Lignin Biodegradation

First models of lignin were sketched in 1970s (Nimz 1974; Adler 1977), but a rigid structure of lignin is still unidentified. Due to lack of information on lignin structure, lignin model (model dimers with p-O-4, p-1, p-5, or p-p linkages) or radioactivelabeling (14C-labeled) synthetic lignin was used to investigate the mechanisms of lignin attack by white-rot basidiomycetes that are able to extensively degrade lignin (Eriksson et al. 1990). Microbial degradation of aromatic ring of lignin remained unsolved until 1985, until Higuchi and co-workers (Higuchi 1997) developed the lignin model 4-ethoxy-3methoxyphenylglycerol-{3-guaiacyl [U-ring 13C, OCD3]} ether and 4-ethoxy-3-methoxyphenylglycerol-{3-syringyl [U-ring 13C, OCD3]} ether as substrates to elucidate the mechanism of aromatic ring cleavage of the model compounds. Other dimers such as 2H, 13C, and 18O and labeled dimers with 18O2, H2, and 18O were also used to elucidate the mechanism of cleavage of aromatic ring. They further supported the mechanism of cleavage of side chains of the aromatic ring.

White-rot basidiomycetes have been found to be the most efficient lignin-degrading microorganisms, such as Coriolus versicolor (Wang et al. 2008), P. chrysosporium, and T. versicolor (Moredo et al. 2003). White-rot fungi are mostly used in biopulping as these fungi degrade lignin extensively resulting into bleached appearance of wood. Ligninolytic enzymes produced by white-rot fungi oxidize the lignin polymer, producing aromatic radicals by (1) C4-ether breakdown, (2) aromatic ring cleavage, (3) C„-Cp breakdown, and (4) demethoxylation, and aldehydes released from C„-Cp breakdown of lignin become the substrate for hydrogen peroxide generation by cyclic redox reactions (Guillen 1994, p. 181; Gutierrez et al. 1994). Mostly, white-rot fungi act on lignocellulose (lignin, cellulose, and hemicel- lulose) simultaneously, whereas some attack selectively (sequentially and preferentially) on lignin and hemicellulose. Examples of such spp. are Cerrena unicolor, Ganoderma australe, Phlebia tremellosa, Pleurotus spp., P. cinnabarinus, Phellinus pini (Martinez et al. 2005), Ceriporiopsis subvermispora (Guerra et al. 2004), Phlebia spp. (Fackler et al. 2006; Arora and Sharma 2009), Physisporinus rivulosus (Hilden et al. 2007), Dichomitus squalens (Fackler et al. 2006), Fomitopsispinicola, Ganoderma lucidum, Lenzites betulinus, Pleurotus eringii, Pleurotus ostreatus, Trametes versicolor (Knezevic et al. 2013). Also Trametes versicolor (Tanaka et al. 1999), Heterobasidium annosum (Daniel et al. 1998), P. chrysosporium (Sanchez 2009), and Irpex lacteus (Xu et al. 2009), and Ascomycota such as Xylaria hypoxy- lon (Martinez et al. 2005) simultaneously degrade cell wall components. Selective lignin degradation has applications in biopulping process and in providing unbounded carbohydrates that are used for animal feed and biofuel production (Mai et al. 2004; Anderson and Akin 2008).

Unlike white-rot fungi which attack selectively lignin or whole lignocellulose, brown-rot fungi, ascomycetes such as Postia placenta, Laetiporus portentosus, Piptoporus betulinus, and Gloeophyllum trabeum, degrade only carbohydrates, while their ability to convert lignin is limited (Martinez et al. 2005). Some plant pathogenic fungi such as Fusarium solani f. sp. glycines produce laccase and lignin peroxidase to degrade lignin. Ustilago maydis, a maize pathogen (Kamper et al. 2006), produces oxidoreductases and hemicellulases and has been studied for increasing the hydrolytic yield (Couturier et al. 2012). In soybean, lignin degradation by fungi causes sudden death syndrome (SDS) (Lozovaya et al. 2007).

Lignin-degrading enzymes are collectively called ligninases. They are classified into two major families: (1) phenol oxidase (laccase) and (2) peroxidases [lignin peroxidase (LiP), manganese peroxidase (MnP), and versatile peroxidase (VP) (Martinez et al. 2005)]. Typically, laccases use oxygen molecule as electron acceptors, whereas peroxidases use hydrogen peroxide as a co-substrate (Mai et al. 2004). It has been studied that P. chrysosporium produces several manganese peroxidases but no laccase (Singh and Chen 2008). According to one of the recent studies, a high lignin degradation enzyme cocktail should contain predominantly peroxidase and low laccase activity (Knezevic et al. 2013).

 
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