Folding mechanism of the metastable serpin α 1-antitrypsin

Abstract

The misfolding of serpins is linked to several genetic disorders including emphysema, thrombosis, and dementia. During folding, inhibitory serpins are kinetically trapped in a metastable state in which a stretch of residues near the C terminus of the molecule are exposed to solvent as a flexible loop (the reactive center loop). When they inhibit target proteases, serpins transition to a stable state in which the reactive center loop forms part of a six-stranded β-sheet. Here,we use hydrogen-deuterium exchange mass spectrometry to monitor region-specific folding of the canonical serpin human α 1-antitrypsin (α 1-AT). We find large differences in the folding kinetics of different regions. A key region in the metastable →stable transition, β-strand 5A, shows a lag phase of nearly 350 s. In contrast, the "B-C barrel" region shows no lag phase and the incorporation of the C-terminal residues into β-sheets B and C is largely complete before the center of β-sheet A begins to fold. We propose this as the mechanism for trapping α 1-AT in a metastable form. Additionally, this separation of timescales in the folding of different regions suggests a mechanism by which α 1-AT avoids polymerization during folding.