Degradation of the lignin model compount springgylglycol-β-guaiacyl ether by Polyporus versicolor and Stereum frustulatum

• Published in: Biochimica et biophysica acta. General subjects Vol. 165; no. 1; pp. 145 - 163
• Main Authors: Kirk, T.Kent, Harkin, John M., Cowling, Ellis B.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 06.08.1968
• ISSN: 0304-4165; 1872-8006
• DOI: 10.1016/0304-4165(68)90199-2

1. 1. Model compounds were used to test the hypothesis that phenol-oxidizing enzymes of white-rot fungi can effect depolymerization of lignin. Studies were made of the effects of syringlyglycol-β-guaiacyl ether of (i) whole cultures of Polyporus versicolor and Stereum frustulatum, (ii) culture filtrates of the former, and (iii) a p-diphenol oxidase ( p-diphenol:O 2 oxidoreductase, EC 1.10.3.2, formerly known as laccase) purified from such filtrates. The fungi were known from previous studies to decompose lignin in wood, but to differ markedly in their production of phenol-oxidizing enzymes. 2. 2. In culture filtrates of P. versicolor, the benzyl alcohol group of syringlyglycol-β-guaiacyl ether was oxidized to a carbonyl group, giving α-guaiacoxyacetosyringone; the same transformation occured in whole cutlures of S. frustulatum. The alkyl-phenyl carbon-to-carbon bond in both syringlyglycol-β-guaiacyl ether and α-guaiacoxyacetosyringone was cleaved by culture filtrates of P. versicolor with formation of guaiacoxyacetaldehyde and guaiacoxyacetic acid, respectively. The syringyl moieties of both parent compounds were converted to 2,6-dimethoxy- p-benzoquinone by culture filtrates of P. versicolor and by whole cultures of S. frustulatum. p-Diphenol oxidase purified from cultures filtrates of P. versicolor effected all of the above reaction. Since neither the filtrates nor the p-diphenol oxidase had any effect on syringlyglycol-β-guaiacyl ether or α-guaiacoxyacetosyringone in a nitrogen atmosphere, the above alkyl-phenyl cleavage reactions of these compounds apparently were oxidative. 3. 3. Whole cultures of P. versicolor and S. frustulatum reduced guaiacoxyacetaldehyde and guaiacoxyacetic acid to 2-guaiacoxyethanol, which accumulated in the cultures and was resistant to further alteration by either organism. The ethanol derivative was also produced from syringlyglycol-β-guaiacyl ether and α-guaiacoxyacetosyringone by both fungi, indicating that the above alkyl-phenyl cleavages also were affected by whole cultures. 4. 4. These experiments indicate that oxidative splitting of the alkyl-phenyl bond in certain phenylpropanoid monomers by phenol oxidases may be important in the depolymerization of lignin by white-rot fungi. A schematic model for sprucelignin was used to estimate that approx. 41% of the phenylpropanoid units in spruce lignin theoretically are vulnerable to the types of cleavage described in this paper.