Inhibitor "double occupancy" in the Q(o) pocket of the chloroplast cytochrome b(6)f complex

Electron paramagnetic resonance (EPR) spectra of the "Rieske" 2Fe-2S cluste r revealed that two molecules of the inhibitor 2,5-dibromo-3-methyl-6-isopr opylbenzoquinone (DBMIB) can bind to each monomer of the spinach cytochrome (cyt) b(6)f complex, both in isolated form and in intact thylakoid membran es. Binding to the high-affinity site, which accounts for the observed inhi bitory effects, caused small shifts in the g(x) transition of the 2Fe-2S cl uster EPR spectrum, similar to those induced by stigmatellin or 2-iodo-6-is opropyl-3-methyl-2',4,4'-trinitrodiphenyl ether (DNP-INT). Occupancy of the low-affinity site was only observed after addition of superstoichiometric amounts of the inhibitor and was accompanied by the appearance of a g = 1.9 4 EPR signal. The shape of the equilibrium binding titration curve, the eff ects on the 2Fe-2S EPR spectrum, and the ability of the DBMIB binding to di splace DNP-INT were consistent with two molecules of DBMIB binding at the Q (o) pocket, with the strongly binding species binding close to the 2Fe-2S c luster. Possible implications of these findings for so-called "double-occup ancy" models for Q(o) site catalysis are discussed.