Photosynthetic oxygen evolution: Effects of the water-soluble polypeptides and inorganic ions.

Abstract

Three water-soluble polypeptides with molecular weights of 17, 23, and 33 kDa and the inorganic ions chloride, calcium and manganese are required for photosynthetic oxygen evolution. Understanding how these components interact with the oxygen evolving complex will aid in an understanding of the overall process of water oxidation. A method was developed for the selective removal of the 17 kDa water-soluble species from isolated PSII membranes. Hydroquinone is shown to be a useful tool for the detection of the amount of 23 kDa water-soluble polypeptide which is functionally attached to the PSII membranes as of the structural shield protecting the manganese complex from exogenous reductants. The 17 kDa species is not part of this shield. Hydroquinone was also used to demonstrate that incubation of the PSII membranes in low pH medium causes the loss of calcium by first releasing the water-soluble polypeptides. In the absence of the 17 and 23 kDa water-soluble polypeptides, cations such as sodium, potassium and cadmium are inhibitors of calcium-activated oxygen evolution with K$\sb{\rm I}$ values of 5mM, 10mM, and 0.3mM respectively. The inorganic ion, tetramethylammonium, does not affect calcium-activated oxygen evolution and as such can be used in place of the inhibitory species in buffer titrations and for chloride additions. The use of sodium in steady state experiments examining the interaction of chloride and calcium in the oxygen evolving complex alters the kinetic parameters that are obtained. Therefore, caution must be used in interpreting data from experimental systems which have inhibitors in them. In the absence of, or at low concentrations of chloride, calcium inhibits photosynthetic oxygen evolution. Only one calcium binding site with a binding constant of 0.03mM is observed in the system examined here. The binding constant of chloride is 5mM. The kinetic mechanism for the interaction of chloride and calcium is best represented as a rapid equilibrium ordered mechanism where calcium binds to the oxygen evolving complex first and chloride can only bind to the calcium containing form of the enzyme.