Biology Reference
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referred to as oxidases, may have a flavinic part such as aryl-alcohol oxidase
(AAO) or coordinate a metal atom, for example, copper as in glyoxal oxidase
(Glox). These enzymes are frequently found in WRF secretomes (
Levasseur
et al., 2008b
).
1. Glyoxal oxidase
A Glox was first described in P. chrysosporium (
Kersten, 1990
), as an extracel-
lular monomeric glycoprotein of molecular mass 68 kDa. Two isoforms were
isolated with pI 4.7 and 4.9, probably corresponding to the two isomeric forms
described by
Kersten et al.(1995)
. Glox is released in the culture medium
of P. chrysosporium in the presence of glucose or xylose, the main sugars of
cell wall polysaccharides. However, these sugars are not oxidized by Glox,
which oxidizes aldehydes or glycoaldehydes produced from lignin oxidation by
peroxidases. Glox has a broad substrate pattern and oxidizes several simple
aldehydes such as glyoxal or methylglyoxal to their corresponding carboxylic
acids. These products are found in lignolytic fungal cultures, confirming their
physiological role in vivo. This redox reaction is coupled to the reduction of O
2
to H
2
O
2
(
Cullen and Kersten, 2004
).
Structurally, the Glox of P. chrysosporium belongs to the copper metal-
loenzyme family and contains a free radical bound to the copper of the active
site, which is similar to that of galactose oxidase (
Whittaker et al., 1996
). In
these enzymes, a radical formed on the side chain of an amino acid is bound
to the copper atom of the active site, forming a pattern characteristic of an
enzyme family known as copper radical oxidases. Four copper ligands were
identified by site-directed mutagenesis: Tyr377, Tyr135, His378 and His471
(
Whittaker et al., 1999
). The purified Glox of P. chrysosporium is reversibly
inactivated in the absence of a peroxidase system. The presence of LiP or of
non-phenolic substrates of LiP restores Glox activity. Conversely, phenolic
compounds prevent the reactivation of Glox by LiP, which suggests a regu-
lation mechanism based on peroxidases, their substrates and products. The
transition between native (inactive) and active forms requires the presence of
oxidants and is linked to the formation of the copper radical complex (
Cullen
and Kersten, 2004
).
2. Aryl-alcohol oxidase
An AAO was first detected in Polystictus versicolor by
Farmer et al. (1960)
.
Similar activities were further identified, purified and characterized in three
Pleurotus species (P. sajor-caju:
Bourbonnais and Paice, 1988;
P. eryngii:
Guillen et al., 1990
; and P. ostreatus:
Sannia et al., 1991
)andinB. adusta
(
Muheim et al., 1990
). These enzymes are quite different from the AAOs
found in Fusarium solani (
Iwahara et al., 1980
), P. chrysosporium (
Asada
