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 (O2•-) and the hydroxyl (HO•) radicals were studied in WT and a tocopherol cyclase (vte1) 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 D1:130E to hydroxyglutamic acid by O2•- at the PheoD1 site. Additionally, D1:246Y was modified to either tyrosine hydroperoxide or dihydroxyphenylalanine by O2•- and HO•, respectively, in the vicinity of the nonheme iron. We propose that α-tocopherol is localized near PheoD1 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 PheoD1 and the nonheme iron (via bicarbonate), and thus protects electron transport in PSII from ROS damage.
- MeSH
- alpha-Tocopherol chemistry metabolism MeSH
- Amino Acids chemistry metabolism MeSH
- Arabidopsis enzymology genetics radiation effects MeSH
- Photosynthesis physiology radiation effects MeSH
- Photosystem II Protein Complex chemistry genetics metabolism MeSH
- Hydroxyl Radical chemistry metabolism MeSH
- Protein Interaction Domains and Motifs MeSH
- Intramolecular Transferases chemistry genetics metabolism MeSH
- Protein Conformation, alpha-Helical MeSH
- Protein Conformation, beta-Strand MeSH
- Oxygen chemistry metabolism MeSH
- Models, Molecular MeSH
- Mutation MeSH
- Oxidation-Reduction MeSH
- Superoxides chemistry metabolism MeSH
- Light MeSH
- Thermodynamics MeSH
- Thermosynechococcus enzymology genetics radiation effects MeSH
- Thylakoids enzymology genetics radiation effects MeSH
- Protein Binding MeSH
- Binding Sites MeSH
- Iron chemistry metabolism MeSH
- Publication type
- Journal Article MeSH
- Research Support, Non-U.S. Gov't MeSH
- Research Support, U.S. Gov't, Non-P.H.S. MeSH
Photosystem II (PSII) is a multi-subunit pigment-protein complex and is one of several protein assemblies that function cooperatively in photosynthesis in plants and cyanobacteria. As more structural data on PSII become available, new questions arise concerning the nature of the charge separation in PSII reaction center (RC). The crystal structure of PSII RC from cyanobacteria Thermosynechococcus vulcanus was selected for the computational study of conformational changes in photosystem II associated to the charge separation process. The parameterization of cofactors and lipids for classical MD simulation with Amber force field was performed. The parametrized complex of PSII was embedded in the lipid membrane for MD simulation with Amber in Gromacs. The conformational behavior of protein and the cofactors directly involved in the charge separation were studied by MD simulations and QM/MM calculations. This study identified the most likely mechanism of the proton-coupled reduction of plastoquinone QB. After the charge separation and the first electron transfer to QB, the system undergoes conformational change allowing the first proton transfer to QB- mediated via Ser264. After the second electron transfer to QBH, the system again adopts conformation allowing the second proton transfer to QBH-. The reduced QBH2 would then leave the binding pocket.
