How Drugs Act Flashcards
Protein Targets for Drug Binding
(5)
- Receptors
- Enzymes
- Specific Circulating Plasma Proteins
- Carrier Molecules (Transporters)
- Ion Channels
. Receptors
Protein molecule which function
to recognize and respond to
endogenous chemical signals
* recognize/bind specific endogenous
ligands
* may also recognize/bind xenobiotics
receptors
Classified based on —
Grouped into — major
superfamilies
ligands
4
Ligand-Gated Ion Channels
— “ionotropic” receptors
G-Protein Coupled Receptors
(4)
— “metabotropic” receptors
— “7 trans-membrane spanning domain” receptors
— “heptahelical” receptors
— “serpentine” receptors
Kinase-Linked & Related Receptors
(2)
— large and heterogeneous group
— single trans-membrane spanning domain
Nuclear Receptors
— “steroid superfamily”
*Ligand-Gated Ion
Channels Composed of
— of these subunits
4-5
Receptor Subtypes
* receptors in given family generally
occur in several molecular varieties or
“subtypes”
(4)
— similar architecture
— significant differences in amino acid
sequence
— often different pharmacologic properties
— nicotinic acetylcholine receptor subtypes
occur in different brain regions and these
differ from subtype in muscle
different genes, different phenotypes
— different genes may encode for different subtypes
same gene, different phenotypes
(2)
— variation may arise from alternative mRNA splicing
— single nucleotide polymorphisms
SKIPPED
— variation may arise from alternative mRNA splicing
(3)
- single gene can give rise to more than one receptor isoform
- splicing can result in inclusion or deletion of one or more mRNA coding regions giving rise to short or long forms of protein
- big role in G-protein coupled receptors
— single nucleotide polymorphisms
- often results in different drug-receptor efficacy
Ligand-Gated Ion Channels
(2)
- share structural features with voltage-
gated ion channels - “ionotropic” receptors
Ligand-Gated Ion Channels
examples
(3)
— nicotinic acetylcholine receptor (nAChR)
— gamma-aminobutyric acid type A receptor
(GABAA)
* inhibitory neurotransmitter
— glutamate receptors [N-methyl-D-aspartate
(NMDA), a-amino-3-hydroxy-5-methylisoxazole-4-propionic
acid (AMPA), and kainate types]
* excitatory neurotransmitter
Ligand-Gated Ion Channels
* nAChR
(8)
— best characterized of all cell-surface receptors
— pentamer: four different polypeptide
subunits
* 2 a, 1 b, 1 g, and 1 d, MW from 43K-50K each
* each subunit crosses plasma membrane 4 times
— acetylcholine binds sites on a subunits
— conformational change occurs
— transient opening of central aqueous channel
— Na+ flow from outside to inside cell
* down electro-chemical gradient
— cell depolarizes
— all occurs in milliseconds
G-Protein Coupled Receptors
(3)
- largest superfamily of receptors
- “metabotropic” receptors
- 7 transmembrane spanning domains
SKIPPED
G-Protein Coupled Receptors
examples
(10)
— muscarinic acetylcholine receptor (mAChR)
— opioid receptors (m, k, d)
— gamma-aminobutyric acid type B receptor (GABAB)
— serotonergic receptors (5-hydroxytryptamine or 5-HT, 1-7 types)
— adrenergic receptors (a and b types)
— angiotensin II receptors (1, 2, 3, 4 types)
— endothelin receptors (A, B, C types)
— histamine receptors (1, 2, 3 types)
— photon receptors (retinal rod and cone)
G-Protein Coupled Receptors
* agonist binds to region inside receptor surrounded by — domains
7 TM
- conformational change in cytoplasmic side
(3)
— spreads cytoplasmic side of 7 TM domains by ~1 nm compared to inactive conformation
— opens cavity in receptors cytoplasmic side
— cavity binds critical regulator surface of the G-Protein
G-Protein receptors sequence
- G-Protein affinity for nucleotide GDP is reduced
— GDP dissociates - GTP binds
— GTP normally higher in concentration than GDP intracellularly - GTP-bound G-Protein dissociates from the receptor
- GTP-bound G-Protein engages downstream mediators (a.k.a. “effectors”)
G-Protein Coupled Receptors
* agonist binds and dissociates …
rapidly
— a few milliseconds
G-Protein Coupled Receptors
activated GTP-bound G-proteins
remain
active much longer
— up to tens of seconds
— this produces significant signal
amplification from one ligand-receptor
interaction
heterogeneity of G-proteins allow for
substantial diversity in GPCR signaling
in various tissues
SKIPPED
G Protein: Gs
Receptor Ligands
Effector/Signaling Pathway
β-Adrenergic amines, histamine, serotonin,
glucagon, and many other hormones
↑ Adenylyl cyclase →↑ cAMP
