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∼ 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. Copyright © 1980, Wiley Blackwell. All rights reserved