Transient peroxide formation by the manganese-containing, redox-active donor side of Photosystem II upon inhibition of O2 evolution with lauroylcholine chloride

Hydrogen Peroxide and Superoxide Anion Radical Photoproduction in PSII Preparations at Various Modifications of the Water-Oxidizing Complex

The photoproduction of superoxide anion radical (O₂^−•) and hydrogen peroxide (H₂O₂) in photosystem II (PSII) preparations depending on the damage to the water-oxidizing complex (WOC) was investigated. The light-induced formation of O₂^−• and H₂O₂ in the PSII preparations rose with the increased destruction of the WOC. The photoproduction of superoxide both in the PSII preparations holding intact WOC and the samples with damage to the WOC was approximately two times higher than H₂O₂. The rise of O₂^−• and H₂O₂ photoproduction in the PSII preparations in the course of the disassembly of the WOC correlated with the increase in the fraction of the low-potential (LP) Cyt b₅₅₉. The restoration of electron flow in the Mn-depleted PSII preparations by exogenous electron donors (diphenylcarbazide, Mn²⁺) suppressed the light-induced formation of O₂^−• and H₂O₂. The decrease of O₂^−• and H₂O₂ photoproduction upon the restoration of electron transport in the Mn-depleted PSII preparations could be due to the re-conversion of the LP Cyt b₅₅₉ into higher potential forms. It is supposed that the conversion of the high potential Cyt b₅₅₉ into its LP form upon damage to the WOC leads to the increase of photoproduction of O₂^−• and H₂O₂ in PSII.

Hydroxyectoine protects Mn-depleted photosystem II against photoinhibition acting as a source of electrons

In the present study, we have investigated the effect of hydroxyectoine (Ect-OH), a heterocyclic amino acid, on oxygen evolution in photosystem II (PS II) membrane fragments and on photoinhibition of Mn-depleted PS II (apo-WOC-PS II) preparations. The degree of photoinhibition of apo-WOC-PS II preparations was estimated by the loss of the capability of exogenous electron donor (sodium ascorbate) to restore the amplitude of light-induced changes of chlorophyll fluorescence yield (∆F). It was found that Ect-OH (i) stimulates the oxygen-evolving activity of PS II, (ii) accelerates the electron transfer from exogenous electron donors (K₄[Fe(CN)₆], DPC, TMPD, Fe²⁺, and Mn²⁺) to the reaction center of apo-WOC-PS II, (iii) enhances the protective effect of exogenous electron donors against donor-side photoinhibition of apo-WOC-PS II preparations. It is assumed that Ect-OH can serve as an artificial electron donor for apo-WOC-PS II, which does not directly interact with either the donor or acceptor side of the reaction center. We suggest that the protein conformation in the presence of Ect-OH, which affects the extent of hydration, becomes favorable for accepting electrons from exogenous donors. To our knowledge, this is the first study dealing with redox activity of Ect-OH towards photosynthetic pigment-protein complexes.

Thomas John Wydrzynski (8 July 1947-16 March 2018)

With this Tribute, we remember and honor Thomas John (Tom) Wydrzynski. Tom was a highly innovative, independent and committed researcher, who had, early in his career, defined his life-long research goal. He was committed to understand how Photosystem II produces molecular oxygen from water, using the energy of sunlight, and to apply this knowledge towards making artificial systems. In this tribute, we summarize his research journey, which involved working on 'soft money' in several laboratories around the world for many years, as well as his research achievements. We also reflect upon his approach to life, science and student supervision, as we perceive it. Tom was not only a thoughtful scientist that inspired many to enter this field of research, but also a wonderful supervisor and friend, who is deeply missed (see footnote*).

Vyacheslav (Slava) Klimov (1945-2017): A scientist par excellence, a great human being, a friend, and a Renaissance man

Vyacheslav Vasilevich (V.V.) Klimov (or Slava, as most of us called him) was born on January 12, 1945 and passed away on May 9, 2017. He began his scientific career at the Bach Institute of Biochemistry of the USSR Academy of Sciences (Akademy Nauk (AN) SSSR), Moscow, Russia, and then, he was associated with the Institute of Photosynthesis, Pushchino, Moscow Region, for about 50 years. He worked in the field of biochemistry and biophysics of photosynthesis. He is known for his studies on the molecular organization of photosystem II (PSII). He was an eminent scientist in the field of photobiology, a well-respected professor, and, above all, an outstanding researcher. Further, he was one of the founding members of the Institute of Photosynthesis in Pushchino, Russia. To most, Slava Klimov was a great human being. He was one of the pioneers of research on the understanding of the mechanism of light energy conversion and of water oxidation in photosynthesis. Slava had many collaborations all over the world, and he is (and will be) very much missed by the scientific community and friends in Russia as well as around the World. We present here a brief biography and some comments on his research in photosynthesis. We remember him as a friendly and enthusiastic person who had an unflagging curiosity and energy to conduct outstanding research in many aspects of photosynthesis, especially that related to PSII.

Two pathways of photoproduction of organic hydroperoxides on the donor side of photosystem 2 in subchloroplast membrane fragments

Earlier the catalase-insensitive formation of organic hydroperoxides (via the interaction of organic radicals produced due to redox activity of P₆₈₀^+· (or TyrZ^·) with molecular oxygen) has been found in Mn-depleted PS2 preparations (apo-WOC-PS2) by Khorobrykh et al. (Biochemistry 50:10658-10665, 2011). The present work describes a second pathway of the photoproduction of organic peroxides on the donor side of PS2. It was shown that illumination of CaCl₂-treated PS2 membranes (deprived of the PS2 extrinsic proteins without removal of the Mn-containing water-oxidizing complex) (CaCl₂-PS2) led to the photoproduction of highly lipophilic organic hydroperoxides (LP-OOH) (in amount corresponding to 1.5 LP-OOH per one reaction center of PS2) which significantly increased upon the addition of exogenous electron acceptor potassium ferricyanide (to 4.2 LP-OOH per one reaction center). Addition of catalase (200 U/ml) before illumination inhibited ferricyanide-induced photoproduction of hydroperoxides while no effect was obtained by adding catalase after illumination or by adding inactivated catalase before illumination. The hydroperoxide photoproduction was inhibited by the addition of exogenous electron donor for PS2, diphenylcarbazide or diuron (inhibitor of the electron transfer in PS2). The addition of exogenous hydrogen peroxide to the CaCl₂-PS2 led to the production of highly lipophilic organic hydroperoxides in the dark (3.2 LP-OOH per one reaction center). We suggest that the photoproduction of highly lipophilic organic hydroperoxides in CaCl₂-PS2 preparations occurs via redox activity of H₂O₂ produced on the donor side of PS2.

Trehalose protects Mn-depleted photosystem 2 preparations against the donor-side photoinhibition

Recently, it has been shown that the addition of 1M trehalose leads to the increase of the rate of oxygen photoconsumption associated with activation of electron transport in the reaction center of photosystem 2 (PS2) in Mn-depleted PS2 membranes (apo-WOC-PS2) [37]. In the present work the effect of trehalose on photoinhibition of apo-WOC-PS2 preparations (which are characterized by a high sensitivity to the donor side photoinhibition of PS2) was investigated. The degree of photoinhibition was estimated by the loss of the capability of exogenous electron donor (sodium ascorbate) to reactivate the electron transport (measured by light-induced changes of chlorophyll fluorescence yield (∆F)) in apo-WOC-PS2. It was found that 1M trehalose enhanced the Mn²⁺-dependent suppression of photoinhibition of apo-WOC-PS2: in the presence of trehalose the addition of 0.2μM Mn²⁺ (corresponding to 2 Mn²⁺ per one reaction center) was sufficient for an almost complete suppression of the donor side photoinhibition of the complex. In the absence of trehalose it was necessary to add 100μM Mn²⁺ to achieve a similar result. The effect of trehalose was observed during photoinhibition of apo-WOC-PS2 at low (15μmolphotons^-1m^-2) and high (200μmolphotons^-1m^-2) light intensity. When Mn²⁺ was replaced by other PS2 electron donors (ferrocyanide, DPC) as well as by Ca²⁺ the protective effect of trehalose was not observed. It was also found that 1M trehalose decreased photoinhibition of apo-WOC-PS2 if the samples contained endogenous manganese (1-2 Mn ions per one RC was enough for the maximum protection effect). It is concluded that structural changes in PS2 caused by the addition of trehalose enhance the capability of photochemical reaction centers of apo-WOC-PS2 to accept electrons from manganese (both exogenous and endogenous), which in turn leads to a considerable suppression of the donor side photoinhibition of PS2.

