A calorimetric study of Ca 2+ binding to two major isotypes of bullfrog parvalbumin

Abstract

Microcalorimetric titrations of the two major isotypes of parvalbumin (PA1 and PA2) from bullfrog skeletal muscle with Ca 2+ in the presence and absence of Mg 2+ 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 (Δ H) of Ca 2+ binding are −33.2 (PA1) and −16.3 kJ mol site (PA2), and the entropy changes (Δ S) are 28 (PA1) and 76 J mol per K (PA2) in the absence of Mg 2+. When 5 mM Mg 2+ is present, the enthalpy change of PA2 (−26.7 kJ mol ) is about twice as large as that in the absence of Mg 2+ whereas that of PA1 (−34.6 kJ mol ) is about the same. The entropy changes are 8 (PA1) and 29 J mol per K (PA2). Both enthalpy and entropy changes are favorable for the Ca 2+-binding reactions of PA1 and PA2 irrespective of the presence of Mg 2+.