isolates is the digestive tracts of insects such as termites and beetles, which are
dominated by symbiotic bacteria [23]. Besides these natural settings, artificial
environments such as compost, waste water sludges, and effluents from pulp and
paper manufacturers are also rich sources of lignin degrading microorganisms, and
enzymes [22]. It has been suggested that biological lignin degradation processes are
a result of a microbial community's undertaking rather than the effort of a single
organism or enzyme. Based on our current understanding, the process starts with
the depolymerization of macropolymer lignin with the help of fungal and/or bacterial oxidoreductases such as lignin peroxidases (LiPs), manganese peroxidases
(MnPs), versatile peroxidases (VPs), dye-decolorizing peroxidases (DyPs), multicopper oxidase (MCOs), and/or laccase. The aromatic compounds that are derived
from the depolymerization step, including syringyl- (S), guaiacyl- (G), p-hydroxyphenyl (H) units of varying combinations, are subsequently funneled into different aromatic catabolism pathways. This step is usually referred to as
mineralization and is carried out mostly by bacteria [17, 19, 22, 24] (see also
Sect. 5). The focus of this section is on currently known enzyme families involved
in the depolymerization step.
Previous studies on lignin-degrading enzymes have been focusing on
wood-degrading fungi such as white-rot fungi and brown-rot fungi, mainly from
Basidiomycota and Ascomycota and with representatives such as Phanerochaete
chrysosporium, Trametes versicolor, and Phlebia radiata. Wood-rotting fungi
occupy diverse ecological habitats. White-rot fungi are more frequent on hardwood
material, while brown-rot fungi are more common on softwood or partially decayed
wood materials [8, 19, 22, 25]. Brown-rot fungi mainly act on the cellulose part of
lignocellulosic biomass and only degrade lignin partially, resulting in oxidized
lignin [17, 21]. On the other hand, white-rot fungi have been found to be able to
degrade lignin completely [8, 22]. Consequently, white-rot fungi are the source of
the majority of ligninolytic enzymes reported and described in the literature so far.
Besides fungi, another eukaryote reported to act on lignin substrates such as
(modified) wheat straw and the lignin fraction from beechwood is the yeast Candida maltosa, even though the involved enzymes and process have not been fully
unraveled [22]. A detailed overview of the most relevant species from which relevant ligninolytic enzymes have been isolated and characterized is given by Janusz
et al. [21].
Recently, more lignin-degrading enzymes originating from bacteria have been
discovered and studied. Bacterial species identified to degrade lignin belong to
different phyla such as Firmicutes (Bacillus sp., Paenibacillus sp.), Proteobacteria
(Pseudomonas sp., Acinetobacter sp., Klebsiella sp.), and Actinobacteria (Streptomyces sp., Rhodococcus sp., Amycolatopsis sp., Nocardia sp.) [8, 21] (Table 1).
Among these three phyla, species from Proteobacteria, including a-, c-, and
d-Proteobacteria, have most often been reported in literature to harbour enzymes
acting on lignin [21, 26]. However, many such discoveries have also been made
from species of the phylum Actinobacteria [21, 27–30]. A few putative laccase
genes have been identified in archaeal species such as Haloferax sp.[22, 26], even
though little is known about the functionalities of these enzymes. Table 1
Recent Advances in Enzymatic Conversion of Lignin …
445
dominated by symbiotic bacteria [23]. Besides these natural settings, artificial
environments such as compost, waste water sludges, and effluents from pulp and
paper manufacturers are also rich sources of lignin degrading microorganisms, and
enzymes [22]. It has been suggested that biological lignin degradation processes are
a result of a microbial community's undertaking rather than the effort of a single
organism or enzyme. Based on our current understanding, the process starts with
the depolymerization of macropolymer lignin with the help of fungal and/or bacterial oxidoreductases such as lignin peroxidases (LiPs), manganese peroxidases
(MnPs), versatile peroxidases (VPs), dye-decolorizing peroxidases (DyPs), multicopper oxidase (MCOs), and/or laccase. The aromatic compounds that are derived
from the depolymerization step, including syringyl- (S), guaiacyl- (G), p-hydroxyphenyl (H) units of varying combinations, are subsequently funneled into different aromatic catabolism pathways. This step is usually referred to as
mineralization and is carried out mostly by bacteria [17, 19, 22, 24] (see also
Sect. 5). The focus of this section is on currently known enzyme families involved
in the depolymerization step.
Previous studies on lignin-degrading enzymes have been focusing on
wood-degrading fungi such as white-rot fungi and brown-rot fungi, mainly from
Basidiomycota and Ascomycota and with representatives such as Phanerochaete
chrysosporium, Trametes versicolor, and Phlebia radiata. Wood-rotting fungi
occupy diverse ecological habitats. White-rot fungi are more frequent on hardwood
material, while brown-rot fungi are more common on softwood or partially decayed
wood materials [8, 19, 22, 25]. Brown-rot fungi mainly act on the cellulose part of
lignocellulosic biomass and only degrade lignin partially, resulting in oxidized
lignin [17, 21]. On the other hand, white-rot fungi have been found to be able to
degrade lignin completely [8, 22]. Consequently, white-rot fungi are the source of
the majority of ligninolytic enzymes reported and described in the literature so far.
Besides fungi, another eukaryote reported to act on lignin substrates such as
(modified) wheat straw and the lignin fraction from beechwood is the yeast Candida maltosa, even though the involved enzymes and process have not been fully
unraveled [22]. A detailed overview of the most relevant species from which relevant ligninolytic enzymes have been isolated and characterized is given by Janusz
et al. [21].
Recently, more lignin-degrading enzymes originating from bacteria have been
discovered and studied. Bacterial species identified to degrade lignin belong to
different phyla such as Firmicutes (Bacillus sp., Paenibacillus sp.), Proteobacteria
(Pseudomonas sp., Acinetobacter sp., Klebsiella sp.), and Actinobacteria (Streptomyces sp., Rhodococcus sp., Amycolatopsis sp., Nocardia sp.) [8, 21] (Table 1).
Among these three phyla, species from Proteobacteria, including a-, c-, and
d-Proteobacteria, have most often been reported in literature to harbour enzymes
acting on lignin [21, 26]. However, many such discoveries have also been made
from species of the phylum Actinobacteria [21, 27–30]. A few putative laccase
genes have been identified in archaeal species such as Haloferax sp.[22, 26], even
though little is known about the functionalities of these enzymes. Table 1
Recent Advances in Enzymatic Conversion of Lignin …
445