BLI binding measurements. Apparent binding rates and affinities for fVIIIa (A), fIXa (B), and Xase (C) (1:1, fVIIIa:fIXa) were determined by BLI using immobilized His₆-tagged lipid nanodiscs. Step-corrected association and dissociation steps were measured for 60 seconds and 30 seconds, respectively, over a serial dilution of each sample (spheres). The association and dissociation binding kinetics were calculated from a global fit using a 1:1 binding analysis (black line) (GraphPad Prism 5.0) and used to calculate K_D values (k_off/k_on). Results are summarized in Table 1.