6 Wood Discoloration

4.5 Lignin Degradation 103
(porphyrin with iron as central atom), needs extracellular H2O2, cleavesC-C
bonds in a number of model compounds, and oxidizes benzyl alcohols to
aldehydes or ketones.
The key reaction of the LiP is a one-electron-oxidation of various nonphenolic
compounds to generate instable aryl radical cations, as the enzyme
delivers two electrons to hydrogen peroxide, which the enzyme then takes back
from each one-phenyl propenoid unit (Kirk 1985; Higuchi 1990). Phenolic
and non-phenolic lignin substructures are attacked, but not the intact lignin
molecule.
The radicals undergo subsequent non-enzymatic reactions to yield a variety
of final products. The radical cations themselves act as oxidants. Thus, LiP
initiates by means of different non-enzymatic reactions the cleavage of Cα-Cβ
bond in the side chain, β-O-4 bond between side chain and next ring, as well
the aromatic ring (Eriksson et al. 1990; Schoemaker et al. 1991; Fig. 4.6). Also,
veratryl alcohol, which is produced independently of lignin degradation, can
be oxidized by LiP to the radical cation, which itself can oxidize lignin (Jennings
and Lysek 1999).
The ligninolytic system of Phanerochaete chrysosporium is not induced by
lignin but appears constitutively as cultures enter the secondary metabolism,
that is, when primary growth ceases because of depletion of nutrients. Secondary
metabolism was triggered by nitrogen, carbon, or sulphur limitation
(review by Highley and Dashek 1998). Lignin cannot be used as only C source,
but in cometabolism with cellulose or hemicellulose. A high O2 concentration
(100% more suitable than 21%) was favorable (Kirk 1988). The enzyme was
excreted by old, autolytic hyphae, but not by arthrospores and chlamydospores
(Lackner et al. 1991).
LiP has been found in several white-rot fungi, e.g., Trametes versicolor,
Phlebia radiata (Dodson et al. 1987) and Bjerkandera adusta (Muheim et al.
1990). There are numerous isomers of LiP with molecular weights of 40 to
47 kDa, which differ in the carbohydrate portion of the protein (Evans 1991).
The enzyme activity of LiP preparations is determined via Cα oxidation of
Fig.4.6. Scheme of reactions initiated
by lignin peroxidase. Cleavage of Cα-Cβ
bond in the side chain (1), β-O-4 bond
between side chain and next ring (2),
and aromatic ring (3)
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