Molecular insights into the anti-browning effect of protocatechualdehyde

Polyphenol oxidase (PPO) is a vital polyphenol-metabolizing enzyme responsible for the enzymatic browning of fruits and vegetables. This study assessed the anti-PPO activity of protocatechualdehyde (PCA), yielding an IC₅₀ value of 94 μM (95% CI: 91.8 to 96.4 μM). Kinetic analysis revealed that PCA acted as a reversible and competitive inhibitor of PPO. Molecular docking indicated that hydrogen bonding and hydrophobic interactions were the main driving forces for PCA-PPO binding. Fluorescence spectroscopy further demonstrated that PCA binding altered the microenvironments around tyrosine and tryptophan residues, leading to conformational rearrangements and partial unfolding of the enzyme, evidenced by significant fluorescence quenching. Molecular dynamics simulation demonstrated that PCA perturbed the secondary structure of PPO and caused stretching of its overall structure, thereby inhibiting the catalytic activity of the enzyme. Furthermore, PCA demonstrated excellent anti-browning activity by regulating phenolic metabolism, reducing membrane lipid peroxidation, enhancing the ascorbic acid-glutathione cycle, and modulating cell wall metabolism to delay cellular senescence. Collectively, these findings offer some valuable insights into the development of PPO inhibitor and anti-browning agent, and provide a theoretical foundation for the potential application of PCA in the area of food preservation.