SKIPPED
G Protein: Gi1, Gi2, Gi3
Receptor Ligands
Effector/Signaling Pathway
α2-Adrenergic amines, acetylcholine
(muscarinic), opioids, serotonin, and many
others
Several, including:
↓ Adenylyl cyclase →↓
cAMP
Open cardiac K+ channels
→↓ heart rate
SKIPPED
G Protein: Golf
Receptor Ligands
Effector/Signaling Pathway
Odorants (olfactory epithelium)
↑ Adenylyl cyclase →↑
cAMP
SKIPPED
G Protein: Go
Receptor Ligands
Effector/Signaling Pathway
Neurotransmitters in brain (not yet
specifically identified)
Not yet clear
SKIPPED
G Protein: Gq
Receptor Ligands
Effector/Signaling Pathway
Acetylcholine (muscarinic), bombesin,
serotonin (5-HT2), and many others
↑ Phospholipase C →↑
IP3, diacylglycerol,
cytoplasmic Ca2+
SKIPPED
G Protein: Gt1, Gt2
Receptor Ligands
Effector/Signaling Pathway
Photons (rhodopsin and color opsins in
retinal rod and cone cells)
↑ cGMP
phosphodiesterase →↓
cGMP (phototransduction)
G-Protein Coupled Receptor
Example
Opioid Pharmacology
* Agonists of Opioid Receptors
— (4)
* Competitive Antagonists of Opioid
Receptors
— (2)
heroin, morphine, oxycodone,
hydrocodone
naloxone, naltrexone
G-Protein Coupled Receptors
* Protease Activated Receptors (PAR)
(2)
— activation of GPCR is normally a result of diffusible ligand in solution acting on a receptor
— but GPCR receptor activation can occur as a result of protease activation
— but GPCR receptor activation can occur as a result of protease activation
(2)
- protease cleaves off part of N-terminal domain of receptor
- “tethered agonist”: remaining attached domain is free to interact with ligand-binding domain
Protease Activated Receptors (PAR)
examples:
(2)
- thrombin: a protease involved in blood clotting activates PAR
- PAR-2 is activated by a protease released from mast cells following degranulation
G-Protein Coupled Receptors
* Desensitization
(2)
— applicable to all GPCRs
— occurs via 2 main mechanisms
* receptor phosphorylation
* receptor internalization
G-Protein Coupled Receptors
* Desensitization
example:
(4)
- b-adrenergic receptors
- b-arrestin phosphorylates receptor
reducing receptor affinity for G-proteins - receptor can then be internalized
- all is rapidly reversible
SKIPPED
G-Protein Coupled Receptors
* Further Intricacies
(5)
- receptor subtypes
» one gene can give rise to multiple
subtypes of receptors through
alternative mRNA splicing - single nucleotide polymorphisms
» one amino acid change can result in
different phenotypes of receptor - cross-talk and collaboration between two
different GPCRs or GPCR with receptor
tyrosine kinase (RTK) - much, much more to be learned
- many new drug targets
Superfamilies of Receptors
* Kinase-Linked & Related Receptors
(4)
— involved mainly in events controlling cell growth and differentiation
— act indirectly by regulating gene transcription
— signal transduction generally involves dimerization of two receptor molecules followed by autophosphorylation of tyrosine residues
— all have large extracellular ligand-binding domain connected via single membrane spanning domain to an intracellular domain which has enzymatic activity
Kinase-Linked & Related Receptors
— all have large extracellular ligand-binding domain connected via
single membrane spanning domain to an intracellular domain which has enzymatic activity
Kinase-Linked & Related Receptors
* three major families
(3)
— Receptor Tyrosine Kinases (RTKs)
— Serine/Threonine Kinases
— Cytokine Receptors
Receptor Tyrosine Kinases (RTKs)
(2)
— intracellular domain has tyrosine kinase
activity
— e.g. epidermal growth factor receptor,
nerve growth factor, Toll-like receptors,
insulin receptors
Receptor Tyrosine
Kinases (RTKs)
— insulin receptor
(2)
- activates PI3 kinase
pathway - activates Mitogen-
Activated Protein (MAP)
Kinase pathway
Insulin
* activates PI3 kinase
pathway
(2)
* activates Mitogen-Activated Protein (MAP)
Kinase pathway
(1)
» turns on or off gene
expression
» activates glycogen
synthesis
» turns on or off gene
expression
SKIPPED
Activated Protein (MAP)
Kinase pathway
» turns on or off gene
expression
Kinase-Linked & Related Receptors
* Serine/Threonine Kinases
(4)
— smaller group than RTKs
— structurally and functionally very similar
to RTKs
— phosphorylate serine and threonine
instead of tyrosine
— e.g. transforming growth factor (TGF)
Kinase-Linked & Related Receptors
* Cytokine Receptors
(3)
— interleukins, interferons, chemokines,
etc
— lack intrinsic enzymatic activity in
intracellular domains!!
