Solution structure and main chain dynamics of the regulatory domain (residues 1-91) of human cardiac troponin C

The three-dimensional structure of calcium-loaded regulatory, i.e. N-terminal, domain (1-91) of human cardiac troponin C (cNTnC) was determined by NMR in water/trifluoroethanol (91:9 v/v) solution. The single-calcium-loaded cardiac regulatory domain is in a "closed"? conformation with comparatively little exposed hydrophobic surface. Difference distance matrices computed from the families of Ca2+-cNTnC, the apo and two-calcium forms of the skeletal TnC (sNTnC) structures reveal similar relative orientations for the N, A, and D helices, The B and C helices are closer to the NAD framework in Ca2+-cNTnC and in apo-sNTnC than in 2 . Ca2+-sNTnC, However, there is an indication of a conformational exchange based on broad N-15 resonances for several amino acids measured at several temperatures. A majority of the amides in the alpha-helices and in the calcium binding loop exhibit very fast motions with comparatively small amplitudes according to the Lipari-Szabo model. A few residues at the N and C termini are flexible. Data were recorded from nonlabeled and N-15-labeled samples, and backbone dynamics was investigated by N-15 T-1,T- T-2, and heteronuclear nuclear Overhauser effect as well as by relaxation interference measurements.