Product specificity is a key advantage of biochemical fuel production, enabling synthesis of bio-advantaged fuels and blendstocks with highly tunable production and combustion characteristics. Historically, production of such bio-advantaged fuels has been constrained by the limited number of tractable, high-flux metabolic pathways to reduced fuel molecules. Recent advances in synthetic biology have greatly increased accessibility of new pathways to metabolic engineering, enabling production of a wider array of structures, many of which are unreachable via traditional chemical synthesis. We seek to demonstrate this bio-advantaged approach via production and characterization of fuel molecules in two key metabolic classes: isoprenoid-derived products and polyketide synthase (PKS)-derived products. Unsaturated C5 isoprenoid alcohols have excellent potential as high-octane blendstocks, but are challenged by production characteristics including product toxicity, volatility, and high solubility in aqueous media. We have overcome these limitations via manipulation of production conditions and product targets, resulting in production and recovery of C5 alcohols at high rate and yield. Canonical isoprenoid-derived molecules are synthesized in C5 units, restricting metabolic flexibility; moving forward, incorporation of newly developed non-canonical C6 and C7 isoprenoid units will allow synthesis of previously uncharacterized molecules with improved fuel properties, including C6 and C7 alcohols, as well as C11 and C16 terpenes. Biological production of diesel-range fuels has traditionally relied on straight-chain, fatty-acid derived fuels, resulting in undesirably high melting points. Utilization of PKS pathways eliminates this challenge by allowing highly controlled branching and functionalization. To demonstrate the viability of this approach, we have developed a system for conversion of biofuels to branched, C5-C7 ketones at titers exceeding 1 g/L. Building on this result, engineering iterative PKS modules for branched chain elongation enables production of highly methylated, diesel fuel candidates with cetane exceeding 40 and predicted melting points below -40°C. A related class of iterative PKSs can be engineered to generate poly-cycloproponated molecules featuring up to 8 iterative cyclopropane units, yielding ultra-high energy density jet and diesel fuel candidates with predicted energy density exceeding 47 MJ/L.

Towards bio-advantaged fuels: Leveraging new biosynthetic pathways for design of next-generation fuels and blendstocks

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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