Extraction and Reconstitution of Photosystem II

New interpretation of the redox properties of cytochrome b559 in photosystem II

A model of heme-quinone redox interaction has been developed for cytochrome b559 in photosystem II. The quinone QC in the singly protonated form may function as an interacting quinone. The electrostatic effect between the charges on the heme iron of the cytochrome and QCH leads to appearance of three forms of the cytochrome with different redox potentials. A simple and effective mechanism of redox regulation of the electron transfer pathways in photosystem II is proposed.

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.

Interaction between the intermediary electron acceptor (pheophytin) and a possible plastoquinone-iron complex in photosystem II reaction centers

Photoreduction of the intermediary electron acceptor, pheophytin (Pheo), in photosystem II reaction centers of spinach chloroplasts or subchloroplast particles (TSF-II and TSF-IIa) at 220 K and redox potential E(h) = -450 mV produces an EPR doublet centered at g = 2.00 with a splitting of 52 G at 7 K in addition to a narrow signal attributed to Pheo([unk]) (g = 2.0033, DeltaH approximately 13 G). The doublet is eliminated after extraction of lyophilized TSF-II with hexane containing 0.13-0.16% methanol but is restored by reconstitution with plastoquinone A (alone or with beta-carotene) although not with vitamin K(1). TSF-II and TSF-IIa are found to contain approximately 2 nonheme Fe atoms per reaction center. Incubation with 0.55 M LiClO(4) plus 2.5 mM o-phenanthroline (but not with 0.55 M LiClO(4) alone) decreases this value to approximately 0.6 and completely eliminates the EPR doublet, but photoreduction of Pheo is not significantly affected. Partial restoration of the doublet (about 25%) was achieved by subsequent incubation with 0.2 mM Fe(2+), but not with either Mn(2+) or Mg(2+). The Fe removal results in the development of a photoinduced EPR signal (g = 2.0044 +/- 0.0003, DeltaH = 9.2 +/- 0.5 G) at E(h) = 50 mV, which is not observed after extraction with 0.16% methanol in hexane. It is ascribed to plastosemiquinone no longer coupled to Fe in photosystem II reaction centers. The results show that a complex of plastoquinone and Fe can act as the stable "primary" electron acceptor in photosystem II reaction centers and that the interaction of its singly reduced form with the reduced intermediary acceptor, Pheo([unk]), is responsible for the EPR doublet.

Discovery of plastoquinones: a personal perspective

The discovery and the rediscovery of plastoquinone (PQ) are described together with the definition of its structure as a 2,3-dimethyl 5 solanosyl benzoquinone. The discovery, by M. Kofler, was a result of a search for Vitamin K. Its rediscovery was made by me, when I was at The Enzyme Institute of the University of Wisconsin, analyzing animals and plants for the newly discovered coenzyme Q. In green plants, I found another lipophilic quinone in addition to coenzyme Q. Some misleading evidence suggested as if the new quinone had coenzyme Q activity in mitochondria, but improved methods gave negative results. When I found that the quinone was concentrated in chloroplasts, I considered a role for it in photosynthesis analogous to the role of coenzyme Q in mitochondria. After moving to the Chemistry Department, University of Texas at Austin, I used a plain light bulb and some spinach chloroplasts to show that PQ could be involved in photosynthetic redox reactions. This effect was supported by Norman Bishop's restoration of chloroplast electron transport after solvent extraction, with PQ and photoreduction studies by E. R. Redfern and J. Friend in R. A. Morton's laboratory in Liverpool, UK. We also found an additional analog of PQ in addition to a second analog found in Wisconsin. We called the new analogs PQB and PQC. Although we found some restoration effects with PQC, the discovery by W. T. Griffiths in Morton's laboratory, that PQB and PQC consisted of six forms of PQ each, made it more likely that the new analogs were breakdown products. Morton's group established the structure of the PQCs as a series of PQs, with a hydroxyl group on the prenyl side chain, and the PQB series as having fatty acids esterified to the hydroxyl groups of PQC. Possible functions of the analogs are also discussed in this article.

