| Abstract: | The effect of antimycin on the ubiquinone cytochrome b-c2 (Q b-c2) oxidoreductase of the photosynthetic bacterium Rhodopseudomonas sphaeroides has been studied under controlled oxidation-reduction potential (Eh) conditions by equilibrium measurements and by rapid kinetic analysis of single turnover flash.induced electron and proton translocations. 1. Antimycin shifts the alpha-band of ferro b50 (lambda max 560 nm) by 1 to 2 nm toward the red but has no apparent effect on the equilibrium oxidation-reduction midpoint potential of the cytochrome. 2. This red shift is proportional to the antimycin added until a "titer" of 0.7 +/- 0.1 antimycin per reaction center (RC) is approached. With a similar titer antimycin essentially abolishes the following millisecond reactions activated by saturating single turnover flashes: reduction of ferri c2, oxidation of ferro b, Phase III of the membrane-potential-indicating band shift of endogenous carotenoid pigments, and the uptake of 1 of the 2 protons taken up per electron transferred. Such titrations indicate that the binding (KD approximately 10(-9) m) and mode of inhibition of antimycin are noncooperative and are independent of the membrane's coupling status and of the pH and Eb over the range in which electron transport is operative. 3. In the presence of excess antimycin a partial recovery of ferri c2 reduction is seen when the intensity of the flash is diminished, but only at Eh values such that Z (a special quinone serving as reductant for ferri c2) is reduced but b50 is oxidized before activation. These results are consistent with the following model. Each Q b-c2 oxidoreductase complex includes one antimycin binding site, one b50, and one Z. These complexes and the c2 . RC complexes, present in an 0.7:1 ratio, are to some degree mobile with respect to each other. Ferri b50 can be reduced either via the quinones of the RC or via Z in a reaction also involving c2. The former route is kinetically dominant in the presence of antimycin, but the latter route is the means for "oxidant-induced reduction" and depends on the collisional interaction of the oxidoreductase and c2 . RC complexes. Antimycin interferes with neither of these two routes but does inhibit the oxidation of ferro b50; all the other inhibitory effects are consequent on this. |
