Two-Electron Reactions S^sub 2^Q^sub B^ [arrow right] S^sub 0^Q^sub B^ and S^sub 3^Q^sub B^ [arrow right] S^sub 1^Q^sub B^ are Involved in Deactivation of Higher S States of the Oxygen-Evolving Complex of Photosystem II

The oxygen-evolving complex of Photosystem II cycles through five oxidation states (S... ...S...), and dark incubation leads to 25% S... and 75% S... This distribution cannot be reached with charge recombination reactions between the higher S states and the electron acceptor Q... We measured flash-induced oxygen evolution to understand how S... and S... are converted to lower S states when the electron required to reduce the manganese cluster does not come from Q... Thylakoid samples preconditioned to make the concentration of the S... state 100% and to oxidize tyrosine Y... 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... and S... are converted to lower S states via recombination from S...Q... and S...Q... and by a slow change of the state of oxygen-evolving complex from S... and S... to S... and S... in reactions with unspecified electron donors. The slow pathway appears to contain two-electron routes, S...Q... ... S...Q..., and S...Q... ... S...Q... 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...Q... ... S...Q... ... S...Q... in PS II-enriched membrane fragments and in thylakoids measured in the presence of artificial electron acceptors. A catalase effect suggested that ... acts as an electron donor for the reaction S...Q... ... S...Q... but added ... did not enhance this reaction. (ProQuest: ... denotes formulae/symbols omitted.)