Sulfite dehydrogenase from
Starkeya novella is an
heterodimer comprising a 40.6 kDa subunit(containing the Mo cofactor) and a smaller 8.8 kDa heme
c subunit. The enzyme catalyses the oxidationof sulfite to sulfate with the natural electron acceptor being cytochrome
c550. Its catalytic mechanism isthought to resemble that found in eukaryotic sulfite oxidases. Using protein film voltammetry and redoxpotentiometry, we have identified both Mo- and heme-centered redox responses from the enzymeimmobilized on a pyrolytic graphite working electrode:
Em,8 (Fe
III/II) +177 mV;
Em,8 (Mo
VI/V) +211 mV and
Em,8 (Mo
V/IV) -118 mV vs NHE; Upon addition of sulfite to the electrochemical cell a steady-statevoltammogram is observed and an apparent Michaelis constant (
Km) of 26(1)
M was determined for theenzyme immobilized on the working electrode surface, which is comparable with the value obtained fromsolution assays.