Heme- and copper-containing terminal oxidases (HCOs) function as cytochrome c oxidases or as quinol oxidases in aerobic respiration but also play a role in oxygen scavenging and in maintaining redox homeostasis. HCOs are diverse in terms of subunit composition, electron donor, and heme type, resulting in different electron and proton transfer characteristics. Based on overall amino acid similarities of the central subunit and specific differences of the proton channels, the HCO superfamily is subdivided into three major families: A, B, and C. The prototypes of family C, comprising ~20% of HCOs, are the cbb3 oxidases. Family-C HCOs contain the central subunit (N) and either one (O) or two (O + P) additional subunits that are completely different from family A and B subunits. This entry represents the N-terminal domain of the P-subunit which is made up of 2 helices PMID:20576851.