Analysis of equilibrium dissociation and unfolding in denaturants of soybean agglutinin and two of its derivatives

Abstract

The equilibrium denaturation of tetrameric soybean agglutinin (SBA) in urea and guanidine hydrochloride (GdnHCl) has been examined by steady-state fluorescence and size-exclusion chromatography. The denaturation of SBA reveals two distinct and separable transitions: dissociation (native tetramer↔tertiary monomer) and unfolding (tertiary monomer↔unfolded monomer). The urea denaturation curves of N-dimethyl and acetyl derivatives of SBA are also similar to unmodified lectin but the midpoints, [ D] 1/2, are shifted to lower denaturant concentrations. The free energy of stabilization of tertiary structure (Δ G u,aq) of SBA is estimated to be 4.5–4.6 kcal mol −1, which shows a decrease by ∼10–15% for both N-dimethyl SBA and acetyl-SBA. The free energy term (Δ G d, aq) for the relative stability of the quaternary structure of SBA and its derivatives shows that the decrease in stability relative to SBA occurs by <10% for N-dimethyl SBA while for acetyl-SBA, this occurs by ∼30%. However, the m values depicting the dependence of free energy on denaturant concentration for SBA and its derivatives are similar for dissociation as well as unfolding, which suggest similar denaturation pathways of unmodified and modified SBA.