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'Plastoquinone' in keywords Facet   Publication Year 1993  [X]
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1Author    H. Adar Kless3, Michal Oren-Shamirb, ItzhakO. Hadc, M. Arvin Edelm, Wim VermaasaRequires cookie*
 Title    Protein Modifications in the D 2 Protein of Photosystem II Affect Properties of the Q B/Herbicide-Binding Environment  
 Abstract    The D 2 protein contains an extended loop (the D-de loop) between helices D and de at the reducing side o f photosystem II (PS II). Characterization o f D 2 mutants o f the cyanobacte­ rium Synechocystis sp. PCC 6803 has indicated that the length and amino acid com position o f the D-de loop are not critical for basic PS II functions, although most o f the residues in that region are conserved phylogenetically. Here we show using herbicide binding and electron-flow inhibition measurements that drastic modifications in the D-de loop o f the D 2 protein modify the interaction o f some PS II-directed herbicides with their binding niche. The stability o f (semi-)reduced Q B in its binding pocket is altered in at least two o f the mutants, as indicated by a shifted peak temperature o f the thermoluminescence signal originating from charge recombi­ nation involving QB. These results suggest a close functional association between the D-de loop o f the D 2 protein and the Q B/herbicide-binding environment, which is viewed as being coordinated mostly by re­ sidues o f the D 1 protein. This represents one o f the first examples o f m odification o f the Q B/ herbicide-binding domain by mutations in the D 2 protein. 
  Reference    Z. Naturforsch. 48c, 185—190 (1993); received November 23 1992 
  Published    1993 
  Keywords    Herbicides, Plastoquinone, Photosystem II, Thermoluminescence, Cyanobacteria 
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 TEI-XML for    default:Reihe_C/48/ZNC-1993-48c-0185.pdf 
 Identifier    ZNC-1993-48c-0185 
 Volume    48 
2Author    Kazuhiko Satoh, Yasuhiro Kashino, Hiroyuki KoikeRequires cookie*
 Title    Electron Transport from QA to Thymoquinone in a Synechococcus Oxygen-Evolving Photosystem II Preparation: Role of QB and Binding Affinity of Thymoquinone to the QB Site  
 Abstract    We have recently shown that binding affinities o f benzoquinones can be estimated by two methods in photosystem (PS) II particles (K. Satoh et al., Biochim. Biophys. Acta 1 1 0 2 ,4 5 -5 2 (1992)). U sing these methods we calculated the binding affinity o f thym oquinone (2-methyl-5-isopropyl-/?-benzoquinone) to the Q B site and studied how the quinone accepts electrons in oxygen-evolving PS II particles isolated from the thermophilic cyanobacteria, Synechococcus elongatus and S. vulcanus. The results are as follows: (1) The binding constant o f thym oqui­ none to the Q B site determined by several methods was around 0.33 m M . (2) At low thym oqui­ none concentrations the quinone was supposed to accept electrons via QB-plastoquinone, whereas at high concentrations the quinone seemed to bind to the QB site and accept an elec­ tron directly from Q~A. Lower rates o f photoreduction o f the quinone at high concentrations were attributed to a slower turnover rate o f the quinone at the QB site than that o f endogenous plastoquinone. (3) A model for the function o f plastoquinone at the Q B site, which can explain all the results, was presented. According to this model, the plastoquinone molecule at the Q B site is not replaced by another plastoquinone molecule. Instead, it transfers electrons to pool plastoquinone molecules by turning over its head group but remaining its long side chain bound to the PS II complexes. 
  Reference    Z. Naturforsch. 48c, 174 (1993); received November 9 1992 
  Published    1993 
  Keywords    Q b Site, Photosystem II, Thym oquinone, Plastoquinone, Synechococcus 
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 TEI-XML for    default:Reihe_C/48/ZNC-1993-48c-0174.pdf 
 Identifier    ZNC-1993-48c-0174 
 Volume    48