L.15/16 Receptor Structure and Function, as drug targets Flashcards
Control and communication come primarily from the _________.
The brain and the spinal column
True or False?
Neurons do connect directly to their target cells.
False.
Neurons do NOT connect directly to their target cells. This is why, there has to be a method of carrying the message across the gap between the nerve ending and the target cell.
Define a neurotransmitter.
They carry signals between nerves; are released from the end of a neuron, they travel across a synapse, bind to receptors on a target cell and then triggers a reaction within a cell
What is a synapse?
the small gap between neurons
What is serotonin?
A neurotransmitter released by a neuron into the synapse. It then binds with specialized proteins called “serotonin receptors” embedded in the cell membrane.
This process leads to a series of secondary effects (constricting smooth muscles, transmitting impulses between nerve cells, and contributing to wellbeing and happiness)
Responsible for maintaining mood balance, so a deficit of serotonin leads to depression
Define circulating hormone
Signaling molecules released from a cell or a gland and into the circulatory system that bind with receptors on or inside the target cells.
Regulate physiology and behavior
How are chemical messengers differientiated?
They are distinguished by the route they travel and by the way they are released.
Regardless of the distinction, their actions when they reach the target cell are the same.
How do chemical messengers work?
They interact with a receptor and message is received. The cell responds to that message and adjusts its internal chemistry accordingly. A biological response results.
‘Switch on’ receptors WITHOUT undergoing a reaction.
Define a receptor
A protein molecule, located mostly in the cell membrane that receives chemical messengers from outside the cell and transmits them into the cell leading to a cellular effect.
Different receptors specific for different chemical messengers.
True or False?
Change in receptor shape results in a signal transduction, leading to a chemical signal being received inside the cell.
True
List the membrane bound receptor super families.
- ION CHANNEL RECEPTORS
- G-PROTEIN COUPLED RECEPTORS
- KINASE LINKED RECEPTORS
What are the characteristics of the neurotransmitter glutamate?
Glutamate is the primary excitatory neurotransmitter in the CNS and is found in almost all neurons.
Glutamate acts on a family of receptors called the glutamatergic receptors. Glutamatergic neurons are
important for learning.
What are the characteristics of the Gamma-amino butyric acid (GABA)?
Gamma-amino butyric acid (GABA) is the primary inhibitory neurotransmitter in the CNS. GABAergic neurons and receptors are found in high concentrations in the cerebral cortex, hippocampus, and
cerebellum. A number of CNS depressants (e.g. benzodiazepines) enhance GABA receptor function.
What is the primary excitatory neurotransmitter in the CNS?
Glutamate
Glutamatergic neurons are important for what?
Learning
What is the primary inhibitory neurotransmitter in the CNS?
Gamma-amino butyric acid (GABA)
Where are GABAergic neurons and receptors found in high concentrations?
Cerebral cortex, hippocampus, and cerebellum.
Does acetylcholine produce an excitatory or inhibitory response in the CNS?
Excitatory
What are receptors that bind acetylcholine called?
Cholinergic receptors
What are the two types of cholinergic receptors?
Nicotinic receptors and Muscarinic receptors
Nicotinic and Muscarinic receptors are what type of receptors?
Cholinergic receptors
Nicotinic receptors can be stimulated by _______.
They have ___ subunits consisting of _____.
Acetylcholine or nicotine
5 subunits: 2 alpha (contains 2 ligand sites), 1 beta, 1 gamma, 1 delta
Muscarinic receptors are involved with what?
Learning, memory, and cognitive function
What are G-Protein Coupled Receptors?
- Single protein with 7 transmembrane regions
- Responsible for activating proteins called G-proteins
Drugs that block the action of acetylcholine at muscarinic receptors produce what?
Amnesia
Loss of cholinergic neurons with muscarinic receptors is thought to be associated with what?
Alzheimer’s Disease.
What are two catecholamines similar in structure?
Dopamine and Norepinephrine.
Dopaminergic pathways are involved in control of what?
Some hormonal systems, motor coordination, and motivation and reward.
How is the “gate” of an ion channel opened?
A Chemical messenger binds to receptor binding site
Induced fit results in further conformational changes, causing each of these helices to rotate the kink points of TM2 away from each other, thus opening up the pore.
