Protein disulfide-isomerase

Protein disulfide isomerase (EC 5.3.4.1), or PDI, is an enzyme in the endoplasmic reticulum (ER) in eukaryotes and the periplasm of bacteria that catalyzes the formation and breakage of disulfide bonds between cysteine residues within proteins as they fold. This allows proteins to quickly find the correct arrangement of disulfide bonds in their fully folded state, and therefore the enzyme acts to catalyze protein folding.

Source: Wikipedia — Protein disulfide-isomerase (CC BY-SA 4.0)