  The lifetimes of non-covalent A:a knob-hole bonds in fibrin probed with the
optical trap-based force-clamp first increases ("catch bonds") and then
decreases ("slip bonds") with increasing tensile force. Molecular modeling of
"catch-to-slip" transition using the atomic structure of the A:a complex
reveals that the movable flap serves as tension-dependent molecular switch.
Flap dissociation from the regulatory B-domain in $\gamma$-nodule and
translocation from the periphery to knob `A' triggers the hole `a' closure and
interface remodeling, which results in the increased binding affinity and
prolonged bond lifetimes. Fluctuating bottleneck theory is developed to
understand the "catch-to-slip" transition in terms of the interface stiffness
$\kappa =$ 15.7 pN nm $^{-1}$, interface size fluctuations 0.7-2.7 nm, knob `A'
escape rate constant $k_0 =$ 0.11 nm$^2$ s$^{-1}$, and transition distance for
dissociation $\sigma_y =$ 0.25 nm. Strengthening of the A:a knob-hole bonds
under small tension might favor formation and reinforcement of nascent fibrin
clots under hydrodynamic shear.
