Initial velocity and equilibrium kinetics of myokinase.

The mechanism of myokinase (ATP-AMP phosphotransferase, EC 2.7.4.3) from rabbit muscle has been investigated by studies of equilibrium isotope exchange and initial velocities. The mechanism for the myokinase reaction is random bi bi. Under most conditions, equilibrium isotope exchange between AMP and ADP is faster than that between ATP and ADP. The rate-limiting steps in the reaction include the dissociations of AMP, ADP, and ATP from the enzyme. Myokinase has two binding sites, one for magnesium-chelated nucleotides and the other for unchelated nucleotides. Michaelis constants for the four reactants are presented. AMP and ADP can form inhibitory complexes of the “dead end” type. Adenosine 5’-monosulfate is a competitive inhibitor with respect to AMP.