Comparison between the nitric oxide reductase family and its aerobic relatives, the cytochrome oxidases

The denitrification pathway has been studied in the hyperthermophilic archaeon Pyrobaculum aerophilum. In contrast with Gram-negative bacteria, all four denitrification enzymes are membrane-bound. P. aerophilum is also the only denitrifyer identified so far in which menaquinol is the electron donor to all four denitrification reductases. The NO reductase NOR) of P. aerophilum belongs to the superfamlily of haem-copper oxidases and is of the qNOR (quinol-dependent) type. Three types of NOR have been purified so far: cNOR (cytochrome c/pseudoazurindependent), qNOR and qCu(A)NOR (qNOR that contains Cu-A at the electron entry site). It is proposed that the NORs and the various cytochrome oxidases have evolved by modular evolution, in view of the structure of their electron donor sites. qNOR is further proposed to be the ancestor of all NORs and cvtochrome oxidases belonging to the superfamily of haern-copper oxidases.