Ag. Roberts et Dm. Kramer, Inhibitor "double occupancy" in the Q(o) pocket of the chloroplast cytochrome b(6)f complex, BIOCHEM, 40(45), 2001, pp. 13407-13412
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.