Gernot Renger (1937-2013): his life, Max-Volmer Laboratory, and photosynthesis research

Gernot Renger (October 23, 1937-January 12, 2013), one of the leading biophysicists in the field of photosynthesis research, studied and worked at the Max-Volmer-Institute (MVI) of the Technische Universität Berlin, Germany, for more than 50 years, and thus witnessed the rise and decline of photosynthesis research at this institute, which at its prime was one of the leading centers in this field. We present a tribute to Gernot Renger's work and life in the context of the history of photosynthesis research of that period, with special focus on the MVI. Gernot will be remembered for his thought-provoking questions and his boundless enthusiasm for science.

Glutathione reductase 2 maintains the function of photosystem II in Arabidopsis under excess light

Glutathione reductase plays a crucial role in the elimination of H(2)O(2) molecules via the ascorbate-glutathione cycle. In this study, we used transgenic Arabidopsis plants with decreased glutathione reductase 2 (GR2) levels to investigate whether this GR2 activity protects the photosynthetic machinery under excess light. The transgenic plants were highly sensitive to excess light and accumulated high levels of H(2)O(2). Photosystem II (PSII) activity was significantly decreased in transgenic plants. Flash-induced fluorescence relaxation and thermoluminescence measurements demonstrated inhibition of electron transfer between Q(A) and Q(B) and decreased redox potential of Q(B) in transgenic plants. Immunoblot and blue native gel analysis showed that the levels of PSII proteins and PSII complexes were decreased in transgenic plants. Analyses of the repair of photodamaged PSII and in vivo pulse labeling of thylakoid proteins showed that the repair of photodamaged PSII is inhibited due to the inhibition of the synthesis of the D1 protein de novo in transgenic plants. Taken together, our results suggest that under excess light conditions, GR2 plays an important role in maintaining both the function of the acceptor side of PSII and the repair of photodamaged PSII by preventing the accumulation of H(2)O(2). In addition, our results provide details of the role of H(2)O(2) in vivo accumulation in photoinhibition in plants.

Substituting Fe for two of the four Mn ions in photosystem II-effects on water-oxidation

We have investigated the interaction of Fe(II) cations with Ca-depleted PSII membranes (PSII[-Ca,4Mn]) in the dark and found that Fe(II) incubation removes 2 of 4 Mn ions from the tetranuclear Mn cluster of the photosynthetic O2-evolving complex (OEC). The reduction of Mn ions in PSII(-Ca,4Mn) by Fe(II) and the concomitant release of two Mn(II) cations is accompanied by the binding of newly generated Fe(III) in at least one vacated Mn site. Flash-induced chlorophyll (Chl) fluorescence yield measurements of this new 2Mn/nFe cluster (PSII[-Ca,2Mn,nFe]) show that charge recombination in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) occurs between Qa (-) and the remaining Mn/Fe cluster (but not YZ (●)) in the OEC, and extraction of 2 Mn occurs uniformly in all PSII complexes. No O2 evolution is observed, but the heteronuclear metal cluster in PSII(-Ca,2Mn,nFe) samples is still able to supply electrons for reduction of the exogenous electron acceptor, 2,6-dichlorophrenolindophenol, by photooxidizing water and producing H2O2 in the absence of an exogenous donor as seen previously with PSII(-Ca,4Mn). Selective extraction of Mn or Fe cations from the 2Mn/nFe heteronuclear cluster demonstrates that the high-affinity Mn-binding site is occupied by one of the iron cations. It is notable that partial water-oxidation function still occurs when only two Mn cations are present in the PSII OEC.

Trehalose stimulation of photoinduced electron transfer and oxygen photoconsumption in Mn-depleted photosystem 2 membrane fragments

It is known that the removal of manganese from the water-oxidizing complex (WOC) of photosystem 2 (PS2) leads to activation of oxygen photoconsumption (OPC) [Khorobrykh et al., 2002; Yanykin et al., 2010] that is accompanied by the formation of organic hydroperoxides on the electron-donor side of PS2 [Khorobrykh et al., 2011]. In the present work the effect of trehalose on the OPC in Mn-depleted PS2 preparations (apo-WOC-PS2) was investigated. A more than two-fold increase of the OPC is revealed upon the addition of 1M trehalose. Drastic (30%-70%) inhibition of the OPC upon the addition of either electron acceptor or electron donor indicates that the trehalose-induced activation of the OPC occurs on both donor and acceptor sides of PS2. A two-fold increase in the rate of superoxide-anion radical photoproduction on the electron-acceptor side of PS2 was also shown. Applying the "variable" chlorophyll fluorescence (ΔF) it was shown that the addition of trehalose induces: (i) a significant increase in the ability of exogenous Mn(2+) to donate electrons to the reaction center of PS2, (ii) slowing down the photoaccumulation of the primary quinone electron acceptor of PS2 (QA(-)) under aerobic conditions, (iii) acceleration of the reoxidation of QA(-) by QB (and by QB(-)) as well as the replacement of QB(2-) by a fully oxidized plastoquinone, and (iv) restoration of the electron transfer between the quinone electron carriers in the so-called "closed reaction centers of PS2" (their content in the apo-WOC-PS2 is 41%). It is suggested that the trehalose-induced increase in efficiency of the O2 interaction with the electron-donor and electron-acceptor sides of apo-WOC-PS2 is due to structural changes leading to both a decrease in the proportion of the "closed PS2 reaction centers" and an increase in the electron transfer rate in PS2.

Enhanced chloroplastic generation of H2O2 in stress-resistant Thellungiella salsuginea in comparison to Arabidopsis thaliana

In order to find some basis of salinity resistance in the chloroplastic metabolism, a halophytic Thellungiella salsuginea was compared with glycophytic Arabidopsis thaliana. In control T.s. plants the increased ratios of chlorophyll a/b and of fluorescence emission at 77 K (F730 /F685 ) were documented, in comparison to A.t.. This was accompanied by a higher YII and lower NPQ (non-photochemical quenching) values, and by a more active PSI (photosystem I). Another prominent feature of the photosynthetic electron transport (PET) in T.s. was the intensive production of H2 O2 from PQ (plastoquinone) pool. Salinity treatment (0.15 and 0.30 M NaCl for A.t. and T.s., respectively) led to a decrease in ratios of chl a/b and F730 /F685 . In A.t., a salinity-driven enhancement of YII and NPQ was found, in association with the stimulation of H2 O2 production from PQ pool. In contrast, in salinity-treated T.s., these variables were similar as in controls. The intensive H2 O2 generation was accompanied by a high activity of PTOX (plastid terminal oxidase), whilst inhibition of this enzyme led to an increased H2 O2 formation. It is hypothesized, that the intensive H2 O2 generation from PQ pool might be an important element of stress preparedness in Thellungiella plants. In control T.s. plants, a higher activation state of carboxylase ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) was also documented in concert with the attachment of Rubisco activase (RCA) to the thylakoid membranes. It is supposed, that a closer contact of RCA with PSI in T.s. enables a more efficient Rubisco activation than in A.t.

