Energy-linked electron transfer reactions in Rhodopseudomonas viridis

Abstract

The particulate fraction of Rhodopseudomonas viridis when supplied with succinate catalyses the reduction of NAD + by light; this reaction is inhibited by uncouplers of oxidative phosphorylation but not by oligomycin. Formation of NADH takes place in the dark when ATP or PP i is supplied. Both light and dark reactions are inhibited by valinomycin and nigericin, when added together, but not by either separately. NADH formation in R. viridis appears to take place by an energy-dependent reversal of electron flow and energy may be conserved in the form of a membrane potential. The addition of ATP caused the oxidation of both C553 and C558 in chromatophores; carbonylcyanide p-trifluoromethoxyphenylhydrazone and oligomycin abolished this oxidation. The NAD + and NADH concentrations at equilibrium in the light-dependent reaction were determined and the oxidation-reduction potential of this couple calculated. From this value it was calculated that under these experimental conditions the energy requirement to form NADH from the succinate/fumarate couple at E h = o V was 9.4 kcal. Particles of R. viridis contained an active transhydrogenase, driven by either light or ATP, that was sensitive to uncouplers of oxidative phosphorylation; the light-driven reaction was insensitive to oligomycin and was inhibited by antimycin A and 2-heptyl-4-hydroxyquinone- N-oxide. R. viridis did not grow aerobically but particles contained NADH oxidase activity that was cyanide sensitive. There was no spectroscopic evidence for cytochromes of the b-type in reduced- minus-oxidised spectra of particles or in pyridine haemochrome spectra of whole cells.