title: Thermal, chemical and pH induced unfolding of turmeric root lectin: modes of denaturation.
abstract: Curcuma longa rhizome lectin, of non-seed origin having antifungal, antibacterial and α-glucosidase inhibitory activities, forms a homodimer with high thermal stability as well as acid tolerance. Size exclusion chromatography and dynamic light scattering show it to be a dimer at pH 7, but it converts to a monomer near pH 2. Circular dichroism spectra and fluorescence emission maxima are virtually indistinguishable from pH 7 to 2, indicating secondary and tertiary structures remain the same in dimer and monomer within experimental error. The tryptophan environment as probed by acrylamide quenching data yielded very similar data at pH 2 and pH 7, implying very similar folding for monomer and dimer. Differential scanning calorimetry shows a transition at 350.3 K for dimer and at 327.0 K for monomer. Thermal unfolding and chemical unfolding induced by guanidinium chloride for dimer are both reversible and can be described by two-state models. The temperatures and the denaturant concentrations at which one-half of the protein molecules are unfolded, are protein concentration-dependent for dimer but protein concentration-independent for monomer. The free energy of unfolding at 298 K was found to be 5.23 Kcal mol-1 and 14.90 Kcal mol-1 for the monomer and dimer respectively. The value of change in excess heat capacity upon protein denaturation (ΔCp) is 3.42 Kcal mol-1 K-1 for dimer. The small ΔCp for unfolding of CLA reflects a buried hydrophobic core in the folded dimeric protein. These unfolding experiments, temperature dependent circular dichroism and dynamic light scattering for the dimer at pH 7 indicate its higher stability than for the monomer at pH 2. This difference in stability of dimeric and monomeric forms highlights the contribution of inter-subunit interactions in the former.
authors.affiliation: Department of Biochemistry, Bose Institute, Calcutta, India.
authors.forename: Himadri
authors.initials: H
authors.lastname: Biswas
authors.affiliation: Department of Biochemistry, Bose Institute, Calcutta, India.
authors.forename: Rajagopal
authors.initials: R
authors.lastname: Chattopadhyaya
mesh_terms: D002152:Calorimetry, Differential Scanning; D030024:Curcuma / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D037102:Lectins / Q000737:chemistry*; D008958:Models, Molecular; D018517:Plant Roots / Q000737:chemistry*; D011489:Protein Denaturation; D017510:Protein Folding; D013696:Temperature; D013816:Thermodynamics
chemical_list: D037102:Lectins