| Co-Investigator(Kenkyū-buntansha) |
UMEZAWA Toshiaki Instructor, Wood Research Institute, Kyoto University, 木材研究所, 助手 (80151926)
SHIMADA Mikio Lecturer, Wood Research Institute, Kyoto University, 木材研究所, 講師 (50027166)
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| Research Abstract |
Chemical structure of lignin, which is formed by sehydrogenative polymerization of p-hydroxy-cinnamyl alcohols, consists of many different types of carbon-to carbon and ether bonds, and is very complex and heterogeneous. Until today, the knowledge of lignin biodegradation has remained to be accumulated. This investigation has elucidated mechanisms for lignin degradation by lignin-degrading fungi, Phanerochaete chrysosporium and Coriolus versicolor, and their enzymes.
First of all, mechanisms for side chain cleavage were investigated. ^<18>O and ^<13>C labeled -O-4 lignin model dimers were synthesized and degraded by the fungi. Mass spectrometric analysis of the degradation products showed occurrence of a novel C -C cleavage. Secondly, we found for the first time aromatic ring cleavage of -O-4 lignin models by the fungi, and that the ring cleavage was catalyzed by lignin peroxidase of p. chrysosporium. Subsequently, an initial product in the ring cleavage of -etherated aryl ring by the enzyme, muconate of arylglycerol, was identified. Based on the tracer experiments with ^<18>O, we proposed the mechanism for the ring cleavage by lignin peroxidase, which involves initial single-electron oxidation, followed by attack by nucleophiles such as H_2O or intramolecular hydroxyl groups, and by coupling by resulting radicals with O_2. The mechanism was the case for aromatic ring cleavage of polymeric lignin by the enzyme.
On the other hand, laccase of C. versicolor was elucidated to catalyze cleavages of side chain and aromatic ring of phenolic lignin model compounds.
Based on these results, mechanisms for lignin degradation by lignin-degrading microorganisms and their enzymes have been elucidated.
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