Fatty acid synthesis in Escherichia coli and its applications towards the production of fatty acid based biofuels

Abstract

The idea of renewable and regenerative resources has inspired research for more than a hundred years. Ideally, the only spent energy will replenish itself, like plant material, sunlight, thermal energy or wind. Biodiesel or ethanol are examples, since their production relies mainly on plant material. However, it has become apparent that crop derived biofuels will not be sufficient to satisfy future energy demands. Thus, especially in the last decade a lot of research has focused on the production of next generation biofuels. A major subject of these investigations has been the microbial fatty acid biosynthesis with the aim to produce fatty acids or derivatives for substitution of diesel. As an industrially important organism and with the best studied microbial fatty acid biosynthesis, Escherichia coli has been chosen as producer in many of these studies and several reviews have been published in the fields of E. coli fatty acid biosynthesis or biofuels. However, most reviews discuss only one of these topics in detail, despite the fact, that a profound understanding of the involved enzymes and their regulation is necessary for efficient genetic engineering of the entire pathway. The first part of this review aims at summarizing the knowledge about fatty acid biosynthesis of E. coli and its regulation, and it provides the connection towards the production of fatty acids and related biofuels. The second part gives an overview about the achievements by genetic engineering of the fatty acid biosynthesis towards the production of next generation biofuels. Finally, the actual importance and potential of fatty acid-based biofuels will be discussed.

Figure 1

Overview of the production of first- and second-generation biofuels.

Figure 2

Biosynthesis and degradation of fatty acids and membrane lipids. ATP and reduction equivalents are colored red if consumed and green if gained. Enzymes are colored blue. FAB, fatty acid biosynthesis; FAD, fatty acid degradation.

Figure 3

Regulation of the lipid metabolism. Activating compounds are colored green, and repressing compounds are shown in red. FAB, fatty acid biosynthesis; FAD, fatty acid degradation.

Figure 4

Metabolic pathways for the production of fatty acids and derived compounds. Enzymes and arrows are shown in green, if the pathway to which they belong departs from the wild-type fatty acid metabolism. The end products are shown in gray boxes. Appropriate deletions or homologous overexpressions within the wild-type pathway are not highlighted in this figure. For details, the reader is referred to the text. FAB, fatty acid biosynthesis; FAD, fatty -acid degradation; Aar, acyl-ACP reductase; Aco, acyl-CoA oxidase; Acr1, acyl-CoA reductase; Adc, aldehyde decarbonylase; Ahr, aldehyde recuctase; AtfA, acyltransferase; Cad, carboxylic acid decarboxylase; Car, carboxylic acid reductase; FadM, thioesterase; PgpB, phosphatidylglycerol phosphate phosphatase; PhaC2, PHA synthase 2; PhaG, (R)-3-hydroxydecanoyl-ACP transacylase; PhaJ, (R)-specific enoyl-CoA hydratase; SHMKS1, methylketone synthase 1; SHMKS2, methylketone synthase 2; TesA, thioesterase.