Ligand Channels and G-protein coupled receptors Flashcards
Drugs can either be _______ mediated or ____________ mediated.
receptor mediated or nonreceptor mediated.
Name a non-receptor mediated drug.
An osmotic laxative. It does not require receptor interactions to provide its effects.
Are general anesthetics receptor mediated or non-receptor mediated?
General anesthetics do have some receptors but effects are so widespread it is also thought that they may act without receptors.
Non receptor mediated drugs tend to act more based on their ______________.
chemical properties
The lock and key model is an example of: (receptor mediated or non-receptor mediated)
receptor mediated
The substance that interacts at the receptor binding site is a: ______________.
ligand
Receptors exist on the cell, not to interact with drug molecules, but to interact with:
some normal endogenous ligand or common environmental material.
Drug molecules that interact with receptors either _______ or __________ normal body compounds.
mimic or inhibit
Receptors and the compounds with which they interact are chemical entities and behave according to:
chemical principles
Receptors are _________ structures with multiple protein subunits.
protein structures with multiple protein subunits.
The binding site on a protein may be just __________ of the protein.
one small part
Define affinity.
how well a particular compound is drawn into and held at the binding site.
The degree of affinity can be calculated by looking at the __________________________.
equilibrium dissociation constant
The smaller the equilibrium dissociation constant, the __________ the affinity of the drug for its receptor.
greater
Particles move around by Brownian motion (random movement). What does this mean for particles and if they will react with a receptor?
It is completely random chance that a drug or even the normal receptor ligand will react with the receptor.
What would increase the chances that a drug will react with a receptor?
by increasing the amount of drug particles present near the receptor site.
The greater the concentration of particles near the receptor, the _________ one is to interact there.
more likely
If the molecule interacts at the receptor, it is said to have ______________ there.
affinity
The stronger the interaction between ligand and receptor the greater the ____________.
affinity
How long will a drug stay attached at the receptor site?
it is unknown. the drug can be knocked off the receptor site by other molecules floating around.
The more drug hanging around the receptor site, the ________ ________ drug will be attached to the receptor site at any given time.
more likely
As the concentration of the drug goes down, the amount of drug hanging around the receptor goes down, and the time the receptor is occupied goes:_______
goes down also.
What are two characteristics of ligands that will affect affinity?
- The ligand’s shape
- its chemical makeup (for example a positively charged ligand will easily react with a negatively charged receptor)
Define intrinsic activity of the a ligand.
Intrinsic activity defines the effects of a ligand when it interacts at a receptor site.
If a ligand has a good affinity for a receptor, will it always have good intrinsic activity?
Not necessarily. Once bound, the ligand may be bound tight but not have a very powerful effect, in fact, sometimes it may not elicit at response at all.
Is there such a thing as a ligand having good intrinsic activity but poor affinity?
Yes, it will elicit a poor response though, because of the poor affinity even if intrinsic activity is high.
In a dose response curve, we typically see low activity, with a slight increase, then a sudden blast of activity. Why is this behavior typical?
Because a certain number of receptors need to be occupied before a response is observed.
Despite the number of receptors occupied, occupying more receptors (will/ will not) always increase a response.
Will not. Every tissue has a limited response. (It is theorized that the excess receptors are used for providing a graded response)
Receptors are made of amino acids that are _____________ to the microenvironment in which they exist.
very sensitive
Are receptors sensitive to the pH surrounding them?
Yes, even slight changes in pH can change the way a receptor functions and how it interacts.
When a drug compound mimics the effect of the endogenous compound for a receptor at that receptor site, and has both good affinity and good intrinsic activity, the drug is classified as a:
agonist
What is a partial agonist?
A drug that mimics the endogenous compound at its receptor site but elicits a weaker response. (because it elicits a weaker response than the endogenous compound, it can be mistaken for an antagonist, but it’s not it’s just a parital agonist).
Remember, an agonist given as a drug ______ the body’s endogenous compound for that receptor site.
displaces
What is meant by physiological agonist?
Compounds that produce the same physiological/ biological effect but act at completely different receptor sites.
Describe how ketamine and propofol are physiological agonists.
Ketamine acts at the NMDA receptor, propofol acts at the GABA receptor. They act at different receptors but both have the same physiological effect of causing loss of consciousness.
What are coagonists?
coagonists are active, not on their own, but in conjunction with multiple compounds, to produce the desired effect.
