2 Molecular aspects of drug action
Drugs produce effects in the body mainly in the following ways: (i) by acting on receptors, (ii) by inhibiting carriers (molecules that transport
one or more ions or molecules across the plasma membrane), (iii) by modulating or blocking ion channels, (iv) by inhibiting enzymes.
binding proteins); also termed metabotropic receptors
RECEPTORS AS TARGETS FOR DRUG ACTION
• receptors linked to ion channels; also termed ionotropic
Receptors are protein molecules in or on cells whose function is to receptors or ligand-gated ion channels
interact with the body’s endogenous chemical messengers • receptors that affect gene transcription
(hormones, neurotransmitters, the chemical mediators of the • receptors linked to enzymes (e.g. kinases, guanylate cyclase,
immune system, etc.) and thus initiate cellular responses. They etc); these mostly initiate a kinase cascade within the cell.
enable the responses of the body’s cells to be coordinated. Drugs
used in medicine make use of these chemical ‘sensors’—either Receptors coupled to G proteins
stimulating them (drugs that do this are termed agonists) or GPCRs occur in the cell membrane and respond in seconds. They
preventing endogenous mediators or agonists from stimulating them have a single polypeptide chain that has seven transmembrane
(drugs that do this are termed antagonists). helices. Signal transduction occurs by activation of particular
There are four types of receptor: G-proteins that modulate enzyme activity or ion channel function
• receptors coupled to G-proteins (GPCR: guanine nucleotide- (Figs 2.1–2.3).
Ion channel Agonist stimulates Agonist stimulates Antagonist has no effect on its
receptor receptor own; blocks action of agonist
R R R
G G Enzyme
G-protein interacts directly with and G-protein activates or
changes the open probability of the inhibits enzyme that gives
ion channel rise to second messengers
cAMP, IP3, etc.
Examples Agonists Antagonists
• Muscarinic receptors in heart K+ permeability
β2-Adrenoceptors Salbutamol Propranolol
and electrical activity
Antagonist: atropine β1-Adrenoceptors Isoprenaline Atenolol
H1 receptors Histamine Mepyramine
• Opiates open K+ channels and excitability in
H2 receptors Histamine Ranitidine
Antagonist: naloxone Opiate µ receptors Morphine Naloxone
Fig. 2.1 Receptors coupled to G-proteins with examples of drugs acting on them. Each receptor couples to several G-proteins
(not shown), resulting in amplification of the response.
G-proteins Targets activated Example of Typical effect Produced by Antagonist
receptor involved agonists
PIP2 Smooth muscle
contraction ( IP3)
Gq Phospholipase C
IP3 Releases Ca2+ from H1-histamine Histamine Mepyramine
A variety of effects Ch. 15
Activates protein due to protein
kinase C phosphorylation
Smooth muscle Adrenaline Ch. 11, Propranolol
relaxation ( cAMP) salbutamol Ch. 24
Activates protein M2-muscarinic Decreased force of Acetylcholine Atropine
kinase A contraction of the
Gi heart ( cAMP)
K+ channels in Increased opening of
M2-muscarinic Cardiac slowing Acetylcholine Atropine
cell membrane the channels resulting
Fig. 2.2 Examples of G-protein-coupled actions. The pathways are shown for three different G-proteins. IP3, inositol trisphosphate,
PIP2, phosphatidylinositol 4,5-bisphosphate.
2 MOLECULAR ASPECTS OF DRUG ACTION
G-proteins are attached to the membrane and consist of 3 subunits a, b and g, the last two being closely associated:
In the free G protein, GDP occupies the binding site on the a-subunit. The a subunit and the b/g complex can each activate intracellular
targets. Subtypes of all 3 subunits exist; the particular subunit determines which targets are activated
R R R
E1 bg E1 bg E1 bg E2
GDP a GDP
1. Agonist interacts with receptor 6. The a-subunit + GDP re-associates with 5. GTP is hydrolysed by the GTPase of the
the bg-subunits, to be back where we a-subunit. The agonist dissociates from
started the receptor
R R R
a a a
bg E1 bg E1
2. The a-subunit (+ GDP) interacts with the 3. GTP replaces GDP 4. The a-subunit + GTP interacts with the
receptor enzyme, activating it. The b/g complex
also activates a target enzyme
Fig. 2.3 The mechanism of the G-protein transduction process.
Activated enzyme indicated by a blue box.
