Direct catalytic electrochemistry of sulfite dehydrogenase: mechanistic insights and contrasts with related Mo enzymes.
Under hydrodynamic electrochemical conditions with slow cyclic voltammetry sweep rates we have been able to probe catalytic events at the molybdenum active site of sulfite dehydrogenase (SDH) from Starkeya novella adsorbed on an edge plane graphite electrode within a polylysine film. The electrochemically driven catalytic behaviour of SDH mirrors that seen in solution assays suggesting that the adsorbed enzyme retains its native activity. However, at high sulfite concentrations, the voltammetric waveform transforms from the expected sigmoidal profile to a peak-shaped response, similar to that reported for the molybdenum enzymes DMSO reductase and nitrate reductase (NarGHI and NapAB) where a redox reaction at the active site has been associated with a switch to lower activity at high overpotentials. This is the first time a similar phenomenon has been observed in a Mo-containing oxidase/dehydrogenase, which raises a number of interesting mechanistic problems. The potential at which the activity of SDH becomes attenuated only emerges at saturating substrate conditions and occurs at a potential (ca. + 320mV vs NHE) well removed from any known redox couple in the enzyme. These results cannot be explained by the same mechanism adopted for DMSO reductase and nitrate reductase catalysis.
Rapson
Trevor D
TD
Centre for Metals in Biology, School of Molecular and Microbial Sciences, University of Queensland, Brisbane, Australia.
Kappler
Ulrike
U
Bernhardt
Paul V
PV
D002384:Catalysis; D020134:Catalytic Domain; D004563:Electrochemistry; D006863:Hydrogen-Ion Concentration; D015394:Molecular Structure; D008982:Molybdenum / Q000737:chemistry*; D013379:Substrate Specificity; D050902:Sulfite Dehydrogenase / Q000737:chemistry* / Q000378:metabolism
D008982:Molybdenum; D050902:Sulfite Dehydrogenase