Effects of Trifluoperazine on Calcium Binding by Calmodulin

Microcalorimetric titrations of calmodulin with Ca" and trifluoperazine (TFP) at various molar ratios have been carried out at 25 "C and at pH 7.0. Ca2+ binding to calmodulin produces heat ( A H ) in the presence of TFP, while heat is absorbed in the absence of TFP. The total heat produced by Ca2+ binding to all four sites is increased at increasing TFP-to-calmodulin ratios, attaining a plateau at about 7. These results indicate that at the higher ratios, the enthalpy changes ( A l l ) associated with Ca2+ binding are affected by TFP molecules bound at both highand low-affinity sites. In addition, the Ca2+ binding reaction of the calmodulin-TFP complex is driven solely by a favorable enthalpy change of -27 kJ/mol of site; the entropy change (As) is -35 J/ mol/K. These thermodynamic changes are opposite to those for TFP-free calmodulin and distinctly different from other Ca2+ binding proteins such as skeletal and cardiac troponin C and parvalbumin, where the reaction is driven by favorable changes of entropy as well as enthalpy.