Fatty Acid-Derived Biofuels and Chemicals Production in Saccharomyces cerevisiae

Abstract

Volatile energy costs and environmental concerns have spurred interest in the development of alternative, renewable, sustainable, and cost-effective energy resources. Environment-friendly processes involving microbes can be used to synthesize advanced biofuels. These fuels have the potential to replace fossil fuels in supporting high-power demanding machinery such as aircrafts and trucks. From an engineering perspective, the pathway for fatty acid biosynthesis is an attractive route for the production of advanced fuels such as fatty acid ethyl esters, fatty alcohols, and alkanes. The robustness and excellent accessibility to molecular genetics make the yeast Saccharomyces cerevisiae a suitable host for the purpose of bio-manufacturing. Recent advances in metabolic engineering, as well as systems and synthetic biology, have now provided the opportunity to engineer yeast metabolism for the production of fatty acid-derived fuels and chemicals.

Keywords: alkanes/alkene; fatty acid ethyl ester; fatty acid metabolism; fatty alcohol; yeast.

Figure 1

Comparison of S. cerevisiae type I (A) and bacterial type II (B) fatty acid synthases is shown. (A) The catalytic reaction cycle of and domain organization of yeast fatty acid synthase. Acetyl-CoA is activated by ACP acyltransferase (AT) and then malonyl-CoA is iteratively fed into the reaction cycle by malonyl/palmitoyl transferase (MPT). The elongation process is consecutively catalyzed by ketoacyl reductase (KR), dehydratase (DH), and enoyl reductase (ER). After several rounds of elongation, the end product is released from the enzyme as a fatty acyl-CoA after back-transfer to CoA from ACP by the double-functional MPT. Desaturation of fatty acyl-CoA takes place in the endoplasmic reticulum and is catalyzed by the Δ9-fatty acid desaturase Ole1, and very long-chain fatty acids (VLFA) are synthesized by chain elongation of saturated acyl-CoAs through a cyclic series of reactions reminiscent of fatty acid de novo synthesis. (B) Bacterial type II fatty acid synthase (FAS) that consists of discrete, monofunctional enzymes. Acetoacetyl-ACP is in prior synthesized for the initiation of chain elongation, and then malonyl-CoA is iteratively fed into the elongation cycle after ACP loading, which is catalyzed by FabD. Desaturation can be performed at the C10 chain length by 3-hydroxydecanoyl-ACP dehydrase (FabA), and the product cis-3-enoyl acyl-ACP bypasses FabI of reduction and goes to FabB for the next round of elongation. Different from yeast fatty acid biosynthesis, the end product is released as a fatty acyl-ACP after several rounds of elongation.

Figure 2

Metabolic pathways for fatty acid-derived biofuel and chemical biosynthesis are shown. Acc1, acetyl-CoA carboxylase; FAS, fatty acid synthase; Pox1, acyl-CoA oxidase; Faa1-4 and Fat1, acyl-CoA synthetase; TES, thioesterase; ACR, fatty acyl-CoA reductase for fatty aldehyde synthesis; FaCoAR, fatty acyl-CoA reductase for fatty alcohol production; CAR, carboxylic acid reductase; FAR, fatty acid reductase; WS, wax ester synthase; ADO, aldehyde-deformylating oxygenase; ADH, alcohol dehydrogenase. The blue marked enzymes responsible for the endogenous fatty acid metabolism, while the green ones transform fatty acids and intermediates to different biofuels and oleo-chemicals.