Functional role of Coenzyme Q in the energy coupling of NADH-CoQ
oxidoreductase (Complex I): Stabilization of the semiquinone state with the
application of inside-positive membrane potential to proteoliposomes
Issue title: The Fifth Conference of the International CoQ10
Association, Kobe 2007 – 50th anniversary of CoQ10 discovery
Affiliations: Dept of Biochemistry and Biophysics, and Johnson
Research Foundation, University of Pennsylvania School of Medicine,
Philadelphia, PA, USA | Department of Life Science, University of Hyogo,
Hyogo, Japan
Note: [] Address for correspondence: Tomoko Ohnishi, Ph.D., Department of
Biochemistry and Biophysics, and Johnson Research Foundation, University of
Pennsylvania School of Medicine, Philadelphia, PA 19104, USA. Tel.: +1 215 898
8024; Fax: +1 215 573 3748; E-mail: Ohnishi@mail.med.upenn.edu
Abstract: Coenzyme Q_{10} (which is also designated as CoQ_{10},
ubiquinone-10, UQ_{10}, CoQ, UQ or simply as Q) plays an important role in
energy metabolism. For NADH-Q oxidoreductase (complex I), Ohnishi and Salerno
proposed a hypothesis that the proton pump is operated by the redox-driven
conformational change of a Q-binding protein, and that the bound form of
semiquinone (SQ) serves as its gate [FEBS Letters 579 (2005) 45–55].
This was based on the following experimental results: (i) EPR signals of the
fast-relaxing SQ anion (designated as Q_{Nf}^{˙ -}) are observable
only in the presence of the proton electrochemical potential (Δμ_{H}^{+});
(ii) iron-sulfur cluster N2 and Q_{Nf}^{˙ -} are directly
spin-coupled; and (iii) their center-to-center distance was calculated as 12Å,
but Q_{Nf}^{˙ -} is only 5Å deeper than N2 perpendicularly to the
membrane. After the priming reduction of Q to Q_{Nf}^{˙ -}, the
proton pump operates only in the steps between the semiquinone anion (Q_{Nf}^{˙ -})
and fully reduced quinone (QH_{2}). Thus, by cycling twice
for one NADH molecule, the pump transports 4H^{+} per 2e^{-}. This hypothesis predicts the following phenomena: (a) Coupled with
the piericidin A sensitive NADH-DBQ or Q_{1} reductase reaction,
Δμ_{H}^{+} would be established; (b) Δμ_{H}^{+} would enhance
the SQ EPR signals; and (c) the dissipation of Δμ_{H}^{+} with
the addition of an uncoupler would increase the rate of NADH oxidation and
decrease the SQ signals. We reconstituted bovine heart complex I, which was prepared at
Yoshikawa's laboratory, into proteoliposomes. Using this system, we succeeded
in demonstrating that all of these phenomena actually took place. We believe
that these results strongly support our hypothesis.
