Calcium binding to troponin C as a primary step of the regulation of contraction. A microcalorimetric approach.

Microcalorimetric titration studies of EF-hand Ca-binding proteins (troponin C, calmodulin and parvalbumins) resulted in the notion that Ca binding to the "active" Ca site, which is involved in the regulation of contraction, induces a characteristic anomalous enthalpy and heat-capacity changes indicating an exposure of hydrophobic residues to the solvent, which enables the proteins to interact with their targets. There is a good agreement between the results of the calorimetric and the structural studies in frog and chicken skeletal troponin C. In both species one of the N-terminal low-affinity Ca-sites is the "active" Ca site regulating muscle contraction. The results from calorimetry have shown, however, that the situation in rabbit skeletal troponin C may be more complex. Moreover, in both calorimetric and structural studies, the situation in cardiac troponin C is quite different. These results suggest the need for further studies to elucidate the mechanism of regulation by Ca. These characteristic changes do not occur in Ca-buffering proteins.