Cytochrome cbb3 oxidase subunit I. 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 mainly in proteobacteria, cbb3 is believed to be a modern enzyme that has evolved independently to perform a specialized function in microaerobic energy metabolism. Subunit I contains a heme-copper binuclear center (the active site where O2 is reduced to water) formed by a high-spin heme and a copper ion. It also contains a low-spin heme, believed to participate in the transfer of electrons to the binuclear center. 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. The fourth subunit (ccoQ) has been shown to protect the core complex from proteolytic degradation by serine proteases. For every reduction of an O2 molecule, eight protons are taken from the inside aqueous compartment and four electrons are taken from cytochrome c on the opposite side of the membrane. The four electrons and four of the protons are used in the reduction of O2; the four remaining protons are pumped across the membrane. This charge separation of four charges contributes to the electrochemical gradient used for ATP synthesis. The polar residues that form the D- and K-pathways in subunit I of other cytochrome c and ubiquinol oxidases are absent in cbb3. The proton pathways remain undefined. A pathway for the transfer of pumped protons beyond the binuclear center also remains undefined. It is believed that electrons are passed from cytochrome c (the electron donor) to the low-spin heme via ccoP and ccoO, respectively, and directly from the low-spin heme to the binuclear center.
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