Flavocytochrome b2 (Baker's yeast). Deuterium isotope effect studied by rapid-kinetic methods as a probe for the mechanism of electron transfer.

The use of DL-[2-2H]lactate in steady-state measurements of ferricyanide reduction by flavocytochrome b2 at 30 degrees C has previously yielded an isotope effect of 5 [F. Lederer (1974) Eur. J. Biochem. 46, 393--399]. We report here studies carried out at 5 degrees C with L-[2-2H]lactate, where flavin and heme reduction were observed in the stopped-flow apparatus, in the absence of acceptor. The generally biphasic reduction curves were analysed according to a new mathematical treatment which allowed us to derive microscopic constants from initial reduction rates. It has thus been possible to determine an isotope effect of 8 on flavin reduction, 6 on heme reduction, compared to 4 in the steady state. Consequently, two slightly rate-limiting steps occur after the first one where the alpha-hydrogen is abstracted. It has also been possible to calculate the substrate association and dissociation rate constants for intact enzyme. The studies were carried out in parallel on intact and cleaved cytochrome b2. The results suggest that proteolysis affects essentially the steps involved in flavin reduction, and not intramolecular electron transfer steps. Moreover, the experimental data obtained at low rates of electron entry have led us to reexamine a previously proposed scheme for electron transfer [Capeillère-Blandin, Bray, Iwatsubo and Labeyrie (1975) Eur. J. Biochem. 54, 549--566]. An alternative model based on computer-simulation studies will be presented in a paper in this journal.