Intermediates involved in the Reduction of SO2: Insight into the mechanism of Sulfite Reductases

Abstract

Sulfite Reductase (SiR) catalyze the reduction of SO₃^2- to H₂S in both biosynthetic sulfur assimilation and dissimilation of sulfate. The mechanism of the 6e^-/6H⁺ reduction of SO₃^2- at the siroheme cofactor is debated and the proposed intermediates involved in this 6e- reduction are yet to be spectroscopically characterized. The reaction of SO₂ with a ferrous iron porphyrin is investigated and two intermediates are trapped and characterized; an initial Fe(III)-SO₂^2- species which undergoes proton assisted S-O bond cleavage to form an Fe(III)-SO species. These species are characterized using a combination of resonance Raman (with ³⁴S labelled SO₂), EPR and DFT calculations. The results obtained help reconciliate the different proposed mechanisms for SiR.