Biological and chemical design of lignin conversion

The utilization of lignin for fungible products remains one of the most imminent challenges in biorefinery. The fundamental challenge in lignin degradation lies in the highly recalcitrant structure that requests redox (oxidation-reduction) reactions to break down the chemical linkages. First, we addressed the challenges by revealing the chemical and biological mechanisms for synergistic lignin degradation via a bacterial and enzymatic system, which significantly improved lignin consumption, cell growth and lipid yield. Chemical analysis suggested that laccase, R. opacus cell and Fenton reaction reagents promoted the degradation of different functional groups, elucidating the chemical basis for the synergistic effects. Second, we further improved lignin depolymerization by establishing a new enzyme-mediator system to enhance the electron transfer in enzymatic lignin depolymerization to achieve more than 35% solubilization of kraft lignin without extreme temperature, pH, and pressure. The study has manifested that an efficient enzyme-mediator system can be exploited to establish a bioprocess to solubilize lignin, cleave lignin linkages, modify the structure, and produce substrates amenable to bioconversion, all of which are vital for improving current processing and conversion methods for lignin based biofuels and biomaterials. Third, based on the aforementioned studies, consolidated lignin conversion was engineered in both Rhodococcus opacus and Pseudomonas putidausing systems biology-guided biodesign. Efficient ligninase secretion system has been established through proteomics-guided optimization of promoter/rbs, secretion peptide, and others. The engineered strains have led to significantly increased cell growth and bioproduct yield on lignin substrates. Overall, the research has opened new avenues for lignin conversion toward various bioproducts.