Function of a bound ubiquinone in Escherichia coli quinoprotein glucose dehydrogenase.

Membrane-bound glucose dehydrogenase (mGDH) is a single integral protein in the respiratory chain in Escherichia coli which oxidizes D-glucose and feeds electrons to ubiquinol oxidase via bulk ubiquinone (UQ). mGDH contains a bound UQ, CoQ8, for its intramolecular electron transfer in addition to pyrroloquinoline quinone (PQQ) as a coenzyme. Pulse radiolysis analysis revealed that the bound UQ exists very close to PQQ at a distance of 11-13 angstroms. Studies on mGDH mutants with substitutions for amino acid residues around PQQ showed that Asp-466 and Lys-493, which are crucial for catalytic activity, interact with bound UQ. Based on these findings, we propose that the bound UQ is involved in the catalytic reaction in addition to the intramolecular electron transfer in mGDH.