Insulin is a small protein; human insulin has a molecular weight of 5808. It is composed of two amino acid chains, connected to each other by disulﬁde linkages. When the two amino acid chains are split apart, the functional activity of the insulin molecule is lost.
Insulin is synthesized in the beta cells by the usual cell machinery for protein synthesis, beginning with translation of the insulin RNA by ribosomes attached to the endoplasmic reticulum to form an insulin preprohormone.
This initial preprohormone has a molecular weight of about 11,500, but it is then cleaved in the endoplasmic reticulum to form a proinsulin with a molecular weight of about 9000; most of this is further cleaved in the Golgi apparatus to form insulin and peptide fragments before being packaged in the secretory granules.