Interaction of ammonia with the water splitting enzyme of photosystem II

The effects of NH3 on the oxygen evolving enzyme have been investigated with EPR and steady-state O2 evolution. The following results were obtained. At low light intensity O2 evolution occurs in all centers even though ammonia is bound. This binding occurs in the S2 state and results in a modification of the multiline signal as reported earlier. However, the oscillations with flash number of the amplitude of the EPR signal are virtually unaffected, indicating that NH3 binding does not prevent S-state advancement. Inhibition of O2 evolution by NH3 measured at light intensities that are nearly saturating for untreated photosystem II is interpreted as being due to a slow down in the rate of S-state cycling. At very high light intensities NH3 is not able to inhibit oxygen evolution presumably because NH3 binding is S state dependent and the susceptible S state (S2) is turned over too quickly. NH3 binding resulting in the modified multiline signal does not occur in S1. When S1 is formed from fully NH3 modified S2 by deactivation or by three further flashes, the S1 state does not have NH3 bound. NH3 thus dissociates easily from S1. Earlier reports of NH3 binding in S1 may be explained by the observation that NH3 binding can occur upon incubation of samples in S2 at temperatures as low as 198 K. Evidence is obtained for an NH3 binding occurring slowly (30 s) in S3. This binding results in a block in S-state advancement as suggested earlier [Velthuys, B. R. (1975) Thesis, University of Leiden]. The results are interpreted in two possible models: (1) NH3 binding in S2 occurs in a substrate site, but it is rapidly exchanged by water upon S4 formation. (2) NH3 binding in S2 is not in a substrate site but instead in a structural site and remains bound while water is oxidized. Inherent in this model is that other NH3 binding sites, i.e., the Cl- site, and the slow NH3 binding site in S3 could be the true substrate sites. Some mechanistic implications are discussed.