Physiological requirements for degradation of lignin and lignin-related substances by Sporotrichum pulverulentum

Microbial demethylation of lignin: Evidence of enzymes participating in the removal of methyl/methoxyl groups

Lignin is an abundant natural plant aromatic biopolymer containing various functional groups that can be exploited for activating lignin for potential commercial applications. Applications are hindered due to the presence of a high content of methyl/methoxyl groups that affects reactiveness. Various chemical and enzymatic approaches have been investigated to increase the functionality in transforming lignin. Among these is demethylation/demethoxylation, which increases the potential numbers of vicinal hydroxyl groups for applications as phenol-formaldehyde resins. Although the chemical route to lignin demethylation is well-studied, the biological route is still poorly explored. Bacteria and fungi have the ability to demethylate lignin and lignin-related compounds. Considering that appropriate microorganisms possess the biochemical machinery to demethylate lignin by cleaving O-methyl groups liberating methanol, and modify lignin by increasing the vicinal diol content that allows lignin to substitute for phenol in organic polymer syntheses. Certain bacteria through the actions of specific O-demethylases can modify various lignin-related compounds generating vicinal diols and liberating methanol or formaldehyde as end-products. The enzymes include: cytochrome P₄₅₀-aryl-O-demethylase, monooxygenase, veratrate 3-O-demethylase, DDVA O-demethylase (LigX; lignin-related biphenyl 5,5'-dehydrodivanillate (DDVA)), vanillate O-demethylase, syringate O-demethylase, and tetrahydrofolate-dependent-O-demethylase. Although, the fungal counterparts have not been investigated in depth as in bacteria, O-demethylases, nevertheless, have been reported in demethylating various lignin substrates providing evidence of a fungal enzyme system. Few fungi appear to have the ability to secrete O-demethylases. The fungi can mediate lignin demethylation enzymatically (laccase, lignin peroxidase, manganese peroxidase, O-demethylase), or non-enzymatically in brown-rot fungi through the Fenton reaction. This review discusses details on the aspects of microbial (bacterial and fungal) demethylation of lignins and lignin-model compounds and provides evidence of enzymes identified as specific O-demethylases involved in demethylation.

Wood stimulates the demethoxylation of [O14CH3]-labeled lignin model compounds by the white-rot fungi Phanerochaete chrysosporium and Phlebia radiata

Mineralization of polymeric wood lignin and its substructures is a result of complex reactions involving oxidizing and reducing enzymes and radicals. The degradation of methoxyl groups is an essential part of this process. The presence of wood greatly stimulates the demethoxylation of a non-phenolic lignin model compound (a [O(14)CH(3)]-labeled beta-O-4 dimer) by the lignin-degrading white-rot fungi Phlebia radiata and Phanerochaete chrysosporium. When grown on wood, both fungi produced up to 47 and 40% (14)CO(2) of the applied (14)C activity, respectively, under air and oxygen in 8 weeks. Without wood, the demethoxylation of the dimer by both fungi was lower, varying between 0.5 and 35%. Addition of nutrient nitrogen together with glucose decreased demethoxylation when the fungi were grown on spruce wood under air. Because the evolution of (14)CO(2) in the absence of wood was poor, the fungi may have preferably used wood as a carbon and nitrogen source. The amount of fungal mycelium, as determined by the ergosterol assay, did not show connection to demethoxylation. P. radiata also showed a high demethoxylation of [O(14)CH(3)]-labeled vanillic acid in the presence of birch wood. The degradation of lignin and lignin-related substances should be studied in the presence of wood, the natural substrate for white-rot fungi.

Bio-degradation of lignin in olive pomace by freshly-isolated species of Basidiomycete

The solid waste (pomace) from olive oil processing cannot be used directly as an animal feed, but it was thought that an appropriate series of fermentations might improve its nutritional value. As a first step, typical samples of pomace were subjected, after an alkaline pre-treatment, to delignification by Phanerochaete chrysosporium (ATCC 19343), Oxysporus sp., Schizophyllum commune, Hyphoderma sp. or Ganoderma sp.; the last four species being freshly isolated from decaying wood collected in a woodland in Jordan. The relative activity of the species was judged by the levels of ligninase or laccase secreted and the extent of lignin degradation under a range of experimental conditions. Oxysporus sp. (ca. 69%) and S. commune (ca. 53%) gave significantly higher levels of breakdown of the lignified material than the other recent isolates. P. chrysosporium (ca. 60%) was not as active as in previously reported studies, and it may be that culturing the fungus on a standard laboratory medium had reduced its ability to generate ligninase. Further work is needed to establish whether the delignified pomace could be further processed into a feed for poultry.

Biodelignification of Lemon Grass and Citronella Bagasse by White-Rot Fungi

Twelve white-rot fungi were grown in solid-state culture on lemon grass ( Cymbopogon citratus ) and citronella ( Cymbopogon winterianus ) bagasse. The two lignocellulosic substrates had 11% permanganate lignin and a holocellulose fraction of 58%. After 5 to 6 weeks at 20°C, nine fungi produced a solid residue from lemon grass with a higher in vitro dry matter enzyme digestibility than the original bagasse; seven did the same for citronella. The best fungus for both substrates was Bondarzewia berkeleyi ; it increased the in vitro dry matter enzyme digestibility to 22 and 24% for lemon grass and citronella, respectively. The increases were correlated with weight loss and lignin loss. All fungi decreased lignin contents: 36% of the original value for lemon grass and 28% for citronella. Practically all fungi showed a preference for hemicellulose over cellulose.

Factors Involved in the Regulation of a Ligninase Activity in Phanerochaete chrysosporium

The regulation of an H 2 O 2 -dependent ligninolytic activity was examined in the wood decay fungus Phanerochaete chrysosporium. The ligninase appears in cultures upon limitation for nitrogen or carbohydrate and is suppressed by excess nutrients, by cycloheximide, or by culture agitation. Activity is increased by idiophasic exposure of cultures to 100% O 2 . Elevated levels of ligninase and, in some cases, of extracellular H 2 O 2 production are detected after brief incubation of cultures with lignins or lignin substructure models, with the secondary metabolite veratryl alcohol, or with other related compounds. It is concluded that lignin degradation (lignin → CO 2 ) by this organism is regulated in part at the level of the ligninase, which is apparently inducible by its substrates or their degradation products.