Exam 2- Drug Target Interactions Part 2 Flashcards
Biochemistry- A receptor
A molecule that transduces a signal
5 classes of receptors
- Nuclear Hormone Receptors
- enzyme linked receptors
- enzyme associated receptors
- ligand gated ion channels
- 2nd Messenger systems (GPCRs)
Nuclear Hormone Receptors
act via intracellular receptors and the drug must cross the cell membrane
- effects can last for hours or days after drug is gone
- Beneficial and adverse effects last longer than drug elimination
Example of Nuclear Hormone Receptors
- Growth hormones
- Steroids like glucocorticoids, sex steroids, thyroid hormones
Most common MOA: binding to DNA via response elements
Enzyme linked receptor
Large transmembrane proteins with extracellular ligand binding domains and intracellular enzyme domains
- common with trophic hormones like insulin
Enzyme linked receptor signaling events
- ligand binds extracellular domain
- conformational change
- dimerization (homo- or hetero-)
- kinase domains together
- phosphorylation event
Example of Enzyme linked receptor
Receptor tyrosine kinase
Ligand induced dimerization and phosphorylation
leads to variety of signaling events triggered
- further effects downstream events like
1. altering membrane permeability to ions
2. changes in gene expression
3. increased nutrient transport and more
intensity and duration of agonist-activation limited by receptor down-regulation
Dimerization may vary
depending upon ligand or cell type
- doesn’t happen until ligand binding
Intensity and duration of agonist-activation is limited by
receptor down regulation
- ligand binds, receptor endocytose and degraded
- New R synthesis takes time, limits drug efficacy
Enzyme Associated Receptor
Mechanism of action is similar to kinase receptors, but, kinase activity is not intrinsic to the receptor - external molecule mediates phosphorylation
Ex. JAK/STAT pathway
Cytokines
large class of molecules that mediate cell-cell communication, particularly involving the immune system
For enzyme associated receptors Activity remains
as long as receptor is dimerized- even if ligand is gone
Ligand-gated ion channel
- ligand binds receptor, opens channel, ions flux down (electro)chemical gradients
- Ion channels are not all the same multiple subtypes
- the ligand binding site is on the extracellular surface
- Effects are turned off immediately once ligand is gone
Second Messenger Systems
- most complicated of the receptor systems
- lead to great signal amplification
- part of signal transduction pathways,
- GPCRs: signaling in 3 parts
3 Parts of G protein Coupled Receptors
- extracellular ligand binds to specific receptor
- G-protein activated on cytoplasmic/intracellular surface (exchanges GDP for GTP)
- G-protein changes activity of some effector like enzyme or ion channel
G-proteins
family of proteins that function as molecular switch and an intermediate
- can be more than one per receptor
- bind and hydrolyze GTP, allows for tremendous signal amplification and increased longevity
- can be both stimulatory and inhibitory
G-proteins are active while
bound to GTP
G-proteins are inactive while
bound to GDP
7TM
7 transmembrane loops
example of GPCR
cAMP
Cyclic adenosine monophosphate
- made by adenylyl cyclase
- example of various second messengers
cAMP what does it do
Mediates hormonal responses
- renal water conversation
- calcium homeostasis
- production of hormones
- smooth muscle relaxation (not skeletal muscle)
Phosphoinisotol (IP3)
- example of 2nd messengers
- G-proteins stimulate phospholipase C which promotes the catabolism of membrane-bound PIP2 into DAG and IP3
IP3 diffuses through
the cytoplasm, binds calmodulin and cascades further
Gs proteins (Stimulatory)
Adenylyl cyclyase -> increased cAMP
Gi1, Gi2, Gi3 effector
Adenylyl cyclyase -> decreased cAMP
Gq effector
mACh, bomesin, 5-HT2
Phospholipase C-> increased IP3
Steps of GPCR
- Ligand binds to receptor
- G-protein is released from the receptor
- G-protein binds to the effector protein
- Change in 2nd messenger
- G-protein hydrolyzes GTP to GDP
Nuclear hormone receptor response time
Slow (hours
Nuclear hormone receptor - how the signal is transduced
Gene transcription (upregulation or downregulation) - induces a change in gene expression
Nuclear hormone receptor - mechanism
Ligand binding results in conformational shift and binding to DNA response elements. Ligand must cross cell membrane
Nuclear hormone receptor - Can the signal outlast the message?
Yes
Ligand gated ion channel - how the signal is transduced
changes in ion transport
Ligand gated ion channel - response time
Very rapid (milliseconds)
Ligand gated ion channel - mechanism
Ligand opens or closes an ion channel. Typically extracellular binding of ligand
Ligand gated ion channel - examples
Nerves, skeletal muscles
Ligand gated ion channel - Can the signal outlast the message?
No
Enzyme-coupled and enzyme-associated receptors - how the signal is transduced
Intracellular enzyme
Enzyme-coupled and enzyme-associated receptors - response time
Moderate (minutes)
Enzyme-coupled and enzyme-associated receptors - mechanism
Extracellular ligand binding results in intracellular enzymatic activity. Typically requires dimerization. Signal can be maintained after ligand leaves
Enzyme-coupled and enzyme-associated receptors - examples
Response to cytokines, growth hormones
Enzyme-coupled and enzyme-associated receptors - can the signal outlast the message?
Yes
GPCR - how the signal is transduced
Second messenger (usually IP3 or cAMP)
GPCR - response time
Rapid (seconds)
GPCR - Mechanism
Extracellular binding to 7-TM results in activation of a GTP-binding protein. Activated G-protein goes on to activate some effector
GPCR - Examples
Taste, smell, endocrine signals
GPCR - can the signal outlast the messenger?
No
Drug-Receptor complex formation leads to intracellular effector molecule phosphorylation
Receptor Dimerization and enzyme activation
Which is a typical response to a G-protein activation?
- Activation of phospholipase C
- activation and inhibition of adenylyl cyclase
-NOT involved in inhibition of phospholipase C
Membrane permeability of the ligand is most imporant for which type of receptor
Nuclear Hormone receptor bc it has to cross the cell membrane
Which type of receptor frequently uses protein phosphorylation to tranduce its signal?
Enzyme linked receptor
Within living systems, EC50 is
a measurement of a biological response of a compound
The bigger the therapeutic window
the better and more safe we consider the drug to be
A low EC50 means
high therapeutic window
ED50
dose at which 50% of patient population exhibits specific effect
- effective dose, 50% of population
agonist activation
TD50 and LD50
toxic and lethal doses for 1/2 the population
TD is used more than LD
TD50- measure of toxicity
NOAEL
No observed adverse events level
Drug selectivity confers
utility, clinically
selectivity is a major factor during drug development as target specific, or receptor specific binding
Tied to the same R and effector
- on target toxicity
- Usually a dose response, optimal dose to “fix” pathological condition
- too much drug exaggerates E
Ex: Diabetes type 1 use of insulin: if overdose -> hypoglycemic coma
Mediated by the same R on different cells/tissues
- on target toxicity
- same receptors exist on multiple cells, may differentiate Beneficial vs. Adverse R
Ex: MOR in CNW vs. GI tract
How to avoid/ mitigate for mediated on the same R
- Use lowest effective dose
- Effect pathology via administration of multiple drugs with different MOAs
- Targeted delivery ( concentrate drug at R for benefit)
Mediated by different R
- off-target toxicity
- Drugs are selective not specific, so toxicities may be mediated by other receptor binding
- Not always bad bc some new drugs/receptors have been discovered bc of these off target R bindings.
- Its a problem and a potential opportunity
Hypotension is an example of
on target toxicity
