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Difficulty: MediumEndocrine and Metabolic Mechanisms

A 54-year-old man with a body mass index of 34 kg/m² comes to the clinic for a follow-up visit regarding his newly diagnosed type 2 diabetes mellitus. He reports struggling to adhere to his prescribed diet and exercise regimen. Laboratory studies show a fasting plasma glucose of 145 mg/dL and a hemoglobin A1c of 7.6%. The physician initiates metformin therapy and explains that the patient's condition is primarily driven by peripheral insulin resistance, which is characterized by a blunted intracellular response to endogenous insulin in target tissues. Place the following molecular events in the correct chronological sequence representing the normal cellular mechanism of insulin-mediated glucose uptake in skeletal muscle, from initial hormone binding to the final physiological effect.

  1. 1Binding of circulating insulin to the extracellular alpha-subunits of the receptor
  2. 2Conformational change leading to autophosphorylation of intracellular beta-subunit tyrosine residues
  3. 3Phosphorylation of intracellular docking proteins, primarily insulin receptor substrates (IRS)
  4. 4Activation of phosphoinositide 3-kinase (PI3K) and subsequent Akt (Protein Kinase B) signaling
  5. 5Exocytosis and translocation of GLUT4 transport vesicles to the plasma membrane

Answer

The correct sequence begins with insulin binding to the extracellular alpha-subunits, followed by beta-subunit autophosphorylation, phosphorylation of IRS proteins, activation of the PI3K/Akt pathway, and finally, the translocation of GLUT4 vesicles to the plasma membrane.
The normal mechanism of insulin action begins with the hormone binding to the extracellular alpha-subunits of its specific membrane receptor. This binding induces a conformational change that rapidly activates the intrinsic tyrosine kinase activity of the transmembrane beta-subunits, causing them to undergo autophosphorylation. The fully active receptor then phosphorylates critical intracellular docking proteins, most notably insulin receptor substrates (IRS-1 and IRS-2). These phosphorylated IRS proteins act as anchors for phosphoinositide 3-kinase (PI3K), which converts membrane PIP2 to PIP3. This ultimately activates Akt (Protein Kinase B), which triggers the final metabolic action in skeletal muscle and adipose tissue: the exocytosis and translocation of intracellular storage vesicles containing GLUT4 transport proteins to the plasma membrane. Once inserted into the membrane, GLUT4 permits the facilitated diffusion of glucose into the cell, thereby lowering plasma glucose levels.

Step-by-Step Solution

1
Identify the primary receptor interaction.
Insulin binds to the alpha-subunits of the receptor.
The alpha-subunits are entirely extracellular and contain the hormone-binding domains.
2
Determine the immediate intracellular consequence of receptor binding.
The beta-subunits undergo autophosphorylation.
The binding event activates the intrinsic tyrosine kinase activity located on the intracellular domains of the beta-subunits.
3
Trace the initial signal transduction steps.
Insulin receptor substrates (IRS) are phosphorylated.
The activated beta-subunit kinase targets downstream docking proteins (IRS) to propagate the signal.
4
Identify the critical kinase pathway activated by IRS.
PI3K and Akt are activated.
Phosphorylated IRS recruits and activates PI3K, initiating the PIP3/Akt cascade essential for metabolic effects.
5
Determine the final physiologic effector mechanism for glucose uptake.
GLUT4 vesicles translocate to the membrane.
Akt signaling mediates the exocytosis of pre-formed vesicles containing GLUT4 into the plasma membrane of myocytes and adipocytes.

Key Concept

The normal intracellular signaling pathway of insulin in skeletal muscle involves a receptor tyrosine kinase cascade that culminates in the membrane insertion of GLUT4 transporters to facilitate glucose uptake.
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Topics

Endocrine and Metabolic Mechanisms

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