Identification of the amino acid position controlling the different enzymatic activities in walnut tyrosinase isoenzymes (jrPPO1 and jrPPO2)
Felix Panis, Annette Rompel
Abstract
Abstract Polyphenol oxidases (PPOs) are ubiquitously distributed among plants, bacteria, fungi and animals. They catalyze the hydroxylation of monophenols (monophenolase activity) and the oxidation of o -diphenols (diphenolase activity) to o -quinones. PPOs are commonly present as an isoenzyme family. In walnut ( Juglans regia ), two different genes ( jr PPO1 and jr PPO2) encoding PPOs have been identified. In this study, jr PPO2 was, for the first time, heterologously expressed in E. coli and characterized as a tyrosinase (TYR) by substrate scope assays and kinetic investigations, as it accepted tyramine and L-tyrosine as substrates. Moreover, the substrate acceptance and kinetic parameters ( k cat and K m values) towards 16 substrates naturally present in walnut were assessed for jr PPO2 (TYR) and its isoenzyme jr PPO1 (TYR). The two isoenzymes prefer different substrates, as jr PPO1 shows a higher activity towards monophenols, whereas jr PPO2 is more active towards o -diphenols. Molecular docking studies performed herein revealed that the amino acid residue in the position of the 1st activity controller (His B1 + 1; in jr PPO1 Asn240 and jr PPO2 Gly240) is responsible for the different enzymatic activities. Additionally, interchanging the 1st activity controller residue of the two enzymes in two mutants ( jr PPO1-Asn240Gly and jr PPO2-Gly240Asn) proved that the amino acid residue located in this position allows plants to selectively target or dismiss substrates naturally present in walnut.