Cosuppression of RBCS3B in Arabidopsis leads to severe photoinhibition caused by ROS accumulation

Cosuppression of an Arabidopsis Rubisco small subunit gene RBCS3B at Arabidopsis resulted in albino or pale green phenotypes which were caused by ROS accumulation As the most abundant protein on Earth, Rubisco has received much attention in the past decades. Even so, its function is still not understood thoroughly. In this paper, four Arabidopsis transgenic lines (RBCS3B-7, 18, 33, and 35) with albino or pale green phenotypes were obtained by transformation with a construct driving expression of sense RBCS3B, a Rubisco small subunit gene. The phenotypes produced in these transgenic lines were found to be caused by cosuppression. Among these lines, RBCS3B-7 displayed the most severe phenotypes including reduced height, developmental arrest and plant mortality before flowering when grown under normal light on soil. Chloroplast numbers in mesophyll cells were decreased compared to WT, and stacked thylakoids of chloroplasts were broken down gradually in RBCS3B-7 throughout development. In addition, the RBCS3B-7 line was light sensitive, and PSII activity measurement revealed that RBCS3B-7 suffered severe photoinhibition, even under normal light. We found that photoinhibition was due to accumulation of ROS, which accelerated photodamage of PSII and inhibited the repair of PSII in RBCS3B-7.

Interaction of molecular oxygen with the donor side of photosystem II after destruction of the water-oxidizing complex

Photosystem II (PSII) is a pigment-protein complex of thylakoid membrane of higher plants, algae, and cyanobacteria where light energy is used for oxidation of water and reduction of plastoquinone. Light-dependent reactions (generation of excited states of pigments, electron transfer, water oxidation) taking place in PSII can lead to the formation of reactive oxygen species. In this review attention is focused on the problem of interaction of molecular oxygen with the donor site of PSII, where after the removal of manganese from the water-oxidizing complex illumination induces formation of long-lived states (P680(+•) and TyrZ(•)) capable of oxidizing surrounding organic molecules to form radicals.

Flash-induced consumption of molecular oxygen on the donor side of photosystem II in Mn-depleted subchloroplast membrane fragments: specific effects of manganese and calcium ions

It has been shown that removal of manganese from the water-oxidizing complex (WOC) of photosystem II (PSII) leads to flash-induced oxygen consumption (FIOC) which is activated by low concentration of Mn(2+) (Yanykin et al., Biochim Biophys Acta 1797:516-523, 2010). In the present work, we examined the effect of transition and non-transition divalent metal ions on FIOC in Mn-depleted PSII (apo-WOC-PSII) preparations. It was shown that only Mn(2+) ions are able to activate FIOC while other transition metal ions (Fe(2+), V(2+) and Cr(2+)) capable of electron donation to the apo-WOC-PSII suppressed the photoconsumption of O2. Co(2+) ions with a high redox potential (E (0) for Co(2+)/Co(3+) is 1.8 V) showed no effect. Non-transition metal ions Ca(2+) by Mg(2+) did not stimulate FIOC. However, Ca(2+) (in contrast to Mg(2+)) showed an additional activation effect in the presence of exogenic Mn(2+). The Ca(2+) effect depended on the concentration of both Mn(2+) and Ca(2+). The Ca effect was only observed when: (1) the activation of FIOC induced by Mn(2+) did not reach its maximum, (2) the concentration of Ca(2+) did not exceed 40 μM; at higher concentrations Ca(2+) inhibited the Mn(2+)-activated O2 photoconsumption. Replacement of Ca(2+) by Mg(2+) led to a suppression of Mn(2+)-activated O2 photoconsumption; while, addition of Ca(2+) resulted in elimination of the Mg(2+) inhibitory effect and activation of FIOC. Thus, only Mn(2+) and Ca(2+) (which are constituents of the WOC) have specific effects of activation of FIOC in apo-WOC-PSII preparations. Possible reactions involving Mn(2+) and Ca(2+) which could lead to the activation of FIOC in the apo-WOC-PSII are discussed.

Production of reactive oxygen species in decoupled, Ca(2+)-depleted PSII and their use in assigning a function to chloride on both sides of PSII

Extraction of Ca(2+) from the oxygen-evolving complex of photosystem II (PSII) in the absence of a chelator inhibits O2 evolution without significant inhibition of the light-dependent reduction of the exogenous electron acceptor, 2,6-dichlorophenolindophenol (DCPIP) on the reducing side of PSII. The phenomenon is known as "the decoupling effect" (Semin et al. Photosynth Res 98:235-249, 2008). Extraction of Cl(-) from Ca(2+)-depleted membranes (PSII[-Ca]) suppresses the reduction of DCPIP. In the current study we investigated the nature of the oxidized substrate and the nature of the product(s) of the substrate oxidation. After elimination of all other possible donors, water was identified as the substrate. Generation of reactive oxygen species HO, H2O2, and O 2 (·-) , as possible products of water oxidation in PSII(-Ca) membranes was examined. During the investigation of O 2 (·-) production in PSII(-Ca) samples, we found that (i) O 2 (·-) is formed on the acceptor side of PSII due to the reduction of O2; (ii) depletion of Cl(-) does not inhibit water oxidation, but (iii) Cl(-) depletion does decrease the efficiency of the reduction of exogenous electron acceptors. In the absence of Cl(-) under aerobic conditions, electron transport is diverted from reducing exogenous acceptors to reducing O2, thereby increasing the rate of O 2 (·-) generation. From these observations we conclude that the product of water oxidation is H2O2 and that Cl(-) anions are not involved in the oxidation of water to H2O2 in decoupled PSII(-Ca) membranes. These results also indicate that Cl(-) anions are not directly involved in water oxidation by the Mn cluster in the native PSII membranes, but possibly provide access for H2O molecules to the Mn4CaO5 cluster and/or facilitate the release of H(+) ions into the lumenal space.

Enhanced sensitivity and characterization of photosystem II in transgenic tobacco plants with decreased chloroplast glutathione reductase under chilling stress

Chloroplast glutathione reductase (GR) plays an important role in protecting photosynthesis against oxidative stress. We used transgenic tobacco (Nicotiana tabacum) plants with severely decreased GR activities by using a gene encoding tobacco chloroplast GR for the RNAi construct to investigate the possible mechanisms of chloroplast GR in protecting photosynthesis against chilling stress. Transgenic plants were highly sensitive to chilling stress and accumulated high levels of H₂O₂ in chloroplasts. Spectroscopic analysis and electron transport measurements show that PSII activity was significantly reduced in transgenic plants. Flash-induced fluorescence relaxation and thermoluminescence measurements demonstrate that there was a slow electron transfer between Q(A) and Q(B) and decreased redox potential of Q(B) in transgenic plants, whereas the donor side function of PSII was not affected. Immunoblot and blue native gel analyses illustrate that PSII protein accumulation was decreased greatly in transgenic plants. Our results suggest that chloroplast GR plays an important role in protecting PSII function by maintaining the electron transport in PSII acceptor side and stabilizing PSII complexes under chilling stress. Our results also suggest that the recycling of ascorbate from dehydroascorbate in the ascorbate-glutathione cycle in the chloroplast plays an essential role in protecting PSII against chilling stress.

