Butanol may be used as a fuel in an internal combustion engine. Because its longer hydrocarbon chain causes it to be fairly non-polar, it is more similar to gasoline than it is to ethanol. Butanol has been demonstrated to work in vehicles designed for use with gasoline without modification. University of California, Berkeley, chemists have engineered bacteria to churn out a gasoline-like biofuel (butanol) at about 10 times the rate of competing microbes, a breakthrough that could soon provide an affordable transportation fuel.
The potential feedstocks are the same as for ethanol: energy crops such as sugar beets, sugar cane, corn grain, wheat and cassava, prospective non-food energy crops such as switchgrass and even guayule in North America, as well as agricultural byproducts such as straw and corn stalks.
The advance is reported in this week's issue of the journal Nature Chemical Biology.
Biobutanol can be produced by fermentation of biomass by the A.B.E. process. The process uses the bacteriumClostridium acetobutylicum, also known as the Weizmann organism.
Various species of the Clostridium bacteria naturally produce a chemical called n-butanol (normal butanol) that has been proposed as a greener substitute for diesel oil and gasoline. While most researchers, including a few biofuel companies, have genetically altered Clostridium to boost its ability to produce n-butanol, others have plucked enzymes from the bacteria and inserted them into other microbes, such as yeast, to turn them into n-butanol factories. Yeast and E. coli, one of the main bacteria in the human gut, are considered to be easier to grow on an industrial scale.
While these techniques have produced promising genetically altered E. coli bacteria and yeast, n-butanol production has been limited.
Chang and her colleagues stuck the same enzyme pathway into E. coli, but replaced two of the five enzymes with look-alikes from other organisms that avoided one of the problems other researchers have had: n-butanol being converted back into its chemical precursors by the same enzymes that produce it.
The basic steps evolved by Clostridium to make butanol involve five enzymes that convert a common molecule, acetyl-CoA, into n-butanol. Other researchers who have engineered yeast or E. coli to produce n-butanol have taken the entire enzyme pathway and transplanted it into these microbes. However, n-butanol is not produced rapidly in these systems because the native enzymes can work in reverse to convert butanol back into its starting precursors.
Chang avoided this problem by searching for organisms that have similar enzymes, but that work so slowly in reverse that little n-butanol is lost through a backward reaction.
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By. Andy Soos
