Biology Reference
In-Depth Information
II. PLANT LACCASES ARE MULTICOPPER
OXIDASES SHOWING BROAD IN VITRO
SUBSTRATE SPECIFICITY
Laccases (EC 1.10.3.2) are a large group of multicopper oxidases occurring in
eukaryotes (higher plants, fungi, animals) and in prokaryotes (
Claus, 2004
).
The localization of plant and fungal laccases is extracellular, whereas bacte-
rial laccases are intracellularly localized (
Diamantidis et al., 2000
).
Laccases are monomeric, dimeric, or tetrameric glycoproteins. These oxi-
dases are copper proteins that are able to catalyse the one-electron oxidation
of a broad range of aromatic or non-aromatic compounds (phenols, anilines,
arylamines, thiols, monolignols, and lignins) with the concomitant four-
electron reduction of molecular oxygen to water. As with other multicopper
oxidases, plant laccases contain four copper atoms (
Messerschmidt et al.,
1992
) that are distributed into three types of copper centres, T1, T2, and T3,
according to their spectroscopic properties (
Bento et al., 2006
).
Although a number of 3D structures of bacterial and fungal laccases have
been resolved (as reviewed in Chapter
8
), there is currently no available crystal-
lographic 3D structure of plant laccases. The only plant laccases structure
reported so far has been obtained by a homology modelling approach using
bioinformatics compared to fungal and bacterial enzymes (
Dwivedi et al.,
2011
). These laccases models are based on sequence conservation and more
particularly on the high conservation of histidine motifs binding copper atoms.
Based on these models, the monomers of plant, fungal, and bacterial enzymes
would all display a similar architecture organized in three sequentially arranged
cupredoxin-like domains mainly formed by beta-barrels (
Dwivedi et al.,2011
).
Fungal and plant laccases are known to be glycoproteins with a sugar-moiety
representing 10-55% of their molecular weight. Plant laccases are more highly
glycosylated than fungal laccases. The carbohydrate region contains monosac-
charides such as hexosamines, glucose, mannose, galactose, fucose, and arabi-
nose (
Rogalski and Leonowicz, 2004
) and glycosylation seems necessary for
secretion, copper retention, proteolytic susceptibility, thermal stability, and
enzymatic activity (
Xu, 1999
). Putative glycosylation sites are identified by the
consensus Asn-X-Thr sequence, even though not all sites are glycosylated. For
example, the Trametes versicolor laccases (LacIIIb) sequence contains six puta-
tive N-glycosylation sites, but only four of them are glycosylated (
Bertrand et al.,
2002
). Fungal laccases have isoelectric points (pI)around4(reviewedin
Rodgers
et al., 2010
), whereas plant laccases pI values are somewhat higher ranging from
5 to 9. Plant laccases show an optimal pH activity in the range of 5-7.5, whereas
fungal laccases have a lower optimal pH ranging between 3.5 and 5.5. Some
biochemical traits of plant and fungal laccases are summarized in
Table I
.
