Transient Formation of a Neutral Ubisemiquinone Radical and Subsequent Intramolecular Electron Transfer to Pyrroloquinoline Quinone in the Escherichia coli Membrane-Integrated Glucose Dehydroge

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• 作者：Kazuo Kobayashi ; Golam Mustafa ; Seiichi Tagawa ; and Mamoru Yamada
• 刊名：Biochemistry
• 出版年：2005
• 出版时间：October 18, 2005
• 年：2005
• 卷：44
• 期：41
• 页码：13567 - 13572
• 全文大小：75K
• 年卷期：v.44,no.41(October 18, 2005)
• ISSN：1520-4995

文摘

The membrane-bound quinoprotein glucose dehydrogenase (mGDH) in Escherichia coli containspyrroloquinoline quinone (PQQ) and participates in the direct oxidation of D-glucose to D-gluconate bytransferring 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-boundenzyme, a hydrated electron reacted with mGDH to form a transient species with an absorption maximumat 420 nm, characteristic of formation of a neutral ubisemiquinone radical. Subsequently, the decay of theabsorbance at 420 nm was accompanied by an increase in the absorbance at 370 nm. Experiments withthe PQQ-free apoenzyme showed no such subsequent absorption changes, although ubisemiquinone wasformed. These results indicate that a pathway for an intramolecular electron transfer from ubisemiquinoneradical at the UQ₈ binding site to PQQ exists in mGDH. The first-order rate constant of this process wascalculated to be equal to 1.2 × 10³ s^-1. These findings are consistent with our proposal that during thecatalytic cycle of mGDH the bound UQ₈ mediates electron transfer from the reduced PQQ to UQ₈ pools.