von Willebrand Factor: Form for Function
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The mechanisms by which von Willebrand factor (VWF) achieves hemostasis lie in its structure. Whereas low-molecular-weight forms have diminished hemostatic potential, ultralarge VWF (ULVWF) in excess is potentially thrombogenic. VWF comprises many subunits, which themselves comprise many repeated domains/assemblies possessing characteristic function(s). Organization of these domains/assemblies into a multimeric structure effectively links and replicates these functions. Each domain/assembly influences the synthesis, assembly, secretion, or hemostatic potential of plasma VWF. The C-terminal CT/CK domain mediates dimerization of VWF subunits in the endoplasmic reticulum, while the N-terminal D1D2 assemblies catalyzes disulfide binding between juxtaposed D3 assemblies in the trans-Golgi, creating multimers. The pH-sensitive domains (A2-CT/CK) allow ULVWF multimers to orderly pack into tubules that unravel upon secretion into the circulation. Hemodynamic forces regulate the conformation of the A2 domain and thus, its accessibility to proteolytic enzyme(s) that regulate VWF's hemostatic potential. Binding to the VWF D'D3 assemblies stabilizes coagulation factor VIII. The VWF A1 and A3 domains facilitate platelet capture onto exposed collagen(s) at sites of vascular injury. Our deeper understanding of VWF provided through the recent growth in VWF structure-function studies may potentially guide novel therapeutics for clotting or bleeding disorders.