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Zorluk: OrtaEndocrine and Metabolic Mechanisms

Emergency medical services bring a 23-year-old male to the emergency department after his roommate found him diaphoretic, lethargic, and disoriented. The patient has a history of type 1 diabetes mellitus and recently increased his running mileage. Capillary blood glucose on arrival is 34 mg/dL. The physician promptly administers an intramuscular injection of a counter-regulatory peptide hormone, resulting in a rapid improvement of the patient's mental status and a steady rise in blood glucose over the next 15 minutes. Arrange the following intracellular and enzymatic steps in the correct chronological sequence detailing the hepatic mechanism by which this treatment reversed the patient's hypoglycemia.

  1. 1Ligand binding to a hepatocyte 7-transmembrane G-protein coupled receptor
  2. 2Exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) on the Gs-alpha subunit
  3. 3Adenylyl cyclase activation leading to cyclic AMP generation
  4. 4Dissociation of regulatory subunits from protein kinase A
  5. 5Phosphorylation and activation of phosphorylase kinase
  6. 6Cleavage of alpha-1,4-glycosidic bonds releasing glucose-1-phosphate

Cevap

The sequence begins with glucagon binding to its 7-transmembrane GPCR, triggering GDP/GTP exchange on the Gs-alpha subunit. This activates adenylyl cyclase to produce cAMP. Cyclic AMP then binds and activates protein kinase A (PKA). PKA phosphorylates phosphorylase kinase, which in turn activates glycogen phosphorylase to cleave alpha-1,4-glycosidic bonds, initiating glycogenolysis.
The correct sequence maps the precise biochemical pathway of glucagon-mediated glycogenolysis. Glucagon is a peptide hormone that binds to a 7-transmembrane Gs-coupled receptor on hepatocytes. This binding induces a conformational change allowing the Gs-alpha subunit to exchange GDP for GTP. The active Gs-alpha subunit stimulates adenylyl cyclase to convert ATP into cAMP. Increased cAMP levels bind to the regulatory subunits of Protein Kinase A (PKA), causing them to dissociate and liberate active catalytic subunits. PKA then phosphorylates and activates phosphorylase kinase. Finally, phosphorylase kinase phosphorylates glycogen phosphorylase, activating it to cleave alpha-1,4-glycosidic linkages in glycogen, yielding glucose-1-phosphate which is converted to free glucose and released into the bloodstream.

Adım Adım Çözüm

1
Identify the administered medication and its target organ.
The medication is intramuscular glucagon, which primarily targets the liver to raise blood glucose.
Glucagon is the standard counter-regulatory peptide hormone used for severe hypoglycemia when intravenous access is unavailable.
2
Determine the type of receptor bound by the medication.
Glucagon binds to a Gs-protein coupled receptor (GPCR).
Peptide hormones typically act via cell surface receptors rather than intracellular receptors.
3
Trace the second messenger cascade activated by a Gs-coupled receptor.
Receptor binding causes Gs-alpha to exchange GDP for GTP. The Gs-alpha-GTP complex stimulates adenylyl cyclase to generate cAMP.
This is the universal signal transduction mechanism for all Gs-coupled GPCRs.
4
Map the downstream kinase cascade triggered by elevated cAMP.
cAMP activates Protein Kinase A (PKA), which phosphorylates phosphorylase kinase, which subsequently activates glycogen phosphorylase.
The metabolic goal is to rapidly break down stored glycogen into glucose-1-phosphate, which requires activating the rate-limiting enzyme glycogen phosphorylase.

Anahtar Kavram

Glucagon acts via a Gs-protein coupled receptor to increase cAMP, activate Protein Kinase A, and initiate a phosphorylation cascade that ultimately stimulates glycogenolysis.
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Endocrine and Metabolic Mechanisms

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