FIGURE 1.

Respiratory chains of denitrification and oxygen reduction. Electron flow is represented by green arrows; substrate flow is represented by blue arrows. Electrons move from NADH to nitrite reductases (NirS or NirK) and nitric oxide reductases (qNor or cNor) or to oxygen oxidoreductase (Complex IV) via Complex I, quinone/quinol pool, Complex III and cytochrome c. Alternative pathways in electron flow (i.e., cytochrome c electron flow to Complex IV and cNor versus quinone flow to qNOr) or in substrate (i.e., oxygen reduction in Complex IV versus nitric oxide (NO) reduction by qNor or cNor) are depicted by dashed green and blue arrows, respectively. Complexes of the oxygen respiratory pathway are drawn in blue and those of the denitrification pathway in orange, except for Complex IV, qNor and cNor, which are in gradient color and represented by a unique module to point out their common evolutionary history.