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

Abstract

The redox cycle of 2,5-dimethoxybenzoquinone (2,5-DMBQ) is proposed as a source of reducing equivalent for the regeneration of Fe 2؉ and H 2 O 2 in brown rot fungal decay of wood. Oxalate has also been proposed to be the physiological iron reductant. We characterized the effect of pH and oxalate on the 2,5-DMBQ-driven Fenton chemistry and on Fe 3؉ reduction and oxidation. Hydroxyl radical formation was assessed by lipid peroxidation. We found that hydroquinone (2,5-DMHQ) is very stable in the absence of iron at pH 2 to 4, the pH of degraded wood. . Catalase and hydroxyl radical scavengers were effective inhibitors of lipid peroxidation, whereas superoxide dismutase caused no inhibition. At a low concentration of oxalate (50 M), ferric ion reduction and lipid peroxidation are enhanced. Thus, the enhancement of both ferric ion reduction and lipid peroxidation may be due to oxalate increasing the solubility of the ferric ion. Increasing the oxalate concentration such that the oxalate/ferric ion ratio favored formation of the 2:1 and 3:1 complexes resulted in inhibition of iron reduction and lipid peroxidation. Our results confirm that hydroxyl radical formation occurs via the 2,5-DMBQ redox cycle.A prerequisite to gaining access to the cellulose and hemicellulose components of woody biomass is the circumvention of the lignin barrier. Filamentous fungi, the predominant degraders of wood, have evolved at least two mechanisms to circumvent this barrier. White rot fungi circumvent the lignin barrier by degrading it with extracellular peroxidases (14, 47), with eventual degradation to the level of CO 2 (28). In contrast, brown rot fungi cannot degrade the lignin component to CO 2 . However, these fungi can access the cellulose components with minimal modification of the lignin. These modifications include demethylation of aryl methoxy groups and ring hydroxylation (for a more extensive review, see reference 29).Due to the limited size of the wood pores and the nonspecific nature of wood degradation, Cowling and Brown (12) suggested that low-molecular-weight oxidants are the initial agents in wood decay. Koenigs (33) showed that a number of wood-decomposing fungi produce H 2 O 2 and noted the similarities between wood treated with the hydroxyl radical and with brown rot fungi (34). Illman et al. (23) subsequently detected the hydroxyl radical in incubations with the brown rot fungus Poria placenta by use of electron spin resonance and spin trapping agents. Further supporting the involvement of the hydroxyl radical is the formation of 3-hydroxy derivatives (the expected products from a hydroxyl radical attack) of phthalic hydrazide in incubations with brown rot fungi (5).The most likely nonphotochemical source of the hydroxyl radical is Fenton's reagent, defined by the following chemistry: 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 h...

Insights into the oxidative degradation of cellulose by a copper metalloenzyme that exploits biomass components

“…From this work, it was suggested that GH61s act directly on cellulose rendering it more accessible to traditional cellulase action (11). Moreover, recent genomic sequencing of the brown rot fungi Postia placenta showed a number of GH61 genes in this organism (13)(14)(15), indicating the widespread nature of this family of enzymes in cellulose degradation. As such, GH61s likely hold major potential for industrial decomposition of cellulosic materials.…”

Laccase and Its Role in Production of Extracellular Reactive Oxygen Species during Wood Decay by the Brown Rot Basidiomycete Postia placenta

“…However, there is an apparent contradiction between these two mechanisms: oxalate is a strong chelator of Fe 3ϩ , and the resulting Fe 3ϩ trioxalate complex has too negative a reduction potential to react readily with methoxyhydroquinones such as 2,5-DMHQ (28,37). In considering this problem, we noted the surprising finding that the P. placenta genome encodes two putative laccases, enzymes that are considered atypical of brown rot fungi (26).…”

Metal Accumulation without Enhanced Oxalate Secretion in Wood Degraded by Brown Rot Fungi

“…Oxalate secretion in response to metals has been demonstrated for brown rot fungi in artificial media (26), and Cu tolerance in wood has been associated with Cu oxalate crystallization (24). Similarly, oxalate has been theorized to chelate excess Fe 3ϩ near hyphae, minimizing radical formation (16), although excessive Fe chelation in wood could impede Fentonbased depolymerization (15,25). It remains uncertain whether metal detoxification in wood is attributable to increased oxalate secretion (resistance) (5), incidental to oxalate production (tolerance) (6,13), or related to other mechanisms (4,10).…”