Superoxide oxidase and reductase activity of cytochrome b559 in photosystem II.

This study provides evidence for the superoxide oxidase and the superoxide reductase activity of cytochrome b(559) (cyt b(559)) in PSII. It is reported that in Tris-treated PSII membranes upon illumination, both the intermediate potential (IP) and the reduced high potential (HP(red)) forms of cyt b(559) exhibit superoxide scavenging activity and interconversion between IP and HP(red) form. When Tris-treated PSII membranes were illuminated in the presence of spin trap EMPO, the formation of superoxide anion radical (O(2)(-)) was observed, as confirmed by EPR spin-trapping spectroscopy. The observations that the addition of enzymatic (superoxide dismutase) and non-enzymatic (cytochrome c, alpha-tocopherol and Trolox) O(2)(-) scavengers prevented the light-induced conversion of IP<-->HP(red) cyt b(559) confirmed that IP and HP(red) cyt b(559) are reduced and oxidized by O(2)(-), respectively. Redox changes in cyt b(559) by an exogenous source of O(2)(-) reconfirmed the superoxide oxidase and reductase activity of cyt b(559). Furthermore, the light-induced conversion of IP to HP(red) form of cyt b(559) was completely inhibited at pH>8 and by chemical modification of the imidazole ring of histidine residues using diethyl pyrocarbonate. We proposed that a change in the environment around the heme iron, induced by the protonation and deprotonation of His(22) residue generates a favorable condition for the oxidation and reduction of O(2)(*-), respectively.