Insulin lispro

Insulin lispro binds to the insulin receptor (IR), a heterotetrameric protein consisting of two extracellular alpha units and two transmembrane beta units. The binding of insulin to the alpha subunit of IR stimulates the tyrosine kinase activity intrinsic to the beta subunit of the receptor. The bound receptor autophosphorylates and phosphorylates numerous intracellular substrates such as insulin receptor substrates (IRS) proteins, Cbl, APS, Shc and Gab 1. Activation of these proteins leads to the activation of downstream signaling molecules including PI3 kinase and Akt. Akt regulates the activity of glucose transporter 4 (GLUT4) and protein kinase C (PKC), both of which play critical roles in metabolism and catabolism. In humans, insulin is stored in the form of hexamers; however, only insulin monomers are able to interact with IR. Reversal of the proline and lysine residues at positions B28 and B29 of native insulin eliminates hydrophobic interactions and weakens some of the hydrogen bonds that contribute to the stability of the insulin dimers that comprise insulin hexamers. Hexamers of insulin lispro are produced in the presence of zinc and m-cresol. These weakly associated hexamers quickly dissociate upon subcutaneous injection and are absorbed as monomers through vascular endothelial cells. These properties give insulin lispro its fast-acting properties.