Cytochrome c oxidase plays a key role in the respiratory chain of mitochondria and aerobic prokaryotes. This electron transfer complex consists of three subunits, SU I/III, IIc and IV, which together form a bundle of 23 transmembrane (TM) helices and a bulky extra-membrane cytochrome c/cupredoxin domain. This family represents subunit IV, which shows two connected TM helices [1]. It is predominantly found in Deinococcus-Thermus.

1: Structural insights into electron transfer in caa3-type cytochrome oxidase. Lyons JA, Aragao D, Slattery O, Pisliakov AV, Soulimane T, Caffrey M; Nature. 2012;487:514-518. PMID:22763450