Engineering Central Metabolism for Production of Higher Alcohol-based Biofuels

Widespread concerns have been raised regarding the need to develop sustainable processes for the production of fuels from renewable resources. While bioethanol production processes have been studied and successfully developed at industrial scales, its inferior fuel properties (e.g., lower energy density and higher hygroscopicity) relative to higher chain alcohols have directed recent interest towards producing C3-C10 alcohols, a more challenging prospect than bioethanol production. Metabolic engineering enabled by systems biology and synthetic biology is an enabling technology in such efforts, and many research examples show great promise for addressing current issues, including engineering enzymes and pathway flux, enhancing cofactor and precursor availability, and improving hosts' tolerance to toxic biofuel products. In this chapter, we discuss the challenges and research efforts towards engineering microbes for optimized production of alcohol-based biofuels, with an emphasis on C3-C10 alcohols produced via central metabolism. Section 1.1 begins with a general introduction and compares relevant properties of different fuels. Section 1.2 discusses two categories of alcohol-producing pathways in detail, including both fermentative (i.e., acetone-butanol-ethanol pathway called the ABE pathway) and non-fermentative (i.e., Ehrlich pathway linked with amino acid metabolism; and reverse β-oxidation pathway). In Section 1.3, engineering strategies to improve higher alcohol production are reviewed, which include (1) enhancing the function of enzymes and pathways as well as cofactor and precursor availability, and (2) addressing product toxicity. Section 1.4 covers successes and challenges towards commercialization of higher alcohol-based biofuels, giving some examples of successful commercialization and current issues and topics such as product separation, host choice, and alternative feedstocks. Section 1.5 concludes this chapter with an outlook on the future of higher alcohol biofuel production.