THE PH-DEPENDENCE OF THE REDOX MIDPOINT POTENTIAL OF THE 2FE2S CLUSTER FROM CYTOCHROME-B6F COMPLEX (THE RIESKE CENTER)

The pH dependence of the redox midpoint potential (E(m)) of the Rieske centre from spinach cytochrome b6f complex was studied by EPR. The E(m) was found to be independent of pH up to pH 8 and to decrease at higher pH values. The slope of the decrease above the pK value was roughly consistent with the involvement of a dissociable proton on the oxidized form of the cluster. The E(m) in the pH-independent region (i.e., < pH 8) was determined to be + 320 mV. This is in contrast to the original value (E(m) = + 290 mV) reported by Malkin and Aparicio (Biochem. Biophys. Res. Commun. 63 (1975) 1157-1160) and confirms the results reported more recently (E(m) = 310-320 mV) by Malkin (FEBS Lett. 131 (1981) 169-172) and Nitschke et al. (Biochim. Biophys. Acta 974 (1989) 223-226).