Tyrosinases (TYR) are copper-containing metalloenzymes that catalyze the hydroxylation and oxidation of phenolic compounds. In humans, a single tyrosinase catalyzes the reaction that initiates melanin biosynthesis. Due to the poor availability of the human enzyme, most experiments in basic and applied research are carried out with the readily available “mushroom tyrosinase”. This crude enzyme preparation is sourced from the globally most often cultivated fungi, the common button mushroom (Agaricus bisporus), which possesses six distinct tyrosinases (AbPPO1–6).

Despite the widespread use of “mushroom tyrosinase”, detailed biochemical data were not available for all of these enzymes. Annette Rompel, University of Vienna, Austria, and colleagues have performed a bacterial expression and biochemical characterization of all six A. bisporus tyrosinases. They are considered isoenzymes, which usually means enzymes with different amino acid sequences that catalyze the same chemical reaction

The team found that all six enzymes accept a wide range of phenolic and catecholic substrates, but show pronounced differences in their specificity and enzymatic reaction rates. For example, the physiologically relevant phenol derivative γ-L-glutaminyl-4-hydroxybenzene, which is accumulated to millimolar concentrations in A. bisporus, is processed very rapidly by AbPPO4, while it is ignored by AbPPO3. This result indicates that isoenzymes may differ in their substrate scope, and therefore, serve altogether different physiological roles. Overall, the work challenges the traditional concept of isoenzymes.

• Mushroom Tyrosinase: Six Isoenzymes Catalyzing Distinct Reactions,
  Matthias Pretzler, Annette Rompel,
  ChemBioChem 2024.
  https://doi.org/10.1002/cbic.202400050