Abstract
Photosynthetic oxygen evolution takes place by water oxidation at the Mn cluster in photosystem II (PSII). Ammonia has been known to be an effective inhibitor of O2 evolution as a water analogue. However, the detailed mechanism of ammonia inhibition is yet to be understood. In this study, we investigated the molecular mechanism of ammonia inhibition by examining the pH dependence of the inhibition rate and the changes in Fourier transform infrared (FTIR) difference spectra. The treatment of 100 mM NH4Cl on either intact or PsbP, PsbQ-depleted PSII membranes of spinach showed decreases in the O2 evolution activity by ~40% at any pH in the pH5.0-8.0 range. S2/S1 FTIR difference spectra exhibited clear changes by NH4Cl treatment in the asymmetric and symmetric COO- stretching regions. The amplitude of the spectral changes as a function of the NH4Cl concentration was correlated with the concentration dependence of the inhibition rate. These results indicate that NH4+ rather than NH3 functions as a major inhibitor and that the interaction of NH4+ with carboxylate ligands to the Mn-cluster or those forming a hydrogen bond network causes inhibition of O2 evolution.
