Insights into the Mechanism of Biological Nitrogen Fixation through Characterization of the Nitrogenase Molybdenum-Iron Protein

Citation

Morrison, Christine Nichole (2017) Insights into the Mechanism of Biological Nitrogen Fixation through Characterization of the Nitrogenase Molybdenum-Iron Protein. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z95B00HX. https://resolver.caltech.edu/CaltechTHESIS:05112017-113214704

Abstract

Nitrogen fixation, the process of converting dinitrogen to ammonia, is performed industrially and biologically by the Haber-Bosch process and nitrogenase, respectively. The resulting ammonia is largely used as fertilizer. Since there is a finite amount of ammonia produced by nitrogenase, we are heavily dependent on the Haber-Bosch process – only two-fifths of the world’s population could be fed without it. Although the importance of the Haber-Bosch process cannot be overstated, our dependence on it has several drawbacks, including significant energy costs (~5% of the annual natural gas consumption), greenhouse gas emissions, and nitrate runoffs. By understanding the biological mechanism of nitrogen fixation, we may be able to (1) develop more efficient nitrogen fixing catalysts to replace those in the Haber-Bosch process or (2) express de novo nitrogen fixing proteins in plants so crops can essentially fertilize themselves. The projects described in this thesis aim to contribute to our understanding of the mechanism of biological nitrogen fixation through structural studies of nitrogenase. Nitrogenase consists of the iron and molybdenum-iron (MoFe) proteins, the latter of which contains the active site, the FeMo-cofactor. Throughout my work, I compare the MoFe proteins from Azotobacter vinelandii (Av1) and Clostridium pasteurianum (Cp1), the two most structurally divergent molybdenum nitrogenases known. Determining the similarities and differences between these proteins may aid our understanding of biological nitrogen fixation. My first project (Chapter III) compares a 1.08 Å Cp1 X-ray structure to a previously published 1.0 Å Av1 structure. I determined that the center atom of the Cp1 FeMo-cofactor is carbon, showing conservation of cofactor structure among molybdenum nitrogenases. Next, I compared substrate pathways in Av1 and Cp1 via Xe pressurization and identification of small molecule binding sites (Chapter IV). My most significant results include the structural and electronic characterization of a reversible protonated resting state of Av1 and Cp1 (Chapter VII).

Thesis Files