Temperature sensitivity of myosin and actomyosin.

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The thermal denaturation of myosin and actomyosin was studied by active site analysis (enzymatic activity) and measurements were related to overall conformational changes (viscosity) over the temperature range 19–65 °C. The role of sulfhydryl (SH) groups on the temperature-induced denaturation of actomyosin was investigated. The temperature of maximum change in the overall conformation (the melting temperature, TM) was unaffected by the binding of F-actin to myosin. For both myosin and actomyosin the TM was 43 ± 2 °C. However, the range of temperature over which large conformational changes were observed was affected by the binding of F-actin to myosin. For myosin and dissociated actomyosin, changes were observed between 37 and 50 °C, while for actomyosin large changes were observed between 20 and 50 °C. With actomyosin there was an irreversible increase in titratable sulfhydryl groups from 19 to 60 °C. Temperature effects on the calcium-activated ATPase were studied. The temperature of maximum enzymatic activity for actomyosin was 45 ± 2 °C, which corresponds to the TM and the temperature at which increases in SH content were apparent. However, for myosin the temperature of maximum enzymatic activity was 33 °C, considerably below the TM. Overall conformational changes were reversible below the TM, while changes in enzymatic activity were reversible below the temperature of maximum enzymatic activity. Actin offers considerable protection to the temperature inactivation of the active site of myosin even though the F-actin–myosin complex is very highly dissociated in the presence of ATP. However, there is no significant stabilization of myosin by F-actin in terms of the temperature sensitivity of the overall conformation.