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

AbstractMicrocalorimetric 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 kJmol site (PA2), and the entropy changes (ΔS) are 28 (PA1) and 76 Jmol per K (PA2) in the absence of Mg2+. When 5 mM Mg2+ is present, the enthalpy change of PA2 (−26.7 kJmol) is about twice as large as that in the absence of Mg2+ whereas that of PA1 (−34.6 kJmol) is about the same. The entropy changes are 8 (PA1) and 29 Jmol 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+