Alpha-2-antiplasmin: a serpin with two separate but overlapping reactive sites.

Although the proteinase inhibitor alpha-2-antiplasmin (alpha 2AP) is known to control the activity of plasmin through rapid formation of stable complexes, it also efficiently inactivates chymotrypsin. These interactions are shown to occur at adjacent, overlapping sites so that plasmin attacks the inhibitor at an Arg364-Met365 peptide bond, while chymotrypsin interacts at a Met365-Ser366 sequence one residue downstream. Thus, a naturally occurring plasma serine proteinase inhibitor can have multiple specificities through interactions at adjacent sites. It also illustrates the potential flexibility of the reactive site loop in this class of inhibitors.