DOCSLIB.ORG

In Photosynthetic Reaction Centers, the Free Energy Difference for Electron Transfer Between Quinones Bound at the Primary and S

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
In Photosynthetic Reaction Centers, the Free Energy Difference for Electron Transfer Between Quinones Bound at the Primary and S Proc. Nati. Acad. Sci. USA Vol. 86, pp. 2658-2662, April 1989 Biophysics In photosynthetic reaction centers, the free energy difference for electron transfer between quinones bound at the primary and secondary quinone-binding sites governs the observed secondary site specificity KATHLEEN M. GIANGIACOMO* AND P. LESLIE DUTTON Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, PA 19104 Communicated by Britton Chance, December 2, 1988 ABSTRACT The secondary quinone-binding site (QB site) enquire further into the specificity of the QB site for quinone of bacterial reaction centers from Rhodobacter sphaeroides is structures. We have questioned in particular the methods of generally regarded to be highly specific for its native ubiqui- assay used to establish the binding strength of a quinone to none-10 molecule. We demonstrate here that this is a miscon- the QB site and have drawn upon the detailed general ception rooted in the kinetic methods used to assay for description for QB function that has been developed by occupancy ofa quinone in the QB site. We show that observance Wraight and Stein (11). of occupancy of the QB site, revealed by kinetic assay, is The methods used to assay for the occupancy of a quinone sensitive to the free-energy difference for electron transfer in the QB site rely entirely on kinetic analysis of various between the quinone at the primary quinone-binding site (QA flash-activated electron-transfer reactions within the reaction site) and the QB site (-AG0 ). For many of the compounds center that are altered by the presence ofa functional quinone previously tested for binding at the QB site, the -AGO. between in the QB site. In these methods, an essential requirement for QA and QB is too small to permit detection of the functional quinone in the QB site. With an increased -AGO. achieved by the recognition of a functional occupant of the QB site is replacing the native ubiquinone-10 at the QA site with lower- electron transfer from the light-generated Q- to QB. potential quinones or by testing higher-potential QB candi- Wraight's model, which is summarized in Fig. 1, introduces dates, it is shown that the QB site binds and functions with the other parameters that describe the interaction and reduction unsubstituted 1,4-benzoquinone, 1,4-naphthoquinone, and of quinones at the QB site. This model includes not only the 9,10-phenanthraquinone, as well as with their various substi- rates of quinone interaction with the QB site (kon and koff), tuted forms. Moreover, quinones with the ortho-carbonyl which define the dissociation constant (Kd = koff/kon), but configuration appear to function in a similar manner to also the rates of electron transfer between quinones occu- quinones with the para-carbonyl configuration. pying the QA and QB sites (kab and kba), which define the free energy of the reaction (-AG0- = RT In kab/kba). It is clear The photochemical reaction center of photosynthetic bacte- from Fig. 1 that, under certain conditions, the dissociation ria is an integral membrane protein that contains four bac- constant measured is not the true Kd but is an apparent value teriochlorophylls (BChls), two bacteriopheophytins (BPhs), (Kd). This situation is evident when equilibrium among the and two quinones associated with discrete catalytic sites, three charge-separated states (upper line of the scheme in designated the primary and secondary quinone-binding sites Fig. 1) is attained under conditions when: (QA and QB, respectively; for review, see ref. 1). After light excitation of a special pair of bacteriochlorophylls (BChl2), kab + kba>> kad and kon[Q] + koff >> kad. Ill an electron is transferred by way of BPh to the quinone at the QA site to form the semiquinone (QA) and the state BChl'- Under these conditions, the Kd is dependent on both the Kd QQB, where BChl' is the oxidized cation radical of BChl2 and the -AG' for electron transfer from QA to QB in the (2). The QA then reduces the quinone bound at the QB site to following way: form BChl'QAQB (3, 4) In the bacterial reaction centers from Rhodobacter sphae- K' = Kd/[1 + exp(-A°G_/RT)]. [2] roides the native ubiquinone-10 (UQ-10) molecules can be reversibly removed from both the QA and QB sites (5, 6). Eq. 2 shows that as -AGC° increases the Kd for quinone at Moreover, the QA site can be functionally reconstituted with the QB site decreases and hence will increase the possibility a wide variety of other natural and synthetic quinone struc- of detecting and measuring the functional presence of a tures (6-8). In contrast, with the exception of one observa- quinone in the QB site. We have used this device to re- tion (9), parallel efforts to reconstitute QB activity have been evaluate the specificity of the QB