1. MOLECULAR MECHANISM OF
ACTION OF DRUGS
by Lee Eun Jin

2. Drugs produce effects in the body mainly in the
following ways:
1. by acting on receptors
2. by inhibiting carriers (molecules that
transport one or more ions or molecules
across the plasma membrane)
3. by modulating or blocking ion channels
4. by inhibiting enzymes.

3. TARGETS FOR DRUG ACTION
PROTEIN TARGETS
• RECEPTORS
• ION CHANNELS
• ENZYMES
• CARRIER MOLECULES (TRANSPORTERS).
NON-PROTEIN TARGETS-Binding, neutralising, osmosis etc.

4. RECEPTORS

5. ION CHANNELS

6. ENZYMES

7. CAR R M E E (T
RIE OL CUL S RANSPORT RS).
E

8. ACTIONS OF DRUGS NOT MEDIATED BY ANY OF THESE
• Therapeutic neutralization of gastric acid by a base
(antacid).
• Drugs: Mannitol -increasing the osmolarity of various
body fluids and causing changes in the distribution of
water to promote diuresis, catharsis, expansion of
circulating volume in the vascular compartment, or
reduction of cerebral edema
• Cholesterol-binding agents,(cholestyramine resin) to
decrease dietary cholesterol absorption.

9. RECEPTORS

10. RECEPTORS
• The term receptor: any cellular macromolecule to which a drug binds
to initiate its effects.
• Receptors are protein molecules in or on cells whose function is to
interact with the body’s endogenous chemical messengers (hormones,
neurotransmitters, the chemical mediators of the immune system,
etc.) and thus initiate cellular responses.
• They enable the responses of the body’s cells to be coordinated
• A molecule which binds (attaches) to a receptor is called a LIGAND ; - a
peptide, or other small molecule, such as a neuorotransmitter,
hormone, chemical/ drug or a toxin.
• A class of cellular macromolecules (cellular proteins) that are
concerned specifically and directly with chemical signaling between
and within cells.
• Combination of a hormone, neurotransmitter, or intracellular
messenger with its receptor(s) results in a change in cellular activity.
• A receptor functions: recognize the particular molecules that
activate (act as receptors for endogenous regulatory ligands)+
Message propagation (alter cell function)

11. RECEPTOR
• Macromolecules that bind mediator
substances and transduce this binding into an
effect, i.e., a change in cell function.
• The component of a cell or organism that
interacts with a drug and initiates the chain
of biochemical events leading to the drug's
observed effects.
• Isolation and characterization -the molecular
basis of drug action.

12. RECEPTORS
• Ligand binding and message propagation (i.e., signalling)
• Functional domains within the receptor: a ligand-binding domain
and an effector domain.
• The regulatory actions of a receptor : Directly on its cellular
target(s), effect or protein(s), or may be conveyed by intermediary
cellular signaling molecules : Transducers.
• The receptor, its cellular target, and any intermediary molecules :
Receptor–effector system or signal-transduction pathway
• An enzyme or transport protein that creates,moves, or degrades a
small metabolite (e.g., a cyclic nucleotide or inositol trisphosphate)
or ion (e.g., Ca2+) : Second messenger. (Neuromediator)
Eg; cAMP.IP3, DAG, PDE etc
• Second messengers : diffuse in the proximity of their binding sites
and convey information to a variety of targets, which can respond
simultaneously to the output of a single receptor binding a single
agonist molecule.

13. • Receptor–transducer–effector–signal termination complexes—
established via protein–lipid and protein–protein interactions.
• Receptors and their associated effector and transducer proteins
also act as integrators of information as they coordinate
signals from multiple ligands with each other and with the
metabolic activities of the cell.
• An important property of physiological receptors : Excellent
targets for drugs- they act catalytically and hence are
biochemical signal amplifiers. The catalytic nature of receptors
is obvious when the receptor itself is an enzyme
• A single agonist molecule binds to a receptor that is an ion
channel, hundreds of thousands to millions of ions flow through
the channel every second.
• Similarly, a single steroid hormone molecule binds to its
receptor and initiates the transcription of many copies of
specific mRNAs, which, in turn, can give rise to multiple copies
of a single protein.

14. • Drugs that bind to physiological receptors and mimic
the regulatory effects of the endogenous signaling
compounds are termed AGONISTS
(Affinity and efficacy: 1)
• Agents those bind to receptors without regulatory
effect, but their binding, blocks the binding of the
endogenous agonist.:
ANTAGONISTS (Affinity1, efficacy 0)
• Agents that are only partly as effective as agonists no
matter the dose employed are: PARTIAL AGONISTS
( Affinity 1, Efficacy <1)
• Agents that stabilize the receptor in its inactive
conformation are termed INVERSE AGONISTS
( Affinity 0; Efficacy -1 )
• The strength of the reversible interaction between a
drug and its receptor, as measured by their dissociation
constant, is defined as the affinity of one for the other.

