Biology Reference
In-Depth Information
laccase are classified as medium. Pycnoporus laccases belong to the group
with the highest redox potential, with values of 0.72-0.8 V (
Sigoillot et al.,
2004; Uzan et al., 2010
), along with T. versicolor, T. villosa and P. ostreatus
laccases. These differences may be explained by variations in the electronic
environment of the type-1 copper, such as the substitution of a methionine by
a leucine in the active site (
Garavaglia et al., 2004; Piontek et al., 2002
).
Laccases catalyse the one-electron oxidation of a wide range of com-
pounds including di-, substituted phenols and polyphenols, and di- and
aromatic amines to form free radicals, which in turn can non-enzymatically
produce dimers, oligomers and polymers (
Baldrian and Valaskova, 2008
).
The reactions of oxidation catalysed by laccase are accompanied by the
reduction of dioxygen to water (
Fig. 6
). One catalytic cycle thus corresponds
to the transfer of four electrons from type-1 copper to the tri-nuclear centre
via a His-Cys-His tripeptide pathway (
Piontek et al., 2002
). In the native
form, laccase is totally oxidized, with four Cu(II) that are totally reduced
Cu
+
Cu
2+
T
1
4 R
+
4 H
+
O
4 RH
O
Cu
+
Cu
+
Cu
2+
Cu
2+
T
3
T
2
Cu
+
Cu
2+
H
2
O
O
2
2H
+
Cu
2+
Cu
+
O
H
O
Cu
2+
Cu
2+
Cu
2+
Cu
2+
2
-
O
-
O
2H
+
2
H
2
O
Cu
+
Cu
2+
Fig. 6. Catalytic cycle of laccase. In the native form of enzyme, all the four
coppers are fully oxidized (Cu
2
þ
). Four electron transfers from four aromatic rings
give the fully reduced form which is further reoxidized by dioxygen (adapted from
Wong, 2008
).
