Tocopherol controls D1 amino acid oxidation by oxygen radicals in Photosystem II.

Photosystem II (PSII) is an intrinsic membrane protein complex that functions as a light-driven water:plastoquinone oxidoreductase in oxygenic photosynthesis. Electron transport in PSII is associated with formation of reactive oxygen species (ROS) responsible for oxidative modifications of PSII proteins. In this study, oxidative modifications of the D1 and D2 proteins by the superoxide anion (O) and the hydroxyl (HO) radicals were studied in WT and a tocopherol cyclase () mutant, which is deficient in the lipid-soluble antioxidant α-tocopherol. In the absence of this antioxidant, high-resolution tandem mass spectrometry was used to identify oxidation of D1:E to hydroxyglutamic acid by O at the Pheo site. Additionally, D1:Y was modified to either tyrosine hydroperoxide or dihydroxyphenylalanine by O and HO, respectively, in the vicinity of the nonheme iron. We propose that α-tocopherol is localized near Pheo and the nonheme iron, with its chromanol head exposed to the lipid-water interface. This helps to prevent oxidative modification of the amino acid's hydrogen that is bonded to Pheo and the nonheme iron (via bicarbonate), and thus protects electron transport in PSII from ROS damage.