Insulin is a circulating peptide hormone that is best known as a critical regulator of glucose levels. It consists of two peptide chains (A and B) that are held together by two disulfide bonds and a third within the A-chain. The first milestone step towards its chemical synthesis was made by Sanger and colleagues who were the first to determine the primary sequence of sheep insulin as well as the precise pairings of the disulfide bonds. However, it was more than ten years before the first total chemical synthesis of bovine insulin was achieved which was via conventional solution phase synthesis of the two individual chains followed by their random combination and folding in solution. Later in that same year, Merrifield applied his newly-developed solid phase peptide synthesis (SPPS) methodology to synthesize bovine insulin within a few days. However the combination of the A- and B-chains invariably led to a low yield of correctly folded peptide. A stepwise disulfide bond formation approach was then introduced by Sieber and co-workers in their complex fragment synthesis of human insulin. In an attempt to overcome the limitations of these approaches, alternative protocols have been subsequently developed, one of which was optimized in our laboratory in which regioselective stepwise formation of the three disulfide bonds is achieved via the use of cysteine S-protecting groups cleaved in different chemical conditions. This review describes the historical course of the advancement of the chemical synthesis of insulin with a particular emphasis on the difficulty and limitations of its assembly.
Keywords: Chemical peptide synthesis, disulfide bonds, insulin, peptide hormone, regioselective
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