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

DOI: 10.1073/pnas.1523575113 DOI Help

Authors: Christian Molitor (Universität Wien) , Stephan Mauracher (Universität Wien) , Annette Rompel (Universität Wien)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Proceedings Of The National Academy Of Sciences , VOL 113 (13) , PAGES E1806 - E1815

State: Published (Approved)
Published: March 2016

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.

Journal Keywords: crystal structure; tyrosinase; catechol oxidase; type III copper protein; mechanism

Subject Areas: Chemistry

Instruments: I04-1-Macromolecular Crystallography (fixed wavelength)

Other Facilities: DESY; ESRF

Added On: 30/03/2016 12:11

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