Folding Pathway of Human α1-Antitrypsin: Characterization of an Intermediate That Is Active but Prone to Aggregation

The folding-unfolding kinetics of human alpha 1-antitrypsin (alpha 1-AT) were examined by monitoring intrinsic Trp fluorescence and extrinsic ANS fluorescence. While the unfolding of alpha 1-AT followed a single exponential phase, refolding exhibited three exponential phases. The fast phase (tau 1r < 40 sec). which was independent of urea concentration, appears to be hydrophobic collapse that may be limited by cis-trans isomerization of prolyl residue. The medium phase (tau 2s = 200 sec) yielded an intermediate (IN), which is capable of elastase binding. The slowest (tau 3r = 1000 sec) phase completes refolding to the native protein, which intersects with the unfolding kinetics at the same urea concentration (1.9 M) as the equilibrium midpoint. Concentration-dependence of the amplitude of major refolding phases indicated that IN is prone to kinetic competition between the on-pathway to native protein and aggregation.