Observation of the 43Ca NMR signals from Ca2+ ions bound to calmodulin, parvalbumin, and troponin C.

Through the combined use of isotopically enriched 43Ca2+, high magnetic field, Fourier transform techniques, and a high-performance probe, the 43Ca NMR signals from ions bound to three proteins have been observed. These proteins are calmodulin, parvalbumin, and troponin C. Both longitudinal (1/T1) and transverse (1/T2) relaxation rates were measured for protein-bound 43Ca2+, and the quadrupole coupling constant and correlation time were obtained from these data. The calculated correlation times are in good agreement with the rotational correlation time for the entire protein molecules, which indicates that the Ca2+-binding sites are relatively rigid.