The Structure of a Plant Tyrosinase from Walnut Leaves Reveals the Importance of "Substrate-Guiding Residues" for Enzymatic Specificity.

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.

Crystallization and preliminary X-ray crystallographic analysis of polyphenol oxidase from Juglans regia (jrPPO1).

Tyrosinase is a type 3 copper enzyme that catalyzes the ortho-hydroxylation of monophenols to diphenols as well as their subsequent oxidation to quinones, which are precursors for the biosynthesis of melanins. The first plant tyrosinase from walnut leaves (Juglans regia) was purified to homogeneity and crystallized. During the purification, two forms of the enzyme differing only in their C-termini [jrPPO1(Asp101-Pro444) and jrPPO1(Asp101-Arg445)] were obtained.

Purification and characterization of tyrosinase from walnut leaves (Juglans regia).

Polyphenol oxidase (PPO) is a type-3 copper enzyme catalyzing the oxidation of phenolic compounds to their quinone derivates, which are further converted to melanin, a ubiquitous pigment in living organisms. In this study a plant originated tyrosinase was isolated from walnut leaves (Juglans regia) and biochemically characterized. It was possible to isolate and purify the enzyme by means of an aqueous two-phase extraction method followed by chromatographic purification and identification.