Fig. 2

The crystal structure of human ERO1α and PDI and their working flow. The crystal structure and illustrative diagram of ERO1α (PDB: 3AQH) (A) and PDI (PDB: 4EKZ) (B). (C) The catalytic cycle of the ERO1α-PDI complex. The PDI a’ domain regulates the affinity of PDI to ERO1α and polypeptides by inducing the spatial rearrangement of the a’ and b’ domains through the conformational change of the x-linker region. Oxidized PDI has higher affinity to polypeptides and binds to them via the b’ domain. Oxidizing equivalents are transferred from the active site disulfide bonds of oxidized PDI to the unfolded polypeptides and PDI is therefore reduced. Reduced PDI shows higher affinity to ERO1α. Consequently, polypeptides dissociate from reduced PDI and are displaced by ERO1α. PDI is re-oxidized by ERO1α and then re-enters into a new catalytic cycle. (D) The electron transport chain within the oxidative folding. PDI oxidizes cysteines in nascent polypeptides to form disulfide bonds and accepts electrons from polypeptides, resulting in the reduction of PDI. Electrons from PDI are passed onto ERO1α leading to the reduction of the outer active site of ERO1α and the oxidation of PDI. Oxidized PDI then goes into a new round, while the outer active site of ERO1α shuffles electrons to the inner active site and onto the adjacent FAD coenzyme. FAD is reduced to FADH₂ upon accepting electrons. As the ultimate acceptor, molecular oxygen accepts electron from FADH₂ with the production of H₂O₂. ERO1α, endoplasmic reticulum oxidoreductase 1 alpha; PDI, protein disulfide isomerase; FAD, flavin adenosine dinucleotide