RECEPTORS – what are they? Langley (1878) suggested presence of specific interaction mechanisms/sites after observing SPECIFIC antagonistic interactions between ‘Pilocarpine & Atropine’ RECEPTORS - Macromolecular PROTEIN/PEPTIDE structures On the Cell Surface, or Transcellular or Intra-cellular Have SPECIFIC 3-D structure & Binding properties Regulate critical Cell Functions – e.g. Enzyme activity Permeability of cell (wall, membrane, etc) Ion Channels activity Carrier functions Template Function, etc. Monomeric - with separate receptor- & DNA-binding domains

1. Drug Receptor Interactions
ANOOP KUMAR
ASSOCIATE PROFESSOR
DEPT. OF PHARMACOLOGY
ISF COLLEGE OF PHARMACY
WEBSITE: - WWW.ISFCP.ORG
EMAIL: anoopisf@gmail.com
ISF College of Pharmacy, Moga
Ghal Kalan, GT Road, Moga- 142001,
Punjab, INDIA
Internal Quality Assurance Cell -
(IQAC)

2. Drug-RECEPTOR Interactions:
RECEPTORS – what are they?
Langley (1878) suggested presence of specific interaction
mechanisms/sites after observing SPECIFIC antagonistic
interactions between ‘Pilocarpine & Atropine’
RECEPTORS -
Macromolecular PROTEIN/PEPTIDE structures
On the Cell Surface, or Transcellular or Intra-cellular
Have SPECIFIC 3-D structure & Binding properties
Regulate critical Cell Functions – e.g. Enzyme activity
Permeability of cell (wall, membrane, etc) Ion Channels
activity Carrier functions Template Function, etc.
2

3. CLASSES (TYPES) OF RECEPTORS:
INTRA-
CELLULAR
TRANSMEMBRANE
ENZYME (KINASE)
LINKED
LIGAND
GATED ION
CHANNEL
G-PROTEIN
COUPLED
Examples:
• STEROIDS
• THYROXIN
•INSULIN •Ach-Nicot
• GABA
• Glutamate
• Aspartate
• Ach-Muscarinic
• Adrenergic
Receptors
Time:
Hours Minutes Milli-Sec Seconds
4321 Drug
Outside
Cell
Inside
Cell
CellMembrane

4. 1: NUCLEAR
RECEPTORS
2: RECEPTOR
KINASES
3: LIGAND-
GATED ION
CHANNELS
4: G-PROTEIN
COUPLED
RECEPTORS
Location Intracellular Membrane Membrane Membrane
Effector Gene
Transcription
Protein Kinases Ion Channel Channel or
Enzyme
Coupling Via DNA Direct Direct G-protein
Examples Steroid
receptors
Insulin, Growth
factors, Cytokine
receptors
N-Ach
receptor,
GABA A
receptor
M-Ach
receptor,
Adrenoceptors
Structure Monomeric -
with separate
receptor- &
DNA-binding
domains
Single -
transmembrane
helix linking
extracellular
receptor domain
to intracellular
kinase domain
Oligomeric
assembly of
subunits
surrounding
central pore
Monomeric,
Dimeric or
structure
comprising
Seven Trans-
membrane
Helices

5. INTRACELLULAR RECEPTORS
for Lipid Soluble agents
Lipid soluble agents
(Corticosteroids, Sex
Steroids, Vitamin D,
Thyroxin etc) cross into the
cell & act on Intracellular
receptors to activate them.
Activated Receptors bind
with specific “Response
Elements” (DNA Sequences)
in the nucleus.
Ligand-Binding
Domain
DNA-binding Domain
(Zn fingers)
Transcription Domain
5

6. contd.
In the Nucleus, they Stimulate Transcription of the
Corresponding Genes  mRNA synthesis  Specific
Proteins are formed  which lead to RESPONSES. That
is why they cause -
SLOW-onset Therapeutic Response (0.5-many hours)
Effects (Therapeutic or ADRs) lasting LONGER even
after plasma Agonist levels fall to zero
Recombinant techniques showed that Corticoids remove
“a restraining factor” on Transcription process by
binding to the specific component of intracytoplasmic
steroid receptor protein
6

