Mechanistic studies of the tyrosinase-catalyzed oxidative cyclocondensation of 2-aminophenol to 2-aminophenoxazin-3-one.

Autor: Washington C; Division of Mathematical and Physical Sciences, Department of Chemistry, Xavier University of Louisiana, New Orleans, LA 70125, USA., Maxwell J; Division of Mathematical and Physical Sciences, Department of Chemistry, Xavier University of Louisiana, New Orleans, LA 70125, USA., Stevenson J; Division of Mathematical and Physical Sciences, Department of Chemistry, Xavier University of Louisiana, New Orleans, LA 70125, USA., Malone G; Division of Mathematical and Physical Sciences, Department of Chemistry, Xavier University of Louisiana, New Orleans, LA 70125, USA., Lowe EW Jr; Center for Structural Biology, Vanderbilt University, Nashville, TN 37232, USA., Zhang Q; Division of Mathematical and Physical Sciences, Department of Chemistry, Xavier University of Louisiana, New Orleans, LA 70125, USA., Wang G; Division of Mathematical and Physical Sciences, Department of Chemistry, Xavier University of Louisiana, New Orleans, LA 70125, USA., McIntyre NR; Division of Mathematical and Physical Sciences, Department of Chemistry, Xavier University of Louisiana, New Orleans, LA 70125, USA. Electronic address: nmcintyr@xula.edu.
Jazyk: angličtina
Zdroj: Archives of biochemistry and biophysics [Arch Biochem Biophys] 2015 Jul; Vol. 577-578, pp. 24-34. Date of Electronic Publication: 2015 May 14.
DOI: 10.1016/j.abb.2015.04.007
Abstrakt: Tyrosinase (EC 1.14.18.1) catalyzes the monophenolase and diphenolase reaction associated with vertebrate pigmentation and fruit/vegetable browning. Tyrosinase is an oxygen-dependent, dicopper enzyme that has three states: Emet, Eoxy, and Edeoxy. The diphenolase activity can be carried out by both the met and the oxy states of the enzyme while neither mono- nor diphenolase activity results from the deoxy state. In this study, the oxidative cyclocondensation of 2-aminophenol (OAP) to the corresponding 2-aminophenoxazin-3-one (APX) by mushroom tyrosinase was investigated. Using a combination of various steady- and pre-steady state methodologies, we have investigated the kinetic and chemical mechanism of this reaction. The kcat for OAP is 75 ± 2s(-1), K(OAP)M = 1.8 ± 0.2mM, K(O2)M =25 ± 4 μM with substrates binding in a steady-state preferred fashion. Stopped flow and global analysis support a model where OAP preferentially binds to the oxy form over the met (k7 ≫ k1). For the met form, His269 and His61 are the proposed bases, while the oxy form uses the copper-peroxide and His61 for the sequential deprotonation of anilinic and phenolic hydrogens. Solvent KIEs show proton transfer to be increasingly rate limiting for kcat/K(OAP)M as [O2] → 0 μM (1.38 ± 0.06) decreasing to 0.83 ± 0.03 as [O2] → ∞ reflecting a partially rate limiting μ-OH bond cleavage (E met) and formation (E oxy) following protonation in the transition state. The coupling and cyclization reactions of o-quinone imine and OAP pass through a phenyliminocyclohexadione intermediate to APX, forming at a rate of 6.91 ± 0.03 μM(-1)s(-1) and 2.59E-2 ± 5.31E-4s(-1). Differences in reactivity attributed to the anilinic moiety of OAP with o-diphenols are discussed.
(Copyright © 2015 Elsevier Inc. All rights reserved.)
Databáze: MEDLINE
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