Roles of the Enantioselective Glutathione S -Transferases in Cleavage of β-Aryl Ether

Abstract

Cleavage of the ␤-aryl ether linkage is the most important process in lignin degradation. Here we characterize the three tandemly located glutathione S-transferase (GST) genes, ligF, ligE, and ligG, from lowmolecular-weight lignin-degrading Sphingomonas paucimobilis SYK-6, and we describe the actual roles of these genes in the ␤-aryl ether cleavage. Based on the identification of the reaction product by electrospray ionization-mass spectrometry, a model compound of ␤-aryl ether, ␣-(2-methoxyphenoxy)-␤-hydroxypropiovanillone (MPHPV), was transformed by LigF or LigE to guaiacol and ␣-glutathionyl-␤-hydroxypropiovanillone (GS-HPV). This result suggested that LigF and LigE catalyze the nucleophilic attack of glutathione on the carbon atom at the ␤ position of MPHPV. High-pressure liquid chromatography-circular dichroism analysis indicated that LigF and LigE each attacked a different enantiomer of the racemic MPHPV preparation. The ligG gene product specifically catalyzed the elimination of glutathione from GS-HPV generated by the action of LigF. This reaction then produces an achiral compound, ␤-hydroxypropiovanillone, which is further degraded by this strain. Disruption of the ligF, ligE, and ligG genes in SYK-6 showed that ligF is essential to the degradation of one of the MPHPV enantiomers, and the alternative activities which metabolize the substrates of LigE and LigG are present in this strain.

Roles for Stress-inducible Lambda Glutathione Transferases in Flavonoid Metabolism in Plants as Identified by Ligand Fishing

“…For example LigG, a GST from the lignin-degrading bacterium Sphingomonas paucimobilis was shown to possess "glutathione lyase" activity, removing GSH from ␣-glutathionyl-␤-hydroxypropiovanillone (18). Similar activities have been demonstrated for aromatic conjugates, for example a purified enzyme from the fungus Phanerochaete chrysosporium with GST-like characteristics catalyzed "glutathione conjugate reductase" activity on S-glutathionyltrichloro-1,4-hydroquinone (19), and a similar "glutathionyl-hydroquinone lyase" activity is catalyzed by the GSTlike enzyme PcpF from Sphingobium chlorophenolicum (20).…”

Characterization of a Phanerochaete chrysosporium Glutathione Transferase Reveals a Novel Structural and Functional Class with Ligandin Properties

“…Etherase activity was also tested using the synthetic substrate ␣-O-methylumbelliferyl-␤-hydroxylpropiovanillone. This compound is an analog of lignin fragments containing an ether linkage that can be cleaved by Lig proteins (17). Using LigF from Sphingobium sp.…”

“…Etherase-like GSTs are fungal specific, although they show weak homology with LigE, a bacterial protein responsible for the cleavage of ␤-aryl ether linkages of lignin in Sphingobium sp. SYK-6 (8,17). The etherase-like class is expanded in wood decaying fungi, because, for instance, 11 etherase-like encoding genes are present in the genome of Postia placenta.…”

Complete genome sequence of the metabolically versatile photosynthetic bacterium Rhodopseudomonas palustris

“…Additional genes that may be useful in bioremediation or biocatalysis include nitrile hydratase (rpa2805 and rpa2806) and amidase (rpa2415) genes, phosphonate utilization genes (rpa0687-rpa0700) and carboxylesterase genes (rpa1568, rpa2627, rpa3893 and rpa4646). The R. palustris genome has 16 glutathione S-transferase genes, some of which may catalyze the cleavage of β-aryl ether bonds 23 .…”