Biology Reference
In-Depth Information
The physiological roles of this enzyme have been associated with the
lignolytic activity of white-rot and related to its capacity to generate hydro-
gen peroxide during its reducing half reaction. The white-rot-produced
hydrogen peroxide might participate to lignin degradation, as co-substrate
of peroxidases and/or in non-enzymatic reactions such as Fenton reactions
(
Ander and Marzullo, 1997
).
Guill´n and Evans (1994)
suggested that the
NADPH-dependent AAO might constitute a redox system in which aryl-
alcohols and aryl-aldehydes produced by the fungus ensured a stable con-
centration of H
2
O
2
for the lignolytic activity.
Marzullo et al.(1995)
reported that the veratryl alcohol oxidase (or AAO) of
P. ostreatus was able to reduce dichlorophenol-indophenol and inhibit or off-
regulate the polymerization of p-OH cinnamic acids by laccases. The study of
the oxidation of lignosulfonates mediated by laccase alone or in association
with AAO revealed that the VA/AAO system is able to prevent the polymeri-
zation of lignin. Both VA and benzylic alcohols released in culture media can
play a key role as co-substrates or supports for redox reactions.
D. CELLOBIOSE DEHYDROGENASES
CDHs (EC 1.1.99.18; cellobiose: [acceptor] 1-oxidoreductase) are extracellular
fungal hemoflavoenzymes produced by white-rot (
Harreither et al., 2009; Li
et al., 1996
:
Moukha et al., 1999; Roy et al., 1996
), brown-rot (
Schmidhalter
and Canevascini, 1993
) and soft-rot fungi (
Schou et al., 1998
), including lignin-
degrading species such as P. chrysosporium. CDH expression is induced by
cellulose culture and also by industrial lignin (such as softwood kraft lignin)
added to the culture medium (
Roy et al., 1996
). Phylogenetic analysis of cdh
gene split CDHs into two groups: class I (mainly CDHs from basidiomycetes)
and class II, produced by ascomycetes/SRF. This last class sometimes contains
a C-terminal type-1 carbohydrate-binding module (CBM;
Henriksson et al.,
1997; Renganathan et al., 1990; Zamocky et al., 2004
).
CDHs are monomeric enzymes containing an N-terminal heme domain and
a C-terminal flavin domain. The flavin domain belongs to the glucose-
methanol-choline oxidoreductase superfamily (
Cavener, 1992
). The heme
domain structure is exclusively found in fungi and contains a cytochrome
b-type heme that presents an unusual heme binding by Met/His (
Hallberg
et al., 2000
). These two regions are linked by a Thr-Ser-rich region
(
Dumonceaux et al., 1998
).
CDHs catalyse the two-electron oxidation of cellobiose (
Fig. 7
) and more
generally cellodextrines, mannodextrines and lactose to the corresponding
lactones (
Henriksson et al., 1998
). Oxidation takes place in the flavin domain
where electrons are transferred to FAD and redistributed using electron