What are selective agonists?
Agonists that are selective for a very specific receptor site. ex. Albuterol is only active at Beta 2 receptor site.
What are nonselective agonists?
they act on many different receptor sites. Ex, isoproterenol acts on both Beta 1 and Beta 2 sites.
What do antagonists do?
Block agonist interaction at the receptor zone
What is a competitive antagonist?
this antagonist interacts at the same receptor site as the normal agonist. Therefore it must possess a powerful affinity, to interact with the receptor, but have no intrinsic activity, so it does not stimulate the receptor.
What is the law of mass action?
The more drug occupying the space around the receptor site, the more likely the receptor is to be occupied by the drug.
So according to the law of mass action, if an agonist drug is present in large quantities around the receptor site, and you give an antagonist, in such an amount that it outnumbers the agonist, which will be more likely to be attached at the receptor site?
The antagonist.
Using the example of neuromuscular blockade, describe how the law of mass action is in effect.
We give vecuronium which is a competitive antagonist at the nicotinic receptor where it blocks Ach (the endogenous ligand) from binding. We reverse it by giving neostigmine, which inhibits the breakdown of Ach and floods the site around the receptor area with Ach, effectively outnumbering the amount of vecuronium there, making AcH more likely to be bound and decreasing the effects of vecuronium.
What are the 2 types of noncompetitive antagonism?
- Irreversible binding
- Allosteric site
What is irreversible binding?
When a drug binds for a very long time, no matter how many agonists you put around it, it will not budge. Even if it’s own concentration drops, it still will not budge. Even if some of the receptors on the cell are occupied by agonists, their collective action will not outweigh the effect of the irreversibly bound agent.
What is meant by allosteric site?
When noncompetitive antagonists interact with the receptor complex at another site, other than the agonist site.
How does allosteric inhibition work?
the binding of an antagonist causes a conformational change at the agonist site causing it to be: no longer available for binding, or no longer able to elicit its effect. (however, the binding to allosteric sites is not irreversible, and as the concentration of allosteric antagonist decreases, the receptor complex may revert back to its normal conformation.
What is allosteric potentiation?
Not a form of antagonism but actually a form of agonism. A compound acts at the allosteric site that actually enhances the affinity of the normal agonist for its binding site.
Decribe up-regulation of receptors using AcH as an example?
If AcH is blocked at the neuromuscular junction, the body senses that, and in reponse to deprivation of the ligand, upregulates the number of receptors available
If a cell has upregulated receptors in response to ligand deprivation, a sudden increase in ligand will:
cause detrimental effects and overstimulation
Describe down-regulation of receptors?
If a cell is flooded with ligand, it can down regulate the number of receptors, but if the source is suddenly cut off, you get less functioning and inadequate receptor stimulation.
Because of the upregulation and downregulation of receptors, many drugs should be ________ and not stopped abruptly, in which case they would cause a massive response.
weaned off
Do all channels in the cell membrane have a receptor mediated response?
No, for example, the inward rectified K+ channel is always open, it depends only on the concentration gradient for K+
Describe ligand gated ion channels.
They may be huge and span the cell membrane. They may require multiple ligands to react at different ligand sites. They may be extracellular or intracellular. Many neurotransmitters act on these receptor sites. Often times they cause an ion channel to open.
What is a metabophore?
Membrane receptor composed of an external binding site and an internal enzymatic component. The action at the external component causes the internal to be activated. It causes things like the conversion of GTP to cGMP which acts as a second messenger, triggering other biological processes.
Describe G-protein coupled receptors.
They control biochemical pathways inside the cell from an external signal. G- protein coupled receptors are not as simple as metabophores, they activate a series of changes that may lead to control of a specific enzyme. This is the most common type of receptor found in cell membranes.
G-protein coupled receptors are (large/ small) proteins with ______ membrane loops.
large proteins with 7 membrane loops.
Give an example of an intracellular receptor.
These are not associated with the cell membrane. ex. is steroid receptors. steroids are lipid soluble and easily cross the cell membrane, they interact inside the cell and cause a dimer to be formed which enters the nucleus and affects DNA transcription
One group of general anesthetics are agents that increase inhibitory pathways on GABA receptors. How does this work? Name some of the drugs in this group.
