Energy transduction in photosynthetic bacteria. XI. Further resolution of cytochromes of b type and the nature of the CO-sensitive oxidase present in the respiratory chain of Rhodopseudomonas capsulata

Abstract

1. In membranes prepared from dark grown cells of Rhodopseudomonas capsulata, five cytochromes of b type ( E′ 0 at pH 7.0 +413±5, +270±5, +148±5, +56±5 and −32±5 mV) can be detected by redox titrations at different pH values. The midpoint potentials of only three of these cytochromes ( b 148, b 56, and b −32) vary as a function of pH with a slope of 30 mV per pH unit. 2. In the presence of a Co/N 2 mixture, the apparent E′ 0 of cytochrome b 270 shifts markedly towards higher potentials (+355 mV); a similar but less pronounced shift is apparent also for cytochrome b 150. The effect of CO on the midpoint potential of cytochrome b 270 is absent in the respiration deficient mutant M6 which possesses a specific lesion in the CO-sensitive segment of the branched respiratory chain present in the wild type strain. 3. Preparations of spheroplasts with lysozyme digestion lead to the release of a large amount of cytochrome c 2 and of virtually all cytochrome cc′. These preparations show a respiratory chain impaired in the electron pathway sensitive to low KCN concentration, in agreement with the proposed role of cytochrome c 2 in this branch; on the contrary, the activity of the CO-sensitive branch remains unaffected, indicating that neither cytochrome c 2 nor the CO-binding cytochrome cc′ are involved in this pathway. 4. Membranes prepared from spheroplasts still possess a CO-binding pigment characterized by maxima at 420.5, 543 and 574 nm and minima at 431, 560 nm in CO-difference spectra and with an α band at 562.5 nm in reduced minus oxidized difference spectra. This membrane-bound cytochrome, which is coincident with cytochrome b 270, can be classified as a typical cytochrome “ o” and considered the alternative CO-sensitive oxidase.