site. successful only with quinones containing the native ubiqui- none (UQ) configuration (10-13). These negative findings to the that the unlike the site, Abbreviations: BPh, bacteriopheophytin; QA, primary quinone- have led sentiment QB site, QA binding site; QB, secondary quinone-binding site; BChl2, special pair is designed to provide stringent binding requirements for the ofbacteriochlorophyll molecules; QA, semiquinone formed in the QA UQ structure (10, 12, 13). However, such a view is not site; QB, semiquinone formed in the QB site; UQ, UQ-10, and UQ-0, entirely consistent with the well-known, broad specificity of ubiquinone, -10, and -0, respectively; - AG0 , free energy difference the QB site for a variety ofherbicide structures (14-16). These for electron transfer from QA to QB; Em, in situ oxidation-reduction apparently unusual characteristics have prompted us to potential; AQ, 9,10-anthraquinone; MeAQ, 1-methyl-9,10-anthraqui- none; BChl, oxidized cation radical of BChl2; cyt c, cytochrome c; BQ, 1,4-benzoquinone. The publication costs of this article were defrayed in part by page charge *To whom reprint requests should be addressed at: B501 Richards payment. This article must therefore be hereby marked "advertisement" Building, 37th and Hamilton Walk, University of Pennsylvania, in accordance with 18 U.S.C. §1734 solely to indicate this fact. Philadelphia, PA 19104. 2658 Downloaded by guest on September 24, 2021 Biophysics: Giangiacorno and Dutton Proc. Natl. Acad. Sci. USA 86 (1989) 2659 kon kab (see ref. 8) with a characteristic rate constant (kad); for example, the kad values for UQ-10 and 9,10-anthraquinone BCh12 QA + [Q] ± BCh12 QA QB >- BCh12 QAQB (AQ) are close to 7 and 70 sec-1, respectively (19). With UQ-10 at both the QA and QB sites, the observed rate constant koff kba for reduction of BChl' is 0.7 sec-1 (21, 22). Under these conditions, the fractional occupancy ofquinone at the QB site is proportional to the fraction of slow phase of the BChl2 kad kad reduction kinetics and is readily observed (10, 11). 2. Ratio ofcyt c oxidized on the second andfirstflashes. kon Z14 With QA present and QB absent in the reaction center, cyt c oxidation occurs only on the first flash (23). However, with BChl2QA + [Q]% BChl2QAQB QB functionally present, the oxidation of Q- by QB permits cyt c oxidation on a second flash. Assuming the QA is koff kinetically capable of reducing QB (see method 3), the second/first flash ratio of cyt c oxidation is governed by QB FIG. 1. Reaction scheme for electron transfer from QA to QB and occupancy and the -AG°_ between QA and (22, quinone binding at the site. QB 24). QB 3. QX to QB electron transfer observed by accompanying electrochromic shifts ofBPh. The formation of Q- induces a MATERIALS AND METHODS shift in the Qy transition of BPh that is distinct from that Reaction Center Preparation. Reaction centers were iso- accompanying the formation of Q- (3, 4). This assay is useful lated from Rb. sphaeroides strain R-26 (17). Extraction of in that it distinguishes between the states BChl'QAQn and UQ-10 from both the QA and QB sites, or the QB site only, was BChl2QAQB and permits the time resolution of electron performed (6) with slight modifications (18). transfer between QA and QB QA Replacement. The procedure of Gunner et al. (8, 18, 19) Chemicals. The quinones used in this work were obtained was used for the reconstitution of QA activity with quinones from several sources as follows: 1-methyl-9,10-anthraqui- other than the native UQ. All procedures were carried out in none (MeAQ) and 2,3-dimethyl-1,4-benzoquinone from J. 10 mM Tris HCI (pH 8.0) under aerobic conditions at 23 ± Malcolm Bruce, University of Manchester; the latter was 1°C. Detergent concentrations were maintained well below also obtained from Chang-An Yu, Oklahoma State Univer- the critical micelle concentration and sufficiently constant to sity; we gratefully acknowledge the compounds provided by have no significant effect on our results (for discussion, see our colleagues. 1,4-Benzoquinone (BQ); 1,4-naphthoqui- ref. 8). none; 9,10-anthraquinone (AQ); 9,10-phenanthraquinone; 3,5- Quinones chosen as QA replacements had in situ oxidation- di-tert-butyl-1,2-benzoquinone; and 2,3-dichloro-1,4-naph- reduction potential (Em) values lower then UQ-10 (18, 19) so thoquinone were purchased from Aldrich. When necessary, that the -AG 0 for electron transfer from a quinone at the QA these compounds were purified by recrystallization from site to a particular quinone at the QB site would be increased. petroleum ether (50-110'C). The purity of all quinones was Displacement of quinone in the QA site by a test quinone established by reverse-phase HPLC. in the QB site was minimized by choosing QA replacements The ferrous form of cyt c was prepared by reducing 1 ml of whose Kd values at the QA site were at least 60-fold tighter 10 mM cyt c (ferric form) (horse heart, type VI from Sigma) than the Kd values at the QA site for quinones chosen to with 400 ,ul offreshly prepared well-buffered 100 mM sodium reconstitute the QB site.
