14D Transferring Information across Membranes

Biochemistry 405 Lecture #14, Module D: Transferring Information Across Membranes

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Key Concepts of Signal Transduction

  1. Modularity:

    • Adaptation of similar structures to respond to different signals.

  2. Specificity:

    • Specific receptors respond to specific ligands.

  3. Amplification:

    • One ligand binding event outside a cell can activate thousands of enzymes inside the cell, resulting in a large and rapid cellular response.

  4. Termination:

    • Mechanisms are in place to turn off a signal once it is no longer needed.

Information Transfer: G-Protein Coupled Receptors (GPCRs)

  • The β2 adrenergic receptor (β2AR) is an example of a 7-trans-membrane receptor which facilitates information transfer within cells.

  • Mechanism of Action:

    1. Ligand Binding: When a ligand binds to the receptor, it stimulates:

    • The association of the G-protein complex with the receptor.

    • The exchange of GDP for GTP on the α-subunit of the G-protein.

    1. GTP Binding: This leads to the dissociation of the G-protein complex and activation of other enzymes or ion channels within the cell (often involving second messengers).

    2. Signal Amplification: A single ligand-bound receptor can activate multiple Gα subunits, thus amplifying the signal.

  1. Inactivation: The slow hydrolysis of GTP inactivates the α-subunit, causing the re-assembly of the heterotrimeric G-Protein complex, serving as a built-in timing mechanism.

  • Epinephrine Response: The binding of epinephrine outside the cell stimulates the production of cAMP and calcium ion (Ca²⁺) influx into the cell.

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Role of G-Protein Coupled Receptors in Signal Amplification

  • GPCRs are crucial for most cellular responses including:

    • Hormonal Responses: e.g., epinephrine, insulin.

    • Neurotransmission: e.g., serotonin, dopamine.

    • Sensation: e.g., light perception, olfaction, taste.

    • Pathological Conditions: e.g., hypertension, cardiac arrhythmia, glaucoma, anxiety, migraine headaches.

  • Notably, 1/3 to 1/2 of all drugs on the market specifically target GPCRs.

Diverse Functions of Heterotrimeric G-Proteins

  • Different heterotrimeric G-Proteins in various tissues lead to distinct physiological responses.

  • Types of Second Messengers:

    • Adenylyl Cyclase: Involved in generating cyclic AMP (cAMP)

    • Ion Channels: Regulation includes Ca²⁺ influx.

    • Phospholipase C Activation:

      • Phospholipase C regulates membrane proteins and can bind to and regulate specific cytoplasmic proteins, generating diacylglycerol (DAG) and inositol trisphosphate (IP₃) from phosphatidylinositol 4,5-bisphosphate (PIP₂).

Insulin and Its Receptor

  • Insulin is classified as a peptide hormone and is disseminated through the circulatory system.

  • The binding of insulin occurs at the extracellular receptor domain.

  • This binding activates the Tyrosine Kinase catalytic domain located inside the cell, as follows:

    1. Receptor Tyrosine Kinases (RTKs): Insulin receptor belongs to a large family of plasma membrane receptors characterized by an extracellular ligand-binding domain linked to an intracellular catalytic domain.

    2. Ligand-Induced Dimerization: Upon ligand binding, either dimerization or a conformational change occurs that brings together the kinase domains.

    3. Self-Activation: This results in a process called auto-phosphorylation, which starts a signaling cascade that activates multiple kinases.

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Signaling Response to Insulin

  • Muscle and Adipose Cell Response:

    • The activation of the PDK1 (PIP₃-dependent protein kinase) leads to a signaling cascade that promotes the movement of Glut4 transporters:

    • These transporters, stored in intracellular vesicles, are translocated to the plasma membrane.

    • This results in increased uptake, storage, and utilization of glucose, evidencing one of the cellular responses to insulin.

Structural Details of Insulin Receptor Activation

  • Understanding the structural changes related to the activation of the Insulin Receptor Tyrosine Kinase (IRK) is essential. The activation loop (colored in green) moves to allow substrates access to the active site:

    • Phosphorylation of Tyrosine Residues: This action locks the activation loop in an “open” conformation which ultimately activates the kinase.

Activation States of the Kinase

  • Inactive State: Represented by the designation of: 1IRK, 1IR3 (crystallographic structures).

  • Active State: The kinase’s N-lobe and C-lobe arrangement allows for full enzymatic function once activated by phosphorylation and appropriate conformational changes.