15. MOLECULAR STRUCTURE OF RECEPTORS : FAMILIES
• The molecular organisation : Four receptor families
• Individual receptors show considerable sequence
variation in particular regions
• Lengths of the main intracellular and extracellular
domains- vary from one to another within the same
family
• The overall structural patterns and associated signal
transduction pathways are very consistent.

16. RECEPTOR HETEROGENEITY AND SUBTYPES
• Receptors within a given family : Molecular varieties, or
subtypes, with similar architecture; differences in their
sequences, pharmacological properties.
• Distinct subtypes occur in different regions/organs, and
these differ from the receptors in other organ Eg: Ach-
Nicotinic
• The sequence variation that accounts for receptor diversity
arises at the genomic level, i.e. different genes give rise to
distinct receptor subtypes.
• A single gene can give rise to more than one receptor isoform.
• After translation from genomic DNA, the mRNA contains
non-coding regions that are excised splicing before the
message is translated into protein.
• Splicing : result in inclusion/deletion of one/more of the mRNA
coding regions, giving rise to long or short forms of the protein.
(eg: GPCR)

17. receptors
• Physiological receptors
• Agonist – primary agonist
-allosteric agonist
-partial agonist
• Antagonist – syntopic
-allosteric
-chemical
-functional

18. RECEPTOR HETEROGENEITY AND SUBTYPES
• Molecular heterogeneity : feature of
receptors- functional proteins in general.
• New receptor subtypes and isoforms : options
for therapy
• Pharmacological viewpoint: To understand
individual drugs action, effects; Molecular
pharmacology.

19. TYPES/FAMILIES OF RECEPTORS
Based on molecular structure and the nature of the
linkage (the transduction mechanism)
 Ligand-gated ion channels (Ionotropic)
Nicotinic acetylcholine receptor, GABA A receptor
 G-protein-coupled receptors
(GPCRs)/Metabotropic
Muscarinic acetylcholine receptor, adrenoceptors
 Kinase( Tyrosine)-linked and related receptors
Insulin, growth factors, cytokine receptors
 Nuclear/ Cytosol receptors
steroids, thyroid hormones, gonadal steroids,vit D

21. MAJOR RECEPTOR FAMILIES
Transmembrane signaling mechanisms.
A. Lignad binds to the extracellular domain of a ligand-gated channel.
B. Ligand binds to a domain of the serpentine receptor, which is coupled to G protein.
C. Ligand binds to the extracellular domain of a receptor that activates a kinase enzyme.
D. Lipid-soluble ligand diffuses across the membrane to interact with its intracellular receptor.

23. Ion Channel Linked
• The molecules responsible for transduction are
ions (e.g., Na+ or Ca2+) that are normally found
outside of cells.
• Binding of a ligand to the receptor results in an
opening of a gate through the plasma membrane
(hyperpolaristaion/depolaristaion) that allows
entrance of the ions (both gate and receptor are
proteins, likely one in the same protein).
• The increased ion concentration in the cytoplasm
propagates
– signal transduction
– results in a direct stimulation of a response.

25. Structure of the nicotinic acetylcholine receptor
(a typical ligand-gated ion channel)
• The five receptor subunits (α2,
β, γ, δ) : a cluster surrounding a
central transmembrane pore
• Contain negatively charged
aminoacids , which makes the
pore cation selective.
• Two acetylcholine binding sites
in the extracellular portion of
the receptor, at the interface
between the α and the adjoining
subunits.
• On ACh binding: kinked α helices
straighten out or swing out of
the way, thus opening the
channel pore.

26. Typical ligand-gated ion channel receptor

29. Ligand-gated Ion Channel Receptor
• The receptor complex consists of five subunits, each
of which contains four transmembrane domains.
• Simultaneous binding of two acetylcholine (ACh)
molecules to the two α-subunits results in opening of
the ion channel, with entry of Na+ (and exit of some
K+), membrane depolarization, and triggering of an
action potential
• The ganglionic N-cholinoceptors apparently consist
only of α and β subunits (α2β2). :
• GABAA subtypeGlutamate and glycine

30. GPCR

31. GPCR
• Typically "serpentine," with seven transmembrane spanning
domains, the third one of which is coupled to the G-protein
effector mechanism.
• The signaling molecule binds to the G-protein coupled
receptor
• This causes a change in the receptor so it is able to bind to
an inactive G protein.
• This causes a GTP to replace a GDP which activates a G
protein.
• Receptor systems coupled via GTP-binding proteins (G-
proteins) to adenylyl cyclase,(converts ATP to a second
messenger cAMP,) that promotes protein phosphorylation by
activating protein kinase A.
• The protein kinase A serves to phosphorylate a set of tissue-
specific substrate enzymes, thereby affecting their activity.

