Intermediates involved in the reduction of SO 2 : insight into the mechanism of sulfite reductases.

Sulfite reductases (SiRs) catalyze the reduction of SO 3 2- to H 2 S in biosynthetic sulfur assimilation and dissimilation of sulfate. The mechanism of the 6e - /6H + reduction of SO 3 2- at the siroheme cofactor is debated, and proposed intermediates involved in this 6e - reduction are yet to be spectroscopically characterized. The reaction of SO 2 with a ferrous iron porphyrin is investigated, and two intermediates are trapped and characterized: an initial Fe(III)-SO 2 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 34 S-labelled SO 2 ), EPR and DFT calculations. Results obtained help reconcile the different proposed mechanisms for the SiRs.