A mechano-reactive coarse-grained model of the blood-clotting agent von Willebrand factor

The von Willebrand Factor (vWF) is a large blood glycoprotein that aids in hemostasis. Within each vWF monomer, the A2 domain hosts a cleavage site for enzyme ADAMTS13, which regulates the size of vWF multimers. This cleavage site can only be exposed when an A2 domain unfolds, and the unfolding reaction energy landscape is highly sensitive to the force conditions on the domain. Based on previous optical tweezer experimental results, we advance here a new activated A2 monomer model (AA2MM) for coarse-grained modeling of vWF that accurately represents the force-based probabilistic change between the unfolded/refolded states. A system of springs is employed to mimic the complex mechanical response of vWF monomers subject to pulling forces. AA2MM was validated by comparing monomer scale simulation results to data from prior pulling experiments on vWF monomer fragments. The model was further validated by comparing multimer scale Brownian dynamics simulation results to experiments using microfluidic chamber microscopy to visualize tethered vWF proteins subject to flow. The A2 domain unfolding reaction was studied in bulk flow simulations (pure shear and elongation flow), giving evidence that elongational flow drives the vWF size regulation process in blood. The mechanoreactive, coarse-grained AA2MM accurately describes the complex mechanical coupling between human blood flow conditions and vWF protein reactivity.