Structural Change of Troponin C Molecule and Its Domains upon Ca2+Binding in the Presence of Mg2+Ions Measured by a Solution X-Ray Scattering Technique

A small-angle X-ray scattering study on troponin C showed that two domains of the molecule move closer to each other and the molecule shrinks along its long axis upon Ca2+binding in the absence of Mg2+ions (Fujisawa, T., Ueki, T., & Iida S. (1988) J. Biochem. 105, 377–383). When Mg2+ions bind to troponin-C, the radius of gyration changes from 27.8 to 24.3 A and the average radius of gyration of the two domains is estimated to be 15.1 A. These radii indicate that the distance between the centers of the two domains is 38.1 A. Such a change is analogous to the previous result for troponin C with two Ca2+ions bound at the high-affinity sites. Thus, the structural behavior of troponin C molecule is essentially the same when Ca2+/Mg2*ions bind to its high-affinity sites. On the other hand, the effect of Ca2+binding to the low-affinity sites in the presence of Mg2+ions is quite different from the previous result. The binding of Ca2+ions causes a dimerization of troponin C molecules with an apparent constant of 511 M-1. Such a characteristic behavior, implying the occurrence of a surface property change, may be related to the physiological role of troponin C molecule in the muscle. The scattering experiments on the tryptic fragments of troponin C also had interesting and important results: the C-domain shrinks, with the radius of gyration changing from 17.0 to 14.9 A while the N-domain swells from 13.9 to 15.0 A upon Ca2+binding. Such an opposite change is consistent with the results of circular dichroism and spectroscopic studies of the domains.