Summary Lignin is the most abundant phenolic polymer; thus, its decomposition by microorganisms is fundamental to carbon cycling on earth. Lignin breakdown is initiated by depolymerization catalysed by extracellular oxidoreductases secreted by white‐rot basidiomycetous fungi. On the other hand, bacteria play a predominant role in the mineralization of lignin‐derived heterogeneous low‐molecular‐weight aromatic compounds. The outline of bacterial catabolic pathways for lignin‐derived bi‐ and monoaryls are typically composed of the following sequential steps: (i) funnelling of a wide variety of lignin‐derived aromatics into vanillate and syringate, (ii) O demethylation of vanillate and syringate to form catecholic derivatives and (iii) aromatic ring‐cleavage of the catecholic derivatives to produce tricarboxylic acid cycle intermediates. Knowledge regarding bacterial catabolic systems for lignin‐derived aromatic compounds is not only important for understanding the terrestrial carbon cycle but also valuable for promoting the shift to a low‐carbon economy via biological lignin valorisation. This review summarizes recent progress in bacterial catabolic systems for lignin‐derived aromatic compounds, including newly identified catabolic pathways and genes for decomposition of lignin‐derived biaryls, transcriptional regulation and substrate uptake systems. Recent omics approaches on catabolism of lignin‐derived aromatic compounds are also described.
Characteristic cylindrical microphase-separated structures have been found for three ABC star-shaped terpolymers. The samples are composed of polystyrene (S), polyisoprene (I), and poly(2vinylpyridine) (P); their volume ratios of I:
Green borole debuts: A heteroaryl‐substituted borole with peripheral thienyl groups has a significantly high‐lying HOMO in addition to a low‐lying LUMO, and thus unusual photophysical and electrochemical properties. These results highlight the highly electron‐donating character of the borole ring.
e Sphingobium sp. strain SYK-6 is able to degrade various lignin-derived biaryls, including a phenylcoumaran-type compound, dehydrodiconiferyl alcohol (DCA). In SYK-6 cells, the alcohol group of the B-ring side chain of DCA is initially oxidized to the carboxyl group to generate 3-(2-(4-hydroxy-3-methoxyphenyl)-3-(hydroxymethyl)-7-methoxy-2,3-dihydrobenzofuran-5-yl) acrylic acid (DCA-C). Next, the alcohol group of the A-ring side chain of DCA-C is oxidized to the carboxyl group, and then the resulting metabolite is catabolized through vanillin and 5-formylferulate. In this study, the genes involved in the conversion of DCA-C were identified and characterized. The DCA-C oxidation activities in SYK-6 were enhanced in the presence of flavin adenine dinucleotide and an artificial electron acceptor and were induced ca. 1.6-fold when the cells were grown with DCA. Based on these observations, SLG_09480 (phcC) and SLG_09500 (phcD), encoding glucose-methanol-choline oxidoreductase family proteins, were presumed to encode DCA-C oxidases. Analyses of phcC and phcD mutants indicated that PhcC and PhcD are essential for the conversion of (؉)-DCA-C and (؊)-DCA-C, respectively. When phcC and phcD were expressed in SYK-6 and Escherichia coli, the gene products were mainly observed in their membrane fractions. The membrane fractions of E. coli that expressed phcC and phcD catalyzed the specific conversion of DCA-C into the corresponding carboxyl derivatives. In the oxidation of DCA-C, PhcC and PhcD effectively utilized ubiquinone derivatives as electron acceptors. Furthermore, the transcription of a putative cytochrome c gene was significantly induced in SYK-6 grown with DCA. The DCA-C oxidation catalyzed by membrane-associated PhcC and PhcD appears to be coupled to the respiratory chain. L ignin, one of the major components of plant cell walls, is a complex phenolic polymer resulting from the oxidative combinatorial coupling of 4-hydroxycinnamyl alcohols (1). Although lignin has various intermolecular linkages between phenylpropane units and contains a number of asymmetric carbons, it is considered to be optically inactive, implying the racemic nature of the lignin backbone (2-4). In nature, lignin is initially decomposed by phenol oxidases such as lignin peroxidase, manganese peroxidase, versatile peroxidase, and laccase secreted by white rot fungi (5-7). Recently, dye-decolorizing peroxidases (Dyp) of Rhodococcus (8) and Amycolatopsis (9) and small laccase of Streptomyces (10) were characterized, and these enzymes have been implicated as being involved in lignin degradation. In addition, bacteria play key roles in the degradation and mineralization of low-molecular-weight aromatic compounds derived from lignin (11, 12). Since fragmented oligomers from lignin consist of stereoisomers that contain various types of intermolecular linkages between phenylpropane units, catabolic enzymes necessary for the conversion of such stereoisomers must have evolved in bacteria to fully utilize structurally and stereochemically complicated l...
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