Sulfite Reductase

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Chemistry 564 Biophysical Chemistry
Joan-Marie Manolakis
Sulfite Reductase Desulfoviridin
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Enzymatic Function

• Reduction of sulfite to sulfide into amino acids
  and cellular metabolites and respiratory pathways
  in an anaerobic bacteria
• Six-electron reduction of SO32- to S2- is
  catalyzed by both assimilatory and dissimilatory
  sulfite reductases

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Components of Desulfovibrio vulgaris

• Two pairs of Fe4S4
• -Pentacoordinate high-spin siroheme opposed to
  low molecular mass assimilatory enzyme that
  posses a low-spin hexacoordinate center
• Bridging ligand
• -Cysteine/Cystine content reflects a large number
  of disulfide linkages and/or cysteine in interior
  of protein
• Siroheme units

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Metallation v. Dematallation US scientists v.
German scientists

• Resonance Raman studies of desulfoviridin
  demonstrate significant shifts for marker band in
  D20

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Metallation v. Dematallation US scientists v.
German scientists
Argument This demonstrates protonation of the
pyrrolic, and pyrroline nitrogens, and therefore
the heme must be demetallated. Rebuttal This
analysis is flawed because coworkers demonstrated
facile exchange of pyrroline CH and CH protons by
1H-NMR in D2O.
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Metallation v. Dematallation US scientists v.
German scientists

• Optical spectra of acid-extracted chromophore are
  purported to demonstrate the presence of
  demetallated siroheme in desulfoviridin

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Metallation v. Dematallation US scientists v.
German scientists
A Referenced spectrum of oxidized enzyme B
Extracted chromophore in 25 pyrimidine C
Extracted chromophore in 0.4 M cyanide
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Metallation v. Dematallation US scientists v.
German scientists
A Acidic acetone extraction and addition of
pyridine (to 25) B Alkaline extraction in 0.1 M
NaOH/25 pyridine
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References
Desulfoviridin, a multimeric-dissimilatory
sulfite reductase from Desulfovibrio vulgaris
(Hildenborough). Purification, characterization,
kinetics and EPR studies. Wolfe BM, Lui SM, Cowan
JA./ Eur J Biochem. 1994 Jul 1223(1)79-89.