title: Stereochemical features of glutathione-dependent enzymes in the Sphingobium sp. strain SYK-6 β-aryl etherase pathway.
abstract: 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 β-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 β-S-thioether linkages. Products of recombinant GSTs (LigE, LigP, and LigF) are β-S-glutathionyl-α-keto-thioethers that are degraded by a β-S-thioetherase (LigG). All three Lig GSTs produced the ketone product (β-S-glutathionyl-α-veratrylethanone) from an achiral side chain-truncated model substrate (β-guaiacyl-α-veratrylethanone). However, when β-etherase assays were conducted with a racemic model substrate, β-guaiacyl-α-veratrylglycerone, LigE- or LigP-catalyzed reactions yielded only one of two potential product (β-S-glutathionyl-α-veratrylglycerone) epimers, whereas the other diastereomer (differing in configuration at the β-position (i.e. its β-epimer)) was produced only in the LigF-catalyzed reaction. Thus, β-etherase catalysis causes stereochemical inversion of the chiral center, converting a β(R)-substrate to a β(S)-product (LigE and LigP), and a β(S)-substrate to a β(R)-product (LigF). Further, LigG catalyzed glutathione-dependent β-S-thioether cleavage with β-S-glutathionyl-α-veratrylethanone and with β(R)-configured β-S-glutathionyl-α-veratrylglycerone but exhibited no or significantly reduced β-S-thioether-cleaving activity with the β(S)-epimer, demonstrating that LigG is a stereospecific β-thioetherase. We therefore propose that multiple Lig enzymes are needed in this β-aryl etherase pathway in order to cleave the racemic β-ether linkages that are present in the backbone of the lignin polymer.
authors: From the Departments of Civil and Environmental Engineering.
authors: Daniel L
authors: DL
authors: Gall
authors: Hoon
authors: H
authors: Kim
authors: Fachuang
authors: F
authors: Lu
authors: Timothy J
authors: TJ
authors: Donohue
authors: Daniel R
authors: DR
authors: Noguera
authors: John
authors: J
authors: Ralph
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D005978:Glutathione / Q000378:metabolism*; D008031:Lignin / Q000737:chemistry / Q000378:metabolism; D010088:Oxidoreductases / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D015398:Signal Transduction; D042301:Sphingomonadaceae / Q000737:chemistry / Q000201:enzymology* / Q000378:metabolism; D013237:Stereoisomerism; D013379:Substrate Specificity; D013438:Sulfhydryl Compounds / Q000737:chemistry / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D011994:Recombinant Proteins; D013438:Sulfhydryl Compounds; D008031:Lignin; D010088:Oxidoreductases; C060093:aryl ether cleaving enzyme; D005978:Glutathione
keywords: Bacterial Metabolism; Enzyme Catalysis; Glutathione; Glutathione S-Transferase; Lignin Degradation; Stereoselectivity; Stereospecificity; Thiol; beta-Aryl Etherase; beta-S-Thioetherase