Patrick

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Patrick An Introduction to Medicinal Chemistry
3/e Chapter 6 PROTEINS AS DRUG
TARGETS RECEPTOR STRUCTURE SIGNAL
TRANSDUCTION Part 2 Sections 6.3 - 6.6
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Contents Part 2 Sections 6.3 -
6.6 3. G-protein-coupled receptors (7-TM
receptors) 3.1. Structure - Single protein
with 7 transmembrane regions 3.2. Ligands
3.3. Ligand binding site - varies depending
on receptor type 3.4. Bacteriorhodopsin
rhodopsin family (2 slides) 3.5. Receptor types
and subtypes (2 slides) 3.6. Signal
transduction pathway a) Interaction of
receptor with Gs-protein (3 slides) b) Interacti
on of ?s with adenylate cyclase (2
slides) c) Interaction of cyclic AMP with
protein kinase A (PKA) (4 slides) 3.7. Glycogen
metabolism - triggered by adrenaline in liver
cells (2 slides) 3.8. GI proteins 3.9. Phosphory
lation 3.10. Drugs interacting with cyclic AMP
signal transduction 3.11. Signal transduction
involving phospholipase C (PLC) (2
slides) 3.12. Action of diacylglycerol (2
slides) 3.13. Action of inositol triphosphate (2
slides) 3.14. Resynthesis of PIP2 29 slides
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3. G-protein-coupled receptors (7-TM receptors)
3.1 Structure - Single protein with 7
transmembrane regions
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3. G-protein-coupled receptors (7-TM receptors)
3.2 Ligands

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

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3. G-protein-coupled receptors (7-TM receptors)
3.3 Ligand binding site - varies depending on
receptor type
A) Monoamines - pocket in TM helices B)
Peptide hormones - top of TM helices
extracellular loops N-terminal chain C)
Hormones - extracellular loops N-terminal chain
D) Glutamate - N-terminal chain
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3. G-protein-coupled receptors (7-TM receptors)
3.4 Bacteriorhodopsin rhodopsin family

• Rhodopsin visual receptor
• Many common receptors belong to this same family
• Implications for drug selectivity depending on
  similarity (evolution)
• Membrane bound receptors difficult to crystallise
• 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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3. G-protein-coupled receptors (7-TM receptors)
3.4 Bacteriorhodopsin rhodopsin family
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3. G-protein-coupled receptors (7-TM receptors)
3.5 Receptor types and subtypes
Reflects differences in receptors which recognise
the same ligand
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3. G-protein-coupled receptors (7-TM receptors)
3.5 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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3. G-protein-coupled receptors (7-TM receptors)
3.6 Signal transduction pathway
a) Interaction of receptor with Gs-protein
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3. G-protein-coupled receptors (7-TM receptors)
3.6 Signal transduction pathway
a) Interaction of receptor with Gs-protein
G-Protein alters shape GDP binding site
distorted GDP binding weakened GDP departs
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3. G-protein-coupled receptors (7-TM receptors)
3.6 Signal transduction pathway
a) Interaction of receptor with Gs-protein
Induced fit G-protein alters shape Complex
destabilised

• Process repeated for as long as ligand bound to
  receptor
• Signal amplification - several G-proteins
  activated by one ligand
• as Subunit carries message to next stage

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3. G-protein-coupled receptors (7-TM receptors)
3.6 Signal transduction pathway
b) Interaction of as with adenylate cyclase
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3. G-protein-coupled receptors (7-TM receptors)
3.6 Signal transduction pathway
b) Interaction of as with adenylate cyclase

• Several 100 ATP molecules converted before as-GTP
  deactivated
• Represents another signal amplification
• Cyclic AMP becomes next messenger (secondary
  messenger)
• Cyclic AMP enters cell cytoplasm with message

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3. G-protein-coupled receptors (7-TM receptors)
3.6 Signal transduction pathway
c) Interaction of cyclic AMP with protein kinase
A (PKA)

• Protein kinase A serine-threonine kinase
• Activated by cyclic AMP
• Catalyses phosphorylation of serine and threonine
  residues on protein substrates
• Phosphate unit provided by ATP

