Pharmacology Flashcards
What is the function of receptors?
They are the sensing elements of the cell
They respond to chemical messengers e.g. hormones, transmitters
They induce a specific response within cells
- induce the release of or synthesis of hormones/transmitters
- Turn on/off a cell
- Increase/decrease gene expression
What does an agonist do?
Activates a receptor
What does an antagonist do?
Prevents a receptor from activating
Draw what happens when an antagonist is present
Draw what happens when an agonist is present
Agonist
Directly leads to ion channel opening and closing
Transduction mechanisms lead to enzyme activation/inhibition, ion channel ovulation and DNA transcription
Antagonist
Leads to no effect because endogenous mediators are blocked
What are ion channels? And how can drugs interact with ion channels?
Ion channels are channels in/on a cell that open to allow ions across.
An increase or decrease in ion concentrations can activate/inhibit specific cell functions e.g.
hormone/ transmitter release and gene transcription.
Drugs can interact with ion channels:
- At the site of the ion channel
- Via intermediate stages involved in channel opening
What does an ion channel blocker do?
Blocks the ion channel so nothing can pass through it.
What does an ion channel modulator do?
Increases or decreases ion channel opening probability
What is the function of enzymes?
Increase the rate of a specific reaction
They act to speed up the formation or breakdown of specific hormones and neurotransmitters e.g. serotonin, dopamine and acetylcholine
What is a enzyme inhibitor?
To stop the action of the enzyme. No reaction occurs.
What is an enzyme false substrate?
To slow an enzyme down a false substrate is introduced. Introduces inert molecules so that it takes longer to break down the active molecules.
For example 10 molecules of serotonin is broken down in 1 minute. But if 10 molecules of inert substances are added to the 10 serotonin molecules, it will take longer to break it down.
What is an enzyme pro-drug?
A Pro-drug is an inactive drug before it reaches the enzyme. The enzyme works on the pro-drug, activates it and then the active drug is produced.
What are the drug actions of enzymes?
Drugs can act to inhibit enzyme function e.g. monoamine oxidase inhibitors increase serotonin
Enzymes can be used as drug activators. This allows a drug to be designed so that it is active only in the regions where a specific enzyme is found.
What is the function of carrier molecules?
Provide transport of small organic molecules across cell membranes.
Facilitate the transport of lipid insoluble molecules e.g. transport of glucose and amino acids into cells and the reuptake of neurotransmitters e.g. noradrenalin and serotonin
How do normal transporters work?
Molecule passes through the cell membrane as normal
How do transport inhibitors work?
Can either block the entrance of the cell so nothing can enter it or it can remain inside the cell membrane so no molecules can pass through.
How do false substrates work for transporters?
A false substrate means that something can be placed in the cell that isn’t usually found in there. Can ‘trick’ the transporter into thinking that the molecule usually lives in the cell.
Can be used in cancer treatment to introduce drugs that kill the cells.
What is the purpose of a receptor?
Explain the two types of response given:
What does the rate of response depend on?
The purpose of a receptor is to elicit a cellular response.
The responses can be:
Rapid e.g. synaptic transmission occurring within milliseconds
Slow: e.g. thyroid hormones induced response occurring over hours and days
The rate of receptor responses depends on the molecular structure and nature of the transduction mechanism.
What is a ligand-gated ion channel? And what do they typically respond to?
Also known as ionotropic receptors or transmitter gated ion channels.
They are similar to ion channels but incorporate a ligand- binding (receptor)
They typically respond to fast acting neurotransmitters e.g. nicotinic acetylcholine receptors, GABA A receptors and NMDA receptors
What are G-protein coupled receptors? What is the response rate?
Also known as metabotropic receptors
They mediate their response via intracellular (located on the cells membrane within the cell) proteins called G-Protein activation
Responses tend to be slower than those observed with ligand-gated ion channels e.g muscarinic acetylcholine receptors and adrenoreceptors
What are kinase-linked receptors?
They are membrane receptors responding to protein meditators.
Linked to an intracellular domain by a single transmembrane helix.
In many cases the intracellular domain is enzymatic in nature e.g. receptors for insulin, cytokines, growth factors
What are nuclear receptors?
They regulate gene transcription
Located in the cytosol
Migrate to the nuclear compartment when a ligand is present e.g. steroid hormones and thyroid hormones
What is the timescales of;
A - ligand-gated ion channels
B- G-protein coupled receptors
C- kinase-linked receptors
D: nuclear receptors
A milliseconds
B seconds
C hours
D days
Draw out the summary of receptor types:
See summary of receptor types in lecture 1
Draw the structure of the 4 receptor families:
See diagram in lecture 1
What is the molecular structure of ligand-gated ion channels?
The best studied is the nicotinic acetylcholine receptor: consists of five subunits, 2 alpha, beta, gamma and delta each of 40-58 kDa
These subunits all show sequence homology
The structure requires two alpha subunits which bind acetylcholine (Ach). For activation two Ach molecules need to bind to the receptor.
Draw out the structure of a nicotine Ach Receptor
See lecture 1 notes
How do Ligand-gated ion channels work?
They control the fastest synaptic events in the nervous system e.g. postsynaptic transmission
They mediate action potentials, indicating that the speed of events is rapid (milliseconds)
E.g. Ach at a neurotransmitter junction or glutamate in the CNS increases Na+ and K+ permeability within cells
A conformational change in the ion channel results in ions being allowed to flow through it
Describe the molecular structure of G- protein coupled receptors
Basic structure comprises of seven transmembrane domains with an extracellular N-terminus and intracellular C-terminus
What is the mechanism of activation of G-protein coupled receptors? Draw the diagram to show this mechanism
Activation occurs via agonist binding, this results in conformational change of the membrane bound receptor.
