Binding and release kinetics of inhibitors of Q- A oxidation in thylakoid membranes

Willem Vermaas, Gerhard Dohnt, Gernot Renger

Research output: Contribution to journalArticle

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Abstract

Oxygen flash yield patterns of dark adapted thylakoid membranes as measured with a Joliot-type O2-electrode indicate that inhibitors that block the oxidation of the reduced primary quinone Q- A of Photosystem II vary greatly in the rate of binding to and release from the inhibitor / QB binding environment. The 'classical' Photosystem-II herbicides like diuron and atrazine exhibit slow binding and release kinetics, whereas, for example, phenolic inhibitors, o-phenanthroline and synthetic quinones are exchanging quite rapidly with QB (about once per second or faster at inhibitor concentrations causing about 50% inhibition of O2 evolution). No general relationship between the efficiency of the inhibitor and the exchange rate is observed; it depends mainly on the type of inhibitor. Based on the classical Kok model, equations are derived in order to calculate oxygen yields evolved by thylakoids in single-turnover flashes as a function of the rate constants of inhibitor binding to and release from the inhibitor / QB binding environment in the presence of an oxidized or semireduced QA · QB or QA · inhibitor complex. Fitting of theoretical and experimental values yields that o-phenanthroline binds much faster to an oxidized than to a semireduced QA · QB complex. This fits very well with the hypothesis that the Q- B affinity to the site is much higher than that of QB. In the case of i-dinoseb, however, inhibitor / quinone exchange seems to occur mainly in the semiquinone state. Possibilities to explain this result are discussed.

Original languageEnglish (US)
Pages (from-to)74-83
Number of pages10
JournalBBA - Bioenergetics
Volume765
Issue number1
DOIs
Publication statusPublished - Apr 26 1984
Externally publishedYes

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Keywords

  • Herbicide
  • Oxygen evolution
  • Photosystem II
  • Plastoquinone

ASJC Scopus subject areas

  • Biophysics

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