Rational design of a potent anticoagulant thrombin

Thrombin acts as a procoagulant when it cleaves fibrinogen and promotes the formation of a fibrin clot and functions as an anticoagulant when it activ ates protein C with the assistance of the cofactor thrombomodulin. The dual function of thrombin in the blood poses the challenge to turn the enzyme i nto a potent anticoagulant by selectively abrogating fibrinogen cleavage. U sing functional and structural data, we have rationally designed a thrombin mutant, W215A/E217A, that cleaves fibrinogen with a value of k(cat)/K-m ab out 20,000-fold slower than wild-type but activates protein C in the presen ce of thrombomodulin with a specificity comparable with wild-type. This mut ant demonstrates for the first time that the relative specificity of thromb in toward fibrinogen and protein C can be completely reversed.