Kinetic mechanism of pyrophosphate-dependent phosphofructokinase from Propionibacterium freudenreichii

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Abstract
Inorganic pyrophosphate dependent D-fructose-6-phosphate 1-phosphotransferase from Propionibacterium freudenreichii was purified to apparent homogeneity by the criterion of silver staining on sodium dodecyl sulfate (SDS) gels. In the direction of phosphorylation of fructose 6-phosphate (F6P), an intersecting initial velocity pattern is obtained when MgPPi is varied at several levels of F6P. In the reverse reaction direction, the reactants are Mg2+, Pi and fructose 1,6-bisphosphate (FDP). Variation of Pi at several levels of Mg2+ and a single level of FDP gives an intersecting pattern. When this pattern is repeated at several additional FDP levels, data are consistent with a fully random terreactant mechanism at pH 8.0 and 25.degree. C. The Keq calculated from the Haldane relationship [(5 .+-. 1.5) .times. 10-3 M] agrees with that determined directly from 31P NMR of the equilibrium mixture [(7 .+-. 2) .times. 10-3 M]. Product inhibition by Pi is competitive vs. either MgPPi or F6P with the other reactant saturating but changes to noncompetitive inhibition when the fixed reactant is decreased to Km levels. Product inhibition by MgPPi is competitive vs. either Pi or FDP with the other reactant saturating but changes to noncompetitive when the fixed reactant is decreased to Km levels. Tagatose 6-phosphate is competitive vs. F6P and noncompetitive vs. MgPPi. Methylenediphosphate is competitive vs. MgPPi and noncompetitive vs. F6P. Sulfate is competitive vs. Pi and noncompetitive vs. FDP, while 2,5-anhydro-D-mannitol 1,6-bisphosphate is competitive vs. FDP and noncompetitive vs. Pi. These data are consistent with a rapid equilibrium random bi-ter mechanism.