Receptors Flashcards
What is the active ingredient of opium?
Morphine
How many types of opiate receptors are there?
3
Which drugs bind to the mu (μ) opiate receptor? (4)
- Morphine
- Heroin
- Codeine
- Fentanyl
What are the endogenous ligands for opiate receptors? (2)
- Endorphins
- Enkephalins
What is a drug?
A chemical of known structure which produces a biological effect when administered to a living organism
What is a biopharmaceutical?
A drug made from DNA (proteins, oligonucleotides, antibodies)
What is a medicine?
A chemical preparation that contains a drug(s) which is administered to produce a therapeutic effect
What are the 2 types of biologics?
- First generation
- Second generation
What are first generation biologics?
Copies of endogenous proteins produced by recombinant DNA technology
What are second generation biologics?
Engineered proteins to improve their performance e.g. humanised monoclonal antibodies
What are the 4 main classes of proteins targeted by drugs?
- Receptors
- Enzymes
- Transporters
- Ion channels
What happens if you increase the dose of drug too much?
Starts to have off-target side effects, toxicity etc.
What is an agonist?
A drug which switches on a receptor when it binds and brings about change in a cell
What is an inverse agonist?
- A drug which binds to the same site as an agonist but reduces receptor activity (opposite response to the agonist)
- Reduces its basal, constitutive activity
What is an antagonist?
- Blocks the activity of the receptor when it binds
- Stops the effect of an agonist/inverse agonist
What kind of receptors are opiate receptors?
G protein coupled
How does lidocaine work?
Blocks voltage gated Na+ channels for analgesia
What is a prodrug?
A drug which is processed by an enzyme to produce an active version of the drug e.g. L-dopa
What is the treatment for opioid overdose?
Naloxone
What are the 4 classes of receptors?
- Ligand gated ion channels (ionotropic)
- G-protein coupled receptors (metabotropic)
- Kinase-linked receptors
- Nuclear receptors
What is the structure of ionotropic receptors? (2)
- Subunits have a few transmembrane domains
- Multiple subunits come together to form the channel
What is the structure of metabotropic receptors? (2)
- 7 transmembrane domains
- Coupled to a G protein
What do kinases do?
Phosphorylate other proteins
Where are nuclear receptors found in a cell?
Cytosol or nucleus
What is the function of nuclear receptors?
Regulate transcription in cells
What are the 3 subunits of G proteins?
Alpha, beta and gamma
What happens when a G protein coupled receptor is activated? (4)
- G protein activated
- Alpha subunit has GDP in its resting state, once activated binds to GTP instead and dissociates from beta-gamma subunit
- Alpha and beta-gamma subunits interact with downstream target proteins
- Alpha breaks down GTP into GDP and binds with the beta-gamma subunit again
What are the 3 main G proteins?
Gs, Gi, Gq
Which receptors do cytokines act on?
Kinase-linked receptors
What are the 2 types of nuclear receptors?
- Class 1
- Class 2
What is the difference between class 1 and 2 nuclear receptors?
Class 1 are outside the nucleus and move in once activated whereas class 2 are already inside the nucleus
What is the structure of class 1 nuclear receptors?
Homodimeric
What is the structure of class 2 nuclear receptors?
Heterodimeric
Which receptors do steroids bind to?
Class 1 nuclear receptors
What is myasthenia gravis?
Autoimmune disease which targets nicotinic receptors
What kind of receptor is the insulin receptor?
Kinase-linked receptor
What is the efficacy of an antagonist?
0
What is the difference between affinity and efficacy?
Affinity is how well the ligand binds to the receptor whereas efficacy is how well the ligand elicits a response
What is Kd? (3)
- Dissociation constant, measure of affinity
- Concentration of drug which occupies 50% of receptors
- Low Kd = high affinity (lower concentration needed to fill 50% of receptors)
What is Kd for a drug with a high affinity for its receptor?
- Very small
- Reverse reaction (dissociation) is very slow compared to the forward reaction (binding)
How is Kd calculated?
Rate of the reverse reaction divided by the rate of the forward reaction
What is Kd for a drug with a low affinity for its receptor?
- Large
- Reverse reaction (dissociation) is fast compared to the forward reaction (binding)
How is occupancy calculated?
Number of receptors occupied /
Total number of receptors
What is occupancy determined by?
Affinity
What is the scale of occupancy?
Between 0 (no receptors occupied) and 1 (all receptors occupied)
What method is used to determine occupancy and therefore affinity?
Radioligand binding assays
How does a radioligand binding assay work?
