Bao Han, Zhang Chunxi, Kawakami Keisuke, Ren Yanan, Shen Jian-Ren, Zhao Jingquan
Laboratory of Photochemistry, Beijing National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China.
Biochim Biophys Acta. 2008 Sep;1777(9):1109-15. doi: 10.1016/j.bbabio.2008.04.044. Epub 2008 May 9.
In intact PSII, both the secondary electron donor (Tyr(Z)) and side-path electron donors (Car/Chl(Z)/Cyt(b)(559)) can be oxidized by P(680)(+)* at cryogenic temperatures. In this paper, the effects of acceptor side, especially the redox state of the non-heme iron, on the donor side electron transfer induced by visible light at cryogenic temperatures were studied by EPR spectroscopy. We found that the formation and decay of the S(1)Tyr(Z) EPR signal were independent of the treatment of K(3)Fe(CN)(6), whereas formation and decay of the Car(+)/Chl(Z)(+) EPR signal correlated with the reduction and recovery of the Fe(3+) EPR signal of the non-heme iron in K(3)Fe(CN)(6) pre-treated PSII, respectively. Based on the observed correlation between Car/Chl(Z) oxidation and Fe(3+) reduction, the oxidation of non-heme iron by K(3)Fe(CN)(6) at 0 degrees C was quantified, which showed that around 50-60% fractions of the reaction centers gave rise to the Fe(3+) EPR signal. In addition, we found that the presence of phenyl-p-benzoquinone significantly enhanced the yield of Tyr(Z) oxidation. These results indicate that the electron transfer at the donor side can be significantly modified by changes at the acceptor side, and indicate that two types of reaction centers are present in intact PSII, namely, one contains unoxidizable non-heme iron and another one contains oxidizable non-heme iron. Tyr(Z) oxidation and side-path reaction occur separately in these two types of reaction centers, instead of competition with each other in the same reaction centers. In addition, our results show that the non-heme iron has different properties in active and inactive PSII. The oxidation of non-heme iron by K(3)Fe(CN)(6) takes place only in inactive PSII, which implies that the Fe(3+) state is probably not the intermediate species for the turnover of quinone reduction.
在完整的光系统II（PSII）中，二级电子供体（酪氨酸自由基Tyr(Z)）和侧路电子供体（类胡萝卜素/叶绿素自由基Chl(Z)/细胞色素b559）在低温下都可被P(680)(+)*氧化。本文通过电子顺磁共振波谱（EPR）研究了受体侧，特别是非血红素铁的氧化还原状态，对低温下可见光诱导的供体侧电子转移的影响。我们发现S(1)Tyr(Z) EPR信号的形成和衰减与K(3)Fe(CN)(6)处理无关，而类胡萝卜素阳离子/叶绿素自由基阳离子Car(+)/Chl(Z)(+) EPR信号的形成和衰减分别与K(3)Fe(CN)(6)预处理的PSII中非血红素铁的Fe(3+) EPR信号的还原和恢复相关。基于观察到的类胡萝卜素/叶绿素自由基氧化与Fe(3+)还原之间的相关性，对0℃下K(3)Fe(CN)(6)氧化非血红素铁的情况进行了定量分析，结果表明约50 - 60%的反应中心产生了Fe(3+) EPR信号。此外，我们发现对苯醌的存在显著提高了Tyr(Z)氧化的产率。这些结果表明，受体侧的变化可显著改变供体侧的电子转移，表明完整的PSII中存在两种类型的反应中心，即一种含有不可氧化的非血红素铁，另一种含有可氧化的非血红素铁。Tyr(Z)氧化和侧路反应在这两种类型的反应中心中分别发生，而非在同一反应中心中相互竞争。此外，我们的结果表明非血红素铁在活性和非活性PSII中具有不同的性质。K(3)Fe(CN)(6)对非血红素铁的氧化仅发生在非活性PSII中，这意味着Fe(3+)状态可能不是醌还原周转的中间物种。