7. Molecular M.O.A. of Corticoids
InabsenceofSteroid,HSP90(Heat
ShockProtein)keepsDNA-Binding
Domainmasked
Steroidenterscytoplasm&attaches
toLigand-BindingDomainthat
triggersthereleaseofHSP90
ThisUnmasksDNA-binding&
Transcription-activatingdomainsof
receptor-proteinfolds(Zincfingers)
SpecificmRNAsynthesis causes
proteinsynthesiswhichcause
RESPONSES
mRNA 
Response
Proteins
Intracellular
Receptor for
Corticoids

8. γ γ
Inactive Monomers
Enzyme
Domain
Recog-
nition
Domain
Ligand Domain
α α
β β
Transmembrane Receptor
protein consist of –
• (a) Extracellular Ligand-
Binding domain (α);
• (b) Trans- & Intracellular
‘β’ domain
• Enzyme (Kinases)
Domain aminoacids (‘γ’)
lie in association with ‘β’
domain
• The inactive receptors
existing as MONOMERS
• Agonist binding causes
Monomers to DIMERIZE
Out
In
Cell Membrane
ENZYME-LINKED TRANSMEMBRANE
(KINASE) RECEPTORS

9. γγγ γ
contd
Inactive Monomers
Enzyme
Kinases
Recog-
nition
Domain
Ligand Domain
P P
Substrate(s) S-Phos
ATP ADP
Ligand
α α
β β
Active
Dimer
• Ligand binding: ‘Inactive Monomers’‘Dimerize’ (activated)
• Activated Enzymes Phosphorylate specific AA residues ‘γ’ in
the substrate (Tyrosine for Insulin)  Responses
Out
In Cell Membrane

10. Kinase-linked receptors
• Receptors for various growth factors incorporate tyrosine
kinase in their intracellular domain.
• Cytokine receptors have an intracellular domain that binds
and activates cytosolic kinases when the receptor is
occupied.
• The receptors all share a common architecture, with a large
extracellular ligand-binding domain connected via a single
membrane-spanning helix to the intracellular domain.
10

11. Kinase-linked receptors
• Signal transduction generally involves dimerisation of
receptors, followed by autophosphorylation of tyrosine
residues. The phosphotyrosine residues act as acceptors
for the SH2 domains of a variety of intracellular proteins,
thereby allowing control of many cell functions.
• They are involved mainly in events controlling cell growth
and differentiation, and act indirectly by regulating gene
transcription.
11

12. Kinase-linked receptors
• Two important pathways are:
• the Ras/Raf/mitogen-activated protein (MAP) kinase
pathway, which is important in cell division, growth and
differentiation
• the Jak/Stat pathway activated by many cytokines, which
controls the synthesis and release of many inflammatory
mediators.
• A few hormone receptors (e.g. atrial natriuretic factor) have
a similar architecture and are linked to guanylate cyclase.
12

13. Central Role of Kinases in Signal
Transduction 13

14. • CaM kinase = Ca2+/calmodulin-
dependent kinase;
• DAG = diacylglycerol;
• GC = guanylate cyclase;
• GRK = GPCR kinase;
• IP3 = inositol trisphosphate;
• PKA = cAMP-dependent protein
kinase;
• PKC = protein kinase C;
• PKG = cGMP-dependent protein
kinase.
14

15. LIGAND GATED RECEPTOR LINKED
ION CHANNELS
• Also called Ionotropic Receptors
• 4-5 Transmembrane peptide sequences
• Ligand binds to Extracellular Ag-binding domain
• Transmembrane Domain enclose an Ion Channel in
Center
• Ex: Ach-Nicotinic-Receptors  Na+ Ion
GABAA-Receptors  Cl- Ion

16. N-Ach-R consists of 5 subunits (2α and 1 each β, γ, δ)
which form a cluster around a Central Trans-membrane
Pore
There are 2 Ach-binding sites in Extracellular part of
receptor at the interface between the α- δ, and α- γ
adjoining subunits.
α-helices forming gate
Ach-Nicotinic Receptor
Pore 0.7 nm diameter 16

17. The lining of PORE is rich in negatively charged amino -acids,
which makes the pore Cation-selective.
• Kinked ‘α’ helices form the GATE
• When Ach binds, KINKS straighten out or swing out of way
• This opens channel pore for Na+ influx  results in
Depolarization.
α-helices forming gate
Ach-Nicotinic
Receptor
-ve Charged
Aminnoacids

