Inhibitory and Non-Inhibitory NH3 Binding at the Water-Oxidizing Manganese Complex of Photosystem II Suggests Possible Sites and a Rearrangement Mode of Substrate Water Molecules

The identity and rearrangements of substrate water molecules in photosystem II (PSII) water oxidation are of great mechanistic interest and addressed herein by comprehensive analysis of NH4+/NH3 binding. Time-resolved detection of O2 formation and recombination fluorescence as well as Fourier transform infrared (FTIR) difference spectroscopy on plant PSII membrane particles reveals the following. (1) Partial inhibition in NH4Cl buffer occurs with a pH-independent binding constant of ∼25 mM, which does not result from decelerated O2 formation, but from complete blockage of a major PSII fraction (∼60%) after reaching the Mn(IV)4 (S3) state. (2) The non-inhibited PSII fraction advances through the reaction cycle, but modified nuclear rearrangements are suggested by FTIR difference spectroscopy. (3) Partial inhibition can be explained by anticooperative (mutually exclusive) NH3 binding to one inhibitory and one non-inhibitory site; these two sites may correspond to two water molecules terminally bound to the ...