Polyphenol-bovine serum albumininteraction and its influence on protein structure

The interaction of polyphenols with proteins depends on the structure and type of protein and polyphenols. This study aims to determine the interaction between different plant polyphenols and bovine serum albumin (BSA), as well as their influence on protein structure, proanthocyanidins (proanthocyanidins, PC), catechin (catechin, C), epigallocatechin gallate (Epigallocatechin gallate, EGCG), and tea polyphenols (TEA polyphenol, TP). The BSA was combined to form a polyphenols-BSA. The interaction of polyphenols with BSA and their effects on protein structure were analyzed by UV spectroscopy, fluorescence spectroscopy, infrared spectroscopy, and molecular docking. The results showed that the maximum absorption peak of BSA at 286 nm all shared an upward trend in a redshift with the increase of PC, C, EGCG and TP added. There was the UV maximum wavelength redshift EGCG (292 nm) >PC (288 nm), C (288 nm), TP (288 nm). The redshifted absorption peak was attributed to the EGCG as a monomer, more accessible to the hydrophobic amino acid residues of protein molecules. The fluorescence intensity of BSA decreased gradually, as the concentration of the four polyphenols increased. The polyphenols were interacted with BSA to make more tyrosine or tryptophan exposure, eventually resulting in the lower fluorescence intensity. There was the fluorescence spectroscopy maximum wavelength redshift EGCG>PC, C>TP (365 nm>364 nm, 364 nm>363 nm). Fluorescence spectrum indicated the static quenching of all four types of polyphenols to BSA. The rate constant of protein quenching of polyphenols was 10¹³ at temperatures of 290 and 300 K. The K_sv of EGCG-BSA decreased most outstandingly, when the temperature increased. The catechol structure in the EGCG molecule was broken or deformed at high temperature. The binding constant K_A with temperature was the same as the quenching constant Ksv . The higher the temperature was, the smaller the binding constant K_A was. All level 10³ demonstrated the weak interaction force between the four polyphenols and BSA. The binding site n was about 1, indicating the presence of a binding site. The fluorescence intensity of BSA gradually decreased with the concentration of the four polyphenols increased. Therefore, both polyphenols were interacted with BSA to make more tyrosine or tryptophan exposure, eventually resulting in lower fluorescence intensity. The infrared spectroscopy indicated that four polyphenols altered the secondary structure of BSA, and then promoted the protein structure stabilization. Amide I band absorption peak (1 657.44 cm⁻¹) occurred the blue-shifted PC>TP>EGCG>C (1 656.15 cm⁻¹>1 632.07 cm⁻¹>1 631.49 cm⁻¹ > 1 631.44 cm⁻¹). Nicotinamide II band peak shape was widening. The hydrogen bond of the BSA increased after the interaction. After the addition of TP, PC, and C, the BSA secondary structure was changed from β-turn, random coil to β-sheet, ɑ-helix, and the relative content of β-sheet, ɑ-helix increases, indicating the enhanced compactness of BSA conformation. Specifically, β -sheet content increased in the order of C-BSA (40.20%)>PC-BSA (39.50%)>EGCG-BSA (39.32%)>TP-BSA (34.04%), indicated that the C was better promoted the secondary structure stability of BSA. Molecular docking results indicated that the binding site was located in the hydrophobic cavity of Sub-domain IIA. Intermolecular interaction forces were mainly hydrogen bonding, and hydrophobic forces, in the order of the Binding energy C (−3.72 kJ/ mol) <EGCG (−1.77 kJ/ mol) <PC (−1.02 kJ/ mol) < TP (−0.38 kJ/ mol). The molecular docking verified that the four polyphenols were bound stably to BSA. The addition of polyphenols made the structure of BSA more compact and stable. The different polyphenols affected the BSA structure, among which C-BSA and EGCG-BSA structures were the most stable. Different plant polyphenols-BSA interactions were clarified to analyze their effects on BSA structure for the better structural stability of the BSA. The interaction of polyphenols and proteins can be expected to regulate the characteristics of proteins during processing, storage and transport. The finding can also provide some theoretical reference for the preparation and development of new polyphenol-protein complex products.