Biology Reference
In-Depth Information
lignolytic enzyme found in the fungal secretome, as no significant peroxidase
activity can be measured. The enzyme is produced in both submerged cultures
(
Eggert et al., 1996; Lomascolo et al., 2003
) and solid-state fermentation of
agro-residues (
Meza et al., 2006; Vikineswary et al., 2006
).
Laccases are multi-copper oxidases containing (at least) four copper
atoms. One of these copper atoms determines the substrates to be oxidized
according to its redox potential, and the three other copper atoms transfer
the electrons to O
2
. Their molecular mass ranges between 54 and 80 kDa.
They are glycosylated, and carbohydrates represent 10-20% of the molecular
mass, although they can reach 49% for B. cinerea (
Slomczynski et al., 1995
).
This high level of glycosylation protects laccases against thermal degradation
and makes them relatively thermostable up to 70
C(
Yaropolov et al., 1994
).
Fungal genomes often contain several genes for laccase, leading to different
isoenzymes that may be constitutive or inducible (
Leonowicz et al., 1978
).
Many aromatic molecules are able to enhance or induce laccase production,
including ferulic acid or VA. Methanol and higher aliphatic alcohols are also
very efficient inducers (
Lomascolo et al., 2003
). However, the effect is strain
dependent and the mechanism of induction is not yet clearly understood.
Fungal laccases are involved in pigment biosynthesis, mainly of phenoxazi-
none derivatives (
Thurston, 1994
). They are involved in cinnabarinic acid
synthesis in P. cinnabarinus (
Eggert et al., 1995
) and in cinnabarin in Pycno-
porus sanguineus (
Smˆnia et al., 1998
), which both have an antimicrobial effect.
Laccases are also involved in fructification, sporulation and phytopathogeni-
city of fungi (
Gianfreda et al., 1999
). Besides these functions, fungal laccases
may play a major role in the depolymerization of lignin (
Hatakka, 1994
).
Laccases are generally monomeric, but dimers and tetramers have been
observed in Agaricus bisporus (
Wood, 1980
), Podospora anserina (
Durrens,
1981
)andTrametes villosa (
Yaver et al., 1996
). The first reported tri-
dimensional structure was an inactive form of Coprinus cinereus laccase, in
which a copper centre was missing (
Ducros et al., 1998
). The structure was
found to be organized into three domains essentially made up of
b
-sheets,
which are strongly associated and envelop the copper centres. This structure
was further confirmed by crystallographic studies on T. versicolor (
Antorini
et al., 2002; Bertrand et al., 2002
), P. ostreatus and Melanocarpus albomyces
(
Hakulinen et al., 2002
).
The active site of laccase is relatively well conserved (
Fig. 5
A). It is
composed of four copper atoms bound to three redox sites (type-1, type-
2 and type-3 Cu pair;
Gianfreda et al., 1999
). The four copper atoms differ in
their characteristic electronic paramagnetic resonance patterns. The type-1
copper, bound to two histidine residues and one cysteine, is responsible for
substrate oxidation and for the redox potential of the enzyme. The type-1
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