Trifluoroethanol-induced conformational change of tetrameric and monomeric soybean agglutinin: Role of structural organization and implication for protein folding and stability

• Published in: Biochimie 2013-02, Vol.95 (2), p.204-214
• Main Authors: Molla, Anisur R., Mandal, Dipak K.
• Format: Article
• Language: English
• Subjects: Circular Dichroism; Glycine max - chemistry; Kinetics; Models, Molecular; Plant Lectins - chemistry; Protein Folding; Protein Multimerization; Protein Stability; Protein Structure, Quaternary; Protein Structure, Secondary; Protein Structure, Tertiary; Solvent perturbation; Solvents - chemistry; Soybean agglutinin; Soybean Proteins - chemistry; Spectroscopy, Fourier Transform Infrared; Temperature; Thermodynamics; Trifluoroethanol - chemistry; Tryptophan - chemistry; β-Sheet lectin
• ISSN: 0300-9084; 1638-6183
• DOI: 10.1016/j.biochi.2012.09.011

2,2,2-Trifuoroethanol (TFE)-induced conformational structure change of a β-sheet legume lectin, soybean agglutinin (SBA) has been investigated employing its exclusive structural forms in quaternary (tetramer) and tertiary (monomer) states, by far- and near-UV CD, FTIR, fluorescence, low temperature phosphorescence and chemical modification. Far-UV CD results show that, for SBA tetramer, native atypical β-conformation transforms to a highly α-helical structure, with the helical content reaching 57% in 95% TFE. For SBA monomer, atypical β-sheet first converts to typical β-sheet at low TFE concentration (10%), which then leads to a nonnative α-helix at higher TFE concentration. From temperature-dependent studies (5–60 °C) of TFE perturbation, typical β-sheet structure appears to be less stable than atypical β-sheet and the induced helix entails reduced thermal stability. The heat induced transitions are reversible except for atypical to typical β-sheet conversion. FTIR results reveal a partial α-helix conversion at high protein concentration but with quantitative yield. However, aggregation is detected with FTIR at lower TFE concentration, which disappears in more TFE. Near-UV CD, fluorescence and phosphorescence studies imply the existence of an intermediate with native-like secondary and tertiary structure, which could be related to the dissociation of tetramer to monomer. This has been further supported by concentration dependent far-UV CD studies. Chemical modification with N-bromosuccinimide (NBS) shows that all six tryptophans per monomer are solvent-exposed in the induced α-helical conformation. These results may provide novel and important insights into the perturbed folding problem of SBA in particular, and β-sheet oligomeric proteins in general. ► Comparison of TFE-induced conformational transition for SBA tetramer and monomer. ► Evidence for nonnative states such as typical β-sheet and α-helix. ► Reduced thermal stability of all nonnative states compared to the native conformer. ► Concomitant quaternary/tertiary structure change with secondary structure changes. ► Implications for non-hierarchical model of protein folding.