Isolation and Purification of a Ubiquinone-Cytochrome b-c_1 Complex from a Photosynthetic Bacterium, Rhodopseudomonas sphaeroides

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A ubiquinone-cytochrome b-c_1 complex was removed from chromatophore membranes of a Rhodopseudomonas sphaeroides green mutant by deoxycholate-cholate treatment of the cromatophores. The complex was purified by ammonium sulfate fractionation and gel filtration. The molecular weight of the purified complex was 240,000 (240 kD) and it was composed of seven subunits with molecular weights of 47 kD, 42 kD, 38 kD, 32 kD, 30 kD, 24 kD and 16 kD. The complex contained 1.54 and 3.42 nmol of cytochrome c_1 and two different cytochrome b species per mg protein, respectively. It also contained 7.07 nmol of ubiquinone, 6.37 nmol of non-heme iron and about 3 nmol of carotenoids per mg protein. No flavins were detected. Heme staining indicated that the 32 kD- and 24 kD-subunits were cytochromes. The midpoint potential of cytochrome c_1 was 245 mV, and the values for the cytochromes b were 60 mV and -75mV at pH7.2. The peak of the α-band of the reduced-minus-oxidized difference spectrum of cytochrome c_1 was located at 552.5 nm, and peaks of the b-type cytochromes with higher and lower midpoint potentials were located at 562 nm and 563 nm. The chemical and the subunit compositions of the purified complex reported here were similar to those obtained for the inner membranes of mitochondria of various organisms.
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