Mechanical transition from :math:`\alpha`-helical coiled coils to :math:`\beta`-sheets in fibrin(ogen)
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Large structural transitions constitute an essential mechanism of protein function and dysfunction, including folding, unfolding, and misfolding, conformational activation and inactivation, aggregation, protein−protein interactions, etc. Secondary structure alterations, including the :math:`\alpha`-helix to :math:`\beta`-sheet (:math:`\alpha`-to-:math:`\beta`) transition, have been suggested as a universal mechanism of protein unfolding. It has been shown experimentally and theoretically that upon mechanical unfolding, protein polymers containing two-stranded :math:`\alpha`-helical coiled coils, such as :math:`\alpha`-keratin, :math:`\alpha`-keratin-like intermediate filaments, wool fibers, vimentin, and desmin intermediate filaments, form structures with :math:`\beta`-sheet architecture. In the :math:`\alpha`-helical coiled coil, two or more right-handed :math:`\alpha`-helices wind around each other to form a left-handed supercoil. 

We characterized the :math:`\alpha`-to-:math:`\beta` transition in :math:`\alpha`-helical coiled-coil connectors of the human fibrin(ogen) molecule using biomolecular simulations of their forced elongation and theoretical modeling. The force (:math:`F`)–extension (:math:`X`) profiles show three distinct regimes: (1) the elastic regime, in which the coiled coils act as entropic springs (:math:`F < 100-125` pN; :math:`X < 7-8` nm); (2) the constant-force plastic regime, characterized by a force-plateau (:math:`F \approx 150` pN; :math:`X \approx 10-35` nm); and (3) the nonlinear regime (:math:`F > 175-200` pN; :math:`X > 40-50` nm). In the plastic regime, the three-stranded :math:`\alpha`-helices undergo a noncooperative phase transition to form parallel three-stranded :math:`\beta`-sheets. The critical extension of the :math:`\alpha`-helices is :math:`0.25` nm, and the energy difference between the :math:`\alpha`-helices and :math:`\beta`-sheets is :math:`4.9` kcal/mol per helical pitch. The soft :math:`\alpha`-to-:math:`\beta` phase transition in coiled coils might be a universal mechanism underlying mechanical properties of filamentous :math:`\alpha`-helical proteins.

The results was published in `J. Am. Chem. Soc. (2012) <http://pubs.acs.org/doi/abs/10.1021/ja3076428>`_.

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