Structural Molecular Origins of Fibrin Mechanics

Abstract

The mechanical properties of fibrin are essential to stop bleeding but also contribute to obstructive thrombi that cause heart attack and stroke. The deformation of fibrin polymer occurs at different spatial scales, including molecular unfolding, which is relatively unexplored. Here, the AFM-induced unfolding of fibrinogen monomers and oligomers was correlated with force-extension curves obtained using MD simulations. The unraveling of fibrin(ogen) comprised reversible extension-contraction of the α-helical coiled-coils and unfolding of the γ-nodules, which occurred at a ∼90-pN force and ∼25-nm peak-to-peak distance. All-atom MD simulations showed a transition from α-helix to β-sheet at higher extensions, confirmed experimentally by FTIR-spectroscopy of deformed fibrin clots. The α-to-β conversions correlated directly with the strain or pressure and were partially reversible at the conditions applied. The spectra characteristics of the nascent inter-chain β-sheets were consistent with protein aggregation and fiber bundling. Further MD studies revealed that the coiled-coils undergo ∼5-50 nm extension and 360-degree unwinding with three distinct regimes. In the linear regime, the coiled-coils unwind but not unfold. In the plastic regime, the triple α-helical segments rewind and re-unwind while undergoing a non-cooperative phase transition to form parallel β-sheets. We conclude that, under extension and/or compression, α-to-β conversion of the coiled-coils occurs in fibrin as a part of forced protein unfolding. These regimes of forced deformation of fibrin provide important qualitative and quantitative characteristics of the molecular mechanisms underlying fibrin mechanical properties at the microscopic and macroscopic scales. Furthermore, these structural characteristics of the dynamic mechanical behavior of fibrin at the nanometer scale determine whether or not clots have the ability to stanch bleeding and if thrombi become obstructive or embolize. Finally, this knowledge of the functional significance of different domains of fibrin(ogen) suggests new approaches for modulation of these properties as potential therapeutic interventions.