The interaction of quinones, herbicides and bicarbonate with their binding environment at the acceptor side of photosystem II in photosynthesis

W.F.J. Vermaas

    Research output: Thesisinternal PhD, WU

    Abstract

    In this thesis experiments are described which are directed towards a further characterization of the interaction of the native bound plastoquinone Q <sub>B</sub> , artificial quinones, herbicides and bicarbonate with their binding environment at the acceptor side of Photosystem II in the thylakoid membrane. The most important thylakoid component involved in binding of, <em>e.g.</em> , herbicides and quinones appears to be the lysine-free, rapidly turned-over 32,000 M <sub>r</sub> protein that is attacked readily by trypsin. This protein, that is involved in creating the herbicide/quinone binding environment is designated in this thesis "ABP-32" (azidoatrazine- binding protein of 32,000 M <sub>r</sub> ). Chapter 3 describes, however, that a lysine-containing protein complex also appears to modify binding of the herbicides atrazine and bromoxynil. This protein complex might be related to the Photosystem II reaction center (Section 3.3). In many earlier reports, where polypeptide staining with Coomassie Brilliant Blue was used for monitoring the polypeptide content of a preparation, herbicide binding was assigned to the wrong 32,000 M <sub>r</sub> protein. The ABP-32 is poorly stainable with Coomassie Brilliant Blue. The other 32,000 M <sub>r</sub> protein associated with Photosystem II is probably related to the water splitting process (Section 3.1).<p/>Chapter 4 shows that herbicides and quinones appear to displace each other from the binding environment in a seemingly competitive fashion. However, after covalent linkage of a quinone to the binding site herbicide binding still occurs, albeit with a low affinity (Section 4.3). This can be taken as evidence of an allosteric interaction between herbicide and quinone binding: upon binding of one the affinity of the other is decreased. This hypothesis is supported by other data in this thesis, which show differential effects on binding of quinones and different types of herbicides. However, we consider the interaction of two related molecules (for example, two herbicides belonging to related chemical groups) for binding to the binding environment to be truly competitive.<p/>Herbicide/quinone interactions were studied not only under equilibrium conditions, but the binding and release rates of the inhibitor to the site were also estimated and calculated (Chapter 5). Herbicides like diuron, atrazine, bromoxynil and phenisopham exchange slowly with the native quinone, whereas phenol-type inhibitors (for example, dinoseb), <em>o</em> -phenanthroline, cyanoacrylates and synthetic quinones exchange faster (>0.1s <sup>-1</sup> at 50 % inhibition of electron transport). In the case of <em>o</em> -phenanthroline a good fit between experimental data and theoretical values calculated from a model of competitive quinone/ inhibitor interaction could be obtained. When using a phenol-type inhibitor, fitting of experimental data and theory was less successful in the sense that the results obtained could not be fitted in a scheme where Q <sub>B</sub> and Q <sub>B</sub> H <sub>2</sub> have a low binding affinity and Q <sub>B</sub><sup>-</sup> has a high affinity. It is possible that this is caused by an interaction between Q <sub>B</sub> and the phenol-type inhibitor, which is not, to a first approximation, competitive.<p/>During the last decade triazine-resistant biotypes of weeds have developed in fields that were sprayed repeatedly with triazine herbicides (for example, atrazine). All triazine-resistant biotypes characterized thus far differ from the "wild type" by one amino acid in the ABP-32. This minor change leads to a large effect on, for example, the affinity of some herbicides and quinones, and on the semiquinone equilibrium between the first electron-accepting quinone in Photosystem II, Q <sub>A</sub> , and Q <sub>B</sub> . This equilibrium is shifted to the Q <sub>A</sub> side considerably in triazine-resistant plants, thus decreasing photosynthetic efficiency under limiting light intensity (Chapter 6).<p/>Photosynthetic electron transport on the acceptor side of Photosystem II can also be modified by bicarbonate, at least in the presence of formate (Chapter 7). Absence of bicarbonate /CO <sub>2</sub> leads to an inhibition of electron transport whereas readdition of HCO <sub>3</sub><sup>-</sup> restores electron flow through Q <sub>B</sub> . The binding site of HCO <sub>3</sub><sup>-</sup> is functionally close to that of herbicides: herbicide affinity is sensitive to CO <sub>2</sub> -depletion and HCO <sub>3</sub><sup>-</sup> -readdition (Section 7.2). Although the precise role of HCO <sub>3</sub><sup>-</sup> in electron transport is not yet known, it is speculated here that HCO <sub>3</sub><sup>-</sup> may be involved in protonation of reduced Q <sub>B</sub> . Bicarbonate depletion also appears to slow down Q <sub>A</sub><sup>-</sup> oxidation by the water splitting system, and to block the reduction of the S <sub>2</sub> and S <sub>3</sub> state of the water splitting system by -probably- a bound quinol (Section 7.3).<p/>In conclusion, this thesis provides many detailed data and analyses, which may add to form a basis for the understanding of the molecular mechanism of ligand binding at the Photosystem II acceptor side and of electron transfer from Q <sub>A</sub> to the plastoquinone pool. At this moment, however, the description of electron transport, inhibition and quinone binding at Photosystem II is still rather phenomenological. For a thorough understanding of the underlying molecular processes much more research, especially interdisciplinary, is required. In this way progress in solving this problem, that contains (bio)physical, (bio)chemical, physiological and genetic components, may best be made.<p/>
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    Supervisors/Advisors
    • Vredenberg, W.J., Promotor
    • Renger, G., Co-promotor, External person
    Award date25 Apr 1984
    Place of PublicationWageningen
    Publisher
    Publication statusPublished - 1984

    Keywords

    • photosynthesis
    • synthesis
    • photochemistry
    • cum laude

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