Conformations of the regulatory domain of cardiac troponin C examined by residual dipolar couplings

Conformations of the regulatory domain of cardiac troponin C (cNTnC) were studied by means of residual dipolar couplings measured from samples dissolved in dilute liquid crystals. Changes in the main chain HN residual dipolar couplings revealed a conformational change in cNTnC due to the complexation with the second binding region (amino acids 148–163) of cardiac troponin I (cTnI). Formation of the complex is accompanied with a molecular realignment in the liquid crystal. The residual dipolar couplings measured for apo‐cNTnC and the complex with TnI were in agreement with the values computed from the corresponding closed and open solution structures, whereas for the calcium‐loaded conformation the correlation and quality factor were only modest. Ca²⁺‐cNTnC may be subject to conformational exchange. The data support the model that cardiac troponin C functions as a calcium‐dependent open–closed switch, such as the skeletal troponin C.