Toward more specific inhibitor for Solanum tuberosum polyphenol oxidase through a structural insight into its activities and inhibition

To prevent enzymatic browning, applying a polyphenol oxidase (PPO) inhibitor is more desirable, especially when the freshness of the product matters. Most of the inhibition studies were done on mushroom tyrosinase (MT) while the literature indicates that MT and PPO of Solanum tuberosum (PPO_sol ) respond differently to the same modulator despite their similar active sites. This research was conducted to deepen our knowledge about PPO_sol and introduce a more specific inhibitor for this enzyme to be used in controlling the enzymatic browning of potatoes. A modified procedure was developed for PPO_sol purification. The enzyme was subjected to some essential physicochemical and kinetics studies. In parallel to the comparable physicochemical properties, homology modeling revealed high structural similarity between Solanum lycopersicum PPO (PPO_sly ) and PPO_sol except for their active site pockets. Accordingly, PPO_sol showed 5.1- and 34-fold higher affinity toward chlorogenic acid compared with two PPO_sly isozymes. Alike PPO_sly , PPO_sol showed monophenolase activity but it was inactive toward L-tyrosine and p-coumaric acid. Based on structural criteria, phthalic acid, cinnamic acid, ferulic acid, and vanillin were selected and thoroughly examined for inhibition of the catecholase activity of PPO_sol . Although all these substances inhibited PPO_sol in mixed-inhibition mode, the results were strongly in favor of vanillin with IC₅₀ < 1.37 mM and K_i < 1.2 mM. PRACTICAL APPLICATIONS: There are subtle structural differences in the active site pockets of polyphenol oxidase (PPOs) of various fruits, vegetables, and crops. Consequently, to introduce an efficient inhibitor for hindering enzymatic browning of crop products, it is essential to have detailed knowledge about the structure and activity of its PPO as the main player of this undesirable phenomenon. Results of this study not only shed light on the physicochemical properties of PPO_sol but can also be used in making various formulations for safe controlling enzymatic browning of potatoes, especially fresh-cut and minimally processed products, and similar crops products during postharvest and the processes of products preparations.