It has been reported that carboxylic acids can
also function as a mediator in the degradation
of high redox potential substrates (Qin et al.
2017). Some aliphatic acids such as malonate,
lactate, and oxalate produced by white rot fungi
can function as metal-chelating agents and
increase the oxidation rate of Mn
2+ to allow
substrate attack (Hofrichter 2002). The chelated
Mn
3+ functions as a LMW diffusible redox
mediator that can attack phenolic lignin to produce unstable radical species. The chelated
Mn
3+ is capable of diffusing into the wood cell
wall to oxidize phenolic lignin compounds, and
this is one of the potential LMW diffusible
species observed by Daniel as cited above (Daniel 2014). MnP has a lower redox potential than
LiP and under physiological conditions can
only oxidize phenolic lignin structures.
Versatile Peroxidases Versatile peroxidases
(VP) are considered to be hybrids between
LiP and MnP (Knop et al. 2016), and they are
enzymes with broad substrate activity. VP can
directly oxidize non-phenolic substrates such
as veratryl alcohol as well as phenolic substrates (Fernandez-Fueyo et al. 2012; Ma ¨kela ¨
et al. 2015). VPs are more widely produced in
white rot fungi than originally considered
when they were first cloned and sequenced in
the year 2000 (Giardina et al. 2000). They are
now known to be produced in many fungi,
such as Phanerochaete chrysosporium, Ceriporiopsis subvermispora, Pleurotus ostreatus, P.
eryngii, and Bjerkandera sp. (Floudas et al.
2012). VPs have been reported to oxidize phenolic and non-phenolic substrates directly
without the aid of a mediator, and they have
the capacity to oxidize aromatic substrates
directly.
Dye-Decolorizing Peroxidases The dye-decolorizing peroxidases (DyP) were first reported in
1995 in a Geotrichum (Ascomycota) species but
have recently become more widely reported in
white rot fungi. The DyP-type peroxidases have
no homology to other known peroxidases. They
are unable to oxidize Mn
2+
, and they are unique
enough that they are classified in their own
superfamily of heme peroxidases (EC
1.11.1.19). They are similar to VPs in that they
can oxidize Reactive Black 5, phenols, and veratryl alcohol, but unlike VPs they also have the
ability to oxidize recalcitrant anthraquinone
dyes (ExPASy 2018; Lauber et al. 2017).
Laccase Unlike the peroxidases, the laccase
enzyme employs a mechanism involving a
four-electron reduction of oxygen to degrade
lignin. Laccase is produced by many microbial
organisms, both degradative and nondegradative, and may play multiple metabolic
roles. Laccase alone is not capable of oxidizing
non-phenolic lignin compounds but can oxidize “mediator” compounds to their radical
forms, and in close proximity to laccase, these
compounds then catalyze the oxidation of nonphenolic lignin. Natural mediators have been
reported, including ortho-diphenols, paradiphenols, methoxysubstituted phenols, diamines, and benzenethiols. Since laccase is a
robust enzyme, its potential for industrial use
is high, and many uses have been studied ranging from the grafting of polymers to lignin to
the production of fiberboard products (Mate
Diana and Alcalde 2017). Several lignin-derived
compounds (Camarero et al. 2005), and also
hydroxamate siderophores from fungi, have
been reported as useful in mediating laccaseaided delignification processes (Niku-Paavola
et al. 2003). Fungi are generally reported to
produce hydroxamic acid siderophores in the
broader literature; however, research on wooddegrading fungi has isolated many catecholate
compounds that are “siderophore-like” in their
structure. Given that the structure of lignin has
more catecholate properties, it would be useful
also to assess the catecholate/phenolate compounds similar to siderophores that are
produced by wood-decaying Basidiomycota
species as laccase mediator compounds. Several
LMW lignin-derivative compounds have now
been reported to have metal reduction capabilities and have been postulated to also function
in electron-transfer capacity for extracellular
enzymes (Tamaru et al. 2019). Laccase-derived
lignin fragments have also been found to
15 Fungi Involved in the Biodeterioration and Bioconversion of Lignocellulose Substrates
383
also function as a mediator in the degradation
of high redox potential substrates (Qin et al.
