The kinetics of activation of myosin ATPase by monovalent cations

The dependence of the kinetic constants K_m, v_max and k₁ of myosin ATPase on the species and concentration of monovalent cations and on pH was analyzed and results were compared with similar data obtained on Mg²⁺ mediated myosin ATPase. A linear relationship between K_m and v_max was observed when myosin ATPase was activated by equal concentration of different alkali cations. This observation was utilized for the calculation of the dissociation constant K_s, the formation rate constant k₁ and the decomposition rate constant k_-1 of the enzyme-substrate complex. The increase in KCl or NH₄Cl concentration causes the measured values of K_m and v_max to increase and that of k₁ to decrease in the monovalent cation activated ATPase. K_m and v_max vary with pH between 6 and 9 in the same way. The value of k₁ is slightly affected by the change in pH. In the Mg²⁺ mediated myosin ATPase K_m and v_max decrease while k₁ increases with increasing Mg²⁺ concentrations. The observations indicate that the monovalent cations enhance the rate of the steady state hydrolysis of ATP and reduce the stability of the myosin-ATP complex while Mg²⁺ inhibits the hydrolysis and improves the stability of this complex. This strongly supports the view that the activation of myosin ATPase with monovalent cations, including the low degree activation with Na⁺ and Li⁺, is essentially different from that with Mg²⁺.