Notes on G-Protein Coupled Receptors and Epinephrine Response

G-Protein Coupled Receptors (GPCR) and Signal Transduction

  • Overview of Activation

    • GPCRs are important receptors activated by ligands (like epinephrine).

    • Ligand binding induces a conformational change in GPCR, activating an associated G protein.

  • G Protein Structure

    • Consists of three subunits: alpha, beta, and gamma.

    • Upon activation, the alpha subunit dissociates from the beta-gamma dimer.

  • Adenylyl Cyclase Activation

    • The activated alpha subunit activates adenylyl cyclase, an enzyme that converts ATP to cyclic AMP (cAMP), a second messenger.

The Role of Cyclic AMP (cAMP)

  • Function of cAMP

    • cAMP acts as a signaling molecule that activates protein kinase A (PKA).

    • It facilitates the conversion of PKA from an inactive state to an active state by binding to its regulatory subunits, allowing catalytic activity.

  • PKA Structure

    • Composed of two subunits:

    • Regulatory subunit (purple) - inhibits PKA in its inactive form.

    • Catalytic subunit (gold) - responsible for phosphorylating target proteins.

  • Activation Mechanism

    • cAMP binding causes the regulatory subunits to dissociate, leaving the catalytic subunit active for phosphorylation.

Fight or Flight Response (Epinephrine)

  • Functions of Epinephrine

    • Commonly known as adrenaline, it plays a crucial role in the body's response to stress.

    • Activation of GPCR by epinephrine leads to key physiological changes necessary for survival.

  • Physiological Responses

    • Increased Heart Rate: Enhances blood flow and oxygen delivery.

    • Vasodilation: Expands blood vessels to facilitate oxygen transport.

    • Decreased Digestion: Constricts smooth muscle in the gastrointestinal tract, slowing down digestion.

    • Pupil Dilation: Enhances vision in critical situations.

  • Energy Utilization

    • The fight or flight response increases glucose availability; glycogen breakdown is stimulated while glucose storage is inhibited.

    • Resulting energy supports immediate physical action needed during emergencies.

Summary of the Response Mechanism

  1. Epinephrine (ligand) binds to GPCR.

  2. GPCR activation leads to G protein dissociation.

  3. Adenylyl cyclase converts ATP to cAMP.

  4. cAMP activates PKA, which phosphorylates various proteins to elicit the fight or flight response.

PKA can activate this enzyme which was previously in an inactive state. It activates this enzyme which activates that enzyme and which breaks glucose out of the glycogen. this gets more energy to do the fight or flight response. PKA shuts off the enzyme that stored your glucose as glycogen.