2017). Some aliphatic acids such as malonate,
lactate, and oxalate produced by white rot fungi
can function as metal-chelating agents and
increase the oxidation rate of Mn
2+ to allow
substrate attack (Hofrichter 2002). The chelated
Mn
3+ functions as a LMW diffusible redox
mediator that can attack phenolic lignin to produce unstable radical species. The chelated
Mn
3+ is capable of diffusing into the wood cell
wall to oxidize phenolic lignin compounds, and
this is one of the potential LMW diffusible
species observed by Daniel as cited above (Daniel 2014). MnP has a lower redox potential than
LiP and under physiological conditions can
only oxidize phenolic lignin structures.
Versatile Peroxidases Versatile peroxidases
(VP) are considered to be hybrids between
LiP and MnP (Knop et al. 2016), and they are
enzymes with broad substrate activity. VP can
directly oxidize non-phenolic substrates such
as veratryl alcohol as well as phenolic substrates (Fernandez-Fueyo et al. 2012; Ma ¨kela ¨
et al. 2015). VPs are more widely produced in
white rot fungi than originally considered
when they were first cloned and sequenced in
the year 2000 (Giardina et al. 2000). They are
now known to be produced in many fungi,
such as Phanerochaete chrysosporium, Ceriporiopsis subvermispora, Pleurotus ostreatus, P.
eryngii, and Bjerkandera sp. (Floudas et al.
2012). VPs have been reported to oxidize phenolic and non-phenolic substrates directly
without the aid of a mediator, and they have
the capacity to oxidize aromatic substrates
directly.
Dye-Decolorizing Peroxidases The dye-decolorizing peroxidases (DyP) were first reported in
1995 in a Geotrichum (Ascomycota) species but
have recently become more widely reported in
white rot fungi. The DyP-type peroxidases have
no homology to other known peroxidases. They
are unable to oxidize Mn
2+
, and they are unique
enough that they are classified in their own
superfamily of heme peroxidases (EC
1.11.1.19). They are similar to VPs in that they
can oxidize Reactive Black 5, phenols, and veratryl alcohol, but unlike VPs they also have the
ability to oxidize recalcitrant anthraquinone
dyes (ExPASy 2018; Lauber et al. 2017).
Laccase Unlike the peroxidases, the laccase
enzyme employs a mechanism involving a
four-electron reduction of oxygen to degrade
lignin. Laccase is produced by many microbial
organisms, both degradative and nondegradative, and may play multiple metabolic
roles. Laccase alone is not capable of oxidizing
non-phenolic lignin compounds but can oxidize “mediator” compounds to their radical
forms, and in close proximity to laccase, these
compounds then catalyze the oxidation of nonphenolic lignin. Natural mediators have been
reported, including ortho-diphenols, paradiphenols, methoxysubstituted phenols, diamines, and benzenethiols. Since laccase is a
robust enzyme, its potential for industrial use
is high, and many uses have been studied ranging from the grafting of polymers to lignin to
the production of fiberboard products (Mate
Diana and Alcalde 2017). Several lignin-derived
compounds (Camarero et al. 2005), and also
hydroxamate siderophores from fungi, have
been reported as useful in mediating laccaseaided delignification processes (Niku-Paavola
et al. 2003). Fungi are generally reported to
produce hydroxamic acid siderophores in the
broader literature; however, research on wooddegrading fungi has isolated many catecholate
compounds that are “siderophore-like” in their
structure. Given that the structure of lignin has
more catecholate properties, it would be useful
also to assess the catecholate/phenolate compounds similar to siderophores that are
produced by wood-decaying Basidiomycota
species as laccase mediator compounds. Several
LMW lignin-derivative compounds have now
been reported to have metal reduction capabilities and have been postulated to also function
in electron-transfer capacity for extracellular
enzymes (Tamaru et al. 2019). Laccase-derived
lignin fragments have also been found to
15 Fungi Involved in the Biodeterioration and Bioconversion of Lignocellulose Substrates
383