Investigation of the plastoquinone pool size and fluorescence quenching in thylakoid membranes and Photosystem II (PS II) membrane fragments

The efficiency of oxidized endogenous plastoquinone-9 (PQ-9) as a non-photochemical quencher of chlorophyll fluorescence has been analyzed in spinach thylakoids and PS II membrane fragments isolated by Triton X-100 fractionation of grana stacks. The following results were obtained: (a) After subjection of PS II membrane fragments to ultrasonic treatment in the presence of PQ-9, the area over the induction curve of chlorophyll fluorescence owing to actinic cw light increases linearly with the PQ-9/PS II ratio in the reconstitution assay medium; (b) the difference of the maximum fluorescence levels, F(max), of the induction curves, measured in the absence and presence of DCMU, is much more pronounced in PS II membrane fragments than in thylakoids; (c) the ratio F(max)(-DCMU)/F(max)(+DCMU) increases linearly with the content of oxidized PQ-9 that is varied in the thylakoids by reoxidation of the pool after preillumination and in PS II membrane fragments by the PQ-9/PS II ratio in the reconstitution assay; (d) the reconstitution procedure leads to tight binding of PQ-9 to PS II membrane fragments, and PQ-9 cannot be replaced by other quinones; (e) the fluorescence quenching by oxidized PQ-9 persists at low temperatures, and (f) oxidized PQ-9 preferentially affects the F695 of the fluorescence emission spectrum at 77 K. Based on the results of this study the oxidized PQ-9 is inferred to act as a non-photochemical quencher via a static mechanism. Possible implications for the nature of the quenching complex are discussed.

Effects of green light on biological systems

Green light (510-565 nm) constitutes a significant portion of the visible spectrum impinging on biological systems. It plays many different roles in the biochemistry, physiology and structure of plants and animals. In only a relatively small number of responses to green light is the photoreceptor known with certainty or even provisionally and in even fewer systems has the chain of events leading from perception to response been examined experimentally. This review provides a detailed view of those biological systems shown to respond to green light, an evaluation of possible photoreceptors and a review of the known and postulated mechanisms leading to the responses.

Redox and acid-base characterization of cytochrome b-559 in photosystem II particles

The redox and acid/base states and midpoint potentials of cytochrome b-559 have been determined in oxygen-evolving photosystem II (PS II) particles at room temperature in the pH range from 6.5 to 8.5. At pH 7.5 the fresh PS II particles present about 2/3 of their cytochrome b-559 in its reduced and protonated (non-auto-oxidizable) high-potential form and about 1/3 in its oxidized and non-protonated low-potential form. Potentiometric reductive titration shows that the protonated high-potential couple is pH-independent (E'0, + 380 mV), whereas the low-potential couple is non-protonated and pH-independent above pH 7.6 (E'0, pH greater than 7.6, + 140 mV), but becomes pH-dependent below this pH, with a slope of -72 mV/pH unit. Moreover, evidence is presented that in PS II particles cytochrome b-559 can cycle, according to its established redox and acid/base properties, as an energy transducer at two alternate midpoint potentials and at two alternate pKa values. Red light absorbed by PS II induces reduction of cytochrome b-559 in these particles at room temperature, the reaction being completely blocked by dichlorophenyldimethylurea.

ESR spectroscopy demonstrates that cytochrome b559 remains low potential in Ca (2+)-reactivated, salt-washed PSII particles

Cytochrome b559 in various Photosystem II preparations was studled by using low temperature ESR spectroscopy. This technique was used because it is able to distinguish high from low potential forms of the cytochrome owing to the g-value differences between these species. Moreover, by using low temperature irradiation to oxidize cyt b559 we have avoided the use of redox mediators. Previous work (Ghanotakis DF., Topper J.N. and Yocum, C.F. (1984) Biochim. Biophys. Acta 767, 524-531) demonstrated that reduction and extraction of manganese of the oxygen evolving complex, which might be expected to alter the redox properties of cyt b559, occurs when certain PSII preparations are exposed to reductants. The ESR data presented here show that a mixture of high potential and lower potential cyt b559 species is observed in the oxygen evolving Photosystem II complex. Treatment of PSII membranes with 0.8 M Tris converts the high potential form(s) to those of lower potential. Exposure of the membranes to 2M NaCl shifts a significant amount of high potential cyt b559 to lower potential form(s); addition of CaCl2 reconstituted oxygen evolution activity but did not restore cyt b559 to its high potential form(s).

