The binding of chloride and acetate to photosystem II (PSII) was examined to elucidate the mechanism of acetate inhibition. The mode of inhibition was studied, and individual binding sites were assigned by steady- state O2 evolution measurements in correlation with electron paramagnetic resonance (EPR) results. Two binding sites were found for acetate, one chloride-sensitive on the electron donor side and one chloride-insensitive on the electron acceptor side. The respective binding constants were as follows: K(Cl) = 0.5 ± 0.2 mM (chloride binding to the donor side), K(I) = 16 ± 5 mM (acetate binding to the donor side), and K(I)' = 130 ± 40 mM (acetate binding to the acceptor side). When acetate was bound to the acceptor side of PSII, 200 K illumination induced a narrowed form of the Q(A)-Fe(II) EPR signal, the yield of which was independent of the chloride concentration. When acetate was bound to the donor side, room-temperature illumination produced the S2Y(Z)· state. EPR measurements showed that both the yield and formation rate of this state increased with acetate concentration. Increasing chloride concentrations slowed the rate of formation of the S2Y(Z)· state, but did not affect the steady-state yield of the S2Y(Z)· state. These findings indicate that the light-induced reactions in acetate-inhibited PSII are modulated by both donor side and acceptor side binding of acetate, while the steady-state yield of the S2Y(Z)· state at the high PSII concentrations used for EPR measurements depends primarily on acceptor side turnover. Our data further support a close proximity of chloride to Y(Z)·, indicating a possible role for chloride in the electron-transfer mechanism at the O2- evolving complex.
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