Interaction between myosin and its substrates

Abstract

Low concentrations of p-chloromercuribenzoate (PCMB), Zn ++, or Cd ++ increase the ATPase and CTPase activities of myosin B but inhibit ITPase and GTPase activities. Higher concentrations of these modifiers inhibit the activities of all the nucleoside triphosphates (NTP). Dialysis against cysteine ethyl ester (CEE) increases the ATPase and CTPase activities of myosin but decreases the ITPase and GTPase activities. The temperature dependence of the maximum velocities of hydrolysis and of the Michaelis-Menten constants also differentiate between ATP-CTP and ITP-GTP. ATP and CTP confer considerable protection against thermal denaturation of myosin, while ITP and GTP confer only a small protection. The shape of the PCMB vs. NTPase activity curve is invariant to the substitution of Mg ++ for Ca ++ in the presence of 0.3 M KCl or to the substitution of Na +, Li +, or NH 4 + for K + in the presence of Ca ++. PCMB inactivates myosin at 37 ° at a rate which is highly sensitive to the PCMB concentration. The inactivated myosin can be restored to activity by dialysis against CEE. These and other observations are interpreted in terms of a conformation change at the active site of myosin.