Tyrosinases and catechol oxidases belong to the polyphenol oxidase (PPO) enzyme family, which is mainly responsible for the browning of fruits. Three cDNAs encoding PPO pro-enzymes have been cloned from leaves of Malus domestica (apple, MdPPO). The three pro-enzymes MdPPO1-3 were heterologously expressed in E. coli yielding substantial amounts of protein and have been characterized with regard to their optimum of activity resulting from SDS, acidic and proteolytic activation. Significant differences were found in the kinetic characterization of MdPPO1-3 when applying different mono- and diphenolic substrates. All three enzymes have been classified as tyrosinases, where MdPPO1 exhibits the highest activity with tyramine (kcat = 9.5 s−1) while MdPPO2 and MdPPO3 are also clearly active on this monophenolic substrate (kcat = 0.92 s−1 and kcat = 1.0 s−1, respectively). Based on the activity, sequence data and homology modelling it is proposed that the monophenolase and diphenolase activity of PPOs can be manipulated by the appropriate combination of two amino acids, which are located within the active site cleft and were therefore named “activity controllers”.

HighlightsThe first PPO gene (cgAUS1) involved in 4-deoxyaurone formation is identified.AUS1 is expressed as latent pro-enzyme in E. coli and purified to homogeneity.Diphenolase activity of AUS1 pro-enzyme is proven using SDS as an activation agent.Gene expression studies suggest a physiological role for AUS1 in aurone formation.

Polyphenol oxidases (PPOs) contain the structurally similar enzymes tyrosinases (TYRs) and catechol oxidases (COs). Two cDNAs encoding pro-PPOs from tomato (Solanum lycopersicum) were cloned and heterologously expressed in Escherichia coli. The two pro-PPOs (SlPPO1-2) differ remarkably in their activity as SlPPO1 reacts with the monophenols tyramine (kcat = 7.94 s−1) and phloretin (kcat = 2.42 s−1) and was thus characterized as TYR, whereas SlPPO2 accepts only diphenolic substrates like dopamine (kcat = 1.99 s−1) and caffeic acid (kcat = 20.33 s−1) rendering this enzyme a CO. This study, for the first time, characterizes a plant TYR and CO originating from the same organism. Moreover, X-ray structure analysis of the latent holo- and apo-SlPPO1 (PDB: 6HQI and 6HQJ) reveals an unprecedented high flexibility of the gatekeeper residue phenylalanine (Phe270). Docking studies showed that depending on its orientation the gatekeeper residue could either stabilize and correctly position incoming substrates or hinder their entrance into the active site. Furthermore, phloretin, a substrate of SIPPO1 (Km = 0.11 mM), is able to approach the active centre of SlPPO1 with both phenolic rings. Kinetic and structural results indicate that phloretin could act as a natural substrate and connote the participation of PPOs in flavonoid-biosynthesis.

Polyphenol oxidase from apricot (Prunus armeniaca) (PaPPO) was purified in its latent form (L-PaPPO), and the molecular weight was determined to be 63 kDa by SDS-PAGE. L-PaPPO was activated in the presence of substrate at low pH. The activity was enhanced by CuSO4 and low concentrations (≤ 2 mM) of SDS. PaPPO has its pH and temperature optimum at pH 4.5 and 45 °C for catechol as substrate. It showed diphenolase activity and highest affinity toward 4-methylcatechol (KM = 2.0 mM) and chlorogenic acid (KM = 2.7 mM). L-PaPPO was found to be spontaneously activated during storage at 4 °C, creating a new band at 38 kDa representing the activated form (A-PaPPO). The mass of A-PaPPO was determined by mass spectrometry as 37 455.6 Da (Asp102 → Leu429). Both L-PaPPO and A-PaPPO were identified as polyphenol oxidase corresponding to the known PaPPO sequence (UniProt O81103) by means of peptide mass fingerprinting.

Tyrosinases (TYRs) catalyze the hydroxylation of phenols and the oxidation of the resulting o‐diphenols to o‐quinones, while catechol oxidases (COs) exhibit only the latter activity. Aurone synthase (AUS) is not able to react with classical tyrosinase substrates, such as tyramine and l‐tyrosine, while it can hydroxylate its natural substrate isoliquiritigenin. The structural difference of TYRs, COs, and AUS at the heart of their divergent catalytic activities is still a puzzle. Therefore, a library of 39 mutants of AUS from Coreopsis grandiflora (CgAUS) was generated and the activity studies showed that the reactivity of the three conserved histidines (HisA2, HisB1, and HisB2) is tuned by their adjacent residues (HisB1+1, HisB2+1, and waterkeeper residue) either to react as stronger bases or / and to stabilize a position permissive for substrate proton shuffling. This provides the understanding for C−H activation based on the type‐III copper center to be used in future biotechnological processes.

