Engineered WE-thrombin and its “anticoagulant” activity profile. Schematic representation of WE-thrombin with active site triad residues indicated in green. The active site cleft is running from left to right with W215A/E217A mutations near the entrance to the active site cleft shown in yellow. The collapse of the 215-217 polypeptide strand blocks access of substrates to the active site resulting in loss of enzymatic activity for all substrates, including procoagulant substrates, cellular substrates, and protease inhibitors. Only in the presence of both thrombomodulin and protein C, and possibly TAFI, is enzymatic activity of WE-thrombin restored, resulting in the generation of activated protein C with anticoagulant and cytoprotective activities, and of activated TAFI with antifibrinolytic and anti-inflammatory activities. “Dead protease” behavior of WE-thrombin, such as competitive inhibition of prothrombinase or blocking the interaction of GPIbα with von Willebrand factor, may contribute to the anticoagulant phenotype of WE-prothrombin/thrombin in vivo.¹  Exosite I (right) and II (left) are shown in blue and residues important for Na⁺ allostery are indicated in red.