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 ( approximately 20%) but high enantioselectivity ( approximately 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 (18)O 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.
Pievo
Roberta
R
Dipartimento di Chimica Inorganica, Metallorganica e Analitica - Università di Milano, Istituto ISTM-CNR 20133 Milano, Italy.
Gullotti
Michele
M
Monzani
Enrico
E
Casella
Luigi
L
D000364:Agaricus / Q000201:enzymology; D002384:Catalysis; D005656:Fungal Proteins / Q000737:chemistry / Q000378:metabolism; D006900:Hydroxylation; D007980:Levodopa / Q000737:chemistry / Q000378:metabolism; D014442:Monophenol Monooxygenase / Q000737:chemistry* / Q000378:metabolism*; D010084:Oxidation-Reduction; D013379:Substrate Specificity; D013440:Sulfides / Q000737:chemistry / Q000378:metabolism; D013454:Sulfoxides / Q000737:chemistry* / Q000378:metabolism*
D005656:Fungal Proteins; D013440:Sulfides; D013454:Sulfoxides; D007980:Levodopa; C093850:methylphenylsulfide; D014442:Monophenol Monooxygenase