— associate and activate other kinases
cytokines
associate and activate other kinases
(3)
- binds and activates Janus Kinase (Jak)
- Jak binds and activates Signal Transducers
and Activators of Transcription
» a.k.a. the “Jak-STAT” pathway - downstream turns on or off gene expression
Superfamilies of Receptors
* Nuclear Receptors (“Steroid
Superfamily”)
(2)
— ligand-activated transcription factors
— ligand-binding and DNA-binding domains
Nuclear Receptors (“Steroid
Superfamily”)
ligand examples:
- estrogens, progestins, androgens,
glucocorticoids, mineralocorticoids, vitamin D,
vitamin A (retinoid receptors), fatty acids, etc.
Nuclear Receptors (“Steroid
Superfamily”)
— two main locations in the cell
(2)
- cytoplasmic
- nuclear
Superfamilies of Receptors
* Nuclear Receptors (“Steroid Superfamily”)
(3)
— cytoplasmic
— nuclear (e.g. fatty acid receptors)
— interact with hormone response elements on genes to regulate gene expression
Superfamilies of Receptors
* Nuclear Receptors (“Steroid Superfamily”)
— cytoplasmic
(4)
- most are bound to Heat Shock Proteins when no ligand is present
- most form homodimers upon ligand binding (e.g. steroid receptors)
- some form heterodimers with Retinoid X Receptor (e.g. thyroid hormone)
- translocate to nucleus to regulate gene expression
Superfamilies of Receptors
* Nuclear Receptors (“Steroid Superfamily”)
— nuclear (e.g. fatty acid receptors)
(2)
- constitutively present in nucleus
- form heterodimers with Retinoid X Receptor (RXR)
Superfamilies of Receptors
* Nuclear Receptors (“Steroid Superfamily”)
* Example:
Androgen Receptors
Enzymes as Drug Targets
* many enzymes serve as drug targets
(2)
— enzymes that are key rate-limiting steps in biochemical reactions are the best drug targets
— strategy is most often to reduce enzyme activity through drug inhibition
non-competitive enzyme inhibitors
— drug may covalently modify enzyme
* aspirin acetylates Cyclooxygenase 1 and 2 to non-competitively and irreversibly inhibits
competitive enzyme inhibitors
(2)
— drug is often a structural analog of the naturally occurring substrate
— example: HMG-CoA Reductase Inhibitors
Enzymes as Drug Targets
* HMG-CoA Reductase Inhibitors
(“Statins”)
(2)
— competitively inhibit rate-limiting step
in cholesterol biosynthesis in liver
— structurally similar to hydroxy-methy-
glutaryl-coenzyme A
Enzymes as Drug Targets
* HMG-CoA Reductase Inhibitors
(“Statins”)
competitively inhibit rate-limiting step
in cholesterol biosynthesis in liver
- liver upregulates LDL receptors thereby
reducing plasma LDL concentrations
Enzymes as Drug Targets
* HMG-CoA Reductase Inhibitors
(“Statins”)
Statins:
- lovastatin, atorvastatin, fluvastatin,
pitavastatin, pravastatin, rosuvastatin,
simvastatin
Disease and/or Symptoms Produced by Elevated Circulating Plasma Proteins
* Tumor Necrosis Factor-a
(2)
— elevated in rheumatoid arthritis, Crohn’s disease, psoriasis, ankylosing spondylitis
— elevated in severe cases of aphthous ulcers
Lowering TNF-a in RA or Crohn’s patients
decreases symptoms and may delay progression
Infliximab and adalimumab
(2)
— monoclonal antibodies that recognizes TNF-a
— bind TNF-a removing it from circulation
Etanercept
— soluble TNF-a receptor that binds TNF-a
Many other examples
— daclizumab –
— mepolizumab –
antibody that binds interleukin-2
antibody that binds interleukin-5
Ion and Small Molecule Transporters
(2)
- important in moving substances across lipid bilayer membranes
- often good drug targets as they regulate key cellular events
SKIPPED
Small Molecule Transporters
(3)
- neurotransmitter uptake (norepinephrine, 5-HT, glutamate, etc.)
