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An allosteric switch controls the procoagulant and anticoagulant activities of thrombin.

Thrombin is an allosteric enzyme existing in two forms, slow and fast, that differ widely in their specificities toward synthetic and natural amide substrates. The two forms are significantly populated in vivo, and the allosteric equilibrium can be affected by the binding of effectors and natural substrates. The fast form is procoagulant because it cleaves fibrinogen with higher specificity; the slow form is anticoagulant because it cleaves protein C with higher specificity. Binding of thrombomodulin inhibits cleavage of fibrinogen by the fast form and promotes cleavage of protein C by the slow form. The allosteric properties of thrombin, which has targeted two distinct conformational states toward its two fundamental and competing roles in hemostasis, are paradigmatic of a molecular strategy that is likely to be exploited by other proteases in the blood coagulation cascade.