An engineered enzyme is the first single biological catalyst that converts carbon dioxide into a renewable form of energy: methane. Surprisingly, the same enzyme can use carbon dioxide to make an important ingredient in plastics as well.
Recycling carbon dioxide by turning it into fuels like methanol (CH3OH) or methane (CH4) might be one way to slow the CO2 accumulation in our atmosphere. But that's quite a challenge, because CO2 is a pretty inert molecule and doesn't readily participate in chemical reactions. So chemists have developed metal-containing catalysts to assist in the reduction reactions that convert it to methane and other carbon-containing small molecules. Alternatively, bacteria can use CO2 to make methane, but they use a series of proteins to catalyze the transformation.
Lance Seefeldt at Utah State University and his colleagues study a bacterial enzyme called a nitrogenase, which reduces nitrogen gas (N2) to ammonia (NH3) with the help of a cluster of iron and molybdenum atoms buried inside the protein. The reduction of carbon dioxide to methane requires a transfer of eight electrons, just as ammonia production does, so the scientists wondered if an altered version of this enzyme could accept and reduce carbon dioxide.
They changed two amino acids in one subunit of this protein. The altered nitrogenase converts carbon dioxide to methane for 20 minutes and then slows down. The enzyme’s reaction rate and the number of reactions it catalyzes are comparable to similar soluble metal catalysts.
But the real surprise to Seefeldt was that the enzyme triggered a more complex reaction: it combined two molecules, carbon dioxide and acetylene, to form propylene, a three-carbon ingredient in many plastics. That particular reaction is new for any catalyst, inorganic or biological, he says.
The scientists want to test other versions of the enzyme to see if it can use CO2 to build other kinds of molecules, too. Enzymes are used as biocatalysts to make some chemicals on an industrial scale, but that’s not Seefeldt’s ultimate goal in engineering this enzyme. He wants to extend this enzyme’s catalytic ability to better understand how the protein works.
Lessons about how the binding site environment helps catalyze a particular reaction might translate into clues that help other scientists build better catalysts for the production of methane and other commercially relevant chemicals. This altered enzyme won’t solve our carbon dioxide or energy problems on its own. But its structure, or that of its yet-to-be-found mutant cousins, might provide some useful hints that do help us address those issues by recycling CO2 through chemistry.
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