Savoring the Flavor of Carbon Dioxide

Nitrogenases are critically important enzymes on our planet, enabling all forms of life to obtain usable nitrogen as ammonia (NH₃). Certain nitrogenases have the capability to transform CO₂ into hydrocarbon chains, making them an intriguing focus for innovative biotechnological applications. A research team in Marburg, Germany, led by Johannes Rebelein from the Max Planck Institute, has conducted an in-depth examination of nitrogenase’s substrate specificity and preferences. Their findings challenge existing knowledge about nitrogenases and illustrate their potential in promoting sustainable bioproduction.

Nitrogen is fundamental for cellular structures, yet most of Earth’s nitrogen exists as unusable gaseous N₂. Only nitrogenases, a specific group of enzymes, can convert N₂ to ammonia, the accessible form of nitrogen for living organisms.

The team, under Johannes Rebelein’s guidance, has recently revealed that some nitrogenases can also process another significant substrate: they convert the greenhouse gas CO₂ into hydrocarbons (such as methane, ethylene, and ethane) as well as formic acid. These products have the potential to serve as energy sources and valuable industrial chemicals. In the interest of developing sustainable and carbon-neutral production methods, the researchers sought to determine how effectively these enzymes can differentiate between CO₂ and N₂, and whether microorganisms that thrive on N₂ can reduce CO₂ under regular physiological conditions.

Two isoenzymes

To explore these questions, the researchers examined the photosynthetic bacterium Rhodobacter capsulatus, which possesses two isoenzymes: molybdenum (Mo) nitrogenase and iron (Fe) nitrogenase, the latter serving as a backup when molybdenum is limited. They isolated both nitrogenases and assessed their CO₂ reduction capabilities through biochemical tests. The results indicated that Fe nitrogenase reduces CO₂ with three times the efficiency of its Mo counterpart, generating formic acid and methane even at normal atmospheric CO₂ levels.

When both enzymes were tested with CO₂ and N₂ simultaneously, a significant difference emerged: while Mo nitrogenase specifically reduces N₂, Fe nitrogenase showed a preference for CO₂ as its substrate. “Typically, higher enzyme reaction rates compromise accuracy. Interestingly, Mo nitrogenase is both quicker and more selective, which benefits N₂ reduction. The less specific Fe nitrogenase and its inclination to favor CO₂ present a compelling opportunity for creating new CO₂ reductases,” explains Frederik Schmidt, a PhD student in Rebelein’s lab and a co-author of the study.

Widespread CO₂ reduction in nature?

The researchers also uncovered another surprising aspect regarding low selectivity. “By analyzing the distribution of electrons among the various products, we discovered that methane and significant amounts of formic acid produced from CO₂ via Fe nitrogenase were released by bacteria even in the absence of additional CO₂ in the culture: the CO₂ generated metabolically was sufficient to drive this conversion. This implies that the CO₂ reduction facilitated by Fe nitrogenase might be more common in nature than previously thought,” notes Niels Oehlmann, co-first author of the research. This indicates that the availability and transfer of one-carbon substrates could impact microbial communities across diverse environments.

These results challenge the conventional perspective of nitrogenases solely as nitrogen-converting enzymes. Photosynthetic bacteria like R. capsulatus that harness light energy to stimulate nitrogenases for CO₂ conversion could significantly influence both environmental outcomes and the transition toward a sustainable circular economy, according to Johannes Rebelein. “Our goal is to capture energy from sunlight through microorganisms’ photosynthesis and store it in the hydrocarbons created by nitrogenase. Moving forward, we aim to further refine iron nitrogenase for effective CO₂ fixation and utilization.”