The mechanism for nitrogenase including all steps

• Published in: Physical chemistry chemical physics : PCCP 2019-07, Vol.21 (28), p.15747-15759
• Main Author: Siegbahn, Per E. M
• Format: Article
• Language: English
• Subjects: Activation; Carbides; Carbon; Carbon - chemistry; Catalysis; Coenzymes - chemistry; Enzyme Activation; Ligands; Metals - chemistry; Molybdenum; Nitrogenase - chemistry; Nitrogenase - metabolism; Protonation; Protons
• ISSN: 1463-9076; 1463-9084; 1463-9084
• DOI: 10.1039/c9cp02073j

The catalytic cofactor of the most common form of nitrogenase contains seven irons and one molybdenum bound together by sulfide bonds. Surprisingly, a central carbide has been demonstrated by experiments. Another noteworthy structural component is a large homocitrate ligand. In recent theoretical studies it has been shown that the central carbide is needed as a place for the incoming protons that are necessary parts of a reduction process. It has also been shown that a role for the homocitrate ligand could be that it may be rotated to release one bond to molybdenum. In the present study, the carbide protonation steps are reinvestigated with similar results to those reported before. The actual activation of N 2 in the E 4 state is an extremely complicated process. It has been found experimentally that two hydrides should leave as H 2 , in a reductive elimination process, to allow N 2 activation in E 4 in an easily reversible step. It is here suggested that after H 2 is released, it is necessary for the metal cofactor to get rid of one proton. This is achieved by protonating the homocitrate and then rotating it to release one of the bonds to Mo. After this rotation, N 2 can bind. In the E 5 step, the homocitrate is rotated back to its original position and remains that way until the end of the catalytic process. The N 2 protonation steps are energetically easy. Since a protonated carbide has never been observed experimentally, it is necessary to also have a mechanism for deprotonating the carbon at the end of the catalytic cycles. Such a mechanism is suggested here. Nitrogen in the air is turned into biologically useful ammonia by the nitrogenase enzyme. The leading member of this group has a cofactor with one molybdenum and seven irons linked together by sulfurs. The structure that binds N 2 has a triply protonated carbide and a rotated homocitrate. Both these structural changes are necessary for the activation.