title: Trifluoroethanol-induced conformational change of tetrameric and monomeric soybean agglutinin: role of structural organization and implication for protein folding and stability.
abstract: 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.
authors.affiliation: Department of Chemistry and Biochemistry, Presidency University, 86/1 College Street, West Bengal, Kolkata 700 073, India.
authors.forename: Anisur R
authors.initials: AR
authors.lastname: Molla
authors.forename: Dipak K
authors.initials: DK
authors.lastname: Mandal
mesh_terms: D002942:Circular Dichroism; D007700:Kinetics; D008958:Models, Molecular; D037121:Plant Lectins / Q000737:chemistry*; D017510:Protein Folding; D055503:Protein Multimerization; D055550:Protein Stability; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012997:Solvents / Q000737:chemistry*; D030262:Soybean Proteins / Q000737:chemistry*; D013025:Glycine max / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature; D013816:Thermodynamics; D014270:Trifluoroethanol / Q000737:chemistry*; D014364:Tryptophan / Q000737:chemistry*; D010835:Phytohemagglutinins
chemical_list: D037121:Plant Lectins; D012997:Solvents; D030262:Soybean Proteins; D014270:Trifluoroethanol; D014364:Tryptophan; C028824:soybean lectin; D010835:Phytohemagglutinins