title: Use of an azido-ubiquinone derivative to identify subunit I as the ubiquinol binding site of the cytochrome d terminal oxidase complex of Escherichia coli.
abstract: The radiolabeled, photoreactive azido-ubiquinone derivative (azido-Q), 3-azido-2-methyl-5-methoxy-6-(3,7-dimethyl-[3H]octyl)- 1,4-benzoquinone, was used to investigate the active site of ubiquinol oxidase activity of the cytochrome d complex, a two-subunit terminal oxidase of Escherichia coli. The azido-Q, when reduced by dithioerythritol, was shown to support enzymatic oxygen consumption by the cytochrome d complex that was 8% of the rate observed with ubiquinol-1. This observation provided the rationale behind further studies of the possible photoinactivation and labeling of the active site by this azido-Q. Ten min of photolysis of the purified cytochrome d complex in the presence of the azido-Q resulted in a 60% loss of the ubiquinol-1 oxidase activity. Uptake of the radiolabeled azido-Q by the cytochrome d complex was correlated to the photoinactivation of the ubiquinol-1 oxidase activity. Both increased linearly during the first 4 min of photolysis and reached 90% of the maximum within 10 min. Photolysis times longer than 10 min resulted in no increase in the maximum of 2 mol of azido-Q incorporated per mol of enzyme. The rate of azido-Q uptake by subunit I, but not subunit II, correlated well with the rate of loss of ubiquinol oxidase activity. Use of ubiquinol-0, which is not oxidized by the enzyme, to competitively inhibit radiolabeling of nonspecific binding sites, resulted in a significant decrease (42%) of azido-Q labeling of subunit II while it did not affect the labeling of subunit I. After photolysis for 4 min, the ratio of radiolabeled azido-Q in subunits I to II of the complex was 4.3 to 1.0. These observations support the conclusion that the ubiquinol substrate binding site is located on subunit I of the cytochrome d complex.
mesh_terms: D001386:Azides / Q000378:metabolism*; D001665:Binding Sites; D003573:Cytochrome b Group; D003580:Cytochromes; D045222:Electron Transport Chain Complex Proteins; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins; D007700:Kinetics; D046911:Macromolecular Substances; D010088:Oxidoreductases / Q000378:metabolism*; D010782:Photolysis; D014451:Ubiquinone / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D001386:Azides; D003573:Cytochrome b Group; D003580:Cytochromes; D045222:Electron Transport Chain Complex Proteins; D029968:Escherichia coli Proteins; D046911:Macromolecular Substances; D014451:Ubiquinone; C048123:3-azido-2-methyl-5-methoxy-6-(3,7-dimethyloctyl)-1,4-benzoquinone; D010088:Oxidoreductases; C040077:cytochrome bd terminal oxidase complex, E coli; C003741:ubiquinol