A team of researchers in France has discovered an algal photoenzyme that catalyzes the decarboxylation of free fatty acids to n-alkanes or -alkenes in response to blue light. In a paper in the journal Science, the researchers suggest that the photoenzyme, which they named fatty acid photodecarboxylase, may be useful in light-driven, bio-based production of hydrocarbons.
Many organisms—photosynthetic and nonphotosynthetic—use light for processes such as growth, development, and metabolism. In most cases, the researchers note, the effects of light on cell physiology are mediated by photoactive proteins, which include light-sensitive ion channels and pumps, photoreceptors, photosynthetic antenna proteins, and light-dependent enzymes. There are two different types of light-dependent enzymes: light-activated, which requires only a flash of light to become active; and photoenzymes, which require a continuous flux of photons to remain catalytically active. Photoenzymes, they note, are rare.
Photoenzymes are useful tools for research and biotechnology. For example, light-controlled catalysis allows the study of rapid enzymatic processes in real time. And, the use of a photoexcited cofactor to design an enzyme with a different catalytic function as well as the development of optogenetics expands the scope of possible applications of enzymes driven by light.
Some microalgae, including Chlorella variabilis NC64A and Chlamydomonas reinhardtii 137C, have the capacity to convert long-chain fatty acids to alkanes or alkenes in a process dependent on light\. Because no homologs to known hydrocarbon-forming enzymes have been found in the genomes of C. variabilis or C. reinhardtii, we hypothesized that microalgae harbor a different type of alkane synthase. Conversion of microalgal fatty acids to hydrocarbons has been mostly studied in Botryococcus braunii. Here, we identify an alkane synthase in the green microalgae C. variabilis and show that it is a photoenzyme belonging to an algae-specific subfamily of proteins that has been conserved throughout algal evolution.—Sorigué et al.
To identify the enzyme responsible for hydrocarbon synthesis, the team used enzyme purification in C. variabilis. The crystal structure of the protein shows a fatty acid–binding site in a hydrophobic tunnel leading to a light-capturing flavin adenine dinucleotide (FAD) cofactor. The decarboxylation is initiated through electron abstraction from the fatty acid by the photoexcited FAD with a quantum yield >80%.
Photoenzymes are an ancestral type of enzyme and may be rare because evolutionary selection has not favored light-driven catalysis. Given the diversity of proteins able to perform synthesis of alkanes, light activation required by the chemistry seems unlikely. We therefore speculate that the light-driven nature of the microalgal alkane synthases is related to their function within the cell.—Sorigué et al.
Damien Sorigué, Bertrand Légeret, Stéphan Cuiné, Stéphanie Blangy, Solène Moulin, Emmanuelle Billon, Pierre Richaud, Sabine Brugière, Yohann Couté, Didier Nurizzo, Pavel Müller, Klaus Brettel, David Pignol, Pascal Arnoux, Yonghua Li-Beisson, Gilles Peltier, Fred Beisson (2017) “An algal photoenzyme converts fatty acids to hydrocarbons,” Science Vol. 357, Issue 6354, pp. 903-907 doi: 10.1126/science.aan6349