Nuclear magnetic resonance study on rabbit skeletal troponin C: calcium-induced conformational change

Rabbit skeletal muscle troponin C (TnC) was investigated by means of 1H NMR in the presence of dithiothreitol that prevents dimerization of the protein. Two-dimensional (2D) 1H NMR spectra were observed in order to assign resonances to specific amino acids. One-dimensional 1H NMR spectra were observed as a function of Ca2+ concentration. The Ca2+-induced spectral change is categorized into two types: type 1 corresponds to the conformational change of the C-terminal-half domain (Ca2+ high-affinity sites) and type 2 to that of the N-terminal-half domain (Ca2+ low-affinity sites). From the 2D NMR spectra and Ca2+ titration data, it was suggested that (1) amide protons of Gly-108, Ile-110, Gly-144, and Ile-146 are hydrogen-bonded when the C-terminal-half domain binds 2 mol of Ca2+ and (2) hydrogen bonds of Gly-108, Ile-110, Gly-144, and Ile-146 are destroyed or weakened when the C-terminal-half domain releases 2 mol of Ca2+. Nuclear Overhauser enhancement difference spectra as well as the Ca2+ titration data suggested that a hydrophobic cluster is formed in the C-terminal-half domain when the C-terminal-half domain binds 2 mol of Ca2+. A hydrophobic cluster exists in the N-terminal-half domain without regard to Ca2+ binding to the N-terminal-half domain. The spectra of Tyr-10 showed both types of spectral change during the Ca2+ titration. The results suggested that Tyr-10 of apo-TnC interacts with the C-terminal-half domain.