Calorimetric and spectroscopic studies of the interactions between insulin and (−)-epigallocatechin-3-gallate

Isothermal titration calorimetry (ITC), circular dichroism (CD) spectroscopy, fluorescence spectroscopy and dynamic light scattering were utilized to investigate the molecular interactions between human insulin and EGCG. The ITC results suggest that the binding stoichiometry is independent of pH, salt concentration and temperature, but increases with the increase of EGCG concentration. The interactions between insulin and EGCG are always enthalpically favorable, indicating that hydrogen bonding is always involved in the binding process. However, with the increase of EGCG concentration, both ΔH and ΔS increase gradually, indicating an enhanced contribution of hydrophobic interaction at the physiological condition. The binding of EGCG to insulin can be promoted by increasing temperature at about 120 mmol/L NaCl and pH values away from insulin's isoelectric point. The CD results indicate that the binding of EGCG slightly changes the secondary structure of insulin. The fluorescence spectra reveal that EGCG binding gives little effect on the polarity of the environments in which tyrosine residues exist and the solvent-accessible hydrophobic surfaces of insulin. The addition of EGCG decreases the hydrodynamic radius of insulin, indicating that EGCG directly binds to the protein

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► The interactions between insulin and EGCG are found always enthalpically favorable.
► Hydrogen bonding is essential in the interactions.
► Both ΔH and ΔS for the binding increase with the increase of EGCG concentration.
► The binding of EGCG only slightly changes the secondary structure of insulin.