Ligninolytic System Of White Rot Fungi

In order to degrade lignin, the white rot fungi have developed an unspecific ligninolytic system consisting of peroxidases and laccases (phenol oxidases; LAC), which degrade lignin in an oxidative process (Hatakka 1994). The peroxidases are heme-containing enzymes and require the presence of hydrogen peroxide (H2O2) to oxidize lignin and lignin-related compounds. Three types of peroxidases have been discovered in white rot fungi: LIP, manganese peroxidase (MnP), and more recently versatile peroxidase (VP)

Table 2 Enzymes and their roles in lignin degradation by white rot fungi

Enzyme

Cofactors or mediators

Role in lignin degradation

References

GLOX AO

Other H2O2

generating enzymes

H2O2, 3,4-dimethoxybenzyl alcohol (veratryl alcohol), 3,4-dimethoxycinnamic acid, 1,2-dimethoxybenzene, and 2-chloro-1,4-dimethoxybenzene

H2O2, Mn2+, organic acids as chelators, and unsaturated fatty acids

H2O2, veratryl alcohol, Mn2+, organic acids as chelators, and unsaturated fatty acids

O2, 3-hydroxyanthranillic acid, and hydroxybenzotriazole

Use electron acceptors including quinones, phenoxy radicals, Fe3+, and Cu2+ to generate lactones from various substrates

Glyoxal and methyl glyoxal

Aromatic alcohols (anisyl and veratryl alcohol)

Many organic compounds

Direct and mediated oxidation of phenolic and nonphenolic lignin structures, respectively. This results in cleavage of Ca-Cp, p-O-4 and aryl-Ca bonds, aromatic ring opening, hydroxylation, and demethoxylation

Mn2+ oxidized to Mn3+; chelated Mn3+ oxidizes phenolic lignin structures and subsequently, alkyl-phenyl cleavage, Ca-Cp cleavage, orbenzylic carbinol oxidation may result; nonphenolic moieties may be co-oxidized when MnP peroxidizes unsaturated fatty acids Still unknown

Phenolic lignin structures are oxidized to phenoxy radicals and subsequently, alkyl-phenyl cleavage or Ca-Cp cleavage may result. Dimethoxylation of several lignin model compounds has been witnessed; nonphenolic lignin structures may be oxidized only in the presence of mediators Reduces aromatic radicals preventing repolymerization, demethoxylation, or hydroxylation of nonphenolic lignin, and reduction of precipitated MnO2

Glyoxal oxidized to glyoxylic acid and concomitant production of H2O2

Aromatic alcohols oxidized to aldehydes with concomitant production of H2O2 O2 reduced to H2O2

For review see Tuor et al. (1995)

For review see Tuor et al. (1995)

Camarero et al. (1999), Mester and Field (1998), and Ruiz-Duenas et al. (1999, 2001)

For review see Tuor et al. (1995)

For review see Henriksson et al. (2000)

Zhao and Janse (1996)

Zhao and Janse (1996)

Urzua et al. (1998) and Zhao and Janse (1996)

Table 3 Selective white rot fungi and their reported ligninolytic enzymes

Enzymes

Table 3 Selective white rot fungi and their reported ligninolytic enzymes

Enzymes

Microorganism

LIP

MnP

Lac

VP

References

P. brevispora

U

U

U

Arora and Gill, (2001), Perez and Jeffries (1990), and Ruttimannet al. (1992)

P. radiata

U

U

U

Niku-Paavolaet al. (1988) and Vares et al. (1995)

P. tremellosa

U

U

U

Bonnarme and Jefferies (1990) and Hatakka et al. (1992, 1993)

I. lacteus

U

U

U

Novotny et al. (2000) and Rothschild et al. (2002)

B. adusta

U

U

U

U

Heinfling et al. (1998a,b), Kimura et al. (1991), and Nakamura et al. (1999)

Bjerkandera sp. strain BOS55

U

U

U

U

Mester and Field (1998), Mester et al. (1995), and ten Have et al. (1998a)

T. versicolor

U

U

U

Dodson et al. (1987), Fahraeus and Reinhammar (1967), and Johansson and Nyman (1987)

P. chrysoporium

U

U

Couto et al. (1999), Glenn et al. (1983), Leisola et al. 1987, and Tien and Kirk (1983b)

P. pini

U

U

Bonnarme and Jefferies (1990)

T. pruinosum

U

U

Waldner et al. (1988)

P. ochraceofulva

U

U

Hatakka (1994) and Vares et al. (1993)

J. separabilima

U

U

Vares et al. (1992)

P. ostreatus

U

U

U

Cohen et al. (2001), Sannia et al. (1986), and Waldner et al. (1988)

P. eryngii

U

U

U

Camarero et al. (1999), Heinfling et al. (1998b), Munoz et al. (1997), and Ruiz-Duenas et al. (1999)

P. sajor-caju

U

U

Bourbonnais and Paice (1989), Buswell et al. (1996), and Fukuzumi 1987

D. squalens

U

U

Perie and Gold (1991) and Perie et al. (1998)

L. edodes

U

U

Forrester et al. (1990) and Leatham and Stahmann (1984)

G. lucidum

U

U

D'Souza et al. (1999)

P. tigrinus

U

U

Golovleva et al. (1993) and Leontievsky et al. (1994)

R. lignosus

U

U

Galliano et al. (1991)

(Camarero et al. 1999; Mester and Field 1998; Ruiz-Duenas et al. 1999; 2001). Laccases are multicopper phenol oxidases, which oxidize phenols and aromatic amines. Rather than H2O2, these enzymes utilize dioxygen (O2) as an oxidant, reducing it by four electrons to water (Call and Mucke 1997). The enzymes involved in lignin degradation along with the roles they play are given in Table 2.

White rot fungi have been classified according to the ligninolytic enzymes they express. Whereas Hatakka suggested that they can be classified into three categories (Hatakka 1994), Tuor et al. classified them into five categories as follows (Tuor et al. 1995): (a) White rot fungi expressing LIP, MnP, and LAC, (b) white rot fungi simultaneously producing MnP and LAC, but not LIP, (c) white rot fungi producing LIP and either MnP or LAC, (d) white rot fungi reported to produce LIP without MnP or LAC, (e) a group which is incompletely characterized and in which neither of the oxidative enzymes have been identified.

However, the continual discovery of enzymes in different white rot fungi species means that classifications of this type are subject to ongoing changes. In particular, the recent discovery of the novel VP in Pleurotus eryngii and

Bjerkandera adusta (Camarero et al. 1999; Mester and Field 1998; Ruiz-Duenas et al. 1999; 2001), warrants re-evaluation of the classification scheme. Selective white rot fungi and the ligninolytic enzymes that they produce are given in Table 3.

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