The complex kinetic properties exhibited by phosphofructokinase from various sources are generally believed to reflect the role of this enzyme in the metabolic control of the glycolytic pathway. In order to understand the significance of such regulatory properties in vivo, we have used isogenic Escherichia coli strains that contain phosphofructokinases with different kinetic properties. These different forms of the enzyme have been characterized in vitro and the effect of the loss of a regulatory property on carbon metabolism and cell growth rate has been evaluated in vivo. In E. coli there are two isoenzymes of phosphofructokinase: isoenzyme-I exhibits sigmoidal kinetics with respect to fructose 6-phosphate, alosteric activation by ADP or GDP, and allosteric inhibition by phosphoenolpyruvate (Blangy et al., 1968; Babul, 1978); the other, isoenzyme-2, shows hyperbolic kinetics with respect to both substrates and is inhibited by MgATP when the assay is performed at low fructose 6-phosphate concentrations. We also have a mutant form of isoenzyme-2, named isoenzyme-2~*, which has a single aminoacid substitution (Tyr~(23)->Asp) in the polypeptide chain. The most striking kinetic feature of this enzyme is the absence of MgATP inhibition at low concentrations of fructose 6-phosphate (Guixe & Babul, 1985).
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