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by Lee Eun Jin
MOLECULAR MECHANISM OF
ACTION OF DRUGS
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Drugs produce effects in the body mainly in the
following ways:
1. by acting on receptors2. 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.
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TARGETS FOR DRUG ACTION
PROTEIN TARGETS RECEPTORS
ION CHANNELS
ENZYMES
CARRIER MOLECULES (TRANSPORTERS).
NON-PROTEIN TARGETS-Binding, neutralising, osmosis etc.
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ION CHANNELS
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ENZYMES
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ARRIER MOLECULES (TRANSPORTERS).
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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, orreduction of cerebral edema
Cholesterol-binding agents,(cholestyramine resin) to
decrease dietary cholesterol absorption.
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RECEPTORS
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RECEPTORS
The term receptor: anycellular macromolecule to which a drug bindsto initiate its effects.
Receptors are protein molecules in or on cells whose function is tointeract with the bodys endogenous chemical messengers (hormones,neurotransmitters, the chemical mediators of the immune system,etc.) and thus initiate cellular responses.
They enable the responses of the bodys cells to be coordinated
A molecule which binds (attaches) to a receptor is called a LIGAND ; - apeptide, or other small molecule, such as a neuorotransmitter,hormone, chemical/ drug or a toxin.
A class of cellular macromolecules (cellular proteins) that areconcerned specifically and directly with chemical signaling betweenand within cells.
Combination of a hormone, neurotransmitter, or intracellularmessenger with its receptor(s) results in a change in cellular activity.
A receptor functions: recognize the particular molecules thatactivate (act as receptors for endogenous regulatory ligands)+Message propagation (alter cell function)
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RECEPTOR
Macromolecules that bind mediator
substances and transduce this binding into aneffect, i.e., a change in cell function.
The component of a cell or organism that
interacts with a drug and initiates the chainof biochemical events leading to the drug's
observed effects.
Isolation and characterization -the molecularbasis of drug action.
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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 :
Receptoreffector 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.
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Receptortransducereffectorsignal termination complexes
established via proteinlipid and proteinprotein interactions.
Receptors and their associated effector and transducer proteins
also act as integrators of information as they coordinatesignals 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 arebiochemical 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.
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Drugs that bind to physiological receptors and mimic theregulatory effects of the endogenous signalingcompounds are termed AGONISTS
(Affinity and efficacy: 1)
Agents those bind to receptors without regulatoryeffect, but their binding, blocks the binding of theendogenous agonist.:
ANTAGONISTS (Affinity1, efficacy 0)
Agents that are only partly as effective as agonists nomatter the dose employed are: PARTIAL AGONISTS
( Affinity 1, Efficacy
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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 extracellulardomains- vary from one to another within the same
family
The overall structural patterns and associated signal
transduction pathways are very consistent.
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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 diversityarises 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)
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receptors
Physiological receptors
Agonist primary agonist
-allosteric agonist
-partial agonist
Antagonist syntopic
-allosteric
-chemical
-functional
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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.
/
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TYPES/FAMILIES OF RECEPTORS
Based on molecular structure and the nature of thelinkage (the transduction mechanism)
Ligand-gated ion channels (Ionotropic)
Nicotinic acetylcholine receptor, GABA A receptor
G-protein-coupled receptors (GPCRs)/MetabotropicMuscarinic acetylcholine receptor, adrenoceptors
Kinase( Tyrosine)-linked and related receptors
Insulin, growth factors, cytokine receptors
Nuclear/ Cytosolreceptors
steroids, thyroid hormones, gonadal steroids,vit D
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MAJOR RECEPTOR FAMILIES
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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.
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Ion Channel Linked
The molecules responsible for transduction are
ions (e.g., Na+ or Ca2+) that are normally foundoutside of cells.
Binding of a ligand to the receptor results in anopening of a gate through the plasma membrane
(hyperpolaristaion/depolaristaion) that allowsentrance of the ions (both gate and receptor areproteins, likely one in the same protein).
The increased ion concentration in the cytoplasmpropagates
signal transduction
results in a direct stimulation of a response.
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Structure of the nicotinic acetylcholine receptor
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Structure of the nicotinic acetylcholine receptor
(a typical ligand-gated ion channel)
The five receptor subunits (2,
,,) : 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 and the adjoining
subunits.
On ACh binding: kinked helicesstraighten out or swing out of
the way, thus opening the
channel pore.
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Typical ligand-gated ion channel receptor
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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 -subunits results in opening of
the ion channel, with entry of Na+ (and exit of some
K+), membrane depolarization, and triggering of anaction potential
The ganglionic N-cholinoceptors apparently consist
only of and subunits (22). :
GABAA subtypeGlutamate and glycine
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GPCR
GPCR
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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 secondmessenger 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.
