Protein-disulfide isomerase (PDI) catalyzes the formation, rearrangement, and breakage of
disulfide bonds and is capable of binding peptides and unfolded proteins in a chaperone-
like manner. In this study we examined which of these functions are required to facilitate
efficient refolding of denatured and reduced proinsulin. In our model system, PDI and also a
PDI mutant having only one active site increased the rate of oxidative folding when present
in catalytic amounts. PDI variants that are completely devoid of isomerase activity are not …