3.D.4.4.1
Cytochrome oxidase including heme A synthase (HAS) and 4 subunits of the cytochrome oxidase.  The 3-d structure of heme A synthase (of 306 aas and 8 TMSs) at 2.2 Å resolution has been solved revealing that the N- and C-terminal halves of HAS consist of four-helix bundles and they align in a pseudo twofold symmetry manner. Each bundle contains a pair of histidine residues and forms a heme-binding domain. The C-half domain binds a cofactor-heme molecule, while the N-half domain is vacant (Niwa et al. 2018). The Sco1 protein, YpmQ, is an accessory protein involved in the assembly of cytochrome c oxidase (Andrews et al. 2004).