyt b 559 equal to 2.9: 1: 0.9: 0.8. In these fractions the photochemical separation of charges could be demonstrated by accumulation of reduced pheophytin (ΔA of 430-440 nm) and by the flash induced formation of P680(+) (ΔA at 820 nm). The relatively slow relaxation kinetics of the latter signal (t1/2 ≈ 1 ms) may suggest that in a substantial fraction of the RCs QA remains bound to the complex.

Isolation and characterisation of the Photosystem two reaction centre complex from a double mutant of Chlamydomonas reinhardtii

A rapid procedure has been developed for the isolation of the photosystem two reaction centre complex (PS II RC) from a double mutant of Chlamydomonas reinhardtii, F54-14, which lacks the Photosystem one complex and the chloroplast ATPase. Thylakoid membranes are solubilised with 1.5% (w/v) Triton X-100 and the PS II RC purified by anion-exchange chromatography using TSK DEAE-650(S) (Merck). The complex has a pigment stoichiometry of approximately six chlorophyll a: two pheophytin a: one cytochrome b-559: one to two β-carotene. It photoaccumulates reduced pheophytin and oxidised P680 in the presence of sodium dithionite and silicomolybdate, respectively. Immunoblotting experiments have confirmed the presence of the D1 and D2 polypeptides in this complex. The α-subunit of cytochrome b-559 was identified by N-terminal sequencing. Comparison of the complex with the PS II RC from pea using SDS-polyacrylamide gel electrophoresis showed that their polypeptide compositions were similar. However, the α-subunit of cytochrome b-559 from C. reinhardtii has a lower apparent molecular weight than the pea counterpart whereas the β-subunit is larger.

Higher plant light-harvesting complexes LHCIIa and LHCIIc are bound by dicyclohexylcarbodiimide during inhibition of energy dissipation

We have investigated the binding to proteins of the photosynthetic apparatus of the carboxy-modifying agent dicyclohexylcarbodiimide, (cHxN)2C; this inhibits the protective dissipation of excess absorbed light energy (qE) by the light-harvesting apparatus of photosystem II (LHCII), suggesting that carboxyl amino-acid side chains within hydrophobic protein domains may be involved in qE. (cHxN)2(14)C was used to label thylakoids and photosystem II particles, so as to identify proteins which may be involved in the detection of lumen pH during qE induction. Of six thylakoid proteins labelled with (cHxN)2C under conditions where qE is efficiently induced, three are associated with photosystem I, and none with the bulk LHCII. PSII-associated label is bound to three minor components of LHCII, identified as LHCIIa (two species) and LHCIIc, as shown by protein sequencing of tryptic fragments of purified complexes. pH titration of qE formation and protein labelling in coupled thylakoids showed that both qE and labelling of LHCIIa increased at pH 7-8.

Irreversible light-induced formation of P680+ and reduced cytochrome b559 in the D1-D2-Cyt b-559 complex at low temperature

Cytochrome b559 in D1-D2-Cyt b-559 complexes from spinach can be photoreduced in the presence of DBMIB at a temperature of 180-240 K upon continuous illumination. The reduction of Cyt b-559 is accompanied by oxidation of P680. At 240 K recombination of P680+ and reduced Cyt b-559 is complete in several seconds. At 220 K and below, the state P680+Cyt b-559red can be trapped for a long time. This indicates that the photoreduced heme is incapable of electron transfer to P680+ at 220 K and below. On the other hand, the chemically reduced heme of Cyt b-559 is oxidized by P680+ at 77 K. These results are consistent with the presence of two kinds of Cyt b-559 hemes in D1-D2-Cyt b-559 complexes which participate in different ways in the photochemical reactions.

Composition and function of cytochrome b559 in reaction centers of photosystem II of green plants

A review of a recent study of the spectral and thermodynamic properties of cytochrome b559 as well as of the electron transfer between b559 and photosystem II reaction center cofactors in isolated D1/D2/cytochrome b559 complex RC-2 is presented. Attention is paid to the existence of intermediary-potential (IP, +150 mV) and extra-low-potential (XLP, -45 mV) hemes located close to the acceptor (quinone) and donor (P680) sides of the reaction center cofactors, respectively. These hemes found in isolated RC-2 probably correspond to the high-potential and low-potential hemes in chloroplasts, respectively. The above location of the hemes is believed to allow the photoreduction of the XLP heme and photooxidation of the IP heme. The electron transfer between the two hemes is discussed in terms of the cyclic electron flow and possible involvement in water splitting.

L protein, encoded by psbL, restores normal functioning of the primary quinone acceptor, QA, in isolated D1/D2/CP47/Cytb-559/I photosystem II reaction center core complex

Plastoquinone-9 (PQ-9)-depleted PSII reaction center core complex, consisting of CP47/D1/D2/Cytb-559/I, was isolated from spinach PSII particles. PQ-9, lipids and several proteins were extracted from the original PSII particles and separated by several steps of chromatography to be reconstituted into the isolated complex. PQ-9 reconstituted in the complex with the help of thylakoid lipids (digalactosyldiglyceride) did not function as QA by itself. However, PQ-9 simultaneously reconstituted with L protein and the thylakoid lipids successfully functioned as QA in the complex. Other proteins of PSII origin, such as CP43, H, K, nuclear encoded 4.1 and 5.0 kDa proteins, are unable to restore the QA activity in the complex.

