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

Water oxidation in Photosystem II occurs at the oxygen-evolving complex (OEC), which cycles through distinct intermediates, S-0-S-4. The inhibitor ammonia selectively binds to the S-2 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 S-2 to S-1. The temperature dependence of biphasic S-2 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 S-2. The model predicts ammonia binds to S-2 rapidly (t(1/2) = 1 ms) with a large equilibrium constant. This finding implies that ammonia decreases the reduction potential of S-2 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.