The remarkable affinity of the microbial enzyme iron nitrogenase for the greenhouse gas CO2 makes it promising for future biotechnologies.
Nitrogenases are among the geochemically most important enzymes on Earth and provide all forms of life with bioavailable nitrogen in the form of ammonia (NH3). Some nitrogenases can also convert CO directly2 in hydrocarbon chains, making them an exciting target for the development of biotechnological processes. A team of researchers in Marburg, Germany, led by Max Planck scientist Johannes Rebelein, has now provided a comprehensive insight into the substrate specificity and preferences of nitrogenase. Their results challenge the current understanding of nitrogenases and highlight their potential for sustainable bioproduction.
Nitrogen is one of the most important building blocks of our cells. However, most of the nitrogen on Earth occurs as gaseous N2 and is chemically useless to cells. Only a single family of enzymes can destroy N2 converted into the bioavailable form of ammonia (NH3): nitrogenases.
Researchers led by Johannes Rebelein from the Max Planck Institute for Terrestrial Microbiology in Marburg have recently discovered that some nitrogenases can also deal with another important substrate: they reduce the greenhouse gas CO2 to hydrocarbons (methane, ethylene, ethane) and formic acid. All these products are potential energy sources and industrially important chemicals. With a view to sustainable, carbon-neutral bioproduction, the team wanted to know: how well can the enzymes distinguish between CO2 and N2? And microorganisms that grow on N2 also reduce CO2 under normal, physiological conditions?
Two isoenzymes
To answer these questions, the researchers focused on the photosynthetic bacterium Rhodobacter capsulatuswhich houses two isoenzymes: the molybdenum (Mo) nitrogenase and the iron (Fe) nitrogenase, which the bacteria need as a reserve in case of a molybdenum deficiency. The researchers isolated both nitrogenases and compared their CO2 reduction using biochemical tests. They found that the Fe nitrogenase actually reduces CO22 three times more efficient than its molybdenum counterpart and produces formic acid and methane at atmospheric CO22 concentrations.
When both enzymes were offered CO2 and N2 At the same time, another important difference became clear: while Mo-nitrogenase selectively reduces N2, Fe-nitrogenase tends to choose CO.2 as substrate. “Normally, a higher reaction rate in enzymes comes at the expense of accuracy. Interestingly, Mo-nitrogenase is both faster and more selective, showing its advantage in N2 decrease. The lower specificity of Fe nitrogenase and its preference for CO2 make it a promising starting point for the development of new CO2 reductases,” says Frederik Schmidt, PhD student in Johannes Rebelein’s laboratory and co-author of the study.
Widespread CO2 nature reduction?
The low selectivity was not the only surprise. ‘We analyzed which fraction of electrons ended up in which product and discovered that methane and high concentrations of formic acid come from CO2 conversion by Fe nitrogenase were secreted by the bacteria even in the absence of additional CO22 was added to the culture: the metabolically obtained CO2 was enough to drive this process. This finding suggests that Fe nitrogenase-catalyzed CO2 This reduction may indeed be widespread in nature,” said Niels Oehlmann, co-first author of the study. This also means that the availability and exchange of single-carbon substrates is likely to influence microbial communities in different environments.
The work challenges the traditional view of nitrogenases as true nitrogen-converting enzymes. Photosynthetic bacteria such as R. capsulatusthat use light energy to stimulate nitrogenases to convert the greenhouse gas CO2could play a key role not only in their impact on the environment, but also in the societal shift towards a sustainable circular economy, says Johannes Rebelein. “The idea is that we can store the energy from sunlight captured by the microorganism’s photosynthetic apparatus in the hydrocarbons produced by nitrogenase. In the future we want to further develop iron nitrogenase to use it for CO2 fixation and use.
