Insights into Substrate Binding to the Oxygen-Evolving Complex of Photosystem II from Ammonia Inhibition Studies

Abstract

Water oxidation in Photosystem II occurs at the oxygen-evolving complex (OEC), which cycles through distinct intermediates, S₀–S₄. The inhibitor ammonia selectively binds to the S₂ state at an unresolved site that is not competitive with substrate water. By monitoring the yields of flash-induced oxygen production, we show that ammonia decreases the net efficiency of OEC turnover and slows the decay kinetics of S₂ to S₁. The temperature dependence of biphasic S₂ decay kinetics provides activation energies that do not vary in control and ammonia conditions. We interpret our data in the broader context of previous studies by introducing a kinetic model for both the formation and decay of ammonia-bound S₂. The model predicts ammonia binds to S₂ rapidly (<i>t</i>_1/2 = 1 ms) with a large equilibrium constant. This finding implies that ammonia decreases the reduction potential of S₂ by at least 2.7 kcal mol^–1 (>120 mV), which is not consistent with ammonia substitution of a terminal water ligand of Mn­(IV). Instead, these data support the proposal that ammonia binds as a bridging ligand between two Mn atoms. Implications for the mechanism of O–O bond formation are discussed