DEGRADATION OF 2,7-DICHLORODIBENZO-PARA-DIOXIN BY THE LIGNIN-DEGRADING BASIDIOMYCETE PHANEROCHAETE-CHRYSOSPORIUM

Under secondary metabolic conditions, the white-rot basidiomycete Phan erochaete chrysosporium degraded 2,7-dichlorodibenzo-p-dioxin (I). The pathway for the degradation of I was elucidated by the characterizati on of fungal metabolites and oxidation products generated by lignin pe roxidase (LiP), manganese peroxidase (MnP), and crude intracellular ce ll-free extracts. The multistep pathway involves the degradation of I and subsequent intermediates by oxidation, reduction, and methylation reactions to yield the key intermediate 1,2,4-trihydroxybenzene (III). In the first step, the oxidative cleavage of the dioxin ring of I, ca talyzed by LiP, generates 4-chloro-1,2-benzoquinone (V), 2-hydroxy-1,4 -benzoquinone (VIII), and chloride. The intermediate V is then reduced to 1-chloro-3,4-dihydroxybenzene (II), and the latter is methylated t o form 1-chloro-3,4-dimethoxybenzene (VI). VI in turn is oxidized by L iP to generate chloride and 2-methoxy-1,4-benzoquinone (VII), which is reduced to 2-methoxy-1,4-dihydroxybenzene (IV). IV is oxidized by eit her LiP or MnP to generate 4-hydroxy-1,2-benzoquinone, which is reduce d to 1,2,4-trihydroxybenzene (III). The other aromatic product generat ed by the initial LiP-catalyzed cleavage of I is 2-hydroxy-1,4-benzoqu inone (VIII). This intermediate is also generated during the LiP- or M nP-catalyzed oxidation of the intermediate chlorocatechol (II). VIII i s also reduced to 1,2,4-trihydroxybenzene (III). The key intermediate III is ring cleaved by intracellular cell extracts to produce, after r eduction, beta-ketoadipic acid. In this pathway, initial oxidative cle avage of both C-O-C bonds in I by LiP generates two quinone products, 4-chloro-1,2-benzoquinone (V) and 2-hydroxy-1,4-benzoquinone (VIII). T he former is recycled by reduction and methylation reactions to genera te an intermediate which is also a substrate for peroxidase-catalyzed oxidation, leading to the removal of a second chlorine atom. Thus, thi s unique pathway results in the removal of both aromatic chlorines bef ore aromatic ring cleavage takes place.