Catalytic Electrochemistry
of Xanthine Dehydrogenase
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Abstract
We report the mediated electrocatalytic voltammetry of
the molybdoenzyme
xanthine dehydrogenase (XDH) from <i>Rhodobacter capsulatus</i> at a thiol-modified Au electrode. The 2-electron acceptor <i>N</i>-methylphenazinium methanesulfonate (phenazine methosulfate,
PMS) is an effective artificial electron transfer partner for XDH
instead of its native electron acceptor NAD<sup>+</sup>. XDH catalyzes
the oxidative hydroxylation of hypoxanthine to xanthine and xanthine
to uric acid. Cyclic voltammetry was used to generate the active (oxidized)
form of the mediator. Simulation of the catalytic voltammetry across
a broad range of substrate and PMS concentrations at different sweep
rates was achieved with the program DigiSim to yield a set of consistent
rate and equilibrium constants that describe the catalytic system.
This provides the first example of the mediated electrochemistry of
a xanthine dehydrogenase (or oxidase) that is uncomplicated by interference
from product oxidation. A remarkable two-step, sequential oxidation
of hypoxanthine to uric acid via xanthine by XDH is observed