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-dihydroxyphenyilalanine (L-dopa) occurs with moderate yields (~20%) but high enantioselectivity (~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 18O2 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.
Tyrosinase catalyzes asymmetric sulfoxidation / R. Pievo, M. Gullotti, E. Monzani, L. Casella. - In: BIOCHEMISTRY. - ISSN 0006-2960. - 47:11(2008), pp. 3493-3498.
Tyrosinase catalyzes asymmetric sulfoxidation
R. PievoPrimo
;M. GullottiSecondo
;
2008
Abstract
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-dihydroxyphenyilalanine (L-dopa) occurs with moderate yields (~20%) but high enantioselectivity (~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 18O2 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.Pubblicazioni consigliate
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