Changes in structure and in interactions of heat-treated bovine beta-lactoglobulin.
Heat stress on structure and ligand binding of beta-LG has been studied by fluorescence, circular dichroism and gel electrophoresis at pH 6.5. Native PAGE gel electrophoresis shows that denaturation of beta-LG is reversible up to 75 degrees C then it becomes irreversible due to aggregation of beta-LG. Formation of aggregated beta-LG is completed at 95 degrees C. Circular dichroism results indicate that formation of aggregated beta-LG is accompanied by the scrambling of disulfide bonds (creation of new intramolecular and intermolecular disulfide bridges and rearrangement of old intramolecular disulfide bridges). Addition of ethanolic retinol causes a change in polarity of the solution and favors transformation of the beta<-->alpha structure. In the presence of retinol, the alpha-helix content of the secondary structure of heat-treated beta-LG is increased and the major portion of its secondary structure is helical. Fluorescence results show that heat-treated beta-LG at 95 degrees C can still bind retinol. The refolding of the tertiary structure of beta-LG heat-denatured at 95 degrees C may recreate a retinol binding site. Surprisingly, the affinity of the new site for retinol is higher than that of native beta-LG; however, the apparent molar ratio is lower than one. The binding properties of beta-LG for terpenoids have been measured after its heat treatment at 20, 75 and 95 degrees C. The intensity of tryptophan emission at 330 nm was changed only in the case of the interaction with beta-ionone. Other ligands probably cannot bind to beta-LG or they bind in a binding site far from the tryptophan residues, hence not affecting its fluorescence.
Mousavi
Seyed Habib-Allah
SH
Laboratory of Biophysical Chemistry, Department of Chemistry, University of Isfahan, Isfahan, 81746-73441, IR Iran.
Bordbar
Abdol-Khalegh
AK
Haertlé
Thomas
T
D000818:Animals; D002417:Cattle; D002942:Circular Dichroism; D004591:Electrophoresis, Polyacrylamide Gel; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007782:Lactoglobulins / Q000737:chemistry* / Q000378:metabolism*; D008024:Ligands; D011485:Protein Binding; D011489:Protein Denaturation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet
D007782:Lactoglobulins; D008024:Ligands