title: Two-electron reactions S2QB -->S0QB and S3QB -->S1QB are involved in deactivation of higher S states of the oxygen-evolving complex of Photosystem II.
abstract: The oxygen-evolving complex of Photosystem II cycles through five oxidation states (S(0)-S(4)), and dark incubation leads to 25% S(0) and 75% S(1). This distribution cannot be reached with charge recombination reactions between the higher S states and the electron acceptor Q(B)(-). We measured flash-induced oxygen evolution to understand how S(3) and S(2) are converted to lower S states when the electron required to reduce the manganese cluster does not come from Q(B)(-). Thylakoid samples preconditioned to make the concentration of the S(1) state 100% and to oxidize tyrosine Y(D) were illuminated by one or two laser preflashes, and flash-induced oxygen evolution sequences were recorded at various time intervals after the preflashes. The distribution of the S states was calculated from the flash-induced oxygen evolution pattern using an extended Kok model. The results suggest that S(2) and S(3) are converted to lower S states via recombination from S(2)Q(B)(-) and S(3)Q(B)(-) and by a slow change of the state of oxygen-evolving complex from S(3) and S(2) to S(1) and S(0) in reactions with unspecified electron donors. The slow pathway appears to contain two-electron routes, S(2)Q(B) -->S(0)Q(B), and S(3)Q(B) -->S(1)Q(B). The two-electron reactions dominate in intact thylakoid preparations in the absence of chemical additives. The two-electron reaction was replaced by a one-electron-per-step pathway, S(3)Q(B) -->S(2)Q(B) -->S(1)Q(B) in PS II-enriched membrane fragments and in thylakoids measured in the presence of artificial electron acceptors. A catalase effect suggested that H(2)O(2) acts as an electron donor for the reaction S(2)Q(B) -->S(0)Q(B) but added H(2)O(2) did not enhance this reaction.
authors.affiliation: Department of Biology, University of Turku, FI-20014 Turku, Finland.
authors.forename: Taras K
authors.initials: TK
authors.lastname: Antal
authors.forename: Päivi
authors.initials: P
authors.lastname: Sarvikas
authors.forename: Esa
authors.initials: E
authors.lastname: Tyystjärvi
mesh_terms: D028464:Cucurbita; D004583:Electrons; D006861:Hydrogen Peroxide / Q000737:chemistry; D007700:Kinetics; D008567:Membranes, Artificial; D008956:Models, Chemical; D010100:Oxygen / Q000737:chemistry*; D010775:Photic Stimulation; D055668:Photochemical Processes; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D020524:Thylakoids / Q000737:chemistry; D014867:Water / Q000737:chemistry
chemical_list: D008567:Membranes, Artificial; D045332:Photosystem II Protein Complex; D014867:Water; D006861:Hydrogen Peroxide; D010100:Oxygen