Propionate 3-nitronate (P3N) is a natural toxin that irreversibly inhibits mitochondrial succinate dehydrogenase. P3N poisoning leads to a variety of neurological disorders and even death. Nitronate monooxygenase (NMO) from Cyberlindnera saturnus (CsNMO) and Pseudomonas aeruginosa PAO1 (PaNMO) serve as paradigms for Class I NMO, which catalyze the oxidation of P3N involving single electron transfer. In this dissertation, the crystallographic structure of CsNMO was solved and demonstrated a highly conserved three-dimensional structure and active site with respect to NMO from PaNMO. The role of conserved residues in the active site of Class I NMO, e.g. Y109, Y254, Y299, Y303, and K307 in PaNMO in substrate binding and catalysis were investigated using site-directed mutagenesis, steady-state kinetics and pH effects on the UV-visible absorption spectrum. The study revealed that a protonated tyrosine is required for binding of the negatively charged P3N substrate. We also report that PaNMO can stabilize both the neutral and anionic semiquinones anaerobically for hours, providing a constant protein environment to study their photochemical and photophysical properties.

Choline oxidase catalyzes two-step oxidation of choline to glycine betaine with betaine aldehyde as an intermediate. The FAD cofactor is covalently attached to the choline oxidase via H99 through an 8α-N³-histidyl linkage. In the active site of choline oxidase, S101 and H466 are located on two extent loops, ~ 4 Å from the flavin C4a atom. In this dissertation, a charge-induced, reversible C4a-S-cysteinyl-8α-N³-histidyl FAD was engineered by replacing S101 with a cysteine. The mechanistic rationale for the stabilization of de novo C4a-S-cysteinyl-flavins was illustrated with rapid kinetics, pH, kinetic isotope effects and proton inventory. A photoinduced transient C4a-N-histidyl-8α-N³-histidyl FAD in choline oxidase wild-type was also observed with the aid of fluorescence excitation spectroscopy. Site-directed mutagenesis, solvent equilibrium isotope effects and pH effects on the stoke shifts of flavin in choline oxidase wild-type demonstrated H466 as the adduct on the C4a atom of flavin upon excitation, and provided a mechanistic rationale involving photoinduced electron transfer (PET) for the formation of the novel photoinduced transient flavin C4a adduct.