1' Receptor superfamilies

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1. Receptor superfamilies

• ION CHANNEL RECEPTORS
• G-PROTEIN COUPLED RECEPTORS
• KINASE LINKED RECEPTORS
• INTRACELLULAR RECEPTORS

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Ion channel receptors (Ligand gated ion channels)
Five glycoprotein subunits traversing cell
membrane
Cationic ion channels for K, Na, Ca2
excitatory Anionic ion channels for Cl-
inhibitory Ex cationic nAChR, iGluR, 5HT3, ATP,
anionic GABAA, Gly
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What determines selectivity of each ion
channel? 1. hydrated size of the ion Li 0.6
A Na 0.95A K 1.33A Rb 1.48A 2.
nature of sidechains lining pore of
channel/selectivity filter Ex K channel
has 8 oxygens Great review article Science
310, 2005, 1461.
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Ion channel receptors (Ligand gated ion channels)
nAChR 255 kDa, non-selective cation channel
Na, K, Ca2
2a, b, g, d subunits
Each subunit highly homologous (40)
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Ion channel receptors (Ligand gated ion channels)
glycine receptor
3a, 2b subunits
anion channels Cl-, SCN-, I-, Br-, NO3-
most relevant physiologically
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Ion channel receptors (Ligand gated ion channels)
Structure of protein subunits (4-TM receptor
subunits)
4 Transmembrane (TM) regions (hydrophobic)
Ex nAChR, 5HT3, glycine, GABAA
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Ion channel receptors (Ligand gated ion channels)
Cartoon of ion channel
Note TM2 of each protein subunit lines the
central pore
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Ion channel receptors (Ligand gated ion channels)
Gating
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Extracellular
Ex iGluR
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Extracellular
Ex ATP receptor (purinoceptors P2X1-P2X7)
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Ion channel receptors (Ligand gated ion channels)
Gating

• Fast response measured in msec
• Ideal for transmission between nerves
• Binding of messenger leads directly to ion flows
  across cell membrane 108 ions/sec
• Ion flow secondary effect (signal transduction)

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G-protein-coupled receptors (7-TM receptors)
Structure - Single protein with 7
transmembrane regions
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G-protein-coupled receptors (7-TM receptors)
Ligands

• Monoamines e.g. dopamine, histamine,
  noradrenaline, acetylcholine (muscarinic)
• Nucleotides
• Lipids
• Hormones
• Glutamate
• Ca2

In general, mediate action of hormones and
slow-acting neurotransmitters
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G-protein-coupled receptors (7-TM receptors)
Ligand binding site - varies depending on
receptor type
A) Monoamines, nucleotides, lipids - pocket in
TM helices B) Peptide hormones (Ex oxytocin,
glucagon, insulin) bind to top of TM helices
extracellular loops N-terminal chain C)
Hormones (Ex epinephrine, norepinephrine,
estrogen, vitamin D) bind to extracellular loops
N-terminal chain D) Glutamate, Ca2 -
N-terminal chain
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G-protein-coupled receptors (7-TM receptors)
Bacteriorhodopsin rhodopsin family

• Rhodopsin visual receptor in retina
• Many common receptors belong to this same family
• Implications for drug selectivity depending on
  similarity (evolution)
• Membrane bound receptors difficult to crystallize
• X-Ray structure of bacteriorhodopsin solved -
  bacterial protein similar to rhodopsin
• Bacteriorhodopsin structure used as template
  for other receptors
• Construct model receptors based on template and
  amino acid sequence
• Leads to model binding sites for drug design
• Crystal structure for rhodopsin now solved -
  better template

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G-protein-coupled receptors (7-TM receptors)
Bacteriorhodopsin rhodopsin family
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G-protein-coupled receptors (7-TM receptors)
Receptor types and subtypes
Reflects differences in receptors which recognize
the same ligand
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G-protein-coupled receptors (7-TM receptors)
Receptor types and subtypes

• Receptor types and subtypes not equally
  distributed amongst tissues.
• Target selectivity leads to tissue selectivity

Heart muscle - b1 adrenergic receptors Fat
cells - b3 adrenergic receptors Bronchial
muscle - a1 b2 adrenergic receptors GI-tract
- a1 a2 b2 adrenergic receptors
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Kinase linked receptors (1 TM)

• Bi-functional receptor / enzyme in 1 molecule
  (activates enzyme directly- doesnt require G
  protein as signaling molecule)
• Activated by hormones (ie insulin), growth
  factors, cytokines (growth factors that regulate
  the differentiation, proliferation, and
  activities of various types of blood cells, ie
  interleukins)
• Over-expression can result in cancer (drug
  target)
• Loss of function developmental defects or
  hormone resistance

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Tyrosine kinase linked receptors
Structure
Extracellular N-terminal chain
Intracellular C-terminal chain
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Tyrosine kinase linked receptors
Reaction catalyzed by tyrosine kinase
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Tyrosine kinase linked receptors
Epidermal growth factor receptor (EGF-
R) (EGF promotes growth and proliferation of
mesenchymal, glial, and epithelial cells)
Induced fit opens tyrosine kinase active sites
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Tyrosine kinase linked receptors
Epidermal growth factor receptor (EGF- R)

• Active site on one half of dimer catalyses
  phosphorylation of Tyr residues on other half
• Dimerisation of receptor is crucial
• Phosphorylated regions act as binding sites for
  further proteins and enzymes
• Results in activation of signalling proteins and
  enzymes
• Message carried into cell

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Tyrosine kinase linked receptors
Insulin receptor (tetrameric complex)
Kinase active site opened by induced fit
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Tyrosine kinase linked receptors
Growth hormone receptor Tetrameric complex
constructed in presence of growth hormone
Kinase active site opened by induced fit
cytoplasmic
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Tyrosine kinase linked receptors
Signalling pathways
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Tyrosine kinase linked receptors
Signalling pathways
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Tyrosine kinase linked receptors
Signalling pathways
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Tyrosine kinase linked receptors
Signalling pathways
Ras member of small G-protein family similar to
a subunit of larger G protein family
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Tyrosine kinase linked receptors
Signalling pathways
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Intracellular receptors

• Chemical messengers must cross cell membrane
• Chemical messengers must be hydrophobic
• Example - steroids and steroid receptors
• 50 members important in directly regulating
  gene
• transcription. Called nuclear hormone
  receptors or
• nuclear transcription factors.

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Intracellular receptors
Structure
CO2H
H2N
Zinc fingers contain cysteine residues (SH) and
histidine residues (NH) to allow for S-Zn and
N-Zn interactions
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Intracellular receptors
Mechanism
1. Messenger crosses membrane
5. Complex binds to DNA
2. Binds to receptor
6. Transcription switched on or off
3. Receptor dimerization
7. Protein synthesis activated or inhibited
4. Binds co-activator protein
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Intracellular receptors
Estrogen receptor
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Intracellular receptors
Estrogen receptor

• Phenol and alcohol of oestradiol are important
  hydrogen binding groups
• Binding site is spacious and hydrophobic
• Phenol group of oestradiol positioned in narrow
  slot
• Orientates rest of molecule

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Intracellular receptors
Estrogen receptor

• Raloxifene is an antagonist (anticancer agent)
• Phenol groups mimic phenol and alcohol of
  estradiol
• Interaction with Asp351 is important for
  antagonist activity
• Side chain prevents receptor helix H12 folding
  over as lid
• AF-2 binding region not revealed
• Co-activator cannot bind

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Intracellular receptors
Estrogen receptor
Tamoxifen (Nolvadex) - anticancer agent which
targets Estrogen receptor