Department of Biological Sciences, Biochemistry and Molecular Biology Section, Louisiana State University, Baton Rouge, LA, 70803, USA.
The Rieveschl Laboratories for Mass Spectrometry, Department of Chemistry, University of Cincinnati, Cincinnati, OH, 45221, USA.
Photosynth Res. 2020 Mar;143(3):263-273. doi: 10.1007/s11120-019-00698-7. Epub 2020 Jan 1.
Reactive oxygen species (ROS) production is an unavoidable byproduct of electron transport under aerobic conditions. Photosystem II (PS II), the cytochrome b/f complex and Photosystem I (PS I) are all demonstrated sources of ROS. It has been proposed that PS I produces substantial levels of a variety of ROS including O, O, HO and, possibly, •OH; however, the site(s) of ROS production within PS I has been the subject of significant debate. We hypothesize that amino acid residues close to the sites of ROS generation will be more susceptible to oxidative modification than distant residues. In this study, we have identified oxidized amino acid residues in spinach PS I which was isolated from field-grown spinach. The modified residues were identified by high-resolution tandem mass spectrometry. As expected, many of the modified residues lie on the surface of the complex. However, a well-defined group of oxidized residues, both buried and surface-exposed, lead from the chl a' of P to the surface of PS I. These residues (PsaB: F, E, M, W, L, and PsaF: L, A,D) may identify a preferred route for ROS, probably O, to egress the complex from the vicinity of P. Additionally, two buried residues located in close proximity to A (PsaB:H and S) were modified, which appears consistent with A being a source of O. Surprisingly, no oxidatively modified residues were identified in close proximity to the 4Fe-FS clusters F, F or F. These cofactors had been identified as principal targets for ROS damage in the photosystem. Finally, a large number of residues located in the hydrophobic cores of Lhca1-Lhca4 are oxidatively modified. These appear to be the result of O production by the distal antennae for the photosystem.
活性氧（ROS）的产生是有氧条件下电子传递不可避免的副产物。光系统 II（PS II）、细胞色素 b/f 复合物和光系统 I（PS I）都是 ROS 的来源。有人提出 PS I 会产生大量的各种 ROS，包括 O 2 、O 2 - 、HO 和可能的•OH；然而，PS I 中 ROS 产生的部位一直是激烈争论的主题。我们假设，接近 ROS 产生部位的氨基酸残基比远离的残基更容易受到氧化修饰。在这项研究中，我们已经鉴定了菠菜 PS I 中的氧化氨基酸残基，该 PS I 是从田间生长的菠菜中分离出来的。通过高分辨率串联质谱法鉴定修饰后的残基。正如预期的那样，许多修饰后的残基位于复合物的表面。然而，一组定义明确的氧化残基，包括埋藏和暴露在表面的残基，从 P 侧的 chl a' 到 PS I 的表面。这些残基（PsaB：F、E、M、W、L 和 PsaF：L、A、D）可能确定了 ROS （可能是 O 2 - ）从 P 附近逸出复合物的首选途径。此外，两个位于 A 附近的埋藏残基（PsaB：H 和 S）被修饰，这似乎与 A 是 O 2 - 的来源一致。令人惊讶的是，在距离 4Fe-FS 簇 F、F 或 F 较近的地方没有鉴定出氧化修饰的残基。这些辅因子已被确定为光系统中 ROS 损伤的主要靶标。最后，大量位于 Lhca1-Lhca4 疏水区的残基被氧化修饰。这些似乎是由于远侧天线产生的 O 2 - 导致的。