-Ligand gated ion channel at the GABA receptor opens, Cl enters the cell, negative charge lowers intracellular voltage, makes cell more resistant to depolarization (hyperpolarizes)
- barbiturates
- propofol
- etomidate
- isoflurane
- desflurane
- sevoflurane (all of these allosterically bind and use GABA at it’s receptor site)
General anethetics like Xenon, Nitrous Oxide, and Ketamine act at which receptor site?
These act to decrease excitatory activity at NMDA glutamate receptor sites and nicotinic acetylcholine receptors sites in the brain and spinal cord.
The two main membrane receptors we studied are:
ligand gated ion channels and g-protein coupled receptors
Cells communicate with each other using __________ which elicit changes in the enzymatic activity of the target cell.
chemical messengers
Action potentials are characteristic of ___________, __________, __________, and _________.
neurons, muscle cells, cardiac cells, and some glands.
Propogation of action potentials from one cell to another is ________ for cell to cell communication.
vital
Action potentials are transmitted from one neuron to the next by the release of ______________.
neurotransmitters
What are the 6 steps that have to occur for an action potential to be transmitted?
- Nerve impulse travels to the nerve end plate and depolarizes it
- Calcium channels open and calcium travels into the nerve endplate
- this causes synaptic vesicles containing AcH to open and release Ach into the synapse
- AcH crosses the synapse to the post synaptic membrane and binds to its receptor (activating ligand gated ion channels)
- Na+ and Cl- gates open causing an influx of these ions into the post synaptic neuron
- Voltage gated channels open, this causes a nerve impulse to occur in the second neuron.
If enough ____________ channels are activated you will get an action potential. These are the ones that propogate an action potential down a neuon.
voltage gated channels
Under normal conditons, the resting membrane potential exists with an overall ________ charge inside the cell compared to the outside of the cell.
negative charge inside
What does an excitatory ion channel do?
It passes positive ions into the cell, thus reducing the charge difference between the inside and the outside which causes depolarization.
What does an inhibitory ion channel do?
It passes negative ions into the cell and increases the charge difference between the outside and the inside of the cell causing hyperpolarization
An agonist drug _______ a receptor.
activates
An antagonist drug _______ a receptor.
inactivates
Ligand-gated ion channels are associated with (fast/ slow) transmission.
Very fast (miliseconds)
Agonist binding at the ligand-gated ion channel _________ the pore.
opens
What are the 2 major families of ligand gated ion channels?
Cys loop receptors and Ionotropic glutamate receptors
Nicotinic acetylcholine receptors, GABA-a receptors, and glycine receptors are all examples of ____________ receptors.
Cys-loop receptors
AMPA receptors, NMDA receptors, and kainate receptors are all examples of ________ receptors.
Ionotropic glutamate receptors
What is the predominate excitatory receptor in the brain?
glutamate
What is the predominate inhibitory receptor in the brain?
GABA, but glycine is also.
Nicotinic receptors are found in the ________ and ___________.
ganglia and in the neuromuscular junction
Define cys loop receptor.
The cys loop ligand gated ion channel superfamily is composed of nicotinic acetylcholine, Gaba- a, GABA- a-p, glycine, and 5HT3 receptors that are composed of 5 protein subunits that form a pentameric arrangement around a central pore
Give some examples of drugs that act at cys loop receptors.
- Nicotine
- Varenicline (Chantrix)
- Muscle relaxants (like succinylcholine)
- anti-epileptic drugs
- anti-anxiety drugs (barbiturates and diazepam)
Give some examples of drugs that act at glutamate receptors.
- aniracetam
- smart drugs
- ketamine
Cys loop receptors are made of ____ subunits.
5
Each subunit on the cys loop has 4 ____________, with the second one forming the ion pore.
transmembrane domains
What are the 5 types of subunits on a cys loop?
alpha, beta, gamma, delta, and epsilon. A cys loop is usually 2 alpha and 3 other beta, gamma, or deltas.
Why is the composition of subunits on the cys loop important?
The subunit composition affects how quickly and what types of ions can pass through the receptor.
Nicotinic acetylcholine receptors and serotonin receptors are ______ ________ channels.
Nonspecific cation. They pass mostly Na+, K+ and sometimes Ca++
Nicotinic acetylcholine receptors and serotonin receptors are (excitatory or inhibitory).
excitatory (depolarizing)
Glycine receptors and GABA-a receptors conduct which ion?
