Heat Capacity and Entropy Changes of Calmodulin Induced by Calcium Binding1

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Abstract
Microcalonmetric titrations were carried out in order to analyze the structural changes of calmodulin caused by Ca 2+ -binding. Measurements were made both in the absence and in the presence of Mg 2+ , at 5, 15, and 25°C. Titrations of calmodulin with Ca 2+ in the absence of Mg 2+ showed that the Ca 2+ -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 Ca 2+ -binding were estimated. In both the absence and the presence of Mg 2+ , Ca 2+ -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 Ca 2+ -free calmodulin. Ca 2+ -binding to calmodulin also gives rise to an increase in vibrational entropy, indicating a “softening” of the overall structure.

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