eBioCO₂n – Electricity-driven CO₂ conversion through synthetic enzyme cascades

Challenges

Not only chemical-catalytic processes can be considered for the current-driven synthesis of chemicals. It is also conceivable to combine CO₂-fixing electron-transmitting biocatalysts with further enzymatic conversion steps in the form of an enzyme cascade for the production of fine chemicals.

Solution approach

The "eBioCO₂n" project, which is being carried out jointly by Fraunhofer and Max Planck researchers, meets this challenge. The aim of this ambitious project is to demonstrate the feasibility of such bioelectrocatalytic syntheses with a demonstrator on a 10 – 100 mL scale. To this end, suitable C0₂-fixing enzymes are to be assembled on electrodes (cathodes) using new molecular architectures and – depending on the target product – combined with other specific enzymes to form continuous and coupled reaction cascades. Recently discovered redox enzymes, enoyl-CoA carboxylases/reductases (ECRs), are used as CO₂-fixing biocatalysts. They are among the most efficient CO₂-converting biocatalysts described so far.

Using hydrogel to transport electrons to the CO₂ fixation enzymes

Special redox enzymes called enoyl-CoA-carboxylases/reductases, developed by Prof. Tobias Erb’s research group at the Max Planck Institute for Terrestrial Microbiology in Marburg, are being used for the biocatalytic CO₂-fixation. These bind CO₂ to a substrate by means of reductive carboxylation, transforming it into a valuable intermediate in the process. The enzymes are among the most efficient biocatalysts ever recorded when it comes to CO₂ conversions.

The CO₂ fixation enzymes are immobilized on electrodes in a custom-made redox-active hydrogel consisting of polymers grafted with viologens. This ensures an efficient, continuous transfer of the electrons supplied by renewable electricity. The basic principles allowing the three-dimensional flow of electrons were developed by a team from the Technical University of Munich (TUM), led by Prof. Nicolas Plumeré. This three-dimensional flow makes it possible to use larger quantities of enzymes, meaning that a higher product yield can be achieved.

Regenerating the cofactor

For the reductive carboxylation reaction to take place, the enzyme enoyl-CoA-carboxylase/reductase requires NADPH as a cofactor. This small, organic molecule acts as either a proton or electron donor, and is used up in the course of every individual reaction. Providing new cofactors every time the synthetic reaction is conducted would be very expensive on a large scale, and as such, it would not be financially viable. As a result, the project team had to find a way of using electricity and the same reaction principles to regenerate the cofactor molecules, so that they would be available for further reaction cycles. Ultimately, they hit on the solution of immobilizing another enzyme in the redox hydrogel to act as a recycling module.

Demonstrating CO₂ incorporation into crotonyl-CoA

With this new approach combining CO₂ reduction, bioelectrocatalytic technology, material chemistry and synthetic biology, the scientists were able to demonstrate the feasibility of the process at a milliliter scale. By enabling regioselective and stereoselective incorporation of CO₂ into crotonyl-CoA, the new approach delivered the most complex product ever achieved in the biocatalytic conversion of CO₂. The results of the studies were published in 2021 in “Angewandte Chemie,” a scientific journal for applied chemistry, and selected as a “hot paper.”

Modular platform technology

As the electro-biocatalytic reaction system can be extended on a modular basis, it can function as a platform technology. Depending on the target molecule (synthesis product), suitable enzymes can be selected from bioinformatic databases, manufactured using biotechnology, and embedded in the hydrogels. Consequently, the eBioCO₂n module can enable the manufacture of various biobased fine chemicals that can be diversified as needed using specific enzyme cascades. This holds great application potential by companies in the pharmaceutical, agrochemical and food industries.

Outlook

In the next steps, the team aims to complete the scaling and modular expansion of the process. Chemical synthesis based on biocatalytic CO₂ fixation is paving the way for a new form of circular economy by offering alternatives to fossil-based raw materials in chemical synthesis processes.