Figure 1.

(a–c) Schematic of how O₂Rs and (d–g) NORs may be functionally integrated in (aerobic and anaerobic) respiratory chains. Nar, Nir and N₂OR stand for nitrate, nitrite and nitrous oxide reductases, respectively, QH₂ denotes quinols. The ‘p-’ and ‘n-’ prefixes denote Nar enzymes the catalytic sites of which are located on the positive (typically outside) and negative (typically inside) faces of the chemiosmotic membrane, respectively. Dotted arrows indicate electron transfer, dashed ones stand for transport of substrates and continuous arrows denote substrate conversions. Lighter shade enzymes and arrows indicate reactions and electron transfer steps, respectively, that may be absent in a given scheme (e.g. nitrate- and N₂O reduction are not part of all chains). Electron donors can be located on either side of the membrane according to their chemical nature and specific chain but these different locations are not indicated in the scheme for reasons of simplicity.