title: Direct evidence for the protonation of aspartate-75, proposed to be at a quinol binding site, upon reduction of cytochrome bo3 from Escherichia coli.
abstract: Aspartate-75 (D75) was recently suggested to participate in a ubiquinone-binding site in subunit I of cytochrome bo(3) from Escherichia coli on the basis of a structural model [Abramson, J., Riistama, S., Larsson, G., Jasaitis, A., Svensson-Ek, M., Laakkonen, L., Puustinen, A., Iwata, S., and Wikström, M. (2000) Nat. Struct. Biol. 7 (10), 910-917]. We studied the protonation state of D75 for the reduced and oxidized forms of the enzyme, using a combined site-directed mutagenesis, electrochemical, and FTIR spectroscopic approach. The D75H mutant is catalytically inactive, whereas the more conservative D75E substitution has quinol oxidase activity equal to that of the wild-type enzyme. Electrochemically induced FTIR difference spectra of the inactive D75H mutant enzyme show a clear decrease in the spectroscopic region characteristic of protonated aspartates and glutamates. Strong variations in the amide I region of the FTIR difference spectrum, however, reflect a more general perturbation due to this mutation of both the protein and the bound quinone. Electrochemically induced FTIR difference spectra on the highly conservative D75E mutant enzyme show a shift from 1734 to 1750 cm(-1) in direct comparison to wild type. After H/D exchange, the mode at 1750 cm(-1) shifts to 1735 cm(-1). These modes, concomitant with the reduced state of the enzyme, can be assigned to the nu(C=O) vibrational mode of protonated D75 and E75, respectively. In the spectroscopic region where signals for deprotonated acidic groups are expected, band shifts for the nu(COO(-))(s/as) modes from 1563 to 1554-1539 cm(-1) and from 1315 to 1336 cm(-1), respectively, are found for the oxidized enzyme. These signals indicate that D75 (or E75 in the mutant) is deprotonated in the oxidized form of cytochrome bo(3) and is protonated upon full reduction of the enzyme. It is suggested that upon reduction of the bound ubiquinone at the high affinity site, D75 takes up a proton, possibly sharing it with ubiquinol.
authors.affiliation: Department of Biochemistry, University of Illinois, 600 South Mathews Avenue, Urbana, Illinois 61801-3364, USA.
authors.forename: P
authors.initials: P
authors.lastname: Hellwig
authors.forename: B
authors.initials: B
authors.lastname: Barquera
authors.forename: R B
authors.initials: RB
authors.lastname: Gennis
mesh_terms: D000577:Amides; D001224:Aspartic Acid / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites / Q000235:genetics; D003573:Cytochrome b Group; D003580:Cytochromes / Q000235:genetics / Q000378:metabolism*; D004563:Electrochemistry; D003576:Electron Transport Complex IV / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics; D029968:Escherichia coli Proteins; D018698:Glutamic Acid / Q000235:genetics / Q000378:metabolism; D006639:Histidine / Q000235:genetics; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism; D011522:Protons; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D014451:Ubiquinone / Q000031:analogs & derivatives* / Q000378:metabolism*
chemical_list: D000577:Amides; D003573:Cytochrome b Group; D003580:Cytochromes; D029968:Escherichia coli Proteins; D011522:Protons; C083850:cytochrome bo3, E coli; D014451:Ubiquinone; D001224:Aspartic Acid; D018698:Glutamic Acid; D006639:Histidine; D010088:Oxidoreductases; C020094:duroquinol oxidase; D003576:Electron Transport Complex IV; C003741:ubiquinol