LECTURE 4
PHARMACOLOGY
Drug Receptors and Pharmacodynamics
The action of a drug on the body,
including receptor interactions,
dose-response phenomena, and
mechanisms of therapeutic and
toxic action.

Receptors




Definitions
Classification
Ligands (Drugs)
Ligand-Receptor interaction


Interaction and consequence
Receptor-mediated mechanism of action

Binding Site


Receptor


Specific and Saturable
Binding Site + Effect
Acceptor

Binding Site + no Effect
DRUG TARGETS
 many drugs inhibit enzymes
Enzymes control a number of metabolic processes
A very common mode of action of many drugs
 in the patient (ACE inhibitors)
 in microbes (sulfas, penicillins)
 in cancer cells (5-FU, 6-MP)
 some drugs bind to:
 proteins (in patient, or microbes)
 the genome (cyclophosphamide)
 microtubules (vincristine)
 most drugs act (bind) on receptors
 in or on cells
 form tight bonds with the ligand
 exacting requirements (size, shape, stereo specificity)
 can be agonists (salbutamol), or antagonists (propranolol)
 receptors have signal transduction methods
1.
enzyme linked
2.
ion channel linked
(multiple actions)
(speedy)
3.
G protein linked
4.
nuclear (gene) linked
(amplifier)
(long lasting)


A macromolecular component of a
cell with which a drug interacts to
produce a response
Usually a protein
1.
2.
3.
4.
Regulatory – change the activity
of cellular enzymes
Enzymes – may be inhibited or
activated
Transport – e.g. Na+ /K+ ATPase
Structural – these form cell parts
B.
Second Messengers
•Small, nonprotein, water-soluble
molecules or ions
•Readily spread throughout the cell by
diffusion
•Two most widely used second messengers
are:
1. Cycle AMP
2. Calcium ions Ca2+
2.
Calcium Ions (Ca2+) and Inositol
Trisphosphate
•Calcium more widely used than cAMP
•used in neurotransmitters, growth
factors, some hormones
•Increases in Ca2+ causes many possible
responses:
•Muscle cell contraction
•Secretion of certain substance
•Cell division
Two benefits of a signal-transduction pathway
A.
1.
Signal amplification
2.
Signal specificity
Signal amplification
•Proteins persist in active form long
enough to process numerous molecules
of substrate
•Each catalytic step activates more
products then in the proceeding steps
Receptors and Drug Action
Receptors:
•Specific areas of cell membranes (proteins, glycoproteins)*
•When bound to ligand, positive or negative biological response
Ligand
cellmembrane with receptor
Biological responce
Extracellular fluid
Cell membrane
Few ex. of free receptors in cytoplasm
Cytoplasma
Organelles
Cell
Nucleus
Drugs that do act on receptors:
Drugs that do not act on receptors:
Acetylcholin
(Neurotransmittor)
Antacids: CaCO3 + HCl
ca. 5Å
Acetylcholin Agonists
Diuretics (osmotic)
O
Pilocarpine
Carbacholin
N
O
Me
N
O
Alkylating agents (cancer)
O
H
H2N
O
N
O
Cl
Cl
Cl
Cl
N
Nu
N
O
N
H
Acetylcholin Antagonists
N
Nu
Atropin
N
Psoralenes
N
HN
O
O
N
O
NH2
R
O
O
R'
O
N
HN
h
N
N
OH
O
O
Cyclopentolat
O
N
R
O
O
O
R'
O
O
O
NH2
N
OH
O
Agonist:
Binds to (have affinity for) receptor
Binding leads to biolog. response
(Agonists have intrinsic activity / efficacy)
Antagonist:
Affinity for receptor
No intrinsic activity
Types of receptors-Mechanism of Action
Superfamily
Endogenous
ligands
General structures
1
Fast neurotransmitters
ex. Acetylcholine
Ligand gated ion channels
2
Slow neurotransmitters. ex. noradrenalin G-Protein coupled receptors
Hormones
3
Insulin
Growth factors
Enzyme coupled receptors
Catalytic receptors
4
Steroid hormones
Thyroid hormones
Vitamin A, D
Cytoplasmic receptors
Ligand gated ion channels
Ligand binding sites
Ligands
Fast neurotransmitters
ex. Acetylcholine (nicotinic receptors)
Membrane
(Phospholipides)
Ion chanel
Fastest intracellular response, ms
Binding of ligand - opening of channel - ion (K+, Na+) in or out of cell - response
Nobel prize chemistry 2003,
Roderick MacKinnon “for structural and mechanistic studies of ion channels”.
http://nobelprize.org/chemistry/laureates/2003/press.html
G-Protein coupled receptors
G-protein: Guanine nucleotide binding protein
Ligand
(Agonist)
Extracellular fluid
Target
Intracellular fluid
Conform. change
Reseptor

