The binding of Von Willebrand Factor to platelets is dependent on the conformation of the A1 domain which binds to platelet GPIbα. This interaction initiates the adherence of platelets to the subendothelial vasculature under the high shear that occurs in pathological thrombosis. We have developed a thermodynamic strategy that defines the A1:GPIbα interaction in terms of the free energies (ΔG values) of A1 unfolding from the native to intermediate state and the binding of these conformational states to GPIbα. We have isolated the intermediate conformation of A1 under nondenaturing conditions by reduction and car-boxyamidation of the disulfide bond. The circular dichroism spectrum of reduction and carboxyamidation A1 indicates that the intermediate has ∼10% less a-helical structure that the native conformation. The loss of a-helical secondary structure increases the GPIbα binding affinity of the A1 domain ∼20-fold relative to the native conformation. Knowledge of these ΔG values illustrates that the A1:GPIbα complex exists in equilibrium between these two thermodynamically distinct conformations. Using this ther-modynamic foundation, we have developed a quantitative allosteric model of the force-dependent catch-to-slip bonding that occurs between Von Willebrand Factor and platelets under elevated shear stress. Forced dissociation of GPIbα from A1 shifts the equilibrium from the low affinity native conformation to the high affinity intermediate conformation. Our results demonstrate that A1 binding to GPIbα is thermodynamically coupled to A1 unfolding and catch-to-slip bonding is a manifestation of this coupling. Our analysis unites thermodynamics of protein unfolding and conformation-specific binding with the force dependence of biological catch bonds and it encompasses the effects of two subtypes of mutations that cause Von Willebrand Disease.
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