Synthetic Biology for Higher Alcohols
Global energy and climate problems have stimulated increased efforts in synthesizing fuels and chemicals from renewable resources. Compared to the traditional biofuel, ethanol, higher alcohols offer advantages as gasoline substitutes because of their higher energy density and lower hygroscopicity. In addition, branched-chain alcohols have higher octane numbers compared to their straight-chain counterparts. However, these alcohols cannot be synthesized economically using native organisms. We have developed a set of keto acid pathways to produce higher alcohols including isobutanol, 1-butanol, 2-methyl-1-butanol, 3-methyl-1-butanol and 2-phenylethanol from renewable carbon source. This strategy leverages the host’s highly active amino acid biosynthetic pathway and diverts its 2-keto acid intermediates for alcohol synthesis. In particular, we have achieved high yield, high specificity production of isobutanol from glucose.
We further developed a non-natural chain-elongation pathway to produce abiotic longer chain keto acids and alcohols by engineering the chain elongation activity of 2-isopropylmalate synthase and altering the substrate specificity of downstream enzymes through rational protein design. When introduced into E. coli, this non-natural biosynthetic pathway produces various long-chain alcohols with carbon number ranging from 5 to 8. This strategy enables the productions of a variety of non-natural metabolites useful as fuels and chemicals.
Direct synthesis of fuels and chemicals from CO2 and sunlight
Current schemes for bioproduction of fuels and chemicals use either plants or algae to convert CO2 to biomass, which is then processed to produce fuels or chemicals. Such schemes require processing of recalcitrant lignocellulose or algal lipid and the efficiency is suboptimal. To synthesize fuels and chemicals directly from CO2, we engineered a photosynthetic organism to produce isobutanol and isobutylaldehyde via the keto acid pathway. The resulting orgnanism produced these compounds with a productivity several-fold higher than current shemes of inderect synthesis of fuels.
References
Atsumi, S.; T. Hanai and J.C. Liao (2008) Non-Fermentative Pathways for Synthesis of Branched-Chain Higher Alcohols as Biofuels, Nature, 451:86-89 Zhang, K., Sawaya, M.R., Eisenberg, D.S. and J.C. Liao (2008) Expanding Metabolism for Biosynthesis of Nonnatural Alcohols, Proc. Natl. Acad. Sci. USA 105: 20653–20658 Atsumi,S.; Higashide, W.; and Liao, J.C. (2009) Direct recycling of carbon dioxide to isobutyraldehyde using photosynthesis, Nat Biotechnol, 27, 1177-1180.