Investigation of Streptomyces antibioticus tyrosinase reactivity toward chlorophenols.
ABSTRACT Tyrosinase (Ty) is a copper-containing enzyme ubiquitously distributed in nature. In recent years, Ty has attracted interest as a potential detoxifying agent for xenobiotic compounds with phenolic structure. Among these, chlorophenols are particularly relevant pollutants, commonly found in waste waters. The activity of Streptomyces antibioticus tyrosinase toward isomeric monochlorophenols was studied. Tyrosinase oxidizes both 3- and 4-chlorophenol to the same product, 4-chloro-1,2-ortho-quinone, which subsequently undergoes a nucleophilic substitution reaction at the chlorine atom by excess phenol to give the corresponding phenol-quinone adduct. By contrast, 2-chlorophenol is not reactive and acts as a competitive inhibitor. Docking calculations suggest that the substrates point to one of the copper atoms of the dinuclear center (copper B) and appear to interact preferentially with one of the two coordinated oxygen atoms. The approach of the substrate toward the active site is favored by a π-stacking interaction with one of the copper-coordinated histidines (His194) and by a hydrogen bonding interaction with the O1 oxygen. With this study, we provide the first characterization of the early intermediates in the biotechnologically relevant reaction of Ty with chlorophenols. Additionally, combining experimental evidences with molecular modeling simulations, we propose a detailed reaction scheme for Ty-mediated oxidation of monochlorophenols.
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ABSTRACT: Backgroundo-Aminophenols have been long recognised as tyrosinase substrates. However their exact mode of interaction with the enzyme's active site is unclear. Properly vic-substituted o-aminophenols could help gain some insight into tyrosinase catalytic mechanism.Methods Eight vic-substituted o-aminophenols belonging to two isomeric series were systematically evaluated as tyrosinase substrates and/or activators and/or inhibitors, by means of spectrophotometric techniques and HPLC-MS analysis. Some relevant kinetic parameters have also been obtained.ResultsFour o-aminophenolic compounds derived from 3-hydroxyorthanilic acid (2-amino-3-hydroxybenzenesulfonic acid) and their four counterparts derived from the isomeric 2-hydroxymetanilic acid (3-amino-2-hydroxybenzenesulfonic acid) were synthesised and tested as putative substrates for mushroom tyrosinase. While the hydroxyorthanilic derivatives were quite inactive as both substrates and inhibitors, the hydroxymetanilic compounds on the contrary all acted as substrates for the enzyme, which oxidised them to the corresponding phenoxazinone derivatives.General significanceBased on the available structures of the active sites of tyrosinases, the different affinities of the four metanilic derivatives for the enzyme, and their oxidation rates, we propose a new hypothesis regarding the interaction between o-aminophenols and the active site of tyrosinase that is in agreement with the obtained experimental results.Graphical abstractView high quality image (54K)Highlights► Two series of vic-substituted o-aminophenols were tested as tyrosinase substrates. ► Only hydroxymetanilic compounds were oxidised by the enzyme. ► A rationale for enzyme/substrate interaction in catalytic cycle was proposed.Biochimica et Biophysica Acta (BBA) - General Subjects 05/2011; 1810(8):799-807. · 3.85 Impact Factor
- Gastroenterology 01/2011; 140(5). · 12.82 Impact Factor
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ABSTRACT: This work introduces some results focused on pointing out the assistance provided by the theory of characteristic modes for designing planar monopole antennas, as it allows the determination of the optimum feed configuration in order to obtain a good impedance bandwidth performance. Square planar monopole antennas with a single feed point, with a shorting pin, and with two feed points are analyzed and compared using this theory.Antennas and Propagation Society International Symposium, 2004. IEEE; 07/2004