Oxyl and Hydroxyl Radical Transfer in Mitochondrial Amidoxime Reducing Component-Catalyzed Nitrite Reduction

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 a 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 ·NO from unprotonated 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 π-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.