The iron nitrogenase reduces carbon dioxide to hydrocarbons and formate – unlocking new pathways for biological carbon capture and conversion

Nitrogenases are best known for catalyzing the reduction of dinitrogen (N₂) to ammonia at a complex metallic cofactor. However, recent advancements in nitrogenase research have revealed numerous promiscuous activities of nitrogenase, which have challenged the conventional perception of nitrogenases as solely N₂-converting enzymes. Remarkably, these include the reduction of carbon dioxide (CO₂), a reaction that offers novel perspectives for the recycling of carbon waste through direct conversion of CO₂ into short-chain hydrocarbons and formate. Among the three nitrogenase isozymes, the iron (Fe) nitrogenase shows the highest wild-type activity for the reduction of CO₂, but the molecular mechanisms facilitating this activity remain elusive. Our research aims to unravel the catalytic principles of the Fe nitrogenase and explore its biocatalytic potential for the conversion of CO₂ into value added products.

Over the past three years, we have pioneered the anaerobic purification of the Fe nitrogenase from Rhodobacter capsulatus, which allowed us to study its reactivity in vitro. By monitoring CO₂ reduction activity in the presence of N₂, we found the Fe nitrogenase to be highly promiscuous for the reduction of CO₂, which results in the formation of formate and methane. Intriguingly, these effects translate in vivo, where we observed an extracellular accumulation of formate and methane by Fe nitrogenase expressing strains upon exposure to CO₂ (Schmidt & Oehlmann et al., Science Advances, 2024). Intrigued by these unique reactivities, we sought to understand the molecular mechanism of the Fe nitrogenase, which lead us to solve a 2.35-Å cryogenic electron microscopy structure of the Fe nitrogenase complex (Schmidt & Schulz et al., Nature Structural & Molecular Biology, 2024). The structure reveals a [Fe₈S₉C-(R)-homocitrate] cluster in the active site and highlights a Fe nitrogenase specific complex architecture.

In conclusion, our results establish the Fe nitrogenase as a CO₂-reducing enzyme and show up new pathways for the utilization of CO₂ as a feedstock chemical. Our structural and biochemical analysis of the Fe nitrogenase provides a solid framework for further investigations on the molecular mechanisms of Fe nitrogenase catalysis. Eventually, we aim to utilize these insights to rationally design, test and build Fe nitrogenase based biocatalysts for the sustainable conversion of CO₂ into value-added products.