Climate change is stimulating research, particularly to develop alternatives to fossil fuels. One such example is the photocatalytic conversion of microalgae fatty acids into hydrocarbons, which represents a promising route to the production of green fuels and other high-value compounds. In fact, like plants, microalgae use CO2 to ultimately synthesise their fatty acids, making the entire process carbon neutral.
In 2017, Biam researchers discovered an enzyme in the microalga Chlorella variabilis with a rare property: when activated by sunlight, it allows microalgae to convert their fatty acids directly into hydrocarbons. Since then, this fatty acid photodecarboxylase (FAP) has mobilised the international scientific community to exploit its full potential in the production of biofuels such as diesel. However, the use of this enzyme in the synthesis of medium-chain hydrocarbons such as gasoline seemed to be ruled out because of its allegedly low activity on C2-C12 fatty acids. Remember that hydrocarbons are organic compounds composed of a carbon chain (C) and hydrogen atoms (H).
An autocatalytic effect that increases the conversion yield of gasoline-type hydrocarbons
Today, the Biam, Frédéric-Joliot institute/I2BC and Institut Polytechnique de Paris teams have shown that the FAP enzyme is more effective on a fatty acid with eight carbon atoms. They observed in vitro that it can convert octanoic acid (C8) four times faster than hexadecanoic acid (C16), the most efficient substrate identified to date. This performance was also observed in vivo, as explained by Pavel Müller, a researcher at CEA-Joliot/I2BC who used time-resolved spectroscopy: "When FAP decarboxylates octanoic acid (C8) in a reaction that produces n-heptane, this n-heptane remains in the binding pocket and mimics the missing part of the long chain, thus helping to increase the quantum yield. This autocatalytic effect makes the photodecarboxylation of medium chain fatty acids (such as C8) almost as efficient as that of native long chain substrates." In addition, replacement of the product by a new substrate is faster for medium chains than for long chains. This results in a tenfold higher rate of PAF production for n-heptane (C7H16), a gasoline-like hydrocarbon, than for n-pentadecane (C15H32).
"These results represent an important step towards the production of gasoline-like hydrocarbons from a biological source and light. We remain fully mobilised to continue optimising the FAP enzyme, which has not yet revealed all its secrets," concludes the researcher.
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