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. (D)</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. (D)</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. (C)</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. (D)</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. (A)</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. (D)</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. (D)</p> Signup and view all the answers

What role do scaffolding proteins play in signaling pathways?

<p>They increase efficiency by grouping involved proteins. (A)</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. (C)</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. (D)</p> Signup and view all the answers

Which factors inhibit the activity of cAMP phosphodiesterase?

<p>Caffeine, theophylline, and theobromine. (C)</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. (A)</p> Signup and view all the answers

What happens to a receptor when signal molecules leave it?

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

What is the primary function of cAMP in cellular signaling?

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

Which process is responsible for signal amplification in cellular responses?

<p>Enzyme cascades that increase activated products. (B)</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. (B)</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. (C)</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. (A)</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. (C)</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. (C)</p> Signup and view all the answers

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

<p>Gα (B)</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. (A)</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. (A)</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. (D)</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. (B)</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. (A)</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. (D)</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. (B)</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. (C)</p> Signup and view all the answers

Which statement accurately describes G-protein activation?

<p>Gα releases GDP and binds GTP to become active. (C)</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. (C)</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. (C)</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. (D)</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. (A)</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. (B)</p> Signup and view all the answers

Flashcards

G-protein-linked receptors

Largest cell-surface receptor family, activated by diverse extracellular signals (hormones, mediators, neurotransmitters).

Signal Molecules

Extracellular molecules that bind to G-protein-linked receptors, varied in structure (proteins, peptides, etc.).

G-protein structure

Heterotrimeric protein with α, β, and γ subunits; α subunit binds GTP & hydrolyzes it to GDP+Pi, α and γ subunits anchor the G-protein to the membrane.

G-protein activation

Extracellular signal binding to the receptor changes receptor's shape. This changed shape activates the associated G-protein.

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Signal transduction

Process whereby a cell converts one kind of signal or stimulus to another.

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GTP hydrolysis

Process where GTP is broken down to GDP and a phosphate group (Pi), inactivating a G-protein.

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Seven-pass transmembrane receptor

A protein that spans the cell membrane seven times.

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Receptor conformational change

A change in the receptor's shape when a signal molecule binds, enabling it to interact with other proteins.

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G-protein activation

A signal transduction pathway where a G-protein is activated by a ligand binding to a receptor, leading to downstream effects.

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Trimeric G-protein

A type of G-protein that consists of three subunits (α, β, and γ) that assemble in an inactive state.

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Adenylate Cyclase

An enzyme that produces cyclic AMP (cAMP) from ATP in response to a specific signal.

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Cyclic AMP (cAMP)

A small intracellular signaling molecule that acts as a second messenger, activating downstream effectors.

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Phospholipase C

An enzyme that produces inositol triphosphate (IP3) and diacylglycerol (DAG) in response to a specific signal.

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GDP-GTP Exchange

A conformational change in the G-protein that facilitates GTP binding and GDP release.

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Second Messenger

A small molecule that relays a signal from a receptor to a target effector inside a cell.

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GPCR

G Protein-Coupled Receptor. A type of transmembrane receptor that activates G-proteins in response to a ligand.

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Signal Amplification

Process where a single hormone molecule triggers a large number of cellular responses, from 10 to a million+.

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G-protein Activation

A G-protein changes shape upon hormone bonding, triggering downstream effects.

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Second Messenger (cAMP)

A small molecule that relays a signal inside the cell after hormone binding.

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Protein Kinase Activation (PKA)

Enzymes activated by cAMP, in turn activating downstream targets.

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Cellular Specificity

Different cells have unique proteins to detect and respond to specific signals or hormones.

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Calmodulin's role

A calcium-responsive protein that plays a major role in biological processes triggered by calcium ions.

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G Protein Function

G proteins relay signals from a receptor to a target protein, like an ion channel or enzyme, mediating cellular responses.

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G-protein targets

Ion channels or membrane-bound enzymes are the targets for G protein subunits, specifically regulated by different subtypes of G proteins.

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Signal Amplification

A process in which enzyme cascades increase the cellular response to a stimulus, with each step having more activated products.

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Specificity in Response

Different G protein types activate different target proteins, enabling a cell to respond appropriately to specific signals.

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Second Messengers

Chemical messengers that amplify and relay intracellular signals initiated by extracellular signal molecules.

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Heart muscle K+ Channels

G proteins regulate the opening of potassium (K+) channels in the heart's plasma membrane, affecting heart beat.

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GTP Hydrolysis

Breaking down GTP (guanosine triphosphate) into GDP (guanosine diphosphate) and phosphate, deactivating G-proteins.

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Signal Termination

Methods a cell uses to stop a signal after it's received.

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Scaffolding Protein Function

Large proteins that organize proteins in a signaling pathway, making signal transduction more efficient.

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Signal Pathway Branching

Multiple branches in a cell signaling process that allow one signal to produce multiple responses.

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G-protein inactivation

GTP hydrolysis inactivates a G-protein by converting GTP to GDP which causes the alpha subunit to detach from the other subunits.

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cAMP Degradation

cAMP is converted back into AMP (inactive) by enzymes called phosphodiesterases, rapidly terminating the signal.

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Receptor Phosphorylation

Activating receptors can be phosphorylated, which prevents it from interacting with G-proteins, turning it off.

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G-protein coupled receptor inactivation

Activated receptors bind to an additional protein (β-arrestin) which causes a signal termination by preventing the interaction of the receptors with G-proteins.

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Signal Efficiency

The ability of cellular systems to generate a proportional response to a signal.

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β-Arrestin's role in signal termination

β-arrestin promotes receptor removal from the membrane via clathrin-mediated endocytosis, effectively turning off the signal. It can also associate with phosphodiesterase, accelerating cAMP to AMP conversion, further contributing to signal deactivation.

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cAMP pathway shut-off

cAMP activates protein kinase A (PKA), which phosphorylates target proteins. Protein phosphatases dephosphorylate these proteins, reversing the signal and returning the system to its original state.

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Vibrio cholera mechanism

Vibrio cholera toxin alters G-protein (Gs) function, preventing its natural GTP hydrolysis. This locks the G-protein in an 'on' state, causing unrelenting cAMP production, resulting in severe diarrhea.

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G-protein function

G-proteins relay signals from extracellular stimuli to intracellular effectors, impacting various cellular processes.

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Intracellular signaling cascades

Intracellular signaling cascades involve a series of molecular interactions to convert extracellular signals into cellular responses.

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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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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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