Understanding the molecular factors governing inhibitor potency and oxygen activation in copper amine oxidases
Shepard, Eric Michael.
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Copper amine oxidases (CuAOs) catalyze the oxidative deamination of primary amines to the corresponding aldehydes, with the concomitant reduction of O2 to H2O2. CuAOs are known to have a multitude of physiological roles, and activity levels are upregulated in several pathological states. With the potential for therapeutic applications, substantial efforts were made to determine the molecular factors governing inhibitor selectivity. As such, several mechanism-based inhibitors were screened against CuAOs from bacterial, yeast, plant, and mammalian sources. The results provided vast insight into the molecular factors governing inhibitor potency in a bacterial (AGAO) and a plant (PSAO) CuAO. This was the defining work in establishing how certain aromatic residues in the substrate channel of AGAO directly control inhibitor potency. Furthermore, this work details the cross-reactivity between CuAOs and monoamine oxidase (MAO) enzymes using the antidepressant MAO-directed inhibitor tranylcypromine, and describes the characterization of a class of compounds which selectively inhibit only CuAOs. In addition to the studies aimed at understanding the molecular factors governing inhibitor binding in CuAOs, this work probes the role of copper during enzymatic reoxidation. Binding and inhibition studies of CuAOs were performed with the monodentate copper ligands cyanide and azide. Studies with cyanide provided the first spectroscopic evidence for cyanohydrin derivitization of TPQ in PSAO and led to the proposal of a detailed mechanism for inhibition of CuAO catalysis by cyanide. Azide studies in PSAO clearly revealed that azide was a competitive inhibitor towards substrate dioxygen, thereby supporting the viability of a redox role for copper. However, azide studies with a yeast CuAO were much more difficult to interpret, as azide was found to significantly inhibit the reaction velocity of both half-reactions. Lastly, temperature jump relaxation measurements of the internal redox equilibrium in AGAO were performed, allowing for the calculation of the electron transfer rate between reduced TPQ and copper(II). The results unequivocally establish that the copper(I)-semiquinone state is a viable catalytic intermediate in the reoxidation reaction of AGAO. Collectively, the results presented in this thesis provide evidence for a redox role of copper during the oxidative half-reaction, although further experimentation is necessary to confirm this.