Heat capacity and entropy changes of calmodulin induced by calcium binding

Microcalorimetric titrations were carried out in order to analyze the structural changes of calmodulin caused by Ca2+-binding. Measurements were made both in the absence and in the presence of Mg2+, at 5, 15, and 25 degrees C. Titrations of calmodulin with Ca2+ in the absence of Mg2+ showed that the Ca2+-binding reaction is endothermic and thus is driven solely by the large entropy change. Following the method of Sturtevant (1977), the magnitudes of the hydrophobic and intramolecular vibrational contributions to the heat capacity and entropy changes of calmodulin on Ca2+-binding were estimated. In both the absence and the presence of Mg2+, Ca2+-binding to every site of calmodulin gives rise to an increase in hydrophobic entropy and thus to an "assembling" of nonpolar groups, which are scattered on the surface of the molecule in Ca2+-free calmodulin. Ca2+-binding to calmodulin also gives rise to an increase in vibrational entropy, indicating a "softening" of the overall structure.