Receptors linked to ion channels (i.e. ionotropic receptors)
Receptors linked to ion channels are located in the cell membrane and respond in milliseconds. The channel forms part of the receptor. The
nicotinic receptor for acetylcholine (see Ch. 10) is an example (Fig. 2.4).
Receptor with Agonist Channel Antagonist The nicotinic receptor has 5 subunits (3 shown).
2 binding sites (ACh) opens inhibits Stimulation by agonist opens the ion channel
for ACh binds binding of and lets cations through
• GABAA receptor: a ligand-gated Cl–
channel (Ch. 32)
• ionotropic glutamate receptor: a cation
channel (Ch. 32)
• 5-HT3 receptor: a ligand-gated cation
channel (Ch. 12)
Fig. 2.4 Examples of receptors linked to ion channels (ionotropic receptors). ACh, acetylcholine.
Receptors linked to gene transcription receptors activate Jak kinases, which, in turn, activate Stat
The receptors that regulate gene transcription are called nuclear transcription factors and these activate gene transcription (Fig. 2.6).
receptors although some are located in the cytosol (e.g.
glucocorticoid receptors) and migrate to the nucleus after binding a
ligand (Fig. 2.5). CARRIERS AS TARGETS FOR DRUG ACTION
Receptors linked to enzymes The classification of membrane transport proteins varies between
These receptors are transmembrane proteins with a large authorities, but in essence there are two main types:
extracellular portion that contains the binding sites for ligands (e.g. • ATP-powered ion pumps
growth factors, cytokines) and an intracellular portion that has • transporters (Table 3.1)
integral enzyme activity—usually tyrosine kinase activity (Fig. 2.6).
Activation initiates an intracellular pathway involving cytosolic and Both are transmembrane proteins. In Rang et al. Pharmacology,
8 nuclear transducers and eventually gene transcription. Cytokine these are termed ‘carriers’.
MOLECULAR ASPECTS OF DRUG ACTION 2
Examples are members of the
steroid superfamily of receptors;
• corticosteroid receptors Gc
• oestrogen and progestogen
receptors R R R Receptor changes
• thyroid hormone receptors
• Vitamin D3 receptors
The Gc/receptor complexes form
dimers before entering the nucleus Complex interacts with
(not shown) DNA and alters gene
Mediator Mediator Transcribed genes induce
Cytoplasm proteins proteins synthesis of some
in plasma mediator proteins and
inhibit synthesis of others
Fig. 2.5 Mechanism of action of a receptor linked to gene transcription. Gc, glucocorticoid; R, receptor.
ATP-powered ion pumps The K+ concentration is 140 mmol/l inside cells and 5 mmol/l
The three principal ion pumps are the sodium pump (the Na /K + + outside. For each molecule of ATP hydrolysed, the sodium pump
ATPase), the calcium pump, and the Na+/H+ pump in the pumps 3Na+ out of the cell and 2K+ in against their chemical
gastric parietal cell, which is the target for the proton pump gradients. (The pump in Fig. 2.7 has simplified stoichiometry.)
inhibitor omeprazole. Here we will concentrate on the sodium
pump. This is important in maintaining cellular osmotic Transporters
balance and cell volume and in maintaining the membrane The main transporters involved in drug action are symporters and
potential. In many cells (e.g. in the myocardium, the nephron) antiporters (exchangers) (see Fig. 2.7).
it is the primary mechanism for transporting Na+ out of the Symporters These use the electrochemical gradient of one ion
cell (Fig. 2.7). (usually Na+) to carry another ion (or molecule or several ions)
A B C
Growth factor receptors
P P P SH2
Tyrosine kinases GTP
Nucleus Kinase 2
Cytoplasm Kinase 3
Agonist binding to 2 SH2-containing ‘adapter’ proteins bind to the
receptors leads to phosphorylated residues in the receptors and activate a
coupling (dimerisation). pathway consisting of Ras, which becomes activated after
The TKs in each receptor exchange of GDP for GTP; this, in turn, activates a cascade
phosphorylate the other of three kinases. The last kinase phosphorylates various
member of the dimer transcription factors, thus activating transcription of the
genes for proliferation and differentiation
Fig. 2.6 Receptors linked to tyrosine kinases, e.g. growth factor receptor.
