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

Significance Catechol oxidases and tyrosinases belong to the family of polyphenol oxidases (PPOs). In contrast to tyrosinases, catechol oxidases were so far defined to lack hydroxylase activity toward monophenols. Aurone synthase (AUS1) is a plant catechol oxidase that specializes in the conversion of chalcones to aurones (flower pigments). We evidence for the first time, to our knowledge, hydroxylase activity for a catechol oxidase (AUS1) toward its natural monophenolic substrate (chalcone). The presented first crystal structure of a plant pro-PPO provides insights into its activation mechanisms, and based on biochemical and structural studies of AUS1, we propose a novel catalytic reaction mechanism for plant PPOs. The proven hydroxylase functionality of AUS1 suggests that other catechol oxidases might also be involved in the plant’s secondary metabolism.