Nucleotide-induced changes in the heat capacity of beef cardiac myosin

The heat of reaction of ATP with beef cardiac myosin has been determined in a flow microcalorimeter at two different temperatures. The reaction heat obtained by extrapolation to infinite flow rate is interpreted to be the heat of formation of the steady-state set of myosin-nucleotide complexes. The large temperature dependence of this heat, an apparent change in heat capacity, could be caused by an isomerization between two myosin conformations. The enthalpies and heat capacities of binding of ADP, AMP, and pyrophosphate have also been measured and are discussed in terms of this model.