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Microcalorimetric studies of the interaction of normal and thyrotoxic cardiac myosins with ATP and ADP.

The heat production of the interaction of normal and thyrotoxic cardiac myosins with ATP and ADP was studied in a microcalorimeter. As compared to the normal protein the thyrotoxic cardiac myosin gave a larger negative enthalpy, entropy, and heat capacity change. These results indicate that the enthalpy, entropy, and heat capacity differences between the two conformational states of myosin are larger in the thyrotoxic cardiac muscle than in the normal control. The ATP induced transition between the conformational states of the cardiac myosins appears to be connected to the powerstroke of the contractile cycle. Therefore the observed thermodynamic differences between the normal and thyrotoxic cardiac myosins may be related to the active site chemistries of these isomyosins and portend the functional abnormality of the thyrotoxic cardiac muscle.