科研之友
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摘要
Background: A bacterial -etherase pathway uses glutathione-dependent enzymes for catabolism of -ether-linked substructures found in lignin. Results: Racemic -ether-linked substructures are stereoselectively converted to thioether-linked monoaromatic compounds by -etherase pathway enzymes. Conclusion: Multiple enzymes with complementary stereochemical features are needed to catabolize racemic lignin substructures. Significance: Understanding the -etherase pathway metabolism of lignin enhances our potential to use lignin for agricultural, industrial, and biotechnological purposes. Glutathione-dependent enzymes play important protective, repair, or metabolic roles in cells. In particular, enzymes in the glutathione S-transferase (GST) superfamily function in stress responses, defense systems, or xenobiotic detoxification. Here, we identify novel features of bacterial GSTs that cleave -aryl ether bonds typically found in plant lignin. Our data reveal several original features of the reaction cycle of these GSTs, including stereospecific substrate recognition and stereoselective formation of -S-thioether linkages. Products of recombinant GSTs (LigE, LigP, and LigF) are -S-glutathionyl--keto-thioethers that are degraded by a -S-thioetherase (LigG). All three Lig GSTs produced the ketone product (-S-glutathionyl--veratrylethanone) from an achiral side chain-truncated model substrate (-guaiacyl--veratrylethanone). However, when -etherase assays were conducted with a racemic model substrate, -guaiacyl--veratrylglycerone, LigE- or LigP-catalyzed reactions yielded only one of two potential product (-S-glutathionyl--veratrylglycerone) epimers, whereas the other diastereomer (differing in configuration at the -position (i.e. its -epimer)) was produced only in the LigF-catalyzed reaction. Thus, -etherase catalysis causes stereochemical inversion of the chiral center, converting a (R)-substrate to a (S)-product (LigE and LigP), and a (S)-substrate to a (R)-product (LigF). Further, LigG catalyzed glutathione-dependent -S-thioether cleavage with -S-glutathionyl--veratrylethanone and with (R)-configured -S-glutathionyl--veratrylglycerone but exhibited no or significantly reduced -S-thioether-cleaving activity with the (S)-epimer, demonstrating that LigG is a stereospecific -thioetherase. We therefore propose that multiple Lig enzymes are needed in this -aryl etherase pathway in order to cleave the racemic -ether linkages that are present in the backbone of the lignin polymer.
- 出版日期2014-3-21
