cell signaling

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Questions and Answers

What role do membrane proteins play in relation to the hydrophobic core of biomembranes?

They facilitate transport of specific molecules across the membrane. (correct)

They create a barrier to all solutes.

They strengthen the hydrophobic core through chemical reactions.

They change the hydrophobic nature of the membrane.

Which type of signaling involves signaling molecules that affect target cells in close proximity?

Endocrine signaling

Autocrine signaling

Paracrine signaling (correct)

Juxtacrine signaling

Which of the following best describes the first step of extracellular signaling?

Binding of signal by a receptor.

Transport of signaling molecule to target cell.

Specific changes in cellular function.

Synthesis of signaling molecule. (correct)

What is the function of second messengers in signal transduction pathways?

To initiate a cascade of reactions leading to cellular responses. Signup and view all the answers

What is an example of a signaling molecule that can act both as a neurotransmitter and a systemic hormone?

Epinephrine Signup and view all the answers

What is typically the first response in a signaling pathway initiated by a hydrophilic chemical signal?

Changing the activity or function of a pre-existing protein. Signup and view all the answers

In which type of signaling do cells respond to substances that they themselves release?

Autocrine signaling Signup and view all the answers

What facilitates the specific changes in cellular function, metabolism, or development following signal transduction?

Activation of transcription factors. Signup and view all the answers

What is the primary role of PDZ domains in cytosolic proteins?

They organize and localize receptors in the plasma membrane. Signup and view all the answers

Which characteristic describes GTP-binding switch proteins?

They belong to the GTPase superfamily. Signup and view all the answers

How do SH3 domains interact with proteins?

By binding to proline-rich sequences. Signup and view all the answers

What are the typical structural features of most cell surface receptors mentioned?

Contain multiple subunits that can bind to PDZ domains. Signup and view all the answers

Which statement accurately describes the role of actin filaments in membrane protein clustering?

They can bind many clusters, facilitating protein localization. Signup and view all the answers

What initiates the activation of GTP-binding proteins?

The release of GDP facilitated by GEF. Signup and view all the answers

In the context of intracellular signaling, what role do protein kinases play?

They phosphorylate target proteins to activate signaling. Signup and view all the answers

What is the effect when GTP is hydrolyzed to GDP in GTP-binding proteins?

It causes the protein to switch to an inactive state. Signup and view all the answers

What is the main function of protein kinases?

They phosphorylate other proteins. Signup and view all the answers

Which second messenger is responsible for activating protein kinase A?

cAMP Signup and view all the answers

What role does Ca2+ play in cellular responses?

It activates a variety of proteins for different cellular responses. Signup and view all the answers

Which of the following is NOT an element of the G Protein Coupled Receptor system?

Intracellular calcium channels Signup and view all the answers

What is one of the short-term effects of GPCR activation in cells?

Modification of existing proteins. Signup and view all the answers

What is the primary function of glucose-6 phosphate in muscle cells?

It enters glycolysis for ATP production. Signup and view all the answers

Which enzyme is responsible for reversing the effects of PKA in glycogen metabolism?

Phosphoprotein phosphatase (PP) Signup and view all the answers

How does PKA enhance glycogen metabolism directly?

By inhibiting glycogen synthase. Signup and view all the answers

What is produced when PIP2 is cleaved by phospholipase C?

DAG and IP3 Signup and view all the answers

What happens to calcium ions (Ca2+) as a result of IP3 binding to its receptor?

They are released from the smooth endoplasmic reticulum (ER). Signup and view all the answers

Epinephrine removal leads to a drop in which molecule, affecting PKA activity?

cAMP Signup and view all the answers

What type of signaling does activation of GPCRs typically amplify?

External signals via second messengers Signup and view all the answers

What role does DAG play in the IP3/DAG signaling pathway?

It recruits protein kinase C (PKC) to the membrane. Signup and view all the answers

What is the primary function of Gα subunit in trimeric G proteins?

To hydrolyze GTP to GDP Signup and view all the answers

Which of the following GPCRs is coupled to an inhibitory G protein?

Muscarinic acetylcholine receptors in cardiac muscle Signup and view all the answers

What role does adenylyl cyclase play in cellular signaling?

It synthesizes cAMP from ATP Signup and view all the answers

How does binding of a ligand to GPCRs affect G proteins?

It leads to the exchange of GDP for GTP on Gα Signup and view all the answers

Which of the following describes a primary consequence of Gβγ subunit activation?

It opens K+ channels to cause hyperpolarization Signup and view all the answers

What is the function of Protein Kinase A (PKA) in cellular signaling?

