Isomerase and chaperone activities of protein disulfide isomerase are both required for its function as a foldase.

Protein disulfide isomerase (PDI) is not only an isomerase catalyzing the formation of native disulfide bond(s) of nascent peptide, but also a molecular chaperone assisting chain folding. The intrinsic chaperone activity of PDI is independent of its isomerase activity as shown by its ability of promoting in vitro reactivation and suppressing aggregation during refolding of denatured proteins containing no disulfide. The -CGHC- active sites of PDI are not required for its chaperone activity and a mutant PDI with no isomerase activity does function in vitro and in vivo. The peptide binding site of PDI is responsible for its chaperone activity. Both isomerase and chaperone activities are required for PDI to function as a foldase in assisting protein folding, in other words, the foldase activity of PDI consists of both isomerase and chaperone activities.