The effects of substrate concentration on the Mg-adenosine triphosphatase activity of myosin

Using myosin, heavy meromyosin, and subfragment-1 the steady state rate of Mg-modified adenosine triphosphatase (Mg-ATPase) was determined over a range of substrate concentrations between 10(-8) M and 5 X 10(-3)M, at 0.5 M and 0.05 M KC1 (pH 7.4 at 20 degrees C). At the substrate concentrations below 10(-5) M, myosin Mg-ATPase was observed to show that two active sites interact, as suggested by the analysis of transient kinetic studies (Walz, F. G., Jr.: J. Theor. Biol. 41, 357-373 (1973)). The increase in the activity at Mg-ATP concentrations higher than 10(-4) M corresponds to the binding of Mg-ATP to myosin sites not responsible for the catalytic action. With heavy meromyosin and subfragment-1, the activity was best expressed by the Michaelis equation. With heavy meromyonsin, the activation at high ATP concentrations is detectable, though not as pronounced as with myosin, but not with subfragment-1.