Proinsulin, a single-chain protein of 86 amino acid residues, is synthesized in the β cells and
enzymatically transformed therein to insulin. The conversion involves the cleavage of a
connecting C-peptide, which contains between 30 and 35 residues, with the number and
sequence varying among different species (human C-peptide consists of 35 residues). The
resulting human insulin consists of two peptide chains, designated A (having 21 residues) and B
(having 30 residues), which are interchain connected by two disulfide bonds. Furthermore, the
A-chain also contains an intrachain disulfide bond between Cys6 and Cys11.
