Reversible Covalent Binding to Cardiac Troponin C by the Ca-Sensitizer Levosimendan

The binding of Ca to cardiac troponin C (cTnC) triggers contraction in heart muscle. In the diseased heart, the myocardium is often desensitized to Ca, which leads to impaired contractility. Therefore, compounds that sensitize cardiac muscle to Ca (Ca-sensitizers) have therapeutic promise. The only Ca-sensitizer used regularly in clinical settings is levosimendan. While the primary target of levosimendan is thought to be cTnC, the molecular details of this interaction are not well understood. In this study, we used mass spectrometry, computational chemistry, and nuclear magnetic resonance spectroscopy to demonstrate that levosimendan reacts specifically with cysteine 84 of cTnC to form a reversible thioimidate bond. We also showed that levosimendan only reacts with the active, Ca-bound conformation of cTnC. Finally, we propose a structural model of levosimendan bound to cTnC, which suggests that the Ca-sensitizing function of levosimendan is due to stabilization of the Ca-bound conformation of cTnC.