Insulin binds its receptor on a hepatocyte. Which event occurs immediately downstream of ligand binding and initiates the signaling cascade?

• A Intrinsic tyrosine kinase activity of the beta subunit causing autophosphorylation, then phosphorylation of IRS proteins ✓
• B Activation of adenylate cyclase and a rise in intracellular cAMP
• C Activation of phospholipase C generating IP3 and a rise in cytosolic calcium
• D Receptor-associated JAK kinases phosphorylate STAT proteins in the cytoplasm

Explanation

The insulin receptor is a ligand-activated tyrosine kinase. Binding triggers conformational change, autophosphorylation of tyrosine residues on the beta subunit, and phosphorylation of insulin receptor substrate proteins, which recruit PI3 kinase and drive Akt-mediated GLUT4 translocation and metabolic effects. cAMP generation and IP3-calcium pathways characterize GPCR-linked hormonal signals, and JAK-STAT signaling belongs to growth hormone and cytokine receptors, making those options category mismatches.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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