Detergent-induced reversible denaturation of the photosystem II reaction center: implications for pigment-protein interactions

Incubation of the D1-D2-cytochrome b559 complex with Triton X-100 modified the protein secondary structure, caused significant spectral modifications, and reduced the formation of light-induced spin-polarized triplet electron paramagnetic resonance (EPR) signal. After 24 h of incubation, the absorption spectrum shifted from 675.5 to 671.5 nm and the fluorescence spectrum shifted from 682 to 672 nm. These shifts were accompanied by an increase in the chlorophyll fluorescence yield and by decreases in the intensity of the circular dichroism in the red region and the secondary electron transport activity. The intensity of the light-induced triplet EPR signal was also markedly reduced in the same experimental conditions. Substitution of dodecyl beta-maltoside for Triton X-100 reversed all the above-mentioned parameters to the values exhibited by the native D1-D2-Cyt b559 complex, including the characteristic triplet EPR signal. We concluded that all observed changes were due to the destruction of P680 with Triton X-100 and to the reestablishment of P680 in the presence of dodecyl beta-maltoside. The easier but certainly not the only possible explanation to all these phenomena is to consider a dimeric structure for P680, at least in its ground state, where interactions take place within the two dimeric chromophores and with the apoprotein. Such a dimeric structure would be very sensitive to small modifications of the P680 domain, which convert the dimer absorbing at 680 into two chlorophyll monomers absorbing near 670 nm. The dodecyl beta-maltoside reestablished the structure of the native P680 domain.

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 Qy 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.

In vivo and in vitro photoinhibition reactions generate similar degradation fragments of D1 and D2 photosystem-II reaction-centre proteins

Isolation of photosystem-II reaction centres from pea leaves after photoinhibitory treatment at low temperature (0-1 degrees C) has provided evidence for the mechanism of degradation of the D1 protein in vivo. These isolated reaction centres did not appear to be spectrally distinct from preparations obtained from control leaves that had not been photoinhibited. Breakdown fragments of both the D1 and D2 proteins were, however, found in preparations isolated from photoinhibited leaves, and showed similarities with those detected when isolated reaction centres were exposed to acceptor-side photoinhibition. Analyses of the origin of D1 fragments indicated that the primary cleavage site of this protein was between transmembrane helices IV and V indicative of the acceptor-side mechanism for photoinhibition. The origins of other D1 protein fragments indicate that some donor-side photoinhibition may also have occurred in vivo under the conditions employed. We have shown that the spectral and functional integrating of the isolated photosystem II reaction centre complex is resistant to proteolytic cleavage by trypsin. Use of a more non-specific protease (subtilisin), however, caused significant destabilisation of the special pair of chlorophylls constituting the primary electron donor, P680, with a consequential loss of functional activity. Thus, it is possible that specific cleavage of photosystem-II reaction-centre proteins may occur in vivo following photoinhibitory damage without a significant change in structural integrity, a conclusion supported by the finding that photodamaged and normal reaction centres were isolated together.

Degradation of D2 protein due to UV-B irradiation of the reaction centre of photosystem II

Exposure of isolated reaction centres of photosystem II to UV-B radiation generates specific breakdown products of the D2 protein. When the quinone, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone is present a 22 kDa fragment containing the N-terminus of the mature protein is generated. Concomitant with the appearance of the N-terminal fragment, two fragments containing the C-terminus of the D2 protein having apparent molecular masses around 10-12 kDa are observed. It is concluded that the primary cleavage occurs in the hydrophilic loop linking putative transmembrane segments IV and V. No such cleavage was observed when silicomolybdate was used as an electron acceptor, suggesting that this UV-B damage is dependent on binding of the added quinone to the QA site.

Characterization of a D1-D2-cyt b-559 complex containing 4 chlorophyll a/2 pheophytin a isolated with the use of MgSO4

A D1-D2-cyt b-559 complex containing 4 chlorophyll alpha, 1 beta-carotene and 1 cytochrome b-559 per 2 pheophytin a has been isolated from spinach with 30% yield using a Q-Sepharose Fast-Flow anion-exchange column equilibrated with 0.1% Triton X-100, 10 mM MgSO4 and 50 mM Tris-HCl (pH 7.2). The preparation was then stabilized with 0.1% dodecyl-beta-D-maltoside. This method gave a yield 10 times higher than that using a Fractogel TSK-DEAE 650(S) column equilibrated with 0.1% Triton X-100, 30 mM NaCl and 50 mM Tris-HCl (pH 7.2). The PS II RC complex was characterized using absorption and fluorescence spectroscopy at 277 and 77 K. A selective reversible bleaching under reducing conditions with maximum at 682 nm, associated with pheophytin a reduction, and light-induced absorption differences with a lifetime of 1.0 ms, ascribed to the triplet state of P680 were measured and indicated that the isolated D1-D2-cyt b-559 complex is active in charge separation. The results are compared with the data obtained for a PS II RC preparation containing 6 chlorophyll alpha, 2 beta-carotene and 1 cyt b-559 per 2 pheophytin a.