Load more

Recommended publications
  • Evolution of Photochemical Reaction Centres
    bioRxiv preprint doi: https://doi.org/10.1101/502450; this version posted December 20, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 1 Evolution of photochemical reaction 2 centres: more twists? 3 4 Tanai Cardona, A. William Rutherford 5 Department of Life Sciences, Imperial College London, London, UK 6 Correspondence to: [email protected] 7 8 Abstract 9 The earliest event recorded in the molecular evolution of photosynthesis is the structural and 10 functional specialisation of Type I (ferredoxin-reducing) and Type II (quinone-reducing) reaction 11 centres. Here we point out that the homodimeric Type I reaction centre of Heliobacteria has a Ca2+- 12 binding site with a number of striking parallels to the Mn4CaO5 cluster of cyanobacterial 13 Photosystem II. This structural parallels indicate that water oxidation chemistry originated at the 14 divergence of Type I and Type II reaction centres. We suggests that this divergence was triggered by 15 a structural rearrangement of a core transmembrane helix resulting in a shift of the redox potential 16 of the electron donor side and electron acceptor side at the same time and in the same redox direction. 17 18 Keywords 19 Photosynthesis, Photosystem, Water oxidation, Oxygenic, Anoxygenic, Reaction centre 20 21 Evolution of Photosystem II 22 There is no consensus on when and how oxygenic photosynthesis originated. Both the timing and the 23 evolutionary mechanism are disputed.
    [Show full text]
  • Chapter 3 the Title and Subtitle of This Chapter Convey a Dual Meaning
    3.1. Introduction Chapter 3 The title and subtitle of this chapter convey a dual meaning. At first reading, the subtitle Photosynthetic Reaction might seem to indicate that the topic of the structure, function and organization of Centers: photosynthetic reaction centers is So little time, so much to do exceedingly complex and that there is simply insufficient time or space in this brief article to cover the details. While this is John H. Golbeck certainly the case, the subtitle is Department of Biochemistry additionally meant to convey the idea that there is precious little time after the and absorption of a photon to accomplish the Molecular Biology task of preserving the energy in the form of The Pennsylvania State University stable charge separation. University Park, PA 16802 USA The difficulty is there exists a fundamental physical limitation in the amount of time available so that a photochemically induced excited state can be utilized before the energy is invariably wasted. Indeed, the entire design philosophy of biological reaction centers is centered on overcoming this physical, rather than chemical or biological, limitation. In this chapter, I will outline the problem of conserving the free energy of light-induced charge separation by focusing on the following topics: 3.2. Definition of the problem: the need to stabilize a charge-separated state. 3.3. The bacterial reaction center: how the cofactors and proteins cope with this problem in a model system. 3.4. Review of Marcus theory: what governs the rate of electron transfer in proteins? 3.5. Photosystem II: a variation on a theme of the bacterial reaction center.