- organic ion transporters (organic acids and bases)
- p-glycoprotein (Multi-Drug Resistance)
p-glycoprotein (Multi-Drug Resistance)
(3)
— protective role in moving potential toxicants out of gastrointestinal epithelial cells back into lumen to prevent absorption
— overexpressed in certain tumor cells leading to drug resistance
— can be blocked by drugs
SKIPPED
p-glycoprotein (Multi-Drug Resistance)
can be blocked by drugs
(2)
- could increase absorption of some drugs
- could potentially increase activity of anti-cancer drugs
Ion Transporters
* many transporters important in renal tubules
— drives water reabsorption and concentration of urine
Ion Transporters
Na+/K+ ATPase important everywhere
— establishes
— requires
— key in all
— often provides the — for other ion transporters
— can be inhibited by
electrochemical gradient by moving Na+ out and K+ in against concentration gradient
energy (ATP) to function
muscle contraction, nervous conduction, ion gradient establishment, etc
driving force
drugs (e.g. digoxin, a cardiac glycoside used for heart failure)
Voltage-Gated Ion Channels
* structure
— very similar in structure and function
to ligand-gated ion channel receptors
Voltage-Gated Ion Channels
(3)
- Ca++ channels (L, T, N types)
- Na+ channels (fast and slow types)
- K+ channels (voltage- and ligand-
gated types)
Voltage-Gated Ion Channels
(3)
- Ca++ channels (L, T, N types)
- Na+ channels (fast and slow types)
- K+ channels (voltage- and ligand-
gated types)
- K+ channels (voltage- and ligand-
gated types)
— produce at least
9 different K+ currents
in heart, vascular smooth muscle, and
other tissues such as pancreas
Voltage-Gated Ion Channels
* voltage-dependent
— channels open or close depending upon
the
— channels change opened/closed or
activated/resting states as electrical
potential changes from
— channels often susceptible to binding by
various compounds, including
electrical gradient (voltage) across the plasma membrane
-90 mV to +10 mV (inside relative to outside)
xenobiotics
channels open or close depending upon
the electrical gradient (voltage) across the
plasma membrane
* resting membrane potential ~ mV
* depolarized membrane potential ~ mV
-90
0
From Molecular Events to Cellular Actions &
Beyond
* Rang & Dale’s Pharmacology
— uses “—” and “—” to understand impact of drugs on
molecular level and cellular level
excitation-contraction coupling
cell secretion
verapamil
— Ca++ Channel Blocker
(6)
- binds to L-type Ca++ channels in heart and vascular smooth
muscle - blocks movement of Ca++ from outside to inside
- reduces cardiac contraction (negative inotropic effect)
- slows cardiac conduction (negative chronotropic effect)
- reduces vascular smooth muscle contraction
- reduces blood pressure
Cardiac Muscle
— contraction of cardiac muscle
(4)
- depolarization of the membrane leads to calcium
influx through L-type calcium channels - results in an increase in intracellular calcium
- calcium stimulates further calcium release from
the sarcoplasmic reticulum (calcium-induced
calcium release) to further increase intracellular
calcium - contractility increases with the increased
availability of calcium for contraction
Cardiac Excitation Contraction & Ca++ Channel Blockers as an example
abbreviations
* ATP =
* RyR =
— ligand-activated Ca++ channel
* CICR =
* SERCA =
cardiac electrical activity
adenosine triphosphate
ryanodine receptor
calcium-induced, calcium release
sarcoplasmic/endoplasmic reticulum Ca++ ATPase
Mechanism of Action
* Cardiac Muscle
— calcium channel blockers
(3)
- calcium channel blockers reduce calcium influx into
the cardiac muscle - thereby reduce intracellular calcium and force of
contraction
» negative inotropic effect - through similar blockade of calcium channels in pace-
maker (SA node), AV node, and Purkinje fibers in the
heart, calcium channel blockers reduce depolarization
and slow conduction of depolarizing waves through
the heart
» negative chronotropic effect
