Laser flash photolysis studies of the kinetics of electron-transfer reactions of Saccharomyces flavocytochrome b2: evidence for conformational gating of intramolecular electron transfer induced by pyruvate binding.

The kinetics of reduction of the flavocytochrome from Saccharomyces cerevisiae by exogenous deazaflavin semiquinones have been investigated by using laser flash photolysis. Direct reduction by deazaflavin semiquinone of both the b2 heme and the FMN cofactor occurred via second-order kinetics with similar rate constants (9 x 10(8) M-1 s-1). A slower, monoexponential, phase of FMN reoxidation was also observed, concurrent with a slow phase of heme reduction. The latter accounted for approximately 20-25% of the total heme absorbance change. Both of these slow phases were protein concentration dependent, yielding identical second-order rate constants (1.1 x 10(7) M-1 s-1), and were interpreted as resulting from intermolecular electron transfer from the FMN semiquinone on one protein molecule to an oxidized heme on a second molecule. Consistent with this conclusion, no slow phase of heme reduction was observed with deflavo-flavocytochrome b2. Upon the addition of pyruvate (but not D-lactate or oxalate), the second-order rate constant for heme reduction was unaffected, but direct reduction of the FMN cofactor was no longer observed. Reduction of the heme cofactor was followed by a slower partial reoxidation, which occurred concomitantly with a monoexponential phase of FMN reduction. Both processes were protein concentration independent and were interpreted as the result of intramolecular electron transfer from reduced b2 heme to oxidized FMN. Potentiometric titrations of the flavocytochrome in the absence and presence of pyruvate demonstrated that the thermodynamic driving force for electron transfer from FMN to heme is much greater in the absence of pyruvate. Despite this, intramolecular electron transfer was only observed in the presence of pyruvate. This result is interpreted in terms of a conformational change induced by pyruvate binding which permits electron transfer between the cofactors. The rate constant for intramolecular electron transfer in the presence of pyruvate was dependent on ionic strength, suggesting the occurrence of electrostatic effects which influence this process.