Molecular mechanisms of production and scavenging of reactive oxygen species by photosystem II

Photosystem II (PSII) is a multisubunit protein complex in cyanobacteria, algae and plants that use light energy for oxidation of water and reduction of plastoquinone. The conversion of excitation energy absorbed by chlorophylls into the energy of separated charges and subsequent water-plastoquinone oxidoreductase activity are inadvertently coupled with the formation of reactive oxygen species (ROS). Singlet oxygen is generated by the excitation energy transfer from triplet chlorophyll formed by the intersystem crossing from singlet chlorophyll and the charge recombination of separated charges in the PSII antenna complex and reaction center of PSII, respectively. Apart to the energy transfer, the electron transport associated with the reduction of plastoquinone and the oxidation of water is linked to the formation of superoxide anion radical, hydrogen peroxide and hydroxyl radical. To protect PSII pigments, proteins and lipids against the oxidative damage, PSII evolved a highly efficient antioxidant defense system comprising either a non-enzymatic (prenyllipids such as carotenoids and prenylquinols) or an enzymatic (superoxide dismutase and catalase) scavengers. It is pointed out here that both the formation and the scavenging of ROS are controlled by the energy level and the redox potential of the excitation energy transfer and the electron transport carries, respectively. The review is focused on the mechanistic aspects of ROS production and scavenging by PSII. This article is part of a Special Issue entitled: Photosystem II.

Production and diffusion of chloroplastic H2O2 and its implication to signalling

Hydrogen peroxide (H(2)O(2)) is recognized as an important signalling molecule. There are two important aspects to this function: H(2)O(2) production and its diffusion to its sites of action. The production of H(2)O(2) by photosynthetic electron transport and its ability to diffuse through the chloroplast envelope membranes has been investigated using spin trapping electron paramagnetic resonance spectroscopy and H(2)O(2)-sensitive fluorescence dyes. It was found that, even at low light intensity, a portion of H(2)O(2) produced inside the chloroplasts can leave the chloroplasts thus escaping the effective antioxidant systems located inside the chloroplast. The production of H(2)O(2) by chloroplasts and the appearance of H(2)O(2) outside chloroplasts increased with increasing light intensity and time of illumination. The amount of H(2)O(2) that can be detected outside the chloroplasts has been shown to be up to 5% of the total H(2)O(2) produced inside the chloroplasts at high light intensities. The fact that H(2)O(2) produced by chloroplasts can be detected outside these organelles is an important finding in terms of understanding how chloroplastic H(2)O(2) can serve as a signal molecule.

Photosynthetic oxygen evolution is not reversed at high oxygen pressures: mechanistic consequences for the water-oxidizing complex

We investigated the effects of elevated O(2) pressure on the production of O(2) by photosynthetic organisms in several species of plants, algae, and a cyanobacterium. Using a noninvasive fluorometry technique to monitor sequential turnover of the photosystem II (PSII) reaction center as a function of O(2) pressures, we showed that none of the reactions of water oxidation are affected by elevated O(2) pressures up to 50-fold greater than atmospheric conditions. Thus, the terminal step of O(2) release from the water oxidation complex (S(4) --> S(0) + O(2) + nH(+)) is not reversible in whole cells, leaves, or isolated thylakoid membranes containing PSII, in contrast to reports using detergent-extracted PSII complexes. This implies that there is no thermodynamically accessible intermediate that can be populated by preventing or reversing the O(2) release step with O(2) at atmospheric pressure. To assess the sensitivity of PSII charge recombination to O(2) pressure, we quantitatively modeled the consequences of two putative perturbations to the catalytic cycle of water oxidation within the framework of the Kok model. On the basis of the breadth of oxygenic phototrophs examined in this study, we conclude that O(2) accumulation in cells or the atmosphere does not suppress photosynthetic productivity through the reversal of water oxidation in contemporary phototrophs and would have been unlikely to influence the evolution of oxygenic photosynthesis.

Production of reactive oxygen species by photosystem II

Photosysthetic cleavage of water molecules to molecular oxygen is a crucial process for all aerobic life on the Earth. Light-driven oxidation of water occurs in photosystem II (PSII) - a pigment-protein complex embedded in the thylakoid membrane of plants, algae and cyanobacteria. Electron transport across the thylakoid membrane terminated by NADPH and ATP formation is inadvertently coupled with the formation of reactive oxygen species (ROS). Reactive oxygen species are mainly produced by photosystem I; however, under certain circumstances, PSII contributes to the overall formation of ROS in the thylakoid membrane. Under limitation of electron transport reaction between both photosystems, photoreduction of molecular oxygen by the reducing side of PSII generates a superoxide anion radical, its dismutation to hydrogen peroxide and the subsequent formation of a hydroxyl radical terminates the overall process of ROS formation on the PSII electron acceptor side. On the PSII electron donor side, partial or complete inhibition of enzymatic activity of the water-splitting manganese complex is coupled with incomplete oxidation of water to hydrogen peroxide. The review points out the mechanistic aspects in the production of ROS on both the electron acceptor and electron donor side of PSII.

Decoupling of the processes of molecular oxygen synthesis and electron transport in Ca2+-depleted PSII membranes

Extraction of Ca(2+) from the O(2)-evolving complex (OEC) of photosystem II (PSII) membranes with 2 M NaCl in the light (PSII(-Ca/NaCl)) results in 90% inhibition of the O(2)-evolution reaction. However, electron transfer from the donor to acceptor side of PSII, measured as the reduction of the exogenous acceptor 2,6-dichlorophenolindophenol (DCIP) under continuous light, is inhibited by only 30%. Thus, calcium extraction from the OEC inhibits the synthesis of molecular O(2) but not the oxidation of a substrate we term X, the source of electrons for DCIP reduction. The presence of electron transfer across PSII(-Ca/NaCl) membranes was demonstrated using fluorescence induction kinetics, a method that does not require an artificial acceptor. The calcium chelator, EGTA (5 mM), when added to PSII(-Ca/NaCl) membranes, does not affect the inhibition of O(2) evolution by NaCl but does inhibit DCIP reduction up to 92% (the reason why electron transport in Ca(2+)-depleted materials has not been noticed before). Another chelator, sodium citrate (citrate/low pH method of calcium extraction), also inhibits both O(2) evolution and DCIP reduction. The role of all buffer components (including bicarbonate and sucrose) as possible sources of electrons for PSII(-Ca/NaCl) membranes was investigated, but only the absence of chloride anions strongly inhibited the rate of DCIP reduction. Substitution of other anions for chloride indicates that Cl(-) serves its well-known role as an OEC cofactor, but it is not substrate X. Multiple turnover flash experiments have shown a period of four oscillations of the fluorescence yield (both the maximum level, F(max), and the fluorescence level measured 50 s after an actinic flash in the presence of DCMU) in native PSII membranes, reflecting the normal function of the OEC, but the absence of oscillations in PSII(-Ca/NaCl) samples. Thus, PSII(-Ca/NaCl) samples do not evolve O(2) but do transfer electrons from the donor to acceptor sides and exhibit a disrupted S-state cycle. We explain these results as follows. In Ca(2+)-depleted PSII membranes, obtained without chelators, the oxidation of the OEC stops after the absorption of three quanta of light (from the S1 state), which should convert the native OEC to the S4 state. An one-electron oxidation of the water molecule bound to the Mn cluster then occurs (the second substrate water molecule is absent due to the absence of calcium), and the OEC returns to the S3 state. The appearance of a sub-cycle within the S-state cycle between S3-like and S4-like states supplies electrons (substrate X is postulated to be OH(-)), explains the absence of O(2) production, and results in the absence of a period of four oscillation of the normal functional parameters, such as the fluorescence yield or the EPR signal from S2. Chloride anions probably keep the redox potential of the Mn cluster low enough for its oxidation by Y(Z)(*).

