Catalytic
Voltammetry of the Molybdoenzyme Sulfite
Dehydrogenase from <i>Sinorhizobium meliloti</i>
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Abstract
Sulfite dehydrogenase from the soil
bacterium <i>Sinorhizobium
meliloti</i> (SorT) is a periplasmic, homodimeric molybdoenzyme
with a molecular mass of 78 kDa. It differs from most other well studied
sulfite oxidizing enzymes, as it bears no heme cofactor. SorT does
not readily reduce ferrous horse heart cytochrome <i>c</i> which is the preferred electron acceptor for vertebrate sulfite
oxidases. In the present study, ferrocene methanol (FM) (in its oxidized
ferrocenium form) was utilized as an artificial electron acceptor
for the catalytic SorT sulfite oxidation reaction. Cyclic voltammetry
of FM was used to generate the active form of the mediator at the
electrode surface. The FM-mediated catalytic sulfite oxidation by
SorT was investigated by two different voltammetric methods, namely,
(i) SorT freely diffusing in solution and (ii) SorT confined to a
thin layer at the electrode surface by a semipermeable dialysis membrane.
A single set of rate and equilibrium constants was used to simulate
the catalytic voltammograms performed under different sweep rates
and with various concentrations of sulfite and FM which provides new
insights into the kinetics of the SorT catalytic mechanism. Further,
we were able to model the role of the dialysis membrane in the kinetics
of the overall catalytic system