From NO to OO: nitric oxide and dioxygen in bacterial respiration.

Nitric oxide reductase (NOR) is a key enzyme in denitrification, reforming the N-N bond (making N2O from two NO molecules) in the nitrogen cycle. It is a cytochrome bc complex which has apparently only two subunits, NorB and NorC. It contains two low-spin cytochromes (c and b), and a high-spin cytochrome b which forms a binuclear center with a non-heme iron. NorC contains the c-type heme and NorB can be predicted to bind the other metal centers. NorB is homologous to the major subunit of the heme/copper cytochrome oxidases, and NOR thus belongs to the superfamily, although it has an Fe/Fe active site rather than an Fe/Cu binuclear center and a different catalytic activity. Current evidence suggests that NOR is not a proton pump, and that the protons consumed in NO reduction are not taken from the cytoplasmic side of the membrane. Therefore, the comparison between structural and functional properties of NOR and cytochrome c- and quinol-oxidizing enzymes which function as proton pumps may help us to understand the mechanism of the latter. This review is a brief summary of the current knowledge on molecular biology, structure, and bioenergetics of NOR as a member of the oxidase superfamily.