Laisk Agu, Eichelmann Hillar, Oja Vello
Tartu Ülikooli Tehnoloogia Instituut, Nooruse tn. 1, Tartu 50411, Estonia.
Tartu Ülikooli Tehnoloogia Instituut, Nooruse tn. 1, Tartu 50411, Estonia.
Biochim Biophys Acta. 2015 Jun-Jul;1847(6-7):565-75. doi: 10.1016/j.bbabio.2015.03.003. Epub 2015 Mar 20.
In sunflower leaves linear electron flow LEF=4O2 evolution rate was measured at 20 ppm O2 in N2. PSII charge separation rate CSRII=aII∙PAD∙(Fm-F)/Fm, where aII is excitation partitioning to PSII, PAD is photon absorption density, Fm and F are maximum and actual fluorescence yields. Under 630 nm LED+720 nm far-red light (FRL), LEF was equal to CSRII with aII=0.51 to 0.58. After FRL was turned off, plastoquinol (PQH2) accumulated, but LEF decreased more than accountable by F increase, indicating PQH2-oxidizing cyclic electron flow in PSII (CEFII). CEFII was faster under conditions requiring more ATP, consistent with CEFII being coupled with proton translocation. We propose that PQH2 bound to the QC site is oxidized, one e- moving to P680+, the other e- to Cyt b559. From Cyt b559 the e- reduces QB- at the QB site, forming PQH2. About 10-15% electrons may cycle, causing misses in the period-4 flash O2 evolution and lower quantum yield of photosynthesis under stress. We also measured concentration dependence of PQH2 oxidation by dioxygen, as indicated by post-illumination decrease of Chl fluorescence yield. After light was turned off, F rapidly decreased from Fm to 0.2 Fv, but further decrease to F0 was slow and O2 concentration dependent. The rate constant of PQH2 oxidation, determined from this slow phase, was 0.054 s(-1) at 270 μM (21%) O2, decreasing with Km(O2) of 60 μM (4.6%) O2. This eliminates the interference of O2 in the measurements of CEFII.
在向日葵叶片中，线性电子流（LEF）=4O₂ 释放速率是在 N₂ 中 20 ppm O₂ 条件下测量的。光系统 II（PSII）电荷分离速率（CSRII）=aII∙PAD∙(Fm - F)/Fm，其中 aII 是激发分配到 PSII 的比例，PAD 是光子吸收密度，Fm 和 F 分别是最大和实际荧光产量。在 630 nm 发光二极管（LED）+720 nm 远红光（FRL）条件下，LEF 等于 CSRII，aII 为 0.51 至 0.58。关闭 FRL 后，质体醌（PQH₂）积累，但 LEF 的下降幅度超过了因 F 增加所能解释的程度，表明在 PSII 中存在 PQH₂ 氧化的循环电子流（CEFII）。在需要更多 ATP 的条件下，CEFII 更快，这与 CEFII 与质子转运相偶联一致。我们提出，结合在醌结合蛋白（QC）位点的 PQH₂ 被氧化，一个电子转移到 P680⁺，另一个电子转移到细胞色素 b559。电子从细胞色素 b559 在醌结合蛋白（QB）位点还原 QB⁻，形成 PQH₂。约 10 - 15%的电子可能循环，导致在第 4 周期闪光 O₂ 释放中出现缺失，并在胁迫下降低光合作用的量子产率。我们还测量了二氧对 PQH₂ 氧化的浓度依赖性，这通过光照后叶绿素荧光产量的下降来表示。光照关闭后，F 迅速从 Fm 下降到 0.2 Fv，但进一步下降到 F₀ 则缓慢且依赖于 O₂ 浓度。从这个缓慢阶段确定的 PQH₂ 氧化速率常数在 270 μM（21%）O₂ 时为 0.054 s⁻¹，以 60 μM（4.6%）O₂ 的米氏常数（Km(O₂)）下降。这消除了 O₂ 在 CEFII 测量中的干扰。
