## 4 Lignin Biodegradation and Ligninolytic Enzymes
### Other Ligninolytic Enzymes in Musluroom  
In addition to the lignin degradation by three main peroxididases described above, fungal genera also produce other ligninolytic enzymes [17; 18; 19], such as dye-decolorizing peroxididases (DyPs: EC 1.11.1.19), which are among the extracellular peroxididases. These peroxididases are not evolutionarily related to the oxidase II family for lignin [126; 127]. A fungal DyP was identified as an extracellular enzyme from a Basidomycete fungus, Geotrichum candidum (now Bjerkandera adusta) [128]. This fungal peroxidase did not have a homology with another peroxidase, named DyP, which can catalyze the decolorization activity of a wide range of dyes, including poor degrading anthraquinone families such as Reactive blue 5 [129]. Comparative studies of DyPs derived from other wood- and litter-degrading fungi revealed that all fungal DyPs efficiently oxidized recalcitrant dyes as well as the phenolic substrate [129]. These peroxididases showed higher activity than LiP and VP toward Reactive Blue 5, Reactive Black 5, and 2,6-DMP. On the other hand, these enzymes displayed lower activity than other fungal peroxididases toward a recalcitrant heterocyclic dye, Azure B, and a nonphenolic compound (veratryl alcohol). Since fungal DyPs have been reported, the similar heme structure protein, YcdB, as a substrate of the twin-arginine translocation system of _Escherichia coli_, was shown to be bacterial DyP [130]. An ever-increasing number of prokaryotic genome data is being gathered, and now several thousand bacterial DyP homologous sequences have been identified and their proteins have been characterized [126, 127, 131]. The protein structures of DyPs revealed the presence of a dimeric \(\alpha/\beta\) ferredoxin-like fold, which is different from any other peroxidase superfamily, which consists of an \(\alpha\)-helical fold domain [126]. Indeed, a phylogenetical analysis of the fungal DyPs displayed that the fungal enzymes are distinct from the bacterial DyPs [126]. Comparative studies of the enzyme activities for both fungal and bacterial DyPs toward a variety of lignin-derived substrates have been performed, and a comparison of the kinetic parameters for a general oxidoreductase substrate, ABTS, showed that the fungal enzyme has enhanced oxidation activity compared with the bacterial enzymes [126].  
Other minor enzymes involved are phenoxy radical-reducing enzymes, such as glyoxal oxidase (EC 1.2.3.5), aryl alcohol oxidase (EC 1.1.3.7), pyranose 2-oxidase (EC1.1.3.10), and cellobiose dehydrogenase (EC 1.1.99.18) within a cofactor heme or flavin adenine dinucleotide (FAD) to generate extracellular hydrogen peroxide [17, 132]. Although these enzymes catalyze different substrates, such as dicarbonyl and methylglyoxal compounds, phenolic and nonphenolic alcohols, and cellobiose in lignin depolymerization, their oxidation reactions commonly produce hydrogen peroxide via the reduction of O\({}_{2}\) to H\({}_{2}\)O\({}_{2}\). These secreted hydrogen peroxides could regulate peroxidase activity in vitro [132]. In addition, heme-thiolate haloperoxidases and FDA-dependent glucose-dehydrogenase have also been known as minor oxidoreductase that catalyze the hydroxylation of organic and aromatic substances and scavenge phenolic radicals and quinone compounds [17, 133, 134].