It's pretty easy to grow vast quantities of microbes. It's a lot harder to convert those microbes into something useful. For example, we've engineered algae that, when starved for nitrogen, will put most of their stored energy into fats. Fats are chemically similar to hydrocarbons, so this is potentially useful for making biofuel. But "similar" isn't really good enough; you still need to process the fats before they can be used as a fuel.

But some researchers may have figured out a way to get biology to convert fats directly to hydrocarbons using nothing more than sunlight. They've identified an enzyme that catalyzes the conversion of fats to long hydrocarbons, which could be used as fuel with no further modification.

Fat vs. fuel

Gasoline is a mixture of different hydrocarbons, which are molecules that contain only hydrogen and carbon. The simplest hydrocarbons are linear chains, with a backbone of carbon atoms where each carbon is linked to, at most, two others (with two or three hydrogen atoms also linked to each carbon). The chemical processing done in refineries ensures that gasoline mostly contains branched hydrocarbons, wherein some carbon atoms are linked to three or four other carbons.

Fats (technically fatty acids) are almost-but-not-quite hydrocarbons. They have long linear chains of carbon atoms, but one of the end carbons is linked to two oxygen atoms. While this may seem like a relatively minor change to a carbon chain that can be 20 or more atoms long, the presence of the oxygen radically changes the chemistry of the molecule. It makes fats more likely to form solids, allows them to mix with water, and turns them into weak acids. All of these properties make fats a terrible option for use in internal combustion engines.

The obvious solution is to simply lop off the terminal carbon that's linked to those oxygens. Unfortunately, from a chemistry standpoint, the link to that carbon is more robust than the other links further down the chain. In other words, most chemical reactions will simply break the chain somewhere at random in the middle, which is not especially useful. Alternatives for getting rid of the oxygens tend to involve multiple steps, each requiring an input of energy and an efficiency less than 100 percent.

Note that the last sentence says "tend to" and not "always." Last year, French researchers discovered that certain species of algae are able to convert fats directly to hydrocarbons. Their new paper describes how they figured out why these algae are able to do so.

Let there be light

The conversion process was potentially interesting from a biology perspective as well. That's because it seemed to involve light as an input. Even though almost all life on Earth depends directly or indirectly on sunlight, remarkably few chemical reactions used by life require light as an input. A search of the literature showed a grand total of three reactions, two of which are used for photosynthesis (the third repairs damaged DNA). If the fat-converting process used light to provide energy, then it would be big news for biology.

To find out, the researchers in France did some old fashioned biochemistry. They took the algae, ground them up, and then ran their contents over a variety of different materials that filter those contents based on different chemical properties, like molecular weight, charge, etc. This left them with a variety of protein solutions, each containing a fraction of the cells' contents, ranging from dozens to hundreds of proteins.

The researchers tested each of these fractions for the fat-converting activity. In the end, they found three fractions that contained the activity: one with 42 proteins, one with 93 proteins, and one with 709 proteins. Each of these proteins was identified, and the researchers discovered that a set of 10 proteins were present in all three fractions. Nine of these proteins were well known, and we knew what they did. The 10th was a mystery, so the researchers focused on it.

They identified its gene and tweaked it so that it worked in bacteria. Once it was transferred to the bacteria, they were also able to convert fats to hydrocarbons. And the researchers could switch the conversion on and off by switching from red to blue light—the reaction required blue light to work. They also purified the protein and fed it fat which had had its carbon labelled with a specific isotope. This isotope showed up in carbon dioxide released by the reaction, confirming that the enzyme was simply lopping off the carbon bonded to the oxygen.

How does the protein manage this trick, and why does it need light to do it? The blue light it uses turns out to be the wavelengths typically absorbed by a chemical called "flavin adenine dinucleotide" (FAD), which is commonly used as a co-factor in many biochemical reactions. And, when the researchers looked at the structure of the enzyme, they found that FAD is present and held in close proximity to the oxygens of the fat molecule.

The authors think that, in this enzyme, light causes FAD to steal an electron from the fat molecule, leaving it in an unstable state. The fat responds by kicking out a carbon and two oxygens, forming carbon dioxide, a process that's very energetically favorable. After that happens, the remaining hydrocarbon steals the electron back from the FAD, resetting the enzyme for use on another fat molecule.

So, because it uses light for energy, the system doesn't need to be supplied with a constant source of chemical energy. That makes it much more convenient for use in processing chemicals than other enzymes, where you typically have to keep refreshing a supply of a chemical energy source (like ATP). It also makes the newly discovered enzyme interesting, since there are so few other examples of light-driven enzymes out there to study.

Finally, the whole system works by light-driven electron transfers. And shifting electrons around is a central feature of a huge range of chemical reactions. This leads the authors to suggest that modifications to the enzyme could get it to catalyze very different chemical reactions. So biohackers could potentially have a field day generating new applications for this system.

Science, 2017. DOI: 10.1126/science.aan6349 (About DOIs).