Maturation of proinsulin to insulin involves

In humans, proinsulin consists of 82 amino acids (21 acids of A chain, 30 amino acids of B chain, the C peptide connecting the two has 35 amino acids). During maturation of proinsulin loses C peptide to form insulin. Hence, the peptide bonds are cleaved.

Mature insulin has A chain (21 amino acids) and B chain (30 amino acids) bound by disulphide bonds.