Commercial cellulosic ethanol: The role of plant-expressed enzymes
The use and production of biofuels has risen dramatically in recent yr. Bioethanol comprises 85% of total global biofuels production, with benefits including reduction of greenhouse gas emissions and promotion of energy independence and rural economic development. Ethanol is primarily made from corn grain in the USA and sugarcane juice in Brazil. However, ethanol production using current technologies will ultimately be limited by land availability, government policy, and alternative uses for these agricultural products. Biomass feedstocks are an enormous and renewable source of fermentable sugars that could potentially provide a significant proportion of transport fuels globally. A major technical challenge in making cellulosic ethanol economically viable is the need to lower the costs of the enzymes needed to convert biomass to fermentable sugars. The expression of cellulases and hemicellulases in crop plants and their integration with existing ethanol production systems are key technologies under development that will significantly improve the process economics of cellulosic ethanol production.
KeywordsBiofuels Cellulosic ethanol Cellulases Pretreatment Biomass Enzymes
Thanks to my colleagues Nancy Nye and Sherrica Morris for help in accessing references and to Karen Bruce, Terry Stone, John Steffens, Tom Bregger, Larry Gasper, and Scott Betts for helpful comments on the manuscript. My apologies to those colleagues whose publications I have not been able to include due to space constraints. The Syngenta Centre for Sugarcane Biofuels Development is supported by Syngenta, the Queensland University of Technology, Farmacule Bioindustries and by a grant from the National and International Research Alliances Program of the Queensland State government.
- Aden, A. Biochemical production of ethanol from corn stover: 2007 state of technology model. Technical report NREL/TP-510-43205, National Renewable Energy Laboratory, Golden, CO USA. http://www.nrel.gov/docs/fy08osti/43205.pdf (accessed 8 Jan 2009); 2008.
- Aden, A.; Ruth, M.; Ibsen, K.; Jechura, J.; Neeves, K.; Sheehan, J.; Wallace, B.; Montague, L.; Slayton, A.; Lukas, J. Lignocellulosic biomass to ethanol process design and economics utilizing co-current dilute acid prehydrolysis and enzymatic hydrolysis for corn stover. Technical report NREL/TP-510-32438 (http://www.nrel.gov/docs/fy02osti/32438.pdf (accessed 8 Jan 2009); 2002.
- Anonymous. Enzyme contract concludes successfully. Ethanol Producer June 2005. http://www.ethanolproducer.com/article.jsp?article_id = 201 (accessed 26 Jan 2009); 2005.
- Anonymous. Food prices: fact versus fiction. Biofuels International 2(5). http://biofuels-news.com/content_item_details.php?item_id = 126 (accessed 4 Jan 2009); 2008.
- Cheryan M.; Mehaia M. A. Ethanol production in a membrane recycle bioreactor. Conversion of glucose using Saccharomyces cerevisiae. Process Biochem. 19: 204–208; 1984.Google Scholar
- Coyle, W. The future of biofuels: a global perspective. Amber Waves 5(5). Available online at http://www.ers.usda.gov/AmberWaves/November07/Features/Biofuels.htm (accessed 7 Jan 2009); 2007.
- Dai Z.; Hooker B. S.; Quesenberry R. D.; Thomas S. R. Optimization of Acidothermus cellulolyticus endoglucanase (E1) production in transgenic tobacco plants by transcriptional, post-transcription and post-translation modification. Transgenic Res. 14: 627–643; 2005. doi: 10.1007/s11248-005-5695-5.PubMedCrossRefGoogle Scholar
- Dale, J. L.; Dugdale, B.; Hafner, G. J.; Hermann, S. R.; Becker, D. K. A construct capable of release in closed circular form from a larger nucleotide sequence permitting site specific expression and/or developmentally regulated expression of selected genetic sequences. International Patent Application WO 01/72996 A1; 2001.Google Scholar
- Dale, J. L.; Dugdale, B.; Hafner, G. J.; Hermann, S. R.; Becker, D. K.; Harding, R. M.; Chowpongpang, S. Construct capable of release in closed circular form from a larger nucleotide sequence permitting site specific expression and/or developmentally regulated expression of selected genetic sequences. US Patent Application US2004/0121430 A1; 2004.Google Scholar
- Dunn-Coleman, N.; Landgon, T.; Morris, P. Manipulation of the phenolic acid content and digestibility of plant cell walls by targeted expression of genes encoding cell wall degrading enzymes. US patent application US2003/0024009 A1; 2001.Google Scholar
- Fulton, L.; Howes, T.; Hardy, J. Biofuels for transport—an international perspective. International Energy Agency, Paris, France. Available online at http://www.iea.org/textbase/nppdf/free/2004/biofuels2004.pdf (accessed 23 Jan 2009); 2004.
