Evidence for a catalytic Mg2+ ion and effect of phosphate on the activity of Escherichia coli phosphofructokinase-2: regulatory properties of a ribokinase family member

Phosphofructokinase-2 (Pfk-2) from Escherichia coli belongs to the ribokinase family of sugar kinases. One of the signatures observed in amino acid sequences from the ribokinase familiy members is the NXXE motif, which locates at the active site in the ribokinase fold. It has been suggested that the effect of Mg2+ and phosphate ions on enzymatic activity, observed in several adenosine kinases and ribokinases, would be a widespread feature in the ribokinase family, with the conserved amino acid residues in the NXXE motif playing a role in the binding of these ions at the active site [Maj, M. C., et al. (2002) Biochemistry 41, 4059-4069]. In this work we study the effect of Mg2+ and phosphate ions on Pfk-2 activity and the involvement of residue E190 from the NXXE motif in this behavior. The kinetic data are in agreement with the requirement of a Mg2+ ion, besides the one present in the metal-nucleotide complex, for catalysis in the wild-type enzyme. Since the response to free Mg2+ concentration is greatly affected in the E190Q mutant, we conclude that this residue is required for the proper binding of the catalytic Mg2+ ion at the active site. The E190Q mutant presents a 50-fold decrease in the kcat value and a 15-fold increment in the apparent Km for MgATP(2-). Inorganic phosphate, typically considered an activator of adenosine kinases, ribokinases, and phosphofructokinases (nonhomologous to Pfk-2) acted as an inhibitor of wild-type and E190Q mutant Pfk-2. We suggest that phosphate can bind to the allosteric site of Pfk-2, producing an inhibition pattern qualitatively similar to MgATP(2-), which can be reversed to some extent by increasing the concentration of fructose-6-P. Given that the E190Q mutant presents alterations in the inhibition by MgATP(2-) and phosphate, we conclude that the E190 residue has a role not only in catalysis but also in allosteric regulation.