    [Show full text]
  • Nourishing and Health Benefits of Coenzyme Q10 – a Review
    Czech J. Food Sci. Vol. 26, No. 4: 229–241 Nourishing and Health Benefits of Coenzyme Q10 – a Review Martina BOREKOVÁ1, Jarmila HOJEROVÁ1, Vasiľ KOPRDA1 and Katarína BAUEROVÁ2 1Institute of Biotechnology and Food Science, Faculty of Chemical and Food Technology, Slovak University of Technology, Bratislava, Slovak Republic; 2Institute of Experimental Pharmacology, Slovak Academy of Sciences, Bratislava, Slovak Republic Abstract Boreková M., Hojerová J., Koprda V., Bauerová K. (2008): Nourishing and health benefits of coen- zyme Q10 – a review. Czech J. Food Sci., 26: 229–241. Coenzyme Q10 is an important mitochondrial redox component and endogenously produced lipid-soluble antioxidant of the human organism. It plays a crucial role in the generation of cellular energy, enhances the immune system, and acts as a free radical scavenger. Ageing, poor eating habits, stress, and infection – they all affect the organism’s ability to provide adequate amounts of CoQ10. After the age of about 35, the organism begins to lose the ability to synthesise CoQ10 from food and its deficiency develops. Many researches suggest that using CoQ10 supplements alone or in com- bination with other nutritional supplements may help maintain health of elderly people or treat some of the health problems or diseases. Due to these functions, CoQ10 finds its application in different commercial branches such as food, cosmetic, or pharmaceutical industries. This review article gives a survey of the history, chemical and physical properties, biochemistry and antioxidant activity of CoQ10 in the human organism. It discusses levels of CoQ10 in the organisms of healthy people, stressed people, and patients with various diseases. This paper shows the distribution and contents of two ubiquinones in foods, especially in several kinds of grapes, the benefits of CoQ10 as nutritional and topical supplements and its therapeutic applications in various diseases.
    [Show full text]
  • Structure of the Cytochrome B6 F Complex of Oxygenic Photosynthesis
    Structure of the Cytochrome b6 f Complex of Oxygenic Photosynthesis The photosynthetic unit of oxygenic photosynthesis data of 3.0 Å from the complex crystal with another is organized as two large multimolecular membrane analogue inhibitor, TDS, was collected at the SBC complexes, photosystem II (PSII) that extracts low- beamline 19ID, APS. The initial model was developed energy electrons from water and photosystem I (PSI) into a 3.4 Å map of the native complex. Final that raises the energy level of such electrons using refinement was carried out with a dataset from a co- light energy to produce a strong reductant, NADPH. crystal with TDS (Figs. 2, 3). The two photosystems operate in a series linked by a Viewed along the membrane normal, the b6f × third multiprotein complex called the cytochrome b6f complex is 90 Å 55 Å within the membrane side, × complex (Fig.1). The cytochrome b6f complex is a and 120 Å 75 Å on the lumen (p)side (Fig. 2). A membrane-spanning protein complex embedded in prominent feature of this structure is an extended the thylakoid membrane of photosynthetic organisms. quinone exchange cavity between the monomers, The molecular weight of the complex is 220,000 as a which exchanges lipophilic plastoquinone in the dimer with 26 transmembrane helices. The b6f complex bilayer center, and also mediates the electron and controls the electron transfer between the plastoquinol proton transfer across the complex. The heme-binding reduced by PSII and the electron carrier protein 4 transmembrane helices core of the b6f complex plastocyanin that associate with PSI.
    [Show full text]
  • Plastoquinol As a Singlet Oxygen Scavenger in Photosystem II ⁎ Jerzy Kruk A, , Achim Trebst B
    Available online at www.sciencedirect.com Biochimica et Biophysica Acta 1777 (2008) 154–162 www.elsevier.com/locate/bbabio Plastoquinol as a singlet oxygen scavenger in photosystem II ⁎ Jerzy Kruk a, , Achim Trebst b a Department of Plant Physiology and Biochemistry, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Gronostajowa 7, 30-387 Kraków, Poland b Plant Biochemistry, Ruhr-University Bochum, D-44780 Bochum, Germany Received 4 July 2007; received in revised form 16 October 2007; accepted 17 October 2007 Available online 25 October 2007 Abstract It has been found that in Chlamydomonas reinhardtii cells, under high-light stress, the level of reduced plastoquinone considerably increases while in the presence of pyrazolate, an inhibitor of plastoquinone and tocopherol biosynthesis, the content of reduced plastoquinone quickly decreases, similarly to α-tocopherol. In relation to chlorophyll, after 18 h of growth under low light with the inhibitor, the content of α-tocopherol was 22.2 mol/1000 mol chlorophyll and that of total plastoquinone (oxidized and reduced) was 19 mol/1000 mol chlorophyll, while after 2 h of high-light stress the corresponding amounts dropped to 6.4 and 6.2 mol/1000 mol chlorophyll for α-tocopherol and total plastoquinone, respectively. The degradation of both prenyllipids was partially reversed by diphenylamine, a singlet oxygen scavenger. It was concluded that plastoquinol, as well as α-tocopherol is decomposed under high-light stress as a result of a scavenging reaction of singlet oxygen generated in photosystem II. The levels of both α-tocopherol and of the reduced plastoquinone are not affected significantly in the absence of the inhibitor due to a high turnover rate of both prenyllipids, i.e., their degradation is compensated by fast biosynthesis.