GDP
G-protein
GTP

+P



GDP
GDP


O
HN



N
N
H2N
N
O
O
O P O P O
O
O
O
n
HO OH
GTP
Responce
n=1; GDP
n=2; GTP
GTP
Subtypes of G-proteins - Targets (Second messenger systems)
Ion channels: G12 Na+ / H+ exchange
Enzymes:
Gi Inhib. Adenylyl cyclase
Gs Stimul. Adenylyl cyclase
Gq Stimul. Phospholipase C
One ligand can bind to more than one type of G-protein coupled rec
second messenger pathways
NH2
NH2
N
N
Adenylyl cyclase
O
P O P
ATP
OH
N
N
N
O
O
O
N
O P O P O P O
O
O
O
O
HO
N
N
O
O P O
O
OH
c-ATP
Activate c-AMP dependent protein kinases
(Phosphoryl. of proteins, i.e. enzymes)
Various responces
(ex. metabolism, cell division)
Subtypes of G-proteins - Targets (Second messenger systems)
Ion channels: G12 Na+ / H+ exchange
Enzymes:
Gi Inhib. Adenylyl cyclase
Gs Stimul. Adenylyl cyclase
Gq Stimul. Phospholipase C
Various processes in cell
Various processes in cell
second messenger pathways
Release Ca2+
Activate protein kinases
O
R
R'
O
O
Phopholipase C
O
O
O
O
P O
OH
O
HO
HO
R
O
O P O
OH
O
O
+
HO
HO
O
P
O
O
OH
O
O
R'
O
P
P
DAG
Diacylglycerol
IP3
Inositol-1,4,5-triphosphate
P
PIP2
Phosphatidylinositol besphophate
Several steps
Li
(Treatmen manic depression)
G protein-linked receptors
Structure:
•Single polypeptide
chain threaded back
and forth resulting in
7 transmembrane å
helices
•There’s a G protein
attached to the
cytoplasmic side of
the membrane
(functions as a
switch).
2004-2005
Enzyme coupled receptors - Catalytic receptors
Ligands: Peptide hormones
1) Binding of Ligand
2) Dimerisation
of reseptor
Tyr kinase
-OH
domain
(Janus kinase)
Phosphoryl of Tyr
-OH
-OH
STATS
protein
O P
O P
O P
O P
STATS
protein
STAT: Signal transducers and activators of transcription
1) Phosphoryl. of STAT
2) Release of STAT in cytoplasma
3) STATto cell nucleus
4) Intitation of transcription
Ion channel
receptors
Structure:
•Protein pores in the
plasma membrane
Cytoplasmic receptors
(not bound to cell membranes)
Lipophil. ligand thru cell membrane
Cell membrane
Cytoplasma
Responce
Cell nucleus
HSP-90
Protein
HSP-90
DNA
DNA
(HSP-90: Heat shock protein)
mRNA
Intracellular receptors
Not all signal receptors are located on the plasma membrane. Some are proteins
located in the cytoplasm or nucleus of target cells.
•
The signal molecule must be able to pass through plasma membrane.
Examples:
~Nitric oxide (NO)
~Steroid (e.g., estradiol, progesterone, testosterone)
and thyroid hormones of animals).

Examples:

Glucocorticoids: Inhibit transcription of COX-2; induce transcription of
Lipocortin

Mineralcorticoids: Regulate expression of proteins involved in renal function

Retinoids (Vit A derivatives): Control embryonic development of limbs and
organs; affect epidermal differentiation => dermatological use (Acne)

