Electron transfer between soluble cytochrem c-554 and purified raction center from the green sulfur bacterium Chlorobium tepidum

Abstract

Green sulfur bacteria contain three kinds of soluble cytochrome c (In Chlorobium limicola f. thiosulfatophilum; cytochrome c-553: flavocytochrome, c-551: e- acceptor of thiosulfate oxidoreductase (30 kDa), and c-555 (10 kDa), respectively). A reaction center (RC) of green sulfur bacteria also contains two molecules of monoheme cytochorme c. Photooxidized P840 is re-reduced by the reaction center-bound cytochorme c-551 with t1/2 of 60-110µs. The 10 kDa-cytochrome c was purified from the green sulfur bacterium Chlorobium tepidum. Its midpoint potential is 148mV. From its a-peak, it is referred to as cytochrome c-554. We measured electron transfer kinitics between purified cytochorme c-554 and RC of C. tepidum using single and multiple flash excitation measurements. The results indicate that cytochrome c-554 can reduce RC bound cytochrome c, but not directly P840. The rate of oxidation of cyt c-554 by oxidized RC was not strongly affected by the ionic strength of the medium.