Kinetic properties of arginine phosphokinase from honeybees, Apis mellifera L. (Hymenoptera, Apidae)

The kinetic properties of honeybee arginine phosphokinase (APK), which catalyzes the reaction: Arginine phosphate + ADP + H⁺ ? arginine + ATP, have been studied.In the direction of ATP synthesis, the pH optimum was around pH 7.2 and the activation energy over the range 18–44 °C was about 10,500 cal/mole. The optimum ratio of Mg²⁺:ADP was about 4:1.In the direction of arginine phosphate (AP) synthesis, the enzyme had a pH optimum around pH 8.3. The energy of activation for the reaction over the range 22–39 °C was about 7500 cal/mole. The optimum ratio of Mg²⁺:ATP was about 1:1.The initial velocities of the reactions in the direction of ATP and AP synthesis were measured at varying concentrations of one substrate while the concentration of the other substrate was held constant at several levels. The double reciprocal plots of the data obtained yielded a series of intersecting lines, indicating that the enzyme has a sequential mechanism. Radioisotope exchange experiment showed that arginine phosphokinase did not catalyze ATP ? ADP exchange in the absence of arginine. Product inhibition studies showed that arginine was competitive with AP and noncompetitive with ADP; whereas ATP was competitive with ADP and noncompetitive with arginine. The results from initial velocity, radioisotope exchange, and product inhibition studies suggested that the enzyme has a rapid equilibrium, random mechanism.