ROLE OF THE P2 RESIDUE IN DETERMINING THE SPECIFICITY OF SERPINS

The importance of the P2 residue in determining serpin specificity was examined by making a series of substitutions in the P2 position of recombinant α1-antichymotrypsin that contained an arginine P1 residue. The importance of the P2 residue in governing the association rate constant (kon) of the serpin varied with the protease examined. For trypsin, the P2 residue played a relatively minor role, whereas the nature of this residue markedly influenced the rates of inhibition of thrombin, factor Xa, and APC. A 1000-fold difference in kon values was observed between the fastest (P2 proline) and the slowest (P2 threonine) inhibitors of thrombin. Similar differences were observed with factor Xa; the best inhibitor (P2 glycine) displayed a 200-fold higher kon value than the poorest (P2 threonine). The nature of the P2 residue also affected whether the interaction of the serpin with the protease resulted in inhibition of the protease or cleavage of the serpin; a P2 proline residue increased the rate of cleavage ...