Tyrosinase is a bifunctional enzyme mediating the o-hydroxylation and two-electron oxidation of monophenols to o-quinones. The monophenolase activity of tyrosinase is much desired for the industrial synthesis of catechols. However, the generally low ratio of monophenolase/diphenolase activity of tyrosinase limited its utilization in the industry. In this study, a novel tyrosinase from Armillaria ostoyae strain C18/9 (AoTyr) was characterized, and the results showed that the enzyme has an optimal temperature of 25°C and an optimal pH of 6. The enzyme has comparable monophenolase and diphenolase activities and exhibits substrate inhibition in both of the activities. In silico analysis and mutagenesis experiments showed that residues 262 and 266 play important roles in modulating the substrate inhibition and enzymatic activities of AoTyr, and the replacement of D262 with asparagine significantly increased the monophenolase/diphenolase catalytic efficiencies (kcat/Km ratios) (1.63-fold) of the enzyme. The results from this study indicated that this novel tyrosinase could be a potential candidate for the industrial biosynthesis of catechols. IMPORTANCE Tyrosinase is able to oxidize various phenolic compounds, and its ability to convert monophenols into diphenols has caught great attention in the research field and industrial applications. However, the utilization of tyrosinase for the industrial synthesis of catechols has been limited due to the fact that the monophenolase activity of most of the known tyrosinases is much lower than the diphenolase activity. In the present study, a novel tyrosinase with comparable monophenolase and diphenolase activities was characterized. The enzyme exhibits substrate inhibition in both monophenolase and diphenolase activities. In silico analysis followed by mutagenesis experiments confirmed the important roles of residues 262 and 266 in the substrate inhibition and activity modulation of the enzyme, and the D262N variant showed an enhanced monophenolase/diphenolase catalytic efficiency ratio compared to the wild-type enzyme.
Keywords: Armillaria ostoyae; diphenolase; monophenolase; mutagenesis; substrate inhibition; tyrosinase.