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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.