Protein of the Week: leghemoglobin
The reduction of nitrogen to ammonia, known as nitrogen fixing, is vital to agriculture:
N2 + 3H2 → 2NH3
Whereas the industrial Haber-Bosch process requires temperatures ~500 oC and hundreds of atmospheres of pressure to overcome the activation energy, some bacteria can accomplish the reaction at soil temperatures and atmospheric pressure. (In fact the nitrogenase complex found in these bacteria is responsible for fixing over 1011 kg of nitrogen every year.) These bacteria are symbionts--in exchange for the abundant ammonia they produce for the legume host, the plant provides huge amounts of energy for the reaction.
However, nitrogenase has a problem with oxygen toxicity. Here again the plant host comes to the rescue by providing leghemoglobin (PDB code 2GDM, structure generated with PyMOL), an oxygen-binding protein:
Leghemoglobin is able to bind oxygen due to the iron-containing heme held in its center.
As its name suggests, leghemoglobin belongs to the globin protein family and its structure resembles that of the mammalian oxygen-binding protein myoglobin (PDB code 1MBO):
Though the structures and functions of the two proteins are quite similar, their sequences have little in common. Furthermore, myoglobin is not found in plants, so it would be a stretch to suggest that leghemoglobin arose from myoglobin. Instead, what we see here is a wonderful example of convergent evolution in which the optimum oxygen-binding structure is found in two very different types of organisms.