Department of Chemistry and Biochemistry, University of North Carolina at Greensboro, Greensboro, NC 27402, USA.
Photosynth Res. 2009 Oct;102(1):7-19. doi: 10.1007/s11120-009-9469-4. Epub 2009 Jul 25.
Photosystem II (PSII), the light-absorbing complex of photosynthesis that evolves oxygen, requires chloride for activation of the oxygen evolving complex (OEC). In this study, fluoride was characterized as an inhibitor of Cl(-)-activated oxygen evolution in higher plant PSII. It was confirmed to be primarily a competitive inhibitor in intact PSII, with Cl(-)-competitive inhibition constant K(i) = 2 mM and uncompetitive inhibition constant K'(1) = 79 mM. A pH dependence study showed that fluoride inhibition was more pronounced at lower pH values. In order to determine the location of the fluoride effect, PSII preparations lacking various amounts of the PsbQ subunit were prepared. The competitive F(-) inhibition constant and the Michaelis constant for Cl(-) activation increased with loss of the PsbQ subunit, while the uncompetitive F(-) inhibition constant was relatively insensitive to loss of PsbQ. The S(2) state EPR signals from PSII lacking PsbQ responded to Ca(2+) and Cl(-) removal and to F(-) treatment similar to intact PSII, with enhancement of the g = 4.1 signal and suppression of the multiline signal, but the effects were more pronounced in PSII lacking PsbQ. Together, these results support the interpretation that the PsbQ subunit has a role in retaining anions within the OEC.
光系统 II（PSII）是光合作用中吸收光的复合物，它进化出氧气，需要氯离子来激活氧气释放复合物（OEC）。在这项研究中，氟化物被表征为高等植物 PSII 中 Cl(-)激活氧气释放的抑制剂。它被确认为完整 PSII 中的主要竞争性抑制剂，Cl(-)竞争性抑制常数 K(i) = 2 mM，非竞争性抑制常数 K'(1) = 79 mM。pH 值依赖性研究表明，氟化物抑制在较低 pH 值下更为明显。为了确定氟化物效应的位置，制备了缺乏不同数量 PsbQ 亚基的 PSII 制剂。缺乏 PsbQ 的 PSII 制剂的竞争性 F(-)抑制常数和 Cl(-)激活的米氏常数随着 PsbQ 亚基的丧失而增加，而非竞争性 F(-)抑制常数对 PsbQ 的丧失相对不敏感。缺乏 PsbQ 的 PSII 的 S(2)状态 EPR 信号对 Ca(2+)和 Cl(-)的去除以及 F(-)处理的反应与完整 PSII 相似，g = 4.1 信号增强，多线信号抑制，但在缺乏 PsbQ 的 PSII 中，这种效应更为明显。综上所述，这些结果支持这样的解释，即 PsbQ 亚基在 OEC 内保留阴离子方面发挥作用。
