Diagram of Insulin Structure. Insulin is a polypeptide hormone produced by cells in the islets of Langerhans in the pancreas. Insulin decreases the levels of glucose in the blood and regulates the metabolism of glucose, fats and proteins. In order to meet the demand for insulin needed by diabetics, the hormone is mass-produced with the aid of genetically engineered bacteria, but can also be taken from pigs and cattle. The diagram to the left illustrates the structure of insulin and proinsulin. Proinsulin is a single polypeptide chain of 86 amino acids that permits correct alignment of three pairs of disulfide bonds. Insulin is derived from proinsulin by cleavage of the C-peptide structure at the dipeptides Arg-Arg and Lys-Arg. Insulin is composed of an A chain of 21 amino acids and a B chain of 30 amino acids, the chains being held together by two disulfide bonds. A third disulfide bond is present within the A chain.