The chloride requirement for photosynthetic oxygen evolution. Analysis of the effects of chloride and other anions on amine inhibition of the oxygen-evolving complex

In the presence of Cl^?, the severity of ammonia-induced inhibition of photosynthetic oxygen evolution is attenuated in spinach thylakoid membranes (Sandusky, P.O. and Yocum, C.F. (1983) FEBS Lett. 162, 339–343). A further examination of this phenomenon using steady-state kinetic analysis suggests that there are two sites of ammonia attack, only one of which is protected by the presence of Cl^?. In the case of Tris-induced inhibition of oxygen evolution only the Cl^? protected site is evident. In both cases the mechanism of Cl^? protection involves the binding of Cl^? in competition with the inhibitory amine. Anions (Br^? and NO^?₃) known to reactive oxygen evolution in Cl^?-depleted membranes also protect against Tris-induced inhibition, and reactivation of Cl^?-depleted membranes by Cl^? is competitively inhibited by ammonia. Inactivation of the oxygen-evolving complex by NH₂OH is impeded by Cl^?, whereas Cl^? does not affect the inhibition induced by so-called ADRY reagents. We propose that Cl^? functions in the oxygen-evolving complex as a ligand bridging manganese atoms to mediate electron transfer. This model accounts both for the well known Cl^? requirement of oxygen evolution, and for the inhibitory effects of amines on this reaction.