Activation of Thermus phosphofructokinase by monovalent cations

The presence of the monovalent cations Tl+, NH+4, K+, Rb+ or Cs+, in decreasing order of potency, produce a marked equivalent increase in the specific enzyme activity of phosphofructokinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) purified from extreme thermophile, Thermus X-1. By contrast, the monovalent cations Li+, Na+ or CH3NH+3 produce no detectable catalyitic activation at concentrations up to 100 mM. The relative potency of these cations suggests that each polypeptide chain in the tetrameric enzyme possesses a cationbinding site having tetragonal symmetry and that the protein ligands are principally hydroxyl or carboxylate oxygens. Only the enzyme-cation complex and not the enzyme by itself exhibits cooperativity with respect to the dependence of catalytic rate on the concentration of the substrate, fructose 6-phosphate. In the presence of subsaturating but not saturating concentrations of substrate, the catalytic activation produced by monovalent cations is also cooperative. Exclusion chromatographic measurements indicate that the enzyme remains tetrameric at catalytic concentrations in the presence or absence of an activating monovalent cation.