Aurone synthase is a catechol oxidase with hydroxylase activity and provides insights into the mechanism of plant polyphenol oxidases

Abstract

Tyrosinases and catechol oxidases belong to the family of polyphenol oxidases (PPOs). Tyrosinases catalyze the o-hydroxylation and oxidation of phenolic compounds, whereas catechol oxidases were so far defined to lack the hydroxylation activity and catalyze solely the oxidation of o-diphenolic compounds. Aurone synthase from Coreopsis grandiflora (AUS1) is a specialized plant PPO involved in the anabolic pathway of aurones. We present, to our knowledge, the first crystal structures of a latent plant PPO, its mature active and inactive form, caused by a sulfation of a copper binding histidine. Analysis of the latent proenzyme's interface between the shielding C-terminal domain and the main core provides insights into its activation mechanisms. As AUS1 did not accept common tyrosinase substrates (tyrosine and tyramine), the enzyme is classified as a catechol oxidase. However, AUS1 showed hydroxylase activity toward its natural substrate (isoliquiritigenin), revealing that the hydroxylase activity is not correlated with the acceptance of common tyrosinase substrates. Therefore, we propose that the hydroxylase reaction is a general functionality of PPOs. Molecular dynamics simulations of docked substrate-enzyme complexes were performed, and a key residue was identified that influences the plant PPO's acceptance or rejection of tyramine. Based on the evidenced hydroxylase activity and the interactions of specific residues with the substrates during the molecular dynamics simulations, a novel catalytic reaction mechanism for plant PPOs is proposed. The presented results strongly suggest that the physiological role of plant catechol oxidases were previously underestimated, as they might hydroxylate their-so far unknown-natural substrates in vivo.crystal structure | tyrosinase | catechol oxidase | type III copper protein | mechanism

Chalcone: A Privileged Structure in Medicinal Chemistry

“…81 Naringenin chalcones are also the building blocks for the biosynthesis of aurone compounds by a plant catechol oxidase, aurone synthase (AURS). 90 These conversions from chalcones to flavanones or aurones could also be realized by chemical reactions, such as the Algar–Flynn–Oyamada reaction. 91 …”

Oxidative discolouration in whole-head and cut lettuce: biochemical and environmental influences on a complex phenotype and potential breeding strategies to improve shelf-life

“…Sellés-Marchart et al (2007) noted that trypsin activation of PPO from loquat fruit altered the pH range of the enzyme. A set of protein domain-swap experiments by Leufken et al (2015) have shown that this response may be associated with the C-terminal region of the protein suggesting that this domain (attached to the N-terminal region by a flexible linker domain) may physically block access to the active site of the enzyme under certain pH conditions (Boeckx et al 2015;Molitor et al 2016).…”

Circular permutation of a bacterial tyrosinase enables efficient polyphenol‐specific oxidation and quantitative preparation of orobol

“…The chalcone specificity of that PPO was rationalized by its insufficient binding affinity for tyrosine or tyramine (Molitor, Mauracher, & Rompel, 2016), an effect that can explain the observed shift in the M/D ratio for cp48.…”