Towards production of short chain fatty acids and adipic acid in Escherichia coli

Short chain fatty acids are industrially interesting building blocks and they are conceivable precursors for biofuels and industrially important diacids, such as adipic acid. Few organisms naturally accumulate short chain fatty acids and no natural metabolic pathway for adipic acid production is known. Here we present our recent efforts to establish Escherichia coli as microbial cell factory for these substances. A synthetic approach employing Golden Gate Cloning to assemble different metabolic pathways derived from Pseudomonas putida and Megasphaera elsdenii was chosen.

The rumen bacterium M. elsdenii readily accumulates a mixture of short chain fatty acids, when cultured under appropriate conditions. Butyric acid, pentanoic acid and hexanoic acid are among the accumulated compounds. However, the precise metabolic pathway leading to these short chain fatty acids has not been known. In order to establish heterologous production we set out to define this metabolic pathway by thorough metabolic analyses of the strain and making use of the genome sequence of M. elsdenii, which we obtained recently. Coversion of carboxylic acids to their corresponding diacids is envisioned with the alk system of P. putida. The central enzyme of this enzyme system is a monooxygenase, which allows the functionalisation of aliphatic side chains, enabling P. putida to grow on alkanes in nature. However, this system also allows the omega oxidation of carboxylic acids, for example the oxidative conversion of hexanoic acid to adipic acid.