HDX-MS of the heavy-chain variant FIXaR333A{165_CT}. FIXaR333A{165_CT} ability to interact with FVIIIa in the FIXa–FVIIIa complex was investigated with HDX-MS. (A-B) FIXa protease domain and EGF-2 (PDB: 2wpm). The deuterium uptake changes are color coded according to their intensity as described in Figure 3. (A) Deuterium uptake differences in FIXaR333A{165_CT} compared with wt-FIXa. (B) Deuterium uptake differences in FIXaR333A{165_CT} compared with FIXaR333A{165_CT} 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 FIXaR333A{165_CT}, in the absence (dark blue) or presence (light blue) of FVIIIa.