Intermediates involved in the reduction of SO2: insight into the mechanism of sulfite reductases

Abstract

Sulfite reductases (SiRs) catalyze the reduction of SO₃²⁻ to H₂S in biosynthetic sulfur assimilation and dissimilation of sulfate. The mechanism of the 6e⁻/6H⁺ reduction of SO₃²⁻ at the siroheme cofactor is debated, and 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)–SO₂²⁻ 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. Results obtained help reconcile the different proposed mechanisms for the SiRs.