Light-induced electron transport pathways in membrane preparations from Rhodopseudomonas capsulata

Abstract

The photosynthetic electron transport chain in Rhodopseudomonas capsulata cells was investigated by studying light-induced noncyclic electron transport from external donors to O 2. Two membrane preparations with opposite membrane polarity, heavy chromatophores and regular chromatophores, were used to characterize this electron transport. It was shown that with lipophylic electron donors such as dichloroindophenol, diaminobenzidine, and phenazine methosulfate the electron transport activities were similar in both types of chromatophores, whereas horse heart cytochrome c, K 4Fe(CN) 6, 3-sulfonic acid phenazine methosulfate, and ascorbate, which cannot penetrate the membrane, were more active in the heavy chromatophores than in the regular chromatophores. Partial depletion of cytochrome c 2 from the heavy chromatophores caused a decrease in the light-induced O 2 uptake from reduced dichloroindophenol or ascorbate. The activity could be restored with higher concentrations of dichloroindophenol or with purified cytochrome c 2 from Rps. capsulata. It is assumed that in the heavy chromatophores the artificial electron donors are oxidized on the cytochrome c 2 level which faces the outside medium. However, cytochrome c 2 is not exposed to the outside medium in the regular chromatophores. Therefore, only lipophylic donors would interact with cytochrome c 2 in this system, while hydrophylic donors would be oxidized by another component of the electron transport chain which is exposed to the external medium. Studies with inhibitors of photophosphorylation show that antimycin A enhances the light-dependent electron transport to O 2 whereas 1:10 phenanthroline inhibited the reaction, but dibromothymoquinone did not affect it. It is assumed that a nonheme iron protein is taking part in this electron transport but not a dibromothymoquinone-sensitive quinone. The terminal oxidase of the light-dependent pathway is different from the two oxidases of the respiratory chain. The ratio between electrons entering the system and molecules of O 2 consumed is 4, which means that the end product of O 2 reduction is H 2O.