Rova M, Mamedov F, Magnuson A, Fredriksson P O, Styring S
Department of Biochemistry, Chemical Center, Lund University, Sweden.
Biochemistry. 1998 Aug 4;37(31):11039-45. doi: 10.1021/bi980381h.
Photoactivation of photosystem II has been studied in the FUD 39 mutant of Chlamydomonas reinhardtii that lacks the 23 kDa extrinsic subunit of photosystem II. We have taken advantage of the slow photoactivation rate of FUD 39, earlier demonstrated in Rova, E. M., et al. [(1996) J. Biol. Chem. 271, 28918-28924], to study events in photosystem II during intermediate stages of the process. By measuring the EPR multiline signal, the decay of the variable fluorescence after single flashes, and electron transfer from water to the QB site, we found a good correlation between the building of a tetrameric Mn cluster, longer recombination times between QA- and the donor side of photosystem II, and the achievement of water splitting ability. An increased rate of electron transfer from QA- to the QB site on the acceptor side of photosystem II, mainly due to enhanced efficiency of binding of QB to its site, was found to precede the building of the Mn cluster. We also showed that TyrD was oxidized simultaneously with this increase in electron-transfer rate. Thus, it appears that photoactivation is sequential, with an increased rate of electron transfer on the acceptor side occurring together with the oxidation of TyrD in the first step, followed by the assembly of the Mn cluster. We suggest that a conformational change of photosystem II is induced early in the photoactivation process facilitating electron transfer from the primary donor to the acceptor side. As a consequence, TyrD, an auxiliary electron donor to P680+/TyrZ*, is oxidized. That this occurs before the Mn cluster is fully functional serves to protect photosystem II against donor side induced photodamage.
在莱茵衣藻的FUD 39突变体中研究了光系统II的光激活，该突变体缺乏光系统II的23 kDa外在亚基。我们利用了FUD 39缓慢的光激活速率（此前在Rova, E. M.等人[(1996) J. Biol. Chem. 271, 28918 - 28924]中有过证明）来研究该过程中间阶段光系统II中的事件。通过测量电子顺磁共振多线信号、单次闪光后可变荧光的衰减以及从水到QB位点的电子转移，我们发现四聚体锰簇的形成、光系统II的QA - 与供体侧之间更长的复合时间以及水裂解能力的实现之间存在良好的相关性。发现光系统II受体侧从QA - 到QB位点的电子转移速率增加（主要是由于QB与其位点结合效率提高）先于锰簇的形成。我们还表明，TyrD的氧化与这种电子转移速率的增加同时发生。因此，光激活似乎是按顺序进行的，第一步是受体侧电子转移速率增加并伴随着TyrD的氧化，随后是锰簇的组装。我们认为在光激活过程早期会诱导光系统II的构象变化，从而促进电子从初级供体转移到受体侧。结果，作为P680 + /TyrZ*辅助电子供体的TyrD被氧化。这发生在锰簇完全发挥功能之前，有助于保护光系统II免受供体侧诱导的光损伤。
