Protein disulfide isomerase is both an enzyme and a chaperone.

Protein disulfide isomerase (PDI) catalyzes the formation of native disulfides of peptide chains from either the reduced form or randomly joined disulfides. So that thiols situated at distant parts of the polypeptide chain can be joined together to form the native disulfides, the polypeptide chain has to be folded, at least to some extent, into the native conformation. It is suggested that PDI promotes folding of the chains as well as formation of the disulfides and plays a role similar to the chaperones in the folding process. PDI is known to be a multifunctional protein and capable of nonspecific peptide binding. These properties are closely connected to its possible function as a chaperone. Thioredoxin, which has an active site sequence similar to that of PDI but lacks the property of peptide binding, is much less efficient as a disulfide isomerase.