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Feature Article

Photo courtesy of:
Rocky Mountain Laboratories, NIAID, NIH

Ethanol may currently be getting all the attention, but a number of researchers have questioned whether higher alcohols such as butanol and isobutanol might not actually make better biofuels. The sticking point has always been the lack of an efficient, non-fossil fuel process for producing these higher alcohols. But now a team of US chemical and biomolecular engineers has developed just such a process, by hijacking amino acid synthesis.

Higher alcohols are so-called because their molecules contain more carbon atoms (four to five) than ethanol (which only has two). As a result, higher alcohols don't suffer from many of the problems that beset ethanol. For a start, their energy density is higher than that of ethanol, being much closer to that of petrol. Furthermore, unlike ethanol, higher alcohols aren't hygroscopic, meaning that they don't take up water from the atmosphere. This property can make ethanol corrosive and therefore difficult to store and transport. Finally, higher alcohols are less volatile than ethanol, meaning that they're less likely to catch fire and explode.

Unfortunately, however, whereas a large number of micro-organisms are able to produce ethanol at fairly high concentrations via fermentation, they can only produce higher alcohols in very small quantities as microbial by-products. Nevertheless, just because micro-organisms don't tend to produce higher alcohols doesn't mean that they can't; it just means that they don't usually need to.

‘Unlike ethanol, organisms are not used to producing these unusual alcohols and there is no advantage for them to do so,' explains James Liao, a professor of chemical and biomolecular engineer at the University of California, Los Angeles (UCLA). But many micro-organisms still possess the potential to produce higher alcohols, it just needs to be harnessed in the right way. In this case, that meant hijacking the process by which micro-organisms create amino acids.

One of the intermediates in this process is a family of organic molecules known as 2-keto acids. Liao realised that these molecules held to the key to the natural production of higher alcohols, because they can be converted to aldehydes by enzymes known as 2-keto-acid decarboxylases (KDC) and then to higher alcohols by enzymes known as alcohol dehydrogenases (ADH). For example, in the normal amino acid biosynthesis pathway glucose is converted into pyruvate, which is then converted into the amino acid valine via 2-ketoisovalerate. Alternatively, however, this 2-ketoisovalerate can be converted to isobutanol via KDC and ADH.

To see whether this hijacking would work in practice, Liao and two colleagues inserted the genes for KDC and ADH from various different micro-organisms into Escherichia coli. On growing these E. coli on glucose-containing media, they found that certain strains, expressing specific versions of the KDC and ADH genes, were able to produce various higher alcohols, including isobutanol and butanol.

They then attempted to optimise the production of these higher alcohols by over-expressing those genes involved in their synthesis and deleting those genes involved in the production of unwanted by-products. In this way, they were able to develop a strain of E. coli that could produce around 60 times more isobutanol than normal.

Following this initial success, UCLA licensed Liao's modified E. coli production process to Gevo, a Californian company developing biofuels based on higher alcohols, at the end of December 2007 . ‘This advanced modification method will enable us to speed up the commercial introduction of advanced biofuels like butanol by several years,' predicts Pat Gruber, Gevo's CEO.

He also envisages a range of other uses for naturally-produced higher alcohols: ‘In addition to being sold as an advanced biofuel, cost effective production of butanol also enables its use as a chemical intermediate.' 

Related Links

Nature, 2008, 451, 86 - 89: "Non-fermentative pathways for synthesis of branched-chain higher alcohols as biofuels"

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.

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