18. Drug Binding Sites in Voltage
Gated Na+ Channels
• Ion Channels have
Muliple sites for Ligand
acting directly on it
• Ion Channels are also
affected INDIRECTLY by
ligands 
GPCRs thru 2nd
Messengers system
e.g. Opioids & β-adr.
affect Ca++ and K+
Channels
Intracellular signals
e.g. Sulfonylureas on
ATP-gated K+
channels
18

19. B G
Cl-
Cl-
Cl- Cl-
Cl- Cl-
LIGAND GATED GABAA-RECEPTOR- Cl-
CHANNELS
• Benzodiazepines (BDZ) [B] are Anxiolytic / Sedatives
Agonists on the BDZ-receptors
• Given alone, however, they do not affect Cl- ion influx
(necessary for Hyperpolarisation)
• GABA [G] acts as Agonists on GABAA-R and opens Cl-
channels  Influx of Cl- ions  Hyperpolarize Cell 
Anxiolytic / Sedative

20. GB
Cl-
Cl-
Cl-
Cl-
Cl-
contd
• When Benzodiazepines [B] and GABA [G] act together,
Cl- ion influx is more efficient than that with GABA alone
• Thus BDZ effects (Anxiolytic, Hypnotic …) occur by
Agonist action on BDZ receptors, which FACILITATE
(Potentiate) GABA action on Chloride Channels
• BDZ-R can also bind with ‘Agonists’ like β-Carbolines
which cause Closure of Cl- Channel  INVERSE AGONIST
[IA]  ANXIOGENIC / CONVULSIOGENIC
G
Cl-
Cl-
Cl-
IA

21. G
Cl-
Cl-
Cl-
F
B
• Flumazenil, [F] BDZ-R Antagonist, blocks BDZ-
Receptors and prevents effect of BDZ [B]. Can be
used to Reverse Overdose with Benzodiazepines
• Flumazenil can block BDZ-R in both states of
conformation – Agonist well as Inverse Agonist
conformations  i.e. Can block effects of BDZ as
well as β-Carbolines
G
Cl-
Cl-
Cl-
F
IA
contd

22. G-PROTEIN COUPLED
RECEPTORS
22

23. 4. G-PROTEIN COUPLED
RECEPTORS (GPCRs)
Sometimes called Metabotropic
Receptors
Hepta-helical (7 Transmembrane
loops) Receptors
G-Proteins are located on the intracytoplasmic
face of cell membrane along with GDP
Called G-Proteins as they interact with GDP/GTP
Agonist binds with specific Extracellular Domain of
GPCReceptor
G-Prot are GOPHER (Go Between) Proteins which carry
‘Ligand-R interaction’ signal to EFFECTORS by diffusing
within the cytoplasm
23
Ag-Binding
Domain
G-Protein
Coupling
Domain

24. Hepta-helical Structure of GPCR

25. G-PROTEIN COUPLED RECEPTORS
(GPCR)
G-Proteins are TRIMERS – consist of α, β and γ subunits.
Resting State: Trimer is attached to cell membrane
‘distant from receptor’ & GDP is anchored to α-subunit.
When Ag acts on Extracellular R-Domain, GTP displaces
GDP
• This activates “α-subunit+GTP” to diffuse away “to
the Effectors and activate them”. The βγ complex
can also bind with effectors.
• The Effectors are usually Enzymes or Ion Channels
• Many subtypes of G-Proteins – Gs, Gi, Gq etc, exist.
Ligands interact with different receptors thru
different G-Prot subtypes causing different end-
results (responses).
25

26. SOME TARGETS FOR G-PROTEINS
 Adenylyl cyclase, the enzyme responsible for
cAMP formation
 Phospholipase C, the enzyme responsible for
inositol phosphate and diacylglycerol (DAG)
formation
 Ion channels, particularly calcium and potassium
channels
 Rho A/Rho kinase, a system that controls the
activity of many signalling pathways controlling cell
growth and proliferation, smooth muscle
contraction, etc.
26

27. E1 E2
βγ
Rec
GDP
α
G-Prot
GTP
E1 E2
βγ
Rec
GDP
α
G-Prot
GTP
Resting State
G-Prot Unattached
Ligand Receptor
Activates G-Prot
E1 E2
βγ
Rec
GTP
α
2nd Messengers /
Ion Channels
RESPONSE
E1 E2
Rec
α
GDP
GTP
G-Prot
(Hydrolysis)
βγ
G-Prot Activate
Effectors
Back to Resting State
G-Proteins
Coupled
Receptors
+ P

