The relaxation rate enhancements of the 23Na nuclei for NaHCO3 solutions of calmodulin and its tryptic peptides TR‐1 and TR‐2 indicate true binding of Na+ ions to these biomolecules. With both TR‐1 ...and TR‐2, Na+ binding occurs in competition with Ca2+ and Mg2+ binding: log KNa∼ 2, log KMg∼ 4, log KCa∼ 6 for TR‐1; and log KNa∼ 2, log KMg∼ 3, log KCa∼ 5 for TR‐2. All the binding constants are systematically greater for binding to TR‐1, as compared to TR‐2. There is also an increase in KNa for TR‐1 of calmodulin as compared to the homologous tryptic fragment of troponin C. The increased binding is identified tentatively with site I of calmodulin. The binding constants KNa, KCa and KMg of calmodulin appear to be finely tuned to the intracellular concentrations of these cations
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∼ 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.
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