receptors Flashcards
(83 cards)
1
Q
examples of drug targets
A
- Enzymes (e.g. Aspirin)
- Nucleic acids (i.e. DNA and RNA; e.g. Anti cancer and antibiotics)
- Carrier Molecules or transporters – these pump ions across cell membranes (e.g. Digoxin)
- Ion Channels – these are proteins/pores across cell membranes that allow ions to enter the cell (e.g. Lignocaine)
- RECEPTORS
2
Q
definition of a receptor
A
Highly specialised proteins embedded in cell membranes that possess structurally defined binding sites which specifically interact with particular complementary sites on the drug molecule
In general terms, a drug receptor is any macromolecule to which a drug or a molecule which naturally exists within the body (neurotransmitter or hormone, for example) binds to cause a response.
3
Q
how do a receptor and an enzyme differ?
A
→ enzyme facilitates a reaction: enabling two molecules to come together etc
→ drugs may BIND to a receptor, and then the receptor enables the cell to generate a response
4
Q
evidence that receptors exist:
A
- Potency – drugs are highly potent as small quantities are required to produce an effect
- Unique actions – drugs can have different actions on different tissues
- Specificity - drugs bind to particular proteins only.
- Antagonism – the actions of drugs can be prevented by other drugs
- Most recently, receptors have been cloned: they were then expressed in cells and shown that specific drugs bind to these in the same manner as native receptors
5
Q
what are the 4 types of receptors:
A
- ligand-gated ion channels/ ionotropic receptors
- metabotropic receptors
- enzyme-linked receptors
- intracellular receptors
6
Q
how quick each receptor takes
A
- ligand-gated ion channels/ ionotropic receptors = ms
- metabotropic receptors = ms-seconds
- enzyme-linked receptors = min
- intracellular receptors = min-hrs
7
Q
IONOTROPIC example and mechanism
A
Example: Na+ channels
Mechanism: Agonist directly opens ion channels → Na+ influx → Activation of conductance
8
Q
METABOTROPIC example and mechanism
A
Mechanism: Agonist → G-Protein Activation → Generation of Second Messenger → Activation of Cell Signaling
9
Q
ENZYME-LINKED example and mechanism
A
Mechanism: Agonist → Phosphorylation of Tyrosines on Key Signaling Molecules → Activation of Cell Signaling
10
Q
steroid mechanism
A
Mechanism: Agonist → Transport to the Nucleus → Activation of transcription and translation
11
Q
what is each different receptor type linked to:
A
different intracellular signalling mechanisms - Depending on the signalling mechanism that the receptor type is linked to, will determine how fast the receptor can exert its action once activated
12
Q
drugs binding to ionotropic receptors produce the fastest response within milliseconds - why
A
This is because the binding of a drug to these receptors leads to the opening of an ion channel and the exchange of ions to produce a very rapid response.
13
Q
drugs acting on metabotropic (G-protein-coupled receptors) take milliseconds to seconds to initiate a response. Why?
A
This is due to the drug receptor complex being associated with G-proteins. The G-proteins need to be activated and these in turn then activate a cascade of intracellular signalling molecules which then has the effect.
14
Q
for enzyme linked and steroid when a drug binds to a receptor, two receptors have to come together, so it takes how long?
A
its slow! therefore the process can take minutes
15
Q
drug-receptor binding can be described simply by the following equation:
A
D + R ↔ DR →Response
16
Q
The magnitude of the response will be dependent upon:
A
- Existence of receptors
- Number of receptors occupied by drugs
- Intracellular signaling cascade
17
Q
why is the drug receptor complex formed quite weak
A
generally, this is a weak interaction, typically hydrogen bonds or van Der Waals
this means there is a tendency for the drug to come off the receptor and bind to another
so binding time is little, and its reversible.
Very few involve covalent bonds which are essentially irreversible.
18
Q
what is potency vs efficacy:
A
potency - ability to bind to receptor, efficacy - ability to bind to produce a response
19
Q
potency equation
A
https://www.notion.so/receptors-15100bb3982d80c58d10c0f244448942?pvs=4#15100bb3982d80a894f6e862e4f29c10
20
Q
what do agonists do
A
bind to receptors and elicit a response.
21
Q
what do antagonists do
A
bind to receptors but do NOT initiate a response.
22
Q
how many types of agonists are there?
A
3
23
Q
what are the types of agonists
A
- full
- partial
- inverse
24
Q
effect of each agonist graph
A
https://www.notion.so/receptors-15100bb3982d80c58d10c0f244448942?pvs=4#15100bb3982d8001bdcdc7d68580188f
25
what are the differences between the types of agonists:
1. Full agonists – able to evoke a maximal response
2. Partial agonists – incapable of evoking a maximal response
3. Inverse agonists - reduce receptor activity by stabilising the inactive form
26
receptors are believed to exist in what two conformational states as per the 2 state model of receptor function:
inactive/resting
active
There is a continuous interchange between the inactive or resting receptor state and the active receptor state:
27
2 state models of receptor function detail:
R (resting state) <-> R* (active state) -> response
agonist: much higher affinity for R* than R
partial agonist will bind to both
inverse agonists: preference for R rather than R*
28
a drug that binds to the receptor when it is in active state (R*) rather than the inactive R state, is called a…
full agonist
In essence, agonists stabilise the receptor in the R*state and is therefore capable of eliciting a maximal response.
