BINDING OF CARBOXYLATE ANIONS AT THE NONHEME FE(II) OF PS-II .2. COMPETITION WITH BICARBONATE AND EFFECTS ON THE Q(A)/Q(B) ELECTRON-TRANSFER RATE

In the companion paper (Petrouleas, V., Deligiannalcis, Y. and Diner, B.A. (1994) Biochim. Biophys. Acta 1188, 260-270) we examined the effects of a number of carboxylate anions on the EPR signals associated with the iron quinone complex, the ligand field parameters of the Fe3+, and the redox properties of the iron. In this paper, we show that three representative anions, glycolate, glyoxylate, and oxalate, compete with NO, formate and bicarbonate for binding to the iron. Furthermore, the bound anions affect diversely the Q(A)/Q(B) electron transfer rate. Glycolate has an extreme effect, similar to what is observed with high levels of formate, and is characterised by a dissociation constant, K-d, similar to 0.5-0.7 mM. Oxalate gives a marked slowing of the rate of Q(A)(-) oxidation on all flashes but preserves a marked oscillation of the rate of period two. Glyoxylate appears to have an intermediate effect. These results offer new information on the stereochemistry of the binding of dissociable ligands to the non-heme iron of PS II and a tool for probing the redox chemistry of the iron and the electron transfer properties of the iron-quinone complex.