33. G-PROTEIN-COUPLED RECEPTORS
• The seven α-helical membrane-spanning domains probably form a
circle around a central pocket that carries the attachment sites for
the mediator substance.
• Binding of the mediator molecule or of a structurally related
agonist molecule induces a change in the conformation of the
receptor protein, enabling the latter to interact with a G-protein (=
guanyl nucleotide-binding protein).
• G-proteins lie at the inner leaf of the plasmalemma and consist of
three subunits designated α, β, and γ.
• There are various G-proteins that differ mainly with regard to their
α-unit. Association with the receptor activates the G-protein,
leading in turn to activation of another protein (enzyme, ion
channel).
• A large number of mediator substances act via G-protein-coupled
receptors

34. G-protein coupled receptors triggers an increase (or, less often, a decrease) in the activity of adenylyl
cyclase.

35. G-Protein coupled effector system
1. Adenylate cyclase-cAMP system
2. Phospholipase-C-inositol phosphate system
3. Ion channels

36. Adenylate cyclase-cAMP system

37. Phospholipase-C-inositol phosphate system

40. Three main variants of GPCRs
1. Gs: Stimulation of Adenyl cyclase
2. Gi: Inibition of Adenyl cyclase
3. Gq: Controls phospholipase-C activity

43. Kinase-linked Receptors
• These receptors are directly linked to:
1. Tyrosine kinase (e.g. receptors for insulin and
various growth factors)
Or
2. Guanylate cyclase (e.g. receptors for atrial
natriuretic peptide)

44. Receptors That Function as Transmembrane Enzymes
Tyrosine kinase linked receptors
• Cell-surface receptors, Membrane-spanning macromolecules
• Bind a large variety of watersoluble ligands, including amines,
amino acids, lipids, peptides, and proteins.
• The ligand-binding domain is connected to the cytoplasmic
domain by a single transmembrane helix.
• In receptors with intrinsic enzymatic activity, the cytoplasmic
domain contains a conserved protein tyrosine kinase (PTK)
core and additional regulatory sequences that are subjected
to autophosphorylation and phosphorylation by heterologous
protein kinases
• Binding of the ligand causes confirmational changes so that
the kinase domains become activated, ultimately leading to
phosphorylation of tissue-specific substrate proteins.
• It initiates a unique cellular response for each
phosphorylated tyrosine.

45. Tyrosine kinase receptor

48. KINASE LINKED RECEPTORS
2.kinase linked receptors

50. Receptors linked to gene transcription
Intracellular Cytosol/ Nulcear Receptors
• Binding of hormones or drugs to receptors
releases regulatory proteins that permit of the
hormone-receptor complex.
• Such complexes interact with response elements
on nuclear DNA to modify gene expression.
• Eg: drugs interacting with glucocorticoid
receptors lead to gene expression of proteins
that inhibit the production of inflammatory
mediators.
• Pharmacologic responses elicited via
modification of gene expression are usually
slower in onset but longer in duration than
other drugs.

51. Mechanism of intracellular receptors (e.g. nuclear receptors).

52. 3.Nuclear receptors

56. Down-regulation of Receptors
• Prolonged exposure to high concentration of
agonist causes a reduction in the number
receptors available for activation.
• This results due to endocytosis or
internalisation of the receptors from the cell
surface

57. Up-regulation of Receptors
• Prolonged occupation of receptors by a
blocker leads to an increase in the number of
receptors with subsequent increase in receptor
sensitivity.
• This is due to externalisation of the receptors
from inside of the cell surface.

58. Spare Receptors
• A drug can produce the maximal response
when even less than 100% of the receptors are
occupied. The remaining unoccupied receptors
are just serving as receptor reserve are called
spare receptors

61. SPARE RECEPTORS
• The production of a maximal tissue response when only a fraction of the
total number of receptors are occupied
• Eg: Acetylcholine analogues in isolated tissues, capable of eliciting
maximal responses at very low occupancies, often less than 1%.
• The mechanism linking the response to receptor occupancy has a
substantial reserve capacity. Such system-said to possess spare receptors,
or a receptor reserve.
• Common with drugs : smooth muscle contraction; less for : RESPONSES-
secretion, smooth muscle relaxation or cardiac stimulation: the effect is
more nearly proportional to receptor occupancy.
• Do not imply any functional subdivision of the receptor pool,
• This surplus of receptors over the number actually needed might seem a
wasteful biological arrangement. It means, however, that a given number
of agonist-receptor complexes, corresponding to a given level of biological
response, can be reached with a lower concentration of hormone or
neurotransmitter than would be the case if fewer receptors were
provided..

