Effects of trifluoperazine on calcium binding by calmodulin. Heat capacity and entropy changes.
The possible structural changes of the calmodulin-trifluoperazine (TFP) complex caused by Ca2+ binding have been analyzed by microcalorimetric titrations. Titrations of calmodulin with Ca2+ in the presence of 8-fold molar excess TFP have been made both in the absence and presence of Mg2+, at pH 7.0, and at 5, 15, and 25 degrees C. At high concentrations of TFP calmodulin forms a complex with TFP even in the absence of Ca2+. The reaction of the calmodulin-TFP complex with Ca2+ is exothermic, both in the presence and absence of Mg2+. In the presence of Mg2+ the reaction is driven almost entirely by a favorable enthalpy change. The magnitudes of the hydrophobic and internal vibrational contributions to the heat capacity and entropy changes of this complex on Ca2+ binding have been estimated by the empirical method of Sturtevant (Sturtevant, J. M. (1977) Proc. Natl. Acad. Sci. U. S. A. 74, 2236-2240). In the presence of Mg2+, the vibrational as well as hydrophobic entropy is slightly increased in a parallel manner by Ca2+ binding to each of the binding sites. In contrast, when Mg2+ is absent, the hydrophobic entropy gradually increases on Ca2+ binding, but the vibrational entropy decreases. These changes of entropy indicate the assembling of non-polar groups on the surface of the complex and suggest that the overall structure is loosened in the presence of Mg2+, but tightened in the absence of Mg2+.
D002118:Calcium / Q000378:metabolism*; D002147:Calmodulin / Q000378:metabolism*; D002151:Calorimetry; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D008433:Mathematics; D013816:Thermodynamics; D014268:Trifluoperazine / Q000494:pharmacology*
D002147:Calmodulin; D014268:Trifluoperazine; D008274:Magnesium; D002118:Calcium