Stabilization of a semiquinone radical at the high‐affinity quinone‐binding site (QH) of the Escherichia coli bo‐type ubiquinol oxidase

FEBS Letters

Anraku, Yasuhiro²  Matsuura, Katsumi³  Mogi, Tatsushi²  Sato-Watanabe, Mariko²  Itoh, Sigeru¹  Miyoshi, Hideto⁴ 

[1] National Institute for Basic Biology, Okazaki National Research Institutes, Myodaiji, Okazaki 444, Japan;Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan;Department of Biology, Faculty of Science, Tokyo Metropolitan University, Minamiohsawa, Hachioji, Tokyo 192-03, Japan;Department of Agricultural Chemistry, Kyoto University, Sakyo-ku, Kyoto 606, Japan

关键词: E. coli bo-type ubiquinol oxidase;    Quinol oxidation;    Semiquinone radical;    EPR;    Potentiometric titration;    Intramolecular electron transfer;    Q8H2;    ubiquinol-8;    QH site;    a high-affinity quinone-binding site;    QL site;    a low-affinity quinol oxidation site;    QH;    ubiquinone-8 bound at the QH site;    Qn;    ubiquinone-n;    Q·−/QH2;    semiquinone/quinol;    Q/Q·−;    quinone/semiquinone;    Q/QH2;    quinone/quinol;   

DOI  :  10.1016/0014-5793(95)01125-X

学科分类：生物化学/生物物理

来源: John Wiley & Sons Ltd.

【 摘 要 】

Reaction of ubiquinone in the high-affinity quinone-binding site (Q_H) in bo-type ubiquinol oxidase from Escherichia coli was revealed by EPR and optical studies. In the Q_H site, ubiquinol was shown to be oxidized to ubisemiquinone and to ubiquinone, while no semiquinone signal was detected in the oxidase isolated from mutant cells that cannot synthesize ubiquinone. The Q_H site highly stabilized ubisemiquinone radical with a stability constant of 1–4 at pH 8.5 and the stability became lower at the lower pH. Midpoint potential of QH₂/Q couple was −2 mV at pH 8.5 and showed −60 mV/pH dependence indicative of 2H⁺/2e⁻ reaction. The E _m was more negative than that of low-spin heme b above pH 7.0. We conclude that the Q_H mediates intramolecular electron transfer from ubiquinol in the low-affinity quinol oxidation site (Q_L) to low-spin heme b. Unique roles of the quinone-binding sites in the bacterial ubiquinol oxidase are discussed.

【 授权许可】

Unknown