In order to elucidate the structural changes of bullfrog skeletal muscle troponin C (TnC) caused by Ca2+ binding, microcalorimetric titrations were performed in both the absence and presence of Mg2+, at pH 7.0, and at 5 degrees, 15 degrees, and 25 degrees C. The results indicate that, in the absence of Mg2+, Ca2+ binding to sites 1 and 2 gives rise to a large hydrophobic effect, a sequestering of nonpolar groups on the surface of molecule to the interior, and a tightening of the molecular structure as a whole. In contrast, hydrophobic groups were exposed from the interior to the surface of molecule and the molecular mobility was increased, upon Ca2+ binding to site 3. Ca2+ binding to site 4 induced much less change in the conformation of TnC molecule than that to the other sites. The presence of 5 mM Mg2+ dramatically reduced the magnitude of the conformational changes in TnC on Ca2+ binding to sites 1 and 2. On the other hand, the conformational changes by Ca2+ binding to sites 3 and 4 were not affected by Mg2+. Ca2+ binding to site 3 of TnC, thus, causes the conformational changes distinctly different from those upon Ca2+ binding to the other sites. The conformational changes upon Ca2+ binding to site 3 of TnC are also clearly distinct from the Ca2(+)-induced conformational changes of parvalbumins, but are similar to those of calmodulin. Site 3 of TnC may, therefore, be the regulatory site in muscle contraction, and the exposure or generation of hydrophobic groups on the surface of molecule as well as the increase in molecular mobility may be the common characteristics of the regulation by Ca2(+)-binding proteins.