Before this century ends, we will definitely need to remove massive amounts of carbon dioxide from the atmosphere. While we already know how to do carbon capture and storage, it takes a reasonable amount of energy and equipment, and someone has to pay for it all. Carbon dioxide withdrawal will be more economical2 Of air if we can turn it into a useful product, Like jet fuel. But such processes also consume a lot of energy, in addition to the raw materials such as hydrogen that take the energy to form.
Plants and a host of microbes have successfully pulled carbon dioxide out of the air and used it to produce all sorts of complex (and valuable!) Chemicals. But the pathways they use to incorporate carbon dioxide2 Not very effective, so they cannot fix enough greenhouse gases or incorporate them into enough product to be particularly beneficial. This prompted a lot of people to look into it Enzyme reengineering This is fundamental to photosynthesis. But a team of European researchers took a completely different approach: engineering an entirely new biochemical pathway involving carbon dioxide carbon.2 In molecules important for the basic metabolism of the cell.
It looks good in theory
On the rare occasions that most biologists think about biochemical pathways, energy is an afterthought. Most cells have enough of them to spare so they can burn off their energy supplies to force forward unlikely pathways for getting the chemicals they want. But stripping carbon from the atmosphere is an entirely different kind of problem. You want it to happen as a central part of the cell’s metabolism rather than an outside pathway on the extremities so that you get a lot of carbon. And you want this to happen in a more efficient way than the options cells already have.
Looking at these points, energy is really important. So some biochemists have worked hard through all the reaction cycles in and around those cycles that usually involve carbon dioxide and have looked at their energies, trying to find the ones that use the least amount of energy to break the strong bonds between carbon and oxygen. Amazingly, none of the researchers’ best findings actually appeared to be in whatever cell we looked at.
The necessary chemical raw materials are present, and are used by other pathways. And there are enzymes that do related things. But as far as we can tell, evolution didn’t bother putting the pieces together.
So the researchers decided that if the development was not at the job level, they would have to take charge.
Its own road
So how do you roll your own biochemical pathway? Previous determination of the non-existent path made the work entered into the new paper a lot easier. This actually marked the beginning of the chemicals that were common in the cell and at every intermediate step. What the researchers had to do was identify enzymes that could transport the chemicals from one step to the next in the pathway. Focus on “can” — remember that the pathway does not exist in nature, so there are no specific enzymes involved in these reactions.
The path itself is rather short, and only takes three steps. In the first case, a common diploid chemical (called glycolate) binds to a cellular cofactor that makes it more reactive. In the second case, the activated glycolate reacts with the carbonate, which is basically a form of carbon dioxide dissolved in the water. Then the co-factor produced by a three-carbon molecule must be split before being used anywhere else in a cell’s metabolism. So the researchers had to find an enzyme for each step.
As for the first step, there are really a lot of enzymes that bind to a cofactor or transfer it from one molecule to another. Researchers tested 11 of them (some are natural, some are pre-engineered) to find the ones that work well on glycols. They found two who did acceptable work – oddly enough, work that did less efficiently turned out to be easier to fix because we already knew something about how to organize it.
Normally, one of the amino acids on a protein is chemically modified to stop the enzymatic activity. So the researchers altered this amino acid so that it could not be modified, and for a good measure, they produced it in a strain of bacteria that was not able to make the modification. This boosted the enzyme’s performance by a factor of 30. They also looked at a related enzyme working on a chemical that was similar in size to glycolate and made a change that would open up the enzyme’s active site where reactions take place. This enzyme gave another 60 percent boost.
To find out that was good enough, the researchers went to purchase another enzyme to catalyze the second step in the pathway, to bind to the new carbon atom. They decided to test a group of enzymes that catalyzed a similar reaction using a chemical slightly larger than glycolate. They found a person with an activity they describe as “very low-key but measurable”.
To give an initial boost, the researcher obtained the structure of the enzyme and made some changes that would increase its ability to interact with glycols. Then they subjected it to a random mutation, and identified a shape with three mutations whose activity was 50 times the “very low but measurable” version.
There are a lot of enzymes that cleave the catalyst from other molecules, so it was easy to test. The researchers found one that worked without much modification, as it ended the pathway with the production of glycerol, a three-carbon molecule closely related to glycerol. Glycerides can be used by a variety of pathways in the cell, many of which lead to larger and more complex molecules.
Good, but not great
From a dynamic perspective, that’s pretty cool. If we compare the natural pathway used by plants with this new one, by some measures it looks very good. The pathway is nearly as strongly favorable as one of the current major pathways for extracting carbon from carbon dioxide, and the vast majority of reactions will be run forward, producing the intended end product rather than digesting it. You will extract twice as much carbon per cycle and use 20 percent less energy to stabilize an equivalent amount of carbon. And unlike the enzyme used in plants, it will not be stopped when oxygen levels rise.
As an added bonus, the researchers showed it could also be incorporated into a pathway that could remove the environmental pollutants used to manufacture PET plastics.
But researchers haven’t tested the new pathway in an organism. All tests were done in solutions using bacteria-derived materials, and in the grand scheme of things, they were not particularly effective. If you have a gram of the required enzymes (which is A. Much Of protein), it would only eliminate 1.3 milligrams of carbon dioxide per minute. This means that it will take 13 hours for a gram of enzymes to withdraw a full gram of carbon dioxide from the atmosphere. And the pathway will need to be constantly replenished with energy to continue the reaction.
In all of these cases, the researchers tested the system extracellularly in a solution made from ingredients derived from bacteria. We have no idea how this path would work – or whether it would – if it were returned inside the cell. But this would be a necessary step if we want this to be, as the authors suggest, “a key to sustainable biostimulation and carbon-neutral bioeconomics.” Both are because living things can take glycerides and build them into the larger chemicals we really want and because forcing an organism to depend on it for carbon is the surest way to allow evolution to get this pathway to work much more efficiently than that. He has before.
Of course, there is no reason to believe that this will not be possible in the end. It is important to acknowledge the importance of this work. While other groups discovered how to improve enzymes to perform entirely new functions, this group took a whole path that had only existed in the calculations and made it a biological fact, leading to a major change in two of the enzymes in the process. It hints at a future where we can get biology to do a lot more than it would likely end up alone.