A combination of electron paramagnetic resonance (EPR) spectroscopy and computational approaches has provided insight into the nature of the reaction coordinate for the one-electron, reduction of nitrite by the mitochondrial amidoxime reducing component (mARC) enzyme. The results show that paramagnetic Mo(V) species is generated when reduced enzyme is exposed to nitrite, and an analysis of the resulting EPR hyperfine parameters confirms that mARC is remarkably similar to the low-pH form of sulfite oxidase:, Two mechanisms for nitrite reduction have been considered. The first shows a modest reaction barrier of 14 kcal/mol for the formation of center dot NO from unprototated, nitrite substrate. In marked contrast, protonation of the substrate oxygen proximal to,Mo in the Mo(IV)-O-N-O substrate-bound species results in barrierless conversion to products. A fragment orbital analysis reveals a high degree of Mo-O(H)-N-O covalency that provides a pi- orbital pathway for one-electron transfer to the substrate and defines orbital constraints on the Mo substrate geometry for productive catalysis in mARC and other pyranopterin molybdenum enzymes that catalyze this one-electron transformation.