A calorimetric study of Ca2+ binding by wheat germ calmodulin. Regulatory steps driven by entropy

A hypothesis has been proposed for Ca(2+)-binding proteins that a regulatory Ca2+ binding step is endothermic and is driven solely by entropy in the absence of Mg2+ (Imaizumi, M., Tanokura, M., and Yamada, K. (1987) J. Biol. Chem. 262, 7963-7966). To confirm this idea, microcalorimetric titrations of wheat germ calmodulin, with Ca2+ in the presence and absence of Mg2+ and with Mg2+ in the absence of Ca2+, have been carried out at 25 degrees C and at pH 7.0. The results indicate that the four binding sites in each molecule are thermodynamically equivalent to one another for both Ca2+ and Mg2+ binding and that the reaction is endothermic for Ca2+ binding in the absence of Mg2+. As compared with bovine brain calmodulin on Ca2+ binding in the absence of Mg2+, Ca2+ binding in both proteins is driven solely by a large favorable entropy change despite unfavorable enthalpy change. Therefore, the above idea seems plausible.