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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
Author(s) -
Ohnishi Tomoko,
Ohnishi S. Tsuyoshi,
ShinzawaIto Kyoko,
Yoshikawa Shinya
Publication year - 2008
Publication title -
biofactors
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.204
H-Index - 94
eISSN - 1872-8081
pISSN - 0951-6433
DOI - 10.1002/biof.5520320103
Subject(s) - semiquinone , oxidoreductase , chemistry , coenzyme q – cytochrome c reductase , stereochemistry , electron paramagnetic resonance , electron transport complex i , reductase , crystallography , proton pump , quinone , enzyme , nuclear magnetic resonance , cytochrome c , biochemistry , atpase , physics , mitochondrion
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 .− Nfare directly spin‐coupled; and (iii) their center‐to‐center distance was calculated as 12Å, but Q .− Nfis only 5Å deeper than N2 perpendicularly to the membrane. After the priming reduction of Q to Nf .− 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 + Hwould be established; (b) Δμ + Hwould enhance the SQ EPR signals; and (c) the dissipation of Δμ + Hwith 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.