Thermodynamics of Ca2+ binding to calmodulin and its tryptic fragments

The binding of Ca2+ to calmodulin and its two tryptic fragments has been studied using microcalorimetry. The binding process is accompanied by the uptake or release of protons, depending on the ionic strength. With no added salt, the total enthalpy change for the binding of four calcium ions to calmodulin is -41 kJ mol-1 but in the presence of 0.15 mM KCl delta Htot is +17 kJ mol-1. The mode of binding of Ca2+ is also completely different with and without added salt. It is also shown that for the C-terminal fragment of calmodulin, TR2C, the drastic reduction in delta Gtot for the binding process on increasing the ionic strength is largely an enthalpic effect. Domain interactions in calmodulin are indicated by the fact that the sum of the enthalpies of calcium binding to the two tryptic fragments is not the same as the total binding enthalpy to calmodulin itself. The binding of Ca2+ to calmodulin has also been studied calorimetrically at different temperatures in the range 21-37 degrees C. delta Cp is large and negative in this interval.