    [Show full text]
  • Regulation of Photosynthetic Electron Transport☆
    Biochimica et Biophysica Acta 1807 (2011) 375–383 Contents lists available at ScienceDirect Biochimica et Biophysica Acta journal homepage: www.elsevier.com/locate/bbabio Review Regulation of photosynthetic electron transport☆ Jean-David Rochaix ⁎ Department of Molecular Biology, University of Geneva, Geneva, Switzerland Department of Plant Biology, University of Geneva, Geneva, Switzerland article info abstract Article history: The photosynthetic electron transport chain consists of photosystem II, the cytochrome b6 f complex, Received 14 September 2010 photosystem I, and the free electron carriers plastoquinone and plastocyanin. Light-driven charge separation Received in revised form 11 November 2010 events occur at the level of photosystem II and photosystem I, which are associated at one end of the chain Accepted 13 November 2010 with the oxidation of water followed by electron flow along the electron transport chain and concomitant Available online 29 November 2010 pumping of protons into the thylakoid lumen, which is used by the ATP synthase to generate ATP. At the other end of the chain reducing power is generated, which together with ATP is used for CO assimilation. A Keywords: 2 Electron transport remarkable feature of the photosynthetic apparatus is its ability to adapt to changes in environmental Linear electron flow conditions by sensing light quality and quantity, CO2 levels, temperature, and nutrient availability. These Cyclic electron flow acclimation responses involve a complex signaling network in the chloroplasts comprising the thylakoid Photosystem II protein kinases Stt7/STN7 and Stl1/STN7 and the phosphatase PPH1/TAP38, which play important roles in Photosystem I state transitions and in the regulation of electron flow as well as in thylakoid membrane folding.
    [Show full text]
  • Light-Harvesting Chlorophyll Pigments Enable Mammalian Mitochondria To
    ß 2014. Published by The Company of Biologists Ltd | Journal of Cell Science (2014) 127, 388–399 doi:10.1242/jcs.134262 RESEARCH ARTICLE Light-harvesting chlorophyll pigments enable mammalian mitochondria to capture photonic energy and produce ATP Chen Xu, Junhua Zhang, Doina M. Mihai and Ilyas Washington* ABSTRACT concentrations, and extends the median life span of Caenorhabditis elegans, upon light exposure; supporting the Sunlight is the most abundant energy source on this planet. However, hypothesis that photonic energy capture through dietary-derived the ability to convert sunlight into biological energy in the form of metabolites may be an important means of energy regulation in adenosine-59-triphosphate (ATP) is thought to be limited to animals. The presented data are consistent with the hypothesis chlorophyll-containing chloroplasts in photosynthetic organisms. that metabolites of dietary chlorophyll modulate mitochondrial Here we show that mammalian mitochondria can also capture light ATP stores by catalyzing the reduction of coenzyme Q. These and synthesize ATP when mixed with a light-capturing metabolite of findings have implications for our understanding of aging, normal chlorophyll. The same metabolite fed to the worm Caenorhabditis cell function and life on earth. elegans leads to increase in ATP synthesis upon light exposure, along with an increase in life span. We further demonstrate the same RESULTS potential to convert light into energy exists in mammals, as Light-driven ATP synthesis in isolated mammalian mitochondria chlorophyll metabolites accumulate in mice, rats and swine when To demonstrate that dietary chlorophyll metabolites can modulate fed a chlorophyll-rich diet. Results suggest chlorophyll type ATP levels, we examined the effects of the chlorophyll molecules modulate mitochondrial ATP by catalyzing the reduction metabolite pyropheophorbide-a (P-a) on ATP synthesis in of coenzyme Q, a slow step in mitochondrial ATP synthesis.
