What are peptide hormones? With the help of schematic diagram, discuss the epinephrine cascade for the glucose release from hepatocytes. (IAS 2021/20 Marks)

Peptide Hormones

• Peptide hormones are composed of multiple amino acids linked by peptide bonds, forming polypeptide chains. They range from small peptides like oxytocin to larger proteins such as insulin.
• Synthesis and Secretion: These hormones are synthesized in the rough endoplasmic reticulum of endocrine cells, processed in the Golgi apparatus, and stored in secretory vesicles. Upon receiving a specific stimulus, they are released into the bloodstream.
• Mechanism of Action: Being water-soluble, peptide hormones cannot cross the lipid bilayer of target cells. Instead, they bind to specific receptors on the cell surface, initiating a cascade of intracellular events that lead to the desired physiological response.

Epinephrine Cascade for Glucose Release from Hepatocytes

• Epinephrine Binding: Epinephrine binds to β-adrenergic receptors on the surface of hepatocytes.
• Activation of G-Protein: This binding activates the G-protein associated with the receptor, leading to the exchange of GDP for GTP on the G-protein's α-subunit.
• Adenylate Cyclase Activation: The activated G-protein α-subunit stimulates adenylate cyclase, an enzyme embedded in the plasma membrane.
• cAMP Production: Adenylate cyclase converts ATP to cyclic AMP (cAMP), a secondary messenger that propagates the signal within the cell.
• Protein Kinase A Activation: Elevated cAMP levels activate protein kinase A (PKA) by binding to its regulatory subunits, causing the release of active catalytic subunits.
• Phosphorylation of Target Enzymes: PKA phosphorylates specific enzymes involved in glycogen metabolism:
  
  • Glycogen Phosphorylase Kinase: Activated by phosphorylation, it subsequently activates glycogen phosphorylase.
  • Glycogen Synthase: Phosphorylation inactivates this enzyme, halting glycogen synthesis.
• Glycogen Breakdown: Activated glycogen phosphorylase catalyzes the breakdown of glycogen into glucose-1-phosphate.
• Glucose Release: Glucose-1-phosphate is converted to glucose-6-phosphate, which is then dephosphorylated to free glucose. This glucose is released into the bloodstream, increasing blood glucose levels to meet the body's energy demands.

Conclusion

The epinephrine cascade for glucose release from hepatocytes is a complex and tightly regulated process that involves the activation of various signaling molecules and enzymes. The molecular mechanisms underlying this cascade is crucial for elucidating the role of peptide hormones in regulating glucose metabolism and maintaining homeostasis in the body.