Structure and Function of the Two Heads of the Myosin Molecule1

Abstract

ATPase activities [EC 3.6.1.3] of myofibrils were measured in 50 mM KC1 and 2 mM MgC1₂ at pH 7.8 in the presence of various concentrations of ATP and free Ca²⁺; ions, using pyruvate kinase [EC 2.7.1.40] and phosphoenolpyruvate as an ATP-regenerating system. Shortening rates of myofibrils were determined by measuring the average lengths of sarcomeres after stopping the contraction by adding 15 mM EDTA. Isometric tensions and ATPase activities of glycerol-treated muscle fibers fixed at the rest length were measured in 50 mM KC1 and 2 mM MgC1₂ at pH 7.8 in the presence of various concentrations of ATP and free Ca²⁺ ions at various temperatures, using creatine kinase [EC 2.7.3.2] and creatine phosphate to regenerate ATP. The following results were obtained: We concluded from the results 7–10 that the intermediates of NTPase for isometric tension development are M₂NTP⇌M_P^NDP in rapid equilibrium, and that the tension development does not depend on the turnover rate of these intermediates.