- Hood E. E.; Love R.; Lane J.; Bray J.; Clough R.; Pappu K.; Drees C.; Hood K. R.; Yoon S.; Ahmad A.; Howard J. A. Subcellular targeting is a key condition for high level accumulation of cellulase protein in transgenic maize seed. Plant Biotechnol. J. 5: 709–719; 2007. doi: 10.1111/j.1467-7652.2007.00275.x.PubMedCrossRefGoogle Scholar
- Hooker B. S.; Dai Z.; Anderson D. B.; Quesenberry R. D.; Ruth M. F.; Thomas S. R. Production of microbial cellulases in transgenic crop plants. In: HimmelM. E.; BakerJ.O.; SaddlerJ. N. (eds) Glycosyl hydrolases for biomass conversion. American Chemical Society, Washington, DC, pp 55–90; 2001.Google Scholar
- Houghton, J.; Weatherwax, S.; Ferrell, J. Breaking the biological barriers to cellulosic ethanol: a joint research agenda. US Department of Energy Office of Science and Office of Energy Efficiency and Renewable Energy 206 pp; 2006.Google Scholar
- Ingram L. O.; Aldrich H. C.; Borges A. C. C.; Causey T. B.; Martinez A.; Morales F.; Saleh A.; Underwood S. A.; Yomano L. P.; York S. W.; Zaldivar J.; Zhou S. Enteric bacterial catalysts for fuel ethanol production. Biotechnol. Prog. 15: 855–866; 1999. doi: 10.1021/bp9901062.PubMedCrossRefGoogle Scholar
- Kline, K. L.; Oladosu, G. A.; Wolfe, A. K.; Perlack, R. D.; Dale, V. H.; McMahon, M. Biofuel feedstock assessment for selected countries. Report ORNL/TM-2007/224 prepared by Oak Ridge National Laboratory for the US Department of Energy (DOE). Available online at http://www.osti.gov/bridge/purl.cover.jsp;jsessionid=FD7D1D5D71C9ACFE53C44AE856766653?purl=/924080-y8ATDg/ (accessed 4 Jan 2009); 2008.
- Lavelle, M.; Garber, K. Fixing the food crisis. US News and World Report, May 19 2008, pp 36–42; 2008.Google Scholar
- Lebel E. G.; Heifetz P. B.; Ward E. R.; Uknes S. J. Transgenic plant expressing a cellulase. US Patent 7361806 B2; 2008.Google Scholar
- Litzen, D.; Dixon, D.; Gilcrease, P.; Winter, R. Pretreatment of biomass for ethanol production. US Patent Application US2006/0141584 A1; 2006.Google Scholar
- Martinot, E. et al. Renewables 2007: World Status Report. Renewable energy policy network for the 21st century (REN21) and Worldwatch Institute, Washington, DC, USA. Available online at http://www.worldwatch.org/files/pdf/renewables2007.pdf (accessed 15 Jan 2009); 2007.
- Rath, A. Focus on raw materials could reap big rewards. Bioenergy Business July/August 2007, pp 16–18; 2007.Google Scholar
- Sainz, M. B.; Dale, J. Towards cellulosic ethanol from sugarcane bagasse. Proceedings of the Australian Society of Sugar Cane Technologists 31: 18–23; 2009.Google Scholar
- Sun Y.; Cheng J. J.; Himmel M. E.; Skory C. D.; Adney W. S.; Thomas S. R.; Tisserat B.; Nishimura Y.; Yamamoto Y. T. Expression and characterization of Acidothermus cellulolyticus E1 endoglucanase in transgenic duckweed Lemna minor 8627. Bioresour. Technol. 98: 2866–2872; 2007. doi: 10.1016/j.biortech.2006.09.055.PubMedCrossRefGoogle Scholar
- Syngenta. The potential of corn amylase for helping to meet US energy needs. http://www.syngenta.com/en/corporate_responsibility/pdf/Fact%20Sheet%20The%20Potental%20of%20CA%20for%20Helping%20Meet%20US%20Energy%20Needs.pdf (accessed 7 February 2009); 2009.
- Taherzadeh M. J.; Karimi K. Enzyme-based hydrolysis processes for ethanol from lignocellulosic materials: a review. BioResources 24: 707–738; 2007.Google Scholar
- Teymouri F.; Alizadeh H.; Laureano-Perez L.; Dale B.; Sticklen M. Effects of ammonia fiber explosion treatment on activity of endoglucanase from Acidothermus cellulolyticus in transgenic plant. Appl. Biochem. Biotechnol. 113–116: 1183–1191; 2004. doi: 10.1385/ABAB:116:1-3:1183.PubMedCrossRefGoogle Scholar
- US Department of Energy. Biomass program: biomass feedstock composition and property database. Available online at http://www1.eere.energy.gov/biomass/feedstock_databases.html (accessed 5 February 2009); 2009.
- Wyman C. E.; Dale B. E.; Elander R. T.; Holtzapple M.; Ladisch M. R.; Lee Y. Y. Comparative sugar recovery data from laboratory scale application of leading pretreatment technologies to corn stover. Bioresour. Technol. 96: 2026–2032; 2005b. doi: 10.1016/j.biortech.2005.01.018.PubMedCrossRefGoogle Scholar