Biology Reference
In-Depth Information
Fe
2+
OH
OH
O
HO
FAD
HO
O
OH
HO
O
OH
OH
Fe
3+
Cellobiose
FADH
Fe
2+
Cellobionolactone
FADH
2
OH
OH
Fe
3+
O
O
HO
HO
O
HO
O
OH
OH
Fig. 7. Catalytic cycle of cellobiose dehydrogenase. FAD oxidizes cellobiose to
cellobionolactone which undergoes spontaneous hydrolysis to produce cellobionic
acid. FADH
2
is further reoxidized by two monoelectronic transfers to the heme part.
The cleavage of heme induces a drastic decrease of CDH activity (
Sigoillot et al.,
2002
). Reduced heme iron may reduce dioxygen or participate in Fenton reaction.
acceptors such as dioxygen, quinones, phenoxyradicals (
Kremer and Wood,
1992; Mason et al., 2002; Samejima and Eriksson, 1992
), or a subsequent
one-electron transfer to the heme domain by intramolecular electron transfer
(
Igarashi et al., 2002
). The heme part is also involved in electron transfer to a
wide variety of substrates acting as electron acceptors including quinones
(
Henriksson et al., 2000a; Westermark and Eriksson, 1975
), metal ions and
organic dyes. When CDHs are reduced, the enzymes can react with dioxygen
to produce hydrogen peroxide in situ (
Pricelius et al., 2009
).
Although most lignocellulolytic fungi produce CDHs, their biological func-
tion is still unclear, but these enzymes participate to a diversity of reactions.
These enzymes are involved in cellulose (
Bao and Renganathan, 1992
)and
lignin (
Dumonceaux et al., 2001
) degradation, in the inhibition of phenol
radical repolymerization (
Ander et al., 1990; Henriksson et al., 1993
), in the
enhancement of MnP turn over (
Roy and Archibald, 1994
) and in the produc-
tion of hydroxyl radicals by a Fenton-type reaction, which is one of the most
often cited CDH-driven reactions. The heme part of CDH may carry out a
Fenton-type reaction with H
2
O
2
and generate hydroxyl radicals by iron reduc-
tion. This hydroxyl radical-generating mechanism might be a key component
of lignocellulose breakdown (
Henriksson et al., 1995, 2000a
). The effect of
CDH on a non-phenolic lignin model has been studied by
Henriksson et al.
(2000b)
. The results indicated that CDH can affect lignins in three ways, the
