Competitive binding of acetate and chloride in photosystem II

The binding of chloride and acetate to photosystem ii (PSII) was examined t o elucidate the mechanism of acetate inhibition. The mode of inhibition was studied, and individual binding sites were assigned by steady-state O-2 ev olution measurements in correlation with electron paramagnetic resonance (E PR) results. Two binding sites were found for acetate, one chloride-sensiti ve 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 (acetat e 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, 20 0 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 conce ntrations slowed the rate of formation of the S2Yz. state, but did not affe ct the steady-state yield of the S2Yz. 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 yi eld of the S2Yz(.) state at the high PSII concentrations used for EPR measu rements depends primarily on acceptor side turnover. Our data further suppo rt a close proximity of chloride to Y-z(.), indicating a possible role for chloride in the electron transfer mechanism at the O-2-evolving complex.