Active Site Distortion Is Sufficient for Proteinase Inhibition by Serpins

Abstract

We report here the x-ray structure of a covalent serpin-protein- ase complex,␣1-proteinase inhibitor (␣1PI) with porcine pancreatic elastase (PPE), which differs from the only other x-ray structure of such a complex, that of ␣1PI with trypsin, in showing nearly com- plete definition of the proteinase. ␣1PI complexes with trypsin, PPE, and human neutrophil elastase (HNE) showed similar rates of deacylation and enhanced susceptibility to proteolysis by exoge- nous proteinases in solution. The differences between the two x-ray structures therefore cannot arise from intrinsic differences in the inhibition mechanism. However, self-proteolysis of purified com- plex resulted in rapid cleavage ofthe trypsin complex, slower cleav- age of the PPE complex, and only minimal cleavage of the HNE complex. This suggests that the earlier ␣1PI-trypsin complex may have been proteolyzed and that the present structure is more likely to be representative of serpin-proteinase complexes. The present structure shows that active site distortion alone is sufficient for inhibition and suggests that enhanced proteolysis is not necessarily exploited in vivo