Acceptor and donor-side interactions of phenolic inhibitors in Photosystem II |
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Authors: | Arthur G Roberts RDavid Britt |
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Institution: | a Institute of Biological Chemistry, Washington State University, 289 Clark Hall, Stadium Way, Pullman, WA 99164-6340, USA b Institute of Molecular Biology, Austrian Academy of Sciences, Billrothstrasse 11, Salzburg A-5020, Austria c Department of Chemistry, University of California, Davis, CA 95616, USA |
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Abstract: | Certain phenolic compounds represent a distinct class of Photosystem (PS) II QB site inhibitors. In this paper, we report a detailed study of the effects of 2,4,6-trinitrophenol (TNP) and other phenolic inhibitors, bromoxynil and dinoseb, on PS II energetics. In intact PS II, phenolic inhibitors bound to only 90-95% of QB sites even at saturating concentrations. The remaining PS II reaction centers (5-10%) showed modified QA to QB electron transfer but were sensitive to urea/triazine inhibitors. The binding of phenolic inhibitors was 30- to 300-fold slower than the urea/triazine class of QB site inhibitors, DCMU and atrazine. In the sensitive centers, the S2QA− state was 10-fold less stable in the presence of phenolic inhibitors than the urea/triazine herbicides. In addition, the binding affinity of phenolic herbicides was decreased 10-fold in the S2QA− state than the S1QA state. However, removal of the oxygen-evolving complex (OEC) and associated extrinsic polypeptides by hydroxylamine (HA) washing abolished the slow binding kinetics as well as the destabilizing effects on the charge-separated state. The S2-multiline electron paramagnetic resonance (EPR) signal and the ‘split’ EPR signal, originating from the S2YZ state showed no significant changes upon binding of phenolic inhibitors at the QB site. We thus propose a working model where QA redox potential is lowered by short-range conformational changes induced by phenolic inhibitor binding at the QB niche. Long-range effects of HA-washing eliminate this interaction, possibly by allowing more flexibility in the QB site. |
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Keywords: | Conformational change Phenolic inhibitor QA redox potential Manganese cluster Electron paramagnetic resonance Redox-active tyrosine |
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