Inhibition of Ascorbic Acid on Lotus Rhizome Polyphenol Oxidase: Inhibition Kinetics and Computational Simulation

Abstract

Polyphenol oxidase(PPO) is widely known to be involved in enzymatic browning reaction in many fruits and vegetables including lotus rhizome with different catalytic mechanisms. In this study, the inhibitory effect and mechanisms of action of ascorbic acid (AA) on the lotus rhizome PPO were investigated using inhibition kinetics and computational simulation. The lotus rhizome PPO was extracted with PBS (pH 7.0), fractionated with ammonium sulphate, concentrated, and purified with DEAE-52(2.6×30 cm) and Sephadex G-75(2.6×60 cm) chromatography. The active fractions were pooled and the PPO activity was determined to be 2627.36Units/mg. AA exhibited inhibition on lotus rhizome PPO with residual activity of 13.79% at concentration of 0.08mM and IC₅₀ of 0.045mM. Kinetic analyses determined by Lineweaver-Burk plots showed that ascorbic acid was reversible and competitive inhibitor to the enzyme. The 3D structure of the lotus rhizome PPO was simulated by SWISS-MODEL program and molecular docking was performed between PPO and its ligands (catehol and AA) by SYBYL-X 2.0. Simulation results showed that AA and catechol compete with the binding site of the PPO active center for its stronger affinity with the enzyme. In conclusion, the AA was established as a competitive inhibitor of lotus rhizome PPO, which provides a theoretical basis for it as an anti-browning agent in storage and preservation of lotus rhizome.

Keywords: Lotus rhizome, Polyphenol oxidase, Computational simulation, Inhibition mechanism