The investigation of respiratory N-oxide reduction as part of a biogeochemical process sustained by prokaryotes has its roots over a century ago and has laid the groundwork for microbial nitric oxide (NO) biology and recognition that NO is of bioenergetic importance in anaerobic environments. NO is an obligatory respiratory substrate of nitrate- and nitrite-denitrifying prokaryotes that release nitrous oxide (N2O) or dinitrogen as products. We witness currently a broadened scope of NO functionality and an increase in awareness that other heme-based NO-metabolizing systems contribute to the overall capability of the prokaryotic cell to cope with NO both in anaerobic and aerobic environments, including the pathogen-host interface. NO reduction of newly recognized physiological importance is catalyzed by the pentaheme nitrite reductase, cytochrome c', flavohemoglobin and flavorubredoxin. Respiratory NO reductases are heme-nonheme Fe proteins that can be classified either in a short-chain group, which are complexes with cytochrome c, or a long-chain group, which have a fused quinol oxidase domain. Even though NO reductases are not proton pumps, both reductase groups are structural homologues of heme-copper oxidases. As a unique case, the short-chain NO reductase of Roseobacter denitrificans acts on oxygen, based presumably on a heme b3–CuB
center. In turn, certain heme-copper oxidases have significant turnover rates with NO. NO reductase mechanisms have been proposed from oxidase active site chemistry. Besides being a respiratory substrate, NO is also a signaling molecule that triggers gene expression of the principal components of NO respiration by members of the Crp-Fnr superfamily of transcription regulators.