Trends in Biotechnology
ReviewBiofuel alternatives to ethanol: pumping the microbial well
Introduction
Conversion of biomass to biofuels has been the subject of intense research efforts since the 1970s. This work has recently gained significant political and scientific momentum owing to concerns about climate change, global energy security and petroleum supply. Global energy usage is projected to nearly double in the next two decades 1, 2, 3, and biological fuel production might serve as a sustainable, carbon-neutral energy source compatible with current engine technology. In an effort to offset increases in consumption and to limit the fossil fuel-related negative impacts on the environment, the US Department of Energy has established the goal of supplanting 30% of gasoline consumption with cellulosic ethanol by 2030 [2]. Similarly, a European Union Directive (2003) aims to replace 5.75% of all gasoline and diesel transport fuels with biofuels by 2010 [4].
Owing to physical and political limitations on arable land, it is believed that future biofuels will, by necessity, originate from abundantly available lignocellulosic biomass [5]. The liberation of monomeric sugars from this biomass is the subject of intensive research efforts that might be the key to the sustainability of any biofuel process. This extremely important area of research has been reviewed elsewhere 6, 7. Here, we concentrate on another important factor for the cost-effective production of biofuels from lignocellulosic feedstocks – namely the conversion of biomass hydrolysates (monosaccharides) to target molecules. We present some recent developments in microbial pathway modifications as a means of producing both currently used and prospective biofuels from agriculturally prevalent sugars.
The use of microbial systems for the production of industrially relevant compounds has seen substantial gains in the past decade as a direct result of the genomics revolution. Further advances in gene regulation, protein engineering, pathway portability, synthetic biology and metabolic engineering will propel the development of cost-efficient systems for biofuel production. The current range of biofuels consists primarily of microbially derived ethanol and plant-based biodiesel (Box 1). Although biodiesel is favored in several European countries, ethanol dominates the majority of the world biofuel market, including that of the United States. The pros and cons associated with any potential biofuel present challenges and opportunities that are best addressed on a case-by-case basis, and these issues are therefore beyond the scope of this review. Instead, we cover advances in metabolic engineering to produce biomolecules that are currently being explored for the development as transportation fuels (Figure 1).
Section snippets
Sugar catabolism and the fermentation pathway
Agriculturally derived lignocellulosic hydrolysates contain predominantly glucose and xylose and small amounts of arabinose, galacturonic acid and rhamnose, in addition to a large percentage of lignin residues (not addressed here). For the economical bioconversion of these feedstocks to commodity chemicals, efficient use of a high percentage of available monosaccharides is crucial. Although glucose metabolism is a nearly universal trait of living organisms, the ability to metabolize other
Major challenges and future directions
Although known metabolic pathways offer several possible avenues for the biosynthesis of fuel molecules, several other factors need to be addressed before they can be applied in an industrial setting. First and foremost are the physical properties of the potential biofuel molecule. These properties have an impact on everything, from its suitability as a fuel to the purification processes and the mode of its transport to consumers. Although discussion of the properties for the biofuels mentioned
Acknowledgements
The authors would like to thank Harry Beller and Dominique Loque for their critical reading of this manuscript. This work was funded by the Joint BioEnergy Institute (JBEI) through a grant from the US Department of Energy and by the Synthetic Biology Engineering Research Center (SynBERC) through a grant from the National Science Foundation.
References (89)
Evolutionary engineering of mixed-sugar utilization by a xylose-fermenting Saccharomyces cerevisiae strain
FEMS Yeast Res.
(2005)Bioproduction of butanol from biomass: from genes to bioreactors
Curr. Opin. Biotechnol.
(2007)Continuous butanol fermentation and feed starch retrogradation: butanol fermentation sustainability using Clostridium beijerinckii BA101
J. Biotechnol.
(2005)Biodiesel fuel production by transesterification of oils
J. Biosci. Bioeng.
(2001)Biodiesel from microalgae
Biotechnol. Adv.
