The Qo-site inhibitor DBMIB favours the proximal position of the chloroplast Rieske protein and induces a pK-shift of the redox-linked proton

The interaction of the inhibitor 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB) with the Rieske protein of the chloroplast b(6)f complex has been studied by EPR, All three redox: states of DBMIB were found to interact with the iron-sulphur cluster. The presence of the oxidised form of DBMIB altered the equilibrium distribution of the Rieske protein's conformational substates, strongly favouring the proximal position close to heme DL, In addition to this conformational effect, DBMIB shifted the pK-value of the redox-linked proton involved in the iron-sulphur cluster's redox transition by about 1.5 pH units towards more acidic values. The implications of these results with respect to the interaction of the native quinone substrate and the Rieske cluster in cytochrome be complexes are discussed. (C) 1999 Federation of European Biochemical Societies.