Biology Reference
In-Depth Information
electron oxidation of aromatic rings, which are then degraded by numerous
non-enzymatic reactions. The main ones are C
a
C
b
cleavages of the lignin
side chains, generating radical structures and aldehydes. In addition, LiPs
can catalyse the oxidation of benzylic alcohol groups to the corresponding
aldehydes or ketones (
Kersten and Cullen, 2007
).
LiPs may also act on lignin in an indirect way, through redox chemical
mediators of low molecular weight. This mechanism was suggested because
veratryl alcohol (VA) is produced at the same time as LiP by P. chrysospor-
ium (
Fenn and Kirk, 1981
). VA can be oxidized by LiP to form a radical
cation, which could then act in lignin degradation as a diffusible redox
mediator. It was also shown that, in the presence of VA, LiP could oxidize
lignin model compounds. However, the role of VA as a diffusible mediator
has been questioned because of the instability of its radical. Its major role
seems to protect LiP from inactivation by H
2
O
2
, by reactivating compound
III or by inhibiting its formation from compound II.
2. Manganese peroxidases
MnPs were discovered in the mid-1980s by two international teams and in
P. chrysosporium which produces several MnP isoenzymes (
Glenn et al.,
1983; Kuwahara et al., 1984
). Their expression was shown to be regulated
by the presence of Mn(II) in the culture medium. They are described as true
ligninases because of their high redox potential. MnPs are extracellular
glycosylated enzymes containing one ferric protoheme IX per molecule.
Their molecular mass ranges from 38 to 62.5 kDa, including 4-18% glycosyl-
ation, and their pI from 2.9 to 7.1. Their maximum activity temperature lies
between 30 and 40
C.
The catalytic cycles of MnPs (
Fig. 2
B) and LiPs are very similar, but Mn-
dependent peroxidases are unique in using Mn(II) as sole reducing substrate
(
Glenn andGold, 1985; Paszczynski et al., 1986
). Addition of H
2
O
2
to the native
enzyme yields a MnP-I compound which is a Fe(IV)-oxo-porphyrin radical
cation (P
8
O
.
þ
). This compound undergoes a first one-electron re-
duction by Mn(II) to give Mn(III) and MnP-II (P
Fe(IV)
¼¼
Fe(IV)
¼¼
O). A second
reduction step then restores native enzyme (P
Fe(III)) and a second Mn(III)
(
Wariishi et al., 1988
). Like LiPs, compound II reacts with additional H
2
O
2
to
form a Fe(III)-superoxo complex (compound III), which is an inactive form.
However, the degree of inactivation is much lower than that of LiPs (
Wariishi
and Gold, 1990
). The reactivation of compound III is mediated by Mn(III),
which interacts by either oxidizing the iron-coordinated superoxide or reacting
with H
2
O
2
in a catalase-type activity (
Timofeevski et al., 1998
).
The crystal structure of the MnP of P. chrysosporium was published
shortly after the LiP structure (
Sundaramoorthy et al., 1994
). These two
