3.D.4.3.2
Cytochrome bd quinol oxidoreductase, CydA/CydB. Borisov et al. (2011) have presented evidence concerning a proton channel connecting the site of oxygen reduction to the bacteria cytoplasm and the molecular mechanism by which a membrane potential is generated. The CydX protein of 37 aas and 1 TMS, is encoded in the cydAB operon and functions as a subunit of the Cytochrome bd oxidase complex, activating its activity (VanOrsdel et al. 2013). The AppX protein of 30 aas and 1 TMS, is a paralogue of CydX and can substitute for it in activating the Cytochrome bd oxidase complex (VanOrsdel et al. 2013).  Cytochrome bd-type quinol oxidases are structurally unrelated to mitochondrial cytochrome c oxidases. Safarian et al. 2019 determined the structure of the E. coli cytochrome bd-I oxidase by single-particle cryo-electron microscopy to a resolution of 2.7 angstroms. The structure contains a previously unknown accessory subunit CydH, the L-subfamily-specific Q-loop domain, a structural ubiquinone-8 cofactor, an active-site density interpreted as dioxygen, distinct water-filled proton channels, and an oxygen-conducting pathway. Comparison with another cytochrome bd oxidase revealed structural divergence in the family, including rearrangement of high-spin hemes and conformational adaption of a transmembrane helix to generate a distinct oxygen-binding site (Safarian et al. 2019). Subunit I of the cytochrome bd quinol oxidase from E. coli has nine transmembrane helices with the O₂ reactive site near the periplasmic surface (Zhang et al. 2004). Two small proteins, YtkA (CtaK; 145 aas; P40768) and YczB (CtaM; 70 aas; O31467; TatAd) facilitate the biogenesis of cytochrome c oxidase in Bacillus subtilis (von Wachenfeldt et al. 2021). The latter protVanOrsdel et al. 2013). The AppX protein of 30 aas and 1 TMS, is a paralogue of CydX and can substitute for it in activating the Cytochrome bd oxidase complex (VanOrsdel et al. 2013).  Cytochrome bd-type quinol oxidases are structurally unrelated to mitochondrial cytochrome c oxidases. Safarian et al. 2019 determined the structure of the E. coli cytochrome bd-I oxidase by single-particle cryo-electron microscopy to a resolution of 2.7 angstroms. The structure contains a previously unknown accessory subunit CydH, the L-subfamily-specific Q-loop domain, a structural ubiquinone-8 cofactor, an active-site density interpreted as dioxygen, distinct water-filled proton channels, and an oxygen-conducting pathway. Comparison with another cytochrome bd oxidase revealed structural divergence in the family, including rearrangement of high-spin hemes and conformational adaption of a transmembrane helix to generate a distinct oxygen-binding site (Safarian et al. 2019). Subunit I of the cytochrome bd quinol oxidase from E. coli has nine transmembrane helices with the O₂ reactive site near the periplasmic surface (Zhang et al. 2004). Two small proteins, YtkA (CtaK; 145 aas; P40768) and YczB (CtaM; 70 aas; O31467; TatAd) facilitate the biogenesis of cytochrome c oxidase in Bacillus subtilis (von Wachenfeldt et al. 2021). The latter protein may also function as a constituent of the Tat system (TC# 2.A.64.3.1).