Mutation of the heme-binding crevice of flavocytochrome b2 from Saccharomyces cerevisiae: altered heme potential and absence of redox cooperativity between heme and FMN centers.

Kinetic and thermodynamic properties of yeast flavocytochrome b2 (EC 1.1.2.3) are modified by the product pyruvate, which binds to the flavosemiquinone (FSQ) form of the prosthetic flavin and decreases the thermodynamic driving force for electron transfer from FSQ to heme. Pyruvate inhibits flavocytochrome b2, but the catalytic competence of pyruvate-ligated FSQ in intramolecular electron transfer to heme is unclear; one kinetic study suggested pyruvate prevented this reaction [Tegoni, M, Janot J.-M., & Labeyrie, F. (1990) Eur. J. Biochem. 190, 329-342], while laser flash photolysis indicated pyruvate was essential [Walker, M. C., & Tollin, G. (1991) Biochemistry 30, 5546-5555]. To address this problem, wild-type (WT) and mutant (L36I) flavocytochromes b2 have been expressed in Escherichia coli. Both forms incorporated heme and FMN prosthetic groups and were catalytically active. The mutation L36I was a conservative substitution within the heme-binding crevice and was designed to alter the midpoint potential (Em) of the heme to alter the pyruvate-FSQ/heme equilibrium. Potentiometric titrations yielded Em values (pH 7.0, 25 degrees C) of +8 and -28 mV for WT and L36I forms, respectively. The FMN midpoint potentials in the absence of pyruvate (-58 mV, n = 2) were identical within experimental error in WT and L36I species and were also identical (+5 mV, n = 1) in the presence of pyruvate. These results indicated the absence of redox cooperativity between FMN and heme.(ABSTRACT TRUNCATED AT 250 WORDS)