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Cytochrome cbb oxidase CcoQ. Cytochrome cbb3 oxidase, the terminal oxidase in the respiratory chains of proteobacteria, is a multi-chain transmembrane protein located in the cell membrane. Like other cytochrome oxidases, it catalyzes the reduction of O2 and simultaneously pumps protons across the membrane. Found exclusively in proteobacteria, cbb3 is believed to be a modern enzyme that has evolved independently to perform a specialized function in microaerobic energy metabolism. The cbb3 operon contains four genes (ccoNOQP or fixNOQP), with ccoN coding for subunit I. Instead of a CuA-containing subunit II analogous to other cytochrome oxidases, cbb3 utilizes subunits ccoO and ccoP, which contain one and two hemes, respectively, to transfer electrons to the binuclear center. ccoQ, the fourth subunit, is a single transmembrane helix protein. It has been shown to protect the core complex from proteolytic degradation by serine proteases. See cd00919, cd01322, or cd01323 for more information on cbb3 oxidase.
Jiyao W et al.(2023). "The conserved domain database in 2023.",