G-Protein-Linked Receptors Overview
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Questions and Answers

What is the primary role of β-arrestin in the signaling process?

  • It activates protein kinase A (PKA).
  • It assists in clathrin-mediated endocytosis of the receptor. (correct)
  • It promotes the binding of receptor to G-protein.
  • It inhibits phosphodiesterase activity.

    • What effect does Vibrio cholerae have on the Gs protein?

  • It stimulates β-arrestin signaling.
  • It enhances GTPase activity.
  • It chemically modifies the Gs protein to avoid hydrolysis. (correct)
  • It converts cAMP back to AMP.

    • Which of the following statements about the cyclic AMP (cAMP) pathway is correct?

  • cAMP induces protein phosphatase activity.
  • cAMP is primarily involved in deactivating ion channels.
  • cAMP directly activates phosphodiesterase.
  • cAMP activates PKA, which then phosphorylates target proteins. (correct)

    • What is the result of prolonged activation of the G protein-coupled receptors (GPCRs) without proper signal termination?

    <p>Chronic dehydration due to excessive fluid loss.</p> Signup and view all the answers

    What happens to the signaling effectiveness when the G protein-coupled receptors are constantly stimulated?

    <p>The GPCR becomes desensitized.</p> Signup and view all the answers

    What role do adenylate cyclases play in signal transduction?

    <p>They convert ATP into cyclic AMP for further signaling.</p> Signup and view all the answers

    How does a single hormone molecule amplify its signal in the described process?

    <p>Through successive activation of multiple G proteins and adenylyl cyclases.</p> Signup and view all the answers

    What is the ultimate response of the signaling pathway described?

    <p>Release of glucose 1-phosphate for energy production.</p> Signup and view all the answers

    What specific function does protein kinase A serve in the signal amplification process?

    <p>It phosphorylates multiple target proteins in response to cAMP binding.</p> Signup and view all the answers

    Why do different cell types respond uniquely to the same signaling molecule?

    <p>They have distinct collections of proteins that allow for varied responses.</p> Signup and view all the answers

    What role do scaffolding proteins play in signaling pathways?

    <p>They increase efficiency by grouping involved proteins.</p> Signup and view all the answers

    What is the first step in the self-inactivation process of G-protein signaling?

    <p>Hydrolysis of GTP to GDP on the Gα subunit.</p> Signup and view all the answers

    Which of the following mechanisms is NOT a form of signal termination?

    <p>Binding of G proteins to the receptor.</p> Signup and view all the answers

    Which factors inhibit the activity of cAMP phosphodiesterase?

    <p>Caffeine, theophylline, and theobromine.</p> Signup and view all the answers

    How does β-arrestin contribute to the termination of signaling?

    <p>By preventing receptor interaction with G proteins.</p> Signup and view all the answers

    What happens to a receptor when signal molecules leave it?

    <p>It reverts to its inactive state.</p> Signup and view all the answers

    What is a major consequence of cAMP being rapidly cleaved by phosphodiesterases?

    <p>Rapid termination of the signaling effect.</p> Signup and view all the answers

    What is the primary function of cAMP in cellular signaling?

    <p>To function as a secondary messenger.</p> Signup and view all the answers

    What role does calmodulin play in G-protein-linked signaling?

    <p>It amplifies Ca2+ signals for various biological processes.</p> Signup and view all the answers

    Which of the following best describes the primary function of G proteins?

    <p>They couple receptor activation to target proteins.</p> Signup and view all the answers

    What initiates the opening of K+ channels in heart muscle cells?

    <p>Binding of acetylcholine to its G-protein-linked receptor.</p> Signup and view all the answers

    Which process is responsible for signal amplification in cellular responses?

    <p>Enzyme cascades that increase activated products.</p> Signup and view all the answers

    What describes a key aspect of the fine-tuning process in G-protein-linked signaling?

    <p>Enhancing overall efficiency through scaffolding proteins.</p> Signup and view all the answers

    What effect does the hydrolysis of bound GTP by the α subunit have on the G protein?

    <p>It re-associates the subunits to form an inactive G protein.</p> Signup and view all the answers

    Which factor influences the specificity of the signal response in G-protein signaling?

    <p>The classification of cell surface receptors.</p> Signup and view all the answers

    Which of the following accurately identifies a consequence of receptor activation in heart muscle cells?

    <p>Opening of K+ channels, leading to hyperpolarization.</p> Signup and view all the answers

    What is a primary characteristic of G-protein-linked receptors?

    <p>They have a structure that traverses the membrane seven times.</p> Signup and view all the answers

    Which subunit is primarily responsible for binding GTP in a G-protein?

    <p>Gα</p> Signup and view all the answers

    Which of the following statements accurately describes the function of G-proteins?

    <p>They operate as a binary switch activated by receptor confirmation.</p> Signup and view all the answers

    What defines the difference between primary and secondary messengers?

    <p>Primary messengers are generated outside the cell, and secondary messengers act inside.</p> Signup and view all the answers

    In the context of G-proteins, what role does the α subunit play after it binds GTP?

    <p>It hydrolyzes GTP into GDP and Pi, facilitating signal termination.</p> Signup and view all the answers

    Which of the following describes a key function of G-proteins in signal transduction?

    <p>They act as switches to transmit signals from receptors to effectors.</p> Signup and view all the answers

    What happens to a G-protein when its α subunit binds to GDP instead of GTP?

    <p>The G-protein becomes inactive and detached from the receptor.</p> Signup and view all the answers

    Which correctly describes the nature of the signal molecules that bind to G-protein-linked receptors?

    <p>They can vary widely, including proteins and derivatives of fatty acids.</p> Signup and view all the answers

    What role does adenylate cyclase play in the signal transduction process?

