Biofuels are defined as “fuel produced from renewable biomass material (Clean Energy Ideas, 2012)” and are oft-touted as a greener and cleaner source of energy. Biofuels currently have great significance as they provide a possible alternative to conventional fossil-fuel burning in the production of energy.
A team of researchers from the US Department of Energy’s Joint BioEnergy Institute (JBEI) were stimulating the Escherichia coli (E.coli) bacteria to use fatty acids to produce “long-chain alkene hydrocarbons (Yarris, 2012)”, which could be turned into diesel fuel, when they noticed that a by-product of this manipulation was an increase in the amount of methyl ketones produced. They decided to further investigate this upon determining that the cetane numbers (CN) – a measure of the ease of ignition and combustion of a diesel fuel – for these ketones were favourable.
GENETICALLY ENGINEERING BACTERIA TO PRODUCE BIOFUELS
Producing larger amount of methyl ketones
E. coli already produces negligible amounts of methyl ketones. The research team has managed to amplify the production rate about 4,000 times through two specific genetic modifications (Yarris, 2012). The first modification was to the metabolic pathway the bacteria uses to break down the fatty acids, beta-oxidation. The second was to one of the proteins present, FadM. (Yarris, 2012).
The genes involved in beta-oxidation which were modified are FadE, which was deleted, and acyl coenzyme A (CoA) and FadB, both of which were over-expressed. The native protein FadM was also over-expressed. As a result of these changes, the beta-Ketoacyl-CoA substrate is overproduced and subsequently processed by FadM to produce beta-Keto acids which (possibly spontaneously) produce methyl ketones (Figure 1). (Goh, E. et al., 2011).
Figure 1: A visual depiction of the modified and genes and their effects on the produced compounds. Green boxes indicate over-expressed genes, while red boxes indicate deleted genes. The believed substrate of the FadM protein is indicated by the blue box. The purple box indicates the final methyl ketone product. (Goh, Baidoo, Keasling, & Beller, 2011).
The methyl ketones produced were within the diesel range (C11 – C15). When the team tested the cetane number of the produced methyl ketone groups, the values were favourable (CN=56.6 and CN=58.4 for 2-undecanone and a 50/50 mix of 2-undecanone and 2-tridecanone, respectively, compared to the US minimum of CN=40 (Goh, E. et al., 2011)). This indicates that these ketones could plausibly be used as biofuels in the future.
Further modifying the bacteria to address the melting point problem
A possible concern in the study is the behaviour of the biofuel at low temperatures. For example, the melting range for 2-undecanone is 11-13°C (ChemWatch, 2009). This would be a problem in cold-temperate locations, as the compound would begin to congeal and solidify at temperatures below this range.
To address this issue, the bacteria was genetically modified to change the cultivation temperature (Goh, E. et al., 2011) to produce methyl ketones with a higher percentage of monounsaturated carbon chains, as they have a lower melting point than the related, saturated, methyl ketones (Yarris, 2012). By lowering the melting point, the team was able to lower the temperature the biofuel could feasibly be used at.
Working to address efficiency
The team will be investigating methods to increase the efficiency of production and to refine desirable fuel properties through experimenting with various carbon-chain lengths and unsaturation levels (Yarris, 2012).
The genetic manipulation of E. coli bacteria to synthesise larger amounts of useable biofuels adds to the currently available biofuel options for use. Furthermore, as the methyl ketones are compounds derived from fatty acids, this research can inform other work being done on fatty acids (Yarris, 2012). As more research is done, biofuels show promise of becoming a clean, renewable energy source in the future.
ChemWatch. (2009, May 8). 2-Undecanone. Retrieved March 15, 2012, from ChemGold III: http://jr.chemwatch.net/chemgold3/testcookie.exe?operation=externalpage&user=qlduni&pwd=AHSr1h&passdirect=y&dummy=0.45473819249209135
Clean Energy Ideas. (2012). Biofuel Definition. Retrieved March 15, 2012, from Clean Energy Ideas: http://www.clean-energy-ideas.com/energy_definitions/definition_of_biofuel.html
Goh, E., Baidoo, E., Keasling, J. & Beller, H. (2011, October 28). Engineering of Bacterial Methyl Ketone Sythesis for Biofuels. Applied and Environmental Microbiology , 70-80.
Reece, J., Urry, L., Cain, M., Wasserman, S., Minorsky, P., & Jackson, R. (2011). Biology. San Francisco: Pearsons Education, Inc.
Yarris, L. (2012, March 13). A Fragrant New Biofuel. Retrieved March 13, 2012, from Berkeley Lab: http://newscenter.lbl.gov/feature-stories/2012/03/13/a-fragrant-new-biofuel/