Date of Award:
Master of Science (MS)
Chemistry and Biochemistry
Nitrogen fixation is a key step of the nitrogen cycle which makes biologically inert N2 gas available for organisms to use in the form of ammonia. Nitrogen fixing microorganisms all contain the same enzyme called nitrogenase which catalyzes the six electron transfers to N2 required for conversion into ammonia. Nitrogenase is a two-component enzyme that contains a cofactor composed of iron and sulfur as well as heavier metals whose identity can be molybdenum, vanadium, or an additional iron atom depending on the variant. The two components of nitrogenase are the MFe protein and the Fe protein. The Fe protein delivers electrons to the MFe protein where N2 conversion to ammonia takes place. Two outstanding topics of interest in the nitrogenase research field are (1) how the iron-sulfur active site cofactor of nitrogenase (FeMoco) interacts with the protein to facilitate electron transfer to substrates, (2) why electron transfer rates are sometimes slowed by N2 despite a long-held understanding that the rate of electron transfer is substrate invariant.
The first question was addressed by using CdS nanoparticles to deliver electrons to the extracted active site cofactor of nitrogenase. This system allows the kinetics of substrate reduction at the cofactor to be investigated in order to understand what role the protein environment plays in reactivity. The CdS:cofactor system was characterized and its feasibility as an investigative tool was examined. The CdS:cofactor reductive system was found to be susceptible to decomposition resulting in the formation of multiple catalytic species; eliminating the current system as a mechanistic investigation tool.
The second question was addressed by investigating what conditions and methods of measurement determine if N2 inhibition of electron transfer is observed. Several conditions were found to determine the extent of N2 inhibition observed including pH, nitrogenase subunit ratio, and the method by which electron transfer is measured. The results of these experiments led to the proposal of a novel, unaccounted-for product of nitrogenase catalysis under N2 as an explanation for N2 inhibition.
Kallas, Hayden, "Investigations of Substrate Reduction by Nitrogenase: Light Powered Substrate Reduction by a CdS:FeMoco System and Understanding Dinitrogen Inhibition of Electron Transfer" (2021). All Graduate Theses and Dissertations. 8076.
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