Title: Effect of protein disulfide isomerase on the rate‐determining steps of the folding of bovine pancreatic ribonuclease A
Abstract: The effects of protein disulfide isomerase (PDI) on the four structured des species that accumulate in the rate‐determining steps of ribonuclease A folding were investigated at pH 8.0 and 15°C. The results indicate that PDI catalyzes the conversion of the kinetically trapped intermediates, des‐[26–84] and des‐[58–110], by reshuffling them into the on‐pathway intermediate, des‐[40–95], and the formation of native protein, by acting as both a chaperone and an oxidase on this on‐pathway intermediate. These results provide the first strong evidence for the mechanism of PDI in the rate‐determining steps of the oxidative folding pathways of ribonuclease A. Our approach, using PDI and blocked PDI, combined with the fast‐blocking 2‐aminoethyl methanethiosulfonate method, may be generally applicable to the clarification of the effect of PDI on folding intermediates.