(2007)Extracellular secretion of free fatty acids by disruption of a fatty acyl-CoA synthetase gene in Saccharomyces cerevisiae
J. Biosci. Bioeng.
(2003)Engineering microbial cell factories for biosynthesis of isoprenoid molecules: beyond lycopene
Trends Biotechnol.
(2007)- et al.
Multi-dimensional gene target search for improving lycopene biosynthesis in Escherichia coli
Metab. Eng.
(2007) Identifying gene targets for the metabolic engineering of lycopene biosynthesis in Escherichia coli
Metab. Eng.
(2005)Engineering of the pyruvate dehydrogenase bypass in Saccharomyces cerevisiae for high-level production of isoprenoids
Metab. Eng.
(2007)
Balancing a heterologous mevalonate pathway for improved isoprenoid production in Escherichia coli
Metab. Eng.
Engineered carotenoid biosynthetic pathways
Methods Enzymol.
Global transcription machinery engineering: a new approach for improving cellular phenotype
Metab. Eng.
Metabolic engineering for the microbial production of 1,3-propanediol
Curr. Opin. Biotechnol.
The commercial production of chemicals using pathway engineering
Biochim. Biophys. Acta
Tolerance of bacteria to organic solvents
Res. Microbiol.
Strain improvement by metabolic engineering: lysine production as a case study for systems biology
Curr. Opin. Biotechnol.
Biofuels. Curr. Biol.
Expanding the metabolic engineering toolbox: more options to engineer cells
Trends Biotechnol.
Non-biodegradable biopolymers from renewable resources: perspectives and impacts
Curr. Opin. Biotechnol.
Bonkers about biofuels
Nat. Biotechnol.
Can biofuels finally take center stage?
Nat. Biotechnol.
Chemical engineering: hybrid routes to biofuels
Nature
Pretreatment of lignocellulosic materials for efficient bioethanol production
Adv. Biochem. Eng. Biotechnol.
Progress and challenges in enzyme development for biomass utilization
Adv. Biochem. Eng. Biotechnol.
Metablic engineering of Saccharomyces cerevisiae for xylose utilization
Adv. Biochem. Eng. Biotechnol.
Development of efficient xylose fermentation in Saccharomyces cerevisiae: xylose isomerase as a key component
Adv. Biochem. Eng. Biotechnol.
Cofermentation of glucose, xylose, and arabinose by genomic DNA-integrated xylose/arabinose fermenting strain of Zymomonas mobilis AX101
Appl. Biochem. Biotechnol.
Bacteria engineered for fuel ethanol production: current status
Appl. Microbiol. Biotechnol.
Alcoholic fermentation of carbon sources in biomass hydrolysates by Saccharomyces cerevisiae: current status
Antonie Van Leeuwenhoek
Development of ethanologenic bacteria
Adv. Biochem. Eng. Biotechnol.
Genetic engineering of ethanol production in Escherichia coli
Appl. Environ. Microbiol.
Cellulosic ethanol: biofuel researchers prepare to reap a new harvest
Science
Construction of an Escherichia coli K-12 mutant for homoethanologenic fermentation of glucose or xylose without foreign genes
Appl. Environ. Microbiol.
Enhanced hydrogen production from glucose using ldh- and frd-inactivated Escherichia coli strains
Appl. Microbiol. Biotechnol.
The photobiological production of hydrogen: potential efficiency and effectiveness as a renewable fuel
Crit. Rev. Microbiol.
Organic acid and solvent production
Butanol fermentation research: upstream and downstream manipulations
Chem. Rec.
The structural biology of type II fatty acid biosynthesis
Annu. Rev. Biochem.
Botryococcus braunii: a renewable source of hydrocarbons and other chemicals
Crit. Rev. Biotechnol.
Botryococcus braunii: a rich source for hydrocarbons and related ether lipids
Appl. Microbiol. Biotechnol.
New pathway for long-chain n-alkane synthesis via 1-alcohol in Vibrio furnissii M1
J. Bacteriol.
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These authors contributed equally.