pH dependence of the isomerase activity of protein disulfide isomerase: insights into its functional relevance.

The isomerase efficacy of the oxidoreductase, protein disulfide isomerase (PDI), has been examined by a simple method. Using this technique, the pH-dependence of relative efficiency of isomerization reactions by PDI has been evaluated and its impact on a key structure-forming step in the oxidative folding pathway of a model protein determined. Results reveal that PDI has a greater relative impact on thiol-disulfide reshuffling (isomerization) reactions and consequently the structure-forming step in oxidative folding at pH 7, as opposed to pH's 8 and 9. These results suggest that PDI, which possesses an anomalously low thiol pKa, is fine-tuned to catalyze oxidative folding in the lumen of the endoplasmic reticulum where the ambient pH of approximately 7 would otherwise retard thioldisulfide exchange reactions and hinder acquisition of the native fold. The pH-dependent impact on isomerization catalysis has important implications for the development of synthetic chaperones for in vivo and in vitro applications.