Denitrification and aerobic respiration depend on the same core respiratory
machinery. This machinery consists of the NADH dehydrogenase
(complex I), the quinone pool, the bc1 complex (complex III) and
cytochrome c. Each of the two pathways adds its own specific modules
to this backbone. Aerobic oxidation requires a terminal oxidase (complex
IV) which accepts electrons either from cytochrome c or the quinone
pool. Denitrification consists of four modules: nitrate reductase, nitrite
reductase, nitric oxide reductase and nitrous oxide reductase. Organisms
that contain at least two or three of these enzymes and produce nitrous
oxide or dinitrogen gas will be referred to as “denitrifiers”.