Oxygen-Linked Electron Transfer and Energy Conversion in Rhodospirillum Rubrum

Abstract

Pigmented Rbodospirillum rubrum cells from dark chemotrophic cultures contain several pathways for the transfer of electrons from reduced substrates to O₂. In order of decreasing H⁺ -translocating efficiency, they are: (i) a cytochrome (oxidase) pathway that is inhibited by low concentrations of KCN and by inhibitors of the cytochrome b.ci complex, but not by CO; (ii) a CO sensitive or alternative (oxidase) pathway that is partly blocked by inhibitors of the cytochrome b.c ₁ complex; and (iii) a third pathway that operates in the presence of CO plus antimycin A and that is absent in the presence of CO plus myxothiazol and in a mutant which lacks rhodoquinone. In addition, a significant fraction of the O₂ uptake activity remains when H⁺ translocation is completely blocked by inhibitors of electron transfer or mutations. Since the rate of respiratory electron transfer appears to be limited at the substrate level, it is difficult to make a direct estimation of the contribution of each pathway to the final rate of respiration. However, from the relative energy-transducing efficiency of the cytochrome and the CO sensitive pathways (as measured by the H⁺/O ratios in O₂ pulses) and the final cell yields of C-limited cultures of wild type and cytochrome-oxidase deficient strains, it seems that the contribution of the cytochrome pathway to the energy-conserving O₂ uptake of pigmented chemotrophic R. rubrum, growing on malic and glutamic acids as carbon sources, is close to 80%.