Polyphenol oxidases (PPOs) are oxidoreductases featuring a di-copper center that are ubiquitously distributed among bacteria, plants fungi, and animals. They catalyze the initial reaction steps in the biosynthesis of melanins. Based on the reactions they catalyze, PPOs are divided into tyrosinases and catechol oxidases. TYRs catalyze the hydroxylation of monophenols to the corresponding o-diphenols (monophenolase activity) and the subsequent oxidation of o-diphenols to the corresponding o-quinones (diphenolase activity), whereas catechol oxidases exclusively exhibit diphenolase activity. In plants, PPO activity is commonly observed as the browning of fruits.
To elucidate the molecular basis for the different reactivities of tyrosinases and catechol oxidases, mutations were applied to key amino acid positions of two tyrosinase isoenzymes (jrPPO1 and jrPPO2, Juglans regia) using site-directed mutagenesis. jrPPO1- and jrPPO2-mutants heterologously expressed in E. coli were kinetically characterized, and demonstrated that two amino acid positions, termed 1st and 2nd activity controller, govern the enzymatic activity. Double mutations applied to the 1st and 2nd activity controller converted jrPPO1 from a tyrosinase into a catechol oxidase. Additionally, the kinetic profiles of jrPPO1 (targeting monophenols) and jrPPO2 (targeting diphenols) were interchanged by interchanging the respective amino acids featured in the positions of the 1st activity controllers.

Two Amino Acid Positions Control the Enzymatic Activity in Walnut Polyphenol Oxidases (jrPPO1 & jrPPO2)

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