Mechanism of shear deformation of a coiled myosin coil: Computer simulation

Polymer coiled coils are composed of entangled linear chains in a helical conformation. Their mechanical characteristics are interesting because these structures are involved in the composition of natural fibrillar structures. The method of molecular dynamics is used for the simulation of stretching at a constant rate for a superhelical fragment of myosin protein composed of two identical α helices containing 126 amino acid residues in each helix. The case of shear deformation of a molecule is considered (the load is applied to the N terminus of one chain and to the C terminus of another chain). In this case of loading, slippage of chains with respect to each other can occur. Deformation of a molecule proceeds in several stages. At the initial stage, the superhelix is unfolded and there is a gradual unfolding of end fragments of individual α helices; this process is accompanied by their displacement with respect to the helical fragment of the neighboring chain. In this case, the reaction force increases. At the second stage of stretching, the process passes to the mechanism of deformation when, in the central part of the molecule, α-helical fragments of both chains unfold. In this region, the reaction-force-extension curve shows a plateau region. Between unfolded fragments, new hydrophobic contacts and hydrogen bonds are formed, and fragments of the β structure emerge. Once all turns of α helices in the central parts of the molecule are unfolded, the mechanism of deformation changes and further extension of a molecule proceeds via straightening of previously unfolded central fragments, a process that is accompanied by an increase in the reaction force. When chains achieve their limiting extension, slippage commences with an accompanying decrease in the reaction force.