singal transduction

SIGNAL TRANSDUCTION

Overview

  • Definition: Signal transduction refers to the process through which cells receive and respond to signals from the environment or other cells, involving a sequence of molecular events that culminate in a cellular response.

  • Key Components: Receptors, ligands, second messengers, and relay molecules.

RECEPTORS RECEIVE THE SIGNAL AND INITIATE CELLULAR RESPONSE

  • Receptor-Ligand Binding: The interaction between a receptor (protein) and a ligand (primary messenger) initiates the cellular response.

    • Receptor: A specific protein that binds to a ligand to undergo a conformational change.

    • Ligand: A molecule (e.g., hormone, neurotransmitter) that binds to a receptor to trigger a response.

  • Cellular Location: This interaction primarily takes place in the cytosol and across the plasma membrane.

LIGAND BINDING ALTERS THE RECEPTOR

  • Ligand binding induces changes in the receptor’s conformation, leading to activation and subsequent signal transduction.

EXAMPLES OF RECEPTORS

G-Protein-Coupled Receptors (GPCRs)

  • Characterization: GPCRs are the largest and diverse group of membrane receptors in eukaryotes.

  • Functions: They play crucial roles in various human physiological functions.

  • Diversity: Humans possess approximately 1,000 distinct GPCRs, each specific to particular signals.

  • Example: The epinephrine receptor, which binds the hormone epinephrine (adrenaline).

Receptor Tyrosine Kinase (RTK)

  • Structure: Contains an a-helix embedded in the membrane with a signal-molecule binding site.

  • Activation Process:

    • Inactive State: Inactive proteins (inactive tyrosine-kinase receptors as monomers) exist before ligand binding.

    • Active State: Binding of a signal molecule activates the receptor, causing dimerization and autophosphorylation (addition of phosphate groups from ATP) for signal propagation.

    • Output: This leads to the activation of downstream proteins and cellular responses.

THE SIGNAL IS PROPAGATED AND "TRANSDUCED"

  • Process Overview:

    1. Reception: The initial interaction between the receptor and ligand occurs at the plasma membrane.

    2. Transduction: The signal is conveyed through different pathways involving various molecules (often termed second messengers).

    3. Response: Final output, resulting in a cellular response such as gene expression or cellular proliferation.

TRANSDUCTION OF THE SIGNAL MEDIATED BY PHOSPHORYLATION

  • Mechanism: Phosphorylation (addition of phosphate groups) by kinase enzymes stands as a common mechanism through which signal transduction pathways operate.

Key Terms

  • Phosphorylation: The process of adding a phosphate group (ATP → ADP, with phosphate group added to proteins).

  • Dephosphorylation: The removal of a phosphate group mediated by phosphatases.

PHOSPHORYLATION CASCADE

  • Description: A sequential activation of proteins through phosphorylation that relays the signal downstream.

  • Diagram:

    • Inactive relay proteins are activated one after another through phosphorylation.

    • This process involves protein kinases alternating between active and inactive states, utilizing ATP for energy.

RECEPTION AND TRANSDUCTION LEADING TO CELLULAR RESPONSE

  • Binding of growth factors triggers a series of activations among relay proteins and kinases.

    • Final Outcome: This culminates in the activation of transcription factors leading to gene expression and production of mRNA, which translates into protein synthesis in the nucleus.

G-PROTEIN ACTIVATION OF SECOND MESSENGERS

  • Mechanism:

    • Activation of G-proteins results in the production of second messengers (e.g., cAMP) that facilitate signal amplification.

    • One signal molecule can activate multiple G-proteins, producing a broader signaling response.

FLOW OF INFORMATION DURING CELL SIGNALLING

Steps of Signaling

  1. Receptor-ligand Binding

  2. Signal Transduction Via Second Messengers

  3. Cellular Responses (e.g., changes in gene expression).

Integration of Signals

  • Multiple pathways can interact:

    • One receptor can activate multiple pathways.

    • Different receptors can activate the same pathway or lead to cross-talk between pathways.

RECEPTORS CAN ACTIVATE DIFFERENT SECOND MESSENGERS

  • Diversity in signaling can lead to varied cellular responses depending on the type of receptor and secondary messengers involved (e.g., Ca²⁺, protein kinase signaling).

DIFFERENT CELLULAR RESPONSES TO THE SAME SIGNAL

  • Variability: Different cells may respond differently to the same signaling molecule based on the receptor type or downstream signaling components.

    • Cell A: Responses lead to a single outcome.

    • Cell B: Pathway branches leading to multiple outcomes.

    • Cell C: Interaction between two pathways (cross-talk).

    • Cell D: Activation of a response due to different receptor types leading to variable outcomes.