Benzene oxygenation and oxidation by the peroxygenase of Agrocybe aegerita

Aromatic peroxygenase (APO) is an extracellular enzyme produced by the agaric basidiomycete Agrocybe aegerita that catalyzes diverse peroxide-dependent oxyfunctionalization reactions. Here we describe the oxygenation of the unactivated aromatic ring of benzene with hydrogen peroxide as co-substrate. The optimum pH of the reaction was around 7 and it proceeded via an initial epoxide intermediate that re-aromatized in aqueous solution to form phenol. Identity of the epoxide intermediate as benzene oxide was proved by a freshly prepared authentic standard using GC-MS and LC-MS analyses. Second and third [per]oxygenation was also observed and resulted in the formation of further hydroxylation and following [per]oxidation products: hydroquinone and p-benzoquinone, catechol and o-benzoquinone as well as 1,2,4-trihydroxybenzene and hydroxy-p-benzoquinone, respectively. Using H218O2 as co-substrate and ascorbic acid as radical scavenger, inhibiting the formation of peroxidation products (e.g., p-benzoquinone), the origin of the oxygen atom incorporated into benzene or phenol was proved to be the peroxide. Apparent enzyme kinetic constants (kcat, Km) for the peroxygenation of benzene were estimated to be around 8 s-1 and 3.6 mM. These results raise the possibility that peroxygenases may be useful for enzymatic syntheses of hydroxylated benzene derivatives under mild conditions.