Since the advent of the Haber-Bosch process to industrially generate ammonia (NH3) in the early 1900s, the widespread usage of synthetic NH3-based fertilizers has caused drastic changes in the balance of the biogeochemical nitrogen cycle. Most importantly, the increased bioavailable NH3 has led to a significant increase in environmentally harmful NxOy species formed primarily through nitrification. Nitrifiers are organisms that derive their metabolic energy via the oxidation of NH3 to nitrite (NO2–), utilizing transition metal-dependent metalloenzymes to perform these transformations. The most prevalent of these organisms are NH3-oxidizing bacteria (AOB) and NH3-oxidizing archaea (AOA). Of particular interest are the enzymes responsible for hydroxylamine (NH2OH) oxidation and nitrous oxide (N2O) formation. Though only recently discovered in 2005, AOA were found to be abundant in most environments where NH3 is available, as well as the primary N2O sources in many. Despite their ubiquity, the enzymology of primary metabolism and general N-oxide transformations in AOA is largely unknown.This thesis describes work toward a biochemical understanding of AOA metabolism and metalloenzymology. Of particular interest were enzymes responsible for NH2OH oxidation, N2O formation, and NO transformations in AOA. Despite the lack of existing biochemical knowledge concerning these enzymes, we have been able to use genomic, transcriptomic, and proteomic data to home in on proteins likely responsible for this chemistry. AOA encode for many proteins containing type 1 (blue) Cu sites, including many multicopper oxidases (MCOs). This thesis presents the structural, spectroscopic, and enzymatic reactivity of some of these MCOs, including a plausible enzymatic origin of N2O produced by AOA. Multiple clades of these MCOs in AOA have the ability to perform the two-electron oxidation of NH2OH to nitroxyl (HNO), which then goes on to form N2O. Mechanistic studies of this unique reactivity were performed using point-mutated variants, revealing the importance of an additional blue Cu site in one clade of these MCOs. Finally, this thesis will present the reactivity of these archaeal MCOs with other substrates, including candid discussion on the potential biological significance of the observed reactivity of these MCOs within the context of what is known about AOA lifestyle traits.