Receptor basics and mechanisms Flashcards
What is tissue selectivity?
Certain tissues exhibit certain receptors, hence why different drugs disproportionately affect different tissues
What is chemical sensitivity?
Only specific drug structures bind to receptors
What is amplification?
Small number of drug/receptor interactions initiate significant biological effects
Define agonist
Drug which binds to a receptor to produce a biology cellular response, therefore have affinity and efficacy
Define antagonist
Drug which binds to a receptor but does not produce a biological cellular response- antagonists bind to receptors and prevent agonists producing effects, therefore these only have affinity
Define affinity
The extent or fraction to which a drug binds to receptors at any given drug concentration
Define efficacy
Ability to initiate a physiological response through interaction with the receptor
Is binding to receptors reversible or irreversible?
Reversible, as most agonists and antagonists bind reversibly, as their they bind to their receptor and then dissociate from it
What is KA?
- Measures affinity
- KA is the affinity at equilibrium, and assesses the affinity of an agonist to a receptor
- Every drug has its own KA value
What does a smaller KA mean?
- The agonist has a greater affinity for a receptor than a drug with a higher KA value. This is because the KA value will show fewer free receptors, meaning most are bound to a agonist-receptor complex.
What is EC50?
- Measures efficacy
- Half maximal effective concentration, describes how well an agonist produces an action assessed by the effective concentration producing 50% of a maximal response.
- Assesses how well an agonist produces an action.
- This value can be used to compare drug potency
What is drug potency determined by?
- Affinity and Efficacy
Affinity (KA) and efficacy (EC50) of an agonist are not…?
Equal, as you don’t need full occupancy of receptors to produce a maximum response
Why don’t receptors need to be full to produce a maximum response?
- This is because receptors amplify signals, so only a small number of drug receptor interactions produce biological effect, hence why drugs work at such low concentrations
What is the effect of competitive antagonism on concentration-response curves?
Antagonist binds to and competes at the same site as the agonist, as both antagonist and agonist compete for the same receptor binding site
Curve shifts to right, as EC50 will increase due to great concentration of agonist needed to have greater affinity than antagonist. Maximal biological response is unchanged.
What are receptors?
- Molecular structures (mainly proteins) that receive input and produce an effect through activation of single transduction pathways
- Are associated with the plasma-membrane and permit communication between the outside and inside of a cell- although some are found inside the cell
- Receptors recognise endogenous chemicals (naturally occurring such as neurotransmitters) and also recognise and bind to drugs
What happens when a chemical signal reaches a receptor
- Chemical signal binds to receptor
- This causes a change in receptor protein conformation
- Signal transduction
- Cellular response
What are the 4 types of receptors?
- Ligand- gated receptors
- G-protein coupled receptors (GPCRs)
- Tyrosine kinose receptors
- Intracellular/ nuclear receptors
What happens when an agonist binds to a receptor?
It produces an agonist receptor reaction, leading to a conformation change in receptor allowing it to initiate a biological response in the cell.
What bonds can drugs form with receptors?
- Hydrogen bonding
- Ionic bonding
- Van der Waal’s forces (London forces)
- Covalent bonding
What bonds allow for easier reversible binding and good dissociation and why?
Hydrogen, ionic and van der waal’s due to their forces being relatively weak
What bond leads to irreversible binding and poor dissociation?
Covalent bonding due to its strong stable bonds
What is the name given to the molecule formed when an agonist or antagonist binds to a receptor?
Agonist/antagonist receptor complex
What happens if there are low levels of agonist or antagonist but lots of receptors?
- There will be few agonist/antagonist complexes, therefore reaction rate towards the complexes (right) is limited
Why will reaction rate eventually reduce?
- The number of receptors is finite, there’s a limit to the number of agonist/antagonist receptor complexes
What are the 4 receptor families?
- Ligand-gated receptors
- G-protein-coupled receptors
- Tyrosine kinase receptors
- Intracellular receptors
What are ligand-gated receptors?
- A class of integral membrane proteins- they form a membrane ion channel to permit the passage of select ions (e.g. Na+/K+).
- Produces a very fast response, in a matter of milliseconds, so is essential for fast communication, such as muscle movement.
What is a ligand?
- A molecule that binds to a receptor (or another molecule), usually a neurotransmitter, hormone etc.
What are properties all membrane receptors have in common?
- Trans-membrane (integral proteins)
What are trans-membrane proteins?
- Amino acids that are hydrophobic, sitting in plasma membrane
- Other areas of the protein are hydrophilic (fine with aqueous environments) and so sit on the outside/inside of the cell
What are the structural features of ligand-gated receptors?
- 5 protein subunits
- Subunits form an ion channel
- N-terminal- this is ligand binding site and there’s a binding site on each of the 5 subunits
- Extracellular site
An example of a ligand gated receptor is nicotinic receptors
What is the signal transduction mechanism of ligand-gated receptors?
- Ligand binds to receptor
- Conformation change in subunits
- Ion channel opens as a pore opens between the subunits, allowing the ions to flow and so there’s an increased ion flux as ions move down the concentration gradient
- Change in cell excitability
What are G protein coupled receptors (GPCRs)?
- A class of integral membrane proteins, represent the largest and most diverse class of receptors
- Produce slower response than ligand-gated, takes seconds to minutes. E.g. heart rate increases, but doesn’t happen instantly
What are the structural features of GPCRs?
