School of Plant Sciences and Food Security, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel.
Institute of Basic Biological Problems of the Russian Academy of Sciences, Federal Research Center, Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences, Pushchino, Russia.
Plant Physiol. 2021 Aug 3;186(4):1848-1858. doi: 10.1093/plphys/kiab221.
Photosynthesis is a vital process, responsible for fixing carbon dioxide, and producing most of the organic matter on the planet. However, photosynthesis has some inherent limitations in utilizing solar energy, and a part of the energy absorbed is lost in the reduction of O2 to produce the superoxide radical (O2•-) via the Mehler reaction, which occurs principally within photosystem I (PSI). For decades, O2 reduction within PSI was assumed to take place solely in the distal iron-sulfur clusters rather than within the two asymmetrical cofactor branches. Here, we demonstrate that under high irradiance, O2 photoreduction by PSI primarily takes place at the phylloquinone of one of the branches (the A-branch). This conclusion derives from the light dependency of the O2 photoreduction rate constant in fully mature wild-type PSI from Chlamydomonas reinhardtii, complexes lacking iron-sulfur clusters, and a mutant PSI, in which phyllosemiquinone at the A-branch has a significantly longer lifetime. We suggest that the Mehler reaction at the phylloquinone site serves as a release valve under conditions where both the iron-sulfur clusters of PSI and the mobile ferredoxin pool are highly reduced.
光合作用是一个至关重要的过程，负责固定二氧化碳，并产生地球上大部分的有机物质。然而，光合作用在利用太阳能方面存在一些固有的限制，吸收的一部分能量会在通过 Mehler 反应将 O2 还原为超氧化物自由基（O2•-）的过程中损失，该反应主要发生在光系统 I（PSI）内。几十年来，人们一直认为 PSI 内的 O2 还原仅发生在远端铁硫簇中，而不是在两个不对称辅助因子分支内。在这里，我们证明在高光强下，PSI 主要通过一个分支（A 分支）的叶绿醌进行 O2 光还原。这一结论源于来自莱茵衣藻的完全成熟野生型 PSI、缺乏铁硫簇的复合物以及一个突变 PSI 的 O2 光还原速率常数对光的依赖性，在该突变 PSI 中，A 分支的叶绿半醌的寿命明显延长。我们认为，在 PSI 的铁硫簇和可移动的铁氧还蛋白池都高度还原的情况下，Mehler 反应在叶绿醌位点起到了释放阀的作用。
