Conformational changes in intact and papain‐modified α1‐proteinase inhibitor induced by guanidinium chloride

Abstract

Equilibrium unfolding-refolding processes of active and proteolytically modified alpha 1-proteinase inhibitor induced by guanidinium chloride were studied. Spectroscopic methods of ultraviolet absorption, fluorescence emission and circular dichroism were used. The functional inhibitor unfolds following a multistate process: a first transition (midpoint at 0.6 M guanidinium chloride) was observed whatever the method used and was attributed to a limited conformational modification of the region including the two tryptophan residues. At higher denaturant concentrations, two other transitions were observed, one in fluorescence (midpoint at 1.7 M guanidinium chloride), attributed to the unfolding of the polypeptide chain in the same region and the other one, observed in circular dichroism and in ultraviolet absorption (midpoint at 2.3 M guanidinium chloride), leading to the totally unfolded protein. Evidence for several intermediates was also obtained with the proteolytically modified inhibitor. If total unfolding is considered, the modified inhibitor was found to be more stable towards the denaturant than the functional form (obtained at 5.5 M and 3.5 M guanidinium chloride, respectively). The unfolding irreversibility observed was attributed to the C-terminal fragment Ser359-Lys394 associated with the main chain of the cleaved inhibitor.