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Stabilization of a semiquinone radical at the high‐affinity quinone‐binding site (Q H ) of the Escherichia coli bo ‐type ubiquinol oxidase
Author(s) -
Sato-Watanabe Mariko,
Itoh Sigeru,
Mogi Tatsushi,
Matsuura Katsumi,
Miyoshi Hideto,
Anraku Yasuhiro
Publication year - 1995
Publication title -
febs letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.593
H-Index - 257
eISSN - 1873-3468
pISSN - 0014-5793
DOI - 10.1016/0014-5793(95)01125-x
Subject(s) - semiquinone , ubiquinol , chemistry , quinone , oxidase test , stereochemistry , photochemistry , coenzyme q – cytochrome c reductase , electron paramagnetic resonance , binding site , biochemistry , enzyme , cytochrome c , nuclear magnetic resonance , mitochondrion , physics
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 QH 2 /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.