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

Abstract

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.