Under normal physiological conditions blood glucose levels are kept under tight control by a series of regulated steps that ensure glucose homeostasis. Upon feeding glucose levels rise and in response to this the body secretes insulin from pancreatic beta-cells into the blood. At physiological concentrations insulin is present in the blood in its monomeric form. The insulin receptor is a tetramer, consisting of two alpha and two beta chains, which are produced by cleavage of a single translated peptide chain (Schenker & Kohanski 1991). Binding of insulin to its receptor occurs on the receptor alpha-subunits. There are two binding domains involved on the receptor (L1 and L2) and it is thought that the amino-terminus of insulin binds with L1 on one of the alpha-subunits and the carboxyterminus with L2 on the other alpha-subunit.
The binding of insulin to its receptor causes a conformational change in the alpha-subunits. This in turn produces a conformational change in the beta-subunits leading to the activation of the intrinsic insulin receptor tyrosine kinase.