Flash-induced electron transport in b- and c-type cytochromes in Rhodospirillum rubrum. Evidence for a Q-cycle

Abstract

Flash-induced electron flow in the cyclic chain in chromatophores of the purple photosynthetic bacterium Rhodospirillu rubrum was studied in order to obtain information about the reactions of various redox components and to test the validity of the Q-cycle model for electron transport. Redox reactions of the b-type cytochromes b-433 and b-428, the c-type cytochrome c-420 and the reaction center bacteriochlorophyll P-870 were observed at redox potentials between −50 and +350 mV in the presence and absence of the electron-transport inhibitors antimycin and n-heptadecylmercapto-6-hydroxy-5,8-quinolinequinone (HMHQQ). At all redox potentials, the reduction of the b-type cytochromes in the presence of antimycin was linked to the reduction of an equal amount of oxidized cytochrome c-420 or P-870 +. These results are interpreted as evidence for the functioning of a Q-cycle in the cyclic electron transport. The Q-cycle is inhibited by the quinone analogue HMHQQ. At potentials around 20 mV, in the absence of antimycin, cytochrome b-433 was reduced in a flash and subsequently oxidized with a simultaneous net oxidation of the dark-reduced cytochrome b-428 and reduction of the flash-oxidized cytochrome c-420. We conclude therefore that cytochrome b-433 has a lower midpoint potential than cytochrome b-428. Presumably, the midpoint potential of cytochrome b-428 is around 50 mV and the midpoint potential of cytochrome b-433 is below 0 mV. The concentration of the b-type cytochromes involved in light reactions was low. There seemed to be approximately one of each per six or eight reaction centers.