Structure of the C-domain of Human Cardiac Troponin C in Complex with the Ca²⁺ Sensitizing Drug EMD 57033

Ca²⁺ binding to cardiac troponin C (cTnC) triggers contraction in heart muscle. In heart failure, myofilaments response to Ca²⁺ are often altered and compounds that sensitize the myofilaments to Ca²⁺ possess therapeutic value in this syndrome. One of the most potent and selective Ca²⁺ sensitizers is the thiadiazinone derivative EMD 57033, which increases myocardial contractile function both in vivo and in vitro and interacts with cTnC in vitro. We have determined the NMR structure of the 1:1 complex between Ca²⁺-saturated C-domain of human cTnC (cCTnC) and EMD 57033. Favorable hydrophobic interactions between the drug and the protein position EMD 57033 in the hydrophobic cleft of the protein. The drug molecule is orientated such that the chiral group of EMD 57033 fits deep in the hydrophobic pocket and makes several key contacts with the protein. This stereospecific interaction explains why the (-)-enantiomer of EMD 57033 is inactive. Titrations of the cCTnC·EMD 57033 complex with two regions of cardiac troponin I (cTnI_34-71 and cTnI_128-147) reveal that the drug does not share a common binding epitope with cTnI _128-147 but is completely displaced by cTnI_34-71. These results have important implications for elucidating the mechanism of the Ca ²⁺ sensitizing effect of EMD 57033 in cardiac muscle contraction.