An international team of researchers has successfully engineered a common intestinal bacterium to produce renewable propane. While they can only produce small amounts at the moment, if the development can be scaled up to a commercially viable process then one day it could potentially lead to a sustainable alternative to fossil fuels. The study has been published in Nature Communications.
Liquid petroleum gas (LPG) is used as a fuel in a variety of applications, from central heating to cooking equipment and vehicles. It’s predominantly made up of either propane or butane which are both hydrocarbon compounds called alkanes. The former is an attractive target as a renewable biofuel since it has an existing global market. It’s also clean-burning and has a naturally lower carbon content than other fuels such as gasoline, diesel and ethanol.
Propane is already produced as a by-product during natural gas processing and petroleum refining, but these are finite resources. Therefore, researchers need to find a way to produce it without tapping into fossil reserves, which is what researchers from Imperial College London and Turku University have been working on.
For the study, the researchers hijacked a pathway used by E. coli to synthesize fatty acids and redirected it towards a synthetic alkane pathway. To do this, they engineered the bacteria to produce three enzymes: a new variant of thioesterase, CAR and aldehyde-deformylating oxygenase (ADO). The first targets a fatty acid called butyric acid and prevents it from entering a pathway that results in its incorporation into membranes. The second converts butyric acid into butyraldehyde, and the third uses this to produce propane.
Scientists had previously tried to use ADO to produce propane, but found that it was very inefficient. By adding an electron-donating molecule, the researchers were able to boost the catalytic capability of the enzyme and also remove native enzymes that naturally break down the products of this new pathway.
While they successfully demonstrated that it is possible to generate propane using this process, at the moment this is more of a proof of concept study since they could only produce small amounts. In order to be used commercially, they would need to produce around 1,000 times more than what they can currently make. However, the fuel that they did produce is ready to be used in an engine straight away.
According to lead researcher Patrik Jones, they are unsure at the moment how precisely the fuel molecules are made. However, further research that's currently underway should shed light on the process, allowing the team to refine it. Jones envisages that they could achieve a commercially viable process in around 5-10 years. Ultimately, they would like to use this system in photosynthetic bacteria, meaning that they could convert solar energy into biofuel.