| Abstract: | Cytochrome c-552 from Chromatium vinosum is an unusual heme protein in that it contains two hemes and one flavin per molecule. To investigate whether intramolecular electron transfer occurs in this protein, we have studied its reduction by external photoreduced flavin by using pulsed-laser excitation. This approach allows us to measure reduction kinetics on the mirosecond time scale. Both fully reduced lumiflavin and lumiflavin semiquinone radical reduce cytochrome c-552 with second-order rate constants of approximately 1.4 x 10(6) M-1s-1 and 1.9 x 10(8) M-1 s-1, respectively. Kinetic and spectral data and the results of similar studies with riboflavin indicate that both the flavin and heme moieties of cytochrome c-552 are reduced simultaneously on a millisecond time scale, with the transient formation of a protein-bound flavin anion radical. This is suggested to be due to rapid intramolecular electron transfer. Further, steric restrictions play an important role in the reduction reaction. Studies were conducted on the redox processes following photolysis of CO-ferrocytochrome c-552 in which the flavin was partly oxidized to resolve the kinetics of electron transfer between the heme and flavin of cytochrome c-552. Based on these results, we conclude that intramolecular electron transfer from ferrous heme to oxidized flavin occurs with a first-order rate constant of greater than 1.4 x 10(6) s-1. |
