Identification of a Factor X-Interactive Site on the Factor VIII A2 Domain

We have experienced a case of acquired hemophilia A with inhibitor recognizing only a factor (F) VIII A2 epitope, and reported the inhibitory mechanism for disappearing FVIII activity (Blood, 124, 4226, 2014). In summary, the patient's inhibitor IgG bound to FVIII A2N (residue 372-562) fragment and inhibited Arg³⁷² cleavage in FVIII by FXa, suggesting that FX(a) bound to FVIII A2 domain. ELISA-based assay showed that FVIII A2 fragment bound to FX (K_d; 338 nM). We hypothesized that FVIII A2 residues 400-429 might be FX binding site according to the 3-D model of FVIII molecule, and prepared synthetic peptides (400-409, 409-419, and 420-429). The 400-409 peptide inhibited the FVIII A2-FX interaction, suggesting that the 400-409 region contributed to FX-interactive site. In this current study, we further performed the localization of a FX-interactive site on the 400-409 region in the A2 domain. A purified FXa generation assay demonstrated the 400-409 peptide decreased the generation of FXa in a dose-dependent manner up to 38% of 100 μM (K_i; 23 ± 9 nM). In comparison, scrambled peptide of 400-409 decreased up to 10% of 100 μM. These data demonstrated that the 400-409 peptide inhibited the generated FXa, suggesting the 400-409 region contributed to regulate the coagulation function. Covalent cross-linking was observed between the biotinylated 400-409 peptide and FX following reaction with EDC (1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide) using SDS-PAGE. This cross-linking formation was blocked by the addition of unlabeled 400-409 peptide. N-terminal sequence analysis of the peptide-FX product demonstrated that two sequential residues (Lys408 and Ser409) could not be detected, supporting that two residues participate in cross-link formation. To confirm the significance of these residues in A2 domain for FX-binding, the mutant forms of the A2 domain, converted to alanine, were expressed in BHK system and purified. Compared with wild type FVIII (K_d; 10 ± 3 nM), the binding affinity of Ser409Ala FVIII mutant for FX was no significant difference (K_d; 14 ± 1 nM) on SPR-based assay. Lys408Ala or Lys408Ala/Ser409Ala double FVIII mutant, however, decreased the binding affinity by 3.6~4.3-fold (K_d; 36 ± 7 or 43 ± 2 nM, respectively), suggesting contribution of Lys408Ala to the binding interaction. For the functional evaluation of the association with FVIII mutants to FX, a FXa generation assay was repeated. Lys408Ala, Ser409Ala, or Lys408Ala/Ser409Ala FVIII mutant reacted with varying concentrations of FX decreased by 1.2~1.6-fold (K_m; 53 ± 12, 69 ± 15, or 65 ± 15 nM, respectively) compared to wild type FVIII (K_m; 43 ± 9 nM), supporting a contribution of these mutants to K_m and overall catalytic efficiency. V_max values were largely unaffected by the mutations with most values within approximately 30% of the wild-type value. On the other hand, K_cat/K_m value of Lys408Ala, Ser409Ala, or Lys408Ala/Ser409Ala FVIII mutant were decreased by 0.5~0.7-fold (K_cat/K_m; 1.0, 1.3, or 0.9 nM^-1min^-1, respectively) compared to wild type FVIII (K_cat/K_m; 1.8 nM^-1min^-1), suggesting low catalytic efficacy of Lys408Ala and Ser409Ala. These results indicate that the 400-409 region in the FVIII A2 domain, and in particular Lys408 and Ser409, may contribute to a unique FX-interactive site.