Essential oxidoreductase that oxidizes proteins in the endoplasmic reticulum to produce disulfide bonds. Acts by oxidizing directly PDI1 isomerase through a direct disulfide exchange. Does not act as a direct oxidant of folding substrate, but relies on PDI1 to transfer oxidizing equivalent. Also able to oxidize directly the PDI related protein MPD2. Does not oxidize all PDI related proteins, suggesting that it can discriminate between PDI1 and related proteins. Reoxidation of ERO1 probably involves electron transfer to molecular oxygen via FAD. Acts independently of glutathione. May be responsible for a significant proportion of reactive oxygen species (ROS) in the cell, thereby being a source of oxidative stress. Enzyme activity is tightly regulated to prevent the accumulation of reactive oxygen species in the endoplasmic reticulum. Reversibly down-regulated by the formation of disulfide bonds between Cys-150 and Cys-295. May function both as a monomer and a homodimer. By unfolded protein response (UPR). The C-terminal part (437-563) is required for the association with the endoplasmic reticulum lumen membrane. The Cys-100/Cys-105 and Cys-352/Cys-355 disulfide bonds constitute the redox-active center. The Cys-100/Cys-105 disulfide bond may accept electron from PDI1 and funnel them to the active site disulfide Cys-352/Cys-355. N-glycosylated. Belongs to the EROs family.