Engineering enzymes to convert CO2

Researchers in London recently successfully replicated the crucial part of the plant enzyme that drives the process of photosynthesis, which captures carbon from the atmosphere. The work could open up new avenues to trap CO2 and pull carbon dioxide back out of the atmosphere in the future. To find out more, we put in a call to Dan Wilson from University College London…

Dan - Nature has been dealing with carbon dioxide for millennia now. So what it has done is evolved to have molecular machines, which we call enzymes. There's a variety of enzymes which take carbon dioxide and convert them into cellular carbon. And this can be useful because we need to be able to build the cells and biomass and sugars and things like that for energy and also so that we can grow as organisms.

Chris - Effectively that's photosynthesis, isn't it? It's using the energy in the sun to grab carbon dioxide from the atmosphere and turn it into a chemical form that feeds everybody.

Dan - That is exactly right. The key thing is that you need to have a carbon source and an energy source. So photosynthesis is famous because it uses light, but you could also use things like thermal energy as well.

Chris - And that reaction centres on enzymes, which effectively drive it. How are you taking your lead from that process or attempting to build on it?

Dan - What we do is we look at data from the enzymes, in particular what's known as x-ray crystallographic structure. So these are 3D images of really large molecules, and what they do is they provide snapshots into what these enzymes actually look like. So we can take these photographs and see where the reaction actually happens. In this case, what we want to do is figure out how an enzyme takes CO2 and then through various transformations along the way, converts it into, in our case, Acetyl-CoA, which is just this form of biomass.

Chris - The aspiration being that if you can produce your own artificial leaf, I suppose you could say that you can make artificial photosynthesis not only solve the carbon dioxide problem, but produce some useful energy molecules in the process.

Dan - Exactly that. You could have some sort of material where you put it inside sunlight, it then takes CO2 and converts it into something else that is quite useful to us as chemists. How the machinery that we can see actually performs the transformations is still very much a mystery. So a lot of the steps along the way where we've captured, say, CO2 and we've started to transform it into something else only exists for very, very short periods of time. And we really struggle to observe what's going on at those points. So what we're trying to do is figure out reasonable mechanisms for how you go from CO2 to something like cellular carbon. What we've learned is you can strip back the enzyme quite significantly. So rather than looking at quite a large molecular structure, you can simplify it to just look at a small section, which we know are the active sites, that is the part where the reactivity is happening. By truncating it just to the bare bones, so just a few of the required structural features, we can still build functioning models of the enzyme itself. What that would mean is that we can in future build simple molecules which perform similar functions to these large enzymes and then hopefully we can employ them to do these sorts of transformations.

Chris - So now you've understood what the key ingredients facilitating this chemical process of turning carbon dioxide into more complicated molecules is, how good is your system? How much CO2 will it convert by when? Is it a rival for what, say, a tree is doing?

Dan - Definitely not even close to what a tree can do. Our catalysts are currently quite early in development so they're not actually industrially ready yet. We're having a lot of trouble with things like air sensitivity because they're very sensitive to oxygen, which means they're not very useful for practical applications yet.

Chris - So it's a start and you've got to start somewhere. So say you get this working and you get it working well, you will have a recipe for molecules, which if you make them can grab CO2 and turn it into useful stuff. So what's the application then? How is that actually deployed industrially or technologically? How would you use what you aim to create?

Dan - The most useful product to come out of a reaction would be methanol and methanol derivatives. So methanol is useful in that it's a liquid, so it's much easier to store than CO2. It also can be used as a solvent in a lot of chemical transformations and quite importantly it can also be burnt as a fuel. So there's a lot of interest in the field right now of developing catalyst switch, take CO2 and selectively and efficiently convert into methanol

Chris - And you can see the road ahead to be able to do that.

Dan - That's a tough question. Yes, we now have a lot of catalysts that can take CO2 and convert it into methanol. The difficulty is where you get the hydrogen atoms from for that reaction. So in an ideal case, what you'd have is some sort of fuel cell type device where you have CO2 or carbon dioxide reduction happening in the presence of water oxidation and the water oxidation would provide protons for the resultant methanol. However, we don't have any technologies which can do both of those things at once with sunlight as the energy source just yet in the next few years that sort of technological development is on the cards.

Chris - And will the molecules that you're making at the moment, will they be easy to make? Is this going to be so difficult to make that it's just economically unviable or is this going to be relatively easy to put it on the shelf?

Dan - What we are trying to learn from the molecules is what structural features are necessary for reactivity. So it's both structural and other features. Things like what transition metal is in there, what the geometry is and things like that. So what we're hoping is that when we learn which of these features is actually important, we can take that understanding and apply it to developing all sorts of different catalysts. So it's not so much about our specific molecules, it's more about understanding principles that will help us.