Leveraging microbial biosynthetic pathways for the generation of 'drop-in' biofuels

Abstract

Advances in retooling microorganisms have enabled bioproduction of 'drop-in' biofuels, fuels that are compatible with existing spark-ignition, compression-ignition, and gas-turbine engines. As the majority of petroleum consumption in the United States consists of gasoline (47%), diesel fuel and heating oil (21%), and jet fuel (8%), 'drop-in' biofuels that replace these petrochemical sources are particularly attractive. In this review, we discuss the application of aldehyde decarbonylases to produce gasoline substitutes from fatty acid products, a recently crystallized reductase that could hydrogenate jet fuel precursors from terpene synthases, and the exquisite control of polyketide synthases to produce biofuels with desired physical properties (e.g., lower freezing points). With our increased understanding of biosynthetic logic of metabolic pathways, we discuss the unique advantages of fatty acid, terpene, and polyketide synthases for the production of bio-based gasoline, diesel and jet fuel.

Figure 1

Fatty acid synthesis of alkanes. (a) In the first step of fatty acid synthesis, acetyl-CoA and malonyl-CoA are transthioesterified to the fatty acid synthase (FAS), and the final product of each round of elongation is a fatty-acyl ACP. A thioesterase (TesA) can cleave the fatty-acyl ACP to generate a free fatty acid that reacts with an acyl-CoA ligase (ACL) to generate fatty-acyl CoA. (b) Fatty aldehydes can be generated from fatty-acyl ACP, free fatty acids, and fatty-acyl CoA through acyl carrier protein reductase (AAR), carboxylic acid reductase (CAR), and acyl CoA reductase (ACR1), respectively. (c) Fatty aldehydes can generate alkanes through aldehyde decarbonylase (ADO) and fatty alcohols through aldehyde reductase (AHR).

Figure 2

Isoprenoid pathway for the production of biobased diesel and jet fuels. (a) Acetyl-CoA generates the building blocks for isoprenoid production through the mevalonate pathway. (b) Farenyl pyrophosphate synthase (FPPS) synthesizes feranyl pyrophosphate (FPP) from two IPP and one DMAPP. Farnesene and bisabolene are synthesized by their respective synthases from FPP. Hydrogenation of each molecule produces biodiesel fuel candidates. (c) Geranyl pyrophosphate synthase (GPPS), converts an IPP and DMAPP molecule to geranyl pyrophosphate (GPP). Limonene and pinene are synthesized by limonene and pinene synthase, respectively. Chemically dimerized pinene and hydrogenated limonene are biobased jet fuel candidates.

Figure 3

Branching through polyketide synthase pathways. (a) An acyltransferase (AT, highlighted in red) selects for methylmalonyl-CoA and transfers it to a phosphopantetheine arm of the acyl carrier protein (ACP). A Claisen condensation reaction of methyl-malonate and the primer chain takes place at the ketosynthase (KS), resulting in a α-methyl group (red). The ACP then shuttles the resulting ketone through the processing domains that reduce (ketoreductase, KR) and dehydrate (dehydratase, DH) the β-ketone, where the enoyl reductase (ER) full reduces the keto group. (b) An alternative approach to generating a α-branched carbon (red) is through utilizing SAM-dependent methyl transferases (MT, highlighted in red).