29
however, some agonists are capable of binding to and stabilising the receptor in both the R and R* states such that although there is full occupancy of the receptor, the efficacy is lower because there are fewer receptors available for conversion to the active state. Such drugs are termed what type of agonists?
partial agonists
30
some drugs bind to the resting state and stabilise the receptor in this state. They effectively reduce the response mediated by full agonists. These are termed
inverse agonists
31
whether or not a drug is an agonist, a partial agonist or an inverse agonist can be determined by measuring its effect on a biological response such as
a change in blood pressure or the state of contraction or relaxation of a piece of muscle.
32
the most common and traditional method by which this was done was by determining whether the drug caused what to contract?
a piece of smooth muscle tissue such as a section of an ileum
33
ileum organ bath set up
the piece of ileum can be set up in an ‘organ bath’ which contains fluid of similar composition to that in the body. The piece of ileum is attached to a transducer using a piece of cotton. The contractions of the ileum can then be measured using a chart recorder
34
when an agonist is added to the bath, what occurs
- when an agonist is added to the bath, what occurs
the tissue will contract. By varying the concentration of agonist that is added to the bath, a graded change in the measured response on a transducer is observed
35
how do we know a maximal response is achieved
When two or more concentrations of the agonist do not produce a further increase in response, it indicates that a maximal response is achieved - see how it gets bigger and bigger? This is the point at which there is no further increase in the measured response despite an increase in the agonist concentration.
https://www.notion.so/receptors-15100bb3982d80c58d10c0f244448942?pvs=4#15100bb3982d80f5ba9efb6f47cf5962
36
graphs of full vs partial vs inverse agonists
https://www.notion.so/receptors-15100bb3982d80c58d10c0f244448942?pvs=4#15100bb3982d80109f65fd5609828fc1
37
how is response measured
The response measured by application of an agonist on a tissue is usually several stages removed from the drug-receptor interaction.
38
drug-receptor interactions often lead to a cascade of intracellular events, which have what effect?
amplify the response: hence, only a small proportion of receptors is required to be occupied to produce a response.
39
as we don’t therefore need a lot of receptors, what happens to them:
therefore held in reserve or are spare.
40
why is it useful to have spare receptors
Physiologically, this can be useful as low concentrations of drugs are required to have their desired effect whilst limiting side effects.
41
what do antagonists do and therefore prevent
these are drugs which bind to receptors, often with high affinity, but do not activate the receptor.
→ Instead, they prevent an agonist from binding to the receptor and activating it
42
- when will a partial agonist act as an antagonist
in the presence of a full agonist because it occupies sites on the receptor thus preventing it from being converted to the R* state.
43
types of antagonists
1. reversible competitive antagonists
2. non-competitive antagonists
44
how does Reversible Competitive Antagonism work
The antagonist competes with agonist for the same binding site on the receptor. They occupy the site and form a drug-receptor complex but no response
45
how can the effect of an antagonist be overcome
by increasing concentrations of agonist
- Example = atropine - used to treat nerve agent poisoning (Salisbury)
46
competitive antagonists are usually reversible, although some may seem irreversible why
they dissociate very slowly from the receptor such that their antagonism appears to be irrevisble
47
concentration response curves are shifted to the right in the presence of the agonist but there is no change in?
EMax (the maximal response). (but an increase in the EC50 value
The slope of the concentration-response curves is NOT altered either.
48
what is a schild plot
- A **Schild plot** is a graphical method used to determine the **potency** of a **competitive antagonist**.
- It is based on the **parallel shift** in the agonist concentration-response curve when a competitive antagonist is added.
49
a schild plot allows derivation of
the antagonist dissociation constant (KA - measure of how potent the antagonist is)
50
what is an assumption of schild plots?
Assumption: only a fraction of receptors occupied by agonists
51
schild plot analysis involves calculating the dose ratio or competitive shift ratio - how?
https://www.notion.so/receptors-15100bb3982d80c58d10c0f244448942?pvs=4#15300bb3982d802aa7b9cac553a14ac4
52
for schild plots: what do we plot on what axis
Constructed by plotting log (DR-1) against log [A, M] Log (DR-1) is on y-axis and log [A, M] is on x-axis
- → The y-intercept cannot be read easily
53
e.g. at the x-axis intercept Log (DR-1) =0 why is this special
This is special as it indicates that at the x- intercept DR = 2
54
Since DR = 1+A/KA, when DR = 2, A=
A=KA.
However, the x-axis is on a log scale.
So the x-intercept = log (KA).
