Differential specificities of polyphenol oxidase from lotus seeds (Nelumbo nucifera Gaertn.) toward stereoisomers, (−)-epicatechin and (+)-catechin: Insights from comparative molecular docking studies

Lotus seeds are highly susceptible to browning because of the catalysis of polyphenol oxidase (PPO), which caused a tremendous waste of lotus resources, so it is urgent to investigate the PPO in lotus seeds. In this study, lotus seed PPO was gradually purified with 26.92 times fold and a 1.43% yield. The molecular weight of lotus seed PPO was tested at 58 kDa, and its three-dimensional (3D) model consisted of eight α-helices, ten β-sheets, and random coils. A total of 14 phenolic compounds were identified in lotus seeds. However, lotus seed PPO showed different affinity towards these phenolic substrates. Even the stereoisomers, (+)-catechin and (−)-epicatechin, demonstrated disparate Km and Vmax values. Thus, the catalytic mechanism between PPO and these stereoisomeric substrates was explored using comparative molecular docking. In contrast to (+)-catechin, (−)-epicatechin formed more Pi-Alkyl interactions and hydrogen bonds with amino acid residues in the hydrophobic pocket of PPO. (−)-Epicatechin had a lower HOMO-LUMO energy gap and higher molecular ovality after entering the active pocket of PPO than (+)-catechin. To conclude, the catalytic mechanism of lotus seed PPO was clarified by comparative molecular docking studies, and (−)-epicatechin was evidenced as the optimal substrate of lotus seed PPO.