This family are the alternative oxidases found in many bacteria which oxidise ubiquinol and reduce oxygen as part of the electron transport chain. This family is the subunit I of the oxidase E. coli has two copies of the oxidase, bo and bd', both of which are represented here In some nitrogen fixing bacteria, e.g. Klebsiella pneumoniae this oxidase is responsible for removing oxygen in microaerobic conditions, making the oxidase required for nitrogen fixation. This subunit binds a single b-haem, through ligands at His186 and Met393 (using SW:P11026 numbering). In addition His19 is a ligand for the haem b found in subunit II

1: The Klebsiella pneumoniae cytochrome bd' terminal oxidase complex and its role in microaerobic nitrogen fixation. Juty NS, Moshiri F, Merrick M, Anthony C, Hill S; Microbiology 1997;143:2673-2683. PMID:9274021