tides in some brown rot species (Riley
et al. 2014), and continued research on
this theory will be important to demonstrate what role expressed glycopeptides
may play in brown rot degradation of
wood.
(ii) Work in the early 1990s through the present by Jellison and Goodell, Hammel and
Houtman, and other groups has demonstrated that brown rot fungi produce
LMW compounds that reduce iron in a
catalytic (repeated) manner, and these
compounds participate in oxidative reactions that depolymerize both cellulose
and lignin. This hypothesis has been
increasingly supported by the fungal degradation research community over the
last 25 years. Initial reports described
these compounds as siderophores
because they were isolated using procedures adapted from those used to isolate
siderophores from bacteria (Jellison et al.
1991). Because siderophore receptor sites
were not identified on the fungal cell
membranes and the catalytic function of
siderophores that was observed in the
compounds isolated from brown rot
fungi has not been widely reported, the
terminology of LMW “chelators” has been
used rather than “siderophore.” The term
“chelator-mediated Fenton” (CMF) system is now used to describe the system
(Goodell et al. 2017; Cragg et al. 2015;
Arantes et al. 2012; Kent et al. 2018) with
the seminal work on the CMF mechanism
in fundamental form described in the
mid-1990s (Goodell et al. 1997). Two
types of chelators, oxalate and a phenolate/hydroquinone, are involved with
some brown rot fungi including perhaps
the most widely studied brown rot fungus
Gloeophyllum trabeum. In this mechanism, oxalate first sequesters oxidized
iron within the low pH environment of
the fungal extracellular matrix (ECM)
produced within the wood cell lumen.
The fungus regulates oxalate concentrations carefully in the cell lumen, and it
has been proposed that oxalate-bound
iron diffuses into the wood cell wall at a
rate regulated by both intracellular and
extracellular fungal mechanisms. Oxalate
will release iron to phenolate/hydroquinone chelators in the environment of the
wood cell wall where pH is higher and
oxalate concentration is lower than in
the immediate environment surrounding
the fungal hyphae. Numerous groups
have demonstrated the production of
2,5-dimethoxyquinone (2,5-DMHQ) and
other chelators such as phenolate/hydroquinone-type chelators. These chelators,
including variegatic acid in some species
(Zhu et al. 2016), are produced by brown
rot fungi and are capable of multiple iron
reduction (MIR) similar to that described
which occurs with methoxylated phenolic
compounds derived from lignin (Tamaru
et al. 2019). These compounds are proposed to diffuse from the fungal hyphae
into the wood cell wall where the higher
pH of the cell wall (5.5–6.0) has then been
shown to permit a transfer of iron from
oxalate to the redox-cycling chelators.
Iron reduction then spontaneously occurs
in the higher pH environment. Once this
chemistry is initiated, a sustained reduction reaction will occur to generate a
stream of hydroxyl radicals (Tamaru
et al. 2019). The production of hydrogen
peroxide occurs naturally in the presence
of ferrous iron (Barb et al. 1951; Varela
and Tien 2003). The generation of hydrogen peroxide (30 mM concentration)
directly from water in microdroplet form
has also been reported to occur (Lee et al.
2019), and similar reactions could also
potentially generate hydrogen peroxide
in the microvoids of the wood cell wall,
particularly when transition metals are
present. The presence of peroxide
assumes adequate oxygenation of the cell
wall although the later systems demonstrates peroxide generation directly from
water microdroplets in oxygen-limited
environments. In any case, because oxygen is a prerequisite for fungal growth, an
15 Fungi Involved in the Biodeterioration and Bioconversion of Lignocellulose Substrates
377
et al. 2014), and continued research on
this theory will be important to demonstrate what role expressed glycopeptides
may play in brown rot degradation of
wood.
(ii) Work in the early 1990s through the present by Jellison and Goodell, Hammel and
Houtman, and other groups has demonstrated that brown rot fungi produce
LMW compounds that reduce iron in a
catalytic (repeated) manner, and these
compounds participate in oxidative reactions that depolymerize both cellulose
and lignin. This hypothesis has been
increasingly supported by the fungal degradation research community over the
last 25 years. Initial reports described
these compounds as siderophores
because they were isolated using procedures adapted from those used to isolate
siderophores from bacteria (Jellison et al.
1991). Because siderophore receptor sites
were not identified on the fungal cell
membranes and the catalytic function of
siderophores that was observed in the
compounds isolated from brown rot
fungi has not been widely reported, the
terminology of LMW “chelators” has been
used rather than “siderophore.” The term
“chelator-mediated Fenton” (CMF) system is now used to describe the system
(Goodell et al. 2017; Cragg et al. 2015;
Arantes et al. 2012; Kent et al. 2018) with
the seminal work on the CMF mechanism
in fundamental form described in the
mid-1990s (Goodell et al. 1997). Two
types of chelators, oxalate and a phenolate/hydroquinone, are involved with
some brown rot fungi including perhaps
the most widely studied brown rot fungus
Gloeophyllum trabeum. In this mechanism, oxalate first sequesters oxidized
iron within the low pH environment of
the fungal extracellular matrix (ECM)
produced within the wood cell lumen.
The fungus regulates oxalate concentrations carefully in the cell lumen, and it
has been proposed that oxalate-bound
iron diffuses into the wood cell wall at a
rate regulated by both intracellular and
extracellular fungal mechanisms. Oxalate
will release iron to phenolate/hydroquinone chelators in the environment of the
wood cell wall where pH is higher and
oxalate concentration is lower than in
the immediate environment surrounding
the fungal hyphae. Numerous groups
have demonstrated the production of
2,5-dimethoxyquinone (2,5-DMHQ) and
other chelators such as phenolate/hydroquinone-type chelators. These chelators,
including variegatic acid in some species
(Zhu et al. 2016), are produced by brown
rot fungi and are capable of multiple iron
reduction (MIR) similar to that described
which occurs with methoxylated phenolic
compounds derived from lignin (Tamaru
et al. 2019). These compounds are proposed to diffuse from the fungal hyphae
into the wood cell wall where the higher
pH of the cell wall (5.5–6.0) has then been
shown to permit a transfer of iron from
oxalate to the redox-cycling chelators.
Iron reduction then spontaneously occurs
in the higher pH environment. Once this
chemistry is initiated, a sustained reduction reaction will occur to generate a
stream of hydroxyl radicals (Tamaru
et al. 2019). The production of hydrogen
peroxide occurs naturally in the presence
of ferrous iron (Barb et al. 1951; Varela
and Tien 2003). The generation of hydrogen peroxide (30 mM concentration)
directly from water in microdroplet form
has also been reported to occur (Lee et al.
2019), and similar reactions could also
potentially generate hydrogen peroxide
in the microvoids of the wood cell wall,
particularly when transition metals are
present. The presence of peroxide
assumes adequate oxygenation of the cell
wall although the later systems demonstrates peroxide generation directly from
water microdroplets in oxygen-limited
environments. In any case, because oxygen is a prerequisite for fungal growth, an
15 Fungi Involved in the Biodeterioration and Bioconversion of Lignocellulose Substrates
377