Natively oxidized amino acid residues in the spinach PS I-LHC I supercomplex.

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.