Inhibitor “Double Occupancy” in the QoPocket of the Chloroplast Cytochromeb6fComplex

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Abstract
Electron paramagnetic resonance (EPR) spectra of the “Rieske” 2Fe−2S cluster revealed that two molecules of the inhibitor 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB) can bind to each monomer of the spinach cytochrome (cyt) b6f complex, both in isolated form and in intact thylakoid membranes. Binding to the high-affinity site, which accounts for the observed inhibitory effects, caused small shifts in the gx transition of the 2Fe−2S cluster EPR spectrum, similar to those induced by stigmatellin or 2-iodo-6-isopropyl-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 = 1.94 EPR signal. The shape of the equilibrium binding titration curve, the effects on the 2Fe−2S EPR spectrum, and the ability of the DBMIB binding to displace DNP-INT were consistent with two molecules of DBMIB binding at the Qo pocket, with the strongly binding species binding close to the 2Fe−2S cluster. Possible implications of these findings for so-called “double-occupancy” models for Qo site catalysis are discussed.