Conformation of the von Willebrand factor/factor VIII complex in quasi-static flow

Von Willebrand factor (VWF) is a plasma glycoprotein that circulates noncovalently bound to blood coagulation factor VIII (fVIII). VWF is a population of multimers composed of a variable number of ∼280 kDa monomers that is activated in shear flow to bind collagen and platelet glycoprotein Ibα. Electron microscopy, atomic force microscopy, small-angle neutron scattering, and theoretical studies have produced a model in which the conformation of VWF under static conditions is a compact, globular “ball-of-yarn,” implying strong, attractive forces between monomers. We performed sedimentation velocity (SV) analytical ultracentrifugation measurements on unfractionated VWF/fVIII complexes. There was a 20% per mg/ml decrease in the weight-average sedimentation coefficient, s(w), in contrast to the ∼1% per mg/ml decrease observed for compact globular proteins. SV and dynamic light scattering measurements were performed on VWF/fVIII complexes fractionated by size-exclusion chromatography to obtain s(w) values and z-average diffusion coefficients, D(z). Molecular weights estimated using these values in the Svedberg equation ranged from 1.7 to 4.1 MDa. Frictional ratios calculated from D(z) and molecular weights ranged from 2.9 to 3.4, in contrast to values of 1.1–1.3 observed for globular proteins. The Mark–Houwink–Kuhn–Sakurada scaling relationships between s(w), D(z) and molecular weight, [Formula: see text] and [Formula: see text] , yielded estimates of 0.51 and –0.49 for a(s) and a(D), respectively, consistent with a random coil, in contrast to the a(s) value of 0.65 observed for globular proteins. These results indicate that interactions between monomers are weak or nonexistent and that activation of VWF is intramonomeric.