28. EFFECTS OF G-Protein
Receptor-Ag InteractionG-PROTEIN MEDIATED EFFECTS mostly involve generation of Chemicals called 2nd
Messengers:
(a) Activation of Adenylyl Cyclase - cAMP pathway:
 Binding to β-adrenoceptors  adenylyl cyclase thru
the Stimulatory G-Protein (Gs) which causes
dissociation of its ‘αs-subunit’ charged with GTP.
 ‘Charged αs-subunit’ activates adenylyl cyclase 
synthesis of cAMP.
 The  cAMP levels produce – * 
Cardiac contractility * Smooth muscle
relaxation (Bronchi, Blood Vessels, Gut, Uterus),
and * Glycogenolysis
 Ex. of drugs  cAMP  Glucagon; β-
Adrenergic drugs (Adrenaline, Salbutamol);
 Adenylyl Cyclase activity is  by Muscarinic drugs
thru Gi-subtype G-Proteins.
28

29. EFFECTS OF RECEPTOR
OCCUPATION BY AGONISTS
G-PROTEIN MEDIATED EFFECTS- 2nd Messengers:
(b) Phospholipase-C: IP3 – DAG Pathway:
 Lead to Contraction, Secretion, Transmitter Release, Neuronal
Excitability, etc.
 Ex: α1–Adrenergic, H1-Histaminic, M1-Muscarinic Effects.
A ligand can produce different effects in different cells by
interacting with different subtypes of G-Proteins:
e.g. Catecholamines respond to Stress by
Increasing Heart Rate thru Gs-coupled β-receptors &
Vasoconstriction in skin thru Gq-coupled α1-receptors
(c) Channel Regulation:
 Ca++, Na+, K+ channels Open / Close .
29

30. G-Proteins Mediated Effects – 2nd
messengers
30

31. βM3
Gq Gs
Aden
Cycl M2
Gi
ATP cAMP
+ _
_
DAG IP3
Ca++
PLC-β
Contraction of Sm. M.
_
G-Proteins mediated 2nd Messengers in Smooth Muscles
Cardiac , Sm.
M. Relaxation,
Glycogenolysis
Protein Kinase C
G-Proteins subtypes 
Gs – Stimulates Target enzymes
Gi – Inhibitory effects
Gq – Activates Phospholipase-C  release
IP3  Ca++ release & PKC 

32. SPARE RECEPTORS
Clark (1930s) observed that –
 Adrenaline / Acetylcholine / Histamine can still produce Maximal
Response when most receptors have been blocked by Irreversible
Antagonist.
 Receptors are said to be "spare" if maximal biologic response can
be elicited at Ag-concentration that does not occupancy the full
complement of available receptors.
 It really indicates that very small % of available receptors are
needed to produce maximal response.
 Spareness of receptors determines the sensitivity of tissue.
 Experimentally, spare receptors may be demonstrated by using
“Irreversible Antagonist” to prevent binding of Agonist to a
proportion of available receptors and showing that high
concentrations of agonist can still produce an undiminished
maximal response.
32

33. SILENT RECEPTORS
 Drugs can bind to molecules that have no direct relation with the action-effect
sequence.
 These binding sites are indeed termed as “Sites of Loss” as this fraction is
not available for action.
 These sites are also called Drug Acceptors
 Most important example is “Binding to Plasma Proteins”
 Other sites can be Tissue Binding sites in those tissues where the primary
action of drug is not expected
 These sites have been called as “SILENT RECEPTORS”
 Indirectly these bindings affect drug response as bound fraction acts as
Storage Site from where drug is released into active free form as the free
fraction levels decline
 Highly plasma protein bound drugs show features like Slow Onset &
Prolonged Duration of action, more displacement Drug-Drug Interactions,
etc.
33

34. RECEPTOR HETROGENEITY &
SUBTYPES
 Receptors within a given family generally occur in several molecular
varieties, or subtypes, with similar architecture but significant differences
in their AMINOACID sequences.
 This results in variation in their pharmacological properties.
 Examples: Ach-N  Nicotinic-N (nervous tissue) &
Nicotinic-M (skeletal muscles)
Beta-adrenoceptors  β1, β2, β3, Alpha-adrenoceptors  α1 & α2 and
their further subtypes α1A, α1C, etc
 Different subtypes / isoforms allow more selective agonists & antagonists
for use in specific disorders
 New subtypes are being discovered regularly, specially after gene-
splicing technology and cloning of receptors
34

35. THANKS