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G-PROTEIN-COUPLED RECEPTORS
The seven -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 ,, and .
There are various G-proteins that differ mainly with regard to their
-unit. Association with the receptor activates the G-protein,
leading in turn to activation of another protein (enzyme, ionchannel).
A large number of mediator substances act via G-protein-coupled
receptors
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G-protein coupled receptors triggers an increase (or, less often, a decrease) in the activity of adenylyl
cyclase.
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G-Protein coupled effector system
1. Adenylate cyclase-cAMP system
2. Phospholipase-C-inositol phosphate system
3. Ion channels
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Adenylate cyclase-cAMP system
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Phospholipase-C-inositol phosphate system
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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)
Receptors That Function as Transmembrane Enzymes
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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 cytoplasmicdomain by a single transmembrane helix.
In receptors with intrinsic enzymatic activity, the cytoplasmicdomain contains a conserved protein tyrosine kinase (PTK)core and additional regulatory sequences that are subjectedto autophosphorylation and phosphorylation by heterologousprotein kinases
Binding of the ligand causes confirmational changes so thatthe kinase domains become activated, ultimately leading tophosphorylation of tissue-specific substrate proteins.
It initiates a unique cellular response for eachphosphorylated tyrosine.
Tyrosine kinase receptor
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Tyrosine kinase receptor
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2 ki li k d
KINASE LINKED RECEPTORS
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2.kinase linked receptorsN S N C O S
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Receptors linked to gene transcription/
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Intracellular Cytosol/ Nulcear Receptors
Binding of hormones or drugs to receptorsreleases regulatory proteins that permit of thehormone-receptor complex.
Such complexes interact with response elementson nuclear DNA to modify gene expression.
Eg: drugs interacting with glucocorticoidreceptors lead to gene expression of proteins thatinhibit the production of inflammatorymediators.
Pharmacologic responses elicited via modificationof gene expression are usually slower in onsetbut longer in duration than other drugs.
Mechanism of intracellular receptors (e g nuclear receptors)
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Mechanism of intracellular receptors (e.g. nuclear receptors).
3 N l t
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3.Nuclear receptors
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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
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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.
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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 calledspare receptors
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SPARE RECEPTORS
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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 elicitingmaximal 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 numberof 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..
RECEPTORS AND DISEASE
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Molecular pharmacology: revealed a number ofdisease states directly linked to receptor
malfunction.
The principal mechanisms:
Autoantibodies directed against receptor proteins
Eg: Myasthenia gravis , - autoantibodies that inactivatenicotinic acetylcholine receptors. Autoantibodies canalso mimic the effects of agonists, as in many cases ofthyroid hypersecretion, caused by activation ofthyrotropin receptors
Mutations in genes encoding receptors andproteins involved in signal transduction.
Mutations of genes encoding GPCRs:
hypoparathyroidism, cancers
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Receptor Related Diseases
Myasthenia Gravis: Antibodies against the cholinergic nicotinic receptors
at motor end plate.
Insulin Resistant Diabetes
Testicular feminisation
Familial Hypercholesterolaemia
ION CHANNELS
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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 functio
n as ionotropic receptors
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Voltage-Operated Channels
VOCs like ROCs are ion channels that are
gated only by voltage.
While ROCs assume only 2 states: Open or
Close; VOCs also assumes a third state called
refractory (inactivated) state.
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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 thechannel.
State Dependent Binding
ION CHANNELS AS TARGETS FOR DRUG ACTION
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ARRIERS AS TARGETS FOR DRUG ACTION.
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Membrane transport proteins
(Transmemebrane Proteins) are two maintypes:
ATP-powered ion pumps
Transporters
Both are transmembrane proteins. , termed
carriers
CARRIERS AS TARGETS FOR DRUG ACTION.
CARRIERS AS TARGETS FOR DRUG ACTION.
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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 pumppumps 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)
CARRIERS AS TARGETS FOR DRUG ACTION.
Carrier molecules (transporters)
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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 thenephron (
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 oppositedirection. 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
CARRIER MOLECULES(TRANSPORTERS) AS TARGETS
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( )
FOR DRUG ACTION
CARRIER MOLECULES(TRANSPORTERS) AS TARGETS FOR DRUGACTION
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ACTION
ENZYMES
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ENZYMES
Drugs can produce effects on enzyme reactions by substrate competitio
n or by reversibly or irreversibly modifying the enzyme
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THANK YOU