Pathogenic α1-antitrypsin polymers are formed by reactive loop-β-sheet A linkage

Abstract

α1-Antitrypsin is the most abundant circulating protease inhibitor and the archetype of the serine protease inhibitor or serpin superfamily. Members of this family may be inactivated by point mutations that favor transition to a polymeric conformation. This polymeric conformation underlies diseases as diverse as α1-antitrypsin deficiency-related cirrhosis, thrombosis, angioedema, and dementia. The precise structural linkage within a polymer has been the subject of much debate with evidence for reactive loop insertion into β-sheet A or C or as strand 7A. We have used site directed cysteine mutants and fluorescence resonance energy transfer (FRET) to measure a number of distances between monomeric units in polymeric α1-antitrypsin. We have then used a combinatorial approach to compare distances determined from FRET with distances obtained from 2.9 x 106 different possible orientations of the α1-antitrypsin polymer. The closest matches between experimental FRET measurements and theoretical structures show conclusively that polymers of α1-antitrypsin form by insertion of the reactive loop into β-sheet A.