Endoplasmic reticulum disulfide reductase involved both in the correct folding of proteins and degradation of misfolded proteins. Required for efficient folding of proteins in the endoplasmic reticulum by catalyzing the removal of non-native disulfide bonds formed during the folding of proteins. Also involved in endoplasmic reticulum-associated degradation (ERAD) by reducing incorrect disulfide bonds in misfolded glycoproteins (By similarity). Thioredoxin domains 3 and 4 are the primary reductase domains. The thioredoxin-like regions Trxb 1 and 2 lack a redox-active CXXC motif.