Plastoquinol generates and scavenges reactive oxygen species in organic solvent: Potential relevance for thylakoids

The present work reports reactions of plastoquinol (PQH2-9) and plastoquinone (PQ-9) in organic solvents and summarizes the literature to understand similar reactions in thylakoids. In thylakoids, PQH2-9 is oxidized by the cytochrome b6/f complex (Cyt b6/f) but some PQH2-9 is also oxidized by reactions in which oxygen acts as an electron acceptor and is converted to reactive oxygen species (ROS). Furthermore, PQH2-9 reacts with ROS. Light enhances oxygen-dependent oxidation of PQH2-9. We examined the oxidation of PQH2-9 via dismutation of PQH2-9 and PQ-9 and scavenging of the superoxide anion radical (O2−) and hydrogen peroxide (H2O2) by PQH2-9. Oxidation of PQH2-9 via dismutation to semiquinone was slow and independent of pH in organic solvents and in solvent/buffer systems, suggesting that intramembraneous oxidation of PQH2-9 in darkness mainly proceeds via reactions catalyzed by the plastid terminal oxidase and cytochrome b559. In the light, oxidation of PQH2-9 by singlet oxygen and by O2− formed in PSI contribute significantly. In addition, Cyt b6/f forms H2O2 with a PQH2-9 dependent mechanism. Measurements of the reaction of O2− with PQH2-9 and PQ-9 in acetonitrile showed that O2− oxidizes PQH2-9, forming PQ-9 and several PQ-9-derived products. The rate constant of the reaction between PQH2-9 and O2− was found to be 104 M−1 s−1. H2O2 was found to oxidize PQH2-9 to PQ-9, but failed to oxidize all PQH2-9, suggesting that the oxidation of PQH2-9 by H2O2 proceeds via deprotonation mechanisms producing PQH−-9, PQ2−-9 and the protonated hydrogen peroxide cation, H3O2+.