Comparative study of the fluorescence yield and of the C550 absorption change at room temperature

The C550 absorption change and the fluorescence yield were studied at room temperature in chloroplasts in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and under conditions in which contributions of P-700 and of the electrochromic effect were neglible. 1. The C550 difference spectrum is a typical band shift with an isobestic point close to 550 nm. 2. The maximum amplitude of C550 absorption change is reached upon the first flash of a series of saturating flashes, unlike the maximum fluorescence yield which is attained after several flashes. 3. The comparison of the induction curves of the C550 change and the fluorescence yield in weak light shows that the fluorescence yield is controlled by two quenchers: one of them (Q1), the redox state of which C550 is a probe, is responsible for the major part of the quenching; the other one (Q2), which is less concentrated and less efficient becomes predominant at the end of the fluorescence induction. 4. Quencher Q2 back-reacts faster than quencher Q1. 5. Two alternative models are discussed in which Q1 and Q2 belong either to the same Photosystem II center or to two different photocenters.

Role of beta-carotene in the reaction centres of photosystems I and II of spinach chloroplasts prepared in non-polar solvents

Spinach chloroplasts have been prepared nonaqueously using non-polar solvents (n-hexane, CCl4, n-hepatane) and the beta-carotene content extracted in a controlled manner. This procedure is reproducible and does not result in large structural or spectral changes of the chloroplasts. The organisation of the chlorophyll-proteins is unaltered, as fragmentation with digitonin results in the appearance of the same fractions as found previously for aqueously-prepared chloroplasts, including the pink zone containing cytochromes f and b6 in the ratio 1 : 2. The chloroplasts possess both Photosystem I activity (P-700 photo-bleaching, and NADP+ photoreduction) and Photosystem II activity (parabenzoquinone reduction with Mn2+ as electron donor, and chlorophyll fluorescence induction). Use of moderate intensity red illumination has allowed a study of the role of beta-carotene in photochemistry separate from its roles in energy transfer and photoprotection. Removal of the fraction of beta-carotene closely associated with the Photosystem I reaction centre caused the rate of NADP+ photoreduction to fall to a low, but significantly non-zero level. Thus, in the complete absence of beta-carotene, photochemistry can still be observed, however the specific association of beta-carotene with the reaction centre is required for maximal rates. We propose that beta-carotene bound at the reaction centre decreases the rate of transfer of excitation energy away from the reaction centre, and increases the rate of photochemistry. It is possible that this occurs via formation of an exciplex between ground state beta-carotene and chlorophyll in the first excited state.

Preparation and characterization of phospholipid-depleted chloroplasts

Spinach class II chloroplasts were treated with snake venom phospholipase A2 in the presence of bovine serum albumin, and separated by sucrose-density centrifugation. The treatment yielded phospholipid-depleted chloroplasts which had lost 82.6% of the original phospholipids. About 20% of the phospholipids of chloroplasts were resistant to enzyme attack. These results suggest that phospholipids exist in two states in chloroplast membranes. In spite of considerable phospholipid depletion, the chloroplast preparations retained a large portion of their photoactivities, i.e. light-induced electron transport, light-induced H+ uptake, and light-induced shrinkage. However, cyclic photophosphorylation was significantly affected with the phospholipid removal.

The role of plastoquinone and beta-carotene in the primary reaction of plant photosystem II

Extraction of Triton Photosystem II chloroplast fragments with 0.2% methanol in hexane for 3 h results in the removal of 90 to 95% of the plastoquinone in the original preparation. The extracted fragments (chlorophyll:plastoquinone ratio, 900: 1) showed no P-680 photooxidation at 15 K after a single laser flash. The extracted fragments also showed no light-induced C-550 absorbance change at 77 K. Reconstitution of the primary reaction of Photosystem II, as evidenced by restoration of low-temperature photooxidation of P-680, could be obtained by the addition of plastoquinone A but not by the addition of beta-carotene. The addition of beta-carotene plus plastoquinone A restored the C-550 absorbance change. These results indicate that plastoquinone functions as the primary electron acceptor of Photosystem II and that beta-carotene does not play a direct role in the primary photochemistry but is required for the C-550 absorbance change.

Light-dependent inhibitory action of p-nitrothiophenol on photosystem II in relation to the redox state of electron carriers

The photosystem-II activity of chloroplasts was inhibited by the treatment with p-nitrothiophenol (NphSH) in the light, and the inhibition was accompanied by a change of the fluorescence spectrum. Aromatic mercaptans examined were active in causing this inhibition and fluorescence change. These effects of p-nitrothiophenol were highly accelerated by blocking the electron transport of the oxidation side of photosystem II by carbonyl-cyanide-m-chlorophenylhydrazone (CCCP) or Tris-HC1 or heat pre-treatment, whereas these were suppressed by blocking the transport on the reduction side by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). It was deduced that the site of NphSH action in the electron transport chain is closer to the reaction center of photosystem II that the blocking site of CCCP or Tris-HC1 or heat, and that such a site in photosystem II is exposed to be modified with NphSH when electron carriers on the oxidation side of photosystem II are oxidized by illumination.

