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