While nitric,oxide (NO, nitrogen monoxide) is a critically important signaling agent, its cellular concentrations must be tightly controlled, generally through its oxidative conversion to nitrite (NO2-) where it is held in reserve to be reconverted as needed. In part, this reaction is mediated by the binuclear heme a(3)/Cu-B active site of cytochrome c oxidase. In this report, the oxidation of NO(g) to nitrite is shown to occur efficiently in new synthetic mu-oxo heme-Fe-III-O-Cu-II(L) constructs being a tridentate or tetradentate pyridyl/alkylamino ligand), and spectroscopic and kinetic investigations provide detailed mechanistic insights. Two new X-ray Structures of mu-oxo complexes have been determined and compared to literature analogs. All mu-oxo complexes react with 2 mol equiv NO(g) to give 1:1 mixtures of discrete [(L)Cu-II(NO2-)](+) plus ferrous heme-nitrosyl compounds; when the first NO(g) equiv reduces the heme center and itself is oxidized to nitrite, the second equiv of NO(g) traps the ferrous heme thus formed. For one mu-oxo heme-Fe-III-O-Cu-II(L),compound, the reaction with NO(g) reveals an intermediate species, ("intermediate"), formally a his-NO adduct, [(NO)(porphyrinate)Fe-II-(NO2-)-Cu-II(L)](+) (lambda(max) = 433 nm), confirmed by cryo-spray ionization mass spectrometry and EPR spectroscopy, along with the observation that cooling a 1:1 mixture of [(L)CUII(NO2-)](+) and heme-Fell(NO) to -125 degrees C leads to association and generation of the key 433 nm UV-vis feature. Kinetic-thermodynamic parameters obtained from low-temperature stopped-flow measurements are in excellent agreement with DFT calculations carried out which describe the sequential addition of NO(g) to the mu-oxo complex.