How Can Acetogenic Bacteria Process Carbon Monoxide?
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Transcript: English(auto-generated)
00:07
According to the Integrated Carbon Observation System, human activities are leading to the release of more than 40 gigatons of CO2 per year in the atmosphere,
00:20
with consequences on the global carbon cycle and climate. 40 gigatons represents 4 million times the weight of the Eiffel Tower and 100 times the weight of the whole human population. Knowing this number, it's quite easy to understand why it's important and we are putting more and more effort into finding ways to reduce CO2 emissions.
00:42
One way to reduce industrial carbon footprint is to use a process which is called biological gas conversion. In this process, we are using organisms, so in the most developed fashion of this process, it's acetogenic bacteria. And these bacteria are using a very ancient metabolism, probably one of the most ancient that appear on Earth,
01:04
that allow them to use directly gases, so H2 and CO2, to produce acetate. The idea of the conversion process is to fed these bacteria with the gases. The bacteria will consume the H2 and CO2, which is in the waste gases, so the greenhouse effect gas for CO2,
01:24
and then will produce acetate. And it's a really interesting feature because then at the end we will obtain cleaner waste gases and also acetate which can be used as an energy source or even for chemical synthesis. One of the main issue of this process is the presence of carbon monoxide or CO in the combustion waste gases.
01:45
Carbon monoxide is a reactive and toxic compound that is harmful for a lot of microorganisms, nearly all, and also for acetogenic bacteria, so it's preventing a correct gas conversion. But there is a subclass, a subgroup of acetogenic bacteria that are able to not only survive to high concentration of carbon monoxide,
02:06
but also to use directly this carbon monoxide to produce acetate, so that these bacteria will also convert the carbon monoxide from the waste gases. We are studying this organism in order to understand how they can survive and how they can be so efficient
02:22
in converting the different gases, including carbon monoxide, how they can survive to the toxic gas, which would represent a really good opportunity for gas conversion. In this study, we assume at first that the major actor of the CO resistant in consumption
02:43
will be the protein which is actually consuming CO in the organism. From other studies, we know that there is one protein that is converting carbon monoxide into carbon dioxide, which is way less toxic and also which can be absorbed by the organism like classical acetogenic bacteria.
03:02
So our idea was that this protein should have really a specificity to be so efficient to consume and to detoxify carbon monoxide. To analyze that, we first took one organism that is able to grow on carbon monoxide, so Clostridium autoethanolgenum,
03:21
we cultivated it in the lab, and the idea was to extract this precise protein, the carbon monoxide hydrogenase, and to study what can make it so specific compared to other enzymes from the same family. So I extracted this protein, and then at the end we studied this protein on a chemical and structural aspect,
03:41
because what characterizes a protein are the reactions it can chemically perform, but also the structure it has, which is important for reactions. And we have all the expertise and the techniques in the laboratory to study both aspects of the protein. And by this way, we managed to understand what is making this carbon hydrogenase so specific.
04:04
We were expecting to find some specificity in the CO-consuming protein from this organism, and indeed we were kind of happy to see that we were right and there were some different key features for this protein. So first, what we realized when just extracting and purifying this protein is that it's not alone.
04:24
So it's in complex with another protein forming a deep unit that is working as only one big thing. With another protein, the acetyl-CoA synthase, which is a protein which is here to sensitize acetyl-CoA, which is an important key actor of the carbon metabolism and also for acetate production.
04:42
So it's a necessary step for acetogenic bacteria. And what was interesting here is that we already know one such CO-dehydrogenase-acetyl-CoA synthase complex from another acetogenic bacterium, which cannot use CO. And in this organism, it's a bit different since you have some carbon monoxide, but it's generated and not consumed.
05:06
Why? Because you need a bit of carbon monoxide in acetogenic bacteria. It's important for acetate production. So whatever you do, you need to have a bit of carbon monoxide. And here, the two proteins are working together. From CO2, the CO-dehydrogenase will produce a bit of carbon monoxide,
05:22
but it's still a toxic compound and it consumes energy to create carbon monoxide. So the solution that has been found is that CO will be produced in a sealed tunnel, which is connecting the two proteins. So one enzyme will produce carbon monoxide that will just diffuse to the other one that will use it for reaction.
