CALORIMETRIC STUDIES ON CALCIUM AND MAGNESIUM BINDING BY TROPONIN-C FROM BULLFROG SKELETAL-MUSCLE

Microcalorimetic titrations were carried out to measure the thennodynamic functions of bullfrog skeletal muscle troponin C (TnC) in the interaction with Ca2, + and Mg2, +, at 25°C and at pH 7.0. Enthalpy titration curves with Ca2, + were composed of three stages both in the presence and in the absence of Mg2, +. The first (0-2 mol Ca2, +/mol TnC) and the third (> 3 mol Ca2, +/mol TnC) stages were exothermic and the second stage (2-3 mol Ca2, +/mol TnC) was endothermic. Mg3,+ affected the first stage to decrease the amount of heat produced but not the second and third stages. The enthalpy titration with Mg2, +, in the absence of Ca2, +, was slightly exothermic initially and then became endothermic beyond 2-3 mol Mg2, +/mol TnC. Absorption of heat was observed throughout the additions of Mg2, + in the presence of 1 mM Ca2, +. The results indicate that bullfrog TnC has two high-affinity Ca2, +-Mg2, + sites, two low-aflinity Ca2, +-specific sites, and two or around two Mg2, +specific sites. Based on the enthalpy and entropy changes, the Ca2, + binding reactions of TnC were classified into three types, indicating thermodynamic variety in the binding sites of the molecule. © 1990 COPYRIGHT, 1990 BY THE JOURNAL OF BIOCHEMISTRY.