Mushroom tyrosinase:: Catalase activity, inhibition, and suicide inactivation

Mushroom tyrosinase exhibits catalase activity with hydrogen peroxide (H2O2) as substrate. In the absence of a one-electron donor substrate, H2O2 is able to act as both oxidizing and reducing substrate. The kinetic parameters V-max and K-m that characterize the reaction were determined from the initial rates of oxygen gas production (V-0(O2)) under anaerobic conditions. The reaction can start from either of the two enzyme species present under anaerobic conditions: met-tyrosinase (E-m) and deoxy-tyrosinase (E-d). Thus, a molecule of H2O2 can reduce Em to Ed via the formation of oxy-tyrosinase (E-ox) (E-m + H2O2 reversible arrow E-ox), E-ox releases oxygen into the medium and is transformed into E-d, which upon binding another molecule of H2O2 is oxidized to E-m. The effect of pH and the action of inhibitors have also been studied. Catalase activity is favored by increased pH, with an optimum at pH = 6.4. Inhibitors that are analogues of o-diphenol, binding to the active site coppers diaxially, do not inhibit catalase activity but do reduce diphenolase activity. However, chloride, which binds in the equatorial orientation to the protonated enzyme (EmH), inhibits both catalase and diphenolase activities. Suicide inactivation of the enzyme by H2O2 has been demonstrated. A kinetic mechanism that is supported by the experimental results is presented and discussed.