Carbon capture and utilization technologies are advancing with promising new methods for converting carbon dioxide into valuable carboxylic acids. Researchers at Tokyo Institute of Technology (Tokyo Tech) have made a significant breakthrough, demonstrating a biocatalyzed carboxylation reaction using both natural and unnatural substrates. This innovative process leverages the Thermoplasma acidophilum NADP+-malic enzyme (TaME) under mild reaction conditions, offering a tailored strategy for selective synthesis through carbon dioxide fixation reactions.

The challenge of reducing atmospheric carbon dioxide (CO₂) extends beyond mere removal; the goal is to transform this greenhouse gas into useful compounds. Emerging carbon capture and utilization (CCU) technologies provide a dual benefit: mitigating global warming and creating commercially valuable chemicals, such as carboxylic acids. However, CO₂'s chemical stability poses significant hurdles, often necessitating reactive reagents, high temperatures, and pressures, which drive up energy costs and reduce sustainability.

In response to these challenges, Associate Professor Tomoko Matsuda and master’s student Yuri Oku from Tokyo Tech's Department of Life Science and Technology explored biocatalysts for CO₂ fixation reactions. Their findings, published online in JACS Au on May 13, 2024, describe a carboxylation reaction conducted under mild conditions with the biocatalyst TaME and gaseous CO₂. This approach successfully carboxylated both a natural substrate, pyruvate, and an unnatural substrate, 2-ketoglutarate.

“Our objective was to develop a TaME-catalyzed carboxylation reaction using only gaseous CO₂ as a CO₂ source and to widen the substrate specificity of TaME for carboxylation,” explains Matsuda. The researchers selected TaME for its robustness and ease of handling, attributes shared by other enzymes from T. acidophilum, known for their high thermal and CO₂-pressure stabilities.

In their experiments, pyruvate was treated with TaME and co-enzyme NADPH under 0.1 MPa CO₂ pressure, initially yielding low results. To enhance this yield, the team introduced two additional co-factors, TaGDH (glucose dehydrogenase) and D-glucose, achieving an 18-fold increase. They also investigated the effects of CO₂ pressure, pH, and substrate concentration on the reaction. Additionally, they successfully performed reductive carboxylation of the unnatural substrate, 2-ketoglutarate, converting it to isocitrate using gaseous CO₂, TaME, TaGDH, and D-glucose.

The biocatalyst-driven method developed in this study demonstrates effective carboxylation of both natural and unnatural substrates at mild temperatures (37 °C) and pressures (0.1 MPa CO₂). This reduces energy consumption and enhances the sustainability of the CCU process. The success with TaME paves the way for selective synthesis of a broader range of carboxylation products, utilizing safer and more environmentally friendly reagents in place of harsh chemicals.

“We believe that our proposed method can be re-engineered to perform a wide range of selective carboxylation reactions using renewable resources, under milder reaction conditions, and with less unwanted by-products and waste, unlocking the possibility of biocatalysis for the utilization of carbon dioxide as a starting material,” concludes Matsuda.