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 effe cts of a number of carboxylate anions on the EPR signals associated wi th the iron quinone complex, the ligand field parameters of the Fe3+, and the redox properties of the iron. In this paper, we show that thre e representative anions, glycolate, glyoxylate, and oxalate, compete w ith 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 hi gh 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 rat e of Q(A)(-) oxidation on all flashes but preserves a marked oscillati on of the rate of period two. Glyoxylate appears to have an intermedia te effect. These results offer new information on the stereochemistry of the binding of dissociable ligands to the non-heme iron of PS II an d a tool for probing the redox chemistry of the iron and the electron transfer properties of the iron-quinone complex.