Flavocytochrome b2 from baker's yeast. Computer-simulation studies of a new scheme for intramolecular electron transfer.

It has been shown recently that the use of L-(+)-[2-2H]lactate as substrate instead of unlabeled L-(+)-lactate induces a lowering of the flavin reduction rate of cytochrome b2 by a factor of 8 [D. Pompon, M. Iwatsubo, and F. Lederer (1980) Eur. J. Biochem. 104, 479--488]. This high isotope effect enabled us to study electron transfer between prosthetic groups at a very low rate of electron entry. The kinetic scheme for electron transfer in cytochrome b2 proposed by Capeillère-Blandin [Eur. J. Biochem. 56, 91--101 (1975)] is examined here in the light of the kinetic data reported in our preceding paper. This study indicates some disagreements, particularly at a low rate of electron entry. New kinetic schemes capable of explaining data obtained with deuterated lactate are proposed. These new schemes differ from that of Capeillère-Blandin in that: (a) the hypothesis of simultaneous prosthetic group reduction for the two protomers of one given dimer is abandoned; (b) the limiting step of the slow phases of heme and flavin reduction is a slow interprotomer electron exchange between a heme pair, a flavin pair or heme and flavin; (c) a rather fast conformational change controlled by the redox state of heme or flavin of one promoter can modulate the rate of electron transfer in another promoter. These new kinetic schemes allow us to determine the rate of intraprotomer and interprotomer electron transfer and to decide precisely how these steps are modified by the proteolytic cleavage of 'intact' enzyme to 'cleaved' enzyme.