Respiratory sulfite reductase that catalyzes the reduction of sulfite to sulfide in a single step, consuming six electrons in the process. Required for sulfite respiration under anaerobic growth conditions. Does not contribute to nitrite reduction. [protein]-disulfide + 2 A + 3 H2O + hydrogen sulfide = [protein]-dithiol + 2 AH2 + H(+) + sulfite Exposure to oxygen reduces copper binding and leads to the formation of a disulfide bond between the two Cys residues that bind the copper ion. Binds 8 heme c groups covalently per monomer. Sulfur metabolism; sulfite reduction. Homotrimer. Loss of growth based on sulfite respiration. The eighth heme binding site has an unusual CXXXXXCH motif. Belongs to the multiheme cytochrome c family.