Binding of the inhibitor NH3 to the oxygen-evolving apparatus of spinach chloroplasts

Abstract

Experiments are described on flash-induced luminescence of isolated spinach chloroplasts after addition of NH4Cl. The results indicate a binding of NH3, presumably in competition with water, in the oxidation states S2 and S3, i.e. the states reached upon illumination of dark-adapted material with one and two flashes, respectively. In the initial state S1, no binding of NH3 occurs. In state S2 the binding of ammonia is rapid (half-time about 0.5 s) and rapidly reversible; in state S3 the binding is slower (half-time about 10 s) and slowly reversible. NH3 bound to S4 prevents the oxidation of water. NH3 bound to S2 decreases the rate of the back reaction of reduced primary acceptor (Q-), indicating a charge stabilization, i.e. a decrease in the redox potential of S2 due to interaction with ammonia. In Tris-washed chloroplasts, the stability of the positive charge generated in a flash is much smaller than in normal chloroplasts and not increased by NH3. On the basis of these observations it is postulated that, in the absence of NH3, states S2 and S3 are stabilized by manganese-coordinated, bound water.