Simulated Thermal Unfolding of the von Willebrand Factor A Domains

Abstract

The A1 and A2 domains of von Willebrand factor (VWF) have important functions: A1 contains a binding site for platelet glycoprotein Ib (GPIb) while A2 contains a cryptic proteolytic site for the VWF-cleavage enzyme, A Disintegrin And Metalloprotease with a ThromboSpondin type 1 motifs 13 (ADAMTS-13). Because the proteolytic site is fully buried in the native A2 structure, A2 needs to be unfolded to expose its proteolytic site for ADAMTS-13 cleavage. To study the unfolding mechanism of the VWF A domains, we used molecular dynamics (MD) to simulate in atomic details the thermal unfolding of A1 and A2 at high temperatures. The thermal unfolding of A1 and A2 appears very different from their unfolding by tensile forces. At 500 K, unfolding of the central β sheet of A2 starts from the two edges and propagates into the center. β4 and β5 in the center are structurally the most stable and unfolded the latest. However, A2 could be unfolded along different pathways and the unfolded A2 structure is highly flexible. By comparison, A1 is unfolded slower than A2 at 500 K. In even longer time, the unfolding of A1 is limited to the edges of the central β sheet, suggesting a protective role of the N–C terminal disulfide bond.