Interaction of plant alkaloid berberine with β-lactoglobulin: an account from spectroscopic, thermodynamic, and small-angle X-ray scattering studies aided by theoretical calculations

Abstract

The binding interaction of milk protein β-lactoglobulin (β-LG) with plant alkaloid berberine (BER) has been thoroughly investigated using spectroscopic, calorimetric, and small-angle X-ray scattering studies aided by computational methods. The electronic UV-Vis and steady-state fluorescence spectroscopy experiments have suggested the creation of a ground-state complex between β-LG and BER. The origin of protein fluorescence is identified to be from Trp19, which is located in the hydrophobic pocket of β-LG. Time-resolved fluorescence spectroscopy confirmed the static quenching of β-LG fluorescence in the presence of BER. Binding parameters allied with the β-LG – BER complex have been estimated from the spectroscopic studies and correlated with the results directly obtained from isothermal titration calorimetry (ITC) studies. The fluorescence resonance energy transfer (FRET) from β-LG to BER has been explored. The synchronous fluorescence measurement has revealed variations in the microenvironment of the tyrosine and tryptophan residues of β-LG in the presence of BER. Circular dichroism (CD) results have revealed no or minimal conformational changes in the secondary structure of β-LG dimer due to the interaction with the BER molecule. The detailed small-angle X-ray scattering (SAXS) data further presages minimal structural alterations of β-LG in the presence of BER. Theoretical docking and molecular dynamics simulation investigations validated the experimental findings. The detailed investigation of β-LG – BER binding interactions may allow β-LG – BER or other protein–alkaloid complexes as potential drug carriers against various diseases as the protein structure is almost retained in the interaction.