Intramolecular electron transfer in Chlorobium thiosulfatophilum flavocytochrome c

The electron-transfer reactions of photoproduced lumiflavin semiquinone and fully reduced lumiflavin with oxidized Chlorobium thiosulfatophilum flavocytochrome c have been studied by using laser flash photolysis. The Chlorobium flavocytochrome c contains one heme and one flavin per Mr 50 000, and thus the possibility exists for intramolecular electron transfer. We find a complex kinetic pattern which is consistent with the transient formation of a spectrally perturbed protein-bound flavin semiquinone which transfers an electron intramolecularly to the heme (k = 1 X 10(3)-1.8 X 10(3) s-1 for the neutral semiquinone, depending upon the pH). Evidence is presented that electron transfer from exogenous lumiflavin to the heme moiety occurs through the protein-bound flavin. We have also performed redox titrations which determine the midpoint potentials of the heme and flavin prosthetic groups at various pH values and the pK values for the semiquinone (6.4) and fully reduced flavin (6.1). Thus, at pH 7, the semiquinone is predominantly in the anionic form at equilibrium. The reactions of Chlorobium flavocytochrome c with photoreduced lumiflavin are similar to those previously found with Chromatium vinosum flavocytochrome c [Cusanovich, M. A., & Tollin, G. (1981) Biochemistry 19, 3343-3347] in that a protein-bound flavin semiquinone is an intermediate in the pathway of reduction. However, the rate constants are substantially different. As a class, the flavocytochromes c appear to operate by analogous mechanisms involving rapid intramolecular transfer between the heme and flavin moieties.