It phosphorylates target proteins Signup and view all the answers

Which type of GPCR is primarily responsible for increasing the levels of cAMP?

β-adrenergic receptors Signup and view all the answers

What is the overall effect of epinephrine on glycogen metabolism?

Stimulates glucose release by activating PKA Signup and view all the answers

Which structure of GPCRs is critical for their function?

The seven transmembrane domains Signup and view all the answers

Which of the following describes the role of second messengers in GPCR signaling?

They amplify the signaling response Signup and view all the answers

What occurs when acetylcholine binds to the cardiac muscarinic receptors?

The heart rate decreases due to K+ outflow Signup and view all the answers

How does the activity of G proteins get terminated?

Through hydrolysis of GTP to GDP Signup and view all the answers

Which structure in the cell is primarily targeted by G protein signaling pathways?

Membrane bound ion channels Signup and view all the answers

Study Notes

G Protein Coupled Receptors (GPCR)

A large family of cell surface receptors that are involved in a wide range of cellular processes.
They are responsible for detecting a variety of extracellular signals, including hormones, neurotransmitters, light, and odorants.
GPCRs are characterized by their seven transmembrane α-helical domains.
The N-terminus of the protein is located on the exoplasmic face of the membrane, and the C-terminus is located on the cytosolic face.
The C3 and C4 domains interact with trimeric G proteins on the cytosolic side of the membrane.

Trimeric G Proteins

Composed of three subunits: α, β, and γ.
The β and γ subunits remain associated and are referred to as the βγ subunit.
The α subunit is a GTPase switch protein that can be in an active (GTP-bound) or inactive (GDP-bound) state.

GPCR Signaling Pathways

When a ligand binds to a GPCR, it activates the associated trimeric G protein.
This activation involves the exchange of GDP for GTP on the α subunit of the G protein, causing it to dissociate from the βγ subunit.
The activated α subunit (α-GTP) or the βγ subunit can interact with downstream effector proteins.
These effector proteins are either membrane-bound ion channels or enzymes that catalyze the production of second messengers.

Examples of GPCR Signaling Pathways

Muscarinic Acetylcholine Receptors in Heart Muscle:
- Acetylcholine binding to muscarinic receptors in the heart is inhibitory, mediated by a Gi protein.
- This leads to the opening of K+ channels, hyperpolarization of the cell, and a decrease in the heart rate.
β-Adrenergic Receptors:
- Couple to the stimulatory Gs protein.
- Activate adenylyl cyclase and increase cAMP levels.
α-Adrenergic Receptors:
- Couple to the inhibitory Gi protein.
- Inactivate adenylyl cyclase and decrease cAMP levels.

Second Messengers in GPCR Signaling

cAMP: A cyclic nucleotide second messenger that activates protein kinase A (PKA).
IP3 (inositol triphosphate) and DAG (diacylglycerol):
- IP3 causes the release of Ca2+ from the endoplasmic reticulum (ER).
- DAG, a lipid-soluble molecule, activates protein kinase C (PKC) in the plasma membrane.

GPCRs and Metabolism

Epinephrine:
- A hormone that plays a role in the body's response to stress.
- Activates different types of GPCRs to regulate glycogen metabolism.
- Binding of epinephrine to β-adrenergic receptors triggers the breakdown of glycogen into glucose.

Glycogen Metabolism

Glycogen: A large glucose polymer stored in the liver and muscle.
Glycogen synthase: An enzyme that builds glycogen.
Glycogen phosphorylase: An enzyme that breaks down glycogen.

Regulation of Glycogen Metabolism

PKA plays a crucial role in regulating glycogen metabolism by phosphorylating and activating glycogen phosphorylase kinase (GPK) and inactivating glycogen synthase.
cAMP levels are critical for regulating PKA activity.

Signal Amplification

GPCR signaling pathways exhibit signal amplification.
One activated GPCR can trigger the activation of a cascade of downstream proteins, resulting in a significant amplification of the initial signal.

Key Concepts

Signal transduction: The process by which cells receive and respond to signals from their environment.
Second messengers: Small intracellular molecules that relay signals from cell surface receptors to downstream effectors.
Protein kinases: Enzymes that add phosphate groups to proteins.
Protein phosphatases: Enzymes that remove phosphate groups from proteins.
GTPases: Switch proteins that are active when GTP-bound and inactive when GDP-bound.
Effector proteins: Proteins that mediate the cellular response to a signal.

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Description

Test your knowledge on G Protein Coupled Receptors (GPCR) and their signaling pathways. This quiz covers the structure of GPCRs, the role of trimeric G proteins, and the mechanisms involved in signal transduction. Perfect for students studying cell biology and molecular signaling.