title: Transient formation of a neutral ubisemiquinone radical and subsequent intramolecular electron transfer to pyrroloquinoline quinone in the Escherichia coli membrane-integrated glucose dehydrogenase.
abstract: The membrane-bound quinoprotein glucose dehydrogenase (mGDH) in Escherichia coli contains pyrroloquinoline quinone (PQQ) and participates in the direct oxidation of D-glucose to D-gluconate by transferring electrons to ubiquinone (UQ). To elucidate the mechanism of ubiquinone reduction by mGDH, we applied a pulse radiolysis technique to mGDH with or without bound UQ8. With the UQ8-bound enzyme, a hydrated electron reacted with mGDH to form a transient species with an absorption maximum at 420 nm, characteristic of formation of a neutral ubisemiquinone radical. Subsequently, the decay of the absorbance at 420 nm was accompanied by an increase in the absorbance at 370 nm. Experiments with the PQQ-free apoenzyme showed no such subsequent absorption changes, although ubisemiquinone was formed. These results indicate that a pathway for an intramolecular electron transfer from ubisemiquinone radical at the UQ8 binding site to PQQ exists in mGDH. The first-order rate constant of this process was calculated to be equal to 1.2 x 10(3) s(-1). These findings are consistent with our proposal that during the catalytic cycle of mGDH the bound UQ8 mediates electron transfer from the reduced PQQ to UQ8 pools.
authors.affiliation: The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan. kobayasi@sanken.osaka-u.ac.jp
authors.forename: Kazuo
authors.initials: K
authors.lastname: Kobayashi
authors.forename: Golam
authors.initials: G
authors.lastname: Mustafa
authors.forename: Seiichi
authors.initials: S
authors.lastname: Tagawa
authors.forename: Mamoru
authors.initials: M
authors.lastname: Yamada
mesh_terms: D003067:Coenzymes; D004926:Escherichia coli / Q000201:enzymology*; D005948:Glucose Dehydrogenases / Q000378:metabolism*; D045542:PQQ Cofactor / Q000378:metabolism*; D011675:Pulse Radiolysis; D013053:Spectrophotometry; D014451:Ubiquinone / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D003067:Coenzymes; D014451:Ubiquinone; D045542:PQQ Cofactor; D005948:Glucose Dehydrogenases; C033955:glucose dehydrogenase (pyrroloquinoline-quinone); C024989:coenzyme Q10