Asymmetric binding of the high-affinity Q(H)(*)(-) ubisemiquinone in quinol oxidase (bo3) from Escherichia coli studied by multifrequency electron paramagnetic resonance spectroscopy

Biochemistry. 2003 May 20;42(19):5632-9. doi: 10.1021/bi034010z.


Ubiquinone-2 (UQ-2) selectively labeled with (13)C (I =(1)/(2)) at either the position 1- or the 4-carbonyl carbon is incorporated into the ubiquinol oxidase bo(3) from Escherichia coli in which the native quinone (UQ-8) has been previously removed. The resulting stabilized anion radical in the high-affinity quinone-binding site (Q(H)(*)(-)) is investigated using multifrequency (9, 34, and 94 GHz) electron paramagnetic resonance (EPR) spectroscopy. The corresponding spectra reveal dramatic differences in (13)C hyperfine couplings indicating a strongly asymmetric spin density distribution over the quinone headgroup. By comparison with previous results on labeled ubisemiquinones in proteins as well as in organic solvents, it is concluded that Q(H)(*)(-) is most probably bound to the protein via a one-sided hydrogen bond or a strongly asymmetric hydrogen-bonding network. This observation is discussed with regard to the function of Q(H) in the enzyme and contrasted with the information available on other protein-bound semiquinone radicals.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Binding Sites
  • Coenzymes
  • Electron Spin Resonance Spectroscopy
  • Escherichia coli / enzymology
  • Hydrogen Bonding
  • Kinetics
  • Models, Molecular
  • Oxidoreductases / chemistry*
  • Oxidoreductases / metabolism*
  • Static Electricity
  • Ubiquinone / analogs & derivatives*
  • Ubiquinone / chemistry*
  • Ubiquinone / metabolism*


  • Coenzymes
  • Ubiquinone
  • Oxidoreductases
  • duroquinol oxidase
  • coenzyme Q10