HEAT CAPACITY CHANGES IN CALCIUM AND MAGNESIUM BINDING TO TROPONIN

Publisher Summary This chapter elaborates a study analyzing heat capacity changes in calcium and magnesium binding to troponin. A microcalorimetric titration with Ca 2+ or Mg 2+ was performed at pH 7 and at different temperatures of 5, 15 and 25°C on rabbit skeletal muscle troponin from which almost all Ca and Mg had been removed by passing a column of chelating resin. The enthalpy changes associated with the binding of both the first and the second 1 mole Ca 2+ per mole metal-free troponin in the absence of Mg show large negative temperature dependence. Thus the heat capacity change (Δ C p °), obtained from the temperature dependence of ΔC° has a large negative value of about −1 kJ mol −1 K −1, which is characteristic of the hydrophobic effect. The binding of the first 1 mole of Mg also shows the similar negative Δ C p °. For the further binding of Ca 2+ Δ C p ° becomes smaller but is substantially negative. The negative Δ C p ° associated with the binding of Ca 2+ to the two high-affinity sites remains substantial in the presence of 1 mM Mg 2+ .