How nature reduces nitrogen - unravelling design features for a nitrogenase mimic

The Nitrogen Cycle converts nitrogen gas into usable forms, such as ammonium, nitrates, and nitrites. Nitrogen and nitrogen compounds are important because they constitute our atmosphere, our bodies as well as our earthly environments. Nitrogen can be recycled into ammonia/ammonium naturally via nitrogen fixing bacteria, or synthetically using the Haber-Bosch process. Nitrogen fixation in bacteria follows the reaction N2 + 8 H+ + 8 e- 2 NH3 + H2 and is powered by the hydrolysis of 16 ATP equivalents. The Haber-Bosch process follows the reaction scheme of N2 + 3 H2 2 NH3 and is powered by using high pressures, temperatures, and catalysts. Using the Haber-Bosch process an optimum yield of 97% ammonium can be obtained. These reactions both use diatomic nitrogen as well as hydrogen, but differ in their final products as bacterial nitrogen fixation releases hydrogen gas as a byproduct. Bacteria in the soil use nitrogen to create energy to grow and reproduce as well as to introduce nitrogen for use by other species. The Haber-Bosch process produces ammonium which can be used for a range of activities such as the production of fertilizers, or even explosive
Haber invented a process that sustains one third of the population on earth: the production of ammonia fertilizer from nitrogen gas. Nitrogen is required for life, a crucial component of both DNA and proteins, but even though nitrogen is the most abundant gas in our atmosphere, our cells can't use it in its atmospheric form, relying on other processes to "fix" that nitrogen into a biologically available form. A few microorganisms possess nitrogenase enzymes that can perform this chemical reaction, and about half of the nitrogen in your body comes from these microorganisms. The other half comes from the Haber-Bosch process (Carl Bosch scaled up Haber's process to large-scale industrial levels).
The Haber-Bosch process consumes vast amounts of energy, and some researchers argue that even tiny improvements in catalyst efficiency could yield big savings. This project aims for a more fundamental shift - finding a catalyst that emulates nature's ability to fix nitrogen gently from the air.
The industrial process relies on a chemical hammer-blow to cleave the bonds of the feedstock gases in one swift stroke, littering the catalyst's surface with nitrogen and hydrogen atoms that rapidly combine to make ammonia. In contrast, nature uses a stream of protons and electrons to unpick dinitrogen one bond at a time - a surgical procedure orchestrated by nitrogenase, an enzyme found in microbes such as soil-dwelling Rhizobia.
Nitrogenase has two major parts: an iron protein that acts as a delivery vehicle for electrons, and a molybdenum-iron protein that uses them to break dinitrogen apart. The electrons ultimately come from reduced ferredoxin, a powerful reducing agent generated during photosynthesis, while adenosine triphosphate (ATP) provides the energy.
Biological nitrogen fixation is far from being a paragon of efficiency. Turning each molecule of nitrogen into ammonia takes at least 16 molecules of ATP, and a quarter of the electrons involved in the reaction are 'wasted' to make hydrogen gas as a by-product. 'These bacteria devote a lot of energy to it, which tells you how important nitrogen fixation is for life. The catalytic heart of the enzyme is called the iron-molybdenum cofactor (FeMo-co), a cluster of seven iron ions, one molybdenum ion, and nine sulfides. Scientists are still unsure how it works - indeed, it was only in 2015 that x-ray crystallography definitively revealed a single carbon ion right in the middle of the cluster.
Based on a mixture of hard data and intuition, the current belief is that a quartet of iron atoms in the FeMo-co forms the catalytically active site. Some are still doubtful that nitrogenase's iron atoms bind and reduce dinitrogen. An alternative view supported by model compounds is that Mo is the key for nitrogen reduction.