Researchers from the University of Turku in Finland, Imperial College London and University College London have devised a synthetic metabolic pathway for producing renewable propane from engineered E. coli bacteria. Propane, which has an existing global market for applications including engine fuels and heating, is currently produced as a by-product during natural gas processing and petroleum refining. A paper on their work is published in Nature Communications.
The new pathway is based on a thioesterase specific for butyryl-acyl carrier protein (ACP), which allows native fatty acid biosynthesis of the Escherichia coli host to be redirected towards a synthetic alkane pathway. Although the initial yields were low, the team was able to identify and to add essential biochemical components in order to boost the biosynthesis reaction, enabling a the E. coli strain to increase propane yield, although the amounts are still far too low for commercialization.
The level of propane that the scientists produced is currently one thousand times less than what would be needed to turn it into a commercial product, so they are now working on refining their newly designed synthetic process.
The new metabolic pathway comprises three key enzymes:
a thioesterase to produce butyric acid;
a carboxylic acid reductase (CAR) to convert butyric acid into butyraldehyde;and
an aldehyde-deformylating oxygenase (ADO) to form the final product, propane.
Previous attempts to use the ADO enzyme have proved disappointing as scientists have been unable to harness the natural power of the enzyme to create cleaner fuel. But the team discovered that by stimulating ADO with electrons they were able to substantially enhance the catalytic capability of the enzyme, and ultimately produce propane.
This propane-generating pathway is able to operate in the presence of oxygen, which opens up avenues for the application of this system in cyanobacteria, a type of bacteria that are powered by solar energy and have minimal nutritional requirements.
The scientists chose to target propane because it can easily escape the cell as a gas, yet requires little energy to transform from its natural gaseous state into a liquid that is easy to transport, store and use.
Although this research is at a very early stage, our proof of concept study provides a method for renewable production of a fuel that previously was only accessible from fossil reserves. Although we have only produced tiny amounts so far, the fuel we have produced is ready to be used in an engine straight away. This opens up possibilities for future sustainable production of renewable fuels that at first could complement, and thereafter replace fossil fuels like diesel, petrol, natural gas and jet fuel.
At the moment, we don’t have a full grasp of exactly how the fuel molecules are made, so we are now trying to find out exactly how this process unfolds. I hope that over the next 5-10 years we will be able to achieve commercially viable processes that will sustainably fuel our energy demands.—Dr. Patrik Jones, Imperial College London, joint corresponding author
This research was funded by a grant from the European Research Council.
Pauli Kallio, András Pásztor, Kati Thiel, M. Kalim Akhtar & Patrik R. Jones (2014) “An engineered pathway for the biosynthesis of renewable propane,” Nature Communications 5, Article number: 4731 doi: 10.1038/ncomms5731