6 Wood Discoloration

4.5 Lignin Degradation 105
genes encoding two laccase isozymes have been cloned and sequenced (also
Eggert et al. 1998). Glyoxal oxidase as a source of extracellular H2O2 was found
to be encoded by a single gene.
Occurrence and distribution of lignin peroxidase inside hyphae and whiterotten
wood were examined by immuno gold labeling (Srebotnik et al. 1988a;
Blanchette et al. 1989; Daniel et al. 1989, 1990; Blanchette and Abad 1992; Kim
et al. 1993). The enzyme is particularly found in the hypha and the extracellular
sheath, and less so in the wood cell wall and then near the hypha. In the cell
wall, it is only considerably present in late degradation stages. It was concluded
from this distribution that the lignin peroxidase attacks rather lignin fragments
that had been set free from the cell wall, than that it binds at the polymeric
lignin inside the intact wall. The primary degradation would have then taken
place by low-molecular compounds like the cation radical of veratrylalcohol,
which diffuses into the wall, produces there lignin fragments, which are then
degraded by ligninase (Evans 1991). It may also assumed that the limited cell
wall degradation starting from the cell lumen in close neighborhood of a hypha
occurs directly by the enzyme towards closely neighboring lignin. This would
agreewiththeearlyresultsoftheerosion-likecellwalldegradationbywhite-rot
fungi (Schmid and Liese 1964; Liese 1970; Fig. 7.2b).
There are many ways that a white-rot fungus could generate hydrogen peroxide
required for LiP and MnP. Extracellular H2O2-producing enzymes are arylalcohol
oxidase (EC 1.1.37), glyoxal oxidase, pyranose 2-oxidase (EC 1.1.3.10),
and cellobiose dehydrogenase. Intracellular enzymes include glucose 1-oxidase
(EC 1.1.3.4) (Leonowicz et al. 1999), pyranose 2-oxidase, and methanol oxidase
(e.g., Daniel et al. 1994; Hyde and Wood 1997; Urzúa et al. 1998). OH 0
may be also formed via hydroquinone redox cycling involving semiquinones
produced by peroxidase or laccase which reduce both Fe(III) and O2 to provide
the components for Fenton-type hydroxyl radical formation. It is not exactly
known which enzyme plays the primary role in supplying H2O2 (Li 2003).
From the only slow microbial decomposition of lignin results its significance
for the formation of humic substances (e.g., Haider 1988; Schlegel 1992) and
also for the lastingness of archaeological woods (Chap. 5.2). The suitability of
lignins as fertilizer and for soil improvement was described by Faix (1992).
Mikulášová and Košíkowá (2002) indicated a potential application of lignin
biopolymers as antimutagenic agents in chemoprevention.
There are some general prerequisites for the action of the degradative systems.
As lignin is a highly oxidized polymer, reductive as well as oxidative
reactions are required to effectively degrade it, both of which must occur aerobically.
These reactions must be balanced or controlled to prevent redox cycling
and free-radical-based polymerization of the degradation products. The oxidizing
and reducing equivalents must be unique and continuously produced
since extracellular regeneration would be improbable. Common biological
compounds for reducing or oxidizing equivalents, such as NADH, which would
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