Biology Reference
In-Depth Information
after four electron transfers from an oxidized substrate (generally electrons
from aromatic rings).
Owing to their relatively low redox potential, the only phenolic com-
pounds that fungal laccases are able to directly oxidize are polyphenols,
monophenols, amino-phenols and aromatic amines (
Thurston, 1994
). They
can oxidize the phenolic units of lignin, as demonstrated by studies on model
molecules (
Higuchi, 1989
). In the presence of dioxygen, laccases are able to
catalyse the formation of phenoxy radicals, which further undergo non-
enzymatic reactions that can result in bond cleavage (
Martinez et al., 2005
).
However, free phenolic lignin units may compose less than 10% of the total
polymer. On this basis, if the role of fungal laccases was limited to the
oxidation of lignin units with free phenolic groups, an efficient and laccase-
driven delignification would be very improbable (
Bourbonnais and Paice,
1990
). However, even if laccases cannot directly oxidize non-phenolic units
of lignin, they are nevertheless able to oxidize high redox potential substrates
in the presence of chemical mediators. The natural mediators involved in lignin
biodegradation remain to be identified, although some lignin-derived phenols
such as acetosyringone or methylsyringate may act as efficient laccase media-
tors (
Camarero et al., 2005
). Interest in laccases for biotechnological applica-
tions increased with the discovery of synthetic mediators (
Bourbonnais et al.,
1995; Fabbrini et al., 2002
). The most studied and most efficient mediators are
ABTS (2,2
0
-azinobis(3-ethylbenzothiazoline-6-sulfonate)), HBT (1-hydroxy-
benzotriazole) and other molecules bearing an N-hydroxy function. In
these conditions, it was demonstrated that the laccase mediator system could
cleave the C
a¼¼
C
b
bond in dimer models of lignin. To make such a reaction
possible, the redox potential of the laccase has to be artificially increased.
This may be done by the displacement of oxidant/reductor equilibrium, that
is, high concentration of oxidized mediator, or by the use of a mediator such
as ABTS, which undergoes spontaneous disproportionation. ABTS (E
0
of
ABTS/ABTS
.
þ
¼
470 mV/ENH) is oxidized by laccase. The oxidation product
ABTS
.
þ
undergoes spontaneous disproportionation to give ABTS and
ABTS
2
þ
(E
0
of ABTS
.
þ
/ABTS
2
þ
¼
885 mV).
C. AUXILIARY ENZYMES FOR HYDROGEN PEROXIDE PRODUCTION
Oxidative lignin degradation mediated by peroxidases implies production
of H
2
O
2
as a result of the biocatalysed reduction of dioxygen to peroxide.
A subsidiary role of H
2
O
2
in wood degradation is related to its ability to
produce hydroxyl radicals (OH
.
) by Fenton-type reactions catalysed by
enzymes such as cellobiose dehydrogenases (CDHs), which may participate
in lignin degradation. Hydrogen peroxide producing enzymes, generally
