Civ / 25B01: Describe how ligands binding to cell membrane receptors can cause a change within that cell

25B01: Exam Report

Describe how ligands binding to cell membrane receptors can cause a change within that cell

25% of candidates passed this question.

This question expected candidates to describe the four mechanisms of transmembrane signalling that are mediated by cell membrane receptors. These are:

  • Ligand gated ion channels that open and close in response to ligand binding
  • G protein coupled and G-protein mediated production of intracellular second messengers
  • Ligand regulated transmembrane enzymes whose intracellular enzymatic activity is allosterically regulated by a ligand binding to extracellular protein binding sites.
  • Transmembrane receptors that bind to and stimulate tyrosine kinase

For each mechanism, a detailed answer would describe the receptor structure, location, an example of a natural ligands and an overview of the signalling pathway and physiological effects.

An example would be: Ligand gated ion channels are part of a membrane-spanning complex of protein subunits forming a channel through the membrane.

Binding of a ligand causes a conformational change allowing passage of an ion(s) down its concentration gradient resulting in a change in the resting membrane potential, depolarization or repolarization.

For example; nicotinic acetylcholine receptors are pentameric structures comprising 5 sub- units.

They are located at the neuromuscular junction and bind acetylcholine to cause opening of Na channels causing membrane depolarisation.
Information on drug-receptor interactions such as agonism, antagonism, receptor affinity and intrinsic activity was not required.

Cii / 25B01: Describe how ligands binding to cell membrane receptors can cause a change within that cell

Answer is specifically asking regarding ligands binding to cell membrane receptors (i.e. do not need to describe voltage gated ion channels or intracellular receptors)

1, Ligand gated ion channels

Structure and location:

  • Part of a membrane-spanning complex of protein subunits forming a channel through the membrane

Signalling Pathway and physiological effects:

  • Open and close in response to ligand binding
  • Binding of a ligand → conformational change → passage of ion(s) down its concentration gradient → change in the resting membrane potential →depolarization or repolarization

Example (1-2 examples and explain one):

  • Pentameric
    • GABA A receptor which allows a Cl channel to form → hyperpolarisation
    • 5HT3 receptor → Na+, K+, Ca2+ influx → depolarisation
    • Nicotinic Ach receptor at the NMJ which allows an Na channel to form → depolarisation of post synaptic membrane
  • Ionotropic glutamate
    • NMDA, AMPA and kainate iontropic ligand gated ion channels
      • They form Na, K and (NMDA only) Ca channels when glutamate binds

2. G protein coupled receptors and G-protein mediated production of intracellular second messengers

Structure and location:

  • 7 transmembrane domain
  • 3 subunits (heterotrimeric) – alpha, beta, gamma
  • Alpha subunits different between subtypes (s, i, q)
  • Called G proteins as they have the ability to bind GTP and GDP
  • Conformational change induced when ligand binds

Signalling Pathway and physiological effects:

  • Binding of a ligand → conformational change → activation of intracellular domain of receptor → activation of enzymatic process at intracellular effector site → change in secondary messenger concentration → action of secondary messenger on substrate
  • Gq → stimulate phospholipase C → PIP2 to IP3 (releases Ca+ from endoplasmic reticulum) + DAG (activates protein kinase C)
  • Gs → activates AC → catalyses conversion of ATP to cAMP → Protein kinase A (PKA)
    • Gs activation →↑AC→cAMP→PKA
      • cAMP roles:
        • Activation of Protein Kinases (i.e. PKA) and (therefore ↑phosphorylation of various proteins/ion channels) (short term effects)
          • ie. Phosphorylation of calcium channels –> ↑calcium influx into the cell, phosphorylation of MLCK
      • Gene transcription proteins and/or gene transcription (long term effects)
  • Gi → inhibits AC

Example:

  • Noradrenaline → alpha- 1→ gq → phospholipase c → ↑ IP3 & DAG → ↑ intracellular calcium → SM contraction → vasoconstriction
  • Hormones that interact with GPCRs
    • FSH, LH, ACTH, TSH, CRH, hCG, ADH, PTH, calcitonin, GHRH, glucagon, histamine, noradrenaline
  • Hormones influenced by IP3
    • GnRH, oxytocin, ADH, TRH, histamine, angiotensin II, gastrin

3. Ligand regulated transmembrane enzymes

Structure and location

  • Single-pass transmembrane receptors with extracellular ligand-binding domain and intracellular enzymatic domain (commonly guanylyl cyclase).

Signalling Pathway and physiological effects:

  • intracellular enzymatic activity is allosterically regulated by a ligand binding to extracellular protein binding sites.
  • Activation of guanylyl cyclase leads to increased cGMP production and downstream effects

Example

  • i.e. ANP/NO → Guanylyl cyclase activation → cGMP → PKG → ↓Ca2+ intracellular release (via phosphorylation of various proteins)

4. Tyrosine Kinase Receptors

Tyrosine = amino acid

Kinase = enzyme that catalyses the transfer of phosphate groups

Structure and location:

  • Transmembrane cell surface receptor/proteins that bind to and stimulate TK receptors
  • Components
    • Extracellular ligand binding domain
    • Transmembrane domain (Single transmembrane α-helix)
    • Intracellular: Tyrosine Kinase Domain (tyrosine residues)

Signalling Pathway and physiological effects:

  • Binding of a ligand (i.e. growth factors or local signalling) → conformational change → Dimerisation (Dimerisation = pairing of kinase domains) → activates the kinase and creates docking sites for intracellular proteins → phosphorylation of tyrosine kinase (phosphate is usually from ATP) → activation of various downstream pathways
  • Intracellular signalling cascade then occurs (MAP Kinase being the most well known)
    • RAS-RAF-MEK/ERK (MAPK) pathway
      • Major pathway for cell growth and differentiation
      • RAS → RAF → MEK → ERK (each kinase phosphorylates and activates the next → amplification)
    • PI3K–AKT–mTOR pathway
      • Cell survival, metabolism, protein synthesis
      • mTOR inhibitors i.e. everolimus for immunosuppression

Example:

  • e. Insulin, IGF1, FGF, PDGF, EGF
  • Insulin → Ras-GTP → MAP3K

Author: Owen Xie