Role of zymogenicity-determining residues of coagulation factor VII/VIIa in cofactor interaction and macromolecular substrate recognition.

Factor VIIa (VIIa) remains in a zymogen-like state following proteolytic activation and depends on interactions with the cofactor tissue factor (TF) for function. Val(21), Glu(154), and Met(156) are residues that are spatially close in available zymogen and enzyme structures, despite major conformational differences in the corresponding loop segments. This residue triad displays unusual side chain properties in comparison to the properties of other coagulation serine proteases. By mutagenesis, we demonstrate that these residues cooperate to stabilize the enzyme conformation and to enhance the affinity for TF. In zymogen VII, however, substitution of the triad did not change the cofactor affinity, further emphasizing the crucial role of the activation pocket in specifically stabilizing the active enzyme conformation. In comparison to VIIa(Q156), the triple mutant VIIa(N21I154Q156) had a stabilized amino-terminal Ile(16)-Asp(194) salt bridge and enhanced catalytic function. However, proteolytic and amidolytic activities of free VIIa variants were not concordantly increased. Rather, a negatively charged Asp at position 21 was the critical factor that determined whether an amidolytically more active VIIa variant also more efficiently activated the macromolecular substrate. These data thus demonstrate an unexpected complexity by which the zymogenicity-determining triad in the activation pocket of VIIa controls the active enzyme conformation and contributes to exosite interactions with the macromolecular substrate.