NMR studies of Ca2+ binding to the regulatory domains of cardiac and E41A skeletal muscle troponin C reveal the importance of site I to energetics of the induced structural changes.

Ca2+ binding to the N-domain of skeletal muscle troponin C (sNTnC) induces an “opening” of the structure [Gagne, S. M., et al. (1995) Nat. Struct. Biol. 2, 784−789], which is typical of Ca2+-regulatory proteins. However, the recent structures of the E41A mutant of skeletal troponin C (E41A sNTnC) [Gagne, S. M., et al. (1997) Biochemistry 36, 4386−4392] and of cardiac muscle troponin C (cNTnC) [Sia, S. K., et al. (1997) J. Biol. Chem. 272, 18216−18221] reveal that both of these proteins remain essentially in the “closed” conformation in their Ca2+-saturated states. Both of these proteins are modified in Ca2+-binding site I, albeit differently, suggesting a critical role for this region in the coupling of Ca2+ binding to the induced structural change. To understand the mechanism and the energetics involved in the Ca2+-induced structural transition, Ca2+ binding to E41A sNTnC and to cNTnC have been investigated by using one-dimensional 1H and two-dimensional {1H,15N}-HSQC NMR spectroscopy. Monitoring the che...