A Mechanism for Production of Hydroxyl Radicals by the Brown-Rot Fungus Coniophora Puteana: Fe(III) Reduction by Cellobiose Dehydrogenase and Fe(II) Oxidation at a Distance from the Hyphae

Abstract

In timber infested by brown-rot fungi, a rapid loss of strength is attributed to production of hydroxyl radicals (HO.). The hydroxyl radicals are produced by the Fenton reaction [Fe(II)/H2O2], but the pathways leading to Fe(II) and H2O2 have remained unclear. Cellobiose dehydrogenase, purified from cultures of Coniophora puteana, has been shown to couple oxidation of cellodextrins to conversion of Fe(III) to Fe(II). Two characteristics of brown rot are release of oxalic acid and lowering of the local pH, often to about pH 2. Modelling of Fe(II) speciation in the presence of oxalate has revealed that Fe(II)-oxalate complexes are important at pH 4-5, but at pH 2 almost all Fe(II) is in an uncomplexed state which reacts very slowly with dioxygen. Diffusion of Fe(II) away from the hyphae will promote conversion to Fe(II)-oxalate and autoxidation with H2O2 as product. Thus the critical Fe(II)/H2O2 combination will be generated at a distance, enabling hydroxyl radicals to be formed without damage to the hyphae.

Degradation of cellulose and hemicelluloses by the brown rot fungus Piptoporus betulinus – production of extracellular enzymes and characterization of the major cellulases

“…The participation of reactive oxygen species and reactive radicals in brown rot fungi has repeatedly been demonstrated (Hyde & Wood, 1997;Jensen et al, 2001;Hammel et al, 2002;Goodell, 2003). P. betulinus did not produce cellobiose dehydrogenase, nor was there a detectable hydrolytic activity of the LMM (<10 kDa) fraction of the culture extracts after the addition of Fe(III), and 2,5-dimethoxybenzoquinone was not found in the cultures (data not shown).…”

“…These include the involvement of quinone cycling (Jensen et al, 2001) and cellobiose dehydrogenase-catalysed reactions (Hyde & Wood, 1997). However, production of reactive radicals has not been found in all brown rot species tested (KlemanLeyer & Kirk, 1994).…”

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

“…We note that Nature uses hydroxyl radical to attack lignin in a different context: brown rot fungi utilise Fenton chemistry to generate hydroxyl radical to attack lignin. 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. 37 If hydroxyl radical attacked at aryl C-1 of an aryl-C3 unit, then C-C fragmentation as shown in Figure 8 would generate compound 7, observed in incubations of SpMnSOD enzymes with both The identification of the Sphingobacterium sp. T2 manganese superoxide dismutases as lignin-oxidising enzymes expands the range of bacterial enzymes capable of lignin oxidation.…”

Degradation of chemically modified Scots pine (Pinus sylvestris L.) with Fenton reagent

“…Fenton reagent (Fenton 1894) is known to play a key role in brown rot decay because it is a diffusible MW low agent that can penetrate the cell wall matrix, even in a chelated form (Koenigs 1974;Kirk et al 1991;Hyde and Wood 1997;Rättö et al 1997;Hammel et al 2002). It can thus reach the non-crystalline regions of cellulose and cleave the glucan chains, which results in tensile strength loss (Green et al 1991;Kleman-Leyer et al 1992;Green and Highley 1997).…”