Engineering Microorganisms Toward the Production of Advanced Bioalcohols: A Review

Since its inception, the primary goal of metabolic engineering has been the manipulation of genetic elements toward the synthesis of high-value products. Today, with the practical and sustainable use of ethanol called into question, engineering microorganisms toward second generation biofuels (specifically alcohols) has become an important focus of academic research. Two approaches have been utilized in the past several decades to accomplish this goal. (1) Exploration into the genetic optimization of natural host pathways has led to higher yields and higher ratios of useful solvents, such as butanol, which satisfy the need for fuels which are more compatible with our current infrastructure. These achievements have been accomplished through rational improvements to the host including the minimization of competitive pathways, optimization of pathway enzymes toward their respective substrates and exploration of hosts with decreased sensitivity to normally toxic product concentrations. (2) Of particular interest in metabolic engineering is the development of innovative biosynthetic pathways for the production of novel fuel analogs and advanced fuel combinations. These are created through recombinant DNA strategies which translocate genetic elements expressing alcohol production pathways into heterologous microorganisms. This provides the opportunity for the selection of more industrially amicable hosts with competitive alcohol production capabilities. Both of these approaches present their own unique challenges. We review here the most important achievements and obstacles faced by the field of metabolic engineering in the development of advanced bioalcohols.