molecular mechanisms of drug action

7/28/2019 Molecular Mechanisms of Drug Action

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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.



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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)


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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molecular mechanisms of drug action

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