Initial steps in the pathway for bacterial degradation of two tetrameric lignin model compounds

We investigated the metabolic route by which a lignin tetramer-degrading mixed bacterial culture degraded two tetrameric lignin model compounds containing beta-O-4 and 5-5 biphenyl structures. The alpha-hydroxyl groups in the propane chain of both phenolic and nonphenolic tetramers were first oxidized symmetrically in two successive steps to give monoketones and diketones. These ketone metabolites were decomposed through C(alpha)(=O)-C(beta) cleavage, forming trimeric carboxyl acids which were further metabolized through another C(alpha)(=O)-C(beta) cleavage. Dehydrodiveratric acid, which resulted from the cleavage of the carbon bonds of the nonphenol tetramer, was demethylated twice. Four metabolites of the phenolic tetramer were purified and identified. All of these were stable compounds in sterile mineral medium, but were readily degraded by lignin tetramer-degrading bacteria along the same pathway as the phenol tetramer. No monoaromatic metabolites accumulated. All metabolites were identified by mass and proton magnetic resonance spectrometry. The metabolic route by which the mixed bacterial culture degraded tetrameric lignin model compounds was different from the route of the main ligninase-catalyzed C(alpha)-C(beta) cleavage by Phanerochaete chrysosporium.