Free-Energy Surfaces of Two Cardiac Thin Filament Conformational Changes during Muscle Contraction

The troponin core is an important regulatory complex in cardiac sarcomeres. Contraction is initiated by a calcium ion binding to cardiac troponin C (cTnC), initiating a conformational shift within the protein, altering its interactions with cardiac troponin I (cTnI). The change in cTnC-cTnI interactions prompts the C-terminal domain of cTnI to dissociate from actin, allowing tropomyosin to reveal myosin-binding sites on actin. Each of the concerted movements in the cardiac thin filament (CTF) is crucial for allowing the contraction of cardiomyocytes, yet little is known about the free energy associated with each transition, which is vital for understanding contraction on a molecular level. Using metadynamics, we calculated the free-energy surface of two transitions in the CTF: cTnC opening in the presence and absence of Ca2+ and cTnI dissociating from actin with both open and closed cTnC. These results not only provide the freeenergy surface of the transitions but will also be shown to determine if the order of transitions in the contraction cycle is important. From our calculations, we found that the calcium ion helps stabilize the open conformation of cTnC and that the C-terminus of cTnI is stabilized by cTnC in the open conformation when dissociating from the actin surface.