Environmental Engineering Reference
In-Depth Information
Fig. 2.3
Mechanism of
action for manganese
peroxidase (MnP).
ox
oxidized state of enzyme.
(Breen and Singleton
1999
)
cals, which lead to decomposition of com-
pounds (Fig.
2.3
). MnP catalyzes the oxida-
tion of several monoaromatic phenols and
aromatic dyes, but depends on both divalent
manganese and certain types of buffers. The
enzyme requirement for high concentrations
of Mn(III) makes its feasibility for wastewa-
ter treatment application doubtful (Karam and
Nicell
1997
). Evidence for the crucial role of
MnP in lignin biodegradation are accumulat-
ing, e.g., in depolymerization of lignin (Warii-
shi et al.
1991
) and chlorolignin (Lackner
et al.
1991
), in demethylation of lignin and
delignification and bleaching of pulp (Paice
et al.
1993
), and in mediating initial steps in
the degradation of high-molecular mass lignin
(Perez and Jeffries
1992
).
c.
Laccase
Laccase (EC 1.10.3.2, benzenediol:oxygen
oxidoreductase) is a multicopper blue oxidase
capable of oxidizing
ortho
- and
para
-diphe-
nols and aromatic amines by removing an
electron and proton from a hydroxyl group to
form a free radical. Laccase in nature can be
found in eukaryotes as fungi (principally by
basidiomycetes), plants, and insects. Howev-
er, in recent years, there is an increasing evi-
dence for the existence in prokaryotes (Claus
2003
). Corresponding genes have been found
in gram-negative and gram-positive bacteria
Azospirillum lipoferum
(Bally et al.
1983
),
Marinomonas mediterranea
(S￡nchez-Amat
and Solano
1997
), and
Bacillus subtilis
(Mar-
tins et al.
2002
).
Laccases not only catalyze the removal of a
hydrogen atom from the hydroxyl group of
methoxy-substituted monophenols,
ortho
- and
para
-diphenols, but can also oxidize other sub-
strates such as aromatic amines, syringaldazine,
and nonphenolic compounds to form free radi-
cals (Bourbonnais et al.
1997
; Li et al.
1999
).
After long reaction times there can be coupling
reactions between the reaction products and even
polymerization. It is known that laccases can cat-
alyze the polymerization of various phenols and
halogen, alkyl- and alkoxy-substituted anilines
(Hoff et al.
1985
). The laccase molecule, as an ac-
tive holoenzyme form, is a dimeric or tetradimer-
ic glycoprotein, usually containing four copper
atoms per monomer, bound to three redox sites
(Fig.
2.4
). The molecular mass of the monomer
ranges from about 50-100 kDa. Typical fungal
laccase is a protein of approximately 60-70 kDa
with acidic isoelectric point around pH 4.0. Sev-
eral laccase isoenzymes have been detected in
many fungal species. Several laccases, however,
exhibit a homodimeric structure, the enzyme
being composed of two identical subunits with a
molecular weight typical for monomeric laccase.
Application of Laccases
The interest in laccases
as potential industrial biocatalysts has particu-
larly increased after the discovery of their abil-
ity to oxidize recalcitrant nonphenolic lignin
compounds (Li et al.
1999
). This capability has
later been shown to be generally applicable to
a number of biotechnological problems; all of
them are related to the degradation or chemi-
cal modification of structurally diverse com-
pounds, being either xenobiotic or naturally
occurring aromatic compounds. Laccase is cur-
rently being investigated by a number of research
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