Cl-
Glycine and GABA-a receptors are (excitatory or inhibitory)
inhibitory (hyperpolarizing)
Where is the agonist binding site typically found in a cys loop?
Agonist binding sites are always between one alpha subunit and one adjacent subunit.
In the inactive state, the second transmembrane domain of the alpha subunit does what?
bows in, obstructing the pore.
Agonist binding does what to the transmembrane domain of a cys loop?
Swings the transmembrane domain out of the way, opening it up so ions can pass through the pore into the cell itself.
Name 3 type of glutamate receptors.
- AMPA
- NMDA
- Kainate receptors
AMPA, NMDA, and kainate receptors are all activated by:
glutamate
A glutamate receptor has how many subunits?
4
Does each binding site on a glutamate subunit need to be activated for the channel to open?
No. For some you only need 2. But for NMDA you need all 4. On NMDA 2 glycine and 2 glutatmate have to be active
Each subunit of a glutamate receptor has ___ transmembrane domains.
4
The pore of the gluatmate receptor is formed by the ________ transmembrane domain.
second
Which ions pass through glutamate receptors
Na+ and K+
NMDA receptors can conduct which ion?
Ca++
The passage of ions (K+, Na+, and Ca++) through glutamate receptors is (inhibitory or excitatory).
excitatory
Each subunit of glutamate has a binding site although not every binding site is____________
for gluatamate
For NMDA receptors, 2 of the binding sites are for ________ and 2 are for_________.
2 for glutatmate and 2 for glycine
The unique thing about NMDA receptor binding sites is that:
all 4 binding sites need to be occupied for the channel to open.
Gycine acts as a ________ at the NMDA receptor binding site.
coagonist for glutamate
_______ receptors play an important role in leaning and memory.
NMDA
At resting membrane potential, NMDA receptors are blocked by?
Mg++. Mg is a big molecule that sits on the pore of the NMDA receptor preventing other ions from entering. Once the NMDA receptor is activated, it kicks the Mg out and allows ions to travel through the channel.
Mg++ blockage of NMDA receptors is ______ dependent.
voltage
Does Ca++ travel through the NMDA receptor channel?
Yes. Ca++ will act as a signal molecule inside the cell. It binds to calmodulin dependent protein kinase II and activates it. This causes more AMPA receptors to go to the surface of the cell which makes the cell more excitatory.
What is an AMPA receptor?
It is a non-NMDA type ionotropic transmembrane receptor for gluatamate that mediates fast synaptic transmission in the CNS
Activation of amphoreceptors is the basis for making ____________ drugs.
smart drugs. Activating amphoreceptors makes you smarter.
Why are NMDA receptors known as “coincidence detectors”?
By NMDA activity, you can tell when a neuron has fired multiple times.
Are G-protein coupled receptors like ion channels?
Not at all. They are totally different, They have no action potentials, they have nothing to do with excitable cells even though they are often found on excitable cells.
G-protein coupled receptors are important in ____________, _____________, and/or ____________.
neuroscience, in modulating metabolism of cell, or telling a cell to divide.
Do G-protein coupled receptors transmit signals more slowly or quicker than ligand- gated ion channels?
Slower
The G-protein coupled receptor has _____ transmembrane receptors.
7
G-coupled proteins are found on _______ of our genes.
over 800 of our genes, they account for 3% of our genome. There are a huge amount of these receptors on our cells.
Is there pharmaceutical research being conducted on G-coupled protein receptors?
Yes, more than half of current pharmaceuticals are targetted at G-protein coupled receptors.
There are 3 classes of G-coupled protein receptors. They are:
- Class A: adrenergic receptors, muscarinic acetylcholine receptors
- Class B: parathyroid hormone receptors
- Class C: metabotropic glutamate receptors, GABA- b receptors.
Most G-coupled protein receptors belong to which class?
Class A
Name 3 types of drugs that act on G-coupled protein receptors.
- Beta blockers (beta adrenergic receptor antagonists)
- Asthma medications (salbutamol) Beta adrenergic receptor agonists
- Opioids (mu-opioid receptor agonists)
In the resting state, without a ligand, the G-protein is _________.
inactive
G- proteins belong to a group of enzymes known as:
GTPases
A G-protein has ___ subunits
3
The alpha subunit of the G protein binds:
GTP to GDP
The beta/ gamma subunit of a G protein does what?
inhibits alpha and prevents it from acting with GTP
When agonist bind to the G-protein coupled receptor, what happens to beta gamma?
