Proinsulin undergoes maturation into active insulin through the action of cellular endopeptidases known as prohormone convertases ( PC1 and PC2 ), as well as the exoprotease carboxypeptidase E .  The endopeptidases cleave at 2 positions, releasing a fragment called the C-peptide , and leaving 2 peptide chains, the B- and A- chains, linked by 2 disulfide bonds. The cleavage sites are each located after a pair of basic residues (lysine-64 and arginine-65, and arginine-31 and −32). After cleavage of the C-peptide, these 2 pairs of basic residues are removed by the carboxypeptidase.  The C-peptide is the central portion of proinsulin, and the primary sequence of proinsulin goes in the order "B-C-A" (the B and A chains were identified on the basis of mass and the C-peptide was discovered later).