Substitution of the gla domain in factor x with that of protein C impairs its interaction with factor VIIa/tissue factor - Lack of comparable effect by similar substitution in factor IX

We previously reported that the first epidermal growth factor-like (EGF1) domain in factor X (FX) or factor IX (FIX) plays an important role in the factor VIIa/tissue factor (FVIIa/TF)induced coagulation. To assess the role of gamma-carboxyglutamic acid (Gla) domains of FX and FIX in FVIIa/TF induced coagulation, we studied four new and two previously described replacement mutants: FXPCGla and FIXPCGla (Gla domain replaced with that of protein C), FXPCEGF1 and FIXPCEGF1 ( EGF1 domain replaced with that of protein C), as well as FXPCGla/EGF1 and FIXPCGla/EGF1 (both Gla and EGF1 domains replaced with those of protein C). FVIIa/TF activation of each FX mutant and the corresponding reciprocal activation of FVII/TF by each FXa mutant were impaired. In contrast, FVIIa/TF activation of FIXPCGla was minimally affected, and the reciprocal activation of FVII/TF by FIXa(PCGla) was normal; however, both reactions were impaired for the FIXPCEGF1 and FIXPCGla/EGF1 mutants. Predictably, FXIa activation of FIXPCEGF1 was normal, whereas it was impaired for the FIXPCGla and FIXPCGla/EGF1 mutants. Molecular models reveal that alternate interactions exist for the Gla domain of protein C such that it is comparable with FIX but not FX in its binding to FVIIa/TF. Further, additional interactions exist for the EGF1 domain of FX, which are not possible for FIX. Importantly, a seven-residue insertion in the EGF1 domain of protein C prevents its interaction with FVIIa/TF. Cumulatively, our data provide a molecular framework demonstrating that the Gla and EGF1 domains of FX interact more strongly with FVIIa/TF than the corresponding domains in FIX.