A study using Stanford Linear Accelerator Center’s (SLAC’s) Linac Coherent Light Source (LCLS) X-ray laser captured how light drives a series of complex structural changes in an enzyme called FAP, which catalyzes the transformation of fatty acids into starting ingredients for solvents and fuels. This drawing captures the starting state of the catalytic reaction. The dark green background represents the protein’s molecular structure. The enzyme’s light-sensing part, called the FAD cofactor, is shown at center right with its three rings absorbing a photon coming from bottom left. A fatty acid at upper left awaits transformation. The amino acid shown at middle left plays an important role in the catalytic cycle, and the red dot near the center is a water molecule. Credit: Damien Sorigué / Université Aix-Marseille
by Jennifer Huber
Although many organisms capture and respond to sunlight, it’s rare to find enzymes — proteins that promote chemical reactions in living things — that are driven by light. Scientists have identified only three so far. The newest one, discovered in 2017, is called fatty acid photodecarboxylase (FAP). Derived from microscopic algae, FAP uses blue light to convert fatty acids into hydrocarbons that are similar to those found in crude oil.
“A growing number of researchers envision using FAPs for green chemistry applications because they can efficiently produce important components of solvents and fuels, including gasoline and jet fuels.” says Martin Weik, the leader of a research group at the Institut de Biologie Structurale at the Université Grenoble Alpes.
Dr. Weik is one of the primary investigators in a new study that has captured the complex sequence of structural changes, or photocycle, that FAP undergoes in response to light, which drives this fatty acid transformation. Researchers had proposed a possible FAP photocycle, but the fundamental mechanism was not understood, partly because the process is so fast that it’s very difficult to measure. Specifically, scientists didn’t know how long it took FAP to split a fatty acid and release a hydrocarbon molecule.
Experiments at the Linac Coherent Light Source (LCLS) at the Department of Energy’s Stanford Linear Accelerator Center (SLAC) National Accelerator Laboratory helped answer many of these outstanding questions. The researchers described their results in Science.
Next steps
Even with this improved understanding of FAP’s photocycle, unanswered questions remain. For example, researchers know carbon dioxide is formed during a certain step of the catalytic process at a specific time and location, but they don’t know if it is transformed into another molecule before leaving the enzyme.
“In future XFEL (LCLS X-ray free-electron laser) work, we want to identify the nature of the products and to take pictures of the process with a much smaller step size so as to resolve the process in much finer detail,” says Dr. Weik. “This is important for fundamental research, but it can also help scientists modify the enzyme to do a task for a specific application.”
Such precision experiments will be fully enabled by upcoming upgrades to the LCLS facility that will increase its pulse repetition rate from 120 pulses per second to 1 million pulses per second, transforming scientists’ ability to track complex processes like this.
Other researchers are already working towards industrial FAP applications, including a group that is designing an economic way to produce gases such as propane and butane.
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