- 1 single protein
- 7 transmembrane regions
- An N-terminal- ligand-binding site-
- A C- terminal- G protein binding site
What is the signal transduction mechanism for GPCRs?
- Ligand binds to receptor
- Activation of G-proteins
- Production of intracellular messengers
- Cellular function
What are tyrosine kinase receptors?
- Class of cell-surface proteins- characterised by their cytoplasmic region’s intrinsic tyrosine kinase activity
- Slow response- can take minutes, hours or days, example is insulin binding to insulin receptors
What are the structural features of tyrosine kinase receptors?
- 1 single protein subunit
- 1 transmembrane domain (these are regions of a protein that are hydrophobic)
- N-terminal- ligand-binding site
- C-terminal- effector binding site
What is the signal transduction mechanism of tyrosine kinase receptors?
- Ligand binding to monomers induces dimersiation -> monomers become 2, allows phosphorylation
- Monomers phosphorylate tyrosine residue in each other
- Phosphorylated intracellular domains bind cellular proteins
- Cellular function
What are intracellular/nuclear receptors?
A class of intracellular proteins characterised by their intracellular location
These have the slowest response, being hours, days, months or longer
Explain the importance of phosphorylation in the tyrosine kinase receptor transduction pathway
- Tyrosine kinase puts a phosphate group on an adjacent tyrosine phosphate
- This causes an area to be the phosphorylated region which allows the binding of cellular proteins
What are the structural features of intracellular receptors?
- Receptor found within cytoplasm of cell- can easily gain entrance to cell
- 1 single protein subunit
- DNA binding site - When activated goes to nucleus + bind to gene in DNA, then C-terminal helps control it.
- N-terminal- binds heat shock protein HSP - binds when ligand bind to but isn’t active
- C- terminal- controls transcription
An example of intracellular receptors is the steroid hormone
What is the signal transduction mechanism of intracellular receptors?
- Ligand (drug, hormone) crosses plasma membrane
- Hormones displaces HSP and binds to N-terminal
- Hormone/ receptor complex enters nucleus and binds to hormone-responsive-element on gene
- Alters gene transcription
What are G proteins?
Guanine nucleotide (GTP/GDP) binding proteins comprised of three subunits (alpha, beta, gamma)
The beta and gamma units allow the protein into the membrane
Describe the cyclic activity of drug-receptor binding produced by G-proteins
- When no drug (ligand) is bound- G protein (alpha, beta and gamma) is bound to receptor, GDP is bound to the alpha subunit
- The drug binds to its g-protein coupled receptor
- There is a change in receptor (G-protein) conformation
- GTP now binds to G alpha subunit. This is because change in interactions (due to drug binding) between G-protein and receptor leads to opening that allows GTP to come in. GTP has higher affinity to alpha subunit than GDP.
- G alpha subunit dissociates from receptor to induce a cellular response
- Intrinsic G alpha subunit has GTPase activity- GTP dephosphorylates to GDP and G-protein, causes alpha, beta and gamma subunits to re-associate and bind with unbound receptor, going back to resting state
What are the different sub-types of the G- alpha subunits?
G- alpha s
G -alpha i
G- alpha q
These are the intracellular responses that occur after the G-alpha has dissociated
What do these different alpha subunits do?
- Interact with specific targets including the enzymes adenylate cyclase (AC) and phospholipase C (PLC). These are the 2 main targets of the alpha subunit pathways.
Describe the function of Gs
Stimulates adneylyl cyclase, which catalyses the conversion of ATP to cyclic AMP (cAMP)
What is cAMP?
- An intracellular messenger
Describe the function of Gi
Inhibits adenylyl cyclase, which will then inhibit the conversion of ATP to cAMP, decreasing cAMP levels
Describe the function of Gq
- Stimulates phospholipase C, which cleaves (hydrolyses) PIP2 (plasma membrane phospholipid) in the cell membrane into IP3 (water soluble) - IP3 acts inside cell to stimulate action of IP3 receptor (a ligand-gated receptor) and calcium moves into the cytosol, within aqueous part of membrane to activate contractile mechanisms - and DAG (triglyceride) - DAG activate PKC (protein kinase C) that modulates calcium levels and contraction
How does IP3 increase calcium levels in cytosol?
- Acts at own receptor inside cell found on calcium stores in sarcoplasmic reticulum, which is ligand-gated, so when activated pores open, allows calcium influx inside cell
What are DAG, IP3 and cAMP?
Intracellular messengers, they trigger signalling cascades that lead to cellular functions and underlying changes to physiological processes
What is the G protein cycle?
- Consists of a G protein-coupled receptor (GPCR) activating the G protein by promoting the exchange of GTP for GDP, which allows the alpha, beta and gamma subunits to separate and activate downstream targets
What are the properties of G-protein-coupled-receptors?
- 1 single protein
- 7 transmembrane regions ( this is where proteins traverse )
- N-terminal - ligand-binding site, outside the cell
- C- terminal- G-protein binding site, inside the cell
- G-protein coupling region- where G proteins bind
What is the signal transduction mechanism of GPCRs?
- Ligand binds to receptor
- G-proteins activated
- Intracellular messengers produced
- Cellular function
