A living organism been engineered to generate propane, the main component of liquid petroleum gas – a development that could see microbes one day pumping out fuel for cars and jets and to heat homes.
Scientists from Imperial College London, UK, and the University of Turku in Finland introduced five new enzymes into E. coli bacteria to divert intermediate compounds involved in fatty acid synthesis into a new metabolic pipeline to make propane (Nature Commun., doi: 10.1038/ncomms5731). A sixth enzyme involved in the process was native to E. Coli.
‘The optimal pathway we found required six genes to be expressed,’ says study author Patrik Jones at Imperial College. ‘The challenging part for us was to generate the four-carbon long butyric acid. We were certain once we had that we could get the remaining two steps working.’
This is a proof-of-concept study and only tiny amounts of propane were generated. Propane is a gas at room temperature and can easily escape E. coli during a liquid-based biotech process; it also requires little energy input for conversion into a liquid for storage or transport.
Apart from E. coli, ‘another natural candidate would be yeast,’ says Jones, adding that further into the future the team is also looking at inserting these pathways into photosynthetic bacteria that could convert solar energy directly into fuel.
Propane is an attractive fuel as it already has a global market and is compatible with existing infrastructure.
‘I was waiting for someone to engineer a pathway for propane production from ADO [aldehyde deformylating oxygenase, one of the key enzymes added to the pathway],’ says Justin Siegel at the University of California Genome Centre, Davis, US. ‘It has been idea I have heard thrown around by many researchers since this class of enzymes was discovered in 2010. It is great to see this idea come to fruition.’
However, ‘there are a huge number of questions for scaling,’ Siegel warns. ‘For this system, the most immediate issue will be getting industrially relevant titer and productivity levels, for example, titers need to be in g/L as opposed to mg/L.’
Asked what a scaled-up process would look like, Siegel says: ‘This is exactly the question the entire biofuels industry is stuck on at the moment. But in a science fiction sense, it would be neat to be able to light up my barbecue by mixing some sugar water with a ‘propane production packet’ containing such an organism and hooking it up to the grill.’