New Insights into the Inhibition of Hesperetin on Polyphenol Oxidase: Inhibitory Kinetics, Binding Characteristics, Conformational Change and Computational Simulation

The inhibitory activity of hesperetin on polyphenol oxidase (PPO) and their interaction characteristics were investigated using multiple spectroscopic methods and computational simulation. Hesperetin, a mixed inhibitor, reversibly inhibited PPO activity, and its half-maximum inhibitory concentration (IC50) values on monophenolase and diphenolase were 80.8 ± 1.4 μM and 776.0 ± 15.5 μM, respectively. Multivariate curve resolution-alternate least squares (MCR-ALS) analysis suggested PPO interacted with hesperetin and formed PPO-hesperetin complex. Hesperetin statically quenched PPO's endogenous fluorescence, and hydrophobic interactions mainly drove their binding. Hesperetin affected the polarity of the microenvironment around the Trp residues in PPO, but had no effect on that around Tyr residues. Circular dichroism (CD) results showed that hesperetin increased α-helix content and decreased β-fold and random coil contents, thus tightening PPO's structure. Molecular docking showed that hesperetin entered the hydrophobic cavity of PPO, bound near the dinuclear copper active center, interacted with Val283, Phe264, His85, Asn260, Val248, and His263 via hydrophobic interactions, formed hydrogen bonds with Met280, His89, and His259 residues and also interacted with Phe292, His61, Phe90, Glu256, His244, Asn260, Phe264, and Gly281 via van der Waals forces. The molecular dynamics simulation results also demonstrated that the addition of hesperetin reduced the stability and hydrophobicity of PPO and increased PPO's structural denseness. Thus, the inhibition of hesperetin on PPO may be because hesperetin bound near the active center of PPO, interacted with the surrounding residues, occupied the binding site for substrate, and induced the changes in PPO's secondary structure, thus inhibiting the catalytic activity of PPO. This study may provide novel views for the inhibition of hesperetin on PPO and theoretical guidance for developing flavonoids as new and efficient PPO inhibitors.