Inhibitory mechanism of salicylic acid on polyphenol oxidase: A cooperation between acidification and binding effects

Salicylic acid is generally considered to combine with polyphenol oxidase (PPO) to inhibit activity and enzymatic browning, while its acidification effect on PPO activity was usually neglected. In this study, the inhibitory mechanism of salicylic acid on PPO was examined from acidification and binding effects by altering the buffer conditions. As the buffer concentration increased, contribution of acidification decreased while the binding effect became more predominant. Salicylic acid exhibited competitive inhibition on PPO, inducing the changes in secondary structure with a reduction in α-helix. Molecular docking results showed that salicylic acid interacted with residues HIS61, HIS85, HIS259, HIS263 and VAL283 through hydrogen bond and hydrophobic interaction. Furthermore, acidic pH enhanced the binding of salicylic acid to PPO with lower binding energy, additional hydrogen bond and electrostatic interactions. Therefore, both acidification and binding effects were important for salicylic acid on PPO inhibition and enzymatic browning control in fruit and vegetables.