Submitted to: Applied Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 29, 2000
Publication Date: N/A
Interpretive Summary: Use of ethanol as a transportation fuel offers benefits such as decreased CO2 emissions, improved air quality, and decreased dependence on petroleum. Currently, 95% of fuel ethanol is produced using yeast to ferment the glucose in cornstarch. Ethanol can also be produced from inexpensive agricultural residues such as corn fiber and sugar cane bagasse. However, these materials contain a mixture of sugars in addition to glucose, and so a biomass to ethanol process requires an organism that, unlike yeast, can metabolize several different sugars. Ethanol-producing E. coli strains will ferment multiple sugars to ethanol, but use of sugars other than glucose is delayed, and often incomplete. Slow and incomplete use of sugars increases the fermentation time and cost and decreases the ethanol yield. To improve fermentation of the multiple sugars derived from biomass, we constructed new E. coli strains with improved use of mixed sugars. The new strains metabolize the pentose sugars xylose and arabinose simultaneously with glucose, rather than sequentially. Because the new E. coli strains ferment pentoses directly, they may provide more efficient production of ethanol from biomass.
Technical Abstract: Use of agricultural biomass, other than cornstarch, to produce fuel ethanol requires a microorganism that can ferment the mixture of sugars derived from hemicellulose. E. coli metabolizes a wide range of substrates and has been engineered to produce ethanol in high yield from sugar mixtures. E. coli metabolizes glucose in preference to other sugars, and as a result, ,utilization of the pentoses in hemicellulose-derived sugar mixtures is delayed and may be incomplete. Residual sugar lowers the ethanol yield and is problematic for downstream processing of fermentation products. Therefore, a catabolite repression mutant that simultaneously utilizes glucose and pentoses would be useful for fermentation of complex substrate mixtures. We constructed ethanologenic E. coli strains with a glucose phosphotransferase (ptsG) mutation and used the mutants to ferment glucose, arabinose, and xylose, singly and in mixtures, to ethanol. Yields swere 87-94% of theoretical for both the wild-type and mutants, but the mutants had an altered pattern of mixed sugar utilization. The parent strain preferentially utilized glucose before metabolizing arabinose and xylose to ethanol. In contrast, the ptsG mutants metabolized the pentoses simultaneously with glucose, rather than sequentially. Based upon fermentations of sugar mixtures, a catabolite repression mutant of ethanologenic E. coli is expected to provide more efficient fermentation of hemicellulose hydrolysates by allowing direct utilization of pentoses.