Abstract
Lignin biosynthesis occurs via radical coupling of guaiacyl and syringyl hydroxycinnamyl alcohol monomers (i.e., "monolignols") through chemical condensation with the growing lignin polymer. With each chain-extension step, monolignols invariably couple at their β-positions, generating chiral centers. Here, we report on activities of bacterial glutathione-S-transferase (GST) enzymes that cleave β-aryl ether bonds in lignin dimers that are composed of different monomeric units. Our data reveal that these sequence-related enzymes from Novosphingobium sp. strain PP1Y, Novosphingobium aromaticivorans strain DSM12444, and Sphingobium sp. strain SYK-6 have conserved functions as β-etherases, catalyzing cleavage of each of the four dimeric α-keto-β-aryl ether-linked substrates (i.e., guaiacyl-β-guaiacyl, guaiacyl-β-syringyl, syringyl-β-guaiacyl, and syringyl-β-syringyl). Although each β-etherase cleaves β-guaiacyl and β-syringyl substrates, we have found that each is stereospecific for a given β-enantiomer in a racemic substrate; LigE and LigP β-etherase homologues exhibited stereospecificity toward β(R)-enantiomers whereas LigF and its homologues exhibited β(S)-stereospecificity. Given the diversity of lignin's monomeric units and the racemic nature of lignin polymers, we propose that bacterial catabolic pathways have overcome the existence of diverse lignin-derived substrates in nature by evolving multiple enzymes with broad substrate specificities. Thus, each bacterial β-etherase is able to cleave β-guaiacyl and β-syringyl ether-linked compounds while retaining either β(R)- or β(S)-stereospecificity.