Signal Reception and Transduction

  • 9-Protein Structure

    • Composed of 3 subunits:
    • 2 Alpha subunits (α)
    • 1 Beta subunit (β)
    • 1 Gamma subunit (γ)

  • Signal Reception: GPCR (G-Protein Coupled Receptors)

    • Acts as a molecular switch.

    • When activated, the alpha subunit detaches to activate an enzyme.

    • Activation Process:

    1. Inactive G protein is bound to GDP.
    2. GTP replaces GDP upon ligand binding, activating the G protein.
    3. The activated G protein activates downstream enzymes, leading to cellular responses.

  • Signal Reception: RTK (Receptor Tyrosine Kinase)

    • Transfers phosphate groups from ATP to specific tyrosine residues on a protein, initiating signal transduction pathways.
    • Can trigger multiple downstream signaling pathways through dimerization of inactive monomers.

  • Ion Channels

    • Act as selective gates that open or close in response to specific signaling molecules (ligands).
    • Mechanism:
    • Closed when no signaling molecule is bound; opens when a ligand binds, allowing ions to flow through, leading to cellular responses.

  • Intracellular Receptors

    • Located within the cytoplasm; bind hydrophobic and small ligands like steroid hormones and thyroid hormones.

    • Function as transcription factors that regulate gene expression by turning genes on/off.

    • Example:

    • Hormone (i.e., aldosterone) enters the cell, binds to the receptor, and the hormone-receptor complex binds to DNA, leading to the synthesis of new proteins.

  • Difference Between Plasma Membrane and Intracellular Receptors:

    • Intracellular Receptors:
    • Bind to hydrophobic ligands (e.g., steroid hormones, Vitamin D).
    • Plasma Membrane Receptors:
    • Bind to hydrophilic ligands (e.g., proteins, amino acids).

  • Phosphorylation Cascade

    • A series of enzymatic reactions where protein kinases phosphorylate proteins, thus amplifying the signal.
    • Activated proteins contain a phosphate group (phosphorylated).
    • Deactivated proteins are those without a phosphate group (dephosphorylated).

  • Transduction via Second Messengers

    • Example: Ligand epinephrine activates adenylate cyclase to produce cAMP, a second messenger.
    • First Messenger: Signaling molecule (e.g., epinephrine) activates G protein-coupled receptors.
      • Activates numerous downstream targets, amplifying the response.
    • Phospholipase C triggers a different pathway resulting in IP3 and DAG, leading to the release of Ca²+ from the endoplasmic reticulum, serving as a second messenger.

  • Summary of the Signaling Process Using Second Messengers:

    1. Signaling molecule binds to receptor.
    2. Activation of G protein followed by production of second messengers.
    3. Activation of downstream kinases leading to cellular responses (e.g., glycogen breakdown into glucose-1-phosphate).