True or False?
With G- protein coupled receptors, ligand binding site on the extracellular portion of the protein varies depending on receptor type.
True
Activation of alpha and beta receptors usually leads to what?
Excitation of the cell.
What is the mechanism of ion channel receptors?
When a chemical messenger binds to the external binding site of the receptor, it causes the protein to change shape, which changes the overall shape of the protein complex, opening the ion channel and allowing a specific ion to pass through.
G-protein coupled receptors have a ______ set of ligands including peptide hormones, neurotransmitters, and odor molecules.
diverse
What is the response time of G-PROTEIN COUPLED RECEPTORS?
Seconds
What is the response time of KINASE LINKED RECEPTORS?
Minutes
The nervous system is specialized too… (basic functions of the nervous system)
Recognize, process, integrate, and react to stimuli in the environment
What is the one receptor superfamily that is not membrane bound and what is its response time?
INTRACELLULAR RECEPTORS
Response time = hours/days
What is the response time of ion channel recepters?
Milliseconds
What are ion channel receptors?
A 5 protein subunits with a Hollow center, Lined with polar amino acids Specific to specific ions (Na+, Ca2+, Cl-, K+)
True or False?
With ion channels receptors, The ion selectivity of different ion channels is NOT dependent on the amino acids lining the ion channel.
False.
the amino acids liniging the channel create the ion selectivity.
True or False?
The receptor protein is part of an ion channel protein complex and is sensitive to an external chemical messenger that controls the lock gate.
True
A Glycine receptor belongs to what receptor superfamily and has how many subunits?
Ion channel receptor
5 total subunits: 3 alpha (with 3 ligand binding sites), 2 beta
Each subunit of an ion channel receptor traverses the cell membrane ____ times having ____ transmembrane regions.
4; 4
True or False?
When it comes to the variable intracellular loop of an ion channel receptor, the length DOES NOT vary between the different types of receptor.
False.
_______ of each protein subunit ‘lines’ the central pore
TM2 - a residue
In the closed state, the kinked points of the TM2 are pointed in what direction?
towards each other
Ion flow is considered a _________. A type of signal transduction.
Secondary effect
What are orphan receptors?
Ligands that are not identified
On a G-protein coupled receptor, Glutamate resides where?
N-terminal chain
On a G-protein coupled receptor, hormones reside in the ______
Extracellular loop and N-terminal chain
On a G-protein coupled receptor, peptide hormones reside where?
top of TM helices, extracellular loops and the N-terminal chain
On a G-protein coupled receptor, monoamides can be found in the _______.
Pocket of the TM helix
In unstimulated cells, the state of G alpha is defined by _________.
its interaction with GDP, G beta-gamma and a G-protein-coupled receptor.
How do G-Protein Coupled Receptors work?
Upon receptor stimulation by a ligand, the state of the receptor changes (induced fit) and activates trimeric G-protein.
G alpha dissociates from the receptor and G beta-gamma, forming a G-α and a G-β-γ complex. GTP replaces the GDP, which leads to G alpha activation.
Activated G-protein bound to GTP activates membrane bound enzyme and the Intracellular reaction catalyzed. G alpha then goes on to activate other molecules in the cell, while G-β-γ complex activate proteins, typically affecting ion channels.
What is the signal protein for a G-protein coupled receptor?
a trimeric G-protein
What helps anchor the trimeric G-protein to the membrane?
Covalently attached lipid tails
These proteins serves a dual role as receptor and enzyme. What are they and how do they acheive this?
Tyrosine Kinase-linked Receptors
The binding of an extracellular ligand causes enzymatic activity on the intracellular side
How do Tyrosine Kinase-linked Receptors work?
- The Receptor binds to the messenger leading to an induced fit
- The Protein changes shape and opens intracellular active site
- Phosphorylation reaction (Tyr phosphorylated) is catalysed within cell, converting ATP (cofactor) to ADP
The catalytic binding region of a Tyrosine Kinase-linked Receptor is _____ in the resting state.
Closed
What is an Epidermal Growth Factor (EGF) receptor?
