Active site structure of the aa3 quinol oxidase of Acidianus ambivalens

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The membrane bound aa3-type quinol:oxygen oxidoreductase from the hyperthermophilic archaeon, Acidianus ambivalens, which thrives at a pH of 2.5 and a temperature of 80°C, has several unique structural and functional features as compared to the other members of the heme-copper oxygen reductase superfamily, but shares the common redox-coupled, proton-pumping function. To better understand the properties of the heme a3-CuB catalytic site, a resonance Raman spectroscopic study of the enzyme under a variety of conditions and in the presence of various ligands was carried out. Assignments of several heme vibrational modes as well as iron-ligand stretching modes are made to serve as a basis for comparing the structure of the enzyme to that of other oxygen reductases. The CO-bound oxidase has conformations that are similar to those of other oxygen reductases. However, the addition of CO to the resting enzyme does not generate a mixed valence species as in the bovine aa3 enzyme. The cyanide complex of the oxidized enzyme of A. ambivalens does not display the high stability of its bovine counterpart, and a redox titration demonstrates that there is an extensive heme-heme interaction reflected in the midpoint potentials of the cyanide adduct. The A. ambivalens oxygen reductase is very stable under acidic conditions, but it undergoes an earlier alkaline transition than the bovine enzyme. The A. ambivalens enzyme exhibits a redox-linked reversible conformational transition in the heme a 3-CuB center. The pH dependence and H/D exchange demonstrate that the conformational transition is associated with proton movements involving a group or groups with a pKa of ∼3.8. The observed reversibility and involvement of protons in the redox-coupled conformational transition support the proton translocation model presented earlier. The implications of such conformational changes are discussed in relation to general redox-coupled proton pumping mechanisms in the heme-copper oxygen reductases.

Original languageEnglish
Pages (from-to)306-320
Number of pages15
JournalBiochimica et Biophysica Acta - Bioenergetics
Issue number1-3
Publication statusPublished - 12 Apr 2004


  • Bioenergetics
  • Heme proteins
  • Midpoint potential
  • Proton translocation
  • Raman spectroscopy


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