Disulfide relay from ERO1 to PDI in the ER. Disulfide bond formation involves electron transfer from the substrate to a disulfide carrier protein (e.g., PDI) and then to a de novo disulfide-generating enzyme (e.g., ERO1). PDI directly transfers a disulfide to two Cys residues of a substrate protein by means of a thiol-disulfide exchange reaction. The reduced form of PDI is reoxidized by ERO1, which relays the oxidizing power from molecular oxygen, via the FAD cofactor, to the reduced form of PDI.