A type of Tyrosine Kinase-linked Receptor that has a bivalent ligand (can bind to 2 receptors simultaneously) that leads to a ligand binding and dimerization.
How does Epidermal Growth Factor (EGF) receptor work?
- It binds to both an active site for EGF and an active site for tyrosine kinase creating a ligand bond and dimerization.
- Active site on one half of dimer catalyses phosphorylation of Tyr residues on the other half
- Phosphorylated regions act as binding sites for further proteins and enzymes
- An external chemical messenger has conveyed its message to the interior of the cell without itself being altered or having to enter the cell.
What is the binding site for an Epidermal Growth Factor (EGF) receptor?
- A binding site for EGF = a protein with 53 AAs and 3 disulfide bonds
- a binding site for tyrosine kinase
What happens if the dimerization is not formed during the binding of a Epidermal Growth Factor (EGF) receptor?
The process goes straight to phosphorylation and the tyrosine kinase is not activated by induced fit.
True or False?
Induced fit opens tyrosine kinase active sites.
True
Define an Insulin Receptor
A type of Tyrosine Kinase-linked Receptor that already exists as a tetramer, a protein with a quaternary structure of four subunits.
An insulin receptor is a preformed, covalently linked tetramer with two extracellular α subunits and two membrane-spanning, tyrosine kinase-containing β subunits.
What is a Growth Hormone (GH) receptor?
a transmembrane protein and type of tyrosine-kinase recptor. The receptorexists in two forms as a full length membrane-bound receptor and as a soluble GH binding protein (GHBP).
True or False?
Growth hormone receptors have no kinase activity before binding of the message.
True.
It is the GH binding and dimerization that causes the binding of kinases via induced fit, forming the tetrameric complex.
How does a growth hormone receptor work?
Upon ligand binding, receptor dimerizes and then binds and activates tyrosine kinase enzymes from the cytoplasm.
Tetrameric complex is constructed and phosphorylation is activated and proceeds as it does with any other tyrosine-kinase receptor and ATP is converted to ADP.
What are Intracellular Receptors?
receptor proteins found on the inside of the cell, typically in the cytoplasm or nucleus.
In most cases, the ligands of intracellular receptors are small, hydrophobic molecules, since they must be able to cross the plasma membrane in order to reach their receptors.
Example: steriods and their receptors
what is the mechanism of Intracellular Receptors?
- A steriod hormone diffuses through the cell membrane.
- The hormone binds to an intracellular receptor, either in the cytoplasm or nucleus. This induced fit changes the shape of the receptor and leads to a dimerization of the ligand-receptor complex
- The dimer binds to a co-activator protein which increases gene expression (unable to bind DNA by itself).
- The whole complex binds to a particular region of the cell’s DNA. The DNA can Recognize two identical sequences separated by a short distance due to two DNA binding sites from two receptors
- The complex interacts with DNA in the nucleus, altering gene expression and cell functioning. Depending on the complex involved, it triggers or inhibits the start of transcription and affects the eventual synthesis of a protein.
What is unique about the binding regions of intracellular receptors?
8 of 9 Cys residues (SH) are involved in binding 2 Zinc ions, Allowing Serin-Zinc interactions which Stabilize and determine the conformation of the DNA binding region
What is the difference between an Agonist vs Antagonist?
An agonist is a drug that binds to the receptor, producing a similar response to the intended chemical and receptor.
an antagonist is a drug that binds to the receptor either on the primary site, or on another site, which all together stops the receptor from producing a response.
Define an Agonist
An Agonist Activates the receptor to produce a biological response.
They mimic the natural messenger using the same induced fit. Bind reversibly and are Receptor activated.
The _______ anchors the receptor in the cell membrane.
membrane-spanning domain
Why is the ligand-binding domain important?
The ligand-binding domain is stimulated by the appropriate ligand and induces a conformational change that activates the catalytic domain. This often results in the initiation of a second messenger cascade.
Define tyrosine-kinase receptors
These are enzyme-linked receptors that are composed of a monomer that dimerizes upon ligand binding.
The dimer is the active form that phosphorylates additional cellular enzymes, including the receptor itself (autophosphorylation).
the protein Gq activates ________.
phospholipase C
What are the functions of the heterotrimeric G proteins Gs and Gi?
