Life performs many amazing feats of chemistry, building complex molecules that can take us years to figure out how to synthesize them. And the thermodynamics of these reactions are often fascinating – in many cases, life lives on the edge, at the risk of critical reactions stalling and deteriorating.
Now some researchers have devised a way to force bacteria to perform a reverse chemical reaction. Instead of breaking down a simple molecule into carbon dioxide, the bacteria absorb carbon dioxide and spew out formic acid, a chemical that already has many uses and can be used as a fuel or to sequester carbon. The secret? Force the bacteria into the raw ingredients for the chemical reaction.
Enzymes and catalysis
The proteins that act as enzymes are nothing more than catalysts. The complex three-dimensional shapes of these proteins stabilize intermediate states of chemical reactions, lowering the energy required to reach them. This essentially lowers the energetic hill that must be climbed to get between a set of reactants and a set of products. But if the total energy of the reactants and products isn’t very different, then that smaller hill will also make things run in the opposite direction: the enzyme will happily form an intermediate of the product and spit out the original reactants.
Life has all kinds of ways to avoid this. In many cases, the reactants end up being used quickly, preventing them from sticking around and getting back into the enzyme. In others, an energy-carrying molecule is used to force the reaction to run in only one direction. But there are plenty of cases where a buildup of the products can occur, rendering the enzyme inactive, catalyzing forward and backward reactions equally. Purified enzymes, given a large supply of the products, can reverse the reaction, producing many of the normal reactants.
That is the intellectual background for the new work. The researchers were presumably looking for an enzyme-catalyzed reaction that normally produces some carbon dioxide as a product. There are many, but the authors then filtered the list by looking for those reactions where the energy difference between the reactants and products is small. One is the digestion of formic acid, the small molecule shown above. Formic acid is essentially carbon dioxide with two hydrogen atoms attached, and E coli has an enzyme that starts with formic acid and spews out CO2 and H2. In this case the energy difference between formic acid and CO2 is about the same as the energy content of H2so that the reaction could proceed in reverse, converting CO2 in formic acid.
But to run the reaction in reverse, many of the end products, hydrogen and carbon dioxide, are needed. To do that, the authors argued, just takes a little pressure. By pressurizing a gas in water, it accelerates the rate at which it dissolves. Since these two gases are very small molecules, they should easily diffuse into cells from an aqueous solution, where they can reach the enzyme.
So the authors set up a bioreactor and set some up E coli in it. They then closed the reactor and pumped in the gases under high pressure. Formic acid began to appear in the liquid the bacteria were in – apparently they eject it from the cell – at a rate proportional to the pressure. So everything they had reasoned in principle turned out to work.
From that moment on, it was mainly a matter of optimizing the yield. One of the problems is that formic acid, as the name implies, is acidic and the levels can eventually reach the point where they would damage the bacteria. So the bioreactor was modified to read the pH of the reaction and inject chemicals that could balance the acidity. E coli also has two other pathways that can digest formic acid, so the team knocked out key genes in those pathways, increasing yield.
By the time they were done, the yield was over 100 percent. Not only did the enzyme convert all of the carbon dioxide the researchers administered, it was also apparently cleaning up some of other cellular processes and converting that as well.
So it is clear that we can do this. Do we want?
It is clear that the supply of hydrogen is the major problem in this reaction. But formic acid has already been considered as a hydrogen storage mechanism and could be used in fuel cells. So if hydrogen production from renewable energy reserves ever becomes economical, formic acid has many advantages. It is a liquid at room temperature, and while it will burn, it is slightly less likely to burn than gasoline. While this won’t remove carbon dioxide from the atmosphere, it will at least prevent more from being put in.
We have also developed another strain E coli grow with formic acid as a carbon source. So it’s possible to combine these two pieces of engineering and force the cells to take in carbon dioxide and hydrogen to survive. This should allow evolution to optimize this enzyme system to work in reverse. But it also opens the door to using carbon dioxide to feed bacteria that produce useful chemicals, including medicines, more complex biofuels and the raw materials for plastics. Plastics, in particular, can help remove some of the carbon we’ve put into the atmosphere.
Current Biology2017. DOI: 10.1016/j.cub.2017.11.050 (About DOIs).
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