Reactive Oxygen Species

Reactive oxygen species and the suggested roles that they play in lignin degradation are given in Table 4.

The production of OH* radicals by white rot fungi is well documented (Barr et al. 1992; Kutsuki and Gold 1982; Tanaka et al. 1999a,b). OH* radicals are very reactive and can attack the subunits of lignin by both abstracting aliphatic Ca-hydrogens and by adding to aromatic rings (Hammel et al. 2002). Typical reactions of OH* radical with the major arylglycerol-ß-aryl ether structure of lignin can result in demethoxylation, ß-O-4 cleavage, hydroxylation, or Ca-oxidation (Hammel et al. 2002). The oxidation of lignin by OH* radicals, therefore, results in diverse reactions, some of which are expected to degrade the polymer. However, it remains unclear whether any wood decay fungus uses OH* radical to oxidize lignin. Hydroxylation of both phenolic and nonphenolic lignin resulting in new phenolic substructures on the lignin polymer may make it susceptible to attack by LAC or MnP (Hilden et al. 2000; Tanaka et al. 1999a).

If white rot fungi produce OH* radicals, then it is also necessary to consider the effects that peroxyl (ROO*) and hydroperoxyl (HOO*) radicals have on lignin, since both of these ROS are expected as secondary radicals when OH* radicals oxidize wood polymers (Hammel et al. 2002). Manganese peroxidase of white rot fungi peroxidizes

Table 4 Reactive oxygen species and their role in lignin degradation by white rot fungi

Reactive oxygen species

Role in lignin degradation

References

Hydroxyl radicals (OH* )

Peroxyl radicals (ROO*) Superoxide radicals (O*_)

Demethoxylation, b-O-4 cleavage, hydroxylation, or Ca-oxidation of nonphenolic structures; hydroxylation of nonphenolic lignin results in the formation of phenolic structures, making it susceptible to attack by MnP and LAC

Oxidation of nonphenolic lignin. Ca-Cb and b-O-4 cleavage

Production of H2O2 by dismutation; Mn2+ oxidation to Mn3+; production of OH* radicals through iron-catalyzed Haber-Weiss reaction; by reacting with phenoxyl radicals produced from lignin model compounds, it can result in oxidative degradation being favored over coupling reactions

Hammel et al. (2002), Hilden et al. (2000), and Tanaka et al. (1999a)

Hammel et al. (2002)

Archibald and Fridovich (1982), Barr et al. (1992), and Gierer et al. (1994)

unsaturated fatty acids, which results in the formation of ROS that include ROO* (Moen and Hammel 1994). ROO* radical-generating systems including MnP in the presence of unsaturated fatty acids were shown to oxidize a nonphenolic (-O-4-linked lignin model dimer to products indicative of hydrogen abstraction (Kapich et al. 1999b). Since, white rot fungi do produce extracellular lipids (Enoki et al. 1999), the formation of ROO* radicals by an MnP dependent mechanism and their involvement in lignin degradation does seem reasonable. The involvement of ROO * radicals in the chemical consumption of 1-(3',4'-dimethoxyphenyl) propene (DMPP) by LIP was confirmed by using the well-known ROO* radical reductant Mn2+ (ten Have et al. 2000). This metal ion severely inhibited the DMPP consumption rate under air, but did not affect the lower enzymatic DMPP consumption rate under N2. In the absence of O2, the Ca-C( cleavage of DMPA to veratryl aldehyde was strongly inhibited and side-chain coupling products (dimers) were formed instead. As a whole, these results suggest that during LIP-catalyzed oxidation of aromatic substrates, O2 is responsible for the formation of reactive ROO* intermediates, which can directly react with other substrate molecules and thereby accelerate consumption rates and also prevents coupling reactions by lowering the pool of carbon-centered radicals accumulating during LIP catalysis.

In contrast to OH* and ROO*, superoxide anion radicals (O*) are unable to oxidize lignin units. However, O*2 produced by white rot fungi can participate in the production of H2O2 via both dismutation (2O**2 + 2H+ = H2O2 + O2) and Mn2+ oxidation with concomitant production of Mn3+ (O*2 + Mn2+ + 2H+ = H2O2 + Mn3+) (Archibald and Fri-dovich 1982). They can also be involved in HO* production through the iron-catalyzed Haber-Weiss reaction (O*2 + H2O2 = HO* + HO2 + O2) (Barr et al. 1992). Furthermore, by reacting with phenoxyl radicals produced from lignin model compounds, they can result in oxidative degradation being favored over coupling reactions (Gierer et al. 1994).

One potential source of O* 2 evolves from the cleavage of oxalate via oxalate decarboxylase (ten Have and Teunissen 2001). Oxalate is produced as a major aliphatic acid by white rot fungi (Makela et al. 2002). Its decomposition results in the formation of CO2 and the formate anion radical (CO*2), which is further oxidized by O2 to give CO2 and O2* 2 or HOO*. Subsequent dismutation of O*2 resulting in the formation of H2O2 indicates that oxalate may serve as a passive sink for production of the latter. Both LIP and MnP are also capable of decomposing oxalate in the presence of VA and Mn2+, respectively (Akamatsu et al. 1990; Shimada et al. 1994) and indeed other organic acids (Hofrichter et al. 1998; Urzua et al. 1998). These reactions account for the observed oxidation of phenol red and kojic acid by MnP in the presence of Mn2+, without exogenous addition of H2O2 (Kuan and Tien 1993; Urzua et al. 1995). The reduction of VA+* + or Mn3+ by oxalate suggests that as long as oxalate coexists with LIP and MnP, it would inhibit lignin degradation. Indeed, oxalate has been shown to strongly reduce the rate of lignin mineralization in ligninolytic cultures of white rot fungi (Akamatsu et al. 1990; Ma et al. 1992; Shimada et al. 1994).

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