Recollections of Thomas John Wydrzynski

In appreciation of his contribution to the Photosystsem II research and commemoration of the book Photosystem II: The Light-Driven Water-Plastoquinone Oxido-Reductase, co-edited with Kimiyuki Satoh, I present here some of my recollections of Thomas John Wydrzynski and by several others with whom he has associated over the years at Urbana (Illinois), Berkeley (California), Standard Oil Company-Indiana (Illinois), Berlin (Germany), Gothenburg (Sweden), and Canberra (Australia). We not only recognize him for his unique career path in Photosystem II research, but also for his qualities as a collaborative scientist working on the only system on Earth that has the ability to oxidize water to molecular oxygen using the energy of sunlight.

Uncovering channels in photosystem II by computer modelling: current progress, future prospects, and lessons from analogous systems

Even prior to the publication of the crystal structures for photosystem II (PSII), it had already been suggested that water, O(2) and H(+) channels exist in PSII to achieve directed transport of these molecules, and to avoid undesirable side reactions. Computational efforts to uncover these channels and investigate their properties are still at early stages, and have so far only been based on the static PSII structure. The rationale behind the proposals for such channels and the computer modelling studies thus far are reviewed here. The need to take the dynamic protein into account is then highlighted with reference to the specific issues and techniques applicable to the simulation of each of the three channels. In particular, lessons are drawn from simulation studies on other protein systems containing similar channels.

Hypothesis: the peroxydicarbonic acid cycle in photosynthetic oxygen evolution

Peroxydicarbonic acid (Podca), a proposed intermediate in photosynthetic oxygen evolution, was synthesized electrochemically. Consistent with literature descriptions of this compound, it was shown to be a highly reactive molecule, spontaneously hydrolyzed to H2O2, as well as susceptible to oxidative and reductive decomposition. In the presence of Mn2+ or Co2+, Podca was quickly broken down with release of O2. The liberation of O2, however, was partially suppressed at high O2 concentrations. In the presence of Ca-washed photosystem II-enriched membranes lacking extrinsic proteins, Podca was decomposed with the release of O2, but only under conditions favoring photosynthetic electron flow (light plus a Hill oxidant). A model is proposed that details how peroxydicarbonic acid could act as an oxygen-evolving intermediate. The hypothesis is consistent with the well-established Kok model and with recent findings related to the chemistry of oxygen evolution.

Damage to the oxygen-evolving complex by superoxide anion, hydrogen peroxide, and hydroxyl radical in photoinhibition of photosystem II

Under strong illumination of a photosystem II (PSII) membrane, endogenous superoxide anion, hydrogen peroxide, and hydroxyl radical were successively produced. These compounds then cooperatively resulted in a release of manganese from the oxygen-evolving complex (OEC) and an inhibition of oxygen evolution activity. The OEC inactivation was initiated by an acceptor-side generated superoxide anion, and hydrogen peroxide was most probably responsible for the transportation of reactive oxygen species (ROS) across the PSII membrane from the acceptor-side to the donor-side. Besides ROS being generated in the acceptor-side induced manganese loss; there may also be a ROS-independent manganese loss in the OEC of PSII. Both superoxide anion and hydroxyl radical located inside the PSII membrane were directly identified by a spin trapping-electron spin resonance (ESR) method in combination with a lipophilic spin trap, 5-(diethoxyphosphoryl)-5-phenethyl-1-pyrroline N-oxide (DEPPEPO). The endogenous hydrogen peroxide production was examined by oxidation of thiobenzamide.

Coordination between manganese and nitrogen within the ligands in the manganese complexes facilitates the reconstitution of the water-oxidizing complex in manganese-depleted photosystem II preparations

The water-oxidizing complex (WOC) within photosystem II (PSII) can be reconstituted with synthetic manganese complexes by a process called photoactivation; however, the key factors affecting the efficiency of synthetic manganese complexes in reconstitution of electron transport and oxygen evolution activity in manganese-depleted PSII remain unclear. In the present study, four complexes with different manganese coordination environments were used to reconstitute the WOC, and an interesting relationship was found between the coordination environment of the manganese atom in the complexes and their efficiency in restoring electron transport and oxygen evolution. If Mn(II) is coordinated to nitrogen atoms within the ligand, it can restore significant rates of electron transport and oxygen evolution; however, if the manganese atom is coordinated only to oxygen atoms instead of nitrogen atoms, it has no capability to restore electron transport and oxygen evolution. So, our results demonstrate that the capability of manganese complexes to reconstitute the WOC is mainly determined by the coordination between nitrogen atoms from ligands and the manganese atom. It is suggested from our results that the ligation between the nitrogen atom and the manganese atom within the manganese complex facilitates the photoligation of the manganese atom to histidyl residues on the apo-protein in manganese-depleted PSII during photoactivation.

A new paradigm for the action of reactive oxygen species in the photoinhibition of photosystem II

Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to the imbalance between the rate of photodamage to PSII and the rate of the repair of damaged PSII. Photodamage is initiated by the direct effects of light on the oxygen-evolving complex and, thus, photodamage to PSII is unavoidable. Studies of the effects of oxidative stress on photodamage and subsequent repair have revealed that reactive oxygen species (ROS) act primarily by inhibiting the repair of photodamaged PSII and not by damaging PSII directly. Thus, strong light has two distinct effects on PSII; it damages PSII directly and it inhibits the repair of PSII via production of ROS. Investigations of the ROS-induced inhibition of repair have demonstrated that ROS suppress the synthesis de novo of proteins and, in particular, of the D1 protein, that are required for the repair of PSII. Moreover, a primary target for inhibition by ROS appears to be the elongation step of translation. Inhibition of the repair of PSII by ROS is accelerated by the deceleration of the Calvin cycle that occurs when the availability of CO(2) is limited. In this review, we present a new paradigm for the action of ROS in photoinhibition.

Effect of bicarbonate on the water-oxidizing complex of photosystem II in the super-reduced S-states

It is shown that the hydrazine-induced transition of the water-oxidizing complex (WOC) to super-reduced S-states depends on the presence of bicarbonate in the medium so that after a 20 min treatment of isolated spinach thylakoids with 3 mM NH(2)NH(2) at 20 degrees C in the CO(2)/HCO(3)(-)-depleted buffer the S-state populations are: 42% of S(-3), 42% of S(-2), 16% of S(-1) and even formal S(-4) state is reached, while in the presence of 2 mM NaHCO(3), the same treatment produces 30% of S(-3), 38% of S(-2), and 32% of S(-1) and there is no indication of the S(-4) state. Bicarbonate requirement for the oxygen-evolving activity, very low in untreated thylakoids, considerably increases upon the transition of the WOC to the super-reduced S-states, and the requirement becomes low again when the WOC returns back to the normal S-states using pre-illumination. The results are discussed as a possible indication of ligation of bicarbonate to manganese ions within the WOC.

Mutagenesis of CP43-arginine-357 to serine reveals new evidence for (bi)carbonate functioning in the water oxidizing complex of Photosystem II

The chlorophyll-binding protein CP43 is an inner subunit of the Photosystem II (PSII) reaction center core complex of all oxygenic photoautotrophs. X-Ray structural evidence places the guanidinium cation of the conserved arginine 357 residue of CP43 within a few Angstroms to the Mn(4)Ca cluster of the water-oxidizing complex (WOC) and has been implicated as a possible carbonate binding site. To test the hypothesis, the serine mutant, CP43-R357S, from Synechocystis PCC 6803 was investigated by PSII variable fluorescence (F(v)/F(m)) and simultaneous flash O(2) yield measurements in cells and thylakoid membranes. The R357S mutant assembles PSII-WOC centers, but is unable to grow photoautotrophically. Reconstitution of O(2) evolution by photoactivation and the occurrence of period-four oscillations of F(v)/F(m) establishes that the R357S mutant contains an assembled Mn(4)Ca cluster, but turnover is impaired as seen by an 11-fold larger Kok double miss parameter and faster decay of upper S states. Using pulsed light to avoid photoinactivation, wild-type cells and thylakoid membranes exhibit a 2-4-fold loss in O(2) evolution rate upon partial bicarbonate depletion under multiple turnover conditions, while the R357S mutant is unaffected by bicarbonate. Arginine R357 appears to function in binding a (bi)carbonate ion essential to normal catalytic turnover of the WOC. The quantum yield of electron donation from the WOC into PSII increases with decreasing turnover rate in R357S mutant cells and involves an aborted two-flash pathway that is distinct from the classical four-flash pattern. We speculate that an altered photochemical mechanism for O(2) production occurs via formation of hydrogen peroxide, by analogy to other treatments that retard the kinetics of proton release into the lumen.

