Studies on the Kinetics and Mechanism of Reduction of Flavodoxin

1 The reactions of the oxidised and semiquinone forms of Peptostreptococcus elsdenii flavodoxin with sodium dithionite have been studied by stopped flow spectrophotometry. At low ionic strength where comproportionation of flavodoxin is slow compared with the overall rate of reduction, the reactions with an excess of dithionite follow first-order kinetics. Semiquinone is not detected during the reduction of oxidised flavodoxin, possibly because the semiquinone is reduced very much faster than the oxidised protein. 2 The rates of reduction are propo~'tional to the square root of the dithionite concentration. This observation suggests that the reducing species is not dithionite itself, but a dissociation product. It is proposed that the reducing species is SO2-. The results support a mechanism in which the reduction of oxidised flavodoxin occurs by two successive one-electron transfers, and is limited by the rate of reduction to the semiquinone level. 3. The reaction of oxidized flavodoxin and fully reduced flavodoxin to form semiquinone is very sensitive to ionic strength and the salt composition of the medium. Increasing ionic strength causes a marked increase in rate. The rate constant at zero ionic strength, calculated by extrapolation, was 100 M -1. rain -1 at 25 °C.