Heat Capacity and Entropy Changes of Calmodulin Induced by Calcium Binding

Abstract

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°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.