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Biological and Chemical Design for Lignin Conversion
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The efficient utilization of lignin for fungible fuels and products represents one of the most imminent challenges in modern biorefinery. My research thesis has mainly focused on developing and optimizing the lignin-to-lipid conversion route. Advanced ^31P nuclear magnetic resonance (NMR) lignin characterization technology, laccase based lignin pretreatment technology, proteomics and synthetic biology tools were applied to this study. First, the study demonstrated that the enzymes and microbes synergized to degrade different functional groups of lignin to promote the bioconversion. The Rhodococcus opacus cell growth increased exponentially in response to the level of laccase treatment, indicating the synergy between laccase and bacterial cells in lignin degradation. NMR analysis suggested that laccase, R. opacus cell and Fenton reaction reagents promoted the degradation of different types of lignin functional groups, elucidating the chemical basis for the synergistic effects. The cell-laccase fermentation led to a 17-fold increase of lipid production. Second, the study showed that enhanced electron transfer via electron mediators can promote the redox reaction to catalyze the lignin bioconversion by laccase. NMR analysis revealed that an efficient enzyme-mediator system can promote the cleavage of most intramolecular and intermolecular cross-links including the condensed chemical linkages, leading to the degradation of lignin to a large extent. The R. opacus PD630 growth was increased by 10^6 fold and the simple batch fermentation could achieve lipid titer at more than 1 g/L. Third, through systems biology-guided biodesign, I have optimized multiple steps of lignin degradation towards to lipid production. An efficient laccase secretion system was established in PD630 for lignin depolymerization through different molecular levels including transcription, translation, signal peptide secretion efficiency and transporters. Moreover, lipid biosynthesis pathway of R. opacus PD630 was optimized based on the proteomics analysis, which led to significantly increased lipid accumulation. The two modules of lignin depolymerization and lipid biosynthesis were then integrated for efficient lignin conversion into lipid. In summary, these studies enabled more efficient conversion of lignin and biorefinery waste to fungible products. The fundamental understanding of chemical and biological processes in lignin conversion can be exploited in different ways to enable various biorefinery product streams from biorefinery waste streams.
Xie, Shangxian (2016). Biological and Chemical Design for Lignin Conversion. Doctoral dissertation, Texas A & M University. Available electronically from