HDX-MS of the light-chain variant FIXaE78K. The ability of FIXaE78K to interact with FVIIIa in the FIXa–FVIIIa complex was investigated with HDX-MS. (A-B) Protease domain and EGF-2 are shown (PDB: 2wpm). The deuterium uptake changes are color coded according to their intensity as described in Figure 3. (A) Deuterium uptake differences in FIXaE78K compared with wt-FIXa. (B) Deuterium uptake differences in FIXaE78K compared with FIXaE78K in the presence of FVIIIa. (C) Example plots comparing deuterium uptake differences between wt-FIXa, in the absence (red) or presence (pink) of FVIIIa, and FIXaE78K, in the absence (dark green) or presence (light green) of FVIIIa.