Tyrosinase catalyzes asymmetric sulfoxidation.

Mushroom tyrosinase was found to catalyze the oxidation of organic sulfides to sulfoxides in the presence of a catechol as cosubstrate, in a reaction which is unprecedented for this enzyme and resembles those performed by external monooxygenases. Only the oxy form of the enzyme is in fact capable of oxidizing the sulfide in a two-electron process, while the resulting met form can only be recycled by reduction with catechol. The cosubstrate competes with the sulfide also in the reaction with oxy-tyrosinase. For this reason, the sulfoxidation of thioanisole in the presence of L-3,4-dihydroxyphenylalanine (L-dopa) occurs with moderate yields (similar to 20%) but high enantioselectivity (similar to 85% e.e.), and favors (S)-methyl phenyl sulfoxide. The enantioselectivity can be further increased to > 90% when excess ascorbic acid is added to the reaction to limit enzyme inactivation by the quinones produced by L-dopa oxidation. An experiment using O-18(2) showed that 18-O incorporation into methyl phenyl sulfoxide was above 95%, confirming that the mechanism of the sulfoxidation involves oxygen transfer from oxy-tyrosinase to the sulfide.