The hemea3/CuB active site of cytochrome c oxidase functions to interconvert nitrite (NO2�) and nitric oxide (NO) as part of its role in regulation of O2 balance. In hypoxia, this binuclear center recycles nitrite to generate nitric oxide, thus down-regulating molecular oxygen consumption. In turn, in normoxia, the same center oxidizes NO back to nitrite. To elucidate the roles of metal centers in this redox interplay between nitrite and nitric oxide, we recently described a heme/Cu assembly that in its partially reduced form facilitates nitrite reduction to nitric oxide. In turn, the fully oxidized form, �-oxo-heme-FeIII-O-CuIIcomplex, oxidizes NO to nitrite. Our findings demonstrated that in this nitrite reductase chemistry, the heme provides the electron while the cupric center serves as a Lewis acid.In order to further investigate the role that the cupric center plays in providing Lewis acid interaction and facilitating nitrite (N�O) bond cleavage for nitrite reduction, copper(II)�nitrito complexes with tridentate and tetradentate ligands were studied. Moreover, to study the role of the reducing ability of the ferrous heme center, different heme-Fe(II) complexes with varying redox character were used. The results show that differing modes of nitrite coordination to the cupric center lead to differing kinetic behavior in nitrite reduction. Also, it has been observed that the reduction ability of the heme center does not play a significant role in the observed overall reaction rate.Furthermore, to obtain detailed mechanistic information/insights regarding the NO oxidase chemistry, different �-oxo-heme-FeIII-O-CuII complexes were utilized. Based on low temperature kinetic data, the key mechanistic steps in terms of heme/copper heterobinuclear structures have been deduced and will be presented and discussed.

Redox interplay between nitrite and nitric oxide interposed by a heme/Cu assembly

248th National Meeting (2014)

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