Structure of Insulin
Insulin is composed of 2 peptide chains i.e. A chain and B chain. Both the chains are linked together by two disulfide bonds, and one disulfide is formed within the A chain. In most species, the A chain consists of 21 amino acids and the B chain of 30 amino acids that means it is composed of 51 amino acids in two peptide chains (A and B). The three-dimensional structure of insulin molecule (insulin monomer) exists in two main conformations. These differ in the extent of helix in the B chain due to phenol or its derivatives.
In acid solutions, the insulin monomer assembles as dimmers (diffuses in the blood) neutral pH and in the presence of zinc ions, as hexamers. The intermediate and long acting insulin has high proportion of hexamers, to delay its action. The sequence of amino acid in insulin varies among species, certain segments are conserved, like positions of the three disulfide bonds, both ends of the A chain and the C-terminal residues of the B chain. These similarities in the amino acid sequence of insulin lead to a three dimensional conformation of insulin that is very similar among species, and insulin from one animal is very likely biologically active in other species. Indeed, pig insulin has been widely used for human.
The first of these molecules to be marketed - called insulin lispro - is engineered such that lysine and proline resting on the C-terminal end of the B chain are reversed; this modification does not alter receptor binding, but minimizes the tendency to form dimmers and hexamers.