The Lig enzymatic system: multi-step cleavage of a lignin model compound

Lignin is the second most abundant natural substance in the world after cellulose, and only a very small part of it is used for the production of value-added products. In fact, after hydrolysis of lignocellulose polysaccharides, lignin remains as a solid residue which represents the most abundant renewable source of aromatic polymers on earth. Several factors restrict the use of lignin: non-uniform structure, peculiar chemical reactivity, and the presence of various organic and inorganic impurities. As a suitable alternative to chemical means, lignin valorization can be obtained by the cleavage of selected bonds under mild conditions by enzymatic approaches. Taking into account that the β-O-4-aryl ether linkages account for approximately 50% of all ether bonds in lignin, non-radical ligninolytic enzymes, such as β-etherases, represent a specific and effective alternative for lignin cleavage and valorization. Five enzymes, LigD and LigL (Cα‐dehydrogenases), LigE and LigF (β‐etherases), and LigG (a glutathione lyase) are involved in the protobacterium Sphingobium sp. SYK-6 β-O-4 aryl-ether cleavage [1]. The biochemical characterization of these recombinant Lig enzymes (efficiently expressed in E. coli) allowed to reach the full bioconversion of a racemic mixture of the lignin linkage model compound (1‐(4‐hydroxy‐3‐methoxyphenyl)‐2‐(2‐methoxyphenoxy)‐1,3‐propanediol) (GGE) [2]. The combination of these oxidative Lig enzymes with known ligninolitic activities (laccases and peroxidases) [3] would provide a great opportunity, as an alternative to the oil source, to transform lignin in promising feedstocks of aromatic compounds which could lead important building blocks for preparative organic chemistry.