Calcium binding by troponin-C. a proton magnetic resonance study

Levine, B.A.; Coffman, D.M.; Thornton, J.M.

Journal of Molecular Biology 115(4): 743-760


ISSN/ISBN: 0022-2836
PMID: 22761
DOI: 10.1016/0022-2836(77)90113-9
Accession: 068518279

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The conformational changes induced by the binding of Ca(II) to rabbit skeletal muscle troponin-C (TNC) have been followed by PMR spectroscopy. Ca(II)-free TNC (apo-TNC) contains definite ordered regions. Ca(II) titration of the high affinity sites causes a large folding of the backbone, some of which involves refolding of an ordered region(s) and changes in several side-chains, e.g., Glu, Asp and Phe. Titration of the low affinity sites does not alter the backbone, but leads to changes among hydrophobic side-chains (1 or more Val, Leu, Ile; 2 or more Phe, Glu and Asp) that define an ordered region(s) of apo-TNC. The rate constants for the conformation changes of the low and high affinity sites are approximately 10 s-1 and < 20 s-1, respectively. Final stages of the titration include a downfield shifted methyl group (likely Ile) and a Phe residue. The thermal stabilities of apo-TNC, TNC .cntdot. Ca2(II) and native TNC were compared. Ca(II) binding by the 2 high affinity sites directs and stabilizes much of the structure. The role of the changes of the low affinity sites, which may activate contraction, are briefly discussed.