Researchers make their own enzyme pathway to get CO₂ out of the air

Before this century is over, we’re almost certainly going to need to pull massive amounts of carbon dioxide back out of the atmosphere. While we already know how to do carbon capture and storage, it takes a fair amount of energy and equipment, and someone has to pay for all that. It would be far more economical to pull CO₂ out of the air if we could convert it to a useful product, like jet fuel. But processes like that also take a lot of energy, plus raw materials like hydrogen that take energy to create.

Plants and a huge range of microbes successfully pull carbon dioxide out of the air and use it to produce all sorts of complicated (and valuable!) chemicals. But the pathways they use to incorporate CO₂ aren’t very efficient, so they can’t fix enough of the greenhouse gas or incorporate it into enough product to be especially useful. That has led a lot of people to look into re-engineering an enzyme that’s central to photosynthesis. But a team of European researchers has taken a radically different approach: engineering an entirely new biochemical pathway that incorporates the carbon of CO₂ into molecules critical for the cell’s basic metabolism.

Sounds good in theory

On the rare occasions that most biologists think about biochemical pathways, energy is an afterthought. Most cells have enough of it to spare that they can afford to burn through their own energy supplies to force rather improbable pathways forward to get the chemicals they want. But grabbing carbon out of the atmosphere represents a very different sort of problem. You want it to happen as a central part of the cell’s metabolism rather than a pathway out on the periphery so that you grab a lot of carbon. And you want it to happen in a way that’s more efficient than the options the cells already have.

Given those focuses, energy really matters. So some biochemists have painstakingly gone through all the reaction cycles in and around the ones that normally incorporate carbon dioxide and looked into their energetics, trying to find the one that uses the least amount of energy to break the strong bonds between carbon and oxygen. Amazingly, one of the best the researchers came up with doesn’t seem to actually exist in any cells we’ve looked at.

The chemical raw materials needed are around, being used by other pathways. And there are enzymes that do related things. But as far as we can tell, evolution has never bothered to put the pieces together.

So the researchers decided that if evolution wasn’t up to the job, they would have to take over.

A pathway of one’s own

So how do you roll your own biochemical pathway? The previous identification of the non-existent pathway made the work that went into the new paper substantially easier. This had already identified starting chemicals that were common in the cell and each intermediate step. What the researchers had to do was identify the enzymes that could move the chemicals from one step to another in the pathway. Emphasis on “could”—remember that the pathway doesn’t exist in nature, so there aren’t any enzymes specialized in these reactions.

The pathway itself is rather short, needing only three steps. In the first, a two-carbon chemical that’s common in cells (called glycolate) is linked to a cellular co-factor that makes it more reactive. In the second, the activated glycolate reacts with carbonate, which is essentially a form of carbon dioxide dissolved in water. The resulting three-carbon molecule then has to have the co-factor cleaved off before it can be used elsewhere in the cell’s metabolism. So the researchers had to find an enzyme for each step.

For the first step, there are already a lot of enzymes that link the co-factor to something or transfer it from one molecule to another. The researchers tested 11 of them (some natural, some previously engineered) to look for ones that worked well on glycolate. They found two that did a passable job—and oddly, the one that did less well turned out to be easier to fix because we already knew something about how it was regulated.

Normally, one of the amino acids on the protein gets chemically modified in order to shut down the enzymatic activity. So the researchers changed this amino acid so it couldn’t be modified and, for good measure, produced it in a strain of bacteria that was unable to perform the modification. This boosted the enzyme’s performance by a factor of 30. They also looked at a related enzyme that acted on a chemical that was similar in size to glycolate and made a change that should open up the enzyme’s active site where the reactions take place. This gave the enzyme another 60 percent boost.

Figuring that was good enough, the researchers went shopping for another enzyme to catalyze the second step in the pathway,…