Insulin is a protein hormone. It was the first protein to be shown to have a precisely defined amino acid sequence. This work was done by Federick Sanger in 1953. As shown above in two colours, the insulin molecule consists of 2 peptide chains. a) The blue coloured chain is the A chain. It has 21 amino acids and a sisulphide bond within itself. b) The yellow colour represents the B chain. It is larger with 30 amino acids. Both chains are connected by 2 disulphide bridges. Porcine and bovine insulins differ from human insulin at 1 and 3 positions of the amino acids respectively.