Nitrogenase molybdenum iron protein
Azotobacter vinelandii (aerobic nitrogen-fixing bacteria)
The atom nitrogen is an important ingredient of life, being a constituent of many natural chemical groups. It is also an important ingredient of most powerful explosives. Devices in airports scanning for explosives check for traces of nitrogen. Finding a way to make the handling of the explosive compound nitroglycerin, in the form of dynamite, safer, earned its inventor, Alfred Nobel a big fortune and the interest paid on this fortune is the source for the Nobel prize. Nitrogen supply problems in Germany of world war I lead Fritz Haber to invent a method to synthesize ammonia from diatomic nitrogen, N2. He was awarded the 1918 Nobel prize in chemistry for it. What he found is that the use of iron oxides helped very much in the conversion of nitrogen to ammonia. In nature the production of ammonia, a main ingredient of fertilizers, falls to nitrogen-fixating bacteria. There is plenty of diatomic nitrogen in the atmosphere (which is mostly O2 and N2) and their job is to turn this atmospheric nitrogen into the more useful ammonia.
Much like Haber with his ammonia synthesis these bacteria make use of iron for this process. The iron, that is part of the catalytic center, is in so-called iron-sulfur (FeS) clusters, made from 4 iron and 4 sulfur atoms which form the corners of an approximate cube. One or more of the sulfurs comes from cysteine sidechains in the protein containing such clusters. Such a protein can be seen here. It is a quite large protein, containing two copies each of two different types of subunits. Such an arrangement is often called a dimer of dimers. Each of these two dimers contains two reaction centers that participate in the process of forming ammonia from nitrogen. As one of the metal clusters also contains a molybdenum atom, the units are distinguished as the Fe-protein and the MoFe-protein units. They work together, using energy by converting adenosine triphosphate (ATP) to adenosine diphosphate (ADP), to split the atomic bond in diatomic nitrogen N2 and add three protons (hydrogen ions) to create two molecules of ammonia NH3.
Protein Data Bank (PDB)
Peters, J.W. Stowell, M.H. Soltis, S.M. Finnegan, M.G. Johnson, M.K. Rees, D.C.; "Redox-dependent structural changes in the nitrogenase P-cluster."; Biochemistry; (1997) 36:1181-1187 PubMed:9063865.
author: Arno Paehler