Abstract

Nitrogenase has a central role in the global nitrogen cycle as the enzyme that catalyzes the reduction of atmospheric N2 to NH3. Fixed nitrogen is generally limiting in the environment and in agriculture, so nitrogenase has received much attention as an alternative to nitrogen fertilizers. Characterizing the mechanism of nitrogenase is the goal of this work. The molybdenum nitrogenase enzyme system is comprised of the MoFe protein and the Fe protein. Interactions between these proteins and nucleotides are crucial to catalysis. An important approach to characterize these interactions is to correlate the kinetics of nitrogenase catalysis to a mechanism based on the properties of the nitrogenase components. Ironically, the most successful kinetic model of nitrogenase was devised by R. N. F. Thorneley and D. J. Lowe (T&L) before any crystal structures of nitrogenase were solved. This work critiques the ability of the T&L model to predict nitrogenase catalysis accurately. Several defects in the model are described, but it is qualitatively correct. A literature review and critique leads to the rational design of a new kinetic model of nitrogenase catalysis. Because of its comprehensive scope and superior detail, this model has the potential to describe nitrogenase catalysis quantitatively. However, the development of this model is an ambitious project only begun in this work, step by step. Some of the areas of study include: an analysis of Fe protein reduction by dithionite; the characterization of a form of Fe protein reduced to the all-ferrous [4Fe-4S]0 state with a novel spin S = 0 state by the in vivo reductant flavodoxin; and a novel account of salt effects that weaken the nitrogenase complex to increase the rate of complex dissociation, the rate-limiting step in nitrogenase catalysis.

Degree

PhD

College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry

Rights

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