6 Wood Discoloration

106 4 Wood Cell Wall Degradation
be difficult to regenerate once released extracellularly are precluded. In vitro,
reduction of manganese dioxide was demonstrated for a ferrireductase system
that includes NADPH-dependent ferrireductase and the iron-binding compound
from Phanerochaete sordida (Hirai et al. 2003). Extracellularly formed
free-radical species are able to diffuse away from their origin and mediate reactions
with the insoluble lignin. The small, diffusible radicals and low-molecular
agents achieve a greater area of reactivity than could be obtained by reactions
catalyzed by enzymes or the fungi directly. The distance of the action from the
hyphae also prevents self-inflicted damage to the fungus (Reading et al. 2003).
The following description of systems to generate low molecular agents is
according to Messner et al. (2003).
In the “manganese peroxidase/Mn(II)/oxalate system”, there are two oneelectron
reducing steps by Mn(II). The Mn(III) formed is chelated and released
from the enzyme by the fungal metabolite oxalate. The relatively stable Mn(III)
oxalate oxidizes phenolic lignin compounds and has been proposed to diffuse
in the wood cell wall.
In the “manganese peroxidase/Mn(II)/oxalate/cellobiose dehydrogenase
system”, CDH is oxidized by O2 and metal ions such as Fe(III) and Cu(II)
yielding H2O2, and Fe(II) or Cu(I) which react with H2O2 to generate hydroxy
radicals which in turn demethoxylate and hydroxylate non-phenolic lignin.
The phenolic lignin formed is then attacked by MnP-generated Mn(III).
In the “manganese peroxidase/Mn(II)/oxalate/lipids system”, lipids extend
the oxidative potential of MnP. Mn(III) promotes peroxidation of unsaturated
fatty acids resulting in the formation of peroxyl radicals which are diffusible,
potentially ligninolytic agents. Mn(III) also abstracts hydrogen from fatty
acids to form acyl radicals. The system depolymerized both phenolic and
non-phenolic lignin (Katayama et al. 2000).
In the “lignin peroxidase/veratryl alcohol system”, the veratryl alcohol radical,
generated during turnover of LiP, was proposed to act as a charge transfer
system in wood. However, its short lifetime may prevent a diffusion into deeper
cell wall areas.
In the “laccase/mediator system”, laccases are combined with low molecular
weight charge transfer agents, so-called mediators. The system is used to bleach
pulp and depolymerized non-phenolic guaiacyl lignin.
In the “glycopeptide system” (Enoki et al. 2003), low-molecular weight
glycosylated peptides produce hydroxy radicals which modify lignin, resulting
in new phenolic, benzyl radical, and cation radical substructures which are
then attacked by LiP, MnP or laccase. The system also depolymerizes the wood
carbohydrates (see Chap. 4.4).
In the “coordinated Cu/peroxide system” (Messner et al. 2003), either hydrogen
peroxide or organic peroxides, is the agent involved at least in the
initial lignin degradation. Cu(II) is reduced to Cu(I) by either H2O2 or reducinggroupsinwood.Cu(I)formswithH2O2
a reactive one-electron oxidant
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