Troponin structure: its modulation by Ca(2+) and phosphorylation studied by molecular dynamics simulations

Area of Science:

• Biophysics
• Molecular Biology
• Computational Biology

Background:

• The crystal structure of human cardiac troponin (cTn) in its Ca(2+) activated state lacks key segments, including the N-terminus of the cTn inhibitory subunit (cTnI).
• Understanding cTn structure and dynamics is crucial for cardiac function.
• Previous computational studies may have been limited by simulation length and model completeness.

Purpose of the Study:

• To investigate the structure and dynamics of human cardiac troponin (cTn) using all-atom molecular dynamics (MD) simulations.
• To compare the unphosphorylated and bis-phosphorylated states of cTn, focusing on Ser23/Ser24 of cTnI.
• To identify the impact of including previously missing segments, like the cTnT C-terminus, on cTn dynamics.

Main Methods:

• All-atom molecular dynamics (MD) simulations.
• Microsecond-scale simulations of wild-type (WT) cTn and bis-phosphorylated (SP23/SP24) cTn.
• Utilized a 419 amino acid cTn model including human cTnC, cTnI, and cTnT sequences, incorporating residues absent in the crystal structure.

Main Results:

• Extended MD simulations (microsecond scale) with adequate water box size (≥25 Å) are necessary for capturing conformational shifts in both native and bis-phosphorylated cTn.
• Inclusion of the cTnT C-terminus alters cTnC-cTnI interactions, impacting overall cTn dynamics.
• Phosphorylation at Ser23/Ser24 of cTnI does not significantly increase protein fluctuations or alter protein-protein interaction profiles.

Conclusions:

• Phosphorylation of cardiac troponin does not enhance its dynamic fluctuations.
• Phosphorylation might lead to Ca(2+) loss by destabilizing Ca(2+) coordination in troponin C's EF hand II.
• Accurate modeling of cTn requires sufficient simulation length and complete structural components.