Link
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Kinase Any of various enzymes that catalyze the
transfer of a phosphate group from a donor, such
as ADP or ATP, to an acceptor. Phosphatase Any
of numerous enzymes that catalyze the hydrolysis
of esters of phosphoric acid and are important in
the absorption and metabolism of carbohydrates,
nucleotides, and phospholipids and in the
calcification of bone. Phosphorylase An enzyme
that catalyzes the production of glucose
phosphate from glycogen and inorganic phosphate.
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3. G-protein-coupled receptors (7-TM receptors)
3.6 Signal transduction pathway
c) Interaction of cyclic AMP with protein kinase
A (PKA)
Link
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3. G-protein-coupled receptors (7-TM receptors)
3.6 Signal transduction pathway
c) Interaction of cyclic AMP with protein kinase
A (PKA)
Protein kinase A - 4 protein subunits - 2
regulatory subunits (R) and 2 catalytic subunits
(C)
Note
Cyclic AMP binds to PKA Induced fit destabilises
complex Catalytic units released and activated
Link
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3. G-protein-coupled receptors (7-TM receptors)
3.6 Signal transduction pathway
c) Interaction of cyclic AMP with protein kinase
A (PKA)
Phosphorylation of other proteins and
enzymes Signal continued by phosphorylated
proteins Further signal amplification
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3. G-protein-coupled receptors (7-TM receptors)
3.7 Glycogen metabolism - triggered by
adrenaline in liver cells
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3. G-protein-coupled receptors (7-TM receptors)
3.7 Glycogen metabolism - triggered by
adrenaline in liver cells
Coordinated effect - activation of glycogen
metabolism - inhibition of glycogen
synthesis Adrenaline has different effects on
different cells - activates fat metabolism in
fat cells
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3. G-protein-coupled receptors (7-TM receptors)
3.8 GI proteins

• Binds to different receptors from those used by
  Gs protein
• Mechanism of activation by splitting is identical
• aI subunit binds adenylate cyclase to inhibit it
• Adenylate cyclase under dual control
  (brake/accelerator)
• Background activity due to constant levels of as
  and ai
• Overall effect depends on dominant G-Protein
• Dominant G-protein depends on receptors activated
  

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3. G-protein-coupled receptors (7-TM receptors)
3.9 Phosphorylation

• Prevalent in activation and deactivation of
  enzymes
• Phosphorylation radically alters intramolecular
  binding
• Results in altered conformations

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3. G-protein-coupled receptors (7-TM receptors)
3.10 Drugs interacting with cyclic AMP signal
transduction
Cholera toxin - constant activation of c.AMP -
diahorrea Theophylline and caffeine -
inhibit phosphodiesterases - phosphodiesterases
responsible for metabolising cyclic AMP -
cyclic AMP activity prolonged
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3. G-protein-coupled receptors (7-TM receptors)
3.11 Signal transduction involving
phospholipase C (PLC)

• Gq proteins - interact with different receptors
  from GS and GI
• Split by same mechanism to give aq subunit
• aq Subunit activates or deactivates PLC
  (membrane bound enzyme)
• Reaction catalysed for as long as aq bound -
  signal amplification
• Brake and accelerator

PIP2
Binding weakened
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3. G-protein-coupled receptors (7-TM receptors)
3.11 Signal transduction involving phospholipase
C (PLC)
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3. G-protein-coupled receptors (7-TM receptors)
3.12 Action of diacylglycerol

• Activates protein kinase C (PKC)
• PKC moves from cytoplasm to membrane
• Phosphorylates enzymes at Ser Thr residues
• Activates enzymes to catalyse intracellular
  reactions
• Linked to inflammation, tumour propagation,
  smooth muscle activity etc

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3. G-protein-coupled receptors (7-TM receptors)
3.12 Action of diacylglycerol
Drugs inhibiting PKC - potential anti cancer
agents
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3. G-protein-coupled receptors (7-TM receptors)
3.13 Action of inositol triphosphate

• IP3 - hydrophilic and enters cell cytoplasm
• Mobilises Ca2 release in cells by opening Ca2
  ion channels
• Ca2 activates protein kinases
• Protein kinases activate intracellular enzymes
• Cell chemistry altered leading to biological
  effect

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3. G-protein-coupled receptors (7-TM receptors)
3.13 Action of inositol triphosphate
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3. G-protein-coupled receptors (7-TM receptors)
3.14 Resynthesis of PIP2
Lithium salts used vs manic depression