Requirement of galactolipids for photosystem I activity in lyophilized spinach chloroplasts

1. The effect of monogalactosyl diacylglycerol and digalactosyl diacylglycerol on reconstitution of Photosystem I activity in heptane-extracted and galactolipase-treated spinach chloroplasts was investigated. 2. Both galactolipids, in a molar ratio with chlorophyll of 2.5, partially restored Photosystem I activity in heptane-extracted chloroplasts. An addition of saturating amounts of plastocyanin caused complete reactivation of Photosystem I. 3. Similarly, with galactolipase-treated chloroplasts, both galactolipids partially restored Phostosystem I activity and additional amounts of plastocyanin were required for complete reactivation. 4. The action of galactolipids on partial reconstitution of Photosystem I supports the suggestion of their structural role in the restoration of thylakoid membranes.

Chlorophyll radical cation in photosystem II of chloroplasts. Millisecond decay at low temperature

We compare the absorption changes, in the near infrared and in the green part of the spectrum, induced in spinach chloroplasts suspensions, at -- 170 degrees C, by continuous light and by flashes. (1) Following flash excitation, an absorption increase peaking at 825 nm which reverses rapidly (t 1/2 = 3.0 ms) is not affected by ferricyanide; it is suppressed when chloroplasts are preilluminated in the presence of 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea (DCMU) and hydroxylamine. The reversion of that signal is simultaneous with a partial back reoxidation of C-550 (fully reduced by the flash) and with partial (about 25%) oxidation of cytochrome b559. The magnitude of the signal peaking at 825 nm (that we attribute to the radical cation of the trap chlorophyll of Photosystem II, acting as a primary electron donor) decreases progressively within a series of successive flashes. (2) An absorption increase (40% of which is slowly reversible) with a broad peak around 810 nm is induced by continuous light or by a flash. It is suppressed by pretreatment with ferricyanide, but it is little affected by the treatment with 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea and hydroxylamine. We attribute it to oxidized P700. (3) With chloroplasts pretreated with 10 mM ferricyanide, an absorption increase, whose magnitude is nearly independent of wavelength between 790 and 870 nm, can be induced by continuous light. One saturating flash produces only 20% of the signal. This absorption change (20% of which is reversible in 30 s) might be due to a secondary donor of Photosystem II.

Primary photochemical reactions in chloroplast photosynthesis

Photosynthesis begins with the absorption of light energy and this absorbed energy is transferred to special sites, termed reaction centres. At these sites, the light energy is transformed into chemical products through an oxidation-reduction reaction that generates the primary reactants, an oxidized pigment molecule (P+) and a reduced electron acceptor (A–) (Clayton, 1972). The subsequent reactions of these species in the dark ultimately results in the formation of chemical products required for the fixation of CO2. In this essay we will discuss the nature of the primary reactants generated in the light reactions of chloroplast photosynthesis, stressing recent advances in the identification and characterization of such reactants.

The photochemical activities and electron carriers of developing barley leaves

The development of photochemical activities in isolated barley plastids during illumination of dark-grown plants has been studied and compared with the behaviour of plastocyanin, cytochromes f, b-559(LP), b-563 and b-559(HP) and pigments P546 (C550) and P700. Electron-transport activity dependent on Photosystem 1 and cyclic photophosphorylation dependent on N-methylphenazonium methosulphate (phenazine methosulphate) were very active relative to the chlorophyll content after only a few minutes of illumination of etiolated leaves, and then rapidly declined during the first few hours of greening. By contrast, Photosystem 2 activity (measured with ferricyanide as electron acceptor) and non-cyclic photophosphorylation were not detectable during the first 2(1/2)h of greening, but then increased in total amount in parallel with chlorophyll. The behaviour of the electron carriers suggested their association with either Photosystem 1 or 2 respectively. In the first group were plastocyanin, cytochrome f and cytochrome b-563, whose concentrations in the leaf did not change during greening, and cytochrome b-559(LP) whose concentration fell to one-half its original value, and in the second group were cytochrome b-559(HP) and pigment P546, the concentrations of which closely followed the activities of Photosystem 2. Pigment P700 could not be detected during the first hour, during which time some other form of chlorophyll may take its place in the reaction centre of Photosystem 1. The plastids started to develop grana at about the time that Photosystem 2 activity became detectable.