PQQ as redox shuttle for quinoprotein glucose dehydrogenase.
The role of pyrroloquinoline quinone (PQQ) as a redox shuttle between an electrode and the active site of soluble quinoprotein glucose dehydrogenase (sGDH) from Acinetobacter calcoaceticus has been investigated using both electrochemical and spectrophotometric methods. Reversible redox behavior of PQQ was observed at cystamine-modified gold electrodes. sGDH is able to reduce free PQQ, i.e. PQQ that is not bound to the enzyme and therefore could act as a mediator between the enzyme and the cystamine-modified electrode. The second order rate constants for the reduction of PQQ by sGDH are 6 x 10(3) M(-1) S(-1) and 64 M(-1) S(-1) in the absence and in the presence of calcium ions, respectively. Similarly, the interaction with a second redox protein is realized via the PQQ shuttle. Using DC voltammetry, the reduction rate of cytochrome c (cyt c) by PQQH2 was determined to be on the order of 10(4) M(-1) S(-1)