05:41
So the rest of the cell will never see any carbon monoxide. The fact that we already know this other protein was really important for us because then we have a template to compare our system. The second thing that we realized when looking at the structure of our protein is that we have the two same proteins but that are organized completely in a different way.
06:01
We checked with several experiments to see if it was really the case or if it could be coming from a mistake or an artifact. But no, it is indeed like that in the cell. So the organization is completely different, with the consequence that the normal sealed tunnel from the other protein cannot be used because it will just lead to nowhere.
06:22
Instead of that, we realized in our protein that there is a completely new tunneling system with multiple entries and exits, which is completely open toward the outside of the protein, which is completely different to the philosophy of the sealed tunnel in the other complex. And when we looked at the different other CO-consuming cytogenic bacteria,
06:42
which are really a few organisms actually, we realized that they seem to all have the same kind of leaky tunneling system as we have in our protein. Which led us to conclude that these organisms should have this kind of system just to maximize the carbon monoxide diffusion to both proteins of the complex.
07:02
And then the external carbon monoxide could be directly diffused to both proteins, to be reduced by the CO dehydrogenase, making energy out of it, and to be used by the acetyl-CoA synthase for acetate production. These kind of features just also led to the idea that this reaction
07:20
should maybe be the only way the organism is deriving energy. That was our idea. And when we checked, it was the case. So this organism is only using consumption of carbon monoxide to make energy. That's the only reaction that can be used for that. And when we look and we dig into the metabolism, we realize that the whole metabolism of the bacterium
07:42
is just organized around that reaction, CO consumption by the CO dehydrogenase. So our findings are relevant on, let's say, two different aspects. At first, the scientific aspect, which is that there is always some surprise that may be unraveled.
08:01
Even in closely related organisms, for example, here, acetogenic bacteria, you may find some specificities in complex, and you may find some really interesting way of combining, of creating new stuff out of something you think you already know perfectly. The second, and I would say more important relevance,
08:21
at least the most valuable one, is the biotechnological aspect. Even if they are not the only CO consumers, bacteria like Clostridium autoethanogenum are by far the most efficient ones. And by be feeding by waste gases, they are consuming H2CO2 and CO, so all the toxics and greenhouse effect gases,
08:41
and they are producing acetate, but also ethanol and butanol. That's a kind of specific features of this kind of acetogenic bacteria, which, by the way, gave its name to this strain, autoethanogenum, which means producing its own ethanol. Which is really interesting, but also one thing to keep in mind here
09:00
is not a by far perspective. There is already large-scale use of this bacterium to convert the waste gases from the steel mill industries. So it's something that is actually working right now, and that have real impact. So every step forward that we are doing in the understanding of the metabolism is a really interesting opportunity to enhance this gas-converting system.
09:25
Moreover, now, in this precise bacterium, there is some genetic tools that are available. Which means that we can now modify the organism to try to enhance the reaction rates, change, switch the metabolic pathways that will maximize ethanol rather than butyl production, et cetera.
09:42
And now we can even induce completely new pathway inside, which will make this organism produce whatever we can imagine. Basically here, because of this use, every small step that we are doing here is a fantastic opportunity to enhance a system which will allow to convert and to detoxify waste gases while producing and creating a new way
10:03
to produce green energy sources and chemical sources. After this work on the CO dehydrogenase from this organism, we are now wondering what are the specificity of all the proteins that are constituting this pathway.
10:20
The CO dehydrogenase may be the most important protein in CO consumption and the whole metabolism, but acetogenase, the pathway of this bacterium, is implicating dozens of proteins. And now that we know that evolution granted the CO dehydrogenase with different specificities, we are expecting to find some interesting features
10:40
as to issuing things in all the other proteins from the pathway. We are trying to now work and purify all the different proteins of the acetogenic pathway. Another thing which is interesting to us, so besides being consumed, the major problem of carbon monoxide is that it's toxic for a whole range of different proteins, especially metalloproteins because of its reactivity.
11:03
And we are wondering how Clostridium metoethanogenum is dealing with the carbon monoxide that is present in these environments and how these enzymes can be resistant or at least resilient to carbon monoxide. And we are interested or deciphering the key features of this resistance in the protein
11:21
that could be then transposed to other protein in other biotechnological applications to have some enzyme that would be more resistant to carbon monoxide.