Department of Biological and Environmental Science, Faculty of Agriculture, Graduate School of Agricultural Science, Kobe University, 1-1 Rokkodai, Nada, 657-8511, Japan.
Graduate School of Biostudies, Kyoto University, Sakyo-ku, Kyoto, 606-8502, Japan.
Photosynth Res. 2019 Mar;139(1-3):487-498. doi: 10.1007/s11120-018-0519-7. Epub 2018 May 22.
The light reactions of photosynthesis are known to comprise both linear and cyclic electron flow in order to convert light energy into chemical energy in the form of NADPH and ATP. Antimycin A (AA) has been proposed as an inhibitor of ferredoxin-dependent cyclic electron flow around photosystem I (CEF-PSI) in photosynthesis research. However, its precise inhibitory mechanism and target site had not been elucidated yet. Here we show that AA inhibits the cyclic (alternative) electron flow via cytochrome b (Cyt b) within photosystem II (CEF-PSII). When AA was applied to thylakoid membranes isolated from spinach leaves, the high potential form of Cyt b, which was reduced in the dark, was transformed into the lower potential forms and readily oxidized by molecular oxygen. In the absence of AA, the reduced Cyt b was oxidized by P680 upon light illumination and re-reduced in the dark, mainly by the electron from the Q site on the acceptor side of PSII. In contrast, AA suppressed the oxidation of Cyt b and induced its reduction under the illumination. This inhibition of Cyt b oxidation by AA enhanced photoinhibition of PSII. Based on the above results, we propose caution regarding the use of AA for evaluating CEF-PSI per se and concurrently propose that AA provides for new insights into, and interpretations of, the physiological importance of Cyt b, rather than that of CEF-PSI in photosynthetic organisms.
光合作用的光反应被认为包括线性和循环电子流，以便将光能转化为 NADPH 和 ATP 的化学能。抗霉素 A（AA）已被提议作为光合作用研究中依赖于铁氧还蛋白的围绕光系统 I（CEF-PSI）的循环电子流（CEF-PSI）的抑制剂。然而，其确切的抑制机制和靶位尚未阐明。在这里，我们表明 AA 通过光合系统 II（CEF-PSII）中的细胞色素 b（Cyt b）抑制循环（替代）电子流。当 AA 施加于从菠菜叶中分离的类囊体膜时，在黑暗中还原的 Cyt b 的高电势形式转变为低电势形式，并容易被分子氧氧化。在没有 AA 的情况下，还原的 Cyt b 在光照下被 P680 氧化，并在黑暗中重新还原，主要由 PSII 受体侧的 Q 位点上的电子还原。相比之下，AA 抑制 Cyt b 的氧化并在光照下诱导其还原。AA 对 Cyt b 氧化的这种抑制增强了 PSII 的光抑制。基于上述结果，我们建议在评估 CEF-PSI 本身时要谨慎使用 AA，并同时提出 AA 为细胞色素 b 而不是光合生物中的 CEF-PSI 的生理重要性提供了新的见解和解释。
