Journal
JOURNAL OF BIOLOGICAL CHEMISTRY
Volume 291, Issue 10, Pages 5234-5246Publisher
ELSEVIER
DOI: 10.1074/jbc.M115.694307
Keywords
enzyme catalysis; enzyme mechanism; enzyme structure; lignin degradation; plant cell wall; protein structure; stereoselectivity; X-ray crystallography; structural enzymology
Categories
Funding
- NIGMS, National Institutes of Health (NIH)
- Office of Science, Office of Basic Energy Sciences, of the United States Department of Energy [DE-AC02-05CH11231]
- United States Department of Energy, Office of Science, Office of Biological and Environmental Research [DE-AC02-05CH11231]
- NIH
- United States Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]
- Michigan Economic Development Corporation
- Michigan Technology Tri-Corridor [085P1000817]
- NCI
- NIH [ACB-12002, AGM-12006, P41GM103399, S10RR027000]
- NIGMS
- University of Wisconsin (Madison, WI)
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Lignin is a combinatorial polymer comprising monoaromatic units that are linked via covalent bonds. Although lignin is a potential source of valuable aromatic chemicals, its recalcitrance to chemical or biological digestion presents major obstacles to both the production of second-generation biofuels and the generation of valuable coproducts from lignin's monoaromatic units. Degradation of lignin has been relatively well characterized in fungi, but it is less well understood in bacteria. A catabolic pathway for the enzymatic breakdown of aromatic oligomers linked via -aryl ether bonds typically found in lignin has been reported in the bacterium Sphingobium sp. SYK-6. Here, we present x-ray crystal structures and biochemical characterization of the glutathione-dependent -etherases, LigE and LigF, from this pathway. The crystal structures show that both enzymes belong to the canonical two-domain fold and glutathione binding site architecture of the glutathione S-transferase family. Mutagenesis of the conserved active site serine in both LigE and LigF shows that, whereas the enzymatic activity is reduced, this amino acid side chain is not absolutely essential for catalysis. The results include descriptions of cofactor binding sites, substrate binding sites, and catalytic mechanisms. Because -aryl ether bonds account for 50-70% of all interunit linkages in lignin, understanding the mechanism of enzymatic -aryl ether cleavage has significant potential for informing ongoing studies on the valorization of lignin.
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