Proposed thermodynamic model predicts non-stoichiometric reactive oxygen species produced with incorrect disulfide bond formation. (a) In the model, forming and breaking an incorrect disulfide bond uses two protein disulfide isomerases (PDIs), one with electron affinity higher (PDIA) and one lower (PDIB) than the incorrect disulfide bond. In the formation phase, electrons are shuttled to molecular oxygen, resulting in ROS formation. In the breaking phase, electrons are passed from NADPH, through glutathione, to the protein. In both cases, electrons move along the electron affinity gradient. The net result is a futile cycle that is required to fix incorrect disulfide bonds, but expends redox energy. (b) The thermodynamic model predicts at fast folding rates near stoichiometric ROS is generated per disulfide bond formed. However, when folding rates are slow, the unfolded protein may go through many futile cycles, resulting in excess ROS. Glutathione (GSH), oxidized glutathione (GSSG), disulfide bond formation (DBF), disulfide bond breaking (DBB).