RESUMO
Cytochrome bd is a quinol oxidase of Escherichia coli under microaerophilic growth conditions. Coupling of the release of protons to the periplasm by quinol oxidation to the uptake of protons from the cytoplasm for dioxygen reduction generates a proton motive force. On the basis of sequence analysis, glutamates 99 and 107 conserved in transmembrane helix III of subunit I have been proposed to convey protons from the cytoplasm to heme d at the periplasmic side. To probe a putative proton channel present in subunit I of E. coli cytochrome bd, we substituted a total of 10 hydrophilic residues and two glycines conserved in helices I and III-V and examined effects of amino acid substitutions on the oxidase activity and bound hemes. We found that Ala or Leu mutants of Arg9 and Thr15 in helix I, Gly93 and Gly100 in helix III, and Ser190 and Thr194 in helix V exhibited the wild-type phenotypes, while Ala and Gln mutants of His126 in helix IV retained all hemes but partially lost the activity. In contrast, substitutions of Thr26 in helix I, Glu99 and Glu107 in helix III, Ser140 in helix IV, and Thr187 in helix V resulted in the concomitant loss of bound heme b558 (T187L) or b595-d (T26L, E99L/A/D, E107L/A/D, and S140A) and the activity. Glu99 and Glu107 mutants except E107L completely lost the heme b595-d center, as reported for heme b595 ligand (His19) mutants. On the basis of this study and previous studies, we propose arrangement of transmembrane helices in subunit I, which may explain possible roles of conserved hydrophilic residues within the membrane.
Assuntos
Citocromos/química , Complexo de Proteínas da Cadeia de Transporte de Elétrons/química , Proteínas de Escherichia coli/química , Ácido Glutâmico/química , Heme/análogos & derivados , Proteínas de Membrana/química , Oxirredutases/química , Subunidades Proteicas/química , Aerobiose/genética , Sequência de Aminoácidos , Substituição de Aminoácidos/genética , Sítios de Ligação/genética , Sequência Conservada/genética , Grupo dos Citocromos b , Citocromos/genética , Citocromos/metabolismo , Complexo de Proteínas da Cadeia de Transporte de Elétrons/genética , Complexo de Proteínas da Cadeia de Transporte de Elétrons/metabolismo , Ativação Enzimática/genética , Proteínas de Escherichia coli/genética , Proteínas de Escherichia coli/metabolismo , Ácido Glutâmico/genética , Ácido Glutâmico/metabolismo , Heme/química , Heme/metabolismo , Leucina/química , Leucina/genética , Leucina/metabolismo , Proteínas de Membrana/genética , Proteínas de Membrana/metabolismo , Dados de Sequência Molecular , Oxirredução , Oxirredutases/genética , Oxirredutases/metabolismo , Oxigênio/química , Oxigênio/metabolismo , Consumo de Oxigênio/genética , Estrutura Secundária de Proteína/genética , Subunidades Proteicas/genética , Subunidades Proteicas/metabolismo , Ubiquinona/análogos & derivados , Ubiquinona/químicaRESUMO
Recent proteome studies on the Escherichia coli membrane proteins suggested that YhcB is a putative third subunit of cytochrome bd-type ubiquinol oxidase (CydAB) (F. Stenberg, P. Chovanec, S.L. Maslen, C.V. Robinson, L.L. Ilag, G. von Heijne, D.O. Daley, Protein complexes of the Escherichia coli cell envelope. J. Biol. Chem. 280 (2005) 34409-34419). We isolated and characterized cytochrome bd from the DeltayhcB strain, and found that the formation of the CydAB heterodimer, the spectroscopic properties of bound hemes, and kinetic parameters for the ubiquinol-1 oxidation were identical to those of cytochrome bd from the wild-type strain. Anion-exchange chromatography and SDS-polyacrylamide gel electrophoresis showed that YhcB was not associated with the cytochrome bd complex. We concluded that YhcB is dispensable for the assembly and function of cytochrome bd. YhcB, which is distributed only in gamma-proteobacteria, may be a part of another membrane protein complex or may form a homo multimeric complex.
Assuntos
Proteínas de Escherichia coli/fisiologia , Escherichia coli/enzimologia , Oxirredutases/metabolismo , Oxirredutases/fisiologia , Eletroforese em Gel de Poliacrilamida , Peso Molecular , Oxirredutases/química , Estrutura Secundária de ProteínaRESUMO
To probe the structure of the quinol oxidation site in loop VI/VII of the Escherichia coli cytochrome bd, we substituted three conserved residues (Gln249, Lys252, and Glu257) in the N-terminal region and three glutamates (Glu278, Glu279, and Glu280) in the first internal repeat. We found that substitutions of Glu257 by Ala or Gln, and Glu279 and Glu280 by Gln, severely reduced the oxidase activity and the expression level of cytochrome bd. In contrast, Lys252 mutations reduced only the oxidase activity. Blue shifts in the 440 and 630 nm peaks of the reduced Lys252 mutants and in the 561 nm peak of the reduced Glu257 mutants indicate the proximity of Lys252 to the heme b(595)-d binuclear center and Glu257 to heme b(558), respectively. Perturbations of reduced heme b(558) upon binding of aurachin D support structural changes in the quinol-binding site of the mutants. Substitutions of Lys252 and Glu257 caused large changes in kinetic parameters for the ubiquinol-1 oxidation. These results indicate that Lys252 and Glu257 in the N-terminal region of the Q-loop are involved in the quinol oxidation by bd-type terminal oxidase.
