Alpha-2-plasmin inhibitor comprises a single domain.

The reaction between plasmin and alpha 2PI (alpha-2-plasmin inhibitor), which constitutes a major regulatory step in fibrinolysis, involves both interactions between the kringle structures of plasmin and a complementary site on the inhibitor, and also between the inhibitor reactive center and the enzyme active site. An attempt was made to distinguish calorimetrically the two functional domains of alpha 2PI. Only one transition was seen, both in the DSC and UV experiments, contrary to the expected two. This transition was unaffected by K4 of plasminogen but was abolished in the presence of anhydrotrypsin. This suggests the major domain structure in alpha 2PI is associated with the reactive center.