PPARs (Peroxisome Proliferation-Activated Receptors): control metabolic
processes:
 PPAR: Target of Fibrates (cholesterol lowering drugs: stimulate oxidation of fatty acids)
 PPAR: Target of Glitazones (anti-diabetic drugs: induce expression of
proteins involved in insulin signaling => improved glucose uptake)
Tyrosine-kinase receptors
Structure:
•Receptors exist as individual polypeptides
•Each has an extracellular signal-binding
site
•An intracellular tail with a number of
tyrosines and a single å helix spanning the
membrane
Receptor subtypes
Most receptor classes - several sub-typesDet somatiske nerves ystem
Each subtypes - differend A(nta)gonists
CNS
Sub types cholinerge receptors
Acetylcholin
(Neurotransmittor)
Det autonome nervesys tem
Det sympatiske Det paras ympatis ke
nervesystem
nervesystem
CNS
CNS
ganglion
Acetylkolin
ca. 5Å
Noradrenalin
O
Synapse
O
O
N
Reseptor
Effektor celle
H
Muscarinergic receptors
M1: G-Protein coupled receptors
Stimulate phospholipase A
4.3 Å
HO
O
O
Nicotinerge
receptors
N
Nmuscle: Ligand gated ion channel
5.9 Å
M2: G-Protein coupled receptors
Inhib. adenylyl cyclase
Incr. Na+/Ca2+
N
H
H
N
O
H
Nneuro: Ligand gated ion channels
Incr. Na+/Ca2+
Spare receptors - Partial agonist
Full agonist
Full responce
Spare receptors
Partial agonist
Weak responce
Full responce
Responce
= Agonist
Absens of full agonist
Responce
Presence of full agonist
= Antagonist
Desensitizing
Receptor and normal amount of ligand = agonist
Overstimulated receptor
Sensitizing
Receptor and normal amount of ligand = Antagonist
Overstimulated receptor
Binding of ligand to receptor
•Covalent bond
•Ionic bond
•Hydrogen bond
•Hydrophobic interaction
Covalent bond
strong; 50-150 kcal/mol,
Normally irreversible bonding
ex. Acetylcholine esterase inhibitors
Acetylkolin-esteras e
N
O
O
Kolin Acyltransferas e
Acetylkolin
(AcCh)
Synapse
Nerve cell
Effector celle
reseptor
Nerve potensial
AcCh
AcCh
AcCh
Ch-acetyltransferase
AcCh (exess)
Ch
AcCh-esterase
Ch
N
Kolin
(Ch)
OH
Reversible inhibitors
O
AcCh
OH
O
N
O
O
k3(AcCh)
OH
O
OH
N
HO
O
OH
Inhibitor
Me2N
O
O
R
O
k3(inhibitor)
NMe2
OH
O
Me2N
HO
OH
R
More difficult to cleave
Reversible inhibitor (drugs): k3 (inhib) < k3(AcCh)
Neostigmine
Pyridostigmine
N
Myasthenia gravis (weak muscles, reduced sensitivity to Acetylcholin
O
O
O
N
N
O
N
Irreversible Inhibitors
Not drugs, nerve gasses, insecticides etc.
Gen structur mustard gasses
L
R2
P
O
R1
N
O
L: Leaving group
R1: alkoksy
R2: alkyl, alkoksy, amino
N
P
F
O
P
O
O
O
O
O
O
Sarin
Tabun
OH
O
Aging
OH
R2
P
O
OR
O
P
R1
O
L
not tox.
Act as mustard gasses
Pralidoxim
OH
HO
N
R1
N Me
Pralidoxim
motgift
OH
OR
O
P
R1
O
N
N Me
O
Malaoxon
P
O
R1
OH
only insects
O
P
O O
S
O
O
Malation
OR
L
O
O
S
P
O S
P
OR
O
Ionic bond
5-10 kcal/mol,
Reversible bonding
Hydrogen bond
2-5 kcal/mol,
Reversible bonding
Hydrophobic interaction
0.5-1 kcal/mol,
Reversible bonding
Acetylcholin
O
O
O
O
H
N
O
O
N
H
Anion
cavity
N
Lipophilic area
H
H-bind acceptor
HO
O
OH
The occupancy theory:
The more receptors sites occupied by ligand, the stronger response
The rate theory:
The more ligand-receptor interact / unit time, the stronger response
The induced-fit theory:
Agonist
Receptor
Conformation A
No ligand bound
+ Agonist
Responce
Conformation B
+ Antagonist
Antagonist
ConformationA
or
Inactive conformation C
The macromolecular pertubation theory:
(induced fit + rate theory)
Responce
The activation -aggregation theory:
Always dynamic equilibrium.
Agonist
Responce
+ Agonist
Responce
Conformation B
(Active)
Antagonist
+ Antagonist
Responce
Inv. Agonist
Responce B
(opposite of responce A)
Responce A
+ Inverse agonist
Dose-Response Relationships
[A] - equil. towards AB
A-B
A + B
L-R
L + R
[L] - equil. towards LR ??
L
L-R
R locked in membrane (do not move freely)
L dissolved in extracellular fluid
R
Reaction on solid - liquid interface
Biological responce


Drugs or endogenous compounds binding to receptors are described
as Ligands.
Ligands are classified into 2 groups
 Agonist: molecule that binds to receptor and produces similar response to
that of the endogenous ligand
 Partial agonist  agonist that produce partial effect

Antagonist: molecule that binds to a receptor, but does not cause a
response
 Competitive
reversible or weak binding
 Non-competitive
non-reversible or strong binding
Ligands (Agonist and Antagonist)

Affinity: the attraction of the drug for
the receptor.
high affinity: low concentrations bind
 low affinity: high concentrations bind
 no affinity: does not bind


Efficacy: the intrinsic activity
Max. effect
 Min. effect

efficacy = 1
efficacy = 0
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