Nitrogen makes up about 78% of Earth’s atmosphere, yet most plants and animals cannot use it directly.
Before nitrogen can help build proteins, DNA and other essential molecules, it must first be converted into ammonia.
This crucial job is carried out by a small group of microbes using special enzymes called nitrogenases.
Now, researchers at the Massachusetts Institute of Technology (MIT) have uncovered why one type of nitrogenase is much better at this task than others.
Their findings, published in the journal Chem, could help scientists develop cleaner ways to produce ammonia for fertilizers and inspire new catalysts for industry.
The ability to convert nitrogen gas into ammonia is one of the most important chemical reactions in nature.
Although nitrogen is abundant in the air, the two nitrogen atoms in each nitrogen gas molecule are held together by an extremely strong triple bond. Breaking this bond is very difficult and normally requires large amounts of energy.
Scientists believe that before nitrogen-fixing microbes evolved around three billion years ago, lightning was one of the few natural processes capable of breaking nitrogen molecules apart. The evolution of nitrogenase enzymes transformed life on Earth by allowing microorganisms to turn atmospheric nitrogen into a form that living organisms could use.
There are three main types of nitrogenase enzymes, each containing different metals. The most efficient versions contain molybdenum, while others use vanadium or only iron. For decades, researchers have wondered why molybdenum makes such a big difference because it does not appear to bind directly to nitrogen during the reaction.
To solve this puzzle, the MIT team carried out two complementary studies using simplified laboratory versions of the enzyme’s active center. These models allowed the researchers to swap different metal atoms into the structure and observe how they affected the enzyme’s behavior.
In the first study, the scientists discovered that only enzyme models containing larger metals, such as molybdenum or tungsten, were able to strongly bind nitrogen gas. Models containing smaller metals like vanadium, chromium or iron could not hold onto nitrogen as effectively and instead favored other chemical reactions.
The second study revealed why this happens. The researchers found that molybdenum changes the electronic properties of nearby iron atoms without directly participating in the reaction itself. Because molybdenum has large electron orbitals, it can interact closely with neighboring iron atoms, making it easier for the iron to share electrons with nitrogen gas.
This electron sharing, known as back-bonding, weakens nitrogen’s powerful triple bond. Once the bond begins to break, hydrogen atoms can attach to the nitrogen, allowing the reaction to continue until ammonia is produced.
The researchers describe this as a form of electronic teamwork between the two metals. Instead of iron carrying out the entire reaction alone, molybdenum helps prepare the iron so it can perform the difficult first step much more efficiently.
Understanding this natural strategy could have important practical benefits. Modern agriculture depends heavily on nitrogen fertilizers, which are largely produced using the Haber-Bosch process. While highly effective, this industrial method requires extremely high temperatures and pressures and consumes large amounts of energy, making it a significant source of greenhouse gas emissions.
By learning how nature performs the same reaction under ordinary conditions, scientists hope to design new catalysts that produce ammonia using much less energy. The findings may also help researchers engineer microbes or enzymes that naturally generate ammonia more efficiently, reducing the need for synthetic fertilizers.
Beyond nitrogen fixation, the study also highlights a broader principle in chemistry: neighboring metals can cooperate in ways that dramatically improve chemical reactions.
The researchers believe this insight could guide the design of many future catalysts inspired by nature’s remarkable ability to perform complex chemistry with extraordinary efficiency.