Reconstruction of the water-oxidizing complex in manganese-depleted Photosystem II using synthetic manganese complexes

The efficiency of a trinuclear and two binuclear manganese complexes in reconstituting electron transport and O(2) evolution activity in Mn-depleted Photosystem II preparations is analyzed. The trinuclear Mn-complex is more efficient than two binuclear Mn-complexes in restoring oxygen evolution, but it is less effective as an electron donor than binuclear Mn-complexes. It is inferred from our results that recovery of electron transport and O(2) evolution with polynuclear Mn-complexes is affected with different factors. Moreover, the trinuclear Mn-complex is extremely sensitive to the addition of CaCl(2). It is suggested that there is an interaction between Ca(2+) and carboxyl within the trinuclear Mn-complex during photoactivation and this interaction benefits the ligation of Mn atom to the apo-WOC and form an active WOC. Binuclear Mn(III)Mn(III) complex shows slightly higher efficiency than binuclear Mn(III)Mn(IV) complex in restoration of O(2) evolution activity. The efficiency of three Mn-complexes in the reconstitution of WOC is in an order: trinuclear Mn(3)(III)>binuclear Mn(III)Mn(III)>binuclear Mn(III)Mn(IV).

Oxidative modifications of the Photosystem II D1 protein by reactive oxygen species: from isolated protein to cyanobacterial cells

Action of reactive oxygen species (ROS) on the isolated D1 protein, a key component of Photosystem II (PSII) complex, was studied and compared with the effect of high irradiance on this protein in mildly solubilized photosynthetic membranes and cells of the cyanobacterium Synechocystis. Whereas singlet oxygen caused mainly protein modification reflected by shift of its electrophoretic mobility, action of hydrogen peroxide and superoxide resulted in generation of specific fragments. Hydroxyl radicals as the most ROS induced fast disappearance of the protein. The results substantiate the ability of ROS to cause direct scission of the D1 peptide bonds. Similar D1 modification, fragmentation and additionally cross-linking with other PSII subunits were observed during illumination or hydrogen peroxide treatment of mildly solubilized thylakoids. Peroxide-induced fragmentation did not occur in thylakoids of the strain lacking a ligand to the nonheme iron, confirming the role of this prosthetic group in the D1-specific cleavage. The D1 modification, fragmentation and cross-linking were suppressed by ROS scavengers, supporting the direct role of ROS in these phenomena. Identical symptoms of the ROS-induced D1 damage were detected in illuminated cells of Synechocystis mutants with a higher probability of ROS formation, documenting the relevance of the in vitro results for the situation in vivo.

The inorganic biochemistry of photosynthetic oxygen evolution/water oxidation

At the request of the organizer of this special edition, we have attempted to do several things in this manuscript: (1) we present a mini-review of recent, selected, works on the light-induced inorganic biogenesis (photoactivation), composition and structure of the inorganic core responsible for photosynthetic water oxidation; (2) we summarize a new proposal for the evolutionary origin of the water oxidation catalyst which postulates a key role for bicarbonate in formation of the inorganic core; (3) we summarize published studies and present new results on what has been learned from studies of 'inorganic mutants' in which the endogenous cofactors (Mn(n+), Ca2+, Cl-) are substituted; (4) the first DeltapH changes measured during the photoactivation process are reported and used to develop a model for the stepwise photo-assembly process; (5) a comparative analysis is given of data in the literature on the kinetics of substrate water exchange and peroxide binding/dismutation which support a mechanistic model for water oxidation in general; (6) we discuss alternative interpretations of data in the literature with a view to forecast new avenues where progress is needed.

Cooperative binding of oxygen to the water-splitting enzyme in the filamentous cyanobacterium Oscillatoria chalybea

In the filamentous cyanobacterium Oscillatoria chalybea photolysis of water does not take place in the complete absence of oxygen. A catalytic oxygen partial pressure of 15x10(-6) Torr has to be present for effective water splitting to occur. By means of mass spectrometry we measured the photosynthetic oxygen evolution in the presence of H(2)(18)O in dependence on the oxygen partial pressure of the atmosphere and analysed the liberations of (16)O(2), (16)O(18)O and (18)O(2) simultaneously. The observed dependences of the light-induced oxygen evolution on bound oxygen yield sigmoidal curves. Hill coefficient values of 3.0, 3.1 and 3.2, respectively, suggest that the binding is cooperative and that four molecules of oxygen have to be bound per chain to the oxygen evolving complex. Oxygen seems to prime the water-splitting reaction by redox steering of the S-state system, putting it in the dark into the condition from which water splitting can start. It appears that in O. chalybea an interaction of oxygen with S(0) and S(1) leads to S(2) and S(3), thus yielding the typical oxygen evolution pattern in which even after extensive dark adaptation substantial amounts of Y(1) and Y(2) are found. The interacting oxygen is apparently reduced to hydrogen peroxide. Mass spectrometry permits to distinguish this highly specific oxygen requirement from the interaction of bulk atmospheric oxygen with the oxygen evolving complex of the cyanobacterium. This interaction leads to the formation H(2)O(2) which is decomposed under O(2) evolution in the light. The dependence on oxygen-partial pressure and temperature is analysed. Structural peculiarities of the cyanobacterial reaction centre of photosystem II referring to the extrinsic peptides might play a role.

Bicarbonate is an essential constituent of the water-oxidizing complex of photosystem II

It is shown that restoration of photoinduced electron flow and O2 evolution with Mn2+ in Mn-depleted photosystem II (PSII) membrane fragments isolated from spinach chloroplasts is considerably increased with bicarbonate in the region pH 5.0-8.0 in bicarbonate-depleted medium. In buffered solutions equilibrated with the atmosphere (nondepleted of bicarbonate), the bicarbonate effect is observed only at pH lower than the pK of H2CO3 dissociation (6.4), which indicates that HCO3- is the essential species for the restoration effect. The addition of just 2 Mn2+ atoms per one PSII reaction center is enough for the maximal reactivation when bicarbonate is present in the medium. Analysis of bicarbonate concentration dependence of the restoration effect reveals two binding sites for bicarbonate with apparent dissociation constant (Kd) of approximately 2.5 microM and 20-34 microM when 2,6-dichloro-p-benzoquinone is used as electron acceptor, while in the presence of silicomolybdate only the latter one remains. Similar bicarbonate concentration dependence of O2 evolution was obtained in untreated Mn-containing PSII membrane fragments. It is suggested that the Kd of 20-34 microM is associated with the donor side of PSII while the location of the lower Kd binding site is not quite clear. The conclusion is made that bicarbonate is an essential constituent of the water-oxidizing complex of PSII, important for its assembly and maintenance in the functionally active state.

