In the midst of the climate crisis, a consequence of the global warming that we are experiencing, there are only two ways to solve it: reducing CO2 emissions and eliminating part of the CO2 that is currently in the atmosphere . These two aspects are the key to developing a carbon-neutral economy and the future circular economy. And photosynthesis may hold the key.
In a natural way, organisms that carry out photosynthesis such as plants are capable of fixing CO2 from the atmosphere and transforming it into biomass, that is, living tissues . The challenge is to come up with something similar that can filter CO2 from the atmosphere and make it usable by technology.
This is the goal of Tobias Erb’s department, ‘Biochemistry and Synthetic Metabolism’ at the Max Planck Institute for Terrestrial Microbiology in Marbug. And one of the ways to solve it can be found in this article published in PNAS magazine .
The current industrial technology can only use the CO2 when it consumes very concentrated many fossil fuels emit CO2 . Unlike this, photosynthesis works directly with the concentrations naturally present in the air (0.04% carbon dioxide gas by volume). The secret lies in the enzymes, the proteins that act as catalysts for very specific chemical reactions, such as the fixation of CO2.
In photosynthesis, this reaction is driven by the RubisCO enzyme. However, the efficiency of natural photosynthesis is not very high, in more than a quarter of the cases, RubisCO metabolizes oxygen from the air which is a very strong competitor of CO2 in this reaction.
Despite this, the annual CO2 fixation by RubisCO is estimated at 400 Gt while at an industrial level, only 0.1 Gt of CO2 is used at high pressure.
Faster and more efficient enzymes than RubisCO in photosynthesis
Because of this, the Max Plank researchers decided to look for more efficient alternatives than RubisCO. Some of those discovered are the Enoyl-CoA carboxylase / reductase enzymes (ECRs) that are much more efficient than RubisCO and are not capable of metabolizing oxygen, unlike RubisCO.
After a long time, they have managed to build a process that works in a test tube. This process fixes CO2 better than nature . Robustness and energy efficiency were the qualities they wanted to give to this artificial photosynthesis. The basis has been nature itself, which serves as a model for molecular biology.
Is there a particular reason why ECRs are more efficient than RubisCO? What spell does it take to create this highly efficient CO2 fixer? Researchers Gabriel Stoffel and Iria Bernhardsgrütter, together with colleagues from Chile and the United States, focused on the bacterium  Kitasatospora setae since this bacterium has the fastest carboxylase known. They analyzed their ECR and by combining structural biology, biochemistry and computer simulations, they were able to understand for the first time how the enzyme binds and converts CO2.
Only four amino acids are enough to control the CO2 molecule. Of these, three of them (asparagine, glutamate, and histidine) hold CO2 in place from two sides; and the fourth amino acid, phenylalanine, which protects the CO2-binding site of water, which could inhibit the reaction.
This finding is very important as it opens up new avenues in research. This ability to bind to CO2 could be transferred to other enzymes, allowing many more possibilities for optimizing photosynthesis.
Bernhardsgrütter had studied in other studies two other enzymes to give flexibility to the protein: Propionyl-CoA synthetase (PCS) and Archaeal Enoyl-CoA reductase (AER).
Improving the CO2 binding capacity
Both enzymes can use CO2 but they only do so with two limitations: their efficiency is over 5% and with highly concentrated CO2. Computer models showed that these enzymes possess one of the four required amino acids and that they were misaligned. Bernhardsgrütter succeeded in changing the amino acids to correct this error in the PCS. Following this change, the CO2 fixation efficiency increased by 20% immediately.
The next problem was to protect the joint site from the water. Again Iria Bernhardsgrütter was able to solve this problem, another amino acid replacement managed to block the access of water to the binding site.
These two changes led to a CO2 fixation rate (carboxylation) of almost 95% . Additionally, similar experiments with AERs reached conversion efficiencies of nearly 90%, much higher than the 5% found in nature.
These findings provide a very detailed view of the molecular control of CO2 in one of the most efficient CO2-binding enzymes. This may help in the future design of catalytic processes to capture and convert CO2 in chemistry and biology . Not only this, but also in the development of sustainable synthetic CO2 fixation cycles for the recovery of other raw materials that are now costly in terms of CO2 emissions.
