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

Abstract

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 (PPOsol ) respond differently to the same modulator despite their similar active sites. This research was conducted to deepen our knowledge about PPOsol 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 PPOsol 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 (PPOsly ) and PPOsol except for their active site pockets. Accordingly, PPOsol showed 5.1- and 34-fold higher affinity toward chlorogenic acid compared with two PPOsly isozymes. Alike PPOsly , PPOsol 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 PPOsol . Although all these substances inhibited PPOsol in mixed-inhibition mode, the results were strongly in favor of vanillin with IC50 < 1.37mM and Ki < 1.2mM. 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 PPOsol 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.