Factor VIII (FVIII) binds to von Willebrand factor (vWF) to form a complex (Lollal P et al. 1988; Leyte A et al. 1989; Vlot et al. 1995). Antibody inhibition data, site-directed deletion and mutagenesis studies suggest that the acidic subdomain a3, C1 & C2 domains of the FVIII light chain together control high affinity binding to vWF (Foster PA et al. 1988; Leyte A et al. 1989, 1991; Shima M et al. 1993; Saenko EL et al. 1994; Saenko EL & Scandella D 1997; Jacquemin M et al. 2000). Structural studies using negative stain electron microscopy (EM) and hydrogen-deuterium exchange mass spectrometry (HDX-MS) have revealed that the vWF TIL’ domain interacts with the FVIII C1 domain, the vWF E’ domain bridges the vWF TIL’ and D3 domains, whereas the vWF D3 domain interacts with the FVIII C1 and C2 domains (Yee A et al. 2015; Chiu PL et al. 2015). In addition, HDX-MS experiments showed that the FVIII a3 subdomain residues V1689-D1697 are directly involved in the interaction (Chiu PL et al. 2015). A combination of NMR spectroscopy and isothermal titration calorimetry (ITC) confirmed direct interaction between the FVIII a3 region and the VWF TIL’ domain mapping it to the residues in the two β-sheet regions on the VWF TIL’ domain (Dagil L et al. 2019). Further, tyrosine sulfation at residue 1699 is required for the interaction of FVIII with vWF (Leyte A et al. 1991). In the absence of sulfation at Y1699 in FVIII, the affinity for vWF was reduced by 5-fold (Leyte A et al. 1991). The nuclear magnetic resonance (NMR) spectrum studies of the complex between FVIII and vWF showed significantly larger residue-specific chemical shift changes when Y1699 was sulfated further highlighting the importance of FVIII sulfation at Y1699 for the binding affinity to vWF (Dagil L et al. 2019). The significance of the sulfation of FVIII at Y1699 in vivo is made evident by the presence of a Y1699F mutation that causes a moderate hemophilia A, likely due to reduced interaction with vWF and decreased plasma half-life (van den Biggelaar M et al. 2011). The vWF stabilizes FVIII, which otherwise has a very short half-life in the blood stream (Kaufman RJ et al. 1988). The interaction of FVIII with vWF allows thrombin to activate the bound FVIII and impedes cleavage of the molecules of nonactivated FVIII by the proteases FXa and activated protein C (APC) (Hamer RJ et al. 1987; Hill-Eubanks DC & Lollar P 1990; Koedam JA et al. 1990; Nogami K et al. 2002). Furthermore, vWF prevents the nonspecific binding of FVIII to the membranes of activated human platelets (Nesheim M et al. 1991; Li X & Gabriel DA 1997).
Factor VIII is a heterodimer containing a heavy and a light polypeptide chain, generated by the proteolytic cleavage of a single large precursor polypeptide (Vehar et al. 1984). Several forms of the heavy chain are found in vivo, all functionally the same but differing in the amount of the B domain removed by proteolysis. The single form annotated here is the shortest one (Eaton et al. 1986; Hill-Eubanks et al. 1989).
It has been demonstrated in in vitro experiments that vWF facilitates the association of FVIII chains and the retention of procoagulant activity in the conditioned medium of cells producing FVIII (Kaufman RJ et al. 1988; Wise RJ et al. 1991). Similar data have been obtained for re-association of FVIII chains in solution (Fay PJ 1988). In vitro, von Willebrand factor (Titani et al. 1986) can form complexes with factor VIII with a 1:1 stoichiometry. The complexes that form in vivo, however, involve large multimers of von Willebrand factor and varied, but always low, proportions of factor VIII (Vlot et al. 1995). A stoichiometry of one molecule of factor VIII associated with 50 of von Willebrand factor is typical in vivo, and is used here to annotate the factor VIII:von Willebrand factor complex.