In Vitro Enzymatic Depolymerization of Lignin with Release of Syringyl, Guaiacyl, and Tricin Units

摘要

New environmentally sound technologies are needed to derive valuable compounds from renewable resources. Lignin, an abundant polymer in terrestrial plants comprised predominantly of guaiacyl and syringyl monoaromatic phenylpro-panoid units, is a potential natural source of aromatic compounds. In addition, the plant secondary metabolite tricin is a recently discovered and moderately abundant flavonoid in grasses. The most prevalent interunit linkage between guaiacyl, syringyl, and tricin units is the beta-ether linkage. Previous studies have shown that bacterial beta-etherase pathway enzymes catalyze glutathione-dependent cleavage of beta-ether bonds in dimeric beta-ether lignin model compounds. To date, however, it remains unclear whether the known beta-e(t)herase enzymes are active on lignin polymers. Here we report on enzymes that catalyze beta-ether cleavage from bona fide lignin, under conditions that recycle the cosubstrates NAD(+) and glutathione. Guaiacyl, syringyl, and tricin derivatives were identified as reaction products when different model compounds or lignin fractions were used as substrates. These results demonstrate an in vitro enzymatic system that can recycle cosubstrates while releasing aromatic monomers from model compounds as well as natural and engineered lignin oligomers. These findings can improve the ability to produce valuable aromatic compounds from a renewable resource like lignin.