Metabolic engineering of Escherichia coli for higher alcohols production: An environmentally friendly alternative to fossil fuels

The extensive use of fossil fuels has contributed to the scarcity of this energy source and to an increase of greenhouse gases emissions to the atmosphere contributing to the global warming phenomenon. To avoid the problems associated to fossil fuels, it becomes necessary to use cleaner and renewable energy sources. However, these new energy sources should compete with the prices of fossil fuels. In the last decades, several efforts have been conducted to produce chemical compounds that can replace fossil fuels using microorganisms. For that, these compounds, normally corresponding to higher alcohols or long-chain fatty acids, must have properties comparable to those of current transportation fuels. However, native organisms cannot synthesize these fuels in a cost-effective way. Owing to that, advances in metabolic engineering, as well as in synthetic and systems biology, appear as a viable alternative for the production of these compounds. Higher alcohols, such as 1-butanol, isobutanol, 2-methyl-1-butanol and 3-methyl-1-butanol, have shown to present comparable properties to those of gasoline, being potential substitutes or additives for this petroleum-derived fuel. This review shows the recent developments in metabolic engineering of Escherichia coli for higher alcohols production and tolerance, emphasizing two different pathways: (i) the fermentative pathway, originally from microorganisms from the genera Clostridia; and (ii) the non-fermentative pathway, also known as the keto acid pathway. Engineering these synthetic pathways in heterologous organisms that are well-known and better suited for large-scale growth and industrial production, normally E. coli and Saccharomyces cerevisiae, can significantly improve advanced biofuels production.