Protein folding - Where do the electrons go?

When a newly synthesized protein folds into the correct three-dimensional conformation, bonds between the thiol groups of cysteine residues - so-called disulphide bonds - cannot form spontaneously. This is because bond formation generates two electrons, which need to be mopped up by an oxidant. Studies in bacteria have identified two members of the electron-transport chain as oxidants that can do this job.