Figure 1: Nitrogenase. PDB: 1M1N
Nitrogenase, an enzyme produced by bacteria in the root nodules of many legumes, performs the important nitrogen fixation reaction:
N≡N → 2 NH3
But why does it deserve your vote?
1. This transformation is incredible. Consider this: the N=N bond is 945 kJ/mol strong. This makes nitrogen gas incredibly difficult to reduce. But nitrogenase does it. It has also been shown to split other molecules in vitro, such as carbon dioxide and ethene.
2. Nitrogenase does it better than Haber-Bosch. If you recall the Haber-Bosch process, you may remember that to perform the same reaction shown above, an iron catalyst, calcium, potassium, and aluminum oxides, temperatures between 300 and 500˚C, and pressures of 15-20 MPa are required. Nitrogenase performs the same reaction under biological temperatures and pressures with 8 electrons, 8 protons and 16 ATP per transformation. Looks like this enzyme has got us beat.
3. Sweet cofactors. Fe-S clusters? Right on. Molybdenum-Fe-S clusters? Even better. What exactly does the molybdenum do? Scientists have yet to provide a definitive answer to that question.
Figure 2: Molybdenum-Iron-Sulfur Cluster in Nitrogenase protein. PDB: 1MIO.
4. 6 bonds to Carbon! To the horror of organic chemists, nitrogenase's ability to split a variety of organic molecules other than nitrogen gas implicates the existence of a six-coordinate atom within the Mo-Fe-S cluster. The reality of this coordination has not been officially confirmed by bioinorganic chemists, but remains an accepted conclusion in many circles.
Figure 3: Schematic drawing of six-coordinate center. Speculation suggests that a carbon atom could exist in this coordination, but the majority of published articles assume it to be a nitrogen atom.
 For more information, feel free to chat with Professor David Benson of the Calvin College Chemistry and Biochemistry Department.