The structure of a plant tyrosinase from Walnut leaves reveals the importance of "substrate-guiding residues" for enzymatic specificity

Abstract

Tyrosinases and catechol oxidases are members of the class of type III copper enzymes. While tyrosinases accept both mono- and o-diphenols as substrates, only the latter substrate is converted by catechol oxidases. Researchers have been working for decades to elucidate the monophenolase/diphenolase specificity on a structural level and have introduced an early hypothesis that states that the reason for the lack of monophenolase activity in catechol oxidases may be its structurally restricted active site. However, recent structural and biochemical studies of this enzyme class have raised doubts about this theory. Herein, the first crystal structure of a plant tyrosinase (from Juglans regia) is presented. The structure reveals that the distinction between mono- and diphenolase activity does not depend on the degree of restriction of the active site, and thus a more important role for amino acid residues located at the entrance to and in the second shell of the active site is proposed. Identity crisis: The first plant tyrosinase structure was solved by the means of X-ray crystallography and reveals that, according to an early theory, this tyrosinase should rather be a catechol oxidase. However, its tyrosinase identity was confirmed by kinetic studies. A binding mechanism is suggested that explains its activity despite its catechol-oxidase-like structure.