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1. RECEPTOR AS DRUG TARGET
Ravish Yadav
RECEPTOR STRUCTURE &
SIGNAL TRANSDUCTION

2. G-protein-coupled receptors (7-TM receptors)
Structure - Single protein with 7 transmembrane regions
Transmembrane
helix
C -Terminal chain
G-Protein
binding region
Variable
intracellular loop
Extracellular
loops
Intracellular loops
N -Terminal chain
HO2C
NH2
VII VI V IV III II IMembrane

3. Ligands
• Monoamines e.g. dopamine, histamine, noradrenaline,
acetylcholine (muscarinic)
• Nucleotides
• Lipids
• Hormones
• Glutamate
• Ca++
G-protein-coupled receptors (7-TM receptors)

4. 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
Ligand
B DCA
G-protein-coupled receptors (7-TM receptors)

5. Signal transduction pathway
a) Interaction of receptor with Gs-protein
GS-Protein - membrane bound protein of 3 subunits (a, b, g)
- aS subunit has binding site for GDP
-GDP bound non covalently
b g
a
GDP
G-protein-coupled receptors (7-TM receptors)

6. ß
a
g
GDP GTP
Ligand
binding
Induced
fit
G-protein
binds
Induced
fit for
G-protein
G-Protein alters shape
GDP binding site distorted
GDP binding weakened
GDP departs
ß
a
g
Ligand
Receptor
G Protein
Cell membrane
ß
a
g
Binding site for G-protein opens
= GDP
a) Interaction of receptor with Gs-protein
Signal transduction pathway

7. ß
a
g
Binding site recognises GTP
GTP binds
Induced fit
G-protein alters shape
Complex destabilised
Fragmentation
and release
ß
a
g
• 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
ß
a
g
a) Interaction of receptor with Gs-protein
Signal transduction pathway

8. as Subunit recombines with b,g dimer
to reform Gs protein
Active site
(closed)
Binding site
for as subunit
cyclic AMPATP
Binding
Induced
fit
Active site
(open)
P
cyclic AMPATP
GTP hydrolysed
to GDP catalysed
by as subunit
as-subunit
Adenylate cyclase
GTP
GDP
as Subunit changes shape
Weaker binding to enzyme
Departure of subunit
Enzyme reverts to inactive
state
Active site
(closed)
Signal
transduction
Signal transduction pathway
b) Interaction of as with adenylate cyclase

9. N
N N
N
NH2
O
OHOH
OP
O
OH
OP
O
OH
OP
O
OH
HO
N
N N
N
NH2
O
OH
O
P O
O OH
ATP
Adenylate cyclase
H H
Cyclic AMP
HH
HH
• 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
b) Interaction of as with adenylate cyclase

10. 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
H
N C
O
OH
H
N C
O
O
P
HO O
OH
H
N C
O
HC OH
CH3
H
N C
O
HC O
CH3
P O
HO OH
Protein
kinase A
Serine
Protein
kinase A
Threonine
H H H H

11. 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.

12. cyclic AMPATP
Adenylate
cyclase
Enzyme
(active)
P
Enzyme
(inactive)
Chemical
reaction
Protein
kinase
Activation
c) Interaction of cyclic AMP with protein kinase A (PKA)

13. 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)
Cyclic AMP binds to PKA
Induced fit destabilises complex
Catalytic units released and activated
Note
C
C
R
R
cAMP
cAMP
binding
sites
catalytic subunit
R
R
C
C
catalytic subunit

14. Phosphorylation of other proteins and enzymes
Signal continued by phosphorylated proteins
Further signal amplification
C
Protein
+ ATP
Protein
+ ADP
P
c) Interaction of cyclic AMP with protein kinase A (PKA)

15. Glycogen metabolism - triggered by adrenaline in liver cells
Catalytic
subunit of
PKA
cAMP
Protein kinase A
C
Inhibitor (inactive)
Inhibitor-P
(active)
Phosphatase
(inhibited)
Glycogen
synthase
(active)
Glycogen
synthase-P
(inactive)
Phosphorylase
kinase (inactive)
Phosphorylase
kinase-P (active)
Phosphorylase b
(inactive)
Phosphorylase a
(active)
Glycogen Glucose-1-phosphate
b-Adrenoreceptor
Adrenaline
adenylate
cyclase
as as

16. Coordinated effect - activation of glycogen
metabolism
- inhibition of glycogen
synthesis
Adrenaline has different effects on different cells
- activates fat metabolism in fat cells
Glycogen metabolism - triggered by adrenaline in liver cells

17. 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

18. Phosphorylation
• Prevalent in activation and deactivation of enzymes
• Phosphorylation radically alters intramolecular binding
• Results in altered conformations
O
NH3
O
P
O
O
O
O
NH3
H
O
Active site
closed
Active site
open
NH3
O
O P
O
O
O

19. 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
Theophylline
N
N
N
H
N
O
H3C
O
CH3
Caffeine
N
N
N
N
O
H3C
O
CH3
CH3

20. 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
a
Active site
(closed)
PLC
Active site
(open)
a
PLC
a
PLC
PIP2
Binding weakened
GTP hydrolysis aq departs
Active site
(closed)
enzyme
deactivated
a
PLC
DG
IP3
a
PLC
PIP2
DG
IP3Phosphate

21. O
HO
O
O
OH
HO
CH2 CH CH2
O O
OH
C CO
R R
O
C O
R
C
R
O
OO
CH2
O
CHCH2
PO O
HO
OH
O
O
HO
O
+
IP3
PIP2
DG
PLC
H
H
HH
H
H
P
P
P
P
P
Signal transduction involving phospholipase C (PLC)
Phosphatidylinositol diphosphate
(integral part of cell membrane)
Inositol triphosphate
(polar and moves
into cell cytoplasm)
Diacylglycerol
(remains in membrane)
R= long chain hydrocarbons = PO3
2-P

22. 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
PKC
DG
Binding
site for DG
Cell membrane
Cytoplasm
PKC moves
to membrane
PKC
DG
Cytoplasm
DG binds to
DG binding site
Active site
closed
PKC
DG
Cytoplasm
Induced fit
opens active site
Enzyme
(inactive)
Enzyme
(active)
Chemical
reaction

23. Drugs inhibiting PKC - potential anti cancer agents
Action of diacylglycerol
O
O
Me
Me
OOH
H
O
O
C
H
Me
OH
Me
Me
C O
OH
H
CHMeO2C
OC
CH
O
CHCO2Me
H H
HO
H
Me
O
CHCHCHCH3CH2CH2
Bryostatin (from sea moss)

24. 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

25. IP3
Calcium
stores
Ca++
Calmodulin
Calmodulin Ca++
Activation
Protein
kinase
Activation
Protein
kinase
Enzyme
(inactive)
Enzyme
(active)
P
Cytoplasm
Cell membrane
Enzyme
(active)
Enzyme
(inactive)
P
Chemical
reaction
Chemical
reaction
Action of inositol triphosphate

26. Resynthesis of PIP2
IP3 + DG PIP2
several
steps
Li+ salts
Inhibition
Lithium salts used vs manic depression