Angle dependent forced separation and rolling calculations for VWF A1 variants bound to GPIbα

Abstract

Binding between the von Willebrand factor (VWF) A1 domain and platelet glycoprotein Ibα (GPIbα) controls platelet capture under flow and changes when disease-associated A1 substitutions alter force-dependent bond lifetimes. We examined how disease-associated A1 substitutions affect bond mechanics under directional loading. Steered molecular dynamics (SMD) showed no significant effect of loading angle on peak separation force, whereas effective secant stiffness showed clear angle dependence. A1 remained structurally stable during pulling, while GPIbα underwent greater angle-dependent deformation and separation. Pooled rank comparisons between each variant and wild type (WT) identified R1334A as the case with lower peak separation force, consistent with disruption of the local R1334 interface, while R1306Q, R1450E, and G1324S showed largely overlapping SMD distributions with WT. For the R1306Q/R1450E rolling comparison, experimentally measured bond lifetimes were fitted with Bell slip kinetics to obtain force-dependent dissociation rates. Angle-dependent SMD mechanics were then used to relate tether extension to bond force, allowing the Bell slip kinetics to be coupled directly to a reduced ellipse rolling model with multiple load-bearing tethers. With increased load sharing, the R1450E/R1306Q velocity ratio reached 0.646, close to the experimental value of 0.607. A separate WT/G1324S calculation reproduced the shift in peak stopped-time stress from 16 to 32 dyn cm−2. Together, the model captured experimentally observed differences in both rolling dynamics and stopped-time behavior.