ABSTRACT
The reaction that this enzyme performs is: N2 + 8H+ + 8e− + 16ATP Æ 2NH3 + H2 + 16 ADP + 16Pi (6.2) Here it is worthwhile to underline the exclusive importance of hydrogen (hydrogen protons) in the nitrogen fixation that is a vitally important process. This process is exclusively essential for life since it fixes nitrogen required to biosynthesize the basic building blocks of life, for example, nucleotides for DNA and RNA and amino acids for proteins. However, to fix nitrogen is not an easy task since an N2 molecule is very stable and it is difficult to break it apart into individual nitrogen atoms. Nitrogen-fixing bacteria have the ability to convert nitrogen gas into ammonia with the help of hydrogen protons, which is easily combined with other raw materials to form the building blocks of proteins and nucleic acids. In nitrogen-fixing bacteria, the enzyme nitrogenase drives the reaction with a large quantity of ATP, and uses a collection of metal ions, including an unusual molybdenum ion, to drive the reaction. Being a strong catalyst, nitrogenase performs this function using hydrogen protons, and is powered by ATP molecules. The nitrogenase complex consists of two proteins encoded by three so-called structural nif genes [105]. The MoFe protein, shown in blue and purple (Fig. 6.1), contains all the necessary machinery to perform the reaction, but requires addition of six electrons for each nitrogen molecule that is split into two ammonia molecules. The Fe protein, shown in green, uses the breakage of ATP to pump these electrons into the MoFe protein. In the typical reaction, two molecules of ATP are consumed for each electron transferred. Nitrogenase also converts hydrogen ions to hydrogen gas at the same time, thus consuming even more ATP in the process. This is a large investment in energy, but necessary if nitrogen is not available in the environment. Fortunately, nitrogen-fixing bacteria are widely spread through the world, and are often found in the vicinity of plants. For instance, legumes have special nodules in their roots that provide an accommodation to the bacteria. The plants provide shelter and even few essential nutrients to make the accommodation of bacteria comfortable so that they can supply nitrogen steadily. At the heart of nitrogenase is an unusual complex of iron, sulfur and molybdenum ions that apparently performs the nitrogen-fixing reaction. A series of cofactors supply this MoFe-cluster with electrons
[105]. As seen in Fig. 6.2 on the left, electrons start at a pair of ATP molecules (two at each end of the dimeric complex), move inwards into the iron-sulfur cluster, then to the P-cluster, and finally to the MoFe-cluster. The three metal clusters are shown in the right side of Fig. 6.2. The MoFe-cluster is at the bottom, with the molybdenum atom in bright red. A homocitrate molecule, shown with white carbon atoms and pink oxygen atoms, helps to stabilize this unusual metal ion [105]. The P-cluster is in the middle and the iron-sulfur cluster of the Fe protein is at the top.
