NP-1 sequestering by VEGFR1 is key to difference in VEGF binding to VEGFR2. (A) Simulations were run with VEGF (0.023 nM) and variable VEGFf for 3 h using standard parameter values (Table 2). VEGFR1 bound to NP-1 or unbound VEGFR1 are shown for Model 1 (VEGFf can bind to VEGFR1 and NP-1 - open) and Model 2 (VEGFf cannot bind to VEGFR1 and NP-1 -filled). Note the direct overlap of the unbound VEGFR1 for the two models. (B) Similar simulations were run but with 0 (solid line) or 1 nM VEGFf for Model 1 (open) and Model 2 (filled) with variable levels of NP-1. VEGF bound to VEGFR2 is shown.