Structural Change of the Troponin C Molecule upon Ca²⁺ Binding Measured in Solution by the X-Ray Scattering Technique

Abstract

The small-angle X-ray scattering technique was used to characterize the overall structural change as well as the state of aggregation of troponin C upon binding various amount of Ca²⁺ ions: in the Ca²⁺-free state and at pCa 6.5 and 4.0. Under these conditions, the forward scattering intensities of troponin C are not much different from each other: i.e., they coincide within 4%. From these intensities, the Ca²⁺-facilitated dimerization of troponin C was not verified, and no appreciable aggregation of troponin C molecules was detected below pCa 4.0. Thus, the small-angle X-ray scattering profiles from troponin C solutions were analyzed assuming a monomeric molecule. The radii of gyration of troponin C were 27.8±0.3 Å, 23.8±0.2 Å, and 22.6±0.1 Å for the Ca²⁺-free state and at pCa 6.5 and 4.0, respectively. The maximum dimension of the molecule decreases from 111 to 98 Å with increasing Ca²⁺ concentration. These results indicate that the troponin C molecule shrinks remarkably as Ca²⁺ ions bind to the high affinity sites of the molecule. Ca²⁺ binding to the low affinity sites, on the other hand, leads to a less pronounced change. Following the interpretation of scattering from the dumbbell-shaped structure (Fujisawa, T., Ueki, T., Inoko, Y., & Kataoka, M. [1987] J. Appl. Cryst. 20, 349-355), the two domains of the molecule move closer to each other. The distance between the centers of the two domains decreases from 46 to 35 Å. The conformation of the two domains also changes upon Ca²⁺ binding: they become more compact, especially in the change from the Ca²⁺-free state to pCa 6.5, in good agreement with an increase of CD and other spectroscopic measurements.