Binary oscillations in the Kok model of oxygen evolution in oxygenic photosynthesis

The flash-induced kinetics of various characteristics of Photosystem II (PS II) in the thylakoids of oxygenic plants are modulated by a period of two, due to the function of a two-electron gate in the electron acceptor side, and by a period of four, due to the changes in the state of the oxygen-evolving complex. In the absence of inhibitors of PS II, the assignment of measured signal to the oxygen-evolving complex or to quinone acceptor side has frequently been done on the basis of the periodicity of its flash-induced oscillations, i.e. four or two. However, in some circumstances, the period four oscillatory processes of the donor side of PS II can generate period two oscillations. It is shown here that in the Kok model of oxygen evolution (equal misses and equal double hits), the sum of the concentrations of the S 0 and S 2 states (as well as the sum of concentrations of S 1 and S 3 states) oscillates with period of two: S 0+S 2→S 1+S 3→S 0+S 2→S 1+S 3. Moreover, in the generalized Kok model (with specific miss factors and double hits for each S-state) there always exist such ε0, ε1, ε2, ε3 that the sum ε0[S0] + ε1[S1] + ε2[S2] + ε3[S3] oscillates with period of two as a function of flash number. Any other coefficients which are linearly connected with these coefficients, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabiGaciaacaqabeaadaqaaqaaaOqaaiqbew7aLzaaja% aaaa!3917!\[\hat \varepsilon \]i = c1εi + c2, also generate binary oscillations of this sum. Therefore, the decomposition of the flash-induced oscillations of some measured parameters into binary oscillations, depending only on the acceptor side of PS II, and quaternary oscillations, depending only on the donor side of PS II, becomes practically impossible when measured with techniques (such as fluorescence of chlorophyll a, delayed fluorescence, electrochromic shift, transmembrane electrical potential, changes of pH and others) that could not spectrally distinguish the donor and acceptor sides. This property of the Kok cycle puts limits on the simultaneous analysis of the donor and acceptor sides of the RC of PS II in vivo and suggests that binary oscillations are no longer a certain indicator of the origin of a signal in the acceptor side of PS II.

Thermoluminescence investigations on the site of action of o-phthalaldehyde in photosynthetic electron transport

Glow curves from spinach leaf discs infiltrated with o-phthalaldehyde (OPA) show significant similarity to those obtained by DCMU treatment which is known to block the electron flow from QA, the stable acceptor of Photosystem II (PS II). In both the cases, the thermoluminescence (TL) peak II (Q band) was intensified significantly, whereas peaks III and IV (B band) were suppressed. Total TL yield of the glow curve remained constant even when the leaf discs were infiltrated with high concentrations of OPA (4 mM) or with DCMU (100 μM), indicating that even at these high concentrations no significant change in the number of species undergoing charge recombination in PS II occurred. However, studies with thylakoids revealed significant differences in the action of OPA and DCMU on PS II. Although OPA, at a certain concentration and time of incubation, reduced the B band intensity by about 50-70%, and completely abolished the detectable oxygen evolution, it still retained the TL flash yield pattern, and, thus, S state turnover. OPA is known to inhibit the oxidoreductase activity of in vitro Cyt b6/f (Bhagwat et al. (1993) Arch Biochem Biophys 304: 38-44). However, in the OPA treated thylakoids the extent of inhibition of O2 evolution was not reduced even in the presence of oxidized tetramethyl-p-phenylenediamine which accepts electrons from plastoquinol and feeds then directly to Photosystem I. This suggests that OPA inhibition is at a site prior to plastoquinone pool in the electron transport chain, in agreement with it being between QA and QB. However, an unusual feature of OPA inhibition is that even though all oxygen evolution was completely suppressed, a significant fraction of PS II centers were functional and turned over with the same periodicity of four in the absence of any added electron donor, an observation which appears to be similar to that reported by Wydrzynski (Wydrzynski et al. (1985) Biochim Biophys Acta 809: 125-136) with lauroylcholine chloride, a lipid analogue compound. The detailed chemistry of OPA inhibition remains to be studied. Since we dedicate this paper to William A. Arnold, discoverer of delayed light and TL in photosynthesis, we have also included in the Introduction, a brief history of how TL work was initiated at BARC (Bombay, India).

Specific degradation of the D1 protein of photosystem II by treatment with hydrogen peroxide in darkness: implications for the mechanism of degradation of the D1 protein under illumination

The D1 protein of the photosystem II (PSII) reaction center has a rapid turnover and is specifically degraded under illumination in vivo. When isolated PSII membranes were treated in darkness with 10 mM hydrogen peroxide (H2O2), an active form of oxygen that is generated at the acceptor side of PSII under illumination, proteins of the PSII reaction center were specifically damaged in almost the same way as observed under illumination with strong light. The D1 protein and, to a lesser extent, the D2 protein were degraded to specific fragments, and cross-linked products (the covalently linked adduct of the D1 protein and the alpha subunit of cytochrome b559 and the heterodimer of the D1 and D2 proteins) were generated concomitantly. The site of cleavage of the D1 protein that gave rise to a major fragment of 22 kDa was located in the loop that connects membrane-spanning helixes IV and V. Treatment with H2O2 caused the same damage to proteins in isolated thylakoids and in core complexes that contained the non-heme iron at the acceptor side, but not in isolated reaction centers depleted of the iron. From these observations and the effects of reagents that are known to interact with the non-heme iron, it is suggested that the damage to proteins is caused by oxygen radicals generated by the non-heme iron in the Fe(II) state in a reaction with H2O2. It is proposed, moreover, that a similar mechanism is operative during the selective and specific degradation of the D1 protein under illumination.

Reconstitution of the water-oxidizing complex in manganese-depleted photosystem II complexes by using synthetic binuclear manganese complexes

The efficiency of synthetic binuclear manganese complexes in reconstituting PS II electron flow and oxygen-evolution capacity was analyzed in PS II enriched preparations deprived of their manganese and of the extrinsic regulatory subunits. Measurements of the variable fluorescence induced by actinic illumination with continuous light led to the following results: (a) the synthetic binuclear complexes are more efficient than MnCl2 in establishing a PS II electron flow; (b) an almost complete restoration is achieved at concentrations of these complexes that correspond with an overall stoichiometry of two manganese per PS II; and (c) the electron flow restored by the binuclear manganese complexes closely resembles that of normal O2-evolving PS II preparations in its resistance to addition of 50 microM EDTA, while that supported by MnCl2 is practically completely suppressed at the same chelator concentration. The rate of O2 evolution was used as a measure of the capability to function as manganese source in reconstitution of the oxygen evolution capacity. It was found that (i) as in the case of PS II electron transport, the synthetic binuclear manganese complexes are significantly more efficient than MnCl2; (ii) with respect to the manganese concentration, the maximum effect is achieved with a mu-oxo bridged binuclear Mn(III) complex (symbolized by M-3) at concentrations corresponding to four manganese per PS II; and (iii) at all concentrations of binuclear manganese complex M-3 a significantly higher restoration of the O2 evolution rate is achieved if the reconstitution assay contains in addition the extrinsic regulatory 33 kDa protein (PS II-O protein).(ABSTRACT TRUNCATED AT 250 WORDS)

The photoproduction of superoxide radicals and the superoxide dismutase activity of Photosystem II. The possible involvement of cytochrome b559

In the present study the light induced formation of superoxide and intrinsic superoxide dismutase (SOD) activity in PS II membrane fragments and D1/D2/Cytb559-complexes from spinach have been analyzed by the use of ferricytochrome c (cyt c(III)) reduction and xanthine/xanthine oxidase as assay systems. The following results were obtained: 1.) Photoreduction of Cyt c (III) by PS II membrane fragments is induced by addition of sodium azide, tetracyane ethylene (TCNE) or carbonylcyanide-p-trifluoromethoxy-phenylhydrazone (FCCP) and after removal of the extrinsic polypeptides by a 1M CaCl2-treatment. This activity which is absent in control samples becomes completely inhibited by the addition of exogenous SOD. 2.) The TCNE induced cyt c(III) photoreduction by PS II membrane fragments was found to be characterized by a half maximal concentration of c1/2=10 μM TCNE. Simultaneously, TCNE inhibits the oxygen evolution rate of PS II membrane fragments with c1/2≈ 3 μM. 3.) The photoproduction of O2 (-) is coupled with H(+)-uptake. This effect is diminished by the addition of the O2 (-)-trap cyt c(III). 4.) D1/D2/Cytb559-complexes and PS II membrane fragments deprived of the extrinsic proteins and manganese exhibit no SOD-activity but are capable of producing O2 (-) in the light if a PS II electron donor is added.Based on these results the site(s) of light induced superoxide formation in PS II is (are) inferred to be located at the acceptor side. A part of the PS II donor side and Cyt b559 in its HP-form are proposed to provide an intrinsic superoxide dismutase (SOD) activity.

