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 mus cle. 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 contrac tile function both in vivo and in vitro and interacts with cTnC in vitro. W e have determined the NMR structure of the 1:1 complex between Ca2+-saturat ed C-domain of human cTnC (cCTnC) and EMD 57033. Favorable hydrophobic inte ractions between the drug and the protein position EMD 57033 in the hydroph obic cleft of the protein. The drug molecule is orientated such that the ch iral group of EMD 57033 fits deep in the hydrophobic pocket and makes sever al key contacts with the protein. This stereospecific interaction explains why the (-)-enantiomer of EMD 57033 is inactive. Titrations of the cCTnC EM D 57033 complex with two regions of cardiac troponin I (CTnI(34-71) and cTn I(128-147)) reveal that the drug does not share a common binding epitope wi th cTnI(128-147) but is completely displaced by cTnI(34-71). These results have important implications for elucidating the mechanism of the Ca2+ sensi tizing effect of EMD 57033 in cardiac muscle contraction.