    [Show full text]
  • Ph DEPENDENCE of the EFFECTS of DIURON, ATRAZINE and DINOSEB on the LUMINESCENT PROPERTIES of THYLAKOID MEMBRANES
    BULG. J. PLANT PHYSIOL., 2001, 27(1–2), 85–100 85 pH DEPENDENCE OF THE EFFECTS OF DIURON, ATRAZINE AND DINOSEB ON THE LUMINESCENT PROPERTIES OF THYLAKOID MEMBRANES Petar Lambrev, Vassilij Goltsev* Department of Biophysics and Radiobiology, Biological Faculty, University of Sofia “St. Kliment Ohridski” Received May 17, 2001 Summary. The effect of the external pH in the range of 5.5 to 8.0 on the activity of three PSII herbicides (diuron, atrazine and dinoseb) was studied in isolated thylakoid membranes by means of prompt and delayed fluores- cence induction kinetics. Well-expressed pH dependence was established for prompt fluorescence parameters (Fv/Fm, Fi/Fm) as well as for delayed fluores- cence parameters (I1, I2). Two types of pH dependence of the herbicide effect were observed. The apparent resistance to herbicides of the urea/triazine type (diuron and atrazine) was found to be maximal at pH 6–6.5, although the amplitude of the herbicide effect was different between diuron and atrazine. The pH dependences for thylakoids treated with the phenolic herbicide, dino- seb, showed a minimum in the herbicide effect at pH 7.5. The mechanism by which pH influences the herbicide sensitivity of thylakoid membranes is supposed to involve pH-induced conformational changes of the QB-binding site on the D1 protein of PSII, resulting in alteration of the affinity to the herbicide molecule. The conformation of the QB-site at a cer- tain pH is characterized by maximal herbicide resistance. The optimal value of pH depends on the type of herbicide, specifically the location of the H- bond between the herbicide molecule and D1.
    [Show full text]
  • Synthesis of Plastoquinoneanalogs and Inhibition of Photosynthetic And
    Proc. Nat. Acad. Sci. USA Vol. 69, No. 12, pp. 3713-3717, December 1972 Synthesis of Plastoquinone Analogs and Inhibition of Photosynthetic and Mammalian Enzyme Systems* (chloroplasts/beef heart/coenzyme Q/electron transport) JAN BQLERt, RONALD PARDINIt, HANNA T. M\IUSTAFAt, KARL FOLKERSt, RICHARD A. DILLEYT, AND FREDERICK L. CRANE: t Stanford Research Institute, Menlo Park, California and Institute for Biomedical Research, The University of Texas at Austin, Texas 78712; and $ Purdue University, Lafayette, Indiana 47907 Contributed by Karl Folkers, September 25, 1972 ABSTRACT New 5-hydroxy- and 5-chloro-6-alkyl-1,4- energy (ATP) and the utilization of the hydrogen of water as benzoquinones with one or two methyl groups on the a source of reducing power (NADPH2). Data from nucleus were synthesized as potential antimetabolites of several plastoquinones for biological research on photosynthetic experimental approaches indicate that plastoquinone does and mammalian enzyme systems; the primary emphasis function in the electron transport system of chloroplasts. was on photosynthesis. The data stem from extraction-restoration studies, spectro- 2,3-Dimethyl-5 - hydroxy - 6 - phytyl - 1,4 - benzoquinone photometric and chemical determination of redox changes, completely inhibited in chloroplasts the water-de- pendent electron transport, but photosystem I was in- correlation with metabolic function, and changes in electron sensitive to this analog. The data are consistent with the transport in mutants, as well as its universal distribution in all interpretation that this analog inhibits electron transport oxygen-producing photosynthetic organisms (3). in the chain prior to the site of electron donation from the To study relationships between structure and activity in ascorbate-dichlorophenolindophenol couple.
    [Show full text]
  • Lecture Inhibition of Photosynthesis Inhibition at Photosystem II
    1 Lecture Inhibition of Photosynthesis Inhibition at Photosystem II 1. General Information The popular misconception is that susceptible plants treated with these herbicides “starve to death” because they can no longer photosynthesize. In actuality, the plants die long before the food reserves are depleted. The photosynthetic inhibitors can be divided into two distinct groups, the inhibitors of Photosystem I and inhibitors of Photosystem II. Both of these groups work in the energy production step of photosynthesis, or the light reactions. Light is required as well as photosynthesis for these herbicides to kill susceptible plants. Herbicides that inhibit Photosystem II do not act as “electron interceptors” or “electron thieves” but rather they bind to the QB protein in the normal electron transport sequence, thereby blocking electron transport to the PQ pool. Herbicides that inhibit Photosystem II are represented by several herbicide families including the symmetrical triazines, triazinones (asymmetrical triazines), substituted ureas, uracils, pyridazinones, phenyl carbamates, nitriles, benzothiadiazoles, phenyl pyridazines, and acid amides. These herbicides have preemergence and/or postemergence activity. ) Note: The pyridazinones, nitrile, and acid amide families of herbicides also have specific herbicides that do not have this mode of action (will be discussed later). 2. Mode of Action See Figure 7.1 (The electron transport chain in photosynthesis and the sites of action of herbicides that interfere with electron transfer in this chain (Q = electron acceptor; PQ = plastoquinone; page 2). Review Photosystem I and II. The specific mode of action is as follows: If foliar applied, herbicide moves through the cuticle into the cell and chloroplast. It binds to the QB protein preventing it from accepting and transferring electrons to the plastoquinone (PQ) pool in Photosystem II.
