Abstract
Alterations in flow during bleeding trigger the von Willebrand factor (VWF) A1 domain to bind to glycoprotein Ibα (GPIbα) on platelets, initiating blood clotting. Using a laser tweezers and a Receptor and Ligand in a Single Molecule (ReaLiSM) construct, we find that force can switch A1 and/or GPIbα to a different state with a faster on-rate; this provides an additional mechanism for activating VWF binding to platelets. Mutations in VWF induce von Willebrand disease (VWD), a common human bleeding disorder. Our results show that force escalates the effects of VWD mutations with respect to on-and off-rates, explaining pathophysiology. Using the effective concentration of receptor and ligand in ReaLiSM to convert single molecule kon (units of s−1) to bulk phase kon (units of s−1M−1) allows the calculation of an equivalent binding affinity. We find that using extrapolated, zero-force kon and koff values from the low-force state of the bond to calculate the binding affinity provides remarkably good agreement with bulk phase measurements.
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