title: Identification of heme and copper ligands in subunit I of the cytochrome bo complex in Escherichia coli.
abstract: The cytochrome bo complex is a terminal ubiquinol oxidase in the aerobic respiratory chain of Escherichia coli (Kita, K., Konishi, K., and Anraku, Y. (1984) J. Biol. Chem. 259, 3368-3374) and functions as a proton pump. It belongs to the heme-copper oxidase superfamily with the aa3-type cytochrome c oxidases in mitochondria and aerobic bacteria. In order to identify ligands of hemes and copper, we have substituted eight conserved histidines in subunit I by alanine and, in addition, His-106, -284, and -421 by glutamine and methionine. Western immunoblotting analysis showed that all the mutations do not affect the expression level of subunit I in the cytoplasmic membrane, indicating that these histidines are not crucial for its stability. A single copy expression vector carrying a single mutation at the invariant histidines, His-106, His-284, His-333, His-334, His-419, and His-421, of subunit I was unable to support the aerobic growth of a strain in which the chromosomal terminal oxidase genes (the cyo and cyd operons) have been deleted. The same mutations caused a complete loss of ubiquinol oxidase activity of the partially purified enzymes. Spectroscopic analysis of mutant oxidases in the cytoplasmic membrane revealed that substitutions of His-106 and -421 specifically eliminated a 563.5 nm peak of the low spin heme and that replacements of His-106, -284, and -419 reduced the extent of the CO-binding high spin heme. These spectroscopic properties of mutant oxidases were further confirmed with partially purified preparations. Atomic absorption analysis showed that substitutions of His-106, -333, -334, and -419 eliminated CuB almost completely. Based on these findings, we conclude that His-106 and -421 function as the axial ligands of the low spin heme and His-284 is a possible ligand of the high spin heme. His-333, -334, and -419 residues are attributed to the ligands of CuB. We present a helical wheel model of the redox center in subunit I, which consists of the membrane-spanning regions II, VI, VII, and X, and discuss the implications of the model.
authors.affiliation: Department of Biology, Faculty of Science, University of Tokyo, Japan.
authors.forename: J
authors.initials: J
authors.lastname: Minagawa
authors.forename: T
authors.initials: T
authors.lastname: Mogi
authors.forename: R B
authors.initials: RB
authors.lastname: Gennis
authors.forename: Y
authors.initials: Y
authors.lastname: Anraku
mesh_terms: D000332:Aerobiosis; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D003300:Copper / Q000378:metabolism*; D004269:DNA, Bacterial / Q000235:genetics; D004227:Dithionite / Q000494:pharmacology; D003576:Electron Transport Complex IV / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D005816:Genetic Complementation Test; D006418:Heme / Q000378:metabolism*; D006801:Humans; D008024:Ligands; D046911:Macromolecular Substances; D008961:Models, Structural; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009838:Oligodeoxyribonucleotides; D010084:Oxidation-Reduction; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D015183:Restriction Mapping; D012689:Sequence Homology, Nucleic Acid
chemical_list: D004269:DNA, Bacterial; D008024:Ligands; D046911:Macromolecular Substances; D009838:Oligodeoxyribonucleotides; D011994:Recombinant Proteins; D004227:Dithionite; D006418:Heme; D003300:Copper; C038862:cytochrome o oxidase; D003576:Electron Transport Complex IV