- Apply radioactive ligand to the tissue containing the receptors and incubate at an appropriate temperature to prevent protein degradation
- Wash off unbound ligand and look at level of radioactivity in the tissue
What is the most common radioactive label used?
Iodine
How is non-specific binding accounted for in radioligand binding assays?
- One sample has the radioactive ligand which is bound to receptors and also non-specific binding sites (plastic etc)
- Unlabelled ligand is added in excess to the other sample, outcompetes the radioligand for the receptors so the radioligand is only bound non-specifically
- Specific binding i.e. occupancy = total radioligand binding in first sample - non-specific radioligand binding in other sample
What is EC50?
The concentration of drug that gives 50% of the maximum response for that tissue
What does ‘receptor reserve’/spare receptors mean?
- Not 100% of receptors need to be occupied to give 100% response i.e. EC50 is lower than Kd
- Spare receptors shift the response curve to the left of the binding curve
How does EC50 relate to potency?
Drugs with a lower EC50 are more potent because a lower concentration of the drug is needed to produce 50% of the max response
What is a partial agonist?
- An agonist that can’t produce the same maximum response as a full agonist
- Efficacy is always lower than the full agonist
What happens to the concentration-response curve if you mix an agonist with a partial agonist?
- Partial agonist behaves like an antagonist
- Causes a shift to the right
What occupancy does a partial agonist need to achieve in order to produce 100% of the max response? (2)
- 100%
- Kd = EC50
What kind of drug has an efficacy of 1?
Full agonist
What 3 properties determine the effect of a drug in a living system?
- Specificity
- Affinity
- Efficacy
How do you determine potency of a drug? (2)
- Potency is relative
- Need to compare full dose-response curves
What are the 5 classes of antagonists?
- Chemical antagonism
- Pharmacokinetic antagonism
- Physiological antagonism
- Non-competitive antagonism
- Competitive antagonism
What is chemical antagonism?
- Where an antagonist binds to the active drug and ‘mops it up’ to prevent its action
- Chemically alters the agonist
- E.g. monoclonal antibodies
What is pharmacokinetic antagonism?
- Where an antagonist reduces the amount of drug absorbed/metabolised/causes excretion
- E.g. opiates inhibit gut motility so reduce absorption of other drugs from the gut, antibiotics can stimulate enzymes involved in warfarin metabolism, diuretics cause increased urine flow
What is physiological antagonism?
- Interaction of 2 drugs with opposing actions, different receptors but same system
- E.g. noradrenaline raises arterial pressure but histamine lowers arterial pressure
What is warfarin?
Anti-coagulant given to reduce the risk of stroke and heart attack
What is non-competitive antagonism?
- Non-competitive antagonists block the effect of an agonist indirectly
- Binds to a different site on the receptor/interferes with a signalling molecule
What is competitive antagonism?
Competes with the agonist for occupancy of the same binding site, blocks the receptor activity
How do you overcome the effects of a competitive antagonist?
Increase the concentration of agonist
What is the effect of adding competitive antagonist on a concentration-response curve for an agonist? (3)
- Parallel shift to the right
- Maximum and slope stays the same
- EC50 increases with increasing antagonist concentration
What is the dose ratio?
- Quantification of the rightward shift in the curve when adding competitive antagonist to an agonist
- How many more times agonist is needed to reach the same response
How is dose ratio calculated?
[Conc. agonist in PRESENCE of antagonist] /
[Conc. agonist in ABSENCE of antagonist]
- Concentrations for the same response e.g. at the EC50 values
What is Schild analysis?
Measure of the affinity of a competitive antagonist
What are the features of Schild plot? (3)
- log(DR - 1) on the y axis
- log[antagonist] on the x axis
- Both linear scales, values are logged
What is the pA2 value?
Measure of affinity
What does a large pA2 value indicate?
High affinity
What does a small pA2 value indicate?
Low affinity
How do you calculate pA2?
x intercept of the Schild plot x -1
How do you calculate Kd from pA2?
Kd = 10^(- pA2)
pA2 = -1 x log(Kd)
What happens to the concentration-response curve for an agonist when you add a partial agonist?
- Partial agonist behaves like a competitive antagonist in the presence of an agonist
- Causes a shift to the right
- Difference is that the partial agonist can cause some signalling on its own but a competitive antagonist never induces signalling
What are irreversible competitive antagonists?
Antagonists which bind permanently to the agonist binding site and can’t be removed by washing the tissue
What happens to the concentration-response curve for an agonist when an irreversible competitive antagonist is added? (3)
- Initial shift to the right and max response can still be reached by adding more agonist due to spare receptors
- Over time max response decreases because all receptors used
- Can’t use Schild analysis
