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