Inhibitor-Quinone Interactions in Reaction Centers From Rhodopseudomonas Sphaeroides (Herbicide, Triazine, Ubiquinone, Photosynthesis, Resistance)
Stein, Randall Ross
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https://hdl.handle.net/2142/70714
Description
Title
Inhibitor-Quinone Interactions in Reaction Centers From Rhodopseudomonas Sphaeroides (Herbicide, Triazine, Ubiquinone, Photosynthesis, Resistance)
Author(s)
Stein, Randall Ross
Issue Date
1985
Department of Study
Physiology and Biophysics
Discipline
Biophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biophysics, General
Abstract
The properties of ubiquinone and inhibitor binding to reaction centers (RCs) from the photosynthetic bacterium Rhodopseudomonas sphaeroides were studied. The charge recombination pathway between the primary electron donor, P, and the primary ubiquinone electron acceptor, Q(,A), or the secondary ubiquinone acceptor, Q(,B), was established, allowing the determination of the free energy change for the electron transfer from Q(,A) to Q(,B). In isolated RCs suspended in the detergent LDAO, this was equivalent to 0.058 V. Electron transfer events between the two quinones were strongly dependent on quinone and detergent concentration. This led to the development of a model in which Q(,B) is in a loose binding equilibrium between the reaction centers and the hydrophobic phase. In vivo, this equilibrium is rapid, but in RC suspensions, the kinetics are limited by slow exchange of quinone between detergent micelles. The model was readily extended to account for the action of many inhibitors of Q(,A)('-) oxidation, including s-triazines and benzonitriles which, in plants, are potent herbicides. These herbicides were shown to act as competitive inhibitors of Q(,B)-binding, by kinetic methods and by direct binding of ('14)C-labelled herbicide. The inhibitors were found to unbind slowly, even in chromatophores, indicating it to be intrinsically slow. When Q(,A) was semi-reduced, the presence of the electron transfer reaction from Q(,A)('-) to Q(,B) stabilized quinone binding and caused herbicide unbinding. Bacterial mutants resistant to the s-triazines were developed. The mutants all exhibited lower affinities for the herbicides at the reaction center, and had altered electrochemical properties of P and Q(,A). One mutant had an appreciably increased ubiquinone content. Analysis of Q(,A)/Q(,B) electron transfer in these mutants suggested that the lower affinity for herbicide was accompanied by lower affinity for Q(,B)('-). This suggested that such inhibitors should be viewed as semi-quinone, rather than quinone, analogs.
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