Functions of lipophilic quinones in photosynthetic reaction sytems of photosynthesis

plastoquinone associated with PS II Preparations isolated from the thermophiliccyanobacterium Sybechococcus sp. were studied1. Photoaffinty labeling of various PS II preparationsclearly demonstrated for the first time that a phenol-type herbicide binds to the 28kda q_b -proteinand the 47kda chlorophyll-carrying protein has a plastoquinone binding site2. Numbers and函数pf plastoquinone molecules associated with PS II preparetions were determined. Threequinone molecules present in the oxygen evolving preparations function as q_a and q_g and possiblyon the water side of the reaction center of PS iii . Chemical composition of the highly purifiedoxygen volving comolexes was分析. The results show that two plastoquinone molecules，其中function as q_a and on the water side of PS II，are needed for oxygen evolution4. A unigue modebenzoquinone reduction was found in the oxygen evolving prepqrations. High rate of oxygen进化was obtained with lipophilic benzoquinoneswhile less lipophilic molecules poor electron acceptors. DCMU strongly inhibited reduction ofall the electron acceptors examined. In contrast，DBMIB suppressed competitively reduction of benzoquinones but not that of ferricyanide anddichlorophenolindophenol . this suggests that benzoquinones bind to the q _ b-site,by replacing plastoquinoneaccept electrons directly from q_a . The arrhenius plot of benzoquinone reduction suggests that Achange in the conformation of the q_b -protein occurs at 19 - 5. Duroquinol oxidation by PS I in thecyanobacterial cells was also inhibited competitively by DBMIB，suggesting a common inhibition mechanism of DBMIB between the ozygen evolving complexes and thecytochrome b_6-f complexes。

Functions of plastoquinone associated with PS II Preparations isolated from the thermophilic cyanobacterium Sybechococcus sp. were studied1. Photoaffinty labeling of various PS II preparations clearly demonstrated for the first time that a phenol-type herbicide binds to the 28 KDa Q_B-protein and the 47 KDa chlorophyll-carrying protein has a plastoquinone binding site2. Numbers and functions pf plastoquinone molecules associated with PS II preparetions were determined. Three quinone molecules present in the oxygen evolving preparations function as Q_A and Q_G and possibly on the water side of the reaction center of PS II3. Chemical composition of the highly purified oxygen evolving comolexes was analyzed. The results show that two plastoquinone molecules, which function as Q_A and on the water side of PS II,are needed for oxygen evolution4. A unigue mode of benzoquinone reduction was found in the oxygen evolving prepqrations. High rate of oxygen evolution was obtained with lipophilic benzoquinones, while less lipophilic molecules were poor electron acceptors. DCMU strongly inhibited reduction of all the electron acceptors examined. In contrast, DBMIB suppressed competitively reduction of benzoquinones but not that of ferricyanide and dichlorophenolindophenol. This suggests that benzoquinones bind to the Q_B-site, by replacing plastoquinone, and accept electrons directly from Q_A. The arrhenius plot of benzoquinone reduction suggests that a change in the conformation of the Q_B-protein occurs at 19゜C5. Duroquinol oxidation by PS I in the cyanobacterial cells was also inhibited competitively by DBMIB, suggesting a common inhibition mechanism of DBMIB between the ozygen evolving complexes and the cytochrome b_6-f complexes.