Calcium and peptide binding to folded and unfolded conformations of cardiac troponin C. Electrospray and Fourier transform ion cyclotron resonance mass spectrometry

The binding of Ca2+ and of a peptide (N-TnI) to human cardiac troponin C (TnC) and its isolated N- and C-terminal domains has been characterized by electrospray ionization Fourier transform ion cyclotron resonance (ESI FT-ICR) mass spectrometry. The peptide N-TnI corresponds to residues 1-29 of the cardiac-specific N-terminal extension of human cardiac troponin I (TnI). The binding of Ca2+ to intact TnC in the absence of the peptide was found to take a bimodal form with preferred stoichiometries of 1 : 1 TnC : Ca2+ and 1 : 3 TnC : Ca2+. It is concluded that TnC existed in two conformational isomers that had different binding affinities for Ca2+: the binding of 3 Ca2+ was characteristic of a folded conformation (TnC(A)) and the binding of 1 Ca2+ was characteristic of a partially unfolded conformation (TnC(B)). Both of these conformations contributed to the 8+ (and other) charge states of TnC, and were distinguished on the basis of their different Ca2+-binding affinities and not on the basis of the charge state. In the presence of the peptide, a complex with 1 : 1 : 1 TnC : peptide : Ca2+ stoichiometry was formed.