Gs = stimulates. Gi = inhibits.
Define a G-protein coupled receptor
G protein-coupled receptors (GPCR) are a large family of integral membrane proteins involved in signal transduction.
Characterized by their seven membrane-spanning α-helices.
The receptors differ in specificity of the ligand-binding
area found on the extracellular surface of the cell.
In order for GPCRs to transmit signals to an effector in the cell, they utilize a heterotrimeric G protein.
G-proteins are named for their intracellular link to guanine nucleotides (GDP and GTP).
What alters the activity of adenylate cyclase in the G-protein receptor pathway?
The activated α subunit alters the activity of adenylate cyclase.
If the α subunit is αs, then the enzyme is activated; if the α subunit is αi, then the enzyme is inhibited.
What are the design requirements of an agonist?
- The agonist must have the correct shape to fit the binding site
- The agonist must have the correct binding groups
- The binding groups must be correctly positioned to interact with complementary binding regions
True or False?
Agonists are designed to have functional groups capable of same interactions and Usually, require same number of interactions.
True
If an agonist drug has 3 of the 3 required binding groups, but positioned in incorrect positions to were only 2 binding groups can bind simultaneously, how will this affect the drugs reactivity?
It will lead to weak activity.
The lack of even one of these interactions would lead to a significant loss in activity.
True or False?
An agonist doesn’t have to have the correct size and shape to fit binding site.
False.
An agonist MUST HAVE the same size and shape to fit the binding site bc an agonist depends on induced fit.
Groups preventing access of an agonist to the binding site are called ________
steric shields or steric blocks
Define Allosteric Modulators
Allosteric Modulators Bind to a site on a receptor other than the ligand binding site and induce a conformational change that increases the binding affinity of the receptor for the target ligand.
They Mimic the action of endogenous modulators
They Binding to an allosteric binding site rather than the ligand binding site
Enhance receptor activity and have an Indirect agonist effect
What is an example of an allosteric modulator?
Benzodiazepines (BDZs)
They are a class of tranquilizer drugs that Cause an allosteric modification of the receptor that results in an increase in GABAA receptor activity.
Bind to distinct benzodiazepine binding sites situated at the interface between the α- and γ-subunits
Define GABA
· γ-aminobutyric acid
· Cause Cl- influx and subsequent membrane hyperpolarization
· Binding site is located between α- and β-subunits
How do Benzodiazepines (BDZs) work?
They mimic the GABAA receptor.
When two molecules of GABA bind to its receptor, the receptor channel opens, and chloride ions rush into the neuron. BSZs mimic this.
BDZs binding increases the frequency of channel opening events which leads to an increase in chloride ion flow
What are partial agonists?
Agents which act as agonists but produce a weaker effect
What is the difference between a full agonist and a partial agonist?
A full agonist Induces a conformational change in the receptor leading to a maximal effect
A partial agonist Induces some degree of conformational change in the receptor leading to a partial effect
How do partial agonists work?
The Agent binds but does not produce the ideal induced fit for maximum effect leading to only a slight shift and partial opening of an ion channel
What is an inverse agonist?
It Binds to the same binding site as an agonist but decreases the activity
What is the prerequisite for an inverse agonist?
the receptor must have a constitutive (inherent) activity (present in the absence of a chemical messenger)
How is an inverse agonist similar to an antagonist?
Same effect as an antagonist by preventing an inherent activity
Properties shared with antagonists
- Bind to ligand binding sites with a different induced fit from the normal ligand
- Receptor is not activated
- Natural ligand is blocked from binding to the binding site
How is an inverse agonist different than an antagonist?
an inverse agonist is a drug that binds to the same receptor as an agonist but induces an opposite pharmacological response to that of the agonist.
A neutral antagonist has no activity in the absence of an agonist or inverse agonist but can block the activity of either.
Properties not shared with antagonists
- Doesn’t Block any inherent activity (present in the absence of a natural ligand) related to the receptor
- Does not compete with the natural ligand
Define an antagonist
Blocks a biological response by binding to a receptor
- Functions to prevent the natural messenger from binding
- Has stronger and/or more binding interactions
- Has a different induced fit
- Receptor not activated
What are reversibile antagonists and how do they work?
