Bacterial Catabolism of β-Hydroxypropiovanillone and β-Hydroxypropiosyringone Produced in the Reductive Cleavage of Arylglycerol-β-Aryl Ether in Lignin

Abstract

sp. strain SYK-6 converts four stereoisomers of arylglycerol-β-guaiacyl ether into achiral β-hydroxypropiovanillone (HPV) via three stereospecific reaction steps. Here, we determined the HPV catabolic pathway and characterized the HPV catabolic genes involved in the first two steps of the pathway. In SYK-6 cells, HPV was oxidized to vanilloyl acetic acid (VAA) via vanilloyl acetaldehyde (VAL). The resulting VAA was further converted into vanillate through the activation of VAA by coenzyme A. A syringyl-type HPV analog, β-hydroxypropiosyringone (HPS), was also catabolized via the same pathway. SLG_12830 (), which belongs to the glucose-methanol-choline oxidoreductase family, was isolated as the HPV-converting enzyme gene. An mutant completely lost the ability to convert HPV and HPS, indicating that is essential for the conversion of both the substrates. HpvZ produced in oxidized both HPV and HPS and other 3-phenyl-1-propanol derivatives. HpvZ localized to both the cytoplasm and membrane of SYK-6 and used ubiquinone derivatives as electron acceptors. Thirteen gene products of the 23 aldehyde dehydrogenase (ALDH) genes in SYK-6 were able to oxidize VAL into VAA. Mutant analyses suggested that multiple ALDH genes, including SLG_20400, contribute to the conversion of VAL. We examined whether the genes encoding feruloyl-CoA synthetase () and feruloyl-CoA hydratase/lyase ( and ) are involved in the conversion of VAA. Only FerA exhibited activity toward VAA; however, disruption of did not affect VAA conversion. These results indicate that another enzyme system is involved in VAA conversion. Cleavage of the β-aryl ether linkage is the most essential process in lignin biodegradation. Although the bacterial β-aryl ether cleavage pathway and catabolic genes have been well documented, there have been no reports regarding the catabolism of HPV or HPS, the products of cleavage of β-aryl ether compounds. HPV and HPS have also been found to be obtained from lignin by chemoselective catalytic oxidation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone/-butyl nitrite/O, followed by cleavage of the β-aryl ether with zinc. Therefore, value-added chemicals are expected to be produced from these compounds. In this study, we determined the SYK-6 catabolic pathways for HPV and HPS and identified the catabolic genes involved in the first two steps of the pathways. Since SYK-6 catabolizes HPV through 2-pyrone-4,6-dicarboxylate, which is a building block for functional polymers, characterization of HPV catabolism is important not only for understanding the bacterial lignin catabolic system but also for lignin utilization.

Lignin catabolic pathways reveal unique characteristics of dye‐decolorizing peroxidases in Pseudomonas putida

“…The following enzymes were induced in A514 when utilizing lignin as the carbon substrate, indicating their roles in β‐Aryl ether degradation (Masai et al ., ; Higuchi et al ., ): Cα‐dehydrogenases [PputA514_2990 (LigD), PputA514_2734 (LigN) and PputA514_6089 (LigL)], three glutathione reductases [PputA514_2989 (LigG) and PputA514_2987‐2986 (LigEF), members of the glutathione‐S‐transferase], β‐hydroxypropiovanillone (HPV) oxidase (PputA514_0720, HpvZ), and aldehyde dehydrogenases (PputA514_2607 and PputA514_3020, ALDHs). Among these, the induced homologous enzymes (LigD, LigN and LigL) are responsible for catalysing all the isomeric forms of β‐Aryl ether biaryls (Higuchi et al ., ), suggesting A514 has the ability to degrade all β‐Aryl ether‐type biaryl stereoisomers. (ii) Diarylpropane degradation pathway (Bugg et al ., ).…”

“…Bacteria are known to play a key role in the mineralization of large amounts of lignin-derived low molecular weight compounds with numerous studies having investigated the catabolic pathways for those aromatic compounds (Masai et al, 2007;Bugg et al, 2011;Díaz et al, 2013). These pathways include ferulate catabolic pathway, β-aryl ether cleavage pathway and biphenyl catabolic pathway (Hara et al, 2003;Masai et al, 2007;Bugg et al, 2011;Higuchi et al, 2018). However, the majority of these reports are focused on the best-characterized lignin-degrading bacteria strains, e.g., Sphingomonas paucimobilis SYK-6 and Streptomyces viridosporus T7A (Hara et al, 2003;Masai et al, 2007;Bugg et al, 2011;Higuchi et al, 2018).…”

“…These pathways include ferulate catabolic pathway, β-aryl ether cleavage pathway and biphenyl catabolic pathway (Hara et al, 2003;Masai et al, 2007;Bugg et al, 2011;Higuchi et al, 2018). However, the majority of these reports are focused on the best-characterized lignin-degrading bacteria strains, e.g., Sphingomonas paucimobilis SYK-6 and Streptomyces viridosporus T7A (Hara et al, 2003;Masai et al, 2007;Bugg et al, 2011;Higuchi et al, 2018). For other ligninolytic bacteria [e.g., P. putida (Jimenez et al, 2002), R. jostii and Streptomyces viridosporus (Moraes et al, 2018)], the catabolic pathways of these compounds are still hypothetical.…”

Roles of two glutathioneS-transferases in the final step of the β-aryl ether cleavage pathway inSphingobiumsp. strain SYK-6

“…Racemic MPHPV and HPV were prepared as described previously (17,27). In preparing (R)-GS-HPV and (S)-GS-HPV, purified LigF (40 µg protein/ml) and LigE (150 µg protein/ml) were incubated in 1 ml reaction mixtures containing 400 µM racemic MPHPV, 5 mM GSH, and buffer A, respectively, for 60 min at 30°C.…”

Roles of two glutathione S-transferases in the final step of the β-aryl ether cleavage pathway in Sphingobium sp. strain SYK-6

“…Stereoisomers of β-aryl ether model compound, GGE, are stereospecifically converted to an achiral C6-C3 monomer, HPV [13][14][15][16] . The resulting HPV is oxidized to vanilloyl acetic acid via vanilloyl acetaldehyde and further catabolized through vanillate 31,37 . Enzymes: LigD, LigL, and LigN, Cα-dehydrogenases; LigF, LigE, and LigP, β-etherases; LigG and LigQ, GSH-removing enzymes; HpvZ, HPV oxidase; ALDHs, aldehyde dehydrogenases; SLG_20400, vanilloyl acetaldehyde dehydrogenase.…”