Ammonia displaces methanol bound to the water oxidizing complex of photosystem II in the S₂ state

Abstract

Ammonia and methanol both bind to the water oxidising complex of photosystem II during its turnover, possibly at sites where water binds during the normal water oxidation process. We have investigated the interaction between these two water analogues at the S 2 state of the water oxidising cycle using electron magnetic resonance techniques. We find evidence that ammonia displaces methanol from its binding site.

Ammonia Triggers Photodamage of Photosystem II in the Cyanobacterium Synechocystis sp. Strain PCC 6803

“…First, the toxicity of ammonia in higher plants is frequently more pronounced at high light intensity (Britto and Kronzucker, 2002), implying that a photoinhibitory process is involved. Second, ammonia was shown in vitro to be a potent inhibitor of the water-splitting reaction in PSII preparations from plants by binding to the Mn cluster (Britt et al, 2004;Evans et al, 2005). These findings are in perfect agreement with the photosensitizing effect of ammonia described here for Synechocystis and suggest that ammonium toxicity could operate in many oxygenic photosynthetic organisms by photosensitizing PSII.…”

“…Furthermore, biochemical studies on isolated PSII particles showed previously that ammonia is a competitive inhibitor of the water-splitting reaction. Ammonia is a known inhibitor of water oxidation because it binds to the OEC as structural analog of the substrate water (Evans et al, 2005;Fang et al, 2005), and it was proposed that ammonia is incorporated into the Mn complex as a bridging ligand Figure 7. Ammonium-dependent accumulation of ftsH2 transcript.…”

18O-Water exchange in photosystem II: Substrate binding and intermediates of the water splitting cycle

“…The different proton exchange rates that were observed in the two S states indicated that the binding affinities changed during the S-state cycle [29]. EPR studies of substrate analogues such as simple alcohols and ammonia, show that the binding of these molecules to the catalytic Mn occurs in a complex interaction with the substrate water [28,[30][31][32].…”

Effects of methanol on the S i -state transitions in photosynthetic water-splitting

“…The possible competition between ammonia and methanol for binding sites at the water-splitting Mn 4 O x Ca cluster of photosystem II has been studied by two groups. Based on FTIR measurements Chu and co-workers concluded that methanol is unable to replace ammonia from its binding site at the Mn 4 O x Ca cluster (Fang et al 2005), whereas for the reverse exchange experiment Evans et al found employing electron-spin echo envelope modulation (ESEEM)-spectroscopy for detection that ammonia can displace methanol from its binding site (Evans et al 2005). These data indicate that ammonia binds more strongly to the Mn 4 O x Ca cluster than methanol, and that these two water analogs have at least one common binding site.…”