polyphenol oxidases (ppos) comprise tyrosinases (tYRs) and catechol oxidases (cos), which catalyse the initial reactions in the biosynthesis of melanin. tYRs hydroxylate monophenolic (monophenolase activity) and oxidize diphenolic (diphenolase activity) substrates, whereas cos react only with diphenols. In order to elucidate the biochemical basis for the different reactions in PPOs, cDNA from walnut leaves was synthesized, the target gene encoding the latent walnut tyrosinase (jrPPO1) was cloned, and the enzyme was heterologously expressed in Escherichia coli. Mutations targeting the two activity controller residues (Asn240 and Leu244) as well as the gatekeeper residue (Phe260) were designed to impair monophenolase activity of jrPPO1. For the first time, monophenolase activity of jrPPO1 towards L-tyrosine was blocked in two double mutants (Asn240Lys/Leu244Arg and Asn240Thr/ Leu244Arg) while its diphenolase activity was partially preserved, thereby converting jrPPO1 into a CO. Kinetic data show that recombinant jrPPO1 resembles the natural enzyme, and spectrophotometric investigations proved that the copper content remains unaffected by the mutations. The results presented herein provide experimental evidence that a precisely tuned interplay between the amino acids located around the active center controls the substrate specificity and therewith the mono-versus diphenolase activity in the type-iii copper enzyme jrPPO1.Tyrosinases (TYRs), catechol oxidases (COs) and aurone synthases (AUSs) represent the polyphenol oxidase (PPO) family, which is an umbrella term for copper metalloenzymes 1-3 containing one type-III copper center. TYRs catalyse the hydroxylation of monophenols to o-diphenols (EC 1.14.18.1, monophenolase activity) as well as the subsequent oxidation of o-diphenols to their corresponding o-quinones (EC 1.10.3.1, diphenolase activity) 1,4 , whereas COs catalyse only the latter reaction, unable to react with monophenolic substrates (Fig. 1). AUSs participate in the formation of aurones from chalcone precursors and are involved in plant secondary metabolism 5,6 . Quinones produced by PPOs usually undergo non-enzymatic reactions, polymerize 7 and finally form melanin products 8,9 . PPOs occur in a broad spectrum of organisms, including archaea 10 , bacteria 11 , fungi 2 , plants 8 and animals 12,13 . In plants, they are believed to be involved in defence mechanisms associated with the formation of browning substances, which is triggered by mechanical damage or wounding 14 , while in animals their reaction products are responsible for coloring of skin, hair and eyes 13 .TYR from Juglans regia (walnut, jrPPO1) is expressed in vivo as a latent 66.8 kDa pro-enzyme consisting of three domains 15 : an N-terminal chloroplast transit peptide (~12 kDa) 15 , the catalytically active domain (~39 kDa) and the C-terminal domain (~16 kDa) that shields the entrance to the catalytic pocket and keeps the enzyme in a latent state. In vivo enzymatic activity is triggered by the removal of the C-terminal domain 16 . Alternati...

Type‐III copper enzymes like polyphenol oxidases (PPOs) are ubiquitous among organisms and play a significant role in the formation of pigments. PPOs comprise different enzyme groups, including tyrosinases (TYRs) and catechol oxidases (COs). TYRs catalyze the o‐hydroxylation of monophenols and the oxidation of o‐diphenols to the corresponding o‐quinones (EC 1.14.18.1). In contrast, COs only catalyze the oxidation of o‐diphenols to the corresponding o‐quinones (EC 1.10.3.1). To date (August 2020), 102 PDB entries encompassing 18 different proteins from 16 organisms and several mutants have been reported, identifying key residues for tyrosinase activity. The structural similarity between TYRs and COs, especially within and around the active center, complicates the elucidation of their modes of action on a structural basis. However, mutagenesis studies illuminate residues that influence the two activities and show that crystallography on its own cannot elucidate the enzymatic activity mode. Several amino acid residues around the dicopper active center have been proposed to play an essential role in the two different activities. Herein, we critically review the role of all residues identified so far that putatively affect the two activities of PPOs.

A concise and efficient total synthesis of the lignan natural product larreatricin as well as an unambiguous assignment of configuration of its enantiomers are reported, resolving a long‐held controversy. Enzyme kinetic studies revealed that different polyphenol oxidases show high and remarkably divergent enantioselective recognition of this secondary metabolite.