Steady-state ATP Hydrolysis Research Articles

The kinetics of activation of myosin ATPase by monovalent cations

The dependence of the kinetic constants K m , v max and k 1 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 Mg 2+ 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 k 1 and the decomposition rate constant k −1 of the enzyme-substrate complex. The increase in KCl or NH 4Cl concentration causes the measured values of K m and v max to increase and that of k 1 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 k 1 is slightly affected by the change in pH. In the Mg 2+ mediated myosin ATPase K m and v max decrease while k 1 increases with increasing Mg 2+ 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 Mg 2+ 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 Mg 2+.