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 degrees C and pH 7.0. The observed enthalpy titration curves were analyzed by the least-squares method. The measured enthalpy changes (delta H) of Ca2+ binding are -33.2 (PA1) and -16.3 kJ/mol site (PA2), and the entropy changes (delta S) are 28 (PA1) and 76 j/mol per K (PA2) in the absence of Mg2+. When 5 mM Mg2+ is present, the enthalpy change of PA2 (-26.7 kJ/mol) is about twice as large as that in the absence of Mg2+, 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 Ca2+-binding reactions of PA1 and PA2 irrespective of the presence of Mg2+.