Nitrogenase is one of those enzymes that play a key role in the fixation of nitrogen. This complex protein is found in many bacteria that live in symbiotic relationships with plants. It can convert N₂ into ammonium ions, thereby helping feed atmospheric nitrogen into food chains. The centre of nitrogenase enzymes contains two cubes made up of iron and sulphur atoms. One of these so-called iron-sulphur clusters, which also contains molybdenum, mediates the actual enzymatic reaction. "Very unusual chemical reactions occur in the enzyme's centre, which consists of only a few inorganic atoms," said Prof. Dr. Oliver Einsle from the Institute of Organic Chemistry and Biochemistry at the University of Freiburg. "This enzyme is able to do naturally what the industrial Haber-Bosch process can only achieve at high pressure and temperatures. Moreover, the Haber-Bosch process generates much less ammonia than the nitrogenase enzyme," said Einsle.

The Haber-Bosch process is used to turn N₂ into ammonium ions which can then be used to produce nitrate fertiliser. However, production of around one t ammonia requires a great deal of energy, corresponding to the quantity of energy required to produce around 1.7 t CO₂. Moreover, only about 50 per cent of the fertiliser produced reaches the plants; the other 50 per cent are degraded by soil bacteria. Scientists who are investigating metalloproteins such as nitrogenase therefore always ensure they are aware of effective industrial use of metalloproteins. "It would be nice if we were able to give plants the ability to convert atmospheric nitrogen into the form required," said Einsle implying that a detailed understanding of bacterial enzymes is needed. The scientists are interested to find out where the educt (i.e. N₂ gas) is stored in the reactive centre of the enzyme. Which intermediary steps are used for the transfer of electrons from iron atoms to nitrogen? One possible approach is to analyse the enzyme's structure. Once the researchers know the conformation of a specific protein, they will be able to make initial assumptions on what happens during the reaction.

Einsle and his team have used crystallography and made good progress with another enzyme, i.e. crimson dinitrogen monoxide reductase (N₂O reductase). This enzyme, whose reactive centre contains a cube of copper ions and hence is lilac in colour, also catalyses a step in the global nitrogen cycle. N₂O reductase catalyses the final reaction of the denitrification reaction, where N₂ is produced from N₂O and released into the atmosphere or reduced once again to ammonium ions. Einsle and his team have clarified the spatial structure of the enzyme. In addition, they have found out how the N₂O molecules are arranged at the copper cube in the centre of the metalloprotein. The researchers now have to clarify the exact mechanism of the reaction and find out whether the process can be simulated in silico.

Bacteria use the enzymes of the nitrogen metabolism to produce energy. The enzymes are located on the bacterial membranes. The reactions lead to the release of protons (i.e. positively charged hydrogen ions), which are pumped across the membranes, a process during which energy is released. The biochemists in Einsle's group are not only interested in investigating the enzymes of the global nitrogen cycle, but are generally interested in the function of membrane proteins. How do the membrane proteins transport protons? How does this process lead to the generation of energy? Einsle and his team have been in Freiburg for two years now where they benefit from working with teams from the faculties of chemistry, medicine, biology and pharmacy. These groups provide Einsle's group with sample material as well as new ideas and questions. In return, Einsle and his team elucidate the structure of the proteins in which their cooperation partners are interested. "People working in many different disciplines contact us to find out what their proteins look like," said Einsle going on to add "such projects have positive outcomes for both sides". X-ray crystallography has become an integral part of modern life sciences research, just as microscopy did many years ago.