Abstract
To date, the most significant sources of biofuels are starch- or sugarcane-based ethanol, which have been industrially produced in large quantities in the USA and Brazil, respectively. However, the ultimate goal of biofuel production is to produce fuels from lignocellulosic biomass-derived sugars with optimal fuel properties and compatibility with the existing fuel distribution infrastructure. To achieve this goal, metabolic pathways have been constructed to produce various fuel molecules that are categorized into fermentative alcohols (butanol and isobutanol), non-fermentative alcohols from 2-keto acid pathways, fatty acids-derived fuels and isoprenoid-derived fuels. This review will focus on current metabolic engineering efforts to improve the productivity and the yield of several key biofuel molecules. Strategies used in these metabolic engineering efforts can be summarized as follows: (1) identification of better enzymes; (2) flux control of intermediates and precursors; (3) elimination of competing pathways; (4) redox balance and cofactor regeneration; and (5) bypassing regulatory mechanisms. In addition to metabolic engineering approaches, host strains are optimized by improving sugar uptake and utilization, and increasing tolerance to toxic hydrolysates, metabolic intermediates and/or biofuel products.
Highlights
Sources of Sugars for Biofuel ProductionEthanol and biodiesels have been industrially produced from biomass by fermentation and chemical trans-esterification of plant oils, respectively
One of the early studies demonstrated the engineering of E. coli as a microbial factory for consolidated bioprocesses (CBP) by expressing enzymes required for both biomass degradation and biofuel synthesis (FAEE, butanol and pinene) [68]
Even though most studies showed that productivity was relatively reduced when biofuels were produced from lignocellulosic biomass (LCB)-derived hydrolysates or sugars, these results suggested that production of advanced biofuels from LCB-derived sugars is currently feasible, and it could be further improved by overcoming limitations that are not intrinsic to the engineered biofuel pathways and by further optimization of the responsible metabolic pathways
Summary
Ethanol and biodiesels have been industrially produced from biomass by fermentation and chemical trans-esterification of plant oils, respectively. Sugarcane-derived sugars (sucrose) have been used for ethanol fermentation in Brazil, and corn-derived starches (glucose) have been the major feedstock in the USA. Since consumption of these feedstocks for biofuel production competes with demands for animal feeds and human consumption [1], lignocellulosic biomass (LCB) has been suggested as an alternative and sustainable feedstock for biofuel industries. Beyond native fermentation pathways or natural biodiesel resources such as vegetable oils, advanced biofuel molecules are synthesized in microbial hosts where heterologous or synthetic metabolic pathways are reconstructed in fermentative hosts. Various pathways and hosts for ethanol and advanced biofuel production will be discussed, with a particular emphasis on metabolic engineering strategies to improve the microbial conversion bioprocess
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