moiety is degraded to form a Kreb’s cycle intermediate and 3-chlorosalicylate that
are released into the culture medium. Thus a co-culture of strain RW1 and strain
JWS is capable of completely degrading 4-chlorodibenzofuran (Schindowski et al.
1991; Arfmann et al. 1997).
In angular pathway, metabolites f, n, p, z3, z4, z5 and z6 are formed, whereas
metabolites z, z1 and z2 are formed non-enzymatically reported in Sphingomonas
RW1, Terrabacter DBF63 and Pseudomonas sp. CA10. Lateral pathways run
through L, x, z4 and t; or m, u, w, z4 intermediary metabolites in Ralstonia sp.
Strain SBUG 290. Metabolites B, c, d, g, h, I, j, k, q, r, s and y are found (Gesell et al.
2004). In DF degradation, the angular pathways are more common. However, very
few reports are available which suggested that some bacteria are able to degrade by
using lateral pathway such as Strain SBUG 290 from Ralstonia sp. (Becher et al.
2000). Not only this, some reports have shown mono-hydroxylation as well as
formation of conjugates with ribose sugar in DF. All reported pathways have been
put together in Fig. 14.2.
Nojiri and Omori (2002) have reviewed thoroughly the molecular basis of
bacterial degradation of dioxins under aerobic conditions and investigated the role
of angular dioxygenation. In this study it was mentioned that dioxin-degrading
strains can actually function in dioxin-contaminated soil, thus providing a bioremediation method for dioxin contamination. As mentioned earlier some studies are also
available about white-rot fungus Phlebia lindtneri that can oxidize DD and DF to 2hydroxy-DD, and 3-hydroxy-DF by a cytochrome P-450 monooxygenase (Mori and
Kondo 2002).
14.8.1 Enzymes Involved in Degradation of Dibenzofuran
Aromatic compounds are degraded by initiation of ring-activating reaction. During
dioxygenolytic activation, two hydroxyl groups are incorporated at the ortho
positions of the aromatic ring. The cis-dihydrodiol dehydrogenases rearomatize
cis-dihydrodiol formed via dioxygenolytic activation. However, in case the aromatic
ring is substituted, the dioxygenation reaction can incorporate the hydroxyl group in
two different ways either at the substituted carbon atom and its neighbouring carbon
atom or on two adjacent unsubstituted carbon atoms. In case of incorporation at
substituted carbon atom and its neighbouring carbon atom, the reaction can be
concomitant with the spontaneous elimination of the substituent.
2-chlorobenzoate;
1,2,4,5-tetrachlorobenzene;
2,2
0 -dichlorobiphenyl
or
sulphoaromatics (haloaromatics) form an unstable dihydrodiol analogues during
degradation process (Engesser et al. 1989; Beil et al. 1998; Haddock et al. 1995).
A dehydrogenase is not required during degradation as it undergoes spontaneous
rearomatization. Similarly, angular dioxygenation of biarylethers forms hemiacetals
(an unstable dihydrodiol analogues) during degradation which undergo spontaneous
rearomatization to form two monocyclic aromatics from simple biarylethers or
dihydroxybiphenyl derivatives from DF and DD, respectively (Engesser et al.
1990; Schmidt et al. 1992). Catalysis by ring hydroxylating dioxygenases requires
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P. K. Jaiswal and J. Gupta
are released into the culture medium. Thus a co-culture of strain RW1 and strain
JWS is capable of completely degrading 4-chlorodibenzofuran (Schindowski et al.
1991; Arfmann et al. 1997).
In angular pathway, metabolites f, n, p, z3, z4, z5 and z6 are formed, whereas
metabolites z, z1 and z2 are formed non-enzymatically reported in Sphingomonas
RW1, Terrabacter DBF63 and Pseudomonas sp. CA10. Lateral pathways run
through L, x, z4 and t; or m, u, w, z4 intermediary metabolites in Ralstonia sp.
Strain SBUG 290. Metabolites B, c, d, g, h, I, j, k, q, r, s and y are found (Gesell et al.
2004). In DF degradation, the angular pathways are more common. However, very
few reports are available which suggested that some bacteria are able to degrade by
using lateral pathway such as Strain SBUG 290 from Ralstonia sp. (Becher et al.
2000). Not only this, some reports have shown mono-hydroxylation as well as
formation of conjugates with ribose sugar in DF. All reported pathways have been
put together in Fig. 14.2.
Nojiri and Omori (2002) have reviewed thoroughly the molecular basis of
bacterial degradation of dioxins under aerobic conditions and investigated the role
of angular dioxygenation. In this study it was mentioned that dioxin-degrading
strains can actually function in dioxin-contaminated soil, thus providing a bioremediation method for dioxin contamination. As mentioned earlier some studies are also
available about white-rot fungus Phlebia lindtneri that can oxidize DD and DF to 2hydroxy-DD, and 3-hydroxy-DF by a cytochrome P-450 monooxygenase (Mori and
Kondo 2002).
14.8.1 Enzymes Involved in Degradation of Dibenzofuran
Aromatic compounds are degraded by initiation of ring-activating reaction. During
dioxygenolytic activation, two hydroxyl groups are incorporated at the ortho
positions of the aromatic ring. The cis-dihydrodiol dehydrogenases rearomatize
cis-dihydrodiol formed via dioxygenolytic activation. However, in case the aromatic
ring is substituted, the dioxygenation reaction can incorporate the hydroxyl group in
two different ways either at the substituted carbon atom and its neighbouring carbon
atom or on two adjacent unsubstituted carbon atoms. In case of incorporation at
substituted carbon atom and its neighbouring carbon atom, the reaction can be
concomitant with the spontaneous elimination of the substituent.
2-chlorobenzoate;
1,2,4,5-tetrachlorobenzene;
2,2
0 -dichlorobiphenyl
or
sulphoaromatics (haloaromatics) form an unstable dihydrodiol analogues during
degradation process (Engesser et al. 1989; Beil et al. 1998; Haddock et al. 1995).
A dehydrogenase is not required during degradation as it undergoes spontaneous
rearomatization. Similarly, angular dioxygenation of biarylethers forms hemiacetals
(an unstable dihydrodiol analogues) during degradation which undergo spontaneous
rearomatization to form two monocyclic aromatics from simple biarylethers or
dihydroxybiphenyl derivatives from DF and DD, respectively (Engesser et al.
1990; Schmidt et al. 1992). Catalysis by ring hydroxylating dioxygenases requires
300
P. K. Jaiswal and J. Gupta