Civ: Explain receptor activity

Definitions

  • Receptor = protein which contains a region that ligand binds to to bring about a response
  • Ligand = a molecule which produces a signal by binding to a target protein
  • Secondary messenger = an intracellular substance that mediates cell activity by relaying a signal from an extracellular molecule i.e. a neurotransmitter

3 main classes of receptors

  1. Alter ion permeability
  2. Produce secondary messengers
  3. Regulate gene transcription

1) Altered Ion Permeability

    • Pentameric
      • 5 membrane spanning units
      • Ligand binds → conformational ∆ → allows channel to open → ions move down [ ] gradient g. nAChR, GABAA
    • Ionotropic glutamate
      • NMDA, AMPA ionotropic ion channels
      • Form Na+, K+ & Ca2+ (NMDA only) when glutamate binds
    • Purinergic receptors
      • PX1 & PX2
      • Activated by ATP
      • Permeable to Na, K, Ca
      • Assoc w mechanosensation & pain

2) Production of Secondary Messengers

Secondary messenger = intracellular substance that mediates cell activity by relaying a signal from an extracellular molecule

  • G proteins
    • Serpentine with 7 transmembrane helical segments α, β, γ subunits
    • Extracellular signal is transduced by G protein & results in intracellular ∆
    • Inactive G Pr bound GDP to a subunit
    • Activation by ligand
    • GDP replaced with GTP
    • GTP α subunit dissociates
    • Activates/inhibits effector protein
    • Effector protein → AC, Phospholipase C, ion channels
    • KA METABROTROPIC (i.e. distinguishing from Ionotropic)
    • Named for a subunit
      • GS → activates AC
      • Gi → inhibits AC
      • Gq → activates Phospholipase C
    • Guanyl Cyclase
      • ANP = peptide hormone of atria
      • Converts GTP → CGMP
      • cGMP phosphorylates protein
    • Membrane tyrosine kinase
      • Ligand binds receptor
      • Intracellular TK residues are phosphorylated, which then phosphorylates other targets e.g Insulin receptor

Nucleic Acid Synthesis

  • Ligand binds receptor
  • Receptor activation → conformational ∆
  • Activated receptor translocates to nucleus
  • Associates with DNA to influence mRNA production
    • Slow
    • Steroids, thyroxine