Biological cleavage of lignin bonds

Newly discovered bacterial enzyme could help valorize lignin

The Science

Optimizing how we break down biomass is critical to developing valuable chemicals from renewable materials such as plant lignin. GLBRC researchers found that the bacteria Novosphingobium aromaticivorans rapidly breaks the β-aryl ether bond commonly found in lignin, and that its enzyme, a Nu-class glutathione S-transferase, performs the critical step of removing the antioxidant glutathione.

The Impact 

Plant lignin contains aromatic compounds that, when released, could be used to make valuable and renewable commodities for the biofuel, chemical, cosmetic, food, and pharmaceutical industries. This study identifies N. aromaticivoransas an attractive organism for studying the β-etherase lignin degradation pathway, as well as a potential biological system for converting lignin biomass into useful commodities. 

Summary 

As a major component of plant cells walls, lignin is a potential renewable source of valuable chemicals. Several sphingomonad bacteria have been identified that can break the β-aryl ether bond connecting most phenylpropanoid units of the lignin heteropolymer. GLBRC researchers testedthree sphingomonads predicted to be capable of breaking the β-aryl ether bond of the dimeric compound guaiacylglycerol-β-guiacyl ether (GGE) and found that Novosphingobium aromaticivorans metabolizes GGE at one of the fastest rates thus far reported. Results from this study indicate that the Nu-class glutathione S-transferase NaGST_Nuis the only enzyme needed for the necessary step of removing glutathione from both (R)- and (S)-β-glutathionyl-γ-hydroxypropiovanillone (GS-HPV) in N. aromaticivorans. Researchers also solved the crystal structure of NaGST_Nuand used molecular modeling to propose a mechanism for the glutathione lyase (deglutathionylation) reaction in which an enzyme-stabilized glutathione thiolate attacks the thioether bond of GS-HPV, and the reaction proceeds through an enzyme-stabilized enolate intermediate. Finally, this research revealed that Nu-class GSTs from Sphingobium sp. SYK-6 (which can also break the β-aryl ether bond) and Escherichia coli (which cannot break the β-aryl ether bond) can also cleave (R)- and (S)-GS HPV, suggesting that glutathione lyase activity may be common throughout this widespread but largely uncharacterized class of glutathione S-transferases. Understanding this bacterial pathway for breaking lignin bonds can aid future efforts to develop microbial systems for converting lignocellulosic biomass into commodities. A recently published companion study [Gall, et al.AEM (2018) DOI: 10.1128/AEM.02076-17] also demonstrated that this enzyme makes it possible to develop a complete in vitroenzymatic system for breaking the β-aryl etherbond of plant lignin.

Contacts (BER PM)

N. Kent Peters
Program Manager, Office of Biological and Environmental Research
kent.peters@science.doe.gov, 301-903-5549 

(PI Contact)

Timothy J. Donohue
University of Wisconsin - Madison
tdonohue@bact.wisc.edu

Funding
This work was supported by U.S. Department of Energy (DOE) Great Lakes Bioenergy Research Center

grants (DOE Office of Science BER DE-FC02-07ER64494 and DE-SC0018409).

Publications

Kontur, W. S. et al.“Novosphingobium aromaticivoransuses a Nu-class glutathione S-transferase as a glutathione lyase in breaking the β-aryl ether bond of lignin.” Journal of Biological Chemistry 293, 4955-4968 (2018) [DOI: 10.1074/jbc.RA117.001268].

Related Links

http://www.jbc.org/content/293/14/4955.full?sid=7ff2b3f2-808e-413b-a644-d9e9b622fd49