Unfolding analysis of the mature and unprocessed forms of Bacillus licheniformis γ-glutamyltranspeptidase

Bacillus licheniformis γ-glutamyltranspeptidase (BlGGT) undergoes an autocatalytic process to generate 44.9 and 21.7 kDa subunits; however, a mutant protein (T399A) loses completely the processing ability and mainly exists as a precursor. For a comprehensive understanding of their structural features, the biophysical properties of these two proteins were investigated by circular dichroism and fluorescence spectroscopy. Tryptophan fluorescence and circular dichroism spectra were nearly identical for BlGGT and T399A, but unfolding analyses revealed that these two proteins had a different sensitivity towards temperature- and guanidine hydrochloride (GdnHCl)-induced denaturation. BlGGT and the unprocessed T399A displayed T(m) values of 61.4°C and 68.1°C, respectively, and thermal unfolding of both proteins was found to be highly irreversible. Fluorescence quenching analysis showed that T399A had a dynamic quenching constant similar to that of the wild-type enzyme. BlGGT started to unfold beyond ∼2.14 M GdnHCl and reached an unfolded intermediate, [GdnHCl](0.5, N - U), at 2.85 M, corresponding to free energy change [Formula: see text] of 12.34 kcal mol( - 1), whereas the midpoint of the denaturation curve for T399A was approximately 3.94 M, corresponding to a [Formula: see text] of 4.45 kcal mol( - 1). Taken together, it can be concluded that the structural stability of BlGGT is superior to that of T399A.