lecture 15 full notes

Lecture 14 - Cell Signaling Cont.

Overview

  • Topics covered include:

    • Epinephrine’s second messenger

    • G-protein coupled receptor (GPCR) signal transduction

    • Enzyme cascades and amplification

    • Receptor tyrosine kinases, Ras, and cancer

    • Hormone receptors

    • Receptor-mediated endocytosis

Liver Cell Response to Epinephrine

  • Liver cells respond to epinephrine by:

    • Breaking down glycogen

    • Producing glucose

    • Correct answers:

    • Both A and B

Production of cAMP

  • The response of liver cells to epinephrine results in:

    • Activation of the enzyme glycogen phosphorylase

    • Secretion of digestive enzymes into the duodenum

    • Opening of sodium (Na+) channels

    • Correct answer: Glycogen phosphorylase activation

Definition of Second Messenger

  • A second messenger is defined as:

    • A diffusible hormone that binds to a receptor on the surfaces of cells, activating a response.

    • The enzyme that catalyzes a reaction in response to ligand binding.

    • A compound secreted by cells in response to ligand binding.

    • A diffusible compound produced upon ligand binding that elicits the cellular response.

Epinephrine Signaling Pathway

  • The second messenger involved in epinephrine signaling is cyclic AMP (cAMP).

  • Binding of epinephrine to its receptor initiates cAMP production.

GPCR Structure and Function

  • The epinephrine receptor is classified as a G-protein-coupled receptor (GPCR), which encompasses the following features:

    • Contains 7 transmembrane domains

    • Binds to heterotrimeric G-proteins consisting of α (alpha), β (beta), and γ (gamma) subunits

    • In the absence of ligand, Gα subunit is bound to GDP (inactive state)

Activation of G-Proteins

  • The binding of epinephrine leads to:

    1. Binding of epinephrine to its receptor

    2. Conformational change in the receptor

    3. Induction of conformational change in the G-protein

    4. GDP is replaced by GTP on Gα subunit (GDP/GTP exchange)

    5. Gα-GTP (active form) dissociates from Gβγ subunits

Enzyme Activation

  • Gα activates adenylyl cyclase:

    • Gα diffuses through the membrane to adenylyl cyclase.

    • Adenylyl cyclase undergoes a conformational change to its active form.

    • Catalyzes conversion of ATP to cAMP + PPi (pyrophosphate).

cAMP and Glycogen Phosphorylase Activation

  • cAMP activates glycogen phosphorylase:

    • Glycogen phosphorylase is phosphorylated in response to epinephrine.

    • Active form contains phosphate; inactive form does not.

    • Phosphorylation requires ATP.

Catalysis of Phosphorylation

  • Kinases facilitate phosphorylation utilizing phosphate from ATP.

    • Phosphorylation is a common regulatory mechanism for proteins, especially enzymes.

    • The process is generally rapid and reversible.

Phosphorylation of Glycogen Phosphorylase

  • Glycogen phosphorylase phosphorylation is catalyzed by phosphorylase kinase.

    • Pathway Summary Illustration

    • Glycogen phosphorylase 'b' (inactive) is converted to glycogen phosphorylase 'a' (active) through phosphorylation.

Activation of Phosphorylase Kinase

  • Phosphorylase kinase also undergoes activation via phosphorylation.

    1. Protein kinase A (PKA) is activated by cAMP.

    2. PKA catalyzes the phosphorylation of phosphorylase kinase.

Enzyme Amplification Mechanism

  • Illustrates enzyme cascades amplifying signals:

    • Example:

    • 1 epinephrine can lead to 20 active adenylyl cyclase

    • 20 ATP → 20 cAMP, which activate 20 PKA

    • Resulting in 100x more phosphorylase kinase active form, inducing glucose release from glycogen.

Types of Cell Responses

  • Responses include:

    • Channel opening

    • Production of a second messenger

    • Direct activation of receptor enzymatic activity

    • Steroid hormones regulating gene transcription

Receptor Tyrosine Kinases (RTKs) Signaling

  • Growth factors signal through RTKs:

    1. Growth factor dimer binds to a pair of receptors

    2. Induction of receptor conformational change

    3. Kinase activation (self-phosphorylation) occurs

    4. GTP-GDP exchange on Ras (small GTPase) leads to cascaded activation

    5. Activation of downstream enzymes and transcription factors

Steroid Hormone Signaling

  • Characteristics of steroid hormones:

    • Diffuse through the cell membrane

    • Bind to intracellular receptors

    • These hormone-receptor complexes regulate gene transcription by binding directly to DNA

Characteristics of Signaling Pathways

  • Key properties:

    • High affinity and specificity between ligand and receptor

    • May involve second messengers or might not

    • Can lead to short-term or long-term cellular changes

    • Enzyme cascades amplify the initial signals significantly

Receptor-Mediated Endocytosis

  • Important for signal transduction in certain ligand-receptor scenarios:

    • Functions in downregulation of response (desensitization)

    • Plays a role in cell uptake of pathogens

Viruses and Receptor-Mediated Endocytosis

  • Mechanism utilized by many viruses to enter cells (e.g., SARS-CoV-2)

    • Example:

    • Spike protein interacts with ACE2 receptor leading to viral entry

Role of Phosphatases

  • Enzymes called phosphatases remove phosphate groups.

    • Their activity can decrease glucose production in response to epinephrine by deactivating signaling pathways.

cAMP Phosphodiesterase Activity

  • cAMP phosphodiesterase catalyzes the conversion of cAMP to AMP.

    • This decreases response in liver cells to epinephrine by reducing available cAMP.

GTPase Activity

  • GTPases hydrolyze GTP to GDP which also mediates the duration of the signaling pathway.

    • In liver cells, this enzymatic activity can decrease response to epinephrine.

Summary

  • Key takeaways from signaling:

    • cAMP functions as a second messenger in the context of epinephrine signaling in liver cells.

    • GPCR signal transduction is generally fast and reversible.

    • Enzyme cascades serve as a mechanism for signal amplification.

    • Receptor Tyrosine Kinases (RTKs) primarily promote cellular proliferation via gene transcription.

    • Steroid hormones uniquely activate gene transcription through direct receptor action.

    • Receptor-mediated endocytosis serves multifaceted roles in signaling and viral entry into cells.