Catalyzes the reduction of disulfide bonds. May function in reducing intermolecular disulfides between proteins and small molecules in the periplasm, or keeping a specific subset of periplasmic proteins reduced, or maintaining the periplasm of Chlamydia in a generally reducing state. Seems to be unable to oxidize thiols into disulfides and does not display disulfide bond isomerase activity. E(0) is -269.7 mV. Monomer.