Reduces sulfate for Cys biosynthesis. AMP + glutathione disulfide + 2 H(+) + sulfite = adenosine 5'-phosphosulfate + 2 glutathione Binds 1 [4Fe-4S] cluster. The C-terminal domain may function as glutaredoxin and mediates the interaction of the enzyme with glutathione (GSH). Active in GSH-dependent reduction of hydroxyethyldisulfide, cystine, dehydroascorbate, insulin disulfides and ribonucleotide reductase. Belongs to the APS reductase family.