title: Sodium-23 nuclear magnetic resonance as an indicator of sodium binding to troponin C and tryptic fragments, in relation to calcium content and attendant conformational changes.
abstract: The relaxation rate enhancements of the 23Na nuclei for NaHCO3 solutions of troponin C and its tryptic peptides TR-1 and TR-2 indicate true binding of Na+ ions to these biomolecules. The low-affinity sites I and II of TR-1 and troponin C are the sites of competitive Na+/Ca2+ binding, below one calcium ion per molecule, with log KNa approximately 2. At low calcium content Na+ ions bind to TR-2 and to troponin C non-competitively with Ca2+ ions; binding of Ca2+ ions to the high-affinity sites III and IV allosterically affects the binding of the Na+ ions: even when sites I and II, located on TR-1 or sites I, II, III, IV of troponin C, are saturated with Ca2+ ions, Na+ ions continue to bind weakly at secondary binding sites.
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D009124:Muscle Proteins / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism; D011487:Protein Conformation; D011817:Rabbits; D012964:Sodium / Q000378:metabolism*; D014336:Troponin / Q000378:metabolism*; D014357:Trypsin
chemical_list: D009124:Muscle Proteins; D010446:Peptide Fragments; D014336:Troponin; D012964:Sodium; D014357:Trypsin; D002118:Calcium