Generation, oxidation by the oxidized form of the tyrosine of polypeptide D2, and possible electronic configuration of the redox states S0, S-1, and S-2 of the water oxidase in isolated spinach thylakoids

Suitable treatment of thylakoids with hydrazine permits a high population of the redox states S0, S-1, and S-2 in the water oxidase. Experiments performed with dark-adapted samples enriched either in the oxidized or reduced form of the redox-active tyrosine, YD, of polypeptide D2 reveal that YoxD is a unique endogenous oxidant within the PS II complex which causes a one-electron abstraction from the water oxidase in states S0, S-1, and S-2, respectively. A kinetic analysis of the period four oscillation of oxygen yield induced by a train of short flashes in dark-adapted samples permits the determination of the rate constants of electron abstraction from the reduced water oxidase by YoxD. A value of 9 x 10(-4) s-1 was found for the oxidation of S0 and S-2, while S-1 becomes oxidized with a rate constant of 4 x 10(-4) s-1 at 20 degrees C and pH 7.2. The redox state S0 generated either from S1 via the three-flash-induced oxidative pathway through S4 or from a one-flash oxidation of the S-1 state obtained by S1 reduction with NH2NH2 exhibits the same kinetics as S0 oxidation by YoxD. On the basis of these findings and data taken from the literature, the electronic configuration of the manganese atoms in the tetranuclear cluster is discussed. It is assumed that the dimer model of two binuclear manganese groups within the tetranuclear cluster is discussed.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoproduction of hydrogen peroxide in Photosystem II membrane fragments: A comparison of four signals

The present study describes the formation of different forms of peroxide in Photosystem II (PS II) by using a chemiluminescence detection technique. Four chemiluminescence signals (A, B, C and D) of the luminolperoxidase (Lu-Per) system, which detects peroxide, are found in illuminated O2-evolving Photosystem II (PS II) membrane fragments isolated from spinach. Signal A ('free peroxide') peaking around 0.2-0.3 s after mixing PS II membrane fragments with Lu-Per is eliminated by catalase or removal of oxygen from the suspension and ascribed to O2 interaction with reduced PS II electron acceptors. In contrast, signal B peaking around 1.5 min remains largely unaffected under anaerobic conditions, as well as in the presence of catalase (20 μg/ml). Under flash illumination the extent of this signal exhibits a weak period four oscillation (maximum at third and 7th flash). Its yield increases up to the third flash, but is close to zero in the fourth flash. An analogous behaviour is observed in flashes 5 to 8. Signal B is ascribed to Lu-Per interaction with the water-oxidizing system being in S2 and/or S3-state. Signal C ('bound peroxide') detected as free peroxide after acid decomposition of illuminated PS II particles is observed on the 1 st flash and oscillates with period 2 with superposition of period 4. It is evidently related to peroxide either released from S2 or formed at S2 upon acid shock treatment. Signal D ('slowly released peroxide') peaking around 2-3 s after mixing is observed in samples after various treatments (LCC-incubation, washing with 1 M NaCl at pH 8 or with 1 M CaCl2, Cl(-)-depletion) that lead to at least partial removal of the extrinsic proteins of 18, 24 and 33 kDa without Mn extraction. The average amplitude of this signal corresponds with a yield of about 0.2 H2O2 molecules per RC and flash. In a flash train, the extent of signal D exhibits an oscillation pattern with a minimum at the 3rd flash. We assume that these treatments increase the release of 'bound' peroxide (upon injection into the Lu-Per assay) either formed in the normal oxidative pathway of the water oxidase in the S2 or the S3-state or give rise to peroxide formation due to higher accessibility of the Mn-cluster to water molecules.

Water cleavage by solar radiation-an inspiring challenge of photosynthesis research

Solar energy exploitation by photosynthetic water cleavage is of central relevance for the development and sustenance of all higher forms of living matter in the biosphere. The key steps of this process take place within an integral protein complex referred to as Photosystem II (PS II) which is anisotropically incorporated into the thylakoid membrane. This minireview concentrates on mechanistic questions related to i) the generation of strongly oxidizing equivalents (holes) at a special chlorophyll a complex (designated as P680) and ii) the cooperative reaction of four holes with two water molecules at a manganese containing unit WOC (water oxidizing complex) resulting in the release of molecular oxygen and four protons. The classical work of Pierre Joliot and Bessel Kok and their coworkers revealed that water oxidation occurs via a sequence of univalent oxidation steps including intermediary redox states Si (i = number of accumulated holes within the WOC). Based on our current stage of knowledge, an attempt is made a) to identify the nature of the redox states Si, b) to describe the structural arrangement of the (four) manganese centers and their presumed coordination and ligation within the protein matrix, and c) to propose a mechanism of photosynthetic water oxidation with special emphasis on the key step, i.e. oxygen-oxygen bond formation. It is assumed that there exists a dynamic equilibrium in S3 with one state attaining the nuclear geometry and electronic configuration of a complexed peroxide. This state is postulated to undergo direct oxidation to complexed dioxygen by univalent electron abstraction with YZ (ox) and simultaneous internal ligand to metal charge transfer.Key questions on the mechanism will be raised. The still fragmentary answers to these questions not only reflect our limited knowledge but also illustrate the challenges for future research.

Increases in peroxide formation by the Photosystem II oxygen evolving reactions upon removal of the extrinsic 16, 22 and 33 kDa proteins are reversed by CaCl2 addition

This communication introduces a new spectrophotometric assay for the detection of peroxide generated by Photosystem II (PS II) under steady state illumination in the presence of an electron acceptor. The assay is based on the formation of an indamine dye in a horseradish peroxidase coupled reaction between 3-(dimethylamino)benzoic acid and 3-methyl-2-benzothiazolinone hydrazone. Using this assay, we found that as the O2 evolution activity of PS II-enriched membrane fragments is decreased by treatments which cause the dissociation of the 33 and/or 23 and 16 kDa extrinsic proteins (i.e., CaCl2-washing, NaCl-washing, lauroylcholine-treatment and ethylene glycol-treatment), light-induced peroxide formation increases. Both the losses of O2 evolution and increases in peroxide formation seen under these conditions are reversed by CaCl2 addition, indicating that the two activities originate from the water-splitting site. However, the increased rates of peroxide formation do not quantitatively match the losses in O2 evolution activity. We suggest that a rapid consumption of the peroxide takes place via a catalase/peroxidase activity at the water-splitting site which competes with both the O2 evolution and peroxide formation reactions. The observed peroxide formation is interpreted as arising from enhanced water accessibility to the catalytic site upon perturbation of the extrinsic proteins which then leads to alternate water oxidation side reactions.