The chloride requirement for photosynthetic oxygen evolution: Factors affecting nucleophilic displacement of chloride from the oxygen-evolving complex

Previous research (Sandusky, P.O. and Yocum, C.F. (1983) FEBS Lett. 162, 339–343 and (1984) Biochim. Biophys. Acta 766, 603–611) has documented a competition between chloride and ammonia or Tris for a binding site within the oxygen-evolving complex of Photosystem II. This competition is in fact a general property of inhibitory amines which is related to their nucleophilicity; this in turn suggests that the binding site is associated with a metal. Only ammonia, of all amines tested, is able to occupy a second binding site which is unrelated to the site of chloride binding; this sterically hindered site may be identical to the site already described for binding of hydroxylamine, hydrazine, and certain of their derivatives (Radmer, R. and Ollinger, O. (1983) FEBS Lett. 152, 39–43). When the interaction between amines, chloride and the inhibitory halide fluoride was examined, steady-state kinetic plotting procedures revealed that amines and fluoride compete for the chloride binding site; binding of one inhibitor precludes the binding of the other. It was also observed that the intensity of inhibitor binding to the oxygen-evolving complex was influenced by the electron acceptor present during assays; stronger inhibition was observed with a PS II-specific electron acceptor (2,5-dichloro-p-benzoquinone) than with an acceptor (ferricyanide) which requires electron transport to the reducing terminus of Photosystem I. These results are interpreted in terms of a model which proposes that the binding site for chloride on the oxidizing side of Photosystem II resides within the pool of functional manganese associated with the oxygen-evolving complex of Photosystem II.