Construction of a flash-activated cyclic electron transport system by using bacterial reaction centers and the ubiquinone-cytochrome b-c1/c segment of mitochondria.

Single-turnover electron transfer within the mitochondrial complex III has been studied by combining, in solution, the isolated complex from bovine heart with detergent-solubilized reaction centers of Rhodopseudomonas sphaeroides. Initiation of electron transfer by short flash activation resulted in the prompt oxidation of cytochrome c and reduction of cytochrome b. The subsequent reduction of ferricytochrome c was observed to be concomitant with the oxidation of the ferrocytochrome b, both reactions being inhibited by the addition of actimycin A. The rate of electron transfer through complex III is dependent upon the ambient redox potential poise in a way that is consistent with the presence of a redox component, presumably analogous to the photosynthetic ubiquinone Qz, which is an obligatory intermediate in electron transfer between cytochromes b and c. These results demonstrate cyclic electron transfer in a constructed assembly of mitochondrial complex III, cytochrome c, and photochemical reaction centers.