It makes the beta gamma go away, so the alpha can react with GTP and activate other things downstream.
Because G proteins are GTPases, they will eventually:
deactivate themselves. It acts as an auto-turn off.
Why would we want a protein to have an auto turn off?
It acts as a fail-safe mechanism that prevents the cell from getting overloaded with signal.
How many different types of G-alpha subunits are there? What are they?
- G- alpha- s
- G- aphpa- i
- G- alpha- q
What do G-alpha s subunits act on?
activate adenyly cyclase and increase cAMP
What do G- alpha i subunits act on?
they inhibit adenyl cylase and decrease cAMP
What do G-alpha q subunits act on?
They activate phospholipase C, phosphoinositol hydrolysis, increase IP3 and DAG, and release of Ca++ from intracellular stores, and activates protein kinase C
When which of the G-alpha proteins is active, would you suspect that the beta subunit is also active?
When G-i alpha subunit is active (the inhibitory one) the beta unit is usually also active.
The G-q coupled protein cleaves lipids to ________ and _________.
IP3 and DAG
adenyl cyclase takes ATP to cAMP, cAMP is a second messenger protein that activates:____________.
transcription factors.
IP3 and DAG causes what to happen to Ca++?
They cause Ca++ to be released from the endoplasmic reticulum to the cytoplasm. Ca++ can be a signalling protein and cause downstream effects.
What does protein kinase C do?
It phosphorylates and activates other proteins, ex. transcription factors for genes.
Do protein kinase C and A activate the same genes?
No, not typically.
What is meant by amplification of a signal from a g-protein coupled receptor?
At each step of signal transmission, the signal could be amplified and stimulate many more reactions.
G protein activation of effector enzymes produces second messengers that:__________________________
modulate the activity of many other enzymes. This is amplification
G-coupled protein receptor will __________ after prolonged exposure to an agonist.
densensitize
______________ _____________ partially explains why drug tolerance develops.
receptor desensitization
When comparing the receptors for opioids in the brain and in the stomach, what is true about their desensitization patterns?
Neuro cells tend to become desensitized more quickly than the ones in the gut. So you can take higher and higher doses, you won’t get any of the neurological pain relief effects but you will get more GI effects.
What is beta arrestin?
Beta arrestin is a protein that causes receptor internalization.
When would beta arrestin become activated?
If desensitization occurs for a prolonged period, other proteins (clathrin AP-2) will be recruited to beta arrestin
What happens when beta arrestin binds to a receptor?
It kicks the alpha protein out so it cannot be activated. it then pulls the receptor into the cell for destruction.
Beta arrestins can do 2 things, they are:
- They can hinder the g protein coupling of agonist-activated receptors resulting in receptor desensitizaton.
- They can also act by activating signalling cascades independently of g-protein activation.
What is one good reason for beta arrestin to act as a scaffold to signalling pathways?
It holds the proteins of the signalling cascade in close proximity to each other to make the signal easier to transmit from protein to protein.
Beta arrestin dependent signalling creates ________ term activation of signalling pathways.
long
Does the activation of beta arrestin signalling pathways require the activation of a G protein.
No
What is a biased ligand?
If a drug activates one signalling pathway more effecitively than another it is said to be a biased ligand.
Is this a biased ligand.
No. A biased ligand activates one pathway more effectively than another. In this image, both pathways are activated equally.
Does this image represent a biased ligand?
Yes, this is a G-protein biased ligand
Does this image represent a biased ligand?
Yes, this is a beta arrestin biased ligand.
Are beta blockers an example of a biased ligand?
Yes. Beta blockers are G alpha biased
Niacin is an effective treatment for high cholesterol. It activates which receptor?
GPR109A
Is Niacin a biased ligand?
No, it is a balanced agonist.
What 2 pathways does niacin follow, and what are the effects of this?
The cholesterol lowering effect is G-protein mediated, but it also causes flusing which is beta arrestin mediated.
Because niacin flushing is distressing and causes some folks to stop taking niacin, new drugs are in the pipeline that will be biased ligand for which protein?
they will be biased for the G-protein so people won’t have to suffer the flushing from the beta arrestin protein activation.