2 MOLECULAR ASPECTS OF DRUG ACTION
1. The sodium pump is 2. The conformation 3. Sodium binds 4. ATP binds to and 5. This conformation
shown with the shown below phosphorylates has a low affinity
binding sites (3 for (sites facing the the cytoplasmic which causes a for Na+ and a high
Na+, 2 for K+) facing cytoplasm) has surface of the change in the affinity for K+
the cytoplasmic high affinity for pump conformation of
surface of the Na+ and lower the ATPase (see 5)
plasma membrane affinity for K+ so that the binding
sites now face
Digitalis glycosides inhibit the
pump by binding to a K+ site
we started K
P P P
11. Potassium 10. The sites now 9. Dephosphorylation 7,8. Potassium binds 6. Sodium is
is released face inwards. occurs, changing released
There is now the ATPase and...
lower affinity conformation
for K+ than Na+
Fig. 2.7 The action of the sodium pump.
across a cell membrane. Drugs can modify this action by occupying Ca2+ exchanger, which exchanges 3Na+ for 1Ca2+ (Fig. 2.8). Note
a binding site (e.g. the action of furosemide (frusemide) on the that this calcium exchanger should be distinguished from the ATP-
Na+/K+/2Cl– symport in the nephron (Fig. 2.8). Similarly, thiazide driven calcium pump and the ligand-gated and voltage-gated Ca2+
diuretics bind to and inhibit the Na+/Cl– symporter in the distal channels (see Fig. 4.1 in Rang et al.). The calcium exchanger is
tubule. crucial in the maintenance of the Ca2+ concentration in blood vessel
Antiporters These use the electrochemical gradient of one ion smooth muscle and cardiac muscle (see Ch. 20). Another example is
(usually Na+) to drive another ion (or molecule) across the the uptake carrier in the noradrenergic varicosity, which transports
membrane in the opposite direction. An important example is the noradrenaline into the cell (see Ch. 11).
Ions bind Transporter gradient of Na+
Cl− Na+ binds
ions releasd in
+ the cell
Ca2+ binds Change of conformation Ions
) inhibits by binding to a Cl− site
Fig. 2.8 Examples of (A) a symporter, and (B) an antiporter.
MOLECULAR ASPECTS OF DRUG ACTION 2
ION CHANNELS AS TARGETS FOR DRUG ACTION
Some drugs produce their actions by directly interacting with ion channels. Three examples are given in Figure 2.9. Note that 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 (see above).
Voltage-gated Na+ channels 2+
Voltage-gated L-type Ca channels ATP-sensitive K+ channels in the
in sensory neurons consist in cardiac myocytes consist of several insulin-secreting pancreatic B cell
of 4 subunits (not shown). subunits (not shown). The channels have binding sites for
Local anaesthetics (LAs) can be blocked by the dihydropyridine sulfonylureas, which are used to
block the channel, stopping calcium antagonists, which bind to a treat non-insulin-dependent
action potential generation site on an =-subunit diabetes (Ch. 27)
KATP channel Sulfonylurea (e.g.
Uncharged LAs reach Calcium
(opens when the glibenclamide)
blocking site from antagonist,
cytosolic ATP acts on binding
outside of membrane e.g. nifedipine
concentration falls) site, blocks
Binding site channel,
Na Ca depolarises cell...
Cytoplasm ...and causes
K+ insulin secretion
Charged LAs reach blocking
site from inside of membrane
Fig. 2.9 Examples of drugs acting directly on ion channels.
ENZYMES AS TARGETS FOR DRUG ACTION
Drugs can produce effects on enzyme reactions by substrate competition or by reversibly or irreversibly modifying the enzyme. Some
examples are given in Table 2.1.
Table 2.1 Drugs acting through alteration of enzyme reactions
Substrate Products Inhibitor ? Substrate
Substrate Enzyme Products Inhibitor Uses
Acetylcholine Acetylcholine esterase Choline; acetate Neostigmine Myasthenia gravis and to reverse neuromuscular block
Arachidonate Cyclooxygenase Prostanoids Aspirin Heart disease and inflammation
Angiotensin (AT)I AT converting enzyme AT II Captopril Hypertension, heart failure, post-infarct
Hypoxanthine Xanthine oxidase Uric acid Allopurinol Gout
HMG-CoA HMG-CoA reductase Mevalonic acid Simvastatin To lower blood cholesterol
Folate Dihydrofolate reductase Tetrahydrofolate Trimethoprim With cotrimoxazole as antibacterial
Thymidine Viral reverse transcriptase Zidovudine HIV infection
Deoxyribonucleotides DNA polymerase DNA Cytarabine Anticancer drug