Lignin Valorization: Two Hybrid Biochemical Routes for the Conversion of Polymeric Lignin into Value-added Chemicals

Weihua Wu,Arul M Varman,Tanmoy Dutta,Bianca Manalansan,Seema Singh,Aymerick Eudes,Dominique Loqué

doi:10.1038/s41598-017-07895-1

Abstract

Naturally, many aerobic organisms degrade lignin-derived aromatics through conserved intermediates including protocatechuate and catechol. Employing this microbial approach offers a potential solution for valorizing lignin into valuable chemicals for a potential lignocellulosic biorefinery and enabling bioeconomy. In this study, two hybrid biochemical routes combining lignin chemical depolymerization, plant metabolic engineering, and synthetic pathway reconstruction were demonstrated for valorizing lignin into value-added products. In the biochemical route 1, alkali lignin was chemically depolymerized into vanillin and syringate as major products, which were further bio-converted into cis, cis-muconic acid (ccMA) and pyrogallol, respectively, using engineered Escherichia coli strains. In the second biochemical route, the shikimate pathway of Tobacco plant was engineered to accumulate protocatechuate (PCA) as a soluble intermediate compound. The PCA extracted from the engineered Tobacco was further converted into ccMA using the engineered E. coli strain. This study reports a direct process for converting lignin into ccMA and pyrogallol as value-added chemicals, and more importantly demonstrates benign methods for valorization of polymeric lignin that is inherently heterogeneous and recalcitrant. Our approach also validates the promising combination of plant engineering with microbial chassis development for the production of value added and speciality chemicals.

Highlights

Ligninolytic microbes an ineffective means for lignin depolymerization and highlights the need for microbial consortia for this task
We have successfully demonstrated the first de novo production process for ccMA and pyrogallol in E. coli from lignocellulosic biomass through two hybrid biochemical routes
By combining chemically catalytic pretreatment of lignin, plant metabolic engineering, and the construction of heterologous synthetic pathways to convert vanillin and PCA into ccMA as well as syringate into pyrogallol, we achieved the yields of ccMA at 0.69 g ccMA/g vanillin and 7.3 mg pyrogallol/g syringate as well as 0.31 g ccMA/g PCA (0.45 mg ccMA/g Tobacco stem, route 2), respectively

Summary

Introduction

Ligninolytic microbes an ineffective means for lignin depolymerization and highlights the need for microbial consortia for this task. In a second approach that avoids the thermochemical approach altogether, making the process environmentally benign without compromising the efficiency and selectivity, we demonstrated a novel hybrid route of lignin valorization combining 1) plant cell wall engineering, and 2) synthetic pathway reconstruction In this biochemical route, the aromatic amino acid pathway, source of phenylalanine for the lignin biosynthesis pathway in a model Tobacco plant was engineered to convert 3-dehydroshikimate, an intermediate of the shikimate pathway, into protocatechuate (highlighted in red, Fig. 1(B)), which was further extracted from the plant biomass as a substrate for an engineered chassis to convert PCA into value-added chemicals, for example cis, cis-muconic acid (ccMA). The engineered strain demonstrated the bioconversion of vanillin into ccMA at high yield

Methods

Results

Conclusion