A calorimetric study of Ca²⁺ binding to two major isotypes of bullfrog parvalbumin

Microcalorimetric titrations of the two major isotypes of parvalbumin (PA1 and PA2) from bullfrog skeletal muscle with Ca²⁺ in the presence and absence of Mg²⁺ have been carried out at 25°C and pH 7.0. The observed enthalpy titration curves were analyzed by the least-squares method. The measured enthalpy changes (ΔH) of Ca²⁺ binding are −33.2 (PA1) and −16.3 site (PA2), and the entropy changes (ΔS) are 28 (PA1) and 76 per K (PA2) in the absence of Mg²⁺. When 5 mM Mg²⁺ is present, the enthalpy change of PA2 (−26.7 ) is about twice as large as that in the absence of Mg²⁺ whereas that of PA1 (−34.6 ) is about the same. The entropy changes are 8 (PA1) and 29 per K (PA2). Both enthalpy and entropy changes are favorable for the Ca²⁺-binding reactions of PA1 and PA2 irrespective of the presence of Mg²⁺.