Abstract
The heat production for the reaction steps of ATP hydrolysis by heavy meromyosin has been reexamined by using a reaction-type calorimeter. So far, the values reported for the heat production have varied substantially among research groups. The present results obtained in 0.3 M KCl, 10 mM MgCl2, and 20 mM Tris-HCl at pH 8.0 and 20 degrees C show that (1) the binding and splitting of ATP on the myosin head are moderately exothermic (delta H = -23 kJ mol-1), (2) the decomposition of the M.ADP.Pi intermediate complex to M.ADP + Pi is strongly exothermic (delta H = -66 kJ mol-1), and (3) the dissociation of ADP from the myosin head is strongly endothermic (delta H = +60 kJ mol-1). These results agree with our previous findings that the decomposition of M.ADP.Pi as well as the binding of ADP to heavy meromyosin is strongly exothermic [Yamada, T., Shimizu, H., & Suga, H. (1973) Bio-chim. Biophys, Acta 305, 642-653].
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