    <p>It catalyzes the conversion of ATP to cyclic AMP.</p> Signup and view all the answers

    Which of the following occurs first upon ligand binding to a GPCR?

    <p>A conformational change in the receptor.</p> Signup and view all the answers

    Which statement accurately describes G-protein activation?

    <p>Gα releases GDP and binds GTP to become active.</p> Signup and view all the answers

    What is the function of cyclic AMP in cellular signaling?

    <p>It acts as a second messenger to amplify the signal.</p> Signup and view all the answers

    What happens during the slow hydrolysis of GTP in a G-protein signaling pathway?

    <p>Gα becomes inactive and reassociates with Gβ and Gγ.</p> Signup and view all the answers

    In G-protein-coupled receptor signaling, which mechanism is primarily responsible for the production of inositol triphosphate?

    <p>Activation of phospholipase C.</p> Signup and view all the answers

    What specifically triggers the active form of Gα in the signaling process?

    <p>It releases GDP and binds GTP.</p> Signup and view all the answers

    What effect does the activated cAMP-dependent protein kinase (PKA) have on cellular pathways?

    <p>It phosphorylates target proteins to regulate their activity.</p> Signup and view all the answers

    Study Notes

    G-Protein-Linked Receptors

    • G-protein-linked receptors are the largest family of cell-surface receptors in mammalian cells
    • They respond to a wide variety of extracellular signals, including hormones, local mediators, and neurotransmitters
    • These signals vary in structure and function, including proteins, small peptides, and derivatives of amino acids or fatty acids
    • Each signal has a specific receptor.
    • All analyzed G-protein-linked receptors have a similar structure: a single polypeptide chain that crosses the lipid bilayer seven times
    • Stimulation of a G-protein-linked receptor activates G-protein subunits
    • The receptor protein changes conformation when a signal molecule binds to it
    • The receptor protein activates a G protein located on the cytoplasmic side of the plasma membrane
    • G-proteins are heterotrimeric, with three subunits: α, β, and γ
    • The α subunit binds GTP and can hydrolyze it to GDP + Pi
    • The α, and γ subunits have lipid anchors that attach the G-protein to the plasma membrane cytosolic surface
    • There are two broad subclasses of trimeric G-protein-activated signal transduction pathways
    • One subclass activates adenylyl cyclase and the other activates phospholipase C
    • Adenylyl cyclase produces the small intracellular signaling molecule, cyclic AMP (cAMP)
    • Phospholipase C produces inositoltriphosphate and diacylglycerol

    Signal Transduction Process via GPCR

    • Extracellular signal molecule binds to a seven-pass transmembrane receptor
    • Receptor protein undergoes a conformational change
    • The receptor protein activates a G protein located on the underside of the plasma membrane
    • The a subunit of the G protein releases GDP and binds GTP
    • The activated a subunit disassociates from the By complex
    • The activated a subunit, along with the free By complex, can regulate target proteins
    • Hydrolysis of GTP to GDP turns the G protein back off and allows the G protein to re-associate as an inactive G protein

    G-Proteins

    • G proteins are similar in structure and activation
    • There are several varieties of G proteins. Each is specific to a particular set of receptors and particular set of downstream target proteins
    • G-proteins activate effector proteins, typically an enzyme

    Some G-Proteins Activate Adenylyl Cyclase

    • Adenylate Cyclase (AC) is a transmembrane protein
    • Adenylate Cyclase is responsible for producing the small intracellular signal molecule cyclic AMP (cAMP)
    • Phospholipase C is an enzyme responsible for producing inositol triphosphate (IP3) and diacylglycerol (DAG)

    Some G-Proteins Activate Phospholipase C

    • Signal molecules activate the G-protein, causing a cascade of events
    • Activation of phospholipase C leads to the production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG)
    • IP3 triggers the release of Ca2+ ions from intracellular stores
    • DAG activates protein kinase C (PKC)

    A Ca2+ Signal Triggers Many Biological Processes

    • Ca2+ has an important role as an intracellular messenger
    • The effects of Ca2+ are largely indirect, mediated through Ca2+-binding proteins
    • The most widespread and common Ca2+-responsive protein is calmodulin
    • Examples of Ca2+-mediated responses include fertilization in eggs

    Fine-Tuning of the Response

    • There are four aspects of fine-tuning to consider
      • Amplifying the signal
      • Specificity of the response
      • Overall efficiency of response
      • Termination of the signal
      • Signal amplification: Enzyme cascades amplify the cell's response -At each step, the number of activated products is much greater than in the preceding step

    What Happens If the Signal Cascade Can't Be Turned Off?

    • Vibrio cholera secretes enterotoxin, which modifies the Gs protein, preventing it from hydrolyzing GTP
    • This permanently activates the stimulatory G protein
    • This causes severe watery diarrhea and dehydration

    Summary: G-PROTEIN-COUPLED RECEPTORS

    • Stimulation of GPCRs activates G-protein subunits.
    • Some G proteins regulate ion channels.
    • Some G proteins activate membrane-bound enzymes.
    • The cyclic AMP (cAMP) pathway activates enzymes and turns genes on and off.
    • The inositol phospholipid pathway increases intracellular Ca2+.
    • Intracellular signaling cascades can create rapid, sensitive, and adaptive responses.

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    G-Protein-Coupled Receptors PDF

    Description

    Explore the fascinating world of G-protein-linked receptors, the largest family of cell-surface receptors in mammals. This quiz covers their structure, function, and how they respond to various extracellular signals including hormones and neurotransmitters. Test your knowledge on the activation of G-proteins and their role in cellular signaling.

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