Modulation of Structure and Dynamics of Cardiac Troponin by Phosphorylation and Mutations Revealed by Molecular Dynamics Simulations

Adrenaline acts on
β1 receptors in the heart muscle
to enhance
contractility, increase the heart rate, and increase the rate of relaxation
(lusitropy) via activation of the cyclic AMP-dependent protein kinase,
PKA. Phosphorylation of serines 22 and 23 in the N-terminal peptide
of cardiac troponin I is responsible for lusitropy. Mutations associated
with cardiomyopathy suppress the phosphorylation-dependent change.
Key parts of troponin responsible for this modulatory system are disordered
and cannot be resolved by conventional structural approaches. We performed
all-atom molecular dynamics simulations (5 × 1.5 μs runs) of the troponin core (419 amino acids) in the presence of
Ca2+ in the bisphosphorylated and unphosphorylated states
for both wild-type troponin and the troponin C (cTnC) G159D mutant.
PKA phosphorylation affects troponin dynamics. There is significant
rigidification of the structure involving rearrangement of the cTnI(1–33)–cTnC
interaction and changes in the distribution of the cTnC helix A/B
angle, troponin I (cTnI) switch peptide (149–164) docking,
and the angle between the regulatory head and ITC arm domains. The
familial dilated cardiomyopathy cTnC G159D mutation whose Ca2+ sensitivity is not modulated by cTnI phosphorylation exhibits a
structure inherently more rigid than the wild type, with phosphorylation
reversing the direction of all metrics relative to the wild type.