Regulation of d-Fructose 1-Phosphate Kinase by Potassium Ion

Purified Aerobacter aerogenes d -fructose 1-phosphate kinase (ATP: d -fructose 1-phosphate 6-phosphotransferase), an enzyme which does not exhibit cooperative kinetics, was activated by K + , Rb + , and NH + 4 , inhibited by Li + , and not significantly affected by Na + or Cs + . The enzyme exhibited some activity when assayed in tetramethylammonium-glycylclycine buffer even in the absence of other monovalent cations, suggesting that the requirement for monovalent cations is not absolute. K + increased the reaction velocity about 4-fold in the presence of excess ATP and d -fructose-1-P, and about 10-fold when both substrates were present at concentrations near their K m values (0.3 m m ). K + also exerted a much greater activating effect under conditions of inhibiting ATP levels than when Mg ++ was present at a sufficiently high level to prevent inhibition by ATP. The K a for K + (about 3 m m ) was not affected by the Mg ++ or ATP concentration, but was increased by decreased levels of d -fructose-1-P. K + decreased the K m values for both ATP and d -fructose-1-P. These kinetics are consistent with Michaelis-Menten theory and suggest a simple model in which the enzyme exists in two active forms, one form predominating in the presence of K + and the other predominating in its absence.