A calorimetric study of Ca2+- and Mg2+-binding by calmodulin

Enthalpy titrations of calmodulin with Ca2+ (or Mg2+) in the presence and the absence of Mg2+ (or Ca2+) have been carried out by microcalorimetry. The binding reactions of both Ca2+ and Mg2+ to calmodulin are endothermic, indicating that the large entropy change gives rise to the strong binding of Ca2+ to calmodulin. The results also indicated that all the four binding sites are Ca2+-Mg2+ sites. These results are markedly different from the findings for troponin C.