Probing the equilibrium denaturation of the serpin α1-antitrypsin with single tryptophan mutants; evidence for structure in the urea unfolded state

The native conformation of proteins in the serpin superfamily is metastable. In order to understand why serpins attain the native state instead of more stable conformations we have begun investigations into the equilibrium-unfolding of α1-antitrypsin. α1-Antitrypsin contains two tryptophan residues, Trp194 and Trp238, situated on the A and B β-sheets, respectively. Site-directed mutagenesis was used to construct two single-tryptophan variants. Both variants were fully active and had similar secondary structure and stabilities to α1-antitrypsin. The denaturation of α1-antitrypsin and its variants was extremely similar when followed by far-UV CD, indicating the presence of a single intermediate. Fluorescence analysis of the unfolding behavior of each single tryptophan variant indicated that the sole tryptophan residue reported the structural changes within its immediate environment. These data suggest that the A β-sheet is expanded in the intermediate state whilst no structural change around the B β-sheet has occurred. In the urea-induced unfolded state, Trp238 does not become fully solvated, suggesting the persistence of structure around this residue. The implications of these data on the folding, misfolding and function of the serpin superfamily are discussed.