Enzymes produced by the mushroom-forming fungus Agrocybe aegerita break down toxic organosulfur compounds by catalyzing the oxidization of the sulfur-containing portion of the molecule.

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Dibenzothiophene (DBT) is an organosulfur compound present in heavier fossil fuels like crude oil, coal tar, and coal. Petroleum spills and combustion of these fuels release the compound into the environment. Like many similar chemicals, DBT is toxic to life and some organisms have evolved methods for partially breaking it down into more degradable compounds. The fungus Agrocybe aegerita produces an enzyme called aromatic peroxygenase which it secretes into its external environment. This enzyme uses hydrogen peroxide to catalyze the oxidation of the sulfur-containing portion of DBY. Though the mechanism has yet to be thoroughly established, the active site on the enzyme seems to stimulate the formation of an epoxide adjacent to the sulfur which then rapidly and spontaneously hydrolyzes into hydroxy-DBTs. The products vary depending on the random way in which water reacts with the transient epoxide form but all are more easily broken down further by spontaneous or enzymatic reactions.

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"A. aegerita oxidized DBT [dibenzothiophene] (110 μM) by 100% within 16 days into eight different metabolites. Among the latter were mainly S-oxidation products (DBT sulfoxide, DBT sulfone) and in lower amounts, ring-hydroxylation compounds (e.g., 2-hydroxy- DBT)...dibenzothiophene (DBT) and its derivatives, are present in most fossil fuels in high amounts. Studies on the biological conversion of these sulfur compounds have focused on the desulfurization owing to the interest of fuel industry to reduce the sulfur content of coal and oil within the refining process...The degradation and biodesulfurization of the model compound DBT." (Aranda et al. 2009:1057)

"Agrocybe aegerita [was] described to produce a new type of extracellular heme-thiolate proteins combining activities of classic peroxidases, haloperoxidases, and P450 monooxygenases. These enzymes are now referred to as aromatic peroxygenases...A. aegerita peroxygenase (AaP)...catalyze[s] the oxidation of phenolic compounds, aryl alcohols, and bromide as well as the oxygenation/hydroxylation of aromatic substrates...DBT is an interesting target molecule for aromatic peroxygenase because it may be oxygenated both at the heterocyclic sulfur and at the benzene rings." (Aranda et al. 2009:1058)

"The main DBT metabolites found in the cultures of both species were sulfoxidation products and in case of A. aegerita to a smaller extent, also ring-hydroxylation products...A total of eight DBT metabolites were detected in liquid cultures of A. aegerita...Similar to naphthalene 1,2-oxide, such epoxides would spontaneously hydrolyze into the corresponding phenolic compounds, i.e., into the detected hydroxy-DBT molecules." (Aranda et al. 2009:1060)

"The results of the present study demonstrate that the heterocyclic aromatic compound dibenzothiophene (DBT) is oxidized both by whole cells of the agaric mushrooms A. aegerita and C. radians as well as by their purified extracellular peroxygenases...A. aegerita completely converted the supplemented DBT and—though sulfoxidation products were the major metabolites—also ring-hydroxylation products (e.g., 2-hydroxy-DBT) were identified in the culture liquid." (Aranda et al. 2009:1063)

"[B]oth AaP and CrP introduced several oxygen functionalities into the DBT ring system (up to four hydroxyl groups)...oxygen came from the peroxide (H2O2) and therefore AaP and CrP acted in these reactions as 'true' peroxygenases...hydroxylation proceeds via initial formation of unstable DBT epoxides whose immediate hydrolysis will lead to different hydroxylation products...Sulfoxidation catalyzed by AaP has been also observed for thioanisol (a monoaromatic thiophenic compound). It was enantioselectively converted into the R-isomers of methylphenyl sulfoxide. Metabolites detected during DBT conversion suggest that A. aegerita and C. radians oxidize DBT via two pathways: S-oxidation leading to the formation of DBT sulfoxide and sulfone as well as ring hydroxylation resulting in differently hydroxylated benzene rings." (Aranda et al. 2009:1064)

Journal article
Conversion of dibenzothiophene by the mushrooms Agrocybe aegerita and Coprinellus radians and their extracellular peroxygenasesAppl Microbiol BiotechnolNovember 27, 2008
Elizabet Aranda, Matthias Kinne, Martin Kluge, René Ullrich, Martin Hofrichter

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