In vitro folding/unfolding of insulin/single-chain insulin

Insulin is a double-chain (designated A and B chain respectively) protein hormone containing three disulfides, while insulin is synthesized in vivo as a single-chain precursor and folded well before being released from B-cells. Although the structure and function of insulin have been well characterized, the progress in oxidative folding pathway studies of insulin has been very slow, mainly due to the difficulties brought about by its disulfide-linked double-chain structure. To overcome these difficulties, we recently studied the in vitro oxidative folding process of two single-chain insulins: porcine insulin precursor (PIP) and human proinsulin (HPI). Based on the analysis of the intermediates captured during folding process, the folding pathways have been proposed for PIP and HPI separately. Similarities between the two folding pathways disclose some common principles that govern the insulin folding process. The following unfolding studies of PIP and HPI further indicate that C-peptide might also function during the folding of proinsulin. Here, we gave a brief review on in vitro folding/unfolding process of insulin and single-chain insulin. The implication of these studies on protein folding has also been discussed.