Free hydroxyl radical is not involved in an important reaction of lignin degradation by Phanerochaete chrysosporium Burds

Abstract

Hydroxyl radical (HO.) has been implicated in the degradation of lignin by Phanerochaete chrysosporium. This study assessed the possible involvement of HO. in degradation of lignin substructural models by intact cultures and by an extracellular ligninase isolated from the cultures. Two non-phenolic lignin model compounds [aryl-C(alpha)HOH-C(beta)HR-C(gamma)H2OH, in which R = aryl (beta-1) or R = O-aryl (beta-O-4)] were degraded by cultures, by the purified ligninase, and by Fenton's reagent (H2O2 + Fe2+), which generates HO. The ligninase and the cultures formed similar products, derived via an initial cleavage between C(alpha) and C(beta) (known to be an important biodegradative reaction), indicating that the ligninase is responsible for model degradation in cultures. Products from the Fenton degradation were mainly polar phenolics that exhibited little similarity to those from the biological systems. Mass-spectral analysis, however, revealed traces of the same products in the Fenton reaction as seen in the biological reactions; even so, an 18O2-incorporation study showed that the mechanism of formation differed. E.s.r. spectroscopy with a spin-trapping agent readily detected HO. in the Fenton system, but indicated that no HO. is formed during ligninase catalysis. We conclude, therefore that HO. is not involved in fungal C(alpha)-C(beta) cleavage in the beta-1 and beta-O-4 models and, by extension, in the same reaction in lignin.

Effect of pH and Oxalate on Hydroquinone-Derived Hydroxyl Radical Formation during Brown Rot Wood Degradation

“…The ability of oxalate to sequester ferric ions may protect brown rot fungi from hydroxyl radicals. Further support for this hypothesis is that white rot fungi, which do not utilize the hydroxyl radical as the major oxidant (30,32), also produce oxalate (39,40,42,49). Despite reports on how the hydroxyl radical may be formed in white rot fungi (4,17,18), it is unlikely that oxalate would have a role in its formation.…”

“…The quinone undergoes cyclic oxidation-reduction reactions, serving as a shuttle for electrons from intracellular donors to extracellular acceptors. Although a similar mechanism has been proposed for white rot fungi (4,17,18) for hydroxyl radical formation, product analysis suggests that hydroxyl radical oxidation is relatively minor in comparison to peroxidase oxidation (30,32). The role of oxalate, ubiquitously found in brown rot fungi, as a chelating agent, and the role of pH, which is altered by the fungus, are not clear.…”

Identification of Manganese Superoxide Dismutase from Sphingobacterium sp. T2 as a Novel Bacterial Enzyme for Lignin Oxidation

“…37,38 There are also literature reports of the production of hydroxyl radical in white rot fungus Phanerochaete chrysosporium, [39][40][41] though subsequent data implied that this is not a major contributing mechanism in white-rot fungal lignin degradation. 42 We therefore propose a possible mechanism shown in Figure 8 for the generation of the observed products from Organosolv lignin. Hydroxyl radical is reported to cause demethoxylation of methoxylated aromatic compounds, via addition of hydroxyl radical to the aromatic ring.…”

Cloning and characterization of the genes encoding the high-affinity iron-uptake protein complex Fet3/Ftr1 in the basidiomycete Phanerochaete chrysosporium

“…Wood contains enough iron to make the generation of hydroxyl radicals through the Fenton reaction feasible (Koenigs, 1974), which contrasts with other biological systems, where iron is normally sequestered in redox-inactive complexes. Although the participation of hydroxyl radicals was long ago postulated in P. chrysosporium (Forney et al, 1982;Kutsuki & Gold, 1982;Bes et al, 1983;Kirk & Nakatsubo, 1983;Faison & Kirk, 1983;Evans et al, 1984), subsequent studies have shown that the attack of lignin model compounds by Fenton chemistry leads to products different from those detected in ligninolytic cultures or by isolated peroxidases (Kirk et al, 1985). Nevertheless, there is evidence that supports a role for Fenton chemistry in the degradation of lignocellulose by P. chrysosporium (Kremer & Wood, 1992a, b;Backa et al, 1993;Wood, 1994;Henriksson et al, 1995;Tanaka et al, 1999).…”