Therefore at this point, Log (KA) = Log [A]
So, KA = antilog (Log [A]), when DR = 2
55
what is pA2?
is defined as the concentration of antagonist that would produce a shift in the DR by 2
56
why do we gave pA2
For convenience and by analogy to pH and pK, Schild developed a definition for antagonistpotency, pA2
57
pA2 =
–log10[KA]: This is equivalent to the x-intercept on the Schild plot
58
higher the pA2 value = more or less potent antagonist?
more potent
59
key points on pA2:
- what does it give us info on?
effectiveness of a REVERSIBLE COMPETITIVE ANTAGONIST
60
pA2 values for a given antagonist acting on a receptor population should be reproducible in the same tissue as well as
diff tissues
61
Specificity: the antagonist will have different pA2 values for different
receptors (e.g. Atropine has a pA2 value of 8.9 when tested against Ach but ~5.6 against histamine receptors)
62
Non-competitive Antagonism
- what is this
binds to a site distinct from the agonist (i.e. an ALLOSTERIC site) or they may interfere with the events producing the response.
63
what does the agonist concentration-response curve look like? for non-competitive antagonism
has a reduction in Emax and in this case, an increase in EC50 as well.
64
for non-competitive antagonism concentration-response curve slope and maximum
reduced in a CONCENTRATION dependent manner.
65
are the effects reversible? for non-competitive antagonism
yes- i.e. upon washout of the antagonist, the maximal effect of the agonist can be restored.
66
non-competitive antagonism example – picrotoxin which binds to…
GABAA receptors at a site distinct from that of the natural agonist, GABA
67
image on how non-competitive antagonism affects conc-response curves:
https://www.notion.so/receptors-15100bb3982d80c58d10c0f244448942?pvs=4#15300bb3982d800281e7db4981c0dee6
68
what are allosteric sites
-some receptors possess binding sites for drugs other than that of the natural ligand/endogenous neurotransmitter (agonist) that activates these receptors.
these allosteric sites are distinct from the agonist binding site, and when occupied, can alter receptor function.
69
how do we know if the drug is an allosteric modulator e.g. what effect does it have?
When the allosteric site is occupied by a drug, it can modify the action of the agonist i.e. the agonist- receptor complex might be able to activate the intracellular signalling cascade faster or slower.
70
how many binding sites does GABA have?
two
71
When the receptor is activated by GABA, what ions are involved
Cl-
72
When the receptor is activated by GABA, what happens to the Cl- ions
they pass through the channel down a concentration gradient into the neuronal cytoplasm causing the membrane potential to become more negative.
73
How is GABA therefore an inhibitory neurotransmitter.
This action results in the neuron becoming less excitable
74
Situated at the interface of the α and g subunits is the binding site for BENZODIAZEPINES, which do what?
potentiate the inhibitory effect of GABA by increasing the affinity of the receptor for GABA and the frequency of ion channel opening.
75
why are benzodiazepines termed POSITIVE ALLOSTERIC MODULATORS,
because they potentiate the effect of GABA + moreover, because they bind to different sites, their effects can be synergistic if administered together.
76
The GABAA receptor also possesses binding sites for other drugs, such as
ethanol, neurosteroids, picrotoxin and zinc: Ethanol and neurosteroids are also positive allosteric modulators whilst zinc is a negative allosteric modulator and picrotoxin a non- competitive antagonist.
77
receptors are dynamic proteins and their number and function within the cell membrane vary with time - why are they described as such?
- Constantly being trafficked to and from the cell membrane.
- The number of receptors that are present on a cell surface at any one time may vary depending on the circumstances.
- In particular, receptors can DESENSITISE or SENSITISE
78
Receptors respond less to agonist stimulation over time.
- This is an issue when constructing concentration-response curves and reasons might be:
a) high concentration of agonist
b) repeated exposure of receptors to agonists with
insufficient time intervals.
79
what is receptor sensitisation: how does it happen?
Many clinically used drugs are antagonists. If these are used on a long-term basis then this can lead to an increase in the number of receptors and therefore an increase in agonist response. Consequently, the treatment is stopped gradually to allow the numbers of receptors to be restored and prevent adverse side effects.
80
what is receptor desensitisation
where responses to a given concentration of an agonist will be reduced compared to that achieved prior to agonist exposure. This can also result in reduced receptor numbers if the agonist exposure is more long-term.
81
Positive Allosteric Modulator (PAM)
This enhances the activity of the receptor. When a PAM binds to the allosteric site, it increases the receptor's response to its normal ligand (agonist). For example, a PAM may make it easier for the receptor to be activated by a neurotransmitter.
82
Negative Allosteric Modulator (NAM):
this reduces or inhibits the activity of the receptor. When a NAM binds to the allosteric site, it decreases the receptor's response to its normal ligand. It may prevent the receptor from being activated as easily by the neurotransmitter or other agonists.
83
An allosteric modulator is?
a molecule that binds to a site on a receptor (different from the active site) to influence the receptor's activity. These modulators can either enhance or inhibit the receptor's function.