- Antagonist binds reversibly to the binding site
- Intermolecular bonds involved in binding
- Different induced fit means receptor is not activated
- Messenger is blocked from the binding site
- The antagonist does not undergo any reaction
- Level of antagonism depends on strength of antagonist binding and concentration
- Increasing the messenger concentration reverses antagonism
True or False?
Antagonists can form binding interactions with binding regions in the binding site not used by the natural messenger.
True
True or False?
Reversibile antagonists have an extra hydrophobic bind region.
True
With reversibile antagonists, the different induced fit results from what?
binding of the natural messenger via the extra hydrophobic binding region
What are the binding particulars of raloxifene?
- Phenol groups mimic phenol and alcohol of estradiol
- Interaction with Asp-351 is important for antagonist activity
- Side chain prevents receptor helix H12 folding over as lid
* Coactivator cannot bind
Define an estrogen receptor and give an example
- Member of the nuclear receptor (NR) superfamily of ligand-activated transcription factors
- Associated with a diverse of diseases including cardiovascular problems, osteoporosis, and breast cancer
Example = estradiol
How does an antagonist work on an estrogen receptor?
Estrogen receptor receives H12, activating estradiol which causes H12 to fold over the binding site like a lid forming a hydrophobic groove and exposing the AF-2 (activating function) regions – coactivator binding site.
The AF-2 regions are Dimerized and nuclear transcription factor allows The coactivator protein to bind as a short α helix to a hydrophobic groove on the surface of the AF-2 regions (ligand-binding domain).
The whole complex then Binds to a specific region of DNA amd Switches on the transcription of a gene resulting in the synthesis of a protein
What are the binding particulars of estradiol?
- Phenol and alcohol are important binding groups
- Binding site is spacious and hydrophobic
- Phenol group is positioned in narrow slot
- Orientates rest of molecule
* Acts as agonist
What is raloxifene?
• an antagonist (anticancer agent)
Define an irreversible antagonist
- Antagonist binds irreversibly to the binding site
- Different induced fit means that the receptor is not activated
- Covalent bond is formed between the drug and the receptor
- Messenger is blocked from the binding site
- Increasing messenger concentration does not reverse antagonism
Define allosteric antagonists
- Antagonist binds reversibly to an allosteric binding site
- Intermolecular bonds formed between antagonist and binding site
- Induced fit alters the shape of the receptor
- Binding site is distorted and is not recognised by the messenger
Increasing messenger concentration does not reverse antagonism
An inverse agonist produces _______ while an agonist produces _______.
inverse agonist = negative intrinsic drug activity (response)
Agonist = positive intrinsic drug activity (response)
List the type of drug that corresponds to each color point on the graph below
Blue = Agonist
Red = Antagonist
Black = Inverse Agonist
In reference to the graph below, if KD for Drug B < KD for Drug A, which drug has the greater affinity?
Drug B has more affinity because its KD is lower. The lower a drug’s KD the greater its affinity, meaning the stronger the drug binds to the receptor.
What is affinity?
How strongly that drug binds to the receptor
Define KD
The dissociation constant.
It has molar units (M), which corresponds to the concentration of ligand [L] at which the binding site on a particular protein is half occupied ([LP] = [P])
What is Efficacy vs Potency?
Efficacy is the Maximum effect that a drug can produce regardless of dose while Potency is the Amount of a drug that is needed to produce a given effect.
Potency is a matter of drug affinity while efficacy is a matter of drug activity.
Is it possible for a drug to be potent (i.e. active in small doses) but have a low efficacy?
Yes.
Potency is the comparative measure of how much drug is required to achieve a defined biological effect – the smaller the dose required, the more potent the drug. While efficacy is the maximum effect that a drug can produce regardless of dose.
So a drug can be very potent, meaning it can cause an effect in a small dosage, but also have low efficacy, meaning it takes a much larger dose of the drug to achieve the optimal effect.
In reference to the graph below, which drug is more potent and which drug has a higher efficacy?
Drug B is more potent because it produced a response at a lower concentration.
Drug A has a higher efficacy because it produced a maximum response while drug B didn’t.