62. RECEPTORS AND DISEASE
• Molecular pharmacology: revealed a number of
disease states directly linked to receptor
malfunction.
The principal mechanisms:
• Autoantibodies directed against receptor proteins
– Eg: Myasthenia gravis , - autoantibodies that inactivate
nicotinic acetylcholine receptors. Autoantibodies can
also mimic the effects of agonists, as in many cases of
thyroid hypersecretion, caused by activation of
thyrotropin receptors
• Mutations in genes encoding receptors and
proteins involved in signal transduction.
– Mutations of genes encoding GPCRs:
hypoparathyroidism, cancers

63. Receptor Related Diseases
• Myasthenia Gravis:
– Antibodies against the cholinergic nicotinic receptors
at motor end plate.
• Insulin Resistant Diabetes
• Testicular feminisation
• Familial Hypercholesterolaemia

64. ION CHANNELS
Some drugs produce their actions by directly interacting with ion channels.
These ion channels transport ions across the plasma membrane.
They are not receptors and should be distinguished from ion channels that
function as ionotropic receptors

65. Voltage-Operated Channels
• VOC’s like ROC’s are ion channels that are
gated only by voltage.
• While ROC’s assume only 2 states: Open or
Close; VOC’s also assumes a third state called
‘refractory’ (inactivated) state.

66. Refractory State
• In this state the channel is unable to ‘open’ (or
reactivate) for a certain period of time even
when the membrane potential returns to a
voltage that normally opens or activates the
channel.
• State Dependent Binding

67. ION CHANNE S AS T
L ARGE S F
T OR
DRUG ACTION

68. CARRIE AS T
RS ARGE S F
T OR DRUG
ACTION.

69. CARRIE AS T
RS ARGE S F
T OR DRUG
ACT ION.
• Membrane transport proteins
(Transmemebrane Proteins) are two main
types:
• ATP-powered ion pumps
• Transporters
• Both are transmembrane proteins. , termed
‘carriers

70. CARRIERS AS TARGETS FOR DRUG ACTION.
ATP-powered ion pumps
• The three principal ion pumps are the sodium pump (the Na+/K+
ATPase), the calcium pump, and the Na+/H+ pump in the gastric
parietal cell, which is the target for the proton pump inhibitor
omeprazole.
• Sodium pump. - important in maintaining cellular osmotic balance
and cell volume and in maintaining the membrane potential.
• In many cells (e.g. in the myocardium, the nephron) it is the
primary mechanism for transporting Na+ out of the cell
• The K+ concentration is 140 mmol/l inside cells and 5 mmol/l
outside. For each molecule of ATP hydrolysed, the sodium pump
pumps 3Na+ out of the cell and 2K+ in against their chemical
gradients
Carrier molecules (transporters)
• The main transporters involved in drug action are symporters and
antiporters (exchangers)

71. Carrier molecules (transporters)
• Symporters These use the electrochemical gradient of one ion (usually
Na+) to carry another ion (or molecule or several ions) across a cell
membrane. Drugs can modify this action by occupying a binding site (e.g.
the action of furosemide (frusemide) on the Na+/K+/2Cl– symport in the
nephron (
• Similarly, thiazide diuretics bind to and inhibit the Na+/Cl– symporter in
the distal tubule.
• Antiporters These use the electrochemical gradient of one ion (usually
Na+) to drive another ion (or molecule) across the membrane in the
opposite direction. An important example is the Ca2+ exchanger, which
exchanges 3Na+ for 1Ca2+
• This calcium exchanger should be distinguished from the ATPdriven
calcium pump and the ligand-gated and voltage-gated Ca2+ channels .
• The calcium exchanger is crucial in the maintenance of the Ca2+
concentration in blood vessel smooth muscle and cardiac muscle
• OtherEg: uptake carrier in the noradrenergic varicosity, which transports
noradrenaline into the cell

72. CARRIER MOLECULES(TRANSPORTERS) AS TARGETS
FOR DRUG ACTION

73. CARRIER MOLECULES(TRANSPORTERS) AS TARGETS FOR DRUG
ACTION

74. ENZYMES
Drugs can produce effects on enzyme reactions by substrate
competition or by reversibly or irreversibly modifying the enzyme

76. THANK YOU