Substrate Binding and Reduction Mechanism of Molybdenum Nitrogenase

As a key constituent of proteins, nucleic acids, and other biomolecules, nitrogen is essential to all living organisms including human beings. Dinitrogen represents the largest pool of nitrogen, about 79% of the Earth’s atmosphere, yet it is unusable by most living organisms due to its inertness. There are two ways to fix this inert dinitrogen to usable ammonia. One is the industrial Haber-Bosch process, which needs to be conducted at high temperature and pressure. This process uses a lot of the non-renewable fossil fuel as the energy source. The other major pathway is the biological nitrogen fixation carried out by some microorganisms called diazotrophs. The usable nitrogen output from this biological pathway ultimately supports an estimated 60% of the human population’s demand for nitrogen.The catalyst responsible for the biological nitrogen fixation is called nitrogenase, the most studied form of which contains molybdenum and iron in its active center, so called molybdenum nitrogenase. The work in this dissertation attempts to understand howthis biological catalyst breaks down dinitrogen to ammonia by application of different modern techniques. Firstly, an approach was developed to understand the stepwise reduction mechanism of dinitrogen to ammonia by molybdenum nitrogenase.The second goal of my research is to understand the roles of iron and molybdenum centers in nitrogenase function. My results using carbon monoxide as a probe for genetically modified molybdenum nitrogenase indicate that iron should be the metal sites functioning for nitrogen fixation. This is further supported by another study aimed at understanding the role of molybdenum during nitrogenase functioning.Moreover, an approach was developed to understand the mechanism for the obligatory production of hydrogen gas when nitrogenase activates dinitrogen for reduction. The same study also suggests possible pathways for the addition of hydrogenous species to nitrogen to produce ammonia.As part of this work, we also found that remodeled nitrogenases can use poisonous carbon monoxide and greenhouse-gas carbon dioxide to produce useful hydrocarbons by coupling one or more small molecules, which is hard to be achieved by other catalysts. Further study of these new reactions might give us deep insights on nitrogenase mechanism and inspire scientists to design better catalysts for relevant industrial processes