1986; Batie et al. 1992). The Class I and III dioxygenases, reductases contain a flavin
chromophore (FAD—Class IA and Class III or FMN—Class IB as a prosthetic
group) and a plant-type iron–sulphur [2Fe-2S] cluster. An additional ferredoxin
component carrier along with a Rieske-type iron–sulphur cluster [2Fe-2S] is present
in Class III that functions as an intermediate electron-transfer. Class II enzyme
reductases contain FAD and either a plant-type or Rieske-type (Class IIA, Class
IIB, respectively) [2Fe-2S] iron–sulphur cluster. Limitation of this classification is
that it only considers electron transport components and eliminates other important
information that should be taken into account such as information on oxygenase. For
example, a monooxygenase reaction catalysing enzyme (2-oxo-1,2dihydroquinoline monooxygenase) and a dioxygenase reaction catalysing enzyme
(benzoate dioxygenase) are classified as Class IB, due to presence of iron-flavoprotein reductase (Rosche et al. 1995). On the other hand, these monooxygenase and
dioxygenase differ in composition, i.e. (α) 6 and (αβ) 3 subunit, respectively. Since the
catalytic α-subunit of oxygenases defines the substrate specificity it should be
considered as superfamily (Beil et al. 1998; Mondello et al. 1997; Gibson and
Parales 2000).
Dibenzofuran 4,4a-dioxygenase is responsible for the stereospecific angular
dioxygenation of DF and DD in RW1 (Sphingomonas sp.). The dibenzofuran
4,4a-dioxygenase was supposed to be a heterodimer (α 2 β 2 ) with molecular weights
of α subunit and β subunit, 45 and 23 kDa, respectively. It has been confirmed by
spectroscopy that the α subunit of a Rieske-type (2Fe-2S) cluster is contained in
oxygenase subunits of ring-activating oxygenases. However, the N-terminal
sequence of α subunit of dioxin dioxygenase exhibits significant differences when
compared to other dioxygenases. In in vitro studies it is clear that DF (into 2,2
0 ,3trihydroxybiphenyl) and DD (into 2,2
0 ,3-trihydroxybiphenyl ether) are degraded by
dibenzofuran 4,4a-dioxygenase that consists of an oxygenase and electron transport
chain. The process of degradation introduces two oxygen atoms from a O 2 molecule
that are added to two vicinal carbon atoms of which one is involved in the bridges
between the two aromatic rings. The purified RedA1 and RedA2 are isofunctional
flavoproteins that transfer electrons from NADH to ferredoxin and oxygenase
component (Bunz and Cook 1993).
The carbazole (1,9a-dioxygenase) characterized from strain CA10 (Pseudomonas
sp.) is capable of catalysing an angular dioxygenation similar to dibenzofuran
4,4adioxygenase of RW1 (Nojiri and Omori 2002).
14.8.2 Genes Involved in Degradation of Dibenzofuran
The catabolic genes involved in the metabolism of DF have been already reported in
many strains such as Sphingomonas HH69, Sphingomonas sp. RW1, Pseudomonas
sp. strain CA10, Terrabacter sp., Pseudomonas aeruginosa and Xanthomonas
maltophilia (Harms et al. 1995; Wittich et al. 1992; Sato et al. 1997; Omori et al.
1997; Schmid et al. 1997; Ishiguro et al. 2000; Coronado et al. 2015) and many
more. However, catabolic genes of very few strains have been well explored.
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P. K. Jaiswal and J. Gupta
chromophore (FAD—Class IA and Class III or FMN—Class IB as a prosthetic
group) and a plant-type iron–sulphur [2Fe-2S] cluster. An additional ferredoxin
component carrier along with a Rieske-type iron–sulphur cluster [2Fe-2S] is present
in Class III that functions as an intermediate electron-transfer. Class II enzyme
reductases contain FAD and either a plant-type or Rieske-type (Class IIA, Class
IIB, respectively) [2Fe-2S] iron–sulphur cluster. Limitation of this classification is
that it only considers electron transport components and eliminates other important
information that should be taken into account such as information on oxygenase. For
example, a monooxygenase reaction catalysing enzyme (2-oxo-1,2dihydroquinoline monooxygenase) and a dioxygenase reaction catalysing enzyme
(benzoate dioxygenase) are classified as Class IB, due to presence of iron-flavoprotein reductase (Rosche et al. 1995). On the other hand, these monooxygenase and
dioxygenase differ in composition, i.e. (α) 6 and (αβ) 3 subunit, respectively. Since the
catalytic α-subunit of oxygenases defines the substrate specificity it should be
considered as superfamily (Beil et al. 1998; Mondello et al. 1997; Gibson and
Parales 2000).
Dibenzofuran 4,4a-dioxygenase is responsible for the stereospecific angular
dioxygenation of DF and DD in RW1 (Sphingomonas sp.). The dibenzofuran
4,4a-dioxygenase was supposed to be a heterodimer (α 2 β 2 ) with molecular weights
of α subunit and β subunit, 45 and 23 kDa, respectively. It has been confirmed by
spectroscopy that the α subunit of a Rieske-type (2Fe-2S) cluster is contained in
oxygenase subunits of ring-activating oxygenases. However, the N-terminal
sequence of α subunit of dioxin dioxygenase exhibits significant differences when
compared to other dioxygenases. In in vitro studies it is clear that DF (into 2,2
0 ,3trihydroxybiphenyl) and DD (into 2,2
0 ,3-trihydroxybiphenyl ether) are degraded by
dibenzofuran 4,4a-dioxygenase that consists of an oxygenase and electron transport
chain. The process of degradation introduces two oxygen atoms from a O 2 molecule
that are added to two vicinal carbon atoms of which one is involved in the bridges
between the two aromatic rings. The purified RedA1 and RedA2 are isofunctional
flavoproteins that transfer electrons from NADH to ferredoxin and oxygenase
component (Bunz and Cook 1993).
The carbazole (1,9a-dioxygenase) characterized from strain CA10 (Pseudomonas
sp.) is capable of catalysing an angular dioxygenation similar to dibenzofuran
4,4adioxygenase of RW1 (Nojiri and Omori 2002).
14.8.2 Genes Involved in Degradation of Dibenzofuran
The catabolic genes involved in the metabolism of DF have been already reported in
many strains such as Sphingomonas HH69, Sphingomonas sp. RW1, Pseudomonas
sp. strain CA10, Terrabacter sp., Pseudomonas aeruginosa and Xanthomonas
maltophilia (Harms et al. 1995; Wittich et al. 1992; Sato et al. 1997; Omori et al.
1997; Schmid et al. 1997; Ishiguro et al. 2000; Coronado et al. 2015) and many
more. However, catabolic genes of very few strains have been well explored.
302
P. K. Jaiswal and J. Gupta