Refer to diagram in lecture 1
What is the G-protein made up of?
Consists of 3 subunits, alpha, beta and gamma
Guanine nucleotides are bound to the alpha subunit of the g-protein
In the inactivation state, the alpha subunit binds GDP and the beta + gamma subunits remain together as a complex.
All three subunits remain membrane bound next to each other
Draw the mechanism of action of the G-protein
Refer to lecture 1 slides
How are G-proteins activated?
Binding of an agonist to the receptor results in a confirmations change that attracts the receptor to the alpha subunit
The GDP bound to the alpha subunit is replaced by GTP (GTP has a higher energy state than GDP due to an extra phosphate group)
The GDP/GTP exchange causes the alpha subunit to dissociate from the beta/gamma complex
What happens after a G-protein is activated?
The GTP bound alpha subunit can then move and associate itself with a target protein such as; enzymes, ion channels or intracellular messengers
This would either activate or inactivate the target protein
How are g-proteins inactivated?
Inactivation occurs by the hydrolysis of GTP to GDP through activity of the GTPase enzyme located within the alpha subunit.
The GDP bound alpha subunit then dissociates from the target protein and reunites with the beta/gamma subunit
How is the G-protein target regulated?
The target molecule regulates the activity of the GTPase enzyme
This means that the extent to which a response is required is under control of the target molecule as opposed to the G- protein
This mechanism results in the amplification of the initial receptor/agonist binding response
What are the three main targets for G-proteins?
- Adenylate cyclase: enzyme responsible for cAMP formation
- Phospholipase C: Enzyme responsible for inositol phosphate and diacylglycerol formation
- ion channels: in particular Ca2+ and K+ channels
Discuss the adenylate cyclase/ cAMP system
Cyclic 3’, 5’ adenosine monophosphate (cAMP) is a second messenger within a signal transduction cascade.
cAMP is synthesised from ATP within a cell by action of a membrane bound enzyme called adenylate cyclase
cAMP is inactivated by a family of enzymes known as phosphodiesterases
Agonist of particular G-protein receptors induce their effects by increasing /decreasing the concentration of cAMP
cAMP is hydrolysed by phosphodiesterases
What is the targets of cAMP?
cAMP is involved in the following cellular responses:
- regulating enzymes involved in energy metabolism, cell division and cell differentiation
- ion transport
- ion channels
- contractile proteins in smooth muscles
These responses are brought about by the activation of protein kinases by cAMP
Draw the example of cAMP targeting
Refer to lecture 1 notes
Describe the phospholipase C/ inositol phosphate system
Phospholipase C activation occurs via G-protein ligand binding
Phospholipase C (PLC) is a membrane bound enzyme responsible for forming the second messengers diacylglycerol (DAG) and 1,4,5-trisphosphate (IP3) from the substrate phosphatidylinositol 4,5-bisphosphate (PIP2)
Draw the IP3 second messenger pathway
Refer to lecture 2
Draw the PLC/PIP2 cycle
Refer to lecture 2 slides
What are the functions of IP3 targets?
Induces the release of Ca2+ ions from intracellular stores, such as the endoplasmic reticulum and mitochondria
Both the endoplasmic reticulum and mitochondria have IP3 receptors located on their cellular membranes
What are the functions of DAG targets?
Activates the membrane bound protein kinase, protein kinase C
Act to increase intracellular Ca2+ concentrations
Draw the summary of G-protein signalling
Refer to lecture 2 slides
What are kinase-linked and related receptors?
These proteins form receptors for various hormones and growth factors
Intracellular signalling involves tyrosine kinase activity
Agonist binding induces the dimerisation and conformational change that leads to tyrosine autophosphorylation
Through further numerous phosphorylation events, gene transcription can be regulated
Draw a diagram to show the transduction in kinase-linked receptors
See lecture 2 slides
What are nuclear receptors?
Receptor mediated DNA transcription
These receptors are located in the cytosol and move into the nucleus once ligand binding has occurred
Agonists pass through the cell membrane (lipophilic e.g. steroids) and bind to these nuclear receptors
Agonist/receptor binding results in specific gene transcription
Response an take a few minutes to a few days
How are ion channels used as drug targets?
Ions cannot penetrate the cell membrane e.g. k+, Na+ and Ca2+
The only way they canoe in/out of the cell is through protein channels
Ion channel are made from proteins that span the width of the membrane
They are characterised by:
- selectivity
- gating properties
- molecular architecture
Discuss the selectivity of ion channels
Cation selective: Na+, K+, Ca2+ or permeable to all three
Anions selective: Cl-
What are voltage gated ion channels?
Open when membrane is depolarised
Selective K+, Na+ and Ca2+ channels
Short lasting responses (milliseconds)
What are ligand gated ion channels?
Activated by agonist binding e.g. GABA, glutamate
Ca2+ activated potassium channels, open when Ca2+ increases
What are calcium release channels?
Ca2+ release from intracellular stores e.g. endoplasmic reticulum and mitochondria
What are store-operated calcium release ion channels?
Calcium induced calcium release via channels located on the cellular membrane.
Mechanism, at present, is poorly understood
How is receptor expression controlled?
Receptor expression can vary within cells
Expression can depend on a number of factors:
- cell cycle
- cell function
- time of day
Regulation is poorly understood
How can disease target receptors?
Disease can target receptors in mainly two ways:
- Auto-antibodies directed against receptor proteins
- Mutations in genes encoding for specific receptors involved in signal transduction
E.g. myasthenia graves is a neuromuscular junction disorder, where auto-antibodies target Ach receptors
Draw a diagram to show how drugs can bind to receptors
See lecture 2 slides
What are bioassays?
Methods for comparing the properties of different substances or the same substance under different conditions
A estimate of the concentration or potency of a substance by measurement of the biological response produced on administration/application
