The Pre-steady State of the Myosin-Adenosine Triphosphate System

Abstract

After mixing ATP with myosin the reaction was stopped by adding 5 per cent trichloroacetic acid, and the amount of P₁ liberated was measured. The change in hydrogen ion concentration was measured by a stopped-flow method at pH 8.3, in which the hydrolysis of ATP by myosin yielded 1 mole of hydrogen ion per mole of ATP hydrolyzed. The following results were obtained from this experiment. 1. At ATP concentrations lower than 1 mole per 4 × 10⁵ g of myosin, both the initial rapid P₁- and hydrogen ion-liberations followed mono-molecular kinetics, and their rates were equal. The rates of P₁- and hydrogen ion-liberation were independent of the ATP concentration when the concentration was lower than 1 mole per 4 ×10⁵ g of myosin, but they increased with increasing ATP concentration at concentrations above this. 2. The amount of initial rapid P₁-liberation increased linearly with the ATP concentration until the amount of added ATP reached about 1 mole per 4 ×10⁵ g of myosin. At higher ATP concentrations, the amount was constant at about 1 mole per 4 ×10⁵g of myosin. 3. At ATP concentrations lower than about 1 mole per 4 ×10⁵ g of myosin, the amount of initial rapid hydrogen ion-liberation also increased with increasing ATP concentration, and reached about 1 mole per 4 × 10⁵ g of myosin at ATP concentrations higher than the stoichiometric amount. 4. The pH-dependence of the rate of initial rapid P₁-liberation was different from that of the rate at the steady state: the initial rate increased with increasing pH and the half maximum value was obtained at pH 6.4. 5. The activation entropy of the initial rapid P₁- and hydrogen ion-liberations was found to be +21.6 cal per mole per deg., while the activation entropy of P₁-liberation at the steady state was −24.8 cal per mole per deg. 6. The rate of the actomyosin type ATPase [EC 3.6. 1.3] reaction increased with increasing pH and the half maximum value was obtained at pH 6.1. The activation entropy of the actomyosin type ATPase reaction was +45.5 cal per mole per deg.