Molecular Mechanisms of Transition from Catch to Slip Bonds in Fibrin

Abstract

Fibrin is the end product of blood clotting that constitutes a proteinaceous three dimensional network providing the filamentous mechanical scaffold of clots and thrombi. Formation of fibrin is essential for stopping bleeding (hemostasis) and thrombotic obstruction of blood vessels (thrombosis). In order to explore how mechanical load impacts fibrin, we carried out optical trap-based single-molecule forced unbinding assays. Surprisingly, the strength of non-covalent A:a knob-hole interactions, most important in fibrin polymerization, first increases with tensile force (catch bonds) and then decreases when the force exceeds a critical value (slip bonds). To provide the structural basis of the catch-slip bond transition, we performed molecular modeling of A:a knob-hole complex. The results revealed that the key structural element, a movable flap (residues γ295-γ305), serves as a tension-dependent sensor. Flap dissociation from the regulatory B-domain in the γ-nodule and subsequent translocation to knob ‘A’ triggers the hole ‘a’ closure resulting in the force-dependent increase of binding affinity and prolonged bond lifetimes. The experimental discovery of bi-phasic catch-slip kinetics of knob-hole bond rupture, supported by simulations, is quantitatively explained using a theory, formulated in terms of dynamic remodeling of the binding interface fluctuating between the low (slip) and high (catch) affinity states. We estimated the stiffness of the binding interface to be 15.7 pN/nm and the width of interface fluctuations to be 0.7-2.7 nm. Strengthening of the A:a knob-hole bonds at small (<30-40 pN) forces might favor formation and reinforcement of nascent fibrin clots subject to hydrodynamic shear in vivo. The novel and unexplored mechano-chemical aspects of fibrin polymerization addressed in this study for the first time further advances our understanding of blood clotting. The work was supported, in part, by the Program for Competitive Growth at Kazan Federal University.