Transient Kinetics of Electron-transfer Reactions of Flavodoxins

Stopped-flow and laser photolysis methods have been used to investigate the rates of electron-transfer reactions of fully reduced riboflavin and the three oxidation states of Clostridium pasteurianum flavodoxin. Both normal and 7,8-dichloroflavin analogues were studied. Redox reagents included oxygen, ferricyanide, ferric EDTA, and several c-type cytochromes as oxidants and the semiquinone of 5-deazariboflavin as a reductant. The dependence of the rate of oxidation of the semiquinone form of the dichloro analogue flavodoxin upon oxidant concentration has provided clear evidence for the existence of a complex in the reaction pathway. Rate constant comparisons demonstrate that dichloro substitution decreases the rate of flavodoxin semiquinone oxidation by at least 1-2 orders of magnitude. The limiting first-order rate constants were found to be dependent on the redox potential of the oxidant, as would be predicted by theory if these were reflecting the actual electron-transfer reaction. Rate constant decreases upon chlorine substitution were also observed for the reduction of both oxidized and semiquinone forms of flavodoxin by deazariboflavin semiquinone. These results, considered in conjunction with the redox potential shift of the flavodoxin produced by the chlorine substitution, provide support for the hypothesis that electron transfer to and from the semiquinone form of the flavodoxin involves direct participation of the dimethylbenzene ring of the flavin. A comparison of oxidation rate constants for free and protein-bound fully reduced flavin suggests that the protein environment does not markedly influence coenzyme reactivity in this oxidation state.