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Biochemistry. 2005 Oct 18;44(41):13567-72.

Transient formation of a neutral ubisemiquinone radical and subsequent intramolecular electron transfer to pyrroloquinoline quinone in the Escherichia coli membrane-integrated glucose dehydrogenase.

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1
The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan. kobayasi@sanken.osaka-u.ac.jp

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.

PMID:
16216080
DOI:
10.1021/bi051347n
[Indexed for MEDLINE]
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