6 Wood Discoloration

104 4 Wood Cell Wall Degradation
veratryl alcohol (with presence of H2O2) to the aldehyde, whose amount is
measured at 310 nm (Faison and Kirk 1985; Schoemaker et al. 1991).
The second enzyme involved in lignin degradation is manganese peroxidase
(MnP) [Mn(II):hydrogen-peroxide oxidoreductase, EC 1.11.1.13], which needs
free phenolic groups at the aromatic ring and does not oxidize veratryl alcohol.
The hemoprotein enzyme was first detected in P. chrysosporium (Glenn and
Gold 1985) and occurs e.g., in Armillaria species, Lentinula edodes, Pleurotus
ostreatus and T. versicolor. It oxidizes in the presence of hydrogen peroxide
Mn(II) to Mn(III), a strong oxidant, which oxidizes phenolic structures by
single-electron-oxidation (Perez and Jeffries 1992; Kofugita et al. 1992; Robene
Soustrade et al. 1992; Chatani et al. 1998; Kamitsuji et al. 1999). MnP polymerizes
more extensively and depolymerizes less than lignin peroxidase (Tanaka
et al. 1999). Treatment of water-insoluble 14 C-labeled milled wheat straw and
milled straw lignin with MnP preparations from the white-rot fungus Nematoloma
frowardii resulted in the direct release of 14 CO2 and in the formation
of soluble 14 C-lignin fragments (Hofrichter et al. 1999). MnP also degraded
polyethylene (Iiyoshi et al. 1998).
For the degradation of native lignin, a fungus must have enzymes, which attack
both phenolic and non-phenolic lignin components (Martinez-Inigo and
Kurek 1997). The lignin peroxidase is most likely responsible for the degradation
of the non-phenolic components and the laccase as well manganese
peroxidases for the oxidation of the phenolic parts (Evans 1991). All together,
there is a variety of oxidative enzymes that may be utilized by white-rot fungi for
lignin degradation (Highley and Dashek 1998). Various enzymes, low molecular
weight agents, free-radical reactions, and metals have been proposed to
participate in lignin degradation (Messner et al. 2003; Reading et al. 2003):
Lignin peroxidase (LiP), manganese peroxidase (MnP), cellobiose dehydrogenase
(CDH), laccases, oxalate, hydrogen peroxide, small molecule mediators,
methyl transferases, and the plasma membrane redox potential are involved
in the degradation systems. There is, however, still some uncertainty on their
accurate participation in lignin degradation.
Progress has been made concerning the molecular genetics of lignin and
cellulose biodegradation by white-rot fungi, primarily with Phanerochaete
chrysosporium, but also with Bjerkandera adusta, Phlebia radiata, and Trametes
versicolor (reviews by Highley and Dashek 1998 and Li 2003). Genes encoding
Lip and MnP have been cloned and sequenced (e.g., Irie et al. 2000). The
total genome sequence of P. chrysosporium has been disclosed by the DoE Joint
Genome Institute in the USA, which has facilitated cDNA cloning of various cellulase
genes from P. chrysosporium and successive production of recombinant
proteins from them (Samejima and Igarashi 2004). The X-ray crystal structures
of both LiP and MnP have also been elucidated. By means of recombinant
DNA techniques, laccase catalysis has been studied, and the crystal structure
of a T2-copper deleted laccase has been reported. In Pycnoporus cinnabarinus,
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