    [Show full text]
  • Physiological Roles of Plastid Terminal Oxidase in Plant Stress Responses
    Review Physiological roles of plastid terminal oxidase in plant stress responses XIN SUN* and TAO WEN Agronomy College, Chengdu Campus, Sichuan Agricultural University, Chengdu 611130, China *Corresponding author (Email, [email protected]) The plastid terminal oxidase (PTOX) is a plastoquinol oxidase localized in the plastids of plants. It is able to transfer electrons from plastoquinone (PQ) to molecular oxygen with the formation of water. Recent studies have suggested that PTOX is beneficial for plants under environmental stresses, since it is involved in the synthesis of photoprotective carotenoids and chlororespiration, which could potentially protect the chloroplast electron transport chain (ETC) from over-reduction. The absence of PTOX in plants usually results in photo-bleached variegated leaves and impaired adaptation to environment alteration. Although PTOX level and activity has been found to increase under a wide range of stress conditions, the functions of plant PTOX in stress responses are still disputed now. In this paper, the possible physiological roles of PTOX in plant stress responses are discussed based on the recent progress. [Sun X and Wen T 2011 Physiological roles of plastid terminal oxidase in plant stress responses. J. Biosci. 36 951–956] DOI 10.1007/s12038-011- 9161-7 1. Introduction important role in chloroplast biogenesis (Carol and Kuntz 2001;Aluruet al. 2006). There was also evidence that PTOX Plastid terminal oxidase (PTOX), a plastid-localized plasto- is the terminal oxidase of chlororespiration and regulates the quinol (PQ)/O2 oxidoreductase, exists widely in photosyn- redox state of the PQ pool (Aluru and Rodermel 2004;Peltier thetic species including algae and higher plants (Carol and and Cournac 2002).
    [Show full text]
  • Time-Independent Structure of the Cytochrome B6f Complex† William A
    Evolution of Photosynthesis: Time-Independent Structure of the Cytochrome b6f Complex† William A. Cramer,* Huamin Zhang, Jiusheng Yan, Genji Kurisu,‡ and Janet L. Smith Department of Biological Sciences, Lilly Hall of Life Sciences, 915 West State Street, Purdue UniVersity, West Lafayette, Indiana 47907-2054 ReceiVed March 22, 2004; ReVised Manuscript ReceiVed April 7, 2004 ABSTRACT: Structures of the cytochrome b6f complex obtained from the thermophilic cyanobacterium Mastigocladus laminosus and the green alga Chlamydomonas reinhardtii, whose appearance in evolution is separated by 109 years, are almost identical. Two monomers with a molecular weight of 110 000, containing eight subunits and seven natural prosthetic groups, are separated by a large lipid-containing “quinone exchange cavity”. A unique heme, heme x, that is five-coordinated and high-spin, with no strong field ligand, occupies a position close to intramembrane heme bn. This position is filled by the n-side bound quinone, Qn, in the cytochrome bc1 complex of the mitochondrial respiratory chain. The structure and position of heme x suggest that it could function in ferredoxin-dependent cyclic electron transport as well as being an intermediate in a quinone cycle mechanism for electron and proton transfer. The significant differences between the cyanobacterial and algal structures are as follows. (i) On the n-side, a plastoquinone molecule is present in the quinone exchange cavity in the cyanobacterial complex, and a sulfolipid is bound in the algal complex at a position corresponding to a synthetic DOPC lipid molecule in the cyanobacterial complex. (ii) On the p-side, in both complexes a quinone analogue inhibitor, TDS, passes through a portal that separates the large cavity from a niche containing the Fe2S2 cluster.
    [Show full text]