Active site modification of factor VIIa affects interactions of the protease domain with tissue factor

In the initiation of coagulation, tissue factor (TF) allosterically activates the serine protease factor VIIa (VIIa) through specific interactions with protease domain residues. These interactions, and consequently affinity for TF, may be influenced by conformational changes in the protease domain that result from zymogen-enzyme transition or occupancy of the active site by tight binding inhibitors. In functional competition and direct binding analysis, we determined affinities for zymogen and enzyme species of wild-type VII and of mutants at protease domain residues that contact TF. We demonstrate that TF binding is not influenced by zymogen activation, indicating that the protease domain of zymogen and enzyme dock similarly with TF. In contrast, active site occupancy enhanced the affinity for TF by predominantly decreasing the dissociation rate of the TF.VIIa complex. Of the three interface residues studied, only Met306 played a major role in the inhibitor-induced increase in affinity. Met306 is also important for transmitting the allosteric changes from TF to the active site, resulting in enhanced catalysis. This study thus provides evidence for a bidirectional conformational interdependence of the interface residue Met306 and the active site of VIIa.