Part of the SoeABC complex that catalyzes the oxidation of sulfite to sulfate. a quinone + H2O + sulfite = a quinol + sulfate a menaquinone + H2O + sulfite = a menaquinol + sulfate Binds 1 [4Fe-4S] cluster. Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Forms a heterotrimeric membrane-bound complex composed of a catalytic heterodimer (SoeAB) and a membrane anchor protein (SoeC). Attached to the membrane by interaction with SoeC. Deletion of the gene causes a strong decrease in sulfite oxidation rate. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family.