Science and technology
A French team [1] has discovered an enzyme which allows microalgae to convert certain of their constituent fatty acids into hydrocarbons using light energy.
This enzyme, which has been named “FAP” (standing for “Fatty Acid Photodecarboxylase”), is of an extremely rare type, as only four enzymes powered by light have been identified to date in the living world. Published on 01/09/2017 in Science, this discovery is all the more important in the context of an energy transition strategy. Indeed, the production of bio-sourced hydrocarbons using atmospheric CO2 has become a major biotechnological challenge because it allows limiting the atmospheric discharge of carbon stored in the subsurface.

Microalgae (Chlorella) viewed under the microscope © VladiDamian

Chlorella is a single-cell freshwater alga which forms part of a number of microalgae which are cultivated on an industrial scale, and have potential for the production of energy-rich molecules. Researchers at the CEA (French Atomic Energy Commission), the CNRS (French National Centre for Scientific Research), the ESRF, INSERM (French National Institute of Health and Medical Research) and the Universities of Aix-Marseille, Grenoble Alps and Paris-Sud have discovered an enzyme in this algae which allows it to convert certain of its constituent fatty acids into hydrocarbons using light energy only. According to the authors: “This is a major advance in the identification of biological mechanisms which permit the conversion of fatty acids in cells into hydrocarbons, and opens up a new option for the synthesis of hydrocarbons by micro-organisms on an industrial scale”.

In this study, published in Science, researchers from the Institute of Biosciences and Biotechnologies of Aix-Marseille (CEA / CNRS / University of Aix-Marseille), have been able to identify this key enzyme for the synthesis of hydrocarbons2, by following its activity and determining a list of potential candidate proteins using a proteomic analysis3 conducted at the laboratory of Large scale biology (CEA / INSERM / University of Grenoble Alps). The expression in the bacterium E. coli of the encoding gene for the main candidate protein has shown evidence of the production of hydrocarbons, thereby demonstrating that this enzyme was both necessary and sufficient for hydrocarbon synthesis. Characteristic analysis of the pure enzyme revealed that it was capable of splitting a fatty acid into a hydrocarbon molecule and a CO2 molecule, and that this activity required light (Figure 1).

Figure 1. Diagram of the reaction catalyzed by FAP. In a single step, it converts a fatty acid into a hydrocarbon by removing the carboxyl group from the carbon chain (decarboxylation reaction). In the absence of light, the enzyme is inactive.

Researchers have also demonstrated that a co-factor4 present in the enzyme permits the capture of blue light. The three-dimensional structure of the enzyme (Figure 2), determined by X-ray diffraction analysis conducted on the fully-automated “MASSIF-1” beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble), and kinetic absorption spectroscopy analyses conducted at the Institute of Integrative Cell Biology (CEA / CNRS / University of Paris-Sud), have allowed the formulation of a model for the mechanism of the enzyme. The fatty acid is positioned in a hydrophobic tunnel, at the end of which the co-factor is located. The latter, upon excitation by blue light, removes an electron from the carboxyl group of the fatty acid, thereby resulting in spontaneous decarboxylation to form a hydrocarbon molecule.

The discovery of this enzyme, named FAP (an English acronym for “Fatty Acid Photodecarboxylase”) is of major and fundamental interest, on the grounds that, to date, only four biocatalysts capable of exploiting light energy (photoenzymes) have been discovered5. FAP is at least ten times more rapid than the best-known enzyme for hydrocarbon synthesis, and employs light, thus potentially providing a highly effective biotechnological tool for the synthesis of hydrocarbons, either by the in vitro conversion of oils, or by the in vivo conversion of the membrane fatty acids of bacteria, yeasts or, ideally, microalgae.

Figure 2. Slice through the three-dimensional structure of FAP. The flavin-derived co-factor (FAD) is situated at the end of a hydrophobic tunnel, where the fatty acid is fixed (in green). The plane section is indicated in black. © CEA-Biam / Pascal Arnoux.

  1. Institute of Biosciences and Biotechnologies of Aix-Marseille (BIAM in Cadarache; CEA / CNRS / University of Aix-Marseille), in collaboration with the Institute of Integrative Cell Biology (I2BC in Saclay; CEA / CNRS / University of Paris-Sud), the Large-scale Biological Laboratory (BGE in Grenoble; CEA / INSERM / University of Grenoble Alps) and the European Synchrotron Radiation Facility (ESRF) in Grenoble.
  2. In this case alkanes and alkenes, wherein the former are hydrocarbons, the carbon chain of which is entirely hydrogen-saturated, and the latter are unsaturated (i.e. with double C=C bonds).
  3. Proteomics involves the study of proteins, and provides access to the genic expression of a cell, a tissue or an organ, by the analysis of proteins and their post-translational modifications.
  4. A co-factor is a non-protein molecule, which is associated with the enzyme and is necessary for its activity. In this case, the co-factor is a flavin derivative, a small organic molecule which absorbs blue light, and is also a constituent of vitamin B2
  5. These are an enzyme for the repair of DNA, an enzyme for chlorophyll synthesis, and the reaction centres for the two photosystems that permit photosynthesis in plants and algae.

Publié le September 1, 2017
Mis à jour le September 4, 2017

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