Biobutanol

Butanol produced from renewable materials is called biobutanol and is the result of fermentation of glucose from corn processing using Clostridium acetobutylicum and other anaerobic bacteria that produce butanol as a fermentative byproduct. Butanol is a potentially interesting biofuel because it is readily blended with gasoline, is poorly miscible with water, so it can be shipped via current pipeline infrastructure, and can be used in all current automobiles. The fermentation proceeds along the same pathway as for ethanol until pyruvate and acetyl-CoA. In butanolproducing bacteria acetyl-CoA is converted to acetoacetyl-CoA by the action of acetyl-CoA-acetyl transferase. Acetoacetyl-CoA is either metabolized to acetone and CO₂ or converted butyryl-CoA, which is the branch point to either butyrate or butanol. Butyrate is further metabolized to acetone and butanol enzymatically yielding multiple byproducts. Many bacteria also produce ethanol, hence yielding an acetone–butanol–ethanol (ABE) process.Subsequently it was discovered that key genes for ABE production are located on large plasmids in C. acetylbutylicum.Research has been aimed at manipulation of the pathway towards butanol at the expense of butyrate, but it is a difficult metabolic challenge.However, one of the largest hurdles is the toxicity of butanol to the cellular membrane and metabolic processes. Resistant mutants have been created as a partial solution,as have genetic engineering approaches.Approaches for improvement include removal of the butanol solvent and recent investigations of alternative substrates besides corn starch have been undertaken.
The genomic sequence of C. acetobutylicum has been determined, allowing further sophisticated investigations to proceed.Work in a number of laboratories has been examining gene expression during butanol fermentation to address the double problems of solvent toxicity
and the delicate balance of butanol and butyric acid accumulation.
The genome data have been used to undertake transcriptomic analysis by the groups of both Blaschek and Papoutsakis, so significant progress might be made. Indeed, DuPont and British Petroleum (BP) announced in June 2006 that they are developing technology for biobutanol production,and certainly they have the capability to apply all the new tools for systems biology to solve the low yield issue with this otherwise interesting biofuel. However, in-depth use of the new biological tool kit (‘the omics’) to this complex metabolic challenge will likely assist further in solving this difficult process problem.