title: The haem b558 component of the cytochrome bd quinol oxidase complex from Escherichia coli has histidine-methionine axial ligation.
abstract: The cytochrome bd ubiquinol oxidase from Escherichia coli is induced when the bacteria are cultured under microaerophilic or low-aeration conditions. This membrane-bound respiratory oxidase catalyses the two-electron oxidation of ubiquinol and the four-electron reduction of dioxygen to water. The oxidase contains three haem prosthetic groups: haem b558, haem b595 and haem d. Haem d is the oxygen binding site, and it is likely that haem d and b595 form a bimetallic site in the enzyme. Haem b558 has been previously characterized spectroscopically as being low spin and has been shown to be located within subunit I (CydA) of this two-subunit enzyme. It is likely that haem b558 is associated with the quinol oxidation site, which has also been shown to be within subunit I. In a previous effort to locate the specific amino acids axially ligated to haem b558, all six histidines within subunit I were altered by site-directed mutagenesis. Only one, histidine-186, was identified as a likely ligand to haem b558. Hence it was suggested that haem b558 could not have bis(histidine) ligation. In the current work, a combination of low-temperature near-infrared magnetic circular dichroism (NIR-MCD) and EPR spectroscopies have been employed to identify the nature of the haem b558 axial ligands. The NIR-MCD spectrum at cryogenic temperatures is dominated by the low-spin haem b558 component of the complex, and the low-energy band near 1800 nm is strong evidence for histidine-methionine ligation. It is concluded that haem b558 is ligated to histidine-186 plus one of the methionines located within subunit I of the oxidase.
authors.affiliation: School of Chemical Sciences, University of East Anglia, Norwich, U.K.
authors.forename: F
authors.initials: F
authors.lastname: Spinner
authors.forename: M R
authors.initials: MR
authors.lastname: Cheesman
authors.forename: A J
authors.initials: AJ
authors.lastname: Thomson
authors.forename: T
authors.initials: T
authors.lastname: Kaysser
authors.forename: R B
authors.initials: RB
authors.lastname: Gennis
authors.forename: Q
authors.initials: Q
authors.lastname: Peng
authors.forename: J
authors.initials: J
authors.lastname: Peterson
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry; D002942:Circular Dichroism; D003573:Cytochrome b Group / Q000737:chemistry*; D003580:Cytochromes / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D045222:Electron Transport Chain Complex Proteins; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins; D006418:Heme / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry; D008715:Methionine / Q000737:chemistry; D019255:NADPH Oxidases; D010088:Oxidoreductases / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D001426:Bacterial Proteins; D003573:Cytochrome b Group; D003580:Cytochromes; D045222:Electron Transport Chain Complex Proteins; D029968:Escherichia coli Proteins; D006418:Heme; D006639:Histidine; C036888:cytochrome b558; D008715:Methionine; D010088:Oxidoreductases; D019255:NADPH Oxidases; C040077:cytochrome bd terminal oxidase complex, E coli