Biology Reference
In-Depth Information
Mn-dependent peroxidases are unique in utilizing Mn(II) as their reducing
substrate, generating Mn(III), which diffuses into the lignocellulose structure
and in turn oxidizes a variety of monomeric phenols. Mn(III) is stabilized by
chelating organic acids such as oxalate, fumarate and malate (
Martinez et al.,
2005
). Owing to the relatively low redox potential of the Mn(II)-Mn(III)
system (around 0.8 V), the Mn(III)-chelator complex is a mild oxidant,
limited to the oxidation of phenolic lignin structures and is hardly able to
directly oxidize non-phenolic compounds. However, MnPs are able to cleave
C
a
C
b
and
b
-aryl ether bonds by indirect oxidation through the peroxida-
tion of lipids. Such a mechanism is supported by studies on non-phenolic
lignin models (
Martinez et al., 2005
) and was observed in cultures of white-
rot on wood. However, the involvement of peroxidation in MnP-driven
lignin degradation has not been clearly demonstrated.
It was demonstrated that when a Mn
2
þ
-binding site was introduced by site-
directed mutagenesis into a P. chrysosporium LiP, the resulting enzyme had
MnP activity (
Mester and Field, 1998
), and, conversely, when the hydroxy-
lated tryptophan residue involved in LiP interaction with VA was introduced
in a position corresponding to Trp171 into MnP, this enzyme acquired LiP
activity (
Timofeevski et al., 1998
).
3. Versatile peroxidases
Versatile peroxidases (VPs) were first described in 1996 (
Martinez et al., 1996
)
before their characterization in P. eryngii (
Ruiz-Duenas et al., 1999
), a fungus
studied for its ability to selectively degrade lignin. They were further isolated
from several other species such as P. ostreatus, Pleurotus pulmonarius, Bjerkan-
dera adusta (
Heinfling et al., 1998a,b,c; Mester and Field, 1998; Ruiz-Duenas
et al., 2001
). VPs are glycoproteins produced as several isoenzymes with a
molecular mass ranging between 40 and 45 kDa and a pI ranging between 3.4
and 3.9 (
Mester and Field, 1998; Ruttiman-Johnson et al., 1994
).
The molecular characterization of VPs revealed structures closer to LiP
than to the MnP isozymes of P. chrysosporium. A Mn(II)-binding site con-
taining Glu/Glu/Asp residues typical of MnP was found near the heme
internal propionate. In addition, a tryptophan residue involved in LiP inter-
action with VA and other aromatic substrates was also found in the protein
structure (
Camarero et al., 1999; Mester and Field, 1998
). Their proximity
allows a direct transfer of electrons fromMn
2
þ
to the heme. By contrast, VA
and other non-phenolic substrates are oxidized at the protein surface by a
relatively long-range electron transfer pathway via a Trp residue (
Fig. 4
).
This mechanism is necessary because many aromatic substrates, including
lignin polymers, cannot penetrate inside the protein to transfer electrons
directly to the cofactor. Therefore, these substrates are oxidized at the
