The process that results in the activation of multiple copies of PKB (also known as AKT) in response to the binding of a single molecule of insulin to its receptor involves several steps:
- When insulin binds to its receptor on the surface of a cell, it activates the receptor's intracellular tyrosine kinase domain.
- The activated receptor then phosphorylates (adds a phosphate group to) intracellular proteins called insulin receptor substrates (IRS).
- The phosphorylated IRS proteins bind to and activate the intracellular enzyme phosphoinositide 3-kinase (PI3K).
- PI3K converts a lipid called phosphatidylinositol 4,5-bisphosphate (PIP2) into another lipid called phosphatidylinositol 3,4,5-trisphosphate (PIP3).
- PIP3 then recruits and activates a protein called PDK1, which phosphorylates and activates PKB.
- Once activated, PKB can phosphorylate and inhibit other proteins, such as those involved in glucose metabolism, to promote the uptake and utilization of glucose by cells.
Insulin can stimulate the activation of PKB but not the activation of PKB because PKB and PKB are two different proteins with distinct functions. PKB is activated by insulin through the process described above, while PKB is activated by other signaling pathways, such as those involving growth factors or stress signals. PKB and PKB also have different target proteins that they phosphorylate and regulate, which contributes to their distinct functions.