Monomer and dimer structures of cytochromebo₃ubiquinol oxidase fromEscherichia coli

AbstractTheE. colicytochromebo3ubiquinol oxidase is a four-subunit heme-copper oxidase that serves as a proton pump in theE. coliaerobic respiratory chain. Despite many mechanistic studies on this protein, it is unclear whether this ubiquinol oxidase functions as a monomer, or as a dimer in a manner similar to its eukaryotic counterparts – the mitochondrial electron transport complexes. In this study, we determined the monomeric and dimeric structures of theE. colicytochromebo3ubiquinol oxidase reconstituted in amphipol by cryogenic electron microscopy single particle reconstruction (cryo-EM SPR) to a resolution of 3.15 Å and 3.46 Å, respectively. We have discovered that the protein can form a dimer in C2 symmetry, with the dimerization interface maintained by interactions between the subunit II of one monomer and the subunit IV of the other monomer. Moreover, the dimerization does not induce significant structural changes in each monomer, except the movement of a loop in subunit IV (residues 67–74).