Chapter 9: Bacterial Enzymes for the Cleavage of Lignin β-Aryl Ether Bonds: Properties and Applications

The β-aryl ether is the most abundant intermonomer linkage in lignin, so identifying the microbial catabolic pathways and enzymes involved in bond cleavage is essential for understanding lignin biodegradation in nature and promising for producing value-added chemicals from lignin using microbial bioreactors. The catabolic pathways and enzyme genes for β-aryl ether cleavage were originally characterized in Sphingobium sp. strain SYK-6, an alphaproteobacterial degrader of lignin-derived aromatics. In SYK-6, the β-aryl ether compound guaiacylglycerol-β-guaiacyl ether (GGE) is converted into β-hydroxypropiovanillone in three reaction steps: (i) stereospecific oxidation of the GGE α-carbon atom by alcohol dehydrogenases to produce α-(2-methoxyphenoxy)-β-hydroxypropiovanillone (MPHPV), (ii) enantioselective ether cleavage via nucleophilic attack of glutathione on the MPHPV β-carbon atom catalyzed by glutathione S-transferases (GSTs), and (iii) removal of the glutathione by GSTs. The studies summarized in this chapter have characterized the catalytic properties and three-dimensional structures of these enzymes and described potential applications for the conversion of lignin.