Electron Transfer Reactions of the Ubiquinol: Cytochrome C-2 Oxidoreductase of Rhodopseudomonas Sphaeroides

Abstract

The electron transfer reactions of the ubiquinol:cytochrome c(,2) oxidoreductase (b-c(,1) complex) of the photosynthetic bacteria Rps. sphaeroides have been investigated through the use of optical spectroscopy and chemical inhibitors of the protein complex.

Two new components, cyts c(,1) and b(,566), have been shown to be part of the complex. The chemical properties and the function of these cytochromes in the complex have been studied. Cyt c(,1) has an E(,m,7) of 260 mV, n = 1, with an alpha band maximum at 552 nm, is bound to the membrane, and is oxidized by cyt c(,2) with a t(, 1/2) of approx. 150 us. Cyt b(,566) E(,m,7) of -90 mV, n = 1, has a double alpha band with maxima at 559 and 566 nm and is reduced only when cyt b(,561) is reduced prior to the flash.

The effects of the inhibitors antimycin and myxothiazol on the cytochromes of the b-c(,1) complex have been investigated. Antimycin has been shown to induce the oxidation of a b-type cytochrome at redox potentials above 100 mV, and to induce a shift to longer wavelengths in the absorption maximum of a b-type cytochrome at redox potentials below 100 mV. The redox and pH dependence of these two effects were studied. Myxothiazol has been shown to shift the midpoint potential of cyt b(,566), shift the absorption maximum of the 566 nm band to shorter wavelengths, and in the presence of antimycin, inhibit the reduction of cyts b(,561) and b(,566). Myxothiazol inhibits the reduction of cyts c(,1) and c(,2), and the Rieske FeS center by the quinone pool. UHDBT binds competitively with myxothiazol to the ubiquinol oxidase site of the complex. This phenomenon has been used to demonstrate that the apo-proteins of cyt b(,566) and the Rieske FeS center form the quinol oxidase site and to measure the rate of oxidation of the Rieske FeS center, t(, 1/2) of approx. 200 us.

A computer program has been written to model the functioning of the b-c(,1) complex in the presence of the inhibitors antimycin, myxothiazol, and UHDBT. This program is based on a Q-cycle mechanism and the thermodynamic properties of the components of the cyclic electron transport chain. The program calculates the extents of oxidation and reduction of the components in the cyclic electron transport chain and can be used to model the system under